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2024 Volume 3
Article Contents
Abstract
Maize is a major staple crop and a model species for plant biology and genetic studies
Hundreds of genomic regions were selected during maize domestication
Domestication genes for morphological traits
Teosinte branched1 (tb1) and increased apical dominance in maize
Teosinte glume architecture 1 (tga1) and reduced glumes in maize
Grassy tillers1 (gt1) and suppression of prolificacy in maize
UPA2 and leaf angle
GLOSSY15 (Gl15) and vegetative phase change
Domestication genes related to identity and morphology of inflorescence
Domestication genes of kernel-related traits
ZmSh1 and seed shattering
Nutritional quality
Domestication genes related to disease resistance and abiotic stresses
Pre-Columbia spread of maize from tropical to temperate
Selective genes underlying agronomic trait improvement during modern maize breeding
Key genes/pathways regulating shade avoidance responses and plant architecture in maize
Complementary and differential selection of heterotic groups during modern maize breeding
Perspectives
Acknowledgments
Conflict of interest
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Chemical profiling and bioactivity evaluation of thymol rich Coleus aromaticus Benth. essential oil

  • 1.

    Department of Chemistry, College of Basic Sciences and Humanities, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, 263145, India

  • 2.

    Medicinal Plants Research and Development Centre, Haldi, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, 263145, India

  • 3.

    Department of Plant Pathology, College of Agriculture, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, 263145, India

  • 4.

    Department of Entomology, College of Agriculture, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, 263145, India

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  • Coleus aromaticus Benth. (Family: Lamiaceae) is a huge perennial, aromatic and succulent herb native to the Indian subcontinent. The dried leaves have an oregano-like texture making them a perfect culinary food supplement to be used as herbal seasoning for meat and other food products. The present study aimed to identify the bioactive components in the essential oil collected from the fresh aerial parts of Coleus aromaticus Benth. Using GC/MS analysis, 12 terpenoid components were identified, accounting for 97.5% of the overall oil content. Thymol (69.6%), p-cymene (3.9%), (E)-caryophyllene (3.7%), carvacrol (3.2%), α-thujene (3.2%), γ-terpinene (2.9%), and carvacrol methyl ether (2.3%) were identified to be the primary constituents in the oil, which was determined to be dominated by oxygenated monoterpenes (72.8%). Additionally, at the highest dose, CAEO showed significant pesticidal activity, inhibiting the egg hatchability of Meloidogyne incognita by 96.9%, immobilizing it by 52.3%, insecticidal activity on Spodoptera litura by 71.13%, and phytotoxic activity on Raphanus raphanistrum seeds by 97.75%. For speculating the potential method of action of CAEO components, the proteins/enzymes namely acetylcholinesterase (PDB ID: 6XYS), carboxylesterase (PDB ID: 5IVH), and acetohydroxyacid synthase (PBD ID: 1YHZ) were employed. The novel aspect of this study was that the herbal spice material was collected during its vegetative stage from the Tarai region of Pantnagar (India) in order to bio-evaluate its nematicidal, herbicidal, and insecticidal effectiveness. It was found that CAEO is an effective alternative source of natural pesticides and opens the way for additional research on its mechanistic techniques and field tests to determine its pesticidal studies.

  • Maize is a major staple crop and a model species for plant biology and genetic studies

    As a major staple crop, today maize accounts for approximately 40% of total worldwide cereal production (http://faostat.fao.org/). Since its domestication ~9,000 years ago from a subgroup of teosinte (Zea mays ssp. parviglumis) in the tropical lowlands of southwest Mexico[1], its cultivating area has greatly expanded, covering most of the world[2]. Human's breeding and utilization of maize have gone through several stages, from landraces, open-pollinated varieties (OPVs), double-cross hybrids (1930s-1950s) and since the middle 1950s, single-cross hybrids. Nowadays, global maize production is mostly provided by single-cross hybrids, which exhibit higher-yielding and better stress tolerance than OPVs and double-cross hybrids[3].

    Besides its agronomic importance, maize has also been used as a model plant species for genetic studies due to its out-crossing habit, large quantities of seeds produced and the availability of diverse germplasm. The abundant mutants of maize facilitated the development of the first genetic and cytogenetic maps of plants, and made it an ideal plant species to identify regulators of developmental processes[4−6]. Although initially lagging behind other model plant species (such as Arabidopsis and rice) in multi-omics research, the recent rapid development in sequencing and transformation technologies, and various new tools (such as CRISPR technologies, double haploids etc.) are repositioning maize research at the frontiers of plant research, and surely, it will continue to reveal fundamental insights into plant biology, as well as to accelerate molecular breeding for this vitally important crop[7, 8].

    Hundreds of genomic regions were selected during maize domestication

    During domestication from teosinte to maize, a number of distinguishing morphological and physiological changes occurred, including increased apical dominance, reduced glumes, suppression of ear prolificacy, increase in kernel row number, loss of seed shattering, nutritional changes etc.[9] (Fig. 1). At the genomic level, genome-wide genetic diversity was reduced due to a population bottleneck effect, accompanied by directional selection at specific genomic regions underlying agronomically important traits. Over a century ago, Beadle initially proposed that four or five genes or blocks of genes might be responsible for much of the phenotypic changes between maize and teosinte[10,11]. Later studies by Doebley et al. used teosinte–maize F2 populations to dissect several quantitative trait loci (QTL) to the responsible genes (such as tb1 and tga1)[12,13]. On the other hand, based on analysis of single-nucleotide polymorphisms (SNPs) in 774 genes, Wright et al.[14] estimated that 2%−4% of maize genes (~800−1,700 genes genome-wide) were selected during maize domestication and subsequent improvement. Taking advantage of the next-generation sequencing (NGS) technologies, Hufford et al.[15] conducted resequencing analysis of a set of wild relatives, landraces and improved maize varieties, and identified ~500 selective genomic regions during maize domestication. In a recent study, Xu et al.[16] conducted a genome-wide survey of 982 maize inbred lines and 190 teosinte accession. They identified 394 domestication sweeps and 360 adaptation sweeps. Collectively, these studies suggest that maize domestication likely involved hundreds of genomic regions. Nevertheless, much fewer domestication genes have been functionally studied so far.

    Figure 1.  Main traits of maize involved in domestication and improvement.
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    Domestication genes for morphological traits

    Teosinte branched1 (tb1) and increased apical dominance in maize

    During maize domestication, a most profound morphological change is an increase in apical dominance, transforming a multi-branched plant architecture in teosinte to a single stalked plant (terminated by a tassel) in maize. The tillers and long branches of teosinte are terminated by tassels and bear many small ears. Similarly, the single maize stalk bears few ears and is terminated by a tassel[9,12,17]. A series of landmark studies by Doebley et al. elegantly demonstrated that tb1, which encodes a TCP transcription factor, is responsible for this transformation[18, 19]. Later studies showed that insertion of a Hopscotch transposon located ~60 kb upstream of tb1 enhances the expression of tb1 in maize, thereby repressing branch outgrowth[20, 21]. Through ChIP-seq and RNA-seq analyses, Dong et al.[22] demonstrated that tb1 acts to regulate multiple phytohormone signaling pathways (gibberellins, abscisic acid and jasmonic acid) and sugar sensing. Moreover, several other domestication loci, including teosinte glume architecture1 (tga1), prol1.1/grassy tillers1, were identified as its putative targets. Elucidating the precise regulatory mechanisms of these loci and signaling pathways will be an interesting and rewarding area of future research. Also worth noting, studies showed that tb1 and its homologous genes in Arabidopsis (Branched1 or BRC1) and rice (FINE CULM1 or FC1) play a conserved role in repressing the outgrowth of axillary branches in both dicotyledon and monocotyledon plants[23, 24].

    Teosinte glume architecture 1 (tga1) and reduced glumes in maize

    Teosinte ears possess two ranks of fruitcase-enclosed kernels, while maize produces hundreds of naked kernels on the ear[13]. tga1, which encodes a squamosa-promoter binding protein (SBP) transcription factor, underlies this transformation[25]. It has been shown that a de novo mutation occurred during maize domestication, causing a single amino acid substitution (Lys to Asn) in the TGA1 protein, altering its binding activity to its target genes, including a group of MADS-box genes that regulate glume identity[26].

    Grassy tillers1 (gt1) and suppression of prolificacy in maize

    Prolificacy, the number of ears per plants, is also a domestication trait. It has been shown that grassy tillers 1 (gt1), which encodes an HD-ZIP I transcription factor, suppresses prolificacy by promoting lateral bud dormancy and suppressing elongation of the later ear branches[27]. The expression of gt1 is induced by shading and requires the activity of tb1, suggesting that gt1 acts downstream of tb1 to mediate the suppressed branching activity in response to shade. Later studies mapped a large effect QTL for prolificacy (prol1.1) to a 2.7 kb 'causative region' upstream of the gt1gene[28]. In addition, a recent study identified a new QTL, qEN7 (for ear number on chromosome 7). Zm00001d020683, which encodes a putative INDETERMINATE DOMAIN (IDD) transcription factor, was identified as the likely candidate gene based on its expression pattern and signature of selection during maize improvement[29]. However, its functionality and regulatory relationship with tb1 and gt1 remain to be elucidated.

    UPA2 and leaf angle

    Smaller leaf angle and thus more compact plant architecture is a desired trait for modern maize varieties. Tian et al.[30] used a maize-teosinte BC2S3 population and cloned two QTLs (Upright Plant Architecture1 and 2 [UPA1 and UPA2]) that regulate leaf angle. Interestingly, the authors showed that the functional variant of UPA2 is a 2-bp InDel located 9.5 kb upstream of ZmRAVL1, which encodes a B3 domain transcription factor. The 2-bp Indel flanks the binding site of the transcription factor Drooping Leaf1 (DRL1)[31], which represses ZmRAVL1 expression through interacting with Liguleless1 (LG1), a SBP-box transcription factor essential for leaf ligule and auricle development[32]. UPA1 encodes brassinosteroid C-6 oxidase1 (brd1), a key enzyme for biosynthesis of active brassinolide (BR). The teosinte-derived allele of UPA2 binds DRL1 more strongly, leading to lower expression of ZmRAVL1 and thus, lower expression of brd1 and BR levels, and ultimately smaller leaf angle. Notably, the authors demonstrated that the teosinte-derived allele of UPA2 confers enhanced yields under high planting densities when introgressed into modern maize varieties[30, 33].

    GLOSSY15 (Gl15) and vegetative phase change

    Maize plants exhibit salient vegetative phase change, which marks the vegetative transition from the juvenile stage to the adult stage, characterized by several changes in maize leaves produced before and after the transition, such as production of leaf epicuticular wax and epidermal hairs. Previous studies reported that Glossy15 (Gl15), which encodes an AP2-like transcription factor, promotes juvenile leaf identity and suppressing adult leaf identity. Ectopic overexpression of Gl15 causes delayed vegetative phase change and flowering, while loss-of-function gl15 mutant displayed earlier vegetative phase change[34]. In another study, Gl15 was identified as a major QTL (qVT9-1) controlling the difference in the vegetative transition between maize and teosinte. Further, it was shown that a pre-existing low-frequency standing variation, SNP2154-G, was selected during domestication and likely represents the causal variation underlying differential expression of Gl15, and thus the difference in the vegetative transition between maize and teosinte[35].

    Domestication genes related to identity and morphology of inflorescence

    A number of studies documented evidence that tassels replace upper ears1 (tru1) is a key regulator of the conversion of the male terminal lateral inflorescence (tassel) in teosinte to a female terminal inflorescence (ear) in maize. tru1 encodes a BTB/POZ ankyrin repeat domain protein, and it is directly targeted by tb1, suggesting their close regulatory relationship[36]. In addition, a number of regulators of maize inflorescence morphology, were also shown as selective targets during maize domestication, including ramosa1 (ra1)[37, 38], which encodes a putative transcription factor repressing inflorescence (the ear and tassel) branching, Zea Agamous-like1 (zagl1)[39], which encodes a MADS-box transcription factor regulating flowering time and ear size, Zea floricaula leafy2 (zfl2, homologue of Arabidopsis Leafy)[40, 41], which likely regulates ear rank number, and barren inflorescence2 (bif2, ortholog of the Arabidopsis serine/threonine kinase PINOID)[42, 43], which regulates the formation of spikelet pair meristems and branch meristems on the tassel. The detailed regulatory networks of these key regulators of maize inflorescence still remain to be further elucidated.

    Domestication genes of kernel-related traits

    Kernel row number (KRN) and kernel weight are two important determinants of maize yield. A number of domestication genes modulating KRN and kernel weight have been identified and cloned, including KRN1, KRN2, KRN4 and qHKW1. KRN4 was mapped to a 3-kb regulatory region located ~60 kb downstream of Unbranched3 (UB3), which encodes a SBP transcription factor and negatively regulates KRN through imparting on multiple hormone signaling pathways (cytokinin, auxin and CLV-WUS)[44, 45]. Studies have also shown that a harbinger TE in the intergenic region and a SNP (S35) in the third exon of UB3 act in an additive fashion to regulate the expression level of UB3 and thus KRN[46].

    KRN1 encodes an AP2 transcription factor that pleiotropically affects plant height, spike density and grain size of maize[47], and is allelic to ids1/Ts6 (indeterminate spikelet 1/Tassel seed 6)[48]. Noteworthy, KRN1 is homologous to the wheat domestication gene Q, a major regulator of spike/spikelet morphology and grain threshability in wheat[49].

    KRN2 encodes a WD40 domain protein and it negatively regulates kernel row number[50]. Selection in a ~700-bp upstream region (containing the 5’UTR) of KRN2 during domestication resulted in reduced expression and thus increased kernel row number. Interestingly, its orthologous gene in rice, OsKRN2, was shown also a selected gene during rice domestication to negatively regulate secondary panicle branches and thus grain number. These observations suggest convergent selection of yield-related genes occurred during parallel domestication of cereal crops.

    qHKW1 is a major QTL for hundred-kernel weight (HKW)[51]. It encodes a CLAVATA1 (CLV1)/BARELY ANY MERISTEM (BAM)-related receptor kinase-like protein positively regulating HKW. A 8.9 Kb insertion in its promoter region was find to enhance its expression, leading to enhanced HKW[52]. In addition, Chen et al.[53] reported cloning of a major QTL for kernel morphology, qKM4.08, which encodes ZmVPS29, a retromer complex component. Sequencing and association analysis revealed that ZmVPS29 was a selective target during maize domestication. They authors also identified two significant polymorphic sites in its promoter region significantly associated with the kernel morphology. Moreover, a strong selective signature was detected in ZmSWEET4c during maize domestication. ZmSWEET4c encodes a hexose transporter protein functioning in sugar transport across the basal endosperm transfer cell layer (BETL) during seed filling[54]. The favorable alleles of these genes could serve as valuable targets for genetic improvement of maize yield.

    In a recent effort to more systematically analyze teosinte alleles that could contribute to yield potential of maize, Wang et al.[55] constructed four backcrossed maize-teosinte recombinant inbred line (RIL) populations and conducted detailed phenotyping of 26 agronomic traits under five environmental conditions. They identified 71 QTL associated with 24 plant architecture and yield related traits through inclusive composite interval mapping. Interestingly, they identified Zm00001eb352570 and Zm00001eb352580, both encode ethylene-responsive transcription factors, as two key candidate genes regulating ear height and the ratio of ear to plant height. Chen et al.[56] constructed a teosinte nested association mapping (TeoNAM) population, and performed joint-linkage mapping and GWAS analyses of 22 domestication and agronomic traits. They identified the maize homologue of PROSTRATE GROWTH1, a rice domestication gene controlling the switch from prostrate to erect growth, is also a QTL associated with tillering in teosinte and maize. Additionally, they also detected multiple QTL for days-to-anthesis (such as ZCN8 and ZmMADS69) and other traits (such as tassel branch number and tillering) that could be exploited for maize improvement. These lines of work highlight again the value of mining the vast amounts of superior alleles hidden in teosinte for future maize genetic improvement.

    ZmSh1 and seed shattering

    Loss of seed shattering was also a key trait of maize domestication, like in other cereals. shattering1 (sh1), which encodes a zinc finger and YABBY domain protein regulating seed shattering. Interesting, sh1 was demonstrated to undergo parallel domestication in several cereals, including rice, maize, sorghum, and foxtail millet[57]. Later studies showed that the foxtail millet sh1 gene represses lignin biosynthesis in the abscission layer, and that an 855-bp Harbinger transposable element insertion in sh1 causes loss of seed shattering in foxtail millet[58].

    Nutritional quality

    In addition to morphological traits, a number of physiological and nutritional related traits have also been selected during maize domestication. Based on survey of the nucleotide diversity, Whitt et al.[59] reported that six genes involved in starch metabolism (ae1, bt2, sh1, sh2, su1 and wx1) are selective targets during maize domestication. Palaisa et al.[60] reported selection of the Y1 gene (encoding a phytoene synthase) for increased nutritional value. Karn et al.[61] identified two, three, and six QTLs for starch, protein and oil respectively and showed that teosinte alleles can be exploited for the improvement of kernel composition traits in modern maize germplasm. Fan et at.[62] reported a strong selection imposed on waxy (wx) in the Chinese waxy maize population. Moreover, a recent exciting study reported the identification of a teosinte-derived allele of teosinte high protein 9 (Thp9) conferring increased protein level and nitrogen utilization efficiency (NUE). It was further shown that Thp9 encodes an asparagine synthetase 4 and that incorrect splicing of Thp9-B73 transcripts in temperate maize varieties is responsible for its diminished expression, and thus reduced NUE and protein content[63].

    Domestication genes related to disease resistance and abiotic stresses

    Teosintes is known to confer superior disease resistance and adaptation to extreme environments (such as low phosphorus and high salinity). de Lange et al. and Lennon et al.[64−66] reported the identification of teosinte-derived QTLs for resistance to gray leaf spot and southern leaf blight in maize. Mano & Omori reported that teosinte-derived QTLs could confer flooding tolerance[67]. Feng et al.[68] identified four teosinte-derived QTL that could improve resistance to Fusarium ear rot (FER) caused by Fusarium verticillioides. Recently, Wang et al.[69] reported a MYB transcription repressor of teosinte origin (ZmMM1) that confers resistance to northern leaf blight (NLB), southern corn rust (SCR) and gray leaf spot (GLS) in maize, while Zhang et al.[70] reported the identification of an elite allele of SNP947-G ZmHKT1 (encoding a sodium transporter) derived from teosinte can effectively improve salt tolerance via exporting Na+ from the above-ground plant parts. Gao et al.[71] reported that ZmSRO1d-R can regulate the balance between crop yield and drought resistance by increasing the guard cells' ROS level, and it underwent selection during maize domestication and breeding. These studies argue for the need of putting more efforts to tapping into the genetic resources hidden in the maize’s wild relatives. The so far cloned genes involved in maize domestication are summarized in Table 1. Notably, the enrichment of transcription factors in the cloned domestication genes highlights a crucial role of transcriptional re-wiring in maize domestication.

    Table 1.  Key domestication genes cloned in maize.
    GenePhenotypeFunctional annotationSelection typeCausative changeReferences
    tb1Plant architectureTCP transcription factorIncreased expression~60 kb upstream of tb1 enhancing expression[18−22]
    tga1Hardened fruitcaseSBP-domain transcription factorProtein functionA SNP in exon (K-N)[25, 26]
    gt1Plant architectureHomeodomain leucine zipperIncreased expressionprol1.1 in 2.7 kb upstream of the promoter region increasing expression[27, 28]
    Zm00001d020683Plant architectureINDETERMINATE DOMAIN transcription factorProtein functionUnknown[29]
    UPA1Leaf angleBrassinosteroid C-6 oxidase1Protein functionUnknown[30]
    UPA2Leaf angleB3 domain transcription factorIncreased expressionA 2 bp indel in 9.5 kb upstream of ZmRALV1[30]
    Gl15Vegetative phase changeAP2-like transcription factorAltered expressionSNP2154: a stop codon (G-A)[34, 35]
    tru1Plant architectureBTB/POZ ankyrin repeat proteinIncreased expressionUnknown[36]
    ra1Inflorescence architectureTranscription factorAltered expressionUnknown[37, 38]
    zflPlant architectureTranscription factorAltered expressionUnknown[40, 41]
    UB3Kernel row numberSBP-box transcription factorAltered expressionA TE in the intergenic region;[44−46]
    SNP (S35): third exon of UB3
    (A-G) increasing expression of UB3 and KRN
    KRN1/ids1/Ts6Kernel row numberAP2 Transcription factorIncreased expressionUnknown[47, 48]
    KRN2Kernel row numberWD40 domainDecreased expressionUnknown[50]
    qHKW1Kernel row weightCLV1/BAM-related receptor kinase-like proteinIncreased expression8.9 kb insertion upstream of HKW[51, 52]
    ZmVPS29Kernel morphologyA retromer complex componentProtein functionTwo SNPs (S-1830 and S-1558) in the promoter of ZmVPS29[53]
    ZmSWEET4cSeed fillingHexose transporterProtein functionUnknown[54]
    ZmSh1ShatteringA zinc finger and YABBY transcription factorProtein functionUnknown[57, 58]
    Thp9Nutrition qualityAsparagine synthetase 4 enzymeProtein functionA deletion in 10th intron of Thp9 reducing NUE and protein content[63]
    ZmMM1Biotic stressMYB Transcription repressorProtein functionUnknown[69]
    ZmHKT1Abiotic stressA sodium transporterProtein functionSNP947-G: a nonsynonymous variation increasing salt tolerance[70]
    ZmSRO1d-RDrought resistance and productionPolyADP-ribose polymerase and C-terminal RST domainProtein functionThree non-synonymous variants: SNP131 (A44G), SNP134 (V45A) and InDel433[71]
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    Pre-Columbia spread of maize from tropical to temperate

    After its domestication from its wild progenitor teosinte in southwestern Mexico in the tropics, maize has now become the mostly cultivated crop worldwide owing to its extensive range expansion and adaptation to diverse environmental conditions (such as temperature and day length). A key prerequisite for the spread of maize from tropical to temperate regions is reduced photoperiod sensitivity[72]. It was recently shown that CENTRORADIALIS 8 (ZCN8), an Flowering Locus T (FT) homologue, underlies a major quantitative trait locus (qDTA8) for flowering time[73]. Interestingly, it has been shown that step-wise cis-regulatory changes occurred in ZCN8 during maize domestication and post-domestication expansion. SNP-1245 is a target of selection during early maize domestication for latitudinal adaptation, and after its fixation, selection of InDel-2339 (most likely introgressed from Zea mays ssp. Mexicana) likely contributed to the spread of maize from tropical to temperate regions[74].

    ZCN8 interacts with the basic leucine zipper transcription factor DLF1 (Delayed flowering 1) to form the florigen activation complex (FAC) in maize. Interestingly, DFL1 was found to underlie qLB7-1, a flowering time QTL identified in a BC2S3 population of maize-teosinte. Moreover, it was shown that DLF1 directly activates ZmMADS4 and ZmMADS67 in the shoot apex to promote floral transition[75]. In addition, ZmMADS69 underlies the flowering time QTL qDTA3-2 and encodes a MADS-box transcription factor. It acts to inhibit the expression of ZmRap2.7, thereby relieving its repression on ZCN8 expression and causing earlier flowering. Population genetic analyses showed that DLF1, ZmMADS67 and ZmMADS69 are all targets of artificial selection and likely contributed to the spread of maize from the tropics to temperate zones[75, 76].

    In addition, a few genes regulating the photoperiod pathway and contributing to the acclimation of maize to higher latitudes in North America have been cloned, including Vgt1, ZmCCT (also named ZmCCT10), ZmCCT9 and ZmELF3.1. Vgt1 was shown to act as a cis-regulatory element of ZmRap2.7, and a MITE TE located ~70 kb upstream of Vgt1 was found to be significantly associated with flowering time and was a major target for selection during the expansion of maize to the temperate and high-latitude regions[77−79]. ZmCCT is another major flowering-time QTL and it encodes a CCT-domain protein homologous to rice Ghd7[80]. Its causal variation is a 5122-bp CACTA-like TE inserted ~2.5 kb upstream of ZmCCT10[72, 81]. ZmCCT9 was identified a QTL for days to anthesis (qDTA9). A Harbinger-like TE located ~57 kb upstream of ZmCCT9 showed the most significant association with DTA and thus believed to be the causal variation[82]. Notably, the CATCA-like TE of ZmCCT10 and the Harbinger-like TE of ZmCCT9 are not observed in surveyed teosinte accessions, hinting that they are de novo mutations occurred after the initial domestication of maize[72, 82]. ZmELF3.1 was shown to underlie the flowering time QTL qFT3_218. It was demonstrated that ZmELF3.1 and its homolog ZmELF3.2 can form the maize Evening Complex (EC) through physically interacting with ZmELF4.1/ZmELF4.2, and ZmLUX1/ZmLUX2. Knockout mutants of Zmelf3.1 and Zmelf3.1/3.2 double mutant presented delayed flowering under both long-day and short-day conditions. It was further shown that the maize EC promote flowering through repressing the expression of several known flowering suppressor genes (e.g., ZmCCT9, ZmCCT10, ZmCOL3, ZmPRR37a and ZmPRR73), and consequently alleviating their inhibition on several maize florigen genes (ZCN8, ZCN7 and ZCN12). Insertion of two closely linked retrotransposon elements upstream of the ZmELF3.1 coding region increases the expression of ZmELF3.1, thus promoting flowering[83]. The increase frequencies of the causal TEs in Vgt1, ZmCCT10, ZmCCT9 and ZmELF3.1 in temperate maize compared to tropical maize highlight a critical role of these genes during the spread and adaptation of maize to higher latitudinal temperate regions through promoting flowering under long-day conditions[72,81−83].

    In addition, Barnes et al.[84] recently showed that the High Phosphatidyl Choline 1 (HPC1) gene, which encodes a phospholipase A1 enzyme, contributed to the spread of the initially domesticated maize from the warm Mexican southwest to the highlands of Mexico and South America by modulating phosphatidylcholine levels. The Mexicana-derived allele harbors a polymorphism and impaired protein function, leading to accelerated flowering and better fitness in highlands.

    Besides the above characterized QTLs and genes, additional genetic elements likely also contributed to the pre-Columbia spreading of maize. Hufford et al.[85] proposed that incorporation of mexicana alleles into maize may helped the expansion of maize to the highlands of central Mexico based on detection of bi-directional gene flow between maize and Mexicana. This proposal was supported by a recent study showing evidence of introgression for over 10% of the maize genome from the mexicana genome[86]. Consistently, Calfee et al.[87] found that sequences of mexicana ancestry increases in high-elevation maize populations, supporting the notion that introgression from mexicana facilitating adaptation of maize to the highland environment. Moreover, a recent study examined the genome-wide genetic diversity of the Zea genus and showed that dozens of flowering-related genes (such as GI, BAS1 and PRR7) are associated with high-latitude adaptation[88]. These studies together demonstrate unequivocally that introgression of genes from Mexicana and selection of genes in the photoperiod pathway contributed to the spread of maize to the temperate regions.

    The so far cloned genes involved in pre-Columbia spread of maize are summarized in Fig. 2 and Table 2.

    Figure 2.  Genes involved in Pre-Columbia spread of maize to higher latitudes and the temperate regions. The production of world maize in 2020 is presented by the green bar in the map from Ritchie et al. (2023). Ritchie H, Rosado P, and Roser M. 2023. "Agricultural Production". Published online at OurWorldInData.org. Retrieved from: 'https:ourowrldindata.org/agricultural-production' [online Resource].
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    Table 2.  Flowering time related genes contributing to Pre-Columbia spread of maize.
    GeneFunctional annotationCausative changeReferences
    ZCN8Florigen proteinSNP-1245 and Indel-2339 in promoter[73, 74]
    DLF1Basic leucine zipper transcription factorUnknown[75]
    ZmMADS69MADS-box transcription factorUnknown[76]
    ZmRap2.7AP2-like transcription factorMITE TE inserted ~70 kb upstream[77−79]
    ZmCCTCCT-domain protein5122-bp CACTA-like TE inserted ~2.5 kb upstream[72,81]
    ZmCCT9CCT transcription factorA harbinger-like element at 57 kb upstream[82]
    ZmELF3.1Unknownwo retrotransposons in the promote[84]
    HPC1Phospholipase A1 enzymUnknown[83]
    ZmPRR7UnknownUnknown[88]
    ZmCOL9CO-like-transcription factorUnknown[88]
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    Selective genes underlying agronomic trait improvement during modern maize breeding

    Subsequent to domestication ~9,000 years ago, maize has been continuously subject to human selection during the post-domestication breeding process. Through re-sequencing analysis of 35 improved maize lines, 23 traditional landraces and 17 wild relatives, Hufford et al.[15] identified 484 and 695 selective sweeps during maize domestication and improvement, respectively. Moreover, they found that about a quarter (23%) of domestication sweeps (107) were also selected during improvement, indicating that a substantial portion of the domestication loci underwent continuous selection during post-domestication breeding.

    Genetic improvement of maize culminated in the development of high planting density tolerant hybrid maize to increase grain yield per unit land area[89, 90]. To investigate the key morphological traits that have been selected during modern maize breeding, we recently conducted sequencing and phenotypic analyses of 350 elite maize inbred lines widely used in the US and China over the past few decades. We identified four convergently improved morphological traits related to adapting to increased planting density, i.e., reduced leaf angle, reduced tassel branch number (TBN), reduced relative plant height (EH/PH) and accelerated flowering. Genome-wide Association Study (GWAS) identified a total of 166 loci associated with the four selected traits, and found evidence of convergent increases in allele frequency at putatively favorable alleles for the identified loci. Moreover, genome scan using the cross-population composite likelihood ratio approach (XP-CLR) identified a total of 1,888 selective sweeps during modern maize breeding in the US and China. Gene ontology analysis of the 5,356 genes encompassed in the selective sweeps revealed enrichment of genes related to biosynthesis or signaling processes of auxin and other phytohormones, and in responses to light, biotic and abiotic stresses. This study provides a valuable resource for mining genes regulating morphological and physiological traits underlying adaptation to high-density planting[91].

    In another study, Li et al.[92] identified ZmPGP1 (ABCB1 or Br2) as a selected target gene during maize domestication and genetic improvement. ZmPGP1 is involved in auxin polar transport, and has been shown to have a pleiotropic effect on plant height, stalk diameter, leaf length, leaf angle, root development and yield. Sequence and phenotypic analyses of ZmPGP1 identified SNP1473 as the most significant variant for kernel length and ear grain weight and that the SNP1473T allele is selected during both the domestication and improvement processes. Moreover, the authors identified a rare allele of ZmPGP1 carrying a 241-bp deletion in the last exon, which results in significantly reduced plant height and ear height and increased stalk diameter and erected leaves, yet no negative effect on yield[93], highlighting a potential utility in breeding high-density tolerant maize cultivars.

    Key genes/pathways regulating shade avoidance responses and plant architecture in maize

    Shade avoidance syndrome (SAS) is a set of adaptive responses triggered when plants sense a reduction in the red to far-red light (R:FR) ratio under high planting density conditions, commonly manifested by increased plant height (and thus more prone to lodging), suppressed branching, accelerated flowering and reduced resistance to pathogens and pests[94, 95]. High-density planting could also cause extended anthesis-silking interval (ASI), reduced tassel size and smaller ear, and even barrenness[96, 97]. Thus, breeding of maize cultivars of attenuated SAS is a priority for adaptation to increased planting density.

    Extensive studies have been performed in Arabidopsis to dissect the regulatory mechanism of SAS and this topic has been recently extensively reviewed[98]. We recently showed that a major signaling mechanism regulating SAS in Arabidopsis is the phytochrome-PIFs module regulates the miR156-SPL module-mediated aging pathway[99]. We proposed that in maize there might be a similar phytochrome-PIFs-miR156-SPL regulatory pathway regulating SAS and that the maize SPL genes could be exploited as valuable targets for genetic improvement of plant architecture tailored for high-density planting[100].

    In support of this, it has been shown that the ZmphyBs (ZmphyB1 and ZmphyB2), ZmphyCs (ZmphyC1 and ZmphyC2) and ZmPIFs are involved in regulating SAS in maize[101−103]. In addition, earlier studies have shown that as direct targets of miR156s, three homologous SPL transcription factors, UB2, UB3 and TSH4, regulate multiple agronomic traits including vegetative tillering, plant height, tassel branch number and kernel row number[44, 104]. Moreover, it has been shown that ZmphyBs[101, 105] and ZmPIF3.1[91], ZmPIF4.1[102] and TSH4[91] are selective targets during modern maize breeding (Table 3).

    Table 3.  Selective genes underpinning genetic improvement during modern maize breeding.
    GenePhenotypeFunctional annotationSelection typeCausative changeReferences
    ZmPIF3.1Plant heightBasic helix-loop-helix transcription factorIncreased expressionUnknown[91]
    TSH4Tassel branch numberTranscription factorAltered expressionUnknown[91]
    ZmPGP1Plant architectureATP binding cassette transporterAltered expressionA 241 bp deletion in the last exon of ZmPGP1[92, 93]
    PhyB2Light signalPhytochrome BAltered expressionA 10 bp deletion in the translation start site[101]
    ZmPIF4.1Light signalBasic helix-loop-helix transcription factorAltered expressionUnknown[102]
    ZmKOB1Grain yieldGlycotransferase-like proteinProtein functionUnknown[121]
     | Show Table
    DownLoad: CSV

    In a recent study to dissect the signaling process regulating inflorescence development in response to the shade signal, Kong et al.[106] compared the gene expression changes along the male and female inflorescence development under simulated shade treatments and normal light conditions, and identified a large set of genes that are co-regulated by developmental progression and simulated shade treatments. They found that these co-regulated genes are enriched in plant hormone signaling pathways and transcription factors. By network analyses, they found that UB2, UB3 and TSH4 act as a central regulatory node controlling maize inflorescence development in response to shade signal, and their loss-of-function mutants exhibit reduced sensitivity to simulated shade treatments. This study provides a valuable genetic source for mining and manipulating key shading-responsive genes for improved tassel and ear traits under high density planting conditions.

    Complementary and differential selection of heterotic groups during modern maize breeding

    Nowadays, global maize production is mostly provided by hybrid maize, which exhibits heterosis (or hybrid vigor) in yields and stress tolerance over open-pollinated varieties[3]. Hybrid maize breeding has gone through several stages, from the 'inbred-hybrid method' stage by Shull[107] and East[108] in the early twentieth century, to the 'double-cross hybrids' stage (1930s−1950s) by Jones[109], and then the 'single-cross hybrids' stage since the 1960s. Since its development, single-cross hybrid was quickly adopted globally due to its superior heterosis and easiness of production[3].

    Single-cross maize hybrids are produced from crossing two unrelated parental inbred lines (female × male) belonging to genetically distinct pools of germplasm, called heterotic groups. Heterotic groups allow better exploitation of heterosis, since inter-group hybrids display a higher level of heterosis than intra-group hybrids. A specific pair of female and male heterotic groups expressing pronounced heterosis is termed as a heterotic pattern[110, 111]. Initially, the parental lines were derived from a limited number of key founder inbred lines and empirically classified into different heterotic groups (such as SSS and NSS)[112]. Over time, they have expanded dramatically, accompanied by formation of new 'heterotic groups' (such as Iodent, PA and PB). Nowadays, Stiff Stalk Synthetics (SSS) and PA are generally used as FHGs (female heterotic groups), while Non Stiff Stalk (NSS), PB and Sipingtou (SPT) are generally used as the MHGs (male heterotic groups) in temperate hybrid maize breeding[113].

    With the development of molecular biology, various molecular markers, ranging from RFLPs, SSRs, and more recently high-density genome-wide SNP data have been utilized to assign newly developed inbred lines into various heterotic groups, and to guide crosses between heterotic pools to produce the most productive hybrids[114−116]. Multiple studies with molecular markers have suggested that heterotic groups have diverged genetically over time for better heterosis[117−120]. However, there has been a lack of a systematic assessment of the effect and contribution of breeding selection on phenotypic improvement and the underlying genomic changes of FHGs and MHGs for different heterotic patterns on a population scale during modern hybrid maize breeding.

    To systematically assess the phenotypic improvement and the underlying genomic changes of FHGs and MHGs during modern hybrid maize breeding, we recently conducted re-sequencing and phenotypic analyses of 21 agronomic traits for a panel of 1,604 modern elite maize lines[121]. Several interesting observations were made: (1) The MHGs experienced more intensive selection than the FMGs during the progression from era I (before the year 2000) to era II (after the year 2000). Significant changes were observed for 18 out of 21 traits in the MHGs, but only 10 of the 21 traits showed significant changes in the FHGs; (2) The MHGs and FHGs experienced both convergent and divergent selection towards different sets of agronomic traits. Both the MHGs and FHGs experienced a decrease in flowering time and an increase in yield and plant architecture related traits, but three traits potentially related to seed dehydration rate were selected in opposite direction in the MHGs and FHGs. GWAS analysis identified 4,329 genes associated with the 21 traits. Consistent with the observed convergent and divergent changes of different traits, we observed convergent increase for the frequencies of favorable alleles for the convergently selected traits in both the MHGs and FHGs, and anti-directional changes for the frequencies of favorable alleles for the oppositely selected traits. These observations highlight a critical contribution of accumulation of favorable alleles to agronomic trait improvement of the parental lines of both FHGs and MHGs during modern maize breeding.

    Moreover, FST statistics showed increased genetic differentiation between the respective MHGs and FHGs of the US_SS × US_NSS and PA × SPT heterotic patterns from era I to era II. Further, we detected significant positive correlations between the number of accumulated heterozygous superior alleles of the differentiated genes with increased grain yield per plant and better parent heterosis, supporting a role of the differentiated genes in promoting maize heterosis. Further, mutational and overexpressional studies demonstrated a role of ZmKOB1, which encodes a putative glycotransferase, in promoting grain yield[121]. While this study complemented earlier studies on maize domestication and variation maps in maize, a pitfall of this study is that variation is limited to SNP polymorphisms. Further exploitation of more variants (Indels, PAVs, CNVs etc.) in the historical maize panel will greatly deepen our understanding of the impact of artificial selection on the maize genome, and identify valuable new targets for genetic improvement of maize.

    Perspectives

    The ever-increasing worldwide population and anticipated climate deterioration pose a great challenge to global food security and call for more effective and precise breeding methods for crops. To accommodate the projected population increase in the next 30 years, it is estimated that cereal production needs to increase at least 70% by 2050 (FAO). As a staple cereal crop, breeding of maize cultivars that are not only high-yielding and with superior quality, but also resilient to environmental stresses, is essential to meet this demand. The recent advances in genome sequencing, genotyping and phenotyping technologies, generation of multi-omics data (including genomic, phenomic, epigenomic, transcriptomic, proteomic, and metabolomic data), creation of novel superior alleles by genome editing, development of more efficient double haploid technologies, integrating with machine learning and artificial intelligence are ushering the transition of maize breeding from the Breeding 3.0 stage (biological breeding) into the Breeding 4.0 stage (intelligent breeding)[122, 123]. However, several major challenges remain to be effectively tackled before such a transition could be implemented. First, most agronomic traits of maize are controlled by numerous small-effect QTL and complex genotype-environment interactions (G × E). Thus, elucidating the contribution of the abundant genetic variation in the maize population to phenotypic plasticity remains a major challenge in the post-genomic era of maize genetics and breeding. Secondly, most maize cultivars cultivated nowadays are hybrids that exhibit superior heterosis than their parental lines. Hybrid maize breeding involves the development of elite inbred lines with high general combining ability (GCA) and specific combining ability (SCA) that allows maximal exploitation of heterosis. Despite much effort to dissect the mechanisms of maize heterosis, the molecular basis of maize heterosis is still a debated topic[124−126]. Thirdly, only limited maize germplasm is amenable to genetic manipulation (genetic transformation, genome editing etc.), which significantly hinders the efficiency of genetic improvement. Development of efficient genotype-independent transformation procedure will greatly boost maize functional genomic research and breeding. Noteworthy, the Smart Corn System recently launched by Bayer is promised to revolutionize global corn production in the coming years. At the heart of the new system is short stature hybrid corn (~30%−40% shorter than traditional hybrids), which offers several advantages: sturdier stems and exceptional lodging resistance under higher planting densities (grow 20%−30% more plants per hectare), higher and more stable yield production per unit land area, easier management and application of plant protection products, better use of solar energy, water and other natural resources, and improved greenhouse gas footprint[127]. Indeed, a new age of maize green revolution is yet to come!

    Acknowledgments

    This work was supported by grants from the Key Research and Development Program of Guangdong Province (2022B0202060005), National Natural Science Foundation of China (32130077) and Hainan Yazhou Bay Seed Lab (B21HJ8101). We thank Professors Hai Wang (China Agricultural University) and Jinshun Zhong (South China Agricultural University) for valuable comments and helpful discussion on the manuscript. We apologize to authors whose excellent work could not be cited due to space limitations.

  • The authors declare that they have no conflict of interest. Haiyang Wang is an Editorial Board member of Seed Biology who was blinded from reviewing or making decisions on the manuscript. The article was subject to the journal's standard procedures, with peer-review handled independently of this Editorial Board member and his research groups.

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  • Cite this article

    Rawat A, Prakash O, Nagarkoti K, Kumar R, Negi MS, et al. 2024. Chemical profiling and bioactivity evaluation of thymol rich Coleus aromaticus Benth. essential oil. Medicinal Plant Biology 3: e007 doi: 10.48130/mpb-0024-0007
    Rawat A, Prakash O, Nagarkoti K, Kumar R, Negi MS, et al. 2024. Chemical profiling and bioactivity evaluation of thymol rich Coleus aromaticus Benth. essential oil. Medicinal Plant Biology 3: e007 doi: 10.48130/mpb-0024-0007
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Chemical profiling and bioactivity evaluation of thymol rich Coleus aromaticus Benth. essential oil

  • 1.

    Department of Chemistry, College of Basic Sciences and Humanities, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, 263145, India

  • 2.

    Medicinal Plants Research and Development Centre, Haldi, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, 263145, India

  • 3.

    Department of Plant Pathology, College of Agriculture, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, 263145, India

  • 4.

    Department of Entomology, College of Agriculture, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, 263145, India

  • Received: 27 October 2023
  • Revised: 25 January 2024
  • Accepted: 22 February 2024
  • Published online: 10 April 2024
Medicinal Plant Biology  3,  Article number: e007  (2024)  |  Cite this article

Abstract: Coleus aromaticus Benth. (Family: Lamiaceae) is a huge perennial, aromatic and succulent herb native to the Indian subcontinent. The dried leaves have an oregano-like texture making them a perfect culinary food supplement to be used as herbal seasoning for meat and other food products. The present study aimed to identify the bioactive components in the essential oil collected from the fresh aerial parts of Coleus aromaticus Benth. Using GC/MS analysis, 12 terpenoid components were identified, accounting for 97.5% of the overall oil content. Thymol (69.6%), p-cymene (3.9%), (E)-caryophyllene (3.7%), carvacrol (3.2%), α-thujene (3.2%), γ-terpinene (2.9%), and carvacrol methyl ether (2.3%) were identified to be the primary constituents in the oil, which was determined to be dominated by oxygenated monoterpenes (72.8%). Additionally, at the highest dose, CAEO showed significant pesticidal activity, inhibiting the egg hatchability of Meloidogyne incognita by 96.9%, immobilizing it by 52.3%, insecticidal activity on Spodoptera litura by 71.13%, and phytotoxic activity on Raphanus raphanistrum seeds by 97.75%. For speculating the potential method of action of CAEO components, the proteins/enzymes namely acetylcholinesterase (PDB ID: 6XYS), carboxylesterase (PDB ID: 5IVH), and acetohydroxyacid synthase (PBD ID: 1YHZ) were employed. The novel aspect of this study was that the herbal spice material was collected during its vegetative stage from the Tarai region of Pantnagar (India) in order to bio-evaluate its nematicidal, herbicidal, and insecticidal effectiveness. It was found that CAEO is an effective alternative source of natural pesticides and opens the way for additional research on its mechanistic techniques and field tests to determine its pesticidal studies.

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    Introduction

    • Humans have used plants and herbs as a source of therapeutic and curative agents since the early ages. Historically, mankind has relied on medicinal and aromatic plant bio-actives to promote overall health and longevity. The growth of herbal plants and their surroundings have been linked to certain factors that qualitatively or quantitatively alter the amount and composition of secondary metabolites, improving the efficacy and bioactive potential of natural products[1,2]. Due to their fragrant character, numerous domestic and foreign exotic species of Lamiaceae have frequently been known in folk medicine. These species have been utilized to treat a variety of skin issues, respiratory infections and digestive disorders. The herbs have noteworthy applications in culinary practices as herbal seasonings[3].

      The aromatic Coleus aromaticus Benth., a huge perennial, and succulent herb that belongs to the genus Coleus and family Lamiaceae is native to the Indian subcontinent and is now widely cultivated in other Asian and South American nations. Asian households frequently employ this traditional aromatic plant[4]. These leaves were also used in cooking due to their powerful perfume and flavor. The dried leaves are used as a herbal seasoning for meat products and other food products, and they have an oregano-like texture which makes them perfect as a culinary food supplement[5]. The fresh herb leaves have a wide range of uses, including the treatment of convulsions, epilepsy, asthma, bronchitis, cough, malarial fever, and hepatitis[6]. These medicinal qualities of C. aromaticus namely antioxidant, anti-inflammatory, analgesic, and anti-microbial properties relate to the biological potential of the essential oil[7−10].

      Thymol, carvacrol, eugenol, and chavicol and other volatile components of the essential oils of C. aromaticus are known for their anti-microbial properties. The oxygenated monoterpenes, carvacrol and thymol are well known for their numerous practical uses in the food and pharmaceutical industries[11]. Additionally, perfume and cosmetics are made from the fragrant oils. Allelopathic potential, antibacterial properties, insecticidal capabilities, free radical scavenging properties, and radio-protective activities are just a few of the numerous bioactivities of the carvacrol/thymol-rich oil[12−15]. The composition of essential oils have been reported to be impacted by various growth settings, phenological stages, varieties, and other factors which in turn affects the biological efficacy of the oil[16,17].

      To the best of our knowledge, Coleus aromaticus has been extensively studied for its biological activities such as antioxidant, anti-inflammatory, and anti-microbial activities but no information regarding its pesticidal capability was found. The primary objective of the present study was to phytochemically characterize the chemical constitution of the aerial parts of C. aromaticus gathered from the agroclimatic region along the foothills of Uttarakhand (India). Further, the essential oil was assessed for its pesticidal activities namely nematicidal activity against Meloidogyne incognita, herbicidal activity against Raphanus raphanistrum, and insecticidal activity against Spodoptera litura. The pesticidal efficacy of the observed major components of the oil was verified using AutoDock software tools on certain proteins/enzymes, i.e., acetylcholinesterase (PDB ID: 6XYS), carboxylesterase (PDB ID: 5IVH), and acetohydroxyacid synthase (PDB ID: 1YHZ).

    Material and methods

      Plant material and oil extraction

    • Fresh aerial parts (leaves with stems) of C. aromaticus Benth. were sourced from experimental farms of Medicinal Plants Research and Development Centre, Haldi, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, India (29°02′14′′ N, 79°48′74′′ W, 243.8 m elevation) in October 2021. A voucher specimen (GBPUH-1038/13-07-2021) was deposited with the Department of Biological Sciences, College of Basic Sciences and Humanities, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, India after the plant was identified by Dr. D.S. Rawat (Plant Taxonomist). The fresh aerial parts of the plant (4,000 g) were chopped and hydrodistilled using a Clevenger-type apparatus for 3−4 h[18] to produce a pale yellow essential oil.

      GC/MS analysis

    • The stored oil was analyzed by GC/MS using a Perkin Elmer gas chromatograph model GC Clarus SQ 8C paired with a single quadrupole mass spectrometer model MS SQ8 to determine the bioactive components. The conditions for the columns were as follows: PE-5 capillary column, with dimensions of 30 m × 0.25 mm I.D × 0.25 µm, working in the electron influence method at 60 eV. Air free helium gas was employed as a carrier gas in addition to a fixed stream of 1.32 ml/min at a volume of 1 µl. The split ratio for a injection volume was 0.02 µl was 1:30. Temperature adjustments were made to bring the ion source and injector source to 210 and 250 °C, respectively. The oven temperature was controlled as follows: the oven temperature was first raised from 60 to 310 °C/min at a rate of 20 °C/min before being isotherm finished for 10 min at 310 °C. MS spectra were captured at 60 eV, with a scan range of 30−1,100 m/z. The results obtained were compared with those of the spectral data received from the Wiley Library and NIST.14 databases[19].

      Evaluation of nematicidal activity

      Isolation, extraction, and identification of nematodes

    • Tomato plant roots infected with root-knot nematodes (Meloidogyne incognita) were gathered from the farmed experimental areas of the Vegetable Research Centre, GBPUA&T, Pantnagar, India. Roots with root-knot nematodes attached to them were cut into short pieces, and they were then placed in a container with a 1.0% NaOCl solution. The suspension was put through a sieve after the bottle was hand-shaken for 5.0 min. The residue was collected from top to bottom sieves 100-mesh and then 400-mesh and put into the 250-ml beaker after being washed with tap water for 1 min. With the use of a counting chamber set up with several eggs or juveniles per mL, the suspension of the fluid was observed[20]. Female perineal patterns were carefully examined in order to identify the species.

      Hatching and mortality test

    • Fresh tomato plant roots infected with root-knot nematodes (M. incognita) were used to prepare a 100 ml suspension of eggs containing 50 eggs per ml in distilled water. Five mL of egg suspension (50 eggs/ml) and 1.0 ml of each concentration of CAEO at 0.25, 0.5, and 1.0 µl/ml were transferred separately in triplicate into blocks of cavity glass (2.5 cm in diameter). Data was observed over the course of 24-, 48-, 72- and 96-h, respectively. In the control groups, 2.0 ml of egg suspension and 1.0 ml water were kept in blocks of hollow glass[21]. Under a stereo optical microscope (Olympus CX3) microscope (40×), the number of eggs that hatched after the 96-h exposure was counted. The percentage (mean%) of the egg hatchability inhibition was found as a function of CAEO activity and the impact of concentrations and time interval.

      M. incognita eggs were placed in distilled water and actively continued for 24 h at room temperature (26 ± 2 °C) to measure the mortality rate. A solution of freshly hatched juveniles (J2) (approx. 50 J2/ml) was made in deionized water. In the block of glass cavity with a diameter of 2.5 cm, 2.0 ml of the suspension of freshly hatched juveniles and 1.0 ml of each concentration of CAEO (0.25, 0.5, and 1.0 µl/ml) were added and kept at room temperature. Three replicates of the experiment were conducted. The block of glass cavity treated as a control contained 1.0 ml of nematode mixture and 1.0 ml of deionized water. Under a light stereo-binocular microscope (Olympus CX3) (6×), the number of deceased juveniles was counted after 72 h of exposure. The percentage (mean%) of dead nematodes used to calculate the immobilization of J2 nematode larvae against CAEO. It was believed that their continued immobility following their submersion in water proved nematode mortality[22].

      Phytotoxic activity

    • To examine the phytotoxic effect demonstrated by CAEO, fresh fungal-treated seeds of Raphanus raphanistrum var sativus (radish) were purchased and obtained from Vegetable Research Centre, Pantnagar, Uttarakhand, India. For a period of four weeks, seeds were kept at room temperature in paper bags. Prior to the experiments, the seeds' viability and capacity for germination were tested. Seed surfaces were sterilized in two-steps (a 30 s 70% ethyl alcohol rinse followed by a 20 min treatment with 10% sodium hypochlorite solution), washed three times with sterile distilled water, and air-dried aseptically in a laminar hood. Ten seeds were put in Petri plates with two layers of filter paper on the surface (Whatman No. 2). First a stock of oil in dimethyl sulfoxide (DMSO)/water (1.0%, v/v) was created in order to make precise concentrations of CAEO in water (250, 500, 750, and 1,000 µl/ml). Ten ml of each concentration were finally added to the Petri dishes. 1.0% DMSO in water was used as the control. All of the studies were repeated twice, and there were five replicates of each treatment. Plastic paraffin film tape was used to seal the Petri dishes containing the seeds. After that, Petri dishes were housed in a germinator with a 16-h photoperiod set at 25 °C. In this experiment, root and shoot lengths as well as germination percentage were measured[23].

      Evaluation of insecticidal activity

      Insects

    • Spodoptera litura eggs lying on castor leaves were obtained from the Crop Research Centre, Pantnagar, Uttarakhand, India, and were confirmed by Dr. R.M. Srivastava (College of Agriculture, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, India). For two to three generations, the eggs were artificially subcultured and cleansed in a dark incubator at 28−30 °C with relative humidity maintained at 70% to 80%. A freshly prepared artificial diet constituting of 120 g soybean powder, 96 g wheat germ, 40 g yeast powder, 32 g agar, 16 g casein, 9.6 g ascorbic acid, 6.0 g potassium sorbate, 2.0 g methylparaben, 1.2 g choline chloride, 0.4 g cholesterol, 0.24 g inositol, 0.08 g vitamin B complex, and 1.280 L H2O was fed to the recently hatched larvae kept in the sterile glass chambers (20 cm × 15 cm × 6 cm). After 5 d, each larva was moved into a separate sterile glass tube (10 cm high and 2 cm in diameter), fed a fresh artificial meal, and kept at room temperature (28−30 °C) until they pupated. Male and female adults were coupled and reared with honey water (15%, w/v) in clean containers (40 cm × 30 cm × 10 cm) following their transformation from the pupal stage. On oiled papers that had been positioned in the containers, the eggs of mated adults were gathered. To create the next generation of larvae, the eggs underwent another treatment. With a photoperiod of 14 L:10 D h, a temperature of 27 ± 0.5 °C, and a relative humidity (RH) of 75% ± 5%, the rearing conditions were maintained. For this investigation, third-instar larvae were employed[24].

      Insecticidal activity via contact activity

    • The drip approach was applied to the contact activity procedure. Unaffected by gender, 5.0 healthy adults with good activity and steady growth were chosen from the reared adults. They were put into a glass activity test container 5.5 cm high and 2.5 cm in diameter. In order to create a serial testing solution, CAEO was dissolved in 1.0% tween 20 water solution. Four concentrations of CAEO (10 to 50 µl/ml) were found in formal experiments in accordance with the findings of preliminary experiments. Five replications of each treatment and control at various concentrations were performed. The test insects' death/survival was examined and noted 24 h later, and irregular activity was taken to mean that the insects had perished[24].

      Molecular docking studies

    • Molecular docking techniques were used to validate all of the pesticide actions. The X-ray crystal structures of the enzymes acetylcholinesterase (PDB ID: 6XYS), carboxylesterase; CaE (PDB: 5IVH), and acetohydroxyacid synthase, AHAS (PDB: 1YHZ) was retrieved from the RCSB protein data bank. The molecular docking studies of thymol on these proteins were carried out using AutoDock4.2 with Discovery Studio and Cygwin64 Terminal tool to determine the binding energy, visualize docking poses, and understand the various ligand-target receptor interactions responsible for the pesticidal activity of CAEO[25].

      Most vertebrates, insects, and nematodes have acetylcholinesterase (AChE), (PDB ID: 6XYS), which is the target for the action of organophosphates and carbamate pesticides. AChE hydrolyzes the neurotransmitter acetylcholine (ACh) to acetic acid and choline at the synapses and neuromuscular junction. As a result, inhibiting AChE causes the nervous system to dysfunction and the nematodes perishes[26].

      Certain plant-derived substances may have an impact on the enzymatic profile of insect pests. Proteinaceous inhibitors, for instance, may impede proteolytic activity and cause abnormal growth and development. By using the protein ligand's three dimensional structure and its affinity for the detoxifying enzyme carboxylesterase (CaE) (PDB ID: 5IVH), which is located in the head capsule of Spodoptera litura larvae, it may be possible to anticipate the hazardous effects of chemical components of botanicals on S. litura[27].

      Numerous commercial herbicides (applied to rice, corn, wheat, and cotton crops) target acetohydroxyacid synthase (AHAS), also known as acetolactate synthase (ALS), with PDB ID: 1YHZ. Low application rates, excellent crop selectivity, and low animals toxicity are the three features that distinguish pesticides as AHAS inhibitors. The AHAS enzyme failed to complete the conversion into isoleucine, leucine, and valine, also known as BCAAs, which is why AHAS inhibitor has an indirect impact on protein synthesis in plants by reducing the production of these branched-chain amino acids[28].

      In silico PASS studies

    • A web-based online software program evaluated the pesticidal activities of the main constituents identified in CAEO. The experiment predicted probable activity (Pa) and probable inactivity (Pi). Using PASS online software, the structures of key constituents were translated into their SMILES forms and utilized to forecast the biological spectrum. Only the activities that have Pa > Pi are thought to be likely for a specific drug prediction.

      Statistical analysis

    • The means ± standard deviation of three parallel measurements represented the experimental results. The statistical calculations used to determine the mean values and standard deviation. Three replicates for three to five concentrations in each sample were used in the experiment to test the nematicidal, insecticidal, and herbicidal activity. The 2-factor and 3-factor CRD (ANOVA) were used to analyze the raw data, and statistical analysis was used to determine the mean values and standard deviation (SD). Percentage data were subjected to angular transformation[29].

    Results and discussion

      Analysis of CAEO by GC-MS

    • A viscous, pale-yellow liquid, with an intense bitter aroma was the product of CAEO at 0.2% (v/w). The GC-MS analysis showed that 12 terpenoid compounds were present, with a total identification rate of 97.67%. Thymol (69.60%) was the predominant component, followed by p-cymene (3.95%), (E)-caryophyllene (3.69%), carvacrol (3.27%), α-thujene (3.25%), γ-terpinene (2.95%) and carvacrol methyl ether (2.26%), which were all in intermediate concentration (Table 1). Figure 1a & b shows the gas chromatogram and mass spectrum of thymol. Oxygenated monoterpene (72.87%), hydrocarbon monoterpene (10.15%), hydrocarbon sesquiterpene (6.17%), and oxygenated sesquiterpene (1.09%) are the different types of these molecules. 7.39% of additional chemicals were found in the oil. The outcomes were consistent with those of an analysis of the chemical variability of aerial parts of C. aromaticus gathered from the experimental farms of Purara, Bagheswar, and Diary farm, Pantnagar conducted by Verma et al.[30]. The thymol content of both the oils ranged from 85.9% to 98.9%. Our findings were consistent with those of Tewari et al.[31], who identified thymol as the main component. The current findings differ from earlier studies published worldwide[11, 12, 32−34], where carvacrol was the main constituent of the aerial section of C. aromaticus. These chemical compositional discrepancies could be caused by geographical distribution, genetic, environmental, developmental, and other factors.

      Table 1.  Chemical composition of CAEO.

      S.N.CompoundR.I. LitR.I. Exp%Mol. formulaM.F.P.
      Monoterpene hydrocarbon
      1.α-thujene9319293.2C10H16M+ = 136; m/z: 121, 119, 105, 93 (100%), 91, 77, 65, 53, 51, 43, 41, 27
      2.p-cymene102210233.9C10H14M+ = 134; m/z: 132, 120, 119 (100%), 103, 91, 77, 65, 55, 41, 39
      3.γ-terpinene105410542.9C10H16M+ = 136; m/z: 121, 119, 107, 105, 93 (100%), 91, 79, 77, 65, 43, 41, 39, 27
      Total (%)10.0
      Monoterpene oxygenated
      4.Thymol1288128369.6C10H14OM+ = 150; m/z: 136, 135 (100%), 115, 91, 79, 77, 65, 51, 39
      5.carvacrol129612973.2C10H14OM+ = 150; m/z: 136, 135 (100%), 117, 107, 91, 77, 65, 51, 39, 27
      Total (%)72.8
      Sesquiterpene hydrocarbon
      6.Bicyclogermacrene150215012.5C15H24M+ = 204;
      m/z: 189, 176, 161, 147, 136, 133, 121, 107, 93 (100%), 79, 67, 53, 41, 39, 29
      7.(E)-caryophyllene142114233.7C15H24M+ = 204; m/z: 175, 147, 133, 120, 107, 93 (100%), 91, 79, 69, 55, 41, 39, 27
      Total (%)6.2
      Sesquiterpene oxygenated
      8.β-eudesmol164816451.1C15H26OM+ = 222; m/z: 189, 175, 141, 131 (100%), 79, 75, 73, 55
      Total (%)1.1
      Others
      9.1-(3-ethyloxiranyl)-ethanone−−2.6C6H10O2M+ = 114; m/z: 85, 71, 57, 44, 43 (100%), 38, 31
      10.Carvacrol methyl ether124712512.3C11H16OM+ = 164; m/z: 161, 149 (100%), 91, 79, 71, 53
      11.Thymyl acetate135513551.3C12H16O2M+ = 192; m/z: 150, 136, 135 (100%), 91,43
      12.Carvacrol ethyl ether145614571.2C12H24OM+ = 184; m/z: 138, 124, 109, 95, 82, 67, 57 (100%), 55, 43, 41, 39, 29
      Total (%)7.4
      Total Composition (%)97.5
      CAEO: Coleus aromaticus essential oil; R.T.: Retention time; R.I. Lit.: Retention index (DB-5 column) acquired from literature; R.I. Exp.: Retention index acquired from experimental data; M.F.P.: Mass Fragmentation Pattern.

      Figure 1. 

      (a) Gas ion chromatogram of CAEO. (b) Mass spectrum of thymol.

      Thymol, the main component in the current study, is an isomeric form of carvacrol and is a phenolic monoterpenoid with a pleasant aroma. It is also found to be a derivative of p-cymene[35]. Thymol is considered to be the marker compound of the Lamiaceae family that is typically found in the Thymus, Oreganum, Coleus, Satureja, and Thymbra. Thymol and carvacrol are popularly utilized as additives in cosmetics, the food industry, perfumery, and aromatherapy due to their pleasant odour and flavour. They are prized for their antioxidant, anti-inflammatory, antibacterial, antispasmolytic, and antitumor activity in the pharmaceutical industries since they are known to be the precursors of thymohydroquinone and thymoquinone. The production of γ-terpinene from geranyl diphosphate (GDP) with the help of P450 monooxygenases and dehydrogenase initiates the whole biosynthetic route of thymol and carvacrol[36].

      According to several studies, C. aromaticus essential oil possesses pharmacological qualities including anti-oxidant activity, anti-diabetic activity, antimicrobial activities, and insecticidal activity[12, 37−39]. In addition, fungicidal, insecticidal, mosquito larvicidal, and antifeedant effects of thymol derived from several plants of the Lamiaceae family have been described[40−43]. The present study evaluated the various pesticide activities of C. aromaticus essential oil.

      In-vitro nematicidal activity of CAEO via mortality and egg hatchability assay

    • In this investigation, the bio-nematicidal potential of the oil was assessed. The oils demonstrated very high levels of inhibition in the case of egg hatchability, with 95.39% at 0.25 µl/ml and 96.87% at 1.0 µl/ml dosing levels (Table 2). A similar dose level was used to test the % mortality of M. incognita 2nd stage larvae. Surprisingly, CAEO was observed to report a moderate mortality rate of 52.32% at a dose of 1.00 µl/ml (Table 3). As the oil was concentrated, the rate at which larvae hatched increased steadily, reflecting the fact that the concentration was a factor in the juvenile hatching of root-knot nematode, M. incognita. In the control setting, a considerable proportion of juveniles hatched, and there was very little mortality. After 72 h and 96 h durations, respectively, the highest concentration of 1.00 µl/ml resulted in the greatest amount of larval mortality and egg hatchability inhibition. As a result, it was discovered that the actions were concentration and time -dependent.

      Table 2.  % Egg hatchability inhibition of CAEO against M. incognita in laboratory conditions.

      Dose (µL/mL)Number of eggs hatched in timeMean% Egg hatchability
      24 h48 h72 h96 h
      0.254.665.667.3311.337.2595.39
      0.504.665.666.668.666.4295.92
      1.003.335.005.665.664.9296.87
      Control106.00143.00173.66207.66157.58 ± 43.35
      S.E.M0.340.290.59
      C.D. 1%1.351.172.34
      C.D. 5%0.990.861.73
      C.V.56.90
      CAEO: Coleus aromaticus essential oil; C.D.: Critical Difference; C.V.: Coefficient of Variance, ** p < 0.05.

      Table 3.  % Mortality of 2nd stage larvae of M. incognita in different concentrations of CAEO.

      Dose (µL/mL)Number of larvae dead in timeMean
      larvae dead      		% Mortality
      24 h48 h72 h
      0.259.3327.3328.3321.66 ± 10.6913.47
      0.5025.0038.3339.3334.22 ± 8.0027.78
      1.0055.6666.0066.3362.66 ± 6.0652.32
      Control2.008.6611.667.44 ± 4.94
      S.E.M.2.052.053.55
      C.D. 1%8.348.3414.45
      C.D. 5%6.096.0910.55
      C.V.15.56
      CAEO: Coleus aromaticus essential oil; C.D.: Critical Difference; C.V.: Coefficient of Variance, ** p < 0.05.

      Acetylcholinesterase enzyme (PDB ID: 6XYS) molecular docking investigations were also carried out to confirm the nematicidal activity testing results. Using a binding energy of -6.20 kcal/mol, root mean square deviation of 96.68 Å and estimated inhibition constant of 28.68 µM, thymol formed strong bonds with the amino acid residues Tyr334, Ser81, and Gly80 through van der Waals forces, Tyr442 and Ile439 through pi-alkyl interactions, and Trp432 through pi-sigma interactions. With a binding energy value of -6.45 kcal/mol, carbofuran was shown to interact with many amino acids when compared to the other ligands that were examined (Fig. 2). After thorough clinical trials, additional research is required to assess the safety of the botanicals for the use in humans.

      Figure 2. 

      Comparative 2D and 3D interactions of thymol and standard drugs with different target proteins used in the study. 6XYS: PBD ID for the crystal structure of enzyme acetylcholinesterase from the gut of Meloidogyne incognita larvae, 5IVH: PDB ID for the crystal structure of enzyme carboxylesterase from the head capsule of Spodoptera litura larvae, 1YHZ: PDB ID for the crystal structure of enzyme acetohydroxyacid synthase (AHAS) from the weed Raphanus raphanistrum sub sativus, amino acid residues in green rings are showing van der Waals interactions, amino acid residues in pink rings are showing pi-alkyl interactions, amino acid residues in purple rings are showing pi-sigma interactions, amino acids in red rings are showing unfavorable bumps.

      The current literature search turned up no accounts on the nematicidal activity of C. aromaticus. Coleus forskohlii belonging to the same genus exhibited nematicidal activity against M. javanica[44]. Even so, several species of Lamiaceae plants, including Mentha pulegium, Origanum vulgare, Origanum dictamnus L., Melissa officinalis, Ruta graveolens, Satureja montana and Thymbra capitata, have been studied for their nematicidal potential[45−47]. Carvacrol was examined for its potent activity against M. incognita as well as its synergistic potency with other terpenes[48]. According to Choi et al.[49] and Abdel-Rahman et al.[50], the main compound in this study, thymol, also showed impressive nematicidal action against Bursaphelenchus xylophilus and Caenorhabditis elegans. Thus, supporting the findings of earlier investigations, the substantial nematicidal activity in the present study can be attributed to the high concentration of thymol.

      Effect of CAEO against Raphanus raphanistrum seed germination

    • To evaluate the bioherbicidal effect of C. aromaticus EOs at various doses, a germination bioassay was conducted. CAEO at 250 µl/ml demonstrated a broad herbicidal spectrum of 63.70% against R. raphanistrum seed germination. With a rise in EOs concentration, the germination inhibition significantly increased. In comparison to the control setup, CAEO showed the maximum germination inhibition rate in R. raphanistrum seeds at the highest concentration of 1000 µl/ml, which was 97.75%. These findings show that CAEO, even at lower doses, had a negative impact on seed germination. Additionally, as compared to the untreated control, all four concentrations dramatically reduced the lengths of the seedlings' roots and shoots (Table 4).

      Table 4.  % Phytotoxic activity of CAEO against R. raphanistrum seeds in laboratory conditions.

      Dose (µL/mL)Number of seeds germinated in different time intervalsMean seed germinated% Growth inhibition% Root growth inhibition% Shoot growth inhibition
      24 h48 h72 h96 h108 h
      2501.662.663.333.664.663.20 ± 1.1263.7074.7991.93
      5000.661.001.662.333.001.73 ± 0.9580.3485.9194.99
      7500.000.661.001.332.001.00 ± 0.7488.6697.3698.76
      10000.000.000.330.330.330.20 ± 0.1897.7598.11100
      Control7.007.0010.0010.0010.008.80 ± 1.640.00.00.0
      Pendimethalin0.00.00.00.00.00.0100.0100.0100.0
      C.D. 1%0.53
      C.D. 5%0.39
      C.V.18.13
      CAEO: Coleus aromaticus essential oil; C.D.: Critical Difference; C.V.: Coefficient of Variance, ** p < 0.05.

      Acetohydroxyacid synthase (AHAS) (PDB ID: 1YHZ) was used in molecular docking studies to corroborate the experimental findings of the herbicidal activity. Using binding energy of −6.02 kcal/mol, root mean square deviation of 97.88 Å and estimated inhibition constant of 38.39 µM, thymol strongly bonded with Tyr334, Ser81, Gly441, and Gly80 amino acid residues with van der Waals forces, Phe330, Trp84, Tyr442, and Ile439 with pi-alkyl interactions, and Trp432 with pi-sigma interactions. With a binding energy of −7.50 kcal/mol, pendimethalin was shown to interact with several amino acids when compared to the examined ligands (Fig. 2). After thorough clinical trials, additional research is required to assess the safety parameters of the botanicals for human use.

      Numerous studies demonstrated that monoterpene enriched essential oils significantly reduced the germination of weed. In the current investigation, practically all CAEO-treated concentrations had a negative impact on seed germination as well as seedling shoot and root length growth. The results presented here also indicated that oxygenated monoterpenes were the predominant class, which is consistent with those of Pinheiro et al.,[51], who discovered that essential oils from Plectranthus amboinicus rich in carvacrol and thymol effectively inhibited the germination of Lactuca sativa and Sorghum bicolor seeds. Kanyal et al.[3] also reported the substantial herbicidal potential of the oxygenated monoterpene-rich Coleus barbatus aerial part essential oil and the monoterpene hydrocarbon-rich C. barbatus root part essential oil. A number of herbal plants in the Lamiaceae family have also demonstrated allelopathic effects in recent studies, including Thymus vulgaris against Xanthium trumarium and Avena sterilis[52], Thymus capitatus against Convolvulus arvensis and Setaria viridis[53], Thymus vulgaris and Satureja hortensis against Chenopodium album, Ambrosia artemisiifolia and Sorghum halepense[54] and Monarda fistulosa, Satureja pilosa, Origanum vulgare, Micromeria dalmatica, Thymus longedentatus, and Artemisa campestris against Lolium perenne and Trifolium pratense[55]. Thymol, the primary component of CAEO and carvacrol, has also been shown to inhibit seed germination in several other plants, including Sinapi sarvensis, Sonchus oleraceus, Amaranthus retroflexus, Centaurea salsotitialis, Lolium rigidum, Raphanus raphanistrum, and Rumex nepalensis[40,56,57] which adequately supports our findings that CAEO has high bioherbicidal activities which affect the seed germination and root and shoot growth of R. raphanistrum.

      In-vitro insecticidal activity of CAEO against S. litura

    • The maximum insect mortality against S. litura was recorded in CAEO at a dose level of 50 µl/ml, which was up to 71.13%. Table 5 presents the comprehensive findings. In the review of the literature, there are no reports on the insecticidal effects of CAEO. The findings are consistent with the studies of earlier researchers. These results imply that CAEO has the potential for the development of novel insecticidal components/chemicals for the management of stored pests and insects.

      Table 5.  % Mortality of S. litura against CAEO in laboratory conditions.

      Dose
      (µL/mL)      		Insects observed alive at different time intervalsMean insect survival% mortality
      12 h24 h36 h
      105.005.005.005.00 ± 0.00
      205.004.334.004.44 ± 0.5111.13
      304.663.663.333.88 ± 0.6922.33
      404.004.003.333.77 ± 0.3824.46
      502.001.331.001.44 ± 0.5171.13
      Control5.005.005.005.00 ± 0.00
      Permethrin0.00.00.00.0100.0
      C.D. 1%0.50.71.3
      C.D. 5%0.40.50.9
      C.V.14.7
      CAEO: Coleus aromaticus essential oil; C.D.: Critical Difference; C.V.: Coefficient of Variance, ** p < 0.05.

      Molecular docking studies were also performed using carboxylesterase enzyme (PDB ID: 5IVH) to corroborate the experimental results of the insecticidal activity. Thymol strongly bonded with Tyr341, Ser293, and Phe295 amino acid residues with van der Waals forces, Leu289 and Phe338 with pi-alkyl whereas Ile294 and Phe297 with pi-sigma interactions using binding energy of −4.61 kcal/mol, root mean square deviation of 107.88 Å and estimated inhibition constant of 416.13 µM. Permethrin was observed to show binding interactions with many amino acids as compared to the tested ligands with a binding energy of −8.78 kcal/mol (Fig. 2). Further clinical trials and research is needed to evaluate the safety of these natural botanicals for human use.

      C. aromaticus has been recommended for its effective efficiency against the stored grain pest, Tribolium castaneum[58]. Essential oil of C. aromaticus along with its major component thymol has also been evaluated for its larvicidal activity against Culex tritaeniorhynchus, Aedes albopictus, and Anopheles subpictus[42]. In another study by Govindaraju et al.,[59], Coleus aromaticus essential oil and its major compound carvacrol against Aedes aegypti, Culex quinquefasciatus, and Anopheles stephensi. In addition, Coleus amboinicus leaf essential oil collected from Andhra Pradesh, India was observed to show insecticidal activity against white termites, Odontotermes obesus Rhamb. and confused flour beetle, Tribolium castaneum[60]. According to reports, thymol and carvacrol found in CAEO exhibit insecticidal activities against a variety of agricultural pests and stored grain insects[61,62]. It can be inferred that the major and minor components of the essential oil may work in synchronous to increase the potency for pesticidal activities.

      In silico PASS studies

    • All the components identified in CAEO were induced to the PASS program which details the pesticidal activities of the components with respect to the probable activity (Pa) and probable inactivity (Pi). A greater Pa value in comparison to Pi (Pa > Pi) validates better activity to be used as a drug. Thymol, the main constituent of the oil showed better results with high anti-helminthic and insecticidal activity which is in accordance with the present results. The Pa and Pi values of the major components are presented in Table 6 showing the insecticidal, antibacterial, antifungal, and anthelmintic activities.

      Table 6.  In silico PASS prediction bioactivities of major compounds in CAEO.

      Major compoundsPredicted biological activities
      Anti-helminthic (nematodes)InsecticidalAnti-fungalAnti-bacterial
      α-thujene0.388 > 0.047−0.337 > 0.0670.130 > 0.098
      p-cymene0.633 > 0.0050.391 > 0.0060.368 > 0.058−
      γ-terpinene0.642 > 0.005−0.443 > 0.0410.325 > 0.051
      thymol0.569 > 0.0080.323 > 0.0130.464 > 0.0370.336 > 0.047
      carvacrol0.722 > 0.0040.351 > 0.0100.449 > 0.0390.319 > 0.053
      bicyclogermacrene0.520 > 0.0140.350 > 0.0100.439 > 0.042−
      (E)-caryophyllene0.333 > 0.0800.368 > 0.0080.582 > 0.0200.437 > 0.023
      β-eudesmol−−0.401 > 0.0490.302 > 0.059
      carvacrol methyl ether0.622 > 0.0050.388 > 0.0070.362 > 0.059−
      thymyl acetate0.775 > 0.0030.327 > 00130.456 > 0.0380.324 > 0.052
      dodecanal0.458 > 0.0250.368 > 0.0080.314 > 0.0750.280 > 0.068
      Pa > Pi, Pa = Probable activity and Pi = Probable inactivity.
    Conclusions

    • The purpose of the current study was to disclose the chemical makeup and for the first time, the possible pesticidal bioactivity of the essential oil found in the aerial portions of C. aromaticus. The unique aspect of this study was that the herbal spice material was collected during its vegetative stage from the Tarai region of Pantnagar in order to bio-evaluate its nematicidal, herbicidal, and insecticidal effectiveness. When compared to other studies of Uttarakhand, the geographical conditions, edaphic and climate characteristics, and experimental setup may have had an influence on the difference in composition observed in the GC-MS analysis. The main component of thymol (69.60%) contributed to oxygenated monoterpenes (72.87%) in the essential oil. Other important compounds identified included p-cymene (3.95%), (E)-caryophyllene (3.69%), carvacrol (3.27%), α-thujene (3.25%), γ-terpinene (2.95%) and carvacrol methyl ether (2.26%). Our results prove that CAEO can also be a viable choice for the management of M. incognita nematodes and Spodoptera litura. The bioactivities were also validated using molecular docking techniques. Further clinical experiments have revealed that the oil can potentially be used as a bio-pesticide.

    Author contributions

    • Avneesh Rawat: Planning original draft, collated the literature and prepared the manuscript. The study was part of his Ph.D. thesis work. Om Prakash: Advisor of Avneesh Rawat, planned the study of the present work, provided research guidance. Kirti Nagarkoti: Helped in preparing the manuscript, Formal analysis. Ravendra Kumar: Co-advisor of the student, helped in executing the experiments. Mahendra Singh Negi: Helped in providing the plant samples for executing the experiments. Satya Kumar: Member of research advisory committee, guided to conduct the nematicidal studies. Ravi Mohan Srivastava: Member of research advisory committee, guided to conduct the entomological studies.

    Data availability

    • The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

    Acknowledgements

    • The authors acknowledge the G. B. Pant University of Agriculture and Technology, Pantnagar, India, for providing academic support and Central Instrumentation Center, University of Petroleum and Energy Studies (UPES), Bidholi campus, Dehradun, for providing facility for GC-MS analysis.

      Conflict of interest

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

      Rights and permissions
      • 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 (2)  Table (6) References (62)
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    Cite this article
    Rawat A, Prakash O, Nagarkoti K, Kumar R, Negi MS, et al. 2024. Chemical profiling and bioactivity evaluation of thymol rich Coleus aromaticus Benth. essential oil. Medicinal Plant Biology 3: e007 doi: 10.48130/mpb-0024-0007
    Rawat A, Prakash O, Nagarkoti K, Kumar R, Negi MS, et al. 2024. Chemical profiling and bioactivity evaluation of thymol rich Coleus aromaticus Benth. essential oil. Medicinal Plant Biology 3: e007 doi: 10.48130/mpb-0024-0007
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  • Introduction
  • Material and methods

  • Plant material and oil extraction

  • GC/MS analysis

  • Evaluation of nematicidal activity

  • Isolation, extraction, and identification of nematodes
  • Hatching and mortality test

  • Phytotoxic activity

  • Evaluation of insecticidal activity

  • Insects
  • Insecticidal activity via contact activity

  • Molecular docking studies

  • In silico PASS studies

  • Statistical analysis

  • Results and discussion

  • Analysis of CAEO by GC-MS

  • In-vitro nematicidal activity of CAEO via mortality and egg hatchability assay

  • Effect of CAEO against Raphanus raphanistrum seed germination

  • In-vitro insecticidal activity of CAEO against S. litura

  • In silico PASS studies

  • Conclusions
  • Author contributions
  • Data availability
  • Acknowledgements
  • Conflict of interest
  • Rights and permissions
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