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Regulatory mechanism of a light-dependent protochlorophyllide oxidoreductase in chlorophyll biosynthesis and environmental adaptation

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  • Chlorophyll is a vital component of photosynthesis and must be produced throughout the plant life cycle. Light-dependent protochlorophyllide oxidoreductase (LPOR) is a pivotal enzyme in the chlorophyll biosynthesis pathway, catalyzing the conversion of Pchlide to Chlide. The presence of different types of LPOR ensures the efficient synthesis of chlorophyll in photosynthetic organisms during the dark-light transition. In addition to the transcriptional, translational, and post-translational regulation of LPOR function under different abiotic stresses, the nature of the substrate also influences LPOR function. Here, a perspective on chlorophyll synthesis and the development of chloroplasts is offered, the importance of LPOR in safeguarding plant light energy utilization is summarized, the gene expression pattern and structural-functional features of LPOR are outlined, as well as the role of LPOR in abiotic stress tolerance response, the catalytic mechanism of LPOR as well as the modulation of LPOR by light signals and other environmental factors are discussed. The aim is to provide references for the cultivation and innovation of plant germplasm resources with stress tolerance.
  • Agricultural management practices impact soil physicochemical properties to a remarkable extent. Degradation of soil health has led to a contraction in agricultural production and soil biodiversity particularly due to conventional farming practices, indiscriminate use of inorganic fertilizers (INF) and inadequate input of residues[1]. Organic or inorganic fertilizers have been regarded as a critical component of agriculture to accomplish global food security goals[2]. The exogenous supply of fertilizers could easily alter soil properties by restoring the nutrients that have been absorbed by the plants[2]. Thus, implementing adequate nutrient management strategies could boost plant yield and sustain plant health. Tillage affects the soil, especially for crop production and consequently affects the agro-ecosystem functions. This involves the mechanical manipulation of the soil to modify soil attributes like soil water retention, evapotranspiration, and infiltration processes for better crop production. Thus, tillage practices coupled with fertilizer inputs may prove a viable strategy to improve soil health components such as nutrient status, biodiversity, and organic carbon.

    Soil serves as a major reservoir of nutrients for sustainable crop production. Intensive cultivation due to growing population burden has led to the decline of soil nutrient status that has adversely affected agricultural production. Various researchers have assessed the soil nutrient budget and the reasons behind decline of nutrient content in soil[3]. Soil management strategies have assisted in overcoming this problem to a greater extent. Tillage practices redistribute soil fertility and improve plant available nutrient content due to soil perturbations[4]. Different tillage and fertilization practices alter soil nutrient cycling over time[5]. Fertilization is an important agricultural practice which is known to increase nutrient availability in soil as well as plants[6]. A report has been compiled by Srivastava et al.[7], which assessed the effectiveness of different fertilizers on soil nutrient status in Indian soils.

    Soil biota has a vital role in the self-regulating functions of the soil to maintain soil quality which might reduce the reliance on anthropogenic activities. Soil microbial activities are sensitive to slight modifications in soil properties and could be used as an index of soil health[8]. Maintenance of microbial activity is essential for soil resilience as they influence a diverse range of parameters and activities including soil structure formation, soil SOM degradation, bio-geochemical cycling of nutrients etc.[9]. Various researchers have identified microbial parameters like microbial biomass carbon (MBC), potentially mineralizable nitrogen (PMN), soil respiration, microbial biomass nitrogen (MBN), and earthworm population as potential predictors of soil quality. Geisseler & Scow[10] have compiled a review on the affirmative influence of long-term mineral fertilization on the soil microbial community.

    Being the largest terrestrial carbon (C) reservoir, soil organic carbon (SOC) plays a significant role in agricultural productivity, soil quality, and climate change mitigation[11]. Manure addition, either solely or along with INF augments SOC content which helps in the maintenance and restoration of SOM more effectively as compared to the addition of INF alone[12]. Enhancement of recalcitrant and labile pools of SOC could be obtained through long-term manure application accentuating the necessity of continuous organic amendments for building up C and maintaining its stability[13]. Generally, compared with manure addition, INF application is relatively less capable of raising SOC and its labile fractions[14]. Alteration in SOC content because of management strategies and/or degradation or restoring processes is more prominent in the labile fraction of soil C[15]. Several fractions of soil C play vital roles in food web and nutrient cycles in soils besides influencing many biological properties of soil[16]. Thus, monitoring the response of SOC and its fractions to various management practices is of utmost importance.

    A positive impact on SOC under manure application coupled with INF in rice-wheat systems has been reported, as compared to sole applications of INFs[17]. Although ploughing and other mechanical disturbances in intensive farming cause rapid OM breakdown and SOC loss[18], additional carbon input into the soil through manure addition and rational fertilization increases carbon content[13]. Wei et al.[19] in light sandy loam soil of China found that the inclusion of crop straw together with inorganic N, P and K fertilizers showed better results for improving soil fertility over sole use of inorganic fertilizers. Zhu et al.[20] studied the influence of soil C through wheat straw, farmyard manure (FYM), green manure, and rice straw on plant growth, yield, and various soil properties and found that the recycling of SOM under intensive cultivation is completely reliant on net OM input and biomass inclusion. However, most of the studies on residue management and organically managed systems could not provide clear views regarding the relations between the quality of OM inputs and biological responses towards it. The disintegration of soil aggregates due to ploughing, use of heavy machinery, and residue removal has been reported widely under conventional tillage (CT) practices[21]. On the contrary, improvement in SOC stabilization has also been observed by some scientists[22]. Under CT, the disintegration of macro-aggregates into micro-aggregates is a prominent phenomenon, while conservation tillage has been identified as a useful practice for increasing macro-aggregates as well as carbon sequestration in agricultural soils[23]. By and large, the ploughing depth (0–20 cm) is taken into consideration for evaluating the impact of tillage and straw retention on soil aggregation[24], while degradation in deeper layers of soil is becoming a major constraint towards soil quality together with crop yield[25].

    Hence, the present review would be useful in determining how tillage practices and inorganic and organic fertilization impact nutrient availability in the soil, microbial composition and SOC fractions besides stocks under different land uses.

    Agricultural production is greatly influenced by nutrient availability and thus nutrient management is required for sustaining higher yields of crop. The term 'nutrient availability' refers to the quantity of nutrients in chemical forms accessible to plant roots or compounds likely to be converted to such forms throughout the growing season in lieu of the total amount of nutrients in the soil. For optimum growth, different crops require specifically designed nutrient ratios. Plants need macronutrients [nitrogen (N), phosphorus (P), potassium (K) in higher concentrations], secondary nutrients [calcium (Ca), magnesium (Mg), sulphur (S) in moderate amounts as compared to macronutrients] and Micronutrients [Zn (zinc), Fe (iron), Cu (copper), B (boron), Mn (manganese), Mo (molybdenum) in smaller amounts] for sustainable growth and production[26]. Fertilizers assist the monitoring of soil nutrient levels by direct addition of required nutrients into the soil through different sources and tillage practices may alter the concentration of available plant nutrients through soil perturbations. Various studies on the influence of fertilization and tillage practices on available plant nutrients have been discussed below.

    Yue et al.[27] reported that long-term fertilization through manure/INF improved the macronutrient content of Ultisol soil in China. Two doses of NPK (2NPK) considerably improved soil properties over a single dose (NPK). Combined application (NPK + OM) resulted in higher hydrolysable N and available P over the sole OM application. The total K content was higher under the treatments NPK, 2NPK and NPK + OM than sole OM treatment, whereas available K was higher in treatments NPK + OM and 2NPK over the sole OM and NPK. Likewise, OM, INF, and OM + INF were evaluated for their potential to regulate the soil macronutrient dynamics. Organic manure significantly improved the soil N content, whereas INF showed comparable results to that of the control treatment. Besides, all the treatments improved available P and exchangeable K concentration[28].

    Hasnain et al.[29] performed comparative studies of different ratios of INF + compost and different application times for the chemical N fertilizer on silty loamy soils of China. The available nitrogen and phosphorous content were greater in conjoint OM + INF application over the bare INF and control application irrespective of N application time. Soil quality substantially improved with increasing ratio of compost and 70:30 (INF to compost ratio) was found to be most suitable to maintain soil fertility and nutrient status. Another study by Liu et al.[30] reported the superior effects of NPK + pig manure and NPK + straw to improve soil available P and K over the control and sole NPK treatments. However, total N concentration did not exhibit any significant variation under any treatment.

    Shang et al.[31] accounted the positive impact of vermicompost and mushroom residue application on grassland soil fertility in China. The addition of organic manures improved available P and K content to a considerable extent. Under moisture-deficit winter wheat-fallow rotation, another study quantified the influence of residue management approaches and fertilizer rate on nutrient accrual. Residue burning resulted in no decline in soil macronutrient content, whereas the perpetual addition of FYM for 84 years significantly improved total N and extractable K and P concentration. Thus, residue incorporation along with FYM application may prove beneficial in reducing the temporal macronutrient decline[32].

    Ge et al.[33] examined the effects of NPK and NPK along with manure (NPKM) addition on the macronutrient status of Hapli-Udic Cambisol soil. The NPKM application resulted in the highest increase in total N, available-P and K concentration as compared to NPK and control. Likewise, mineral fertilization reduction and partial substitution with organic amendments have posed a significant influence on soil macronutrient status. Soil available P and K decreased after INF reduction[34]. Chen et al.[35] evinced that integrated application of manure and mineral fertilizers to red clay soil (typical Ultisols) improved hydrolyzed nitrogen and available P due to an increase in the decomposition of organic matter (OM) and N bio-fixation than sole mineral fertilizers and control.

    A long-term experiment was carried out out by Shiwakoti et al.[36] to ascertain the influence of N fertilization and tillage on macronutrient dynamics in soil. Nitrogen fertilization produced higher crop biomass which might have improved total N and P concentration in soil. Moreover, the reduced interaction between soil colloids and residue or greater cation exchange sites due to tillage practices could have augmented K concentration in 0−10 cm soil depth. Likewise, among tillage systems combined organic (poultry manure) and inorganic (lime and fertilizers) fertilization, no-tillage, and reduced tillage with organic fertilization resulted in higher availability of P owing to minimal disturbance of soil which decreases contact surface between phosphate ions and adsorption sites. Greater losses of K in runoff water under NT resulted in lower K availability under NT than CT[37].

    The influence of tillage systems on soil nutrient dynamics showed that minimal soil disturbances under zero tillage prohibited redistribution of soil nutrients and resulted in the highest available N, P, and K in the surface soil[38]. The influence of tillage timing on soil macronutrient status has also been assessed under tillage treatments that are fall tillage (FT), spring tillage (ST), no tillage (NT), and disk/chisel tillage (DT/CT) on mixed mesic Typic Haploxerolls soil. All the tillage systems differed in the quantity of residues generated. Thus, variation in the decomposition of crop residue and mineralization of SOM resulted in variable rates of nutrient release. The FT and ST had the highest N content over DT/CT and NT systems at corresponding depth. The N content also decreased with soil depth irrespective of tillage treatment. The available P and extractable K were highest under NT at the top 10 cm soil depth and increased over time[39]. Residue management in combination with tillage treatments (ST and CT) has been reported to affect the soil macronutrient status in Bangladesh. Tillage treatments enhanced the total N content to a considerable extent. Moreover, 3 years of residue retention led to a higher concentration of total N, available P and K in the soil.

    The combinations of N, P, and K in different ratios together with two rates of organic fertilizer (OF) applied on the aquic Inceptisol having sandy loam texture influenced the micronutrient status of the soil[40]. Soil Zn content decreased with time when no fertilizer was applied as compared to organic fertilizer (OF) application. The mineral fertilizer treatments led to a substantial increase in DTPA-extractable micronutrients in the soil. The higher micronutrient concentration due to higher OM highlights the importance of maintaining OM for soil fertility and higher crop production. Further studies revealed that long-term application of sole N fertilizers led to a significant decline in total Zn and Cu, whereas Mn and Fe status improved through atmospheric deposition. Phosphorus and OF addition along with straw incorporation markedly increased total Zn, Cu, Fe, and Mn. The DTPA-extractable Mn, Zn, Fe, and Cu were also higher in OF treatment, thus demonstrating the beneficial effects of constant OM application for maintaining the nutrient status of soil[41].

    López-Fando & Pardo[42] quantified the impact of various tillage practices including NT, CT, minimum tillage (MT), and zone-tillage (ZT) on soil micronutrient stocks. Tillage systems did exhibit a significant influence on plant available Fe stocks in the topsoil; however, diminished with depth under ZT, NT and MT. Manganese was higher in NT and ZT at all depths and increased with soil depth. Zinc was highest under NT and other results did not vary significantly as in the case of Cu. The SOC levels were also found to be responsible to affect micronutrients due to tillage practices. Likewise, in Calciortidic Haploxeralf soil the distribution of soil micronutrients (Zn, Mn, Fe, Cu) was ascertained under different tillage practices (CT, MT, and NT). The micronutrient status was highest under NT in the upper layers due to the higher SOC level[43].

    Sharma & Dhaliwal[44] determined that the combined application of nitrogen and rice residues facilitated the transformation of micronutrients (Zn, Mn, Fe, Cu). Among different fractions, the predominant fractions were crystalline Fe bound in Zn, Mn, and Cu and amorphous Fe oxide in Fe with 120 kg N ha˗1 and 7.5-ton rice residue incorporation. The higher content of occluded fractions adduced the increment in cationic micronutrient availability in soil with residue incorporation together with N fertilization due to increased biomass. Rice straw compost along with sewage sludge (SS) and INF also affected the micronutrient availability under the RW cropping system. Nitrogen fertilization through inorganic fertilizers and rice straw compost and sewage sludge (50% + 50%) improved soil micronutrient status due to an increase in SOM over sole NPK fertilizers[45]. Earlier, Dhaliwal et al.[46] in a long-term experiment determined that different combinations of NPK along with biogas slurry as an organic source modified the extractable micronutrient status of the soil.

    A comparative study was carried out by Dhaliwal et al.[47] to ascertain the long-term impact of agro-forestry and rice–wheat systems on the distribution of soil micronutrients. The DTPA-extractable and total Cu, Zn, Fe, and Mn were greater in the RW system due to the reduced conditions because of rice cultivation. Under the RW system Zn removal was higher which was balanced by continuous Zn application. The higher availability of Fe under the RW system was due to reduced conditions. Contrarily, Mn was greater under the agro-forestry system owing to nutrient recycling from leaf litter.

    The long-term impact of integrated application of FYM, GM, WCS (wheat-cut straw) and INF on the soil micronutrients (Zn, Mn, Cu, and Fe) have been studied by Dhaliwal et al.[48]. The FYM application substantially improved DTPA-extractable Zn status followed by GM and WCS, whereas Cu content was maximum in the plots with OM application. The highest Fe concentration was recorded in treatment in which 50% recommended N supplied through FYM. This could be ascribed to the release of micronutrients from OM at low soil pH.

    Shiwakoti et al.[49] studied the dual effects of tillage methods (MP, DP, SW) and variable rates of N (0, 45, 90, 135 and 180 kg ha−1) on the distribution of micronutrients under a moisture-deficit winter wheat-fallow system. The soil Mn content was highest under the DP regime. Inorganic N application reduced Cu content in the soil. Comparative studies with adjacent undisturbed grass pasture indicated the loss of Zn and Cu to a significant extent. Thus, DP along with nitrogen added through inorganic fertilizers could improve micronutrient concentration in the soil. Moreover, the results implied that long-term cultivation with nitrogen fertilization and tillage results in the decline of essential plant nutrients in the soil. Thus, organic amendments along with INF may prove an effective approach to increase soil micronutrient content. In another study conducted by Lozano-García & Parras-Alcántara[50] tillage practices such as NT under apple orchard, CT with the wheat-soybean system and puddling (PD) in the rice-rice cropping system were found to affect nutrient status. Under CT, Cu content was lowest and Zn content was highest. On the contrary, puddling caused an increase in Fe and Mn concentration owing to the dispersion of soil aggregates which reduced the percolation of water and created an anaerobic environment thereby enhancing the availability of Fe and Mn.

    Tillage practices along with gypsum fertilization have been known to affect secondary nutrient concentrations in soil. In a long-term experiment, FYM application showed maximum response to increased S concentration due to the maximum addition of OM through FYM over other treatments as S is an essential component of OM and FYM[32]. Higher Mg content was recorded in FYM and pea vine treatments because the application of organic matter through organic manure or pea vines outright led to Mg accrual. The lower Mg concentration in topsoil than the lower layers was due to the competition between Mg and K for adsorbing sites and thus displacement of Mg by K. Han et al.[28] while ascertaining the impact of organic manures and mineral fertilizers (NPK) on soil chemical attributes determined that INF application reduced exchangeable calcium, whereas no significant changes were exhibited in the magnesium concentrations. The OM application significantly increased both the calcium and magnesium concentrations in the soil.

    While ascertaining the effect of different tillage treatments such as CT, NT, and MT on exchangeable and water-soluble cations, Lozano-García & Parras-Alcántara[50] recorded that NT had greater content of exchangeable Ca2+ and Mg2+ than MT and CT. The exchangeable Ca2+ decreased with depth, however, opposite results were observed for Mg2+ which might be due to the higher uptake of Mg2+ by the crop. On another note, there might be the existence of Mg2+-deficient minerals on the surface horizon. Alam et al.[51] studied the temporal effect of tillage systems on S distribution in the soil and observed that available S was 19%, 31%, and 34% higher in zero tillage than in minimum tillage, conventional tillage, and deep tillage, respectively.

    Kumar et al.[38] appraised the impact of tillage systems on surface soil nutrient dynamics under the following conditions: conventional tillage, zero till seeding with bullock drawn, conventional tillage with bullock drawn seeding, utera cropping and conservation tillage seeding with country plough and observed that tillage had a significant impact on the available S content. Compared with conventional tillage, zero and minimum tillage had higher S content as there was none or limited tillage operations which led to the accumulation of root stubble in the soil that decomposed over time and increased S concentration.

    Soil is considered a hotspot for microbial biodiversity which plays an important role in building a complex link between plants and soil. The microbial components exhibit dynamic nature and, therefore, are characterized as good indicators of soil quality[52]. These components include MBC, MBN, PMN and microbial respiration which not only assist in biological transformations like OM conversion, and biological nitrogen fixation but also increase nutrient availability for crop uptake. Management strategies such as fertilizer inputs and tillage practices may exert beneficial effects on soil biota as discussed below.

    Soil is an abode to a considerable portion of global biodiversity. This biodiversity not only plays a pivotal role in regulating soil functions but also provides a fertile ground for advancing global sustainability, especially agricultural ventures. Thus, the maintenance of soil biodiversity is of paramount importance for sustaining ecosystem services. Soil biodiversity is the diverse community of living creatures in the soil that interact not only with one another but also with plants and small animals to regulate various biological activities[53]. Additionally, it increases the fertility of soil by converting organic litter to SOM thereby enhancing SOC content. Thus, the SOM measures the number and activity of soil biota. Furthermore, the quality and amount of SOC, as well as plant diversity have a considerable impact on the soil microbial community structure[54].

    Dangi et al.[55] ascertained the impact of integrated nutrient management and biochar on soil microbial characteristics and observed that soil amended with biochar or the addition of organic manures influenced microbial community composition and biomass and crop yield. After two years, the higher rates of biochar significantly enhanced the levels of gram-positive and gram-negative bacterial phospholipid fatty acid (PLFA), total arbuscular mycorrhizal fungal (AMF) than lower rates, unfertilized and non-amended soil. Luan et al.[56] conducted a comparison study in a greenhouse to assess the effects of various rates of N fertilizer and kinds (inorganic and organic) on enzyme activities and soil microbial characteristics. Microbial growth (greater total PLFAs and microbial biomass carbon) and activity were promoted by manure substitution of mineral fertilizer, particularly at a higher replacement rate. On account of lower response in bacterial over fungal growth, manure addition led to a greater fungi/bacteria ratio. Furthermore, manure application significantly enhanced microbial communities, bacterial stress indicators and functional diversity. Lazcano et al.[57] determined the influence of different fertilization strategies on microbial community structure and function, soil biochemical properties and crop yield three months after addition of fertilizer. The integrated fertilizer regimes augmented microbial growth with improved enzyme activity as compared to sole inorganic amendments. Bacterial growth showed variable response with variation in fertilizer regime used whereas fungal growth varied with the amount of fertilizer added. Compared to mineral fertilizers, manure application led to a rapid increase in PLFA biomarkers for gram-negative bacteria. The organic amendments exhibited significant effects even at small concentration of the total quantity of nutrients applied through them; thus, confirming the viability of integrated fertilizer strategies in the short term.

    Kamaa et al.[58] assessed the long-term effect of crop manure and INF on the composition of microbial communities. The organic treatments comprised of maize (Zea mays) stover (MS) at 10 t ha−1 and FYM @ 10 t ha−1, INF treatments (120 kg N, 52.8 kg P-N2P2), integrated treatments (N2P2 + MS, N2P2 + FYM), fallow plot and control. The treatment N2P2 exhibited unfavourable effects on bacterial community structure and diversity that were more closely connected to the bacterial structure in control soils than integrated treatments or sole INF. In N2P2, fungal diversity varied differently than bacterial diversity but fungal diversity was similar in the N2P2 + FYM and N2P2 + MS-treated plots. Thus, the total diversity of fungal and bacterial communities was linked to agroecosystem management approaches which could explain some of the yield variations observed between the treatments. Furthermore, a long-term experiment was performed by Liu et al.[59] to study the efficiency of pig manure and compost as a source for N fertilization and found unique prokaryotic communities with variable abundance of Proteobacteria under compost and pig manure treatments.

    Recently, Li et al.[60] assessed the influence of different tillage practices (no-tillage, shallow tillage, deep tillage, no-tillage with straw retention, shallow tillage with straw retention and deep tillage with straw retention) on microbial communities and observed that tillage practices improved the bacterial Shannon index to a greater extent over the no-tillage plots in which the least value was recorded. Another research study by He et al.[61] reported the effect of tillage practices on enzyme activities at various growth stages. Across all the growth stages, enzyme activities of cellobiohydrolase (CBH), β-xylosidase (BXYL), alkaline phosphatase (AP), β-glucosidase (BG), β-N-acetylglucosamines (NAG) were 17%−169%, 7%−97%, 0.12%−29%, 3%−66%, 23%−137% greater after NT/ST, NT, ST, ST/PT, and PT/NT treatments as compared to plow tillage. The NT/ST treatment resulted in highest soil enzyme activities and yield, and thus was an effective and sustainable method to enhance soil quality and crop production.

    Microbes play a crucial role in controlling different soil functions and soil ecology and microbial community show significant variation across as well as within the landscape. On average, the total biomass of microbes exceeds 500 mg C kg soil−1[62]. Microbial biomass carbon is an active constituent of SOM which constitutes a fundamental soil quality parameter because SOM serves as a source of energy for microbial processes and is a measure of potential microbial activity[48,63]. Soil systems that have higher amounts of OM indicate higher levels of MBC. Microbial biomass carbon is influenced by many parameters like OM content in the soil, land use, and management strategies[64]. The MBC and soil aggregate stability are strongly related because MBC integrates soil physical and chemical properties responds to anthropogenic activities.

    Microbial biomass is regarded as a determinative criterion to assess the functional state of soil. Soils having high functional diversity of microbes which, by and large, occurs under organic agricultural practices, acquire disease and insect-suppressive characteristics that could assist in inducing resistance in plants[65]. Dou et al.[66] determined that soil microbial biomass C (SMBC) was 5% to 8% under wheat-based cropping systems and zero tillage significantly enhanced SMBC in the 0−30 cm depth, particularly in the upper 0 to 5 cm. According to Liang et al.[67], SMBC and soil microbial biomass N (SMBN) in the 0−10 cm surface layer were greater in the fertilized plots in comparison to the unfertilized plots on all sampling dates whereas microbial biomass C and N were highest at the grain filling stage. Mandal et al.[68] demonstrated that MBC also varied significantly with soil depth. Surface soil possessed a maximum MBC value than lower soil layers due to addition of crop residues and root biomass on the surface soil. The MBC content was highest with combined application of INF along with farmyard manure and GM, whereas untreated plots showed minimum MBC values. The incorporation of CR slows down the rate of mineralization processes; therefore, microbes require more time to decompose the residues and utilize the nutrients released[69]. On the other hand, incorporation of GR having a narrow C:N ratio enhances microbial activity and consequently accelerates mineralization in the soil. Malviya[70] also recorded that the SMBC contents were significantly greater under RT than CT, regardless of soil depth which was also assigned to residue incorporation which increases microbial biomass on account of higher carbon substrate in RT.

    Naresh et al.[71] studied the vertical distribution of MBC under no-tillage (NT), shallow (reduced) tillage and normal cultivated fields. A shallow tillage system significantly altered the tillage induced distribution of MBC. In a field experiment, Nakhro & Dkhar[72] examined the microbial populations and MBC in paddy fields under organic and inorganic farming approaches. The organic source used was a combination of rock phosphate, FYM and neem cake, whereas a mixture of urea, muriate of potash and single super phosphate was used as an inorganic source. The organically treated plots exhibited the highest MBC compared to inorganically treated plots and control. Organic carbon exhibited a direct and significant correlation with bacterial and fungal populations. The addition of organic fertilizers enhanced the content of SOC and consequently resulted in higher microbial count and MBC. Ramdas et al.[73] investigated the influence of inorganic and organic sources of nutrients (as minerals or INF) applied over a five-year period on SOC, MBC and other variables. It was observed that the addition of FYM and conjoint application of paddy straw (dry) and water hyacinth (PsWh) (fresh) significantly increased the SOC content than vermicompost, Chromolaena adenophorum (fresh) and Glyricidia aculeate (fresh), and Sesbania rostrata (fresh).

    Xu et al.[74] evaluated the influence of long-term fertilization strategies on the SOC content, soil MBN, soil MBC, and soil microbial quotient (SMQ) in a continuous rice system and observed that MBC at the main growth stages of early and late rice under 30% organic matter and 70% mineral fertilizer and 60% organic matter and 40% mineral fertilizer treatments was greater as compared to mineral fertilizer alone (MF), rice straw residues and mineral fertilizer (RF), and no fertilizer (CK) treatments. However, SMBC levels at late growth stages were greater in comparison to early growth stages. A recent study by Xiao et al.[75] demonstrated that increasing tillage frequency (no-tillage, semi-annual tillage, and tillage after every four months, two months, and one month) decreased soil MBC. Microbial biomass carbon content was significantly greater in no-till treatment (597 g kg−1) than in tillage every four months (421 g kg−1), two months (342 g kg−1) and one month (222 g kg−1). The decrease in the content of MBC in association with tillage practices is due to soil perturbations which enhanced soil temperature, diminished soil moisture content, and resulted in the destruction of microbial habitat and fungal hyphae. Therefore, the MBC content eventually affected the N cycle.

    Li et al.[76] reported that in comparison to CT, NT and RT resulted in increased MBC content and NT significantly increased MBC by 33.1% over CT. Furthermore, MBC concentration was 34.1% greater in NT than RT. The increase in MBC concentration was correlated with the results of increase in SOC concentration. Site-specific factors including soil depth and mean annual temperature significantly affected the response ratio of MBC under NT as compared to the duration of NT.

    Microbial biomass nitrogen (MBN) is a prominent indicator of soil fertility as it quantifies the biological status of soil. Soil MBN is strongly associated with organic matter of the soil. The nitrogen in MBN has a rapid turnover rate thereby reflecting the changes in management strategies way before the transformations in total N are discernable[77].

    In an experiment on continuous silage maize cultivation with crop rotation, Cerny et al.[78] observed that organic fertilizers exerted an affirmative influence on the soil MBN. During the application of organic manure MBN decreased, but there was higher MBN content as compared to control. However, addition of mineral nitrogenous fertilizers exerted an adverse effect on MBN content in experiments with maize. El-Sharkawi[79] recorded that organic matter-treated pots resulted in maximum MBN content than urea-treated pots. The sludge application enhanced total MBN and, therefore, could implicitly benefit crop production particularly in poor soils[18]. Sugihara et al.[80] observed that during the grain-filling stage in maize, residue and/or fertilizer addition exerted a pronounced influence on soil microbial dynamics; however, a clear effect of residue and ⁄or fertilizer addition was not observed. Microbial biomass nitrogen reduced dramatically from 63–71 to 18–33 kg N ha˗1 and C:N ratio at the same time increased more than ten-fold in all plots.

    Malik et al.[81] apprised that the organic amendments significantly enhanced MBN concentrations up to 50% more than the unamended soil. Wang et al.[82] evaluated the influence of organic materials on MBN content in an incubation and pot experiment with acidic and calcareous soils. The results revealed that MBN content which was affected by the different forms of organic amendments, increased by 23.37%−150.08% and 35.02%−160.02% in acidic and calcareous soils, respectively. The MBN content of both soils decreased with the increase in the C/N ratio of the organic materials, though a higher C/N ratio was effective for sustaining a greater MBN content for a very long time.

    Dhaliwal & Bijay-Singh[52] observed higher MBN levels in NT soils (116 kg ha−1) than in cultivated soils (80 kg ha−1). Kumar et al.[83] ascertained that in surface layer, MBN content was 11.8 mg kg−1 in CT which increased to 14.1 and 14.4 mg kg−1 in ZT and RT without residue retention and 20.2, 19.1 and 18.2 mg kg−1 in ZT, RT and CT with residue incorporation, respectively (Table 1). In the subsurface layer, the increased tendency on account of tillage and crop residue retention was identical to those of 0−15 cm layer but the magnitude was comparatively meagre (Table 1). In comparison to control, the persistent retention of crop residues led to significant accrual of MBN in the surface layer.

    Table 1.  Effect of different treatments on contents of various fractions of soil organic carbon[38].
    TreatmentsPMN (mg kg−1)MBC (mg kg−1)MBN (mg kg−1)DOC (mg kg−1)
    Depths (cm)
    0−1515−300−1515−300−1515−300−1515−30
    Tillage practices
    ZTR12.411.2562.5471.120.218.9198.6183.6
    ZTWR8.57.6350.4302.114.112.6167.1159.2
    RTR10.69.9490.2399.319.117.2186.4171.6
    RTWR7.66.6318.1299.814.413.7159.5148.7
    CTR9.38.5402.9354.418.216.6175.9168.9
    CT6.75.6307.9289.511.89.7142.5134.6
    Nitrogen management
    Control3.62.8218.3202.910.810.4103.792.3
    80 kg N ha−15.34.4241.1199.414.912.2128.3116.9
    120 kg N ha−18.97.6282.7220.916.516.1136.8123.6
    160 kg N ha−19.88.4343.9262.919.418.1164.8148.9
    200 kg N ha−110.49.7346.3269.622.721.7155.7136.4
    ZTR = Zero tillage with residue retention, ZTWR = Zero tillage without residue retention; RTR = Reduced tillage with residue retention, RTWR = Reduced tillage without residue retention, CTR = Conventional tillage with residue incorporation; CT = Conventional tillage without residue incorporation.
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    Xiao et al.[75] determined that the MBN content decreased with tillage treatment having highest value in no tillage treatment, however, the difference among the treatments was negligible. Soil perturbations decreased the aggregate size and thus lower the soil aeration and exposure of fresh organic matter which restricted the growth of microorganisms. The results also concluded that MBN content is highly sensitive to tillage. Ginakes et al.[84] assessed the impact of zone tillage intensity on MBN in a corn-kura clover cropping sequence. Microbial biomass nitrogen was influenced by time and type of tillage treatment. Temporal studies revealed that MBN was higher after tillage treatment than the values possessed before tillage. Under different tillage treatments, higher values were recorded in ST (shank-till) and DT (double-till) over NT and RZT (zone-till) treatments.

    Another biological parameter, PMN, is a crucial parameter of soil fertility due to its association with soil N supply for crop growth. Also, PMN indicates the status of soil microbial community associated with PMN, whether it is improving or degrading. Forest soils are characterized by greater levels of PMN than CT receiving conventional chemical fertilizers which could be assignable to improved microbial activity in the former soils than the latter[48,77]. Aulakh et al.[85] assessed the effect of various combinations of fertilizer N, P, FYM and wheat residue (WR) applied to soybean and soybean residues added to wheat under CT and CA. The added fertilizers of N and P, FYM, and crop residue enhanced the mean weight diameter and water-stable aggregates thus favoured the development of macro-aggregates. The treatment INF + FYM + crop residue performed better among all the treatments. The net flux of mineral nitrogen from the mineralizable fraction is used to measure potentially mineralizable N which indicates the balance between mineralization and immobilization by soil microbes[77]. Nitrogen mineralization is widely used to assess the ability of SOM to supply inorganic nitrogen in the form of nitrate which is the most common form of plant-available nitrogen. Kumar et al.[83] observed an increase in PMN which was higher in surface soil than sub-surface soil thereby implying that high OC accumulation on account of crop residue retention was the most probable cause.

    Verma & Goyal[86] assessed the effect of INM and organic manuring on PMN and observed that PMN was substantially affected by different organic amendments. Potentially mineralizable nitrogen varied between 19.6−41.5 mg kg−1 soil with greater quantity (2.5%) in vermicompost applied plots than FYM treated plots. The INF treatments resulted in lower PMN content which could be due to nutrient immobilization by microbes. Mahal et al.[87] reported that no-till resulted in higher PMN content than conventional tillage treatments. This trend was due to the maintenance of SOM due to the residue cover and reduction of soil erosion under no-tillage system[88]. On the contrary, tillage practices led to the loss of SOC owing to loosened surface soil and higher mineralization of SOM.

    Soil respiration is referred as the sum of CO2 evolution from intact soils because of the respiration by soil organisms, mycorrhizae and roots[89]. Various researchers have proposed soil respiration as a potential indicator of soil microbial activity[52,77]. Gilani & Bahmanyar[90] observed that addition of organic amendments enhanced soil respiration more than the control and synthetic fertilizer treatments. Moreover, among organic amendment treatments, highest soil respiration was observed in sewage-sludge treated soils. Under controlled conditions in saline-sodic soil, Celis et al.[91] reported that sewage sludge resulted in a higher soil respiration rate than mined gypsum and synthetic gypsum. The application of gypsum because of minimal organic matter intake had little effect on soil respiration. The addition of organic matter especially during early spring led to higher microbial biomass and soil respiration albeit diminished levels of nitrate-N. Moreover, SOM hinders the leaching of nitrate ions thereby resulting in a better soil chemical environment[71].

    Faust et al.[92] observed that microbial respiration was associated with volumetric water content. The respiration declined with less availability of water, thus the lesser the tillage intensity, the more the volumetric water content which consequently resulted in higher microbial respiration. Another study by Bongiorno et al.[93] reflected the influence of soil management intensity on soil respiration. Reduced tillage practices resulted in 51% higher basal respiration than CT. Furthermore, this investigation suggested that microbial catabolic profile could be used as a useful biological soil quality indicator. Recently, Kalkhajeh et al.[94] ascertained the impact of simultaneous addition of N fertilizer and straw-decomposing microbial inoculant (SDMI) on soil respiration. The SDMI application boosted the soil microbial respiration which accelerated the decomposition of straw due to N fertilization. The C/N ratio did not affect the microbial respiration at elongation and heading stages, whereas N fertilization enhanced the microbial respiration to a greater extent than the unfertilized control. Additionally, the interaction between sampling time and basal N application significantly affected microbial respiration.

    Gong et al.[95] apprised the effect of conventional rotary tillage and deep ploughing on soil respiration in winter wheat and observed that deep ploughing resulted in a higher soil respiration rate than conventional rotary tillage. Soil moisture content and temperature are the dominating agents influencing soil respiration which is restricted by the soil porosity.

    Soil organic carbon plays a vital role in regulating various soil functions and ecosystem services. It is influenced by numerous factors like tillage practices and fertilization. Moreover, modified management practices may prove beneficial to avoid SOC loss by increasing its content. An exogenous supply of fertilizers may alter the chemical conditions of soil and thus result in transformation of SOC. Tillage practices lead to frequent soil disturbances which reduce the size of soil aggregates and accelerate the oxidation of SOC thereby reducing its content. The literature on the influence of fertilization and tillage practices on the transformation of SOC is discussed below.

    Soil organic carbon is a major part of the global carbon cycle which is associated not only with the soil but also takes part in the C cycling through vegetation, oceans and the atmosphere (Figs 1 & 2). Soil acts as a sink of approximately 1,500 Pg of C up to 1 m depth, which is greater than its storage in the atmosphere (approximately 800 Pg C) and terrestrial vegetation (500 Pg C) combined[96]. This dynamic carbon reservoir is continuously cycling in diverse molecular forms between the different carbon pools[97]. Fertilization (both organic and mineral) is one of the crucial factors that impart a notable influence on OC accretion in the soil. Many researchers have studied the soil C dynamics under different fertilizer treatments. Though inorganic fertilizers possess the advantage of easy handling, application and storage, they do not contribute to soil organic carbon. On the contrary, regardless of management method, plant residues are known to increase organic carbon content.

    Figure 1.  Impact of different fertilization regimes on abundance of the microbial biomarker groups . Error bars represent the standard error of the means and different letters indicate significant differences at p < 0.05 among treatments. Source: Li et al.[60].
    Figure 2.  Soil organic carbon (SOC) dynamics in the global carbon cycle.

    Katkar et al.[98] reported a higher soil quality index under conjunctive nutrient management strategies comprising addition of compost and green leaves along with mineral nutrients. Mazumdar et al.[99] investigated the impact of crop residue (CR), FYM, and leguminous green manure (GM) on SOC in continuous rice-wheat cropping sequence over a 25-year period. At the surface layer, the maximum SOC content was recorded under NPK + FYM than NPK + CR and NPK + GM treatments. SOC was significantly lower under sole application of INFs (NPK) than the mixed application of organic and inorganic treatments. A higher range of SOC content was recorded at a depth of 0.6 m in the rice-wheat system (1.8–6.2 g kg−1) in farmyard manure (FYM)-treated plots than 1.7–5.3 g kg−1 under NPK, and 0.9–3.0 g kg−1 in case of unfertilized plots[100]. In a research study Dutta et al.[101] reported that rice residue had a higher decomposition rate (k¼ 0.121 and 0.076 day−1) followed by wheat (0.073 and 0.042 day−1) and maize residues (0.041 day−1) when their respective residues placed on soil surface than incorporated in the soils. Naresh et al.[102] found FYM and dhaincha as GM/ sulphitation press mud (SPM) treatments are potent enough to enhance the SOC. Maximum SOC content was noted in 0–5 cm depth that reduced gradually along the profile. In surface soil, the total organic content (TOC) under different treatments varied with source used to supply a recommended dose of nitrogen (RDN) along with conventional fertilizer (CF).

    Cai et al.[103] ascertained that long-term manure application significantly improved SOC content in different size fractions which followed the sequence: 2,000–250 μm > 250–53 μm > 53 μm fraction. Naresh et al.[22] determined that mean SOC content increased from 0.54% in control to 0.65% in RDF and 0.82% in RDF + FYM treatment and improved enzyme activity; thus, ultimately influenced nutrient dynamics under field conditions. The treatments RDF + FYM and NPK resulted in 0.28 Mg C ha−1 yr−1 and 0.13 Mg C ha−1 yr−1, respectively and thus higher sequestration than control. Zhao et al.[104] determined that in the surface layer, significant increase in SOC content in each soil aggregate was noticed under straw incorporation treatments over no straw incorporated treatments (Fig. 3). Moreover, the aggregate associated OC was significantly higher in the surface layer than the sub-surface layer. The highest increment in aggregate-associated OC was noted in both maize and wheat straw (MR-WR) added plots followed by MR and least in WR. Besides, all of the three straw-incorporated treatments exhibited notable increase in SOC stock in each aggregate fraction in the surface layer of the soil. In the subsurface (20−40 cm) layer under MR-WR, significant rise in SOC stock of small macro-aggregates was observed, whereas there was a reduction in SOC stock in the silt + clay fraction than other treatments. The straw-incorporated treatments increased the quantity of mineral-associated organic matter (mSOM) and intra-aggregate particulate organic matter, (iPOM) within small macro-aggregates and micro-aggregates especially in the topmost layer of the soil.

    Figure 3.  Distribution of OC in coarse iPOM (intra-aggregate particulate organic matter) fine iPOM, mSOM (mineral-associated matter), and free LF (free light fraction) of small macro-aggregates and micro-aggregates in the 0–20 cm and 20–40 cm soil layers under MR-WR (return of both maize and wheat straw), MR (maize straw return), WR (wheat straw return). Different lowercase and uppercase letters indicate significant differences at p < 0.05 among treatments and depths respectively[104].

    Srinivasarao et al.[105] reported that SOC content was reduced with the addition of INFs (100% RDN) alone as compared to the conjunctive application of inorganic and organic or sole FYM treatments. Earlier, Srinivasarao et al.[106] reported that FYM treated plots exhibited greater per cent increase in SOC stock than mineral fertilized plots and control. Tong et al.[107] ascertained that the application of NP and NPK significantly improved SOC stocks. On the contrary, fertilized soils could also exhibit decrease in carbon content than control. Naresh et al.[108] determined that higher biomass C input significantly resulted in greater particulate organic carbon (POC) content. Zhang et al.[109] ascertained that long-term addition of NPK and animal manures significantly improved SOC stocks by a magnitude of 32%−87% whereas NPK and wheat/ and or maize straw incorporation enhanced the C stocks by 26%−38% than control. Kamp et al.[110] determined that continuous cultivation without fertilization decreased SOC content by 14% than uncultivated soil. However, super optimum dose of NPK, balanced NPK fertilization and integration of NPK with FYM not only improved SOC content but also SOC stocks over the first year. In conventionally tilled cotton-growing soils of southern USA, Franzluebbers et al.[111] estimated that carbon sequestration averaged 0.31 ± 0.19 Mg C ha−1 yr−1. Mandal et al.[112] reported maximum SOC stock in the surface layer of the soil (0–15 cm) which progressively diminished with depth in each land use system. A significant decrease in SOC stock along the profile depth was also observed by Dhaliwal et al.[47] in both croplands and agroforestry. In the topmost soil layer, highest SOC stock was recorded in rice–fallow system while the lowest was in the guava orchard[112].

    Nath et al.[113] determined that there was accrual of higher TOC in surface layers as compared to lower layers of soil under paddy cultivation. This accrual could be adduced to left-over crop residues and remnant root biomass which exhibited a decreasing trend with soil depth. Das et al.[114] determined that integrated use of fertilizers and organic sources resulted in greater TOC as compared to control or sole fertilizer application. Fang et al.[115] observed that the cumulative carbon mineralization differed with aggregate size in top soils of broad-leaved forests (BF) and coniferous forests (CF). However, in deep soil it was greater in macro-aggregates as compared to micro-aggregates in BF but not in CF (Fig. 4). By and large, the percent SOC mineralized was greater in macro-aggregates as compared to micro-aggregates. Dhaliwal et al.[100] ascertained that SOC accrual was considerably influenced by residue levels and tillage in surface soil (0−20 cm); albeit no variation was observed at lower depth (20−40 cm). The SOC content was greater in zero-tilled and permanently raised beds incorporated with residues as compared to puddled transplanted rice and conventionally planted wheat. Pandey et al.[116] reported that no-tillage prior to sowing of rice and wheat increased soil organic carbon by 0.6 Mg C ha–1 yr–1. The carbon sequestration rate on account of no-tillage or reduced tillage ranged between 0−2,114 kg ha–1 yr–1 in the predominant cropping system of South Asia, Xue et al.[117] observed that the long-term conventional tillage, by and large, exhibited a significant decline in SOC owing to degradation of soil structure, exposing protected soil organic matter (intra-soil aggregates) to microbes. Therefore, the adoption of no-tillage could hamper the loss of SOC thereby resulting in a greater or equivalent quantity of carbon in comparison to CT (Fig. 5).

    Figure 4.  (a) Soil aggregate fractions of two depths in two restored plantations of subtropical China, (b) organic carbon and (c) its mineralization from various soil aggregates within 71 d at various soil depths in two restored plantations of subtropical China. Error bars show the standard error of the mean. The different letters represent significant differences among the different soil aggregate fractions within a depth at p < 0.05[115].
    Figure 5.  The concentrations of (a) SOC, (b) total nitrogen (TN), and (c) soil C:N ratio for 0–50 cm profile under different tillage treatments in 2012 and 2013. NT = no-till with residue retention; RT = rotary tillage with residue incorporation; PT = plow tillage with residue incorporation; and PT0 = plow tillage with residue removed. The lowercase letters indicate statistical difference among treatments at p < 0.05[117].

    Singh et al.[118] determined that carbon stock in the 0-40 cm layer increased by 39, 35 and 19% in zero-tilled clay loam, loam, and sandy loam soils, respectively as compared to conventional tilled soils over a period of 15 years. Kuhn et al.[119] also apprised about the advantages of NT over CT vis-a-vis SOC stocks across soil depths. In the surface layer (0−20 cm) NT, by and large, resulted in higher SOC stocks as compared to CT; however, SOC stocks exhibited a declining trend with soil depth, in fact, became negative at depths lower than 20 cm. Sapkota et al.[120] observed that over a period of seven years, direct dry-seeded rice proceeded by wheat cultivation with residue retention enhanced SOC at 0-60 cm depth by a magnitude of 4.7 and 3.0 t C ha−1 in zero-tillage (ZTDSR-ZTW + R) and without tillage (PBDSR-PBW + R), respectively. On the contrary, the conventional tillage rice-wheat cropping system (CTR-CTW) decreased the SOC up to 0.9 t C ha−1 (Table 2).

    Table 2.  Influence of tillage and crop establishment methods on SOC stock and its temporal variation under rice–wheat system[120].
    Tillage and crop establishment methodsDepths (m)
    0–0.050.05–0.150.15–0.30.3–0.60–0.6
    Total SOC (t/ha)
    CTR-CTW3.5e7.1c8.77.026.2c
    CTR-ZTW3.9d7.6bc8.86.526.7c
    ZTDSR-CTW4.2d7.5bc9.26.327.3c
    ZTDSR-ZTW4.9c8.9ab8.26.228.2bc
    ZTDSR-ZTW+R6.1a9.0ab9.86.831.8a
    PBDSR-PBW+R5.5b9.3a9.36.030.1ab
    MSD0.41.72.01.42.49
    Treatment effect
    (p value)
    < 0.0010.040.1580.267< 0.001
    Initial SOC content3.6 ±
    0.15
    8.1 ±
    1.39
    8.78 ±
    1.07
    6.7 ±
    0.73
    27.1 ±
    1.21
    Change in SOC over seven years (t/ha)
    CTR-CTW−0.16−0.99−0.040.28−0.90
    CTR-ZTW0.28−0.500.01−0.20−0.41
    ZTDSR-CTW0.62−0.570.45−0.340.16
    ZTDSR-ZTW1.340.84−0.62−0.461.09
    ZTDSR-ZTW+R2.490.961.040.164.66
    PBDSR-PBW+R1.891.220.51−0.642.98
    CTR-CTW = Conventionally tilled puddled transplanted rice followed by conventionally tilled wheat, CTR-ZTW = Conventionally tilled puddled transplanted rice followed by zero-tilled wheat, ZTDSR-CTW = Zero-tilled direct dry-seeded rice followed by conventionally tilled wheat, ZTDSR-ZTW = Zero-tilled direct dry-seeded rice followed by zero-tilled wheat, ZTDSR-ZTW+R = Zero-tilled direct dry-seeded rice followed by zero-tilled wheat with residue retention, PBDSR-PBW+R = Direct dry-seeded rice followed by direct drilling of wheat both on permanent beds with residue retention, MSD, minimum significant difference. Significant different letters indicate significant differences at p < 0.05.
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    Labile organic carbon (LC) is that fraction of SOC that is rapidly degraded by soil microbes, therefore, having the highest turnover rate. This fraction can turn over quickly on account of the change in land use and management strategies. From the crop production perspective, this fraction is crucial as it sustains the soil food cycle and, hence, considerably impacts nutrient cycling thereby altering soil quality and productivity. Short-term management could influence the labile fraction of carbon[121]. However, some site-specific problems and regional factors may influence their distribution in soil layers[102].

    Banger et al.[122] observed significant alteration in labile pools of C, for instance, particulate organic matter (POM), water-soluble C (WSC) and light fraction of C (LFC) because of the addition of fertilizers and/or FYM over a 16-year period. Particulate organic matter, LFC and WSC contributed 24%–35%, 12%–14% and 0.6%–0.8%, respectively, towards SOC. The increase in concentration of SOC including its pools like POC and the sequestration rate due to integrated nutrient management was also reported by Nayak et al.[123]. Gu et al.[124] observed that mulch-treated soils (straw and grass mulch) had significantly greater levels of LOC, POC, DOC and EOC as compared to no mulch-treated soils which could be adduced to the addition of straw, root and its sections into the soil. The content of labile C fractions across all treatments exhibited a decreasing trend with soil depth[23, 102, 125].

    In a long-term experiment, Anantha et al.[126] observed that the total organic carbon apportioned into labile carbon, non-labile, less labile, and very labile carbon constituted around 18.7%, 19.3%, 20.6% and 41.4% of the TOC, respectively (Table 3). Zhu et al.[20] determined that straw incorporation had a substantial impact on TOC and labile C fractions of the soil which were greater in straw incorporated treatments as compared to non-straw treatments across all the depths. Wang et al.[127] reported that the light fraction organic carbon (LFOC) and DOC were significantly greater in the straw-applied treatments than the control by a magnitude of 7%–129% for both the early and late season rice. The treatments NPK + FYM or NPK + GR + FYM resulted in greater content of very labile and labile C fractions whereas non-labile and less labile fractions were greater in control and NPK + CR treatment. There was 40.5% and 16.2% higher C build-up in sole FYM treated plots and 100% NPK + FYM, respectively over control. On the other hand, a net depletion of 1.2 and 1.8 Mg ha−1 in carbon stock was recorded under 50% NPK and control treatments, respectively. Out of the total C added through FYM, only 28.9% was stabilized as SOC, though an external supply of OM is a significant source of soil organic carbon[69]. Hence, to sustain the optimum SOC level at least an input of 2.3 Mg C ha−1 y−1 is required. A comparatively greater quantity of soil C in passive pools was observed in 100% NPK + FYM treatment. The increase in allocation of C into the passive pool was about 33%, 35%, 41% and 39% of TOC in control, suboptimal dose, optimal dose and super optimal dose of NPK which indicates that the concentration of passive pools increased with an increase in fertilization doses. Water-soluble carbon (WSC) was 5.48% greater in the upper soil layer as compared to lower layer of soil. In surface soil (0−15 cm), the values of light fraction carbon (LFC) were 81.3, 107.8, 155.2, 95.7, 128.8, 177.8 and 52.7 mg kg−1 in ZT without residue retention, ZT with 4 t ha−1 residue retention, ZT with 6 t ha−1 residue retention, FIRB without residue addition and FIRB with 4 and 6 t ha−1 residue addition and CT, respectively (Table 4). Tiwari et al.[128] determined that the decrease in POC was due to reduction in fine particulate organic matter in topsoil whereas decrement in dissolved organic carbon was observed largely in subsoil. Therefore, in surface soils fine POC and LFOC might be regarded as preliminary evidence of organic C alteration more precisely, while DOC could be considered as a useful indicator for subsoil. Reduction in allocations of fine POC, LFOC and DOC to SOC caused by tillage and straw management strategies indicated the decline in quality of SOC. A higher SOC concentration was recorded in the conjoint application of INF + FYM (0.82%) and sole application of INF (0.65%) than control (0.54%). Kumar et al.[83] reported that the CT without residue retention had significantly lower labile carbon fractions (27%–48%) than zero-tillage with 6-ton residue retention. Moreover, residue-retained fertilized treatments had significantly greater labile fractions of C than sole fertilized treatments[125]. Kumar et al.[83] reported highest change in DOC in zero-till with residue retention (28.2%) in comparison to conventional tillage practices. In ZT, absence of soil perturbations resulted in sustained supply of organic substrata for soil microbes which increases their activity. On the contrary, CT practices resulted in higher losses of C as CO2 due to frequent disturbances.

    Table 3.  Oxidisable organic carbon fractions in soils (g kg−1) at different layers[126].
    TreatmentDepths (cm)
    0−1515−3030−45Total
    Very Labile C
    Control3.6 ± 0.5c1.4 ± 0.3b1.3 ± 0.2a6.3 ± 0.4b
    50% NPK4.6 ± 0.3bc2.1 ± 0.7ab1.5 ± 0.1a8.1 ± 0.9a
    100% NPK4.4 ± 0.3bc2.3 ± 0.2a1.4 ± 0.5a8.0 ± 0.7a
    150% NPK5.0 ± 0.2ab2.6 ± 0.2a1.5 ± 0.1a9.0 ± 0.3a
    100% NPK + FYM4.8 ± 0.2ab2.0 ± 0.2ab1.3 ± 0.3a8.1 ± 0.2a
    FYM5.9 ± 1.3a2.2 ± 0.2a1.4 ± 0.3a9.5 ± 1.6a
    Fallow4.2 ± 0.7bc1.5 ± 0.5b0.7 ± 0.3b6.3 ± 0.8b
    Lbile C
    Control2.4 ± 0.3a1.0 ± 0.2a0.8 ± 0.4a4.2 ± 0.6a
    50% NPK1.7 ± 0.4ab0.9 ± 0.5a0.7 ± 0.2a3.3 ± 0.7a
    100% NPK1.8 ± 0.4ab0.8 ± 0.5a0.6 ± 0.3a3.2 ± 0.8a
    150% NPK1.2 ± 0.3b0.7 ± 0.2a0.9 ± 0.2a2.8 ± 0.4a
    100% NPK + FYM1.9 ± 0.3ab0.7 ± 0.2a0.7 ± 0.3a3.4 ± 0.2a
    FYM2.5 ± 0.9a0.7 ± 0.3a0.7 ± 0.2a3.9 ± 0.9a
    Fallow2.2 ± 1.0ab1.0 ± 0.3a1.0 ± 0.4a4.1 ± 1.1a
    Less labile C
    Control1.5 ± 0.3c0.6 ± 0.4c0.4 ± 0.0c2.6 ± 0.7d
    50% NPK1.8 ± 0.1c0.4 ± 0.1c0.5 ± 0.2c2.7 ± 0.1cd
    100% NPK2.5 ± 0.3ab0.8 ± 0.1bc1.1 ± 0.2ab4.4 ± 0.1b
    150% NPK2.6 ± 0.2a0.9 ± 0.1bc0.4 ± 0.2c3.9 ± 0.1b
    100% NPK + FYM2.7 ± 0.6a1.5 ± 0.2a1.4 ± 0.1a5.6 ± 0.7a
    FYM1.9 ± 0.7bc1.7 ± 0.2a1.0 ± 0.2b4.5 ± 0.7ab
    Fallow1.5 ± 0.3c1.3 ± 0.7ab0.9 ± 0.4b3.8 ± 1.2bc
    Non labile C
    Control1.2 ± 0.5b1.2 ± 0.3a0.2 ± 0.2b2.6 ± 0.5b
    50% NPK1.2 ± 0.9b1.7 ± 0.8a0.7 ± 0.4ab3.5 ± 1.8ab
    100% NPK1.3 ± 0.6b1.5 ± 0.6a0.5 ± 0.2ab3.3 ± 1.0ab
    150% NPK1.4 ± 0.3b1.5 ± 0.2a0.8 ± 0.1a3.7 ± 0.3ab
    100% NPK + FYM2.0 ± 0.8b1.3 ± 0.1a0.3 ± 0.3ab3.5 ± 0.7ab
    FYM3.7 ± 1.3a1.0 ± 0.2a0.5 ± 0.5ab5.1 ± 1.9a
    Fallow2.1 ± 0.2b1.4 ± 0.7a0.4 ± 0.2ab3.9 ± 0.9ab
    Values in the same column followed by different letters are significantly different at p < 0.001, ± indicates the standard deviation values of the means.
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    Table 4.  Influence of tillage and nitrogen management on distribution of carbon fractions in soil[83].
    TreatmentsWSOC
    (g kg−1)
    SOC
    (g kg−1)
    OC
    (g kg−1)
    BC
    (g kg−1)
    POC
    (mg kg)
    PON
    (mg kg−1)
    LFOC
    (mg kg−1)
    LFON
    (mg kg−1)
    Depths (cm)
    0−1515−300−1515−300−1515−300−1515−300−1515−300−1515−300−1515−300−1515−30
    Tillage practices
    ZTR28.826.223.119.39.619.134.694.281342.8967.9119.5108.1194.7154.814.812.3
    ZTWR25.324.618.414.87.877.213.763.19981.1667.494.686.5120.5104.711.810.3
    RTR27.025.922.418.28.688.174.133.871230.2836.9109.797.8170.9144.913.711.6
    RTWR23.721.818.114.27.667.073.122.96869.4604.482.676.6107.197.39.78.6
    CTR26.124.421.817.48.497.963.823.481099.1779.498.489.3143.8115.912.810.9
    CT21.820.916.113.16.215.642.892.63617.5481.869.257.690.873.69.67.9
    Nitrogen management
    Control21.114.916.113.16.135.481.581.07709.7658.631.726.3123.9104.36.45.8
    80 kg N ha−128.321.217.814.76.466.162.461.75860.7785.668.456.2132.8116.17.66.9
    120 kg N ha−129.522.119.116.17.256.713.262.18952.2808.989.578.5150.6127.69.78.6
    160 kg N ha−130.223.120.818.27.757.283.822.661099.5823.896.883.4168.5145.710.29.8
    200 kg N ha−131.125.421.318.77.937.484.153.421153.1898.4103.997.3176.2152.911.710.6
    WSOC = Water soluble organic carbon, SOC = Total soil organic carbon, OC = Oxidizable organic carbon, BC =Black carbon, POC = particulate organic carbon, PON = particulate organic nitrogen, LFOC = labile fraction organic carbon, and LFON = labile fraction organic nitrogen. ZTR = Zero tillage with residue retention, ZTWR = Zero tillage without residue retention; RTR = Reduced tillage with residue retention, RTWR = Reduced tillage without residue retention, CTR = Conventional tillage with residue incorporation; CT = Conventional tillage without residue incorporation.
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    The soil characteristics such as plant available nutrients, microbial diversity and soil organic carbon transformation are dwindling on account of intensive cultivation under conventional tillage practices, therefore, demand relevant management approaches for soil and crop sustainability. Long-term application of organic amendments significantly increases soil properties by increasing plant available macro, micro, secondary nutrients and soil organic C, whereas the increase in organic C by INF application is, by and large, due to increment in organic C content within macro-aggregates and in the silt + clay compartments. The soil organic carbon and other plant available nutrients were significantly greater in conservation tillage systems as compared to conventional tillage (CT) that conservation approaches could be an exemplary promoter of soil productivity by modifying soil structure thereby protecting SOM and maintaining higher nutrient content. The mean concentration of different fractions of carbon MBN, PMN and soil respiration under integrated nutrient management treatments was higher as compared with to control. Therefore, the conjoint use of organic manures or retention of crop residues with inorganic fertilizers is imperative to reduce the depletion of SOC while sustaining crop production as a realistic alternative. Future research should focus mainly on the usage of organic and mineral fertilizers in conjunction with conservation tillage approaches to sustain the soil environment.

    The authors confirm contribution to the paper as follows: study conception and design: Dhaliwal SS, Shukla AK, Randhawa MK, Behera SK; data collection: Sanddep S, Dhaliwal SS, Behera SK; analysis and interpretation of results: Dhaliwal SS, Gagandeep Kaur, Behera SK; draft manuscript preparation: Dhaliwal SS, walia, Shukla AK, Toor AS, Behera SK, Randhawa MK. All authors reviewed the results and approved the final version of the manuscript.

    Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

    The support rendered by the Departemnt of Soil Science, PAU, Ludhiana, RVSKVV, Gwailor, CSSRI, Karnal, IISS, Bhopal, School of Organic Farming, PAU Ludhiana and Washington State University, USA is fully acknowledged .

  • The authors declare that they have no conflict of interest.

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  • Cite this article

    Wang Q, Gao J, Chen J, Tan X, Liu C, et al. 2024. Regulatory mechanism of a light-dependent protochlorophyllide oxidoreductase in chlorophyll biosynthesis and environmental adaptation. Technology in Agronomy 4: e023 doi: 10.48130/tia-0024-0019
    Wang Q, Gao J, Chen J, Tan X, Liu C, et al. 2024. Regulatory mechanism of a light-dependent protochlorophyllide oxidoreductase in chlorophyll biosynthesis and environmental adaptation. Technology in Agronomy 4: e023 doi: 10.48130/tia-0024-0019

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Regulatory mechanism of a light-dependent protochlorophyllide oxidoreductase in chlorophyll biosynthesis and environmental adaptation

Technology in Agronomy  4 Article number: e023  (2024)  |  Cite this article

Abstract: Chlorophyll is a vital component of photosynthesis and must be produced throughout the plant life cycle. Light-dependent protochlorophyllide oxidoreductase (LPOR) is a pivotal enzyme in the chlorophyll biosynthesis pathway, catalyzing the conversion of Pchlide to Chlide. The presence of different types of LPOR ensures the efficient synthesis of chlorophyll in photosynthetic organisms during the dark-light transition. In addition to the transcriptional, translational, and post-translational regulation of LPOR function under different abiotic stresses, the nature of the substrate also influences LPOR function. Here, a perspective on chlorophyll synthesis and the development of chloroplasts is offered, the importance of LPOR in safeguarding plant light energy utilization is summarized, the gene expression pattern and structural-functional features of LPOR are outlined, as well as the role of LPOR in abiotic stress tolerance response, the catalytic mechanism of LPOR as well as the modulation of LPOR by light signals and other environmental factors are discussed. The aim is to provide references for the cultivation and innovation of plant germplasm resources with stress tolerance.

    • The demand for food has sharply increased because of the growing population[1,2]. Improving the LUE of crops represents the primary option for increasing crop yield potential[3]. LUE refers to the efficiency by which a crop produces biomass from absorbed light energy, which is measured by the accumulation rate of photosynthetically active radiation in relation to biomass per unit of intercepted or absorbed light[4]. The improvement of LUE depends on the rate of photosynthesis and the efficiency of converting light energy into fixed carbon[5]. The principal methods of improving LUE include (i) extending photosynthetic time, (ii) increasing the area of photosynthesis, and (iii) increasing the rate of photosynthesis. Therefore, enhancing photosynthetic efficiency is crucial for increasing yield[6]. Photosynthesis is the process by which plants and photosynthetic bacteria use light energy and CO2 to produce carbohydrates and O2[7]. During photosynthesis, plants convert light energy into chemical energy, which is used for plant growth. Chlorophyll plays an important role in photosynthesis in higher plants and has multiple functions in this process[8]. Related proteins bind with chlorophyll and form complexes that capture, convert, and redirect light energy[8]. Chl a and Chl b are the primary substances of the core protein complex and light-harvesting antenna protein complex of photosystems, respectively[9].

    • Chlorophyll biosynthesis is a complex process completed by multiple enzymes (Fig. 1). Accurate and stable chlorophyll synthesis is critical for plant growth and development because free chlorophyll and its precursors, the tetrapyrrole compounds can be photosensitive and phototoxic to cells. This effect is most pronounced in the photosystem II reaction center, which is highly exposed to oxidative damage[1012]. Chlorophyll synthesis consists of two parts. The first part is the common pathway of tetrapyrrole substance synthesis, starting from the synthesis of glutamyl tRNA and ending with Pro. This pathway involves the enzymes glutamine tRNA reductase, delta-amino ketones pentanoic acid dehydratase, uroporphyrinogen III synthase, coproporphyrinogen III oxidase, and protoporphyrinogen oxidase. The second aspect involves Pro chelation with Mg2+ in the chlorophyll synthesis pathway, requiring involved enzymes Mg-chelatase, Mg-protoporphyrin IX methyltransferase, POR, chlorophyllide a oxygenase, chlorophyll synthase, and chlorophyllase. The rate-limiting enzyme POR performs a crucial catalytic role in chlorophyll synthesis by transforming Pchlide into Chlide[13]. LPOR also participates in chloroplast development and serves as the primary protein component in PLBs of etioplasts[14,15]. Two types of POR exist in nature: DPOR (EC 1.3.7.7) and LPOR (EC 1.3.1.33)[16]. From evolutionary terms, DPOR is the older enzyme, a multisubunit enzyme containing three separate subunits, and very similar to nitrogen-fixing enzymes. DPOR catalyzes Pchlide reduction in an ATP-dependent manner and is oxygen-sensitive, and is present in non-flowering land plants, algae, and cyanobacteria[17]; LPORs are thought to have evolved in cyanobacteria about 2 billion years ago as a result of increased atmospheric oxygen levels and are not sensitive to oxygen[18]. LPOR is present in a wide range of organisms, including plants, algae, cyanobacteria, and anaerobic photosynthetic bacteria[1921]. However, only LPOR is present in angiosperms, indicating that induction with light is necessary for angiosperms to produce chlorophyll[22,23]. These characteristics make LPOR essential in chlorophyll biosynthesis and chloroplast development in angiosperms.

      Figure 1. 

      Chlorophyll biosynthesis pathway. 1: glutamine-tRNA reductase, 2: delta-amino ketones pentanoic acid dehydratase, 3: urinary porphyrins original III synthetase, 4: coproporphyrin III oxidase, 5: protoporphyrin oxidase, 6: Mg-chelating enzyme, 7: Mg-protoporphyrin IX methyltransferase, 8: the original chlorophyll acid ester oxidoreductase, 9: chlorophyll acid ester a oxygenase, 10: chlorophyll synthase, 11: chlorophyllase, 12: chlorophyllide a oxygenase, 13: ferrous chelase.

      A multitude of investigations have demonstrated that LPOR plays a pivotal role in the response to abiotic stress, influencing the biosynthesis of chlorophyll, and exhibiting a certain level of stress resistance. A synthesis was conducted of the research conducted domestically and internationally on LPOR, encompassing its structural characteristics, catalytic reaction mechanism, optical signal, temperature, moisture, and the factors influencing its activity.

    • LPOR is a single polypeptidase that is encoded within the nucleus. It has a transporter protein segment at its N-terminus. This transporter protein segment transports LPOR into the plastid after it is transcribed. LPOR is highly similar to the SDR family and needs photoactivation to reduce Pchlide and perform biological functions[24]. Previous studies investigated LPOR in several species, including Arabidopsis thaliana[25], Brassica oleracea[26], Oryza sativa[27], Hordeum vulgare[28], Zea mays[29], Nicotiana tabacum[30], Cucumis sativus[31], and Pisum sativum[32]. Pchlide, the precursor of chlorophyll synthesis, failed to undergo the requisite reduction in time, thereby allowing the production of a substantial amount of ROS upon exposure to light, which resulted in the oxidative damage and chlorophyll bleaching of the seedling[33]. In addition, the accumulation of ROS in chloroplasts also impairs the de novo synthesis of protein D1 (also known as photosystem b-a or PsbA), which is essential for PSII repair[34]. The enzyme LPOR catalyzes the conversion of pchlide into chlide, a process which enables the developing seedling to gain the capacity to perform photosynthesis[35]. The production of chlorophyll is thus facilitated, thus allowing the seedling to grow in an autotrophic manner[15].

      The different gene expression patterns of LPOR subtypes prevent direct photooxidative harm in yellow seedlings when exposed to light. In Arabidopsis, three LPOR subtypes, namely, LPORA, LPORB, and LPORC were identified[36,37]. At the early stage of plant development, AtPORA and AtPORB are expressed during etiolation in Arabidopsis seedlings[35]. AtPORB and AtPORC are synthesized abundantly in slightly mature seedlings and, subsequently, in mature plants[35]. Upon light exposure, the expression levels of AtPORB and AtPORC are directly correlated to Chl a content and the stacking of thylakoids in seedlings[38,39]. The cpSRP43 as a chaperone, stabilizes the enzyme and provides the optimal quantity of PORB during leaf greening and heat shock[40]. In contrast, cpSRP54 enhances its binding to the thylakoid membrane, thus ensuring a sufficient level of metabolic flux during late chlorophyll biosynthesis[40]. Two LPOR subtypes, namely, OsPORA and OsPORB, were identified in rice[27]. During early leaf development, OsPORA is expressed in darkness, whereas OsPORB is expressed throughout the entire leaf development process regardless of light conditions[41]. Previous studies demonstrated that although OsPORA and OsPORB have overlapping biochemical functions, the response of OsPORB to constant light or physiological functions during reproductive growth cannot be substituted with OsPORA[27]. Barley also has two LPOR subtypes[13]. In vitro measurement revealed that HvPORA is expressed in etiolated seedlings, and its expression is downregulated after light exposure[42]. On the contrary, HvPORB is expressed during morphogenesis in leaf development regardless of light conditions[42]. In addition, barley etioplast contains a distinctive light-harvesting complex called LHPP, of which HvPORA is an essential component[42]. The LHPP complex is prepared in advance for the LPOR-catalyzed transformation of Pchlide under light to prevent the free Pchlide phototoxicity after light exposure, which causes photobleaching of seedlings[43].

    • A comparison of the amino acid sequences of LPOR in barley and Synechocystis reveals that the conserved Cys residue sequence is closely linked to the binding and catalysis of substrate Pchlide conversion. Point mutation experiments on various Cys residues revealed that Cys276 is the active site for Pchlide binding. Additionally, Cys303 functions as a pigment-binding site with low affinity. Both Cys residues participate in the assembly and stabilization of PORB in the etioplast[44,45].

      LPOR’s structure, along with other SDR family members[24], contains the Rossmann-fold structure that binds dinucleotides[46]. The structure has three flexible regions positioned at amino acid residues 146–160, 228–255, and 284–291. Binding to the NADPH binding site of LPOR is closely associated with amino acid residues 146–160 and 228–255, whereas amino acid residues 284–291 plays a crucial role in regulating substrate Pchlide binding. In contrast to the other SDR family members, the SDR proteins stand out for utilizing Asn-Ser-Tyr-Lys to facilitate proton transfer from tetrads, leading to the production of stable reaction intermediates[46]. However, LPOR employs Thr residues, particularly Thr145, to replace the Ser residues. The structural modeling of the ternary enzyme-substrate complexes constructed from crystal and electron microscopy data also confirmed the differences in the orientation of LPOR to the substrate Pchlide and the structure of the LPOR active site[4749]. Further results indicate that the conserved Tyr and Gln residues in LPOR are essential for Pchlide binding, while the active site Cys residue is crucial for both hydride and proton transfer reactions in LPOR[48].

      LPOR participates in both chlorophyll biosynthesis and chloroplast development[50]. Chloroplasts originate from proplastids, and etioplasts are transitional forms of chloroplast development. Etioplasts are characterized by the lack of chlorophyll and possession of a distinctive membrane structure called PLBs[15]. LPOR is the main protein constituent of PLBs, comprising more than 90% of the total protein content of PLBs[51]. Carotenoids act in parallel with DET1 to regulate the transcriptional formation of LPOR and plastids PLBs, thereby controlling chloroplast development[52]. PLBs have diverse LHPP complexes with distinct absorption spectra, namely, LPOR-Pchlide633, LPOR-Pchlide640, and LPOR-Pchlide655, with peak absorptions at 633, 640, and 655 nm, respectively[26,53]. The photoactive binary complex of LPOR-Pchlide655 binds to NADPH after light detection and catalyzes Pchlide reduction, leading to light conversion and gradual formation of Chl[53]. Moreover, the decomposition of the PLB lattice structure initiates grana stacking, leading to the complete development of chloroplasts[50]. Nonetheless, LPOR-Pchlide633, which is not photoactive, can degrade after exposure to light, thereby triggering an outburst of ROS in chloroplasts. Severe instances of such degradations may result in cell death[32].

    • In plants, Pchlide reduction is an important rate-limiting step within the chlorophyll synthesis pathway. In angiosperms lacking DPOR, chlorotic seedlings rely on LPOR to turn green. LPOR requires light-induced NADPH as a reducing agent to catalyze the Pchlide conversion. In regular plant development, LPOR-Pchlide-NADPH forms a ternary complex that accumulates during the dark morphogenesis of the etioplast; the NADPH hydride on the nicotinamide ring is transferred to the C17 position of Pchlide through light induction (Fig. 2). Then, the conserved Tyr residue transfers a proton to the C18 position of Pchlide. Thus, the C17 and C18 double bonds of the Pchlide-D ring are reduced, resulting in the creation of Chlide. Then, Chlide undergoes esterification and further modifications to form Chl a and Chl b[47,54].

      Figure 2. 

      The C17–C18 double bond of Pchlide is restored to chlorophyll by light.

    • LPOR is a protein encoded in the nucleus, synthesized into large pPORs in the cytoplasm, and then modified to enter the plastid[55] (Fig. 3). The process of LPORs entering chloroplasts is distinct[56]. Prior studies verified that pPORA relies on substrates to enter chloroplasts, whereas pPORB does not require any substrate for entry[57]. TOC33 is an essential core component in the complex of PORA and PORB import channels in cotyledons and leaves[56,58]. PTC52 is a unique Pchlide a oxygenase complex located in the plastid envelope. It is responsible for associating the synthesis of Pchlide b with the import of pPORA[59]. In RNAi plants lacking PTC52 transcripts and proteins, pPORA cannot be imported to plastids normally. This phenomenon causes an excessive accumulation of Pchlide and leads to ROS accumulation and cell death during greening[59]. In addition, CDF1 present on thylakoids and capsules interacts with the LPOR subtype and plays a crucial role in the introduction and stability of LPOR[55]. Deletion of CDF1 leads to a decrease in LPOR protein accumulation, which normally hinders chlorophyll synthesis, damages PLB formation, affects chloroplast development under light, causes photobleaching of plants under light, and inhibits plant growth[55,60]. Various pathways of LPOR import into plastids guarantee normal chloroplast development and chlorophyll biosynthesis. This finding suggests that factors beyond the components of the core complex of the import channel participates in the transportation of nuclear-encoded plastid proteins. Plastid-localized membrane-bound factors, such as TTP1, play a role in LPOR-directed import into chloroplasts. TTP1 deficiency leads to the accumulation of glutamate receptors, enhances pentosamine ketoglutarate synthesis and reduces POR levels, which in turn leads to increased sensitivity to reactive oxygen species and slower greening of yellowing seedlings[61].

      Figure 3. 

      Regulatory networks of the PORA, PORB, and PORC genes in A. thaliana. Positive interactions are indicated by arrows, negative interactions by blunt ends, and the direction of protein movement by arched arrows. Helices represent mRNA, and ellipses represent proteins.

    • For the reduction of Pchlide to occur, LPOR catalysis requires photoactivation[23]. The catalytic efficiency of the reaction varies with different light qualities, and the quantum yield of the reaction is 3–7 times higher under red light (647 nm) than under blue light (407 nm)[62]. Moreover, LPOR's catalytic reaction efficiency varies when combined with different substrates. Several studies suggested that cabbage’s photobleaching is primarily caused by short-wavelength light (625–630 nm) of 7 μmol photons m−2∙s−1. Light with wavelengths longer than 630–640 nm causes bleaching and photoreduction, whereas that with wavelengths higher than 640 nm predominantly causes photoreduction[26]. Similar conclusions can be reached through various methods, such as behavioral spectroscopy, pigment content measurement, and kinetic analysis. These studies indicated that under low light intensity, the excitation energy of the short-wavelength-absorption-type Pchlide causes the photoreduction of long-wavelength-absorption-type Pchlide; in comparison, photobleaching occurs under high photochemical light[63].

      The active site of LPOR plays a critical role in its catalytic activity. Tyr275 and Lys279, which are essential active sites of LPOR, along with four conserved Cys residues involved in substrate binding, have a very important function in the operation of LPOR. Tyr275 and Lys279 regulate the efficiency of LPOR photoactive state formation by participating in the coordination of NADPH and Pchlide at enzyme catalytic sites[64]. LPOR photoactivity is regulated by several Cys binding sites with different substrate affinities[58]. Any mutation at these sites can affect LPOR photoactivity[65].

      Furthermore, proteins interacting with LPOR regulate their enzyme activity in various ways. The structural stability of the LHPP complex is crucial for plant greening. The presence of TLs has been demonstrated to influence the activation of LPORs in response to regulators[66]. Research indicated that galactosyl diacylglycerol impacts the catalytic reaction of LPOR by influencing the creation and breakdown of the Pchlide-LPOR-NADPH complex, which affects the biosynthesis of plant chlorophyll[67]. Feedback regulation is usual in chlorophyll biosynthesis. Fluorescent protein is one of the tetrapyrrole biosynthesis negative feedback regulators in higher plants. It directly interacts with LPOR and cooperates with LPOR negative feedback to regulate chlorophyll biosynthesis[68]. The protein LIL3 regulates plant greening through direct interaction with LPOR. The mutation in lil3 causes a considerable loss of LPOR protein, affecting its posttranslational modifications, which lead to abnormal chlorophyll synthesis[69]. Pchlide transformation catalyzed by LPOR involves protein phosphorylation. Previous research demonstrated that certain plastid ADP-dependent kinase impacts the membrane association of LPOR via reversible protein phosphorylation regulates PLB formation, promotes light-dependent diffusion, and ultimately facilitates chloroplast development[70].

    • Light is necessary for angiosperms to turn green, and the reaction involved is Pchlide reduction[56]. Several researchers examined prevalent transcription factors in light signaling, and they summarized and predicted the availability of multiple light response elements on the promoter of genes involved in chlorophyll biosynthesis, further highlighting the importance of light signaling in chlorophyll synthesis in plants[71]. Under dark conditions, PORC expression is extremely reduced or completely inhibited because of the suppression of PIF3, histone deacetylase 1, and SCL proteins. Research indicated that the phytochrome proteins PHYA and PHYB, which are photosensitive receptors found in plants, play a role in chlorophyll biosynthesis. When exposed to white light, PHYA and PHYB positively regulates LPOR, promoting chlorophyll biosynthesis[33]. Terminal flower 2 protein located downstream of the PHYA signal regulates the expression of PORA and promotes chlorophyll synthesis[72]. Under shading or low-intensity far-red light, the expression levels of PORB and PORC in the phyA mutant is substantially suppressed[73,74]. Far-red light exposure leads to the far-red-blocked greening phenomenon in plants. PHYA suppresses LPOR gene expression under far-red light, resulting in irreversible plastid damage that restrains proper greening ability of seedlings[75]. The sigma factor is a nuclear-encoded protein regulated by PHYA that participates in the regulation gene expression in chloroplasts and influences reverse signaling from plastids to nuclei to promote plant greening and plastid development[75].

      In the dark, PORA activity is initiated by acetylation in the presence of HDAC, which regulates chlorophyll synthesis[76]. Meanwhile, phytochrome interacting factor 1 binds to the G-box DNA sequence element (CACGTG) of the PORC promoter and positively regulates PORC expression[77] (Fig. 3). After exposure to light, PIF3 is phosphorylated and inactivated, and histone H4 is acetylated[78]. Moreover, light facilitates the expression of miR171 and hinders gibberellic acid synthesis, thereby promoting the expression of DELLA protein. However, miR171 and DELLA inhibit the SCL transcription, resulting in a significant increase in the PORC expression under light[79,80] (Fig. 3). In addition, light positively regulates the expression of the transcription factor HY5, which binds directly to PORC to promote its gene expression[81]. HY5 interacts with PHYB in the dark through COP1/SPA1; after exposure to light, the transcription factor HY5 is released. HY5 cooperates with the biological clock of PIFs to regulate the transcription level of PORC[81,82] (Fig. 3).

      The dark-to-light transition enables plants to transition from the skotomorphogenesis to the photomorphogenesis state. This process is often accompanied by changes in phytohormones, which regulate the expression of different PORs in different ways[83]. EIN 3 and EIN 3-like 1 positively regulate PORA and PORB[84]. Cytokinins significantly enhance the transcription of POR mRNA and accelerate plant greening, whereas abscisic acid has the opposite effect[85]. Auxin binds to the promoters of PORA and GUN5 through ARF2 and ARF7 to inhibit their expression directly, with the help of IAA14[86]. Additionally, growth hormones are known to inhibit chlorophyll biosynthesis. Inhibition of their expression directly inhibits chlorophyll biosynthesis[86]. In Arabidopsis, the structures of PORA and PORB are mostly the same, except for the initial transit peptide. However, they perform different functions and cannot replace each other. PORA solely performs during the initial stage of light exposure in yellowing seedlings, and light significantly inhibits PORA. Following light exposure, the expression of PORA declines rapidly, whereas PORB stays constantly expressed.[25].

      The regulatory mechanisms of PORA, PORB, and PORC are interconnected and not completely independent. When ethylene is applied under light, EIN3 regulates the transfer of COP1 from the cytoplasm to the nucleus. Thus, the activity of extranuclear COP1 is blocked, and the expression of PORC is inhibited. When ethylene is absent under light, COP1 mainly exists in the cytoplasm, and HY5 initiates PORC transcription[87] (Fig. 3).

      The expression of LPOR mRNA exhibits noteworthy cyclic variations[88]. Reveille 1 directly binds to the PORA promoter through the EE motif (AAAATATCT) and regulates the transcription of PORA[88] (Fig. 3). AtPORB expression is regulated by the biological clock, whereas AtPORC expression is independent of the biological clock. This finding corresponds to the regulation of OsPORB expression observed in rice grown under short-day conditions. The LPOR enzyme in cucumber is encoded by a single gene, with its expression under light being six times greater than that under dark treatment[31]. Upon exposure to light, the LPOR level decreases slightly, followed by a gradual increase in LPOR expression from 3 to 12 h[31].

      Research on light signal regulation of LPOR primarily focuses on plants turning green during the transition from darkness to light. Further experiments must be conducted to determine whether changes occur in the expression, content, and enzyme activity of LPOR during shading (including changes in light intensity and quality) and whether it is an essential enzyme affecting chlorophyll biosynthesis during variations in the light environment.

    • Changes in LPOR enzyme activity are one of the important factors affecting chlorophyll biosynthesis under abiotic stress[89,90] (Table 1). Shading environments are ubiquitous in nature. Increasing the content of photosynthetic pigments and reducing Chl a/b are important shade tolerance mechanisms for plants[91]. However, the molecular mechanism regulating chlorophyll synthesis under shade has not yet been studied. Previous proteomic studies indicated that compared with the level of LPOR protein in the soybean seedling leaves grown under normal light, that in the soybean seedling leaves grown under shade increases[9294]. This increase may be a significant reason for the augmentation observed in plants’ chlorophyll content under shade conditions.

      Table 1.  Effects of different stresses on the LPOR activity, protein, and transcription levels.

      Abiotic stress Species Response to stress (transcript and protein expression and enzyme activity) Ref.
      Water Rice LPOR content decreases. [90]
      Salt/drought Peanut The expression of AhPORA is downregulated during drought and upregulated during postdrought recovery through AhGLK. [95]
      Rice LPOR activity is downregulated by 60% in salt-treated seedlings. [89]
      Chill Rice and LPOR activity is downregulated. [41]
      Corydalis bungeana Turcz. LPOR's transcript and protein content slightly decline at 4 °C but dramatically decrease at −4 °C with time. [96]
      Wheat and cucumber LPOR level is not reduced in light-exposed chill-stressed seedlings. [97]
      Heat Wheat and cucumber LPOR content is greatly reduced in response to light in heat-stressed seedlings. [97]
      Shade Camellia sinensis L. and soybean LPOR is significantly upregulated after shading, but downregulated by low R/FR ratio [93,98,99]
      Rice OsPORA expression is repressed by light, and OsPORB expression is rapidly upregulated by high-light treatment. [100]

      When plants undergo the process of greening, low temperatures significantly hinder chlorophyll biosynthesis, leading to a decline in chlorophyll accumulation. However, the extent of this decline differs between various species[97]. One of the primary factors contributing to this phenomenon is the inhibition of the conversion of Pchlide to Chlide, which significantly diminishes LPOR activity and downregulates protein and transcriptional expression levels at low temperatures[41,97,101], This results in the obstruction of chlorophyll synthesis and the accumulation of ROS at low temperatures[41]. These ROS subsequently have the potential to cause oxidative damage during the greening process[41]. However, spraying exogenous carotenoids can improve the downregulation of LPOR transcriptional levels at low temperatures, thereby reducing their impact on chlorophyll biosynthesis[102]. Furthermore, the exogenous application of ALA and H2S was found to significantly enhance the content of chlorophyll and its upstream precursors[103]. Previous research showed that LPOR plays an important role in the cold resistance of plants[96,104,105]. CbPORB is resistant to cold in Chorispora bungeana[105]. CbPORB transcription and protein content decrease slightly at 4 °C but significantly decrease over time at −4 °C. Conversely, in A. thaliana[106] and wheat[107], low temperatures upregulate the HY5 expression at the transcriptional level, and HY5 regulates the transfer of COP1 from the nucleus to the cytoplasm, thereby promoting PORC expression (Fig. 3). A comparison of winter wheat XN1376 with its albino line XN1376B revealed that the expression of TaPOR2D in albino leaves with methylation of its promoter at low temperatures was an important factor influencing chlorophyll accumulation at low temperatures[108]. The restricted decrease in Pchlide is the primary cause of the impact on chlorophyll biosynthesis during periods of high-temperature stress[97,101,109,110]. Although the activity of the LPOR enzyme in green seedlings increases under high-temperature conditions[101], the LPOR protein content decreases significantly[97], resulting in a decrease in chlorophyll content. Developing seedlings could regulate the balance between ROS and Chl levels by regulating the production of LPOR enzymes[111]. PORB plays a significant role in the thermoregulation of chlorophyll biosynthesis in phototrophic seedlings and FCA (Flowering Control Locus A) induces the expression of PORA and PORB by promoting the DNA accessibility of RNA polymerase II to the gene promoters, thereby maintaining protein levels at a constant temperature[111]. Some studies suggest that melatonin can enhance plant stress resistance and improve the impact of heat stress on plant chlorophyll synthesis by upregulating the PORA expression[110]. A high-temperature stress-responsive protein, Ta2CP, was discovered in a heat-adapted wheat variety that is also involved in regulating chlorophyll biosynthesis under high-temperature stress[109]. The results indicated that Ta2CP positively regulates chlorophyll biosynthesis via interaction with TaPORB. Silencing Ta2CP expression downregulates TaPORB expression and decreases chlorophyll content, whereas Ta2CP overexpression upregulates TaPORB expression and increases chlorophyll content.

      Under salt stress, the decrease in LPOR enzyme activity is a key contributor to the decline in chlorophyll levels[89]. The expression of genes related to chlorophyll synthesis and photosynthesis in peanuts (Arachis hypogaea) decreases significantly under drought stress. During recovery, the transcript and protein expression levels of AhPORA upregulate significantly, leading to the recovery of chlorophyll biosynthesis and photosynthesis[95]. This finding is consistent with the results observed in rice, where the accumulation of chlorophyll in seedlings developed under water stress is significantly reduced because of a decrease in the accumulation of intermediate precursors for chlorophyll synthesis. In particular, the decrease in the activity of LPOR enzymes, protein, and gene expression leads to damage to the Shibata shift, resulting in a decrease in Pchlide photoreduction[90]. Similarly, when cucumbers experience water stress, the LPOR enzyme content and the transcriptional content directly impact chlorophyll accumulation[112]. Arsenic significantly reduced the growth rate, chlorophyll content, and photosynthetic rate of melon plants. In contrast, iron oxide nanoparticles and selenium treatments up-regulated the expression of chlorophyll synthase and LPOR and increased the chlorophyll content of melon plants under arsenic stress[113]. In summary, LPOR plays a crucial role in regulating chlorophyll biosynthesis during adverse conditions. Plants improve their stress response capability by regulating their LPOR transcription, protein level expression, and enzyme activity. However, the regulatory mechanism is not completely clear. Thus, further exploration is needed.

    • In angiosperms, chlorophyll synthesis is dependent on light-induced activity of LPOR, which reduces Pchlide (Fig. 2) and promotes chloroplast development. Different types of LPOR were identified in multiple species, with varying expression patterns. The different expression patterns and the dependence of LPOR activity on the type of substrate can optimize preparations in the dark to ensure efficient chlorophyll synthesis with minimal impact on photosynthesis. The current research on light has focused on the greening process during the transition from darkness to light. A large number of studies have gradually revealed the regulatory mechanisms of light signaling factors. However, the complexity of light variations in natural environments, such as shaded or densely planted areas in forests, as well as cultivation techniques including strip cropping, result in varying degrees of light intensity and light quality. The precise regulation of LPOR in a variable environment and the efficient synthesis of the optimal amount of chlorophyll to ensure the utilization of light energy by plants remain unclear. Additionally, numerous studies have demonstrated that LPOR plays a pivotal role in abiotic stress response by regulating chlorophyll synthesis at the transcriptional and protein levels (Fig. 3, Table 1). However, the regulatory mechanisms at the posttranslational level have not been extensively investigated. Consequently, further investigation will provide a theoretical foundation for the breeding and development of plant germplasm resources with stress tolerance.

    • The authors confirm contribution to the paper as follows: draft manuscript preparation: Wang Q; manuscript revision: Gao J, Chen J, Tan X, Liu C, Yu L, Yang F, Yang W; conceptualization, funding acquision: Yang F, Yang W. All authors reviewed the results and approved the final version of the manuscript.

    • Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

      • This work was supported by the National Natural Science Foundation of China (32071963).

      • The authors declare that they have no conflict of interest.

      • Copyright: © 2024 by the author(s). Published by Maximum Academic Press, Fayetteville, GA. This article is an open access article distributed under Creative Commons Attribution License (CC BY 4.0), visit https://creativecommons.org/licenses/by/4.0/.
    Figure (3)  Table (1) References (113)
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    Wang Q, Gao J, Chen J, Tan X, Liu C, et al. 2024. Regulatory mechanism of a light-dependent protochlorophyllide oxidoreductase in chlorophyll biosynthesis and environmental adaptation. Technology in Agronomy 4: e023 doi: 10.48130/tia-0024-0019
    Wang Q, Gao J, Chen J, Tan X, Liu C, et al. 2024. Regulatory mechanism of a light-dependent protochlorophyllide oxidoreductase in chlorophyll biosynthesis and environmental adaptation. Technology in Agronomy 4: e023 doi: 10.48130/tia-0024-0019

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