Figures (2)  Tables (3)

    • Figure 1. 

      Melatonin biosynthetic pathways in plants. Solid lines indicate enzymes or oxidants identified in plants, whereas dashed lines indicate enzymes or oxidants identified in animals but not yet confirmed in plants. AFMK: N1-acetyl-N2-formyl-5-methoxyknuramine, AMK: N-acetyl-5-methoxyknuramine, ASDAC: N-acetylserotonin deacetylase, ASMT: N-acetylserotonin methyltransferase, COMT: caffeic acid O-methyltransferase, IAA: indole-3-acetic acid, IDO: indoleamine 2,3-dioxygenase, IPA: indole-3-pyruvate, M2H: melatonin 2-hydroxylase, M3H: melatonin 3-hydroxylase, ROS: reactive oxygen species, RNS: reactive nitrogen species, SNAT: serotonin N-acetyltransferase, TAA: tryptophan aminotransferase of Arabidopsis, TDC: tryptophan decarboxylase, T5H: tryptamine 5-hydroxylase, TPH: tryptophan hydroxylase, YUC: YUCCA (the YUCCA gene encodes a flavin monooxygenase-like enzyme), 2-ODD: 2-oxoglutarate-dependent dioxygenase, 2-OHM: 2-hydroxymelatonin, 3-OHM: 3-hydroxymelatonin, 4-OHM: 4-hydroxymelatonin, 6-OHM: 6-hydroxymelatonin.

    • Figure 2. 

      Schematic illustrating plant growth, development, and physiological activities regulated by melatonin under drought conditions. ABA: abscisic acid, AFMK: N1-acetyl-N2-formyl-5-methoxyknuramine, APX: ascorbate peroxidase, BR: brassinosteroids, CAT: catalase, CTK: cytokinin, DREB: dehydration-responsive element-binding protein, GA: gibberellic acid, GPA: G protein alpha subunit, GPX: glutathione peroxidase, IAA: indole-3-acetic acid, JA: jasmonic acid, MAPK: mitogen-activated protein kinase, MKK: MAPK kinase, MAPKKK: MKK kinase, PMTR1: phyto-melatonin receptor 1, RNS: reactive nitrogen species, ROS: reactive oxygen species, SA: salicylic acid, SOD: superoxide dismutase, TFs: transcription factors, 2-OHM: 2-hydroxymelatonin, 3-OHM: 3-hydroxymelatonin, 5-MT: 5-methoxytryptamine.

    • Object species name Melatonin biosynthesis genes Function of melatonin Ref.
      Apple
      (Malus domestica Borkh.)      		 MdASMT9 Overexpression of MdASMT9 also reduces abscisic acid accumulation through promoting MdWRKY33-mediated transcriptional inhibition of MdNCED1 and MdNCED3, thus inducing stomatal opening for better heat dissipation. [18]
      Arabidopsis thaliana L. AtSNAT2 snat2 knockout mutant exhibits delayed flowering and reductions in leaf area and biomass. [19]
      Barley
      (Hordeum vulgare L.)      		 HvCOMT1, HvCOMT2, and HvCOMT3 Reducing thioacidolysis yields in the COMT RNAi lines are an indication of changes to lignin structure with a greater proportion of resistant bonds in the lignin. [20]
      Grapevine
      (Vitis vinifera L.)      		 VvASMT1 Ectopic overexpression of VvASMT1 in Nicotiana benthamiana significantly enhances melatonin production and increased tolerance to salt and osmotic stresses. [21]
      Malus zumi Mats. MzASMT1 Overexpression of MzASMT1 increases melatonin levels and drought tolerance in Arabidopsis. [22]
      Rice
      (Oryza Sativa L.)      		 OsASMT1, OsASMT2, and OsASMT3 All three ASMT mRNAs are simultaneously induced in treatments with abscisic and methyl jasmonic acids. [23]
      Rice
      (Oryza Sativa L.)      		 OsCOMT Overexpression of OsCOMT significantly delays leaf senescence at the grain-filling stage by inhibiting the degradation of chlorophyll and chloroplast. OsCOMT plays a positive role in the vascular development of rice. OsCOMT is a positive regulator of grain yield. [24]
      Rice
      (Oryza Sativa L.)      		 OsSNAT1 OsSNAT1-overexpressing rice plants increases resistance to cadmium and senescence stresses. [25]
      Rice
      (Oryza Sativa L.)      		 OsSNAT2 SNAT2 RNAi lines indicates a decrease in melatonin and a dwarf phenotype with erect leaves by exhibiting photomorphogenic phenotypes such as inhibition of internodes and increased expression of light-inducible CAB genes in the dark. [26]
      Tobacco
      (Nicotiana tabacum L.)      		 NtCOMT1 Overexpression of NtCOMT1 promotes drought resistance by increasing melatonin content. [27]
      Watermelon
      (Citrullus lanatus L.)      		 ClCOMT1 Overexpression of ClCOMT1 enhances transgenic Arabidopsis tolerance against cold, drought, and NaCl. [28]
      Tomato
      (Solanum lycopersicum L.)      		 SlCOMT1 Overexpression of COMT1 significantly enhances the capacity of the tomato to reduce fungicide carbendazim phytotoxicity and residue. [29]
      Wheat
      (Triticum aestivum L.)      		 TaCOMT Overexpression of the wheat TaCOMT gene enhances drought tolerance and increases the content of melatonin in transgenic Arabidopsis. [30]
      Cassava
      (Manihot esculenta Crantz)      		 MeTDC2, MeASMT2, and MeASMT3 MeWRKY20/75 interacts with 3 melatonin synthesis enzymes (MeTDC2, MeASMT2/3) and positively regulates endogenous melatonin accumulation. [55]
      Cassava
      (Manihot esculenta Crantz)      		 MeTDC2, MeT5H, MeASMT1, MeTDC1, MeASMT2, MeASMT3, and MeSNAT MeRAV1 and MeRAV2 positively regulate 7 melatonin biosynthesis genes (MeTDC2, MeT5H, MeASMT1, MeTDC1, MeASMT2, MeASMT3, and MeSNAT) and the endogenous melatonin level in plant disease resistance against cassava bacterial blight. [56]

      Table 1. 

      Regulation of melatonin biosynthesis genes to enhance stress tolerance in plants.

    • Species name Melatonin concentration Function of exogenous melatonin under drought stress Ref.
      Apple
      (Malus domestica Borkh.)      		 Root application (100 μM) Helping to maintain the better function of PSII and controlling the burst of hydrogen peroxide to delay the leaf senescence under drought. [3]
      Cotton
      (Gossypium hirsutum L.)      		 Foliar spray (100 μM) Down-regulating chlorophyll degradation-related genes and senescence marker genes (GhNAC12 and GhWRKY27/71). Improving photosynthetic efficiency, reducing chlorophyll degradation and ROS accumulation, and inhibiting ABA synthesis, thereby delaying drought-induced leaf senescence in cotton. [4]
      Wheat
      (Triticum aestivum L.)      		 Root application (100 μM) Alleviating photosynthetic and cell membrane damage by maintaining low levels of hydrogen peroxide. [13]
      Rice
      (Oryza Sativa L.)      		 Seed priming (100 μM) Promoting the germination rate and improving the biomass of rice seed shoots and roots. Alleviating the oxidative damage of rice seeds caused by drought stress. [33]
      Alfalfa
      (Medicago sativa L.)      		 Foliar spray (100 μM) Boosting antioxidant enzyme activities, improving photosynthetic performance, and accumulating total soluble sugar and proline content. [36]
      Maize
      (Zea mays L.)      		 Foliar spray (100 μM), and
      root application (50 μM)      		 Reducing the reactive oxygen species burst and enhancing the photosynthetic activity. [37]
      Pepper
      (Capsicum annuum L.)      		 Foliar spray (100 μM) Reducing oxidative stress and improving nitrogen metabolism by activating various enzymes such as nitrate reductase, nitrite reductase, glutamine synthetase, and glutamine dehydrogenase activities. [38]
      Tomato
      (Solanum lycopersicum L.)      		 Seed priming (100 μM) Increasing stomatal conductance, photochemical efficiency, and the antioxidant system, and also reducing the cellular content of toxic substances. [39]
      Tomato
      (Solanum lycopersicum L.)      		 Foliar spray (100 μM), and
      root application (100 μM)      		 Alleviating the inhibition of drought stress on the gas exchange parameters and the leaf net photosynthetic rate, protecting the thylakoid membrane from damage, and strengthening the ATP-synthase activity. [40]
      Two Citrus cultivars Foliar spray (100 μM) Increasing total flavonoid and total phenolic contents under severe drought stress. [41]
      Wheat
      (Triticum aestivum L.)      		 Root application (500 μM) Decreasing membrane damage, more intact grana lamella of chloroplast, higher photosynthetic rate, and maximum efficiency of photosystem II, as well as higher cell turgor and water-holding capacity. [42]
      Wheat
      (Triticum aestivum L.)      		 Root application (100 μM) Improving fine root, lateral root, and root hair, plant height, dry weight, net photosynthesis, and stomatal aperture of leaves. [43]
      Cotton
      (Gossypium hirsutum L.)      		 Foliar spray (100 and 200 μM) Improving the translocation of carbon assimilates to drought-stressed anthers, regulating the carbohydrate balance of drought-stressed anthers to improve male fertility. [44]
      Kiwifruit
      (Actinidia chinensis Planch)      		 Root application (100 μM) Improving photosynthesis by inhibiting stomatal closure, enhancing light energy absorption, and promoting electron transport in PSII. [45]
      Maize (Zea mays L.) Foliar spray (100 μM) Increasing the accumulation of flavonoid metabolites, particularly apigenin, luteolin, and quercetin. Upregulating the expression of genes related to flavonoid biosynthesis (PAL, C4H, 4CL, HCT, CHS, CHI, F3′5′H, and DFR), activates drought-responsive transcription factors (ERFs, NACs, MYBs, and bHLHs). [46]
      Moldavian balm
      (Dracocephalum moldavica)      		 Foliar spray (100 μM) Increasing soluble sugar content, malondialdehyde content, and lipoxygenase activity, non-enzyme antioxidants (including flavonoid, polyphenol compounds, and anthocyanin) under moderate and severe drought stress. [47]
      Maize (Zea mays L.) Root application (100 μM) Improving the photosynthetic activities and alleviated the oxidative damages of maize seedlings under the drought stress. [48]
      Rapeseed
      (Brassica napus L.)      		 Root application (50 μM) Alleviating the seedling growth inhibition and increasing the leaf area and fresh and dry weights of roots and shoots under drought stress. [49]
      Rice
      (Oryza Sativa L.)      		 Root application (100 μM) Promoting root, shoot length, fresh and dry weight, and increasing chlorophyll contents. [50]
      Soybean
      (Glycine max L.)      		 Foliar spray or root application (50 and 100 μM) Improving photosynthetic activity, reduction of abscisic acid and drought-induced oxidative damage. [51]
      Tomato
      (Solanum lycopersicum L.)      		 Root application (150 μM) Affecting stomatal conductance and the activity of ROS scavenging enzymes. [52]
      Tomato
      (Solanum lycopersicum L.)      		 Root application
      (100 and 500 μM)      		 Carbon monoxide is a downstream signal molecule of melatonin-enhanced drought resistance by promoting chlorophyll biosynthesis. [59]
      Cotton
      (Gossypium hirsutum L.)      		 Seed priming (10 μM) Increasing photosynthetic activity, water-use efficiency, and nitrogen metabolism. Upregulating the expression of the autophagosome marker [lipidated (ATG8-PE)]. [63]
      Tomato
      (Solanum lycopersicum L.)      		 Root application (100 μM) Improving the seedlings growth, root characteristics, leaf photosynthesis and antioxidant machinery. [64]
      Creeping bentgrass
      (Agrostis stolonifera)      		 Foliar spray (20 μM) Increasing visual quality, photochemical efficiency, chlorophyll content, and relative water content. Up-regulating and chlorophyll-degradation genes, and cytokinin-signaling and synthesis genes. [82]
      Naked oats
      (Avena nuda L.)      		 Foliar spray (100 μM) Increasing the chlorophyll content and photosynthetic rate of leaves, increasing expression of PYL, PP2C, ABF, SNRK2, and IAA. [84]
      Barley
      (Hordeum vulgare L.)      		 Root application (2 mM) Exogenous melatonin increases the relative abundance of the bacterial community in carbohydrate and carboxylate degradation while decreasing the relative abundance in the pathways of fatty acid and lipid degradation and inorganic nutrient metabolism under drought. [89]

      Table 2. 

      Exogenous melatonin enhances drought tolerance in plants.

    • Species name Regulation objects Function of melatonin under drought conditions Ref.
      Cotton
      (Gossypium hirsutum L.)      		 Decreased ABA Melatonin can effectively enhance the antioxidant enzyme system, improve photosynthetic efficiency, reduce chlorophyll degradation and ROS accumulation, and inhibit ABA synthesis, thereby delaying drought-induced leaf senescence in cotton. [4]
      Wheat
      (Triticum aestivum L.)      		 Increased JA Up-regulating jasmonic acid biosynthesis genes and increasing jasmonic acid contents to mitigate drought stress. [13]
      Maize (Zea mays L.) Increased GA and IAA; decreased ABA Melatonin increases GA and IAA while reducing the ABA levels in leaves under drought conditions. [69]
      Apple
      (Malus domestica Borkh.)      		 Decreased ABA Melatonin scavenges H2O2 and reduces ABA by upregulating MdNCED3 and downregulating MdCYP707A1 and MdCYP707A2 to re-open stomatal under drought conditions. [79]
      Maize (Zea mays L.) Reduced ABA Reducing ABA accumulation and inducing stomatal reopening by inhibiting up-regulation of NCED1, and up-regulating ABA catabolic genes ABA8ox1 and ABA8ox3. [80]
      Soybean (Glycine max L.) Decreased ABA in leaves and increased ABA in roots Melatonin-received plant leaves accumulates less ABA but roots content higher ABA. Melatonin significantly suppresses ABA biosynthesis and signaling gene expression in soybean exposed to drought stress. [81]
      Creeping bentgrass
      (Agrostis stolonifera)      		 Increased CTK Melatonin synergistically interacts with cytokinins to suppress drought-induced leaf senescence. Increasing endogenous cytokinins content and upregulating cytokinins signal transduction genes and transcription factors. [82]
      Perennial ryegrass
      (Lolium perenne L.)      		 Increased BR Brassinosteroid plays a critical role in the melatonin-mediated mitigation of cold and drought stress by triggering antioxidant activities as well as enhancing the photosynthetic capacities. Brassinosteroid biosynthesis and it signaling pathway are induced by melatonin. [83]
      Barley
      (Hordeum vulgare L.)      		 Increased SA, GA, CK, and IAA; decreased ABA Melatonin increases levels of SA, GA, CK, and IAA, as well as a decrease in abscisic acid to enhance stress tolerance in barley. [84]
      Cotton
      (Gossypium hirsutum L.)      		 Increased GA and reduced ABA Increasing the germination rate, germination potential, radical length, and fresh weight, as well as the activities of superoxide dismutase (SOD), peroxidase (POD), and α-amylase. Melatonin alleviates drought stress by reducing ABA content and increasing GA3 content. [85]
      Maize (Zea mays L.) Increased JA Up-regulating jasmonic acid biosynthesis genes and increasing jasmonic acid contents to mitigate drought stress. [86]

      Table 3. 

      Melatonin regulatory pathways under drought conditions.