Coordinated accumulation of (+)-delta-cadinene synthase mRNAs and gossypol in developing seeds of Gossypium hirsutum and a new member of the cad1 family from G. arboreum

Functional characterization of a novel terpene synthase GaTPS1 involved in (E)-α-bergamotene biosynthesis in Gossypium arboreum

Terpenoids in plants are mainly synthesized by terpene synthases (TPSs), which play an important role in plant-environment interactions. Gossypium arboreum is one of the important cotton cultivars with excellent pest resistance, however, the biosynthesis of most terpenoids in this plant remains unknown. In this study, we performed a comparative transcriptome analysis of leaves from intact and Helicoverpa armigera-infested cotton plants. The results showed that the H. armigera infestation mainly induced the JA signaling pathway, ten TPS genes were differentially expressed in G. arboreum leaves. Among them, a novel terpene synthase, GaTPS1, was heterologously expressed and functionally characterized in vitro. The enzymatic reaction indicated that recombinant GaTPS1 was primarily responsible for the production of (E)-α-bergamotene. Moreover, molecular docking and site-directed mutagenesis analysis demonstrated that two amino acid residues, A412L and Y535F, distinctly influenced the catalytic activities and product specificity of GaTPS1. The mutants GaTPS1-A412L and GaTPS1-Y535F resulted in a decrease in the proportion of products (E)-α-bergamotene and D-limonene, while an increase in the proportion of products (E)-β-farnesene, α-pinene and β-myrcene. Our findings provide valuable insights into understanding the molecular basis of terpenoid diversity in G. arboreum, with potential applications in plant metabolism regulation and the improvement of resistant cotton cultivars.

Genome-wide identification of terpenoid synthase family genes in Gossypium hirsutum and functional dissection of its subfamily cadinene synthase A in gossypol synthesis

Plant terpenoid synthase (TPS) family genes participate in metabolite synthesis, hormones, gossypol, etc. Here, we genome-widely identified TPS family genes in 12 land plant species. Four hundred and thirty TPS-related genes were divided into seven subfamilies. The TPS-c in Bryophytes was suggested to be the earliest subfamily, followed by the TPS-e/f and TPS-h presence in ferns. TPS-a, the largest number of genes, was derived from monocotyledonous and dicotyledonous plants. Collinearity analysis showed that 38 out of the 76 TPS genes in G. hirsutum were collinear within G. arboreum and G. raimondii. Twenty-one GhTPS-a genes belong to the cadinene synthase (GhCDN) subfamily and were divided into five groups, A, B, C, D, and E. The special cis-elements in the promoters of 12 GhCDN-A genes suggested that the JA and ethylene signaling pathways may be involved in their expression regulation. When 12 GhCDN-A genes were simultaneously silenced through virus-induced gene silencing, the glandular color of GhCDN-A-silenced plants was lighter than that of the control, supported by a gossypol content decrease based on HPLC testing, suggesting that GhCDN-A subgroup genes participate in gossypol synthesis. According to RNA-seq analysis, gossypol synthesis-related genes and disease-resistant genes in the glandular variety exhibited upregulated expression compared to the glandless variety, whereas hormone signaling-related genes were downregulated. All in all, these results revealed plant TPS gene evolution rules and dissected the TPS subfamily, GhCDN-A, function in gossypol synthesis in cotton.

NM, Jebakani K. S, Selvaraj S, Pramitha J. L, Selvaraj R, Petchiammal K. I, Kather Sheriff S, Thinakaran J, Rathinamoorthy S, Kumar P. R. Genetic manipulation of anti-nutritional factors in major crops for a sustainable diet in future

The consumption of healthy food, in order to strengthen the immune system, is now a major focus of people worldwide and is essential to tackle the emerging pandemic concerns. Moreover, research in this area paves the way for diversification of human diets by incorporating underutilized crops which are highly nutritious and climate-resilient in nature. However, although the consumption of healthy foods increases nutritional uptake, the bioavailability of nutrients and their absorption from foods also play an essential role in curbing malnutrition in developing countries. This has led to a focus on anti-nutrients that interfere with the digestion and absorption of nutrients and proteins from foods. Anti-nutritional factors in crops, such as phytic acid, gossypol, goitrogens, glucosinolates, lectins, oxalic acid, saponins, raffinose, tannins, enzyme inhibitors, alkaloids, β-N-oxalyl amino alanine (BOAA), and hydrogen cyanide (HCN), are synthesized in crop metabolic pathways and are interconnected with other essential growth regulation factors. Hence, breeding with the aim of completely eliminating anti-nutrition factors tends to compromise desirable features such as yield and seed size. However, advanced techniques, such as integrated multi-omics, RNAi, gene editing, and genomics-assisted breeding, aim to breed crops in which negative traits are minimized and to provide new strategies to handle these traits in crop improvement programs. There is also a need to emphasize individual crop-based approaches in upcoming research programs to achieve smart foods with minimum constraints in future. This review focuses on progress in molecular breeding and prospects for additional approaches to improve nutrient bioavailability in major crops.

Comparative Transcriptome Analysis Reveals Genes Associated with the Gossypol Synthesis and Gland Morphogenesis in Gossypium hirsutum

Gossypium hirsutum is an important source of natural textile fibers. Gossypol, which is a sesquiterpenoid compound mainly existing in the cotton pigment glands, can facilitate resistance to the stress from diseases and pests. The level of gossypol in the cotton is positively correlated to the quantity of pigment glands. However, the underlying regulatory mechanisms of gossypol synthesis and gland morphogenesis are still poorly understood, especially from a transcriptional perspective. The transcripts of young leaves and ovules at 30 DPA of the glanded plants and glandless plants were studied by RNA-Seq and 865 million clean reads were obtained. A total of 34,426 differentially expressed genes (DEGs) were identified through comparative transcriptome analysis. Genes related to gossypol synthesis or gland morphogenesis displayed significant differential expression between the two cultivars. Functional annotation revealed that the candidate genes related to catalytic activity, the biosynthesis of secondary metabolites, and biomolecular decomposition processes. Our work herein unveiled several potential candidate genes related to gossypol synthesis or gland morphogenesis and may provide useful clues for a breeding program of cotton cultivars with low cottonseed gossypol contents.

Genome-wide identification and characterization of terpene synthase genes in Gossypium hirsutum

Terpenoids are widely distributed in plants and play important roles in the regulation of plant growth and development and in the interactions between plants and both the environment and other organisms. However, terpene synthase (TPS) genes have not been systematically investigated in the tetraploid Gossypium hirsutum. In this study, whole genome identification and characterization of the TPS family from G. hirsutum were carried out. Eighty-five TPS genes, including 47 previously unidentified genes, were identified in the G. hirsutum genome and classified into 5 subfamilies according to protein sequence similarities, as follows: 43 GhTPS-a, 29 GhTPS-b, 4 GhTPS-c, 7 GhTPS-e/f, and 2 GhTPS-g members. These 85 TPS genes were mapped onto 19 chromosomes of the G. hirsutum genome. Segmental duplications and tandem duplications contributed greatly to the expansion of TPS genes in G. hirsutum and were followed by intense purifying selection during evolution. Indentification of cis-acting regulatory elements suggest that the expression of TPS genes is regulated by a variety of hormones. RNA sequencing (RNA-seq) expression profile analysis revealed that the TPS genes had distinct spatiotemporal expression patterns, and several genes were highly and preferentially expressed in the leaves of cotton with gossypol glands (glanded cotton) versus a glandless strain. Virus-induced gene silencing (VIGS) of three TPS genes yielded plants characterized by fewer, smaller, and lighter gossypol glands, which indicated that these three genes were responsible for gland activity. Taken together, our results provide a solid basis for further elucidation of the biological functions of TPS genes in relation to gland activity and gossypol biosynthesis to develop cotton cultivars with low cottonseed gossypol contents.

GhCDNC and GhCYP706B1 genes mediate gossypol biosynthesis in upland cotton

BACKGROUND

In cotton the identification and characterization of natural defense is a cost-effective, sustainable, and environment-friendly strategy to combat cotton pests. The secondary metabolites traits in cotton plant i.e., toxic gossypol glands play significant role for development and self-defense mechanism. To utilize gossypol in breeding implements, the understanding of gossypol initiation biosynthesis genes has vital importance at reproductive organ development stages.

METHODS

Cotton germplasm of 100 genotypes screened visually based on gossypol glandedness and a core set of ten genotypes was developed. Further three genotypes FH-330 (high glanding), F-280 (low glanding) and IRMA-197 (glandless) were used for determining the transcript abundance of twelve gossypol biosynthesis genes.

RESULTS

Out of 100, germplasm categorized as (76) high glanding, (22) medium glanding), one genotype for each (low glanding) and (glandless) category. Real-time qPCR analysis revealed varied expression patterns among selected three genotypes. Out of twelve, three genes CYP706B1, CDNC and 2ODD-1 had strong expression levels in all tested tissues in high glanded genotype, while, slight or no expression of these genes was recorded in low glanding and glandless genotype, respectively. The shell of developing boll (10, 20, 30 DPA), and developing embryo (20, 30 DPA) showed substantially medium to maximum expression, respectively while high to medium expression was recorded in sepals and leaf tissue.

CONCLUSIONS

Our study demonstrated that CYP706B1, CDNC and 2ODD-1 are the most promising genes involved in gossypol biosynthesis. Developing boll shell, developing embryo, leaf and sepal also have significant ability to synthesize gossypol. This will provide scientists a way to manipulate gossypol contents in economically important organs of cotton plant for targeted breeding.

Cotton roots are the major source of gossypol biosynthesis and accumulation

BACKGROUND

Gossypol is a specific secondary metabolite in Gossypium species. It not only plays a critical role in development and self-protection of cotton plants, but also can be used as important anti-cancer and male contraceptive compound. However, due to the toxicity of gossypol for human beings and monogastric animals, the consumption of cottonseeds was limited. To date, little is known about the gossypol metabolism in cotton plants.

RESULTS

In this study, we found that cotyledon was the primary source of gossypol at the seed germination stage. But thereafter, it was mainly originated from developing roots. Grafting between glanded and glandless cotton as well as sunflower rootstocks and cotton scion revealed that gossypol was mainly synthesized in the root systems of cotton plants. And both glanded and glandless cotton roots had the ability of gossypol biosynthesis. But the pigment glands, the main storage of gossypol, had indirect effects on gossypol biosynthesis. In vitro culture of root and rootless seedling confirmed the strong gossypol biosynthesis ability in root system and the relatively weak gossypol biosynthesis ability in other organs of the seedling. Expression profiling of the key genes involved in the gossypol biosynthetic pathway also supported the root as the major organ of gossypol biosynthesis.

CONCLUSIONS

Our study provide evidence that the cotton root system is the major source of gossypol in both glanded and glandless cottons, while other organs have a relatively weak ability to synthesize gossypol. Gossypol biosynthesis is not directed related to the expression of pigment glands, but the presence of pigment glands is essential for gossypol accumulation. These findings can not only clarify the complex regulation network of gossypol metabolism, but it could also accelerate the crop breeding process with enhanced commercial values.

Genome-wide analysis of genetic variations between dominant and recessive NILs of glanded and glandless cottons

Cotton is an important economic crop in worldwide. It produces fiber for the textile industry and provides cottonseeds with high-quality protein and oil. However, the presence of gossypol limits the utilization of cottonseed. Two pairs of cotton near isogenic lines (NILs) with different pigment glands, i.e., Coker 312 vs Coker 312 W and CCRI12 vs CCRI12W, exhibit different gossypol contents. The glandless traits of Coker 312 W and CCRI12W are controlled by recessive and dominant genes, respectively. However, knowledge regarding the genomic variations in the NILs is limited. Therefore, the NILs genomes were resequenced and the sequencing depths were greater than 34×. Compared with the TM-1 genome, numerous SNPs, Indels, SVs, and CNVs were discovered. KEGG pathway analysis revealed that genes with SNPs and Indels from the recessive NILs and genes with Indels from the dominant NILs shared only one enriched pathway, i.e., the sesquiterpenoid and triterpenoid biosynthesis pathway, which is relevant to gossypol biosynthesis. Expression analysis revealed that key genes with variations that participate in the gossypol biosynthesis and pigment gland formation pathways had different expression patterns among the dominant, recessive glandless and glanded plants. The expression levels in the glanded organs were higher than those in their NILs. Altogether, our results provide deeper insight into cotton NILs with different pigment glands.

Characterization of gossypol biosynthetic pathway

Gossypol and related sesquiterpene aldehydes in cotton function as defense compounds but are antinutritional in cottonseed products. By transcriptome comparison and coexpression analyses, we identified 146 candidates linked to gossypol biosynthesis. Analysis of metabolites accumulated in plants subjected to virus-induced gene silencing (VIGS) led to the identification of four enzymes and their supposed substrates. In vitro enzymatic assay and reconstitution in tobacco leaves elucidated a series of oxidative reactions of the gossypol biosynthesis pathway. The four functionally characterized enzymes, together with (+)-δ-cadinene synthase and the P450 involved in 7-hydroxy-(+)-δ-cadinene formation, convert farnesyl diphosphate (FPP) to hemigossypol, with two gaps left that each involves aromatization. Of six intermediates identified from the VIGS-treated leaves, 8-hydroxy-7-keto-δ-cadinene exerted a deleterious effect in dampening plant disease resistance if accumulated. Notably, CYP71BE79, the enzyme responsible for converting this phytotoxic intermediate, exhibited the highest catalytic activity among the five enzymes of the pathway assayed. In addition, despite their dispersed distribution in the cotton genome, all of the enzyme genes identified show a tight correlation of expression. Our data suggest that the enzymatic steps in the gossypol pathway are highly coordinated to ensure efficient substrate conversion.

Fine mapping and candidate gene analysis of the dominant glandless gene Gl 2 (e) in cotton (Gossypium spp.)

Dominant glandless gene Gl 2 (e) was fine-mapped to a 15 kb region containing one candidate gene encoding an MYC transcription factor, sequence and expression level of the gene were analyzed. Cottonseed product is an excellent source of oil and protein. However, this nutrition source is greatly limited in utilization by the toxic gossypol in pigment glands. It is reported that the Gl 2 (e) gene could effectively inhibit the formation of the pigment glands. Here, three F2 populations were constructed using two pairs of near isogenic lines (NILs), which differ nearly only by the gland trait, for fine mapping of Gl 2 (e) . DNA markers were identified from recently developed cotton genome sequence. The Gl 2 (e) gene was located within a 15-kb genomic interval between two markers CS2 and CS4 on chromosome 12. Only one gene was identified in the genomic interval as the candidate for Gl 2 (e) which encodes a family member of MYC transcription factor with 475-amino acids. Unexpectedly, the results of expression analysis indicated that the MYC gene expresses in glanded lines while almost does not express in glandless lines. These results suggest that the MYC gene probably serves as a vital positive regulator in the organogenesis pathway of pigment gland, and low expression of this gene will not launch the downstream pathway of pigment gland formation. This is the first pigment gland-related gene identification in cotton and will facilitate the research on glandless trait, cotton MYC proteins and low-gossypol cotton breeding.

Rhizobacteria activates (+)-δ-cadinene synthase genes and induces systemic resistance in cotton against beet armyworm (Spodoptera exigua)

Gossypol is an important allelochemical produced by the subepidermal glands of some cotton varieties and important for their ability to respond to changing biotic stress by exhibiting antibiosis against some cotton pests. Plant growth-promoting rhizobacteria (PGPR) are root-colonizing bacteria that increase plant growth and often elicit defence against plant pathogens and insect pests. Little is known about the effect of PGPR on cotton plant-insect interactions and the potential biochemical and molecular mechanisms by which PGPR enhance cotton plant defence. Here, we report that PGPR (Bacillus spp.) treated cotton plants showed significantly higher levels of gossypol compared with untreated plants. Similarly, the transcript levels of the genes (i.e. (+)-δ-cadinene synthase gene family) involved in the biosynthesis of gossypol were higher in PGPR-treated plants than in untreated plants. Furthermore, the levels of jasmonic acid, an octadecanoid-derived defence-related phytohormone and the transcript level of jasmonic acid responsive genes were higher in PGPR-treated plants than in untreated plants. Most intriguingly, Spodoptera exigua showed reduced larval feeding and development on PGPR-treated plants. These findings demonstrate that treatment of plants with rhizobacteria may induce significant biochemical and molecular changes with potential ramifications for plant-insect interactions.

Gossypol: phytoalexin of cotton

Sesquiterpenoids are a class of 15-carbon secondary metabolites that play diverse roles in plant adaptation to environment. Cotton plants accumulate a large amount of sesquiterpene aldehydes (including gossypol) as phytoalexins against pathogens and herbivores. They are stored in pigment glands of aerial organs and in epidermal layers of roots. Several enzymes of gossypol biosynthesis pathway have been characterized, including 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) and farnesyl diphosphate synthase (FPS) that catalyze the formation of the precursor farnesyl diphosphate (FPP), (+)-δ-cadinene synthase (CDN) which is the first enzyme committed to gossypol biosynthesis, and the downstream enzymes of CYP706B1 and methyltransferase. Expressions of these genes are tightly regulated during cotton plants development and induced by jasmonate and fungi elicitors. The transcription factor GaWRKY1 has been shown to be involved in gossypol pathway regulation. Recent development of new genomic platforms and methods and releases of diploid and tetraploid cotton genome sequences will greatly facilitate the elucidation of gossypol biosynthetic pathway and its regulation.

Isolation and characterization of terpene synthases in cotton (Gossypium hirsutum)

Cotton plants accumulate gossypol and related sesquiterpene aldehydes, which function as phytoalexins against pathogens and feeding deterrents to herbivorous insects. However, to date little is known about the biosynthesis of volatile terpenes in this crop. Herein is reported that 5 monoterpenes and 11 sesquiterpenes from extracts of a glanded cotton cultivar, Gossypium hirsutum cv. CCRI12, were detected by gas chromatography-mass spectrometry (GC-MS). By EST data mining combined with Rapid Amplification of cDNA Ends (RACE), full-length cDNAs of three terpene synthases (TPSs), GhTPS1, GhTPS2 and GhTPS3 were isolated. By in vitro assays of the recombinant proteins, it was found that GhTPS1 and GhTPS2 are sesquiterpene synthases: the former converted farnesyl pyrophosphate (FPP) into β-caryophyllene and α-humulene in a ratio of 2:1, whereas the latter produced several sesquiterpenes with guaia-1(10),11-diene as the major product. By contrast, GhTPS3 is a monoterpene synthase, which produced α-pinene, β-pinene, β-phellandrene and trace amounts of other monoterpenes from geranyl pyrophosphate (GPP). The TPS activities were also supported by Virus Induced Gene Silencing (VIGS) in the cotton plant. GhTPS1 and GhTPS3 were highly expressed in the cotton plant overall, whereas GhTPS2 was expressed only in leaves. When stimulated by mechanical wounding, Verticillium dahliae (Vde) elicitor or methyl jasmonate (MeJA), production of terpenes and expression of the corresponding synthase genes were induced. These data demonstrate that the three genes account for the biosynthesis of volatile terpenes of cotton, at least of this Upland cotton.

Identification and characterization of (E)-β-caryophyllene synthase and α/β-pinene synthase potentially involved in constitutive and herbivore-induced terpene formation in cotton

Cotton (Gossypium hirsutum L.) plants damaged by insects emit a blend of volatiles, including monoterpenes and sesquiterpenes, which can directly repel herbivores and/or indirectly protect the plant by attracting natural enemies of the herbivores. To understand the molecular basis of terpene biosynthesis and regulation in cotton, two terpene synthase genes, GhTPS1 and GhTPS2, were heterologously expressed and characterized. Recombinant GhTPS1 accepted farnesyl pyrophosphate as substrate and produced (E)-β-caryophyllene and α-humulene. GhTPS2 was characterized as a monoterpene synthase which formed α-pinene and β-pinene using geranyl pyrophosphate as substrate. Quantitative real-time PCR analysis revealed that GhTPS1 and GhTPS2 gene expression was elevated after methyl jasmonate (MeJA) treatment in cotton leaves. Moreover, feeding of the green plant bug Apolygus lucorum, a major cotton pest in northern China, resulted in increased GhTPS2 expression in young leaves, suggesting that GhTPS2 might be involved in plant defense in cotton.

Transcriptome sequencing and differential gene expression analysis of delayed gland morphogenesis in Gossypium australe during seed germination

The genus Gossypium is a globally important crop that is used to produce textiles, oil and protein. However, gossypol, which is found in cultivated cottonseed, is toxic to humans and non-ruminant animals. Efforts have been made to breed improved cultivated cotton with lower gossypol content. The delayed gland morphogenesis trait possessed by some Australian wild cotton species may enable the widespread, direct usage of cottonseed. However, the mechanisms about the delayed gland morphogenesis are still unknown. Here, we sequenced the first Australian wild cotton species (Gossypiumaustrale) and a diploid cotton species (Gossypiumarboreum) using the Illumina Hiseq 2000 RNA-seq platform to help elucidate the mechanisms underlying gossypol synthesis and gland development. Paired-end Illumina short reads were de novo assembled into 226,184, 213,257 and 275,434 transcripts, clustering into 61,048, 47,908 and 72,985 individual clusters with N50 lengths of 1,710 bp, 1544 BP and 1,743 bp, respectively. The clustered Unigenes were searched against three public protein databases (TrEMBL, SwissProt and RefSeq) and the nucleotide and protein sequences of Gossypiumraimondii using BLASTx and BLASTn. A total of 21,987, 17,209 and 25,325 Unigenes were annotated. Of these, 18,766 (85.4%), 14,552 (84.6%) and 21,374 (84.4%) Unigenes could be assigned to GO-term classifications. We identified and analyzed 13,884 differentially expressed Unigenes by clustering and functional enrichment. Terpenoid-related biosynthesis pathways showed differentially regulated expression patterns between the two cotton species. Phylogenetic analysis of the terpene synthases family was also carried out to clarify the classifications of TPSs. RNA-seq data from two distinct cotton species provide comprehensive transcriptome annotation resources and global gene expression profiles during seed germination and gland and gossypol formation. These data may be used to further elucidate various mechanisms and help promote the usage of cottonseed.

Metabolic engineering of gossypol in cotton

Cotton has long been known as a fiber plant. Besides the cotton fiber, the cottonseed oil and cottonseed protein are two other major products of cotton plants. However, the applications of the cottonseed oil and protein are limited because of the presence of toxic gossypol, which is unsafe for human and monogastric animal consumption. Meanwhile, gossypol in cotton increases the plant defense response to insect herbivores and pathogens. Consequently, gossypol has been extensively used in clinical trials in biomedical science. Over the last few years, major advances have occurred in both understanding and practice with regard to molecular regulation of gossypol pathway in cotton plant or hairy root culture. This review highlights a few major recent and ongoing developments in metabolic engineering of gossypol, as well as suggestions regarding further advances needed.

Molecular characterization and functional analysis of a necrosis- and ethylene-inducing, protein-encoding gene family from Verticillium dahliae

Verticillium dahliae Kleb. is a hemibiotrophic, phytopathogenic fungus that causes wilt disease in a wide range of crops, including cotton. Successful host colonization by hemibiotrophic pathogens requires the induction of plant cell death to provide the saprophytic nutrition for the transition from the biotrophic to the necrotrophic stage. In this study, we identified a necrosis-inducing Phytophthora protein (NPP1) domain-containing protein family containing nine genes in a virulent, defoliating isolate of V. dahliae (V592), named the VdNLP genes. Functional analysis demonstrated that only two of these VdNLP genes, VdNLP1 and VdNLP2, encoded proteins that were capable of inducing necrotic lesions and triggering defense responses in Nicotiana benthamiana, Arabidopsis, and cotton plants. Both VdNLP1 and VdNLP2 induced the wilting of cotton seedling cotyledons. However, gene-deletion mutants targeted by VdNLP1, VdNLP2, or both did not affect the pathogenicity of V. dahliae V592 in cotton infection. Similar expression and induction patterns were found for seven of the nine VdNLP transcripts. Through a comparison of the conserved amino acid residues of VdNLP with different necrosis-inducing activities, combined with mutagenesis-based analyses, we identified several novel conserved amino acid residues, in addition to the known conserved heptapeptide GHRHDWE motif and the cysteine residues of the NPP domain-containing protein, that are indispensable for the necrosis-inducing activity of the VdNLP2 protein.

Isolation of a (+)- delta-cadinene synthase gene CAD1-A and analysis of its expression pattern in seedlings ofGossypium arboreum L

The cotton sesquiterpene cyclase, (+)-delta-cadinene synthase, is encoded by a gene family, which can be divided into two subfamilies:CAD1-A and CAD1-C. The geneCAD1-A was isolated fromG. arboreum. In situ hybridization performed on seven-day-old cotton seedlings localized transcripts of both the CAD1 -A and CAD1 -C mainly in lateral root primordium and apical ground meristem, vascular tissues of emerging lateral roots, and also in procambium and some subepidermal cells of the hypocotyl. The CAD1 -A promoter showed a similar tissue-specificity in transgenic tobacco plants. Histochemistry showed occurrence of sesquiterpene aldehydes in outer cells of the lateral root tips, as well as in pigment glands. The CAD1 gene expression in G.arboreum seedlings and the spatial pattern of sesquiterpene biosynthesis constitute a chemical defense machinery in cotton seedlings.

Cotton plants expressing CYP6AE14 double-stranded RNA show enhanced resistance to bollworms

RNA interference (RNAi) plays an important role in regulating gene expression in eukaryotes. Previously, we generated Arabidopsis and tobacco plants expressing double-stranded RNA (dsRNA) targeting a cotton bollworm (Helicoverpa armigera) P450 gene, CYP6AE14. Bollworms fed on transgenic dsCYP6AE14 plants showed suppressed CYP6AE14 expression and reduced growth on gossypol-containing diet (Mao et al., in Nat Biotechnol 25: 1307-1313, 2007). Here we report generation and analysis of dsRNA-expressing cotton (Gossypium hirsutum) plants. Bollworm larvae reared on T2 plants of the ds6-3 line exhibited drastically retarded growth, and the transgenic plants were less damaged by bollworms than the control. Quantitative reverse-transcription polymerase chain reaction (RT-PCR) showed that the CYP6AE14 expression level was reduced in the larvae as early as 4 h after feeding on the transgenic plants; accordingly, the CYP6AE14 protein level dropped. These results demonstrated that transgenic cotton plants expressing dsCYP6AE14 acquired enhanced resistance to cotton bollworms, and that RNAi technology can be used for engineering insect-proof cotton cultivar.

Monoterpene and sesquiterpene synthases and the origin of terpene skeletal diversity in plants

The multitude of terpene carbon skeletons in plants is formed by enzymes known as terpene synthases. This review covers the monoterpene and sesquiterpene synthases presenting an up-to-date list of enzymes reported and evidence for their ability to form multiple products. The reaction mechanisms of these enzyme classes are described, and information on how terpene synthase proteins mediate catalysis is summarized. Correlations between specific amino acid motifs and terpene synthase function are described, including an analysis of the relationships between active site sequence and cyclization type and a discussion of whether specific protein features might facilitate multiple product formation.

Construction of cDNA library of cotton mutant (Xiangmian-18) library during gland forming stage

Gossypol, a secondary metabolite stored in the glands of cotton, protecting cottonseed from consumption of human and monogastric animal. This ability is unique to the tribe Gossypieae. Although the relationship between gossypol and pigment gland has been studied for a long time, the development mechanism of pigment gland has not been investigated at molecular level. Here we described a simple and efficient method for constructing a normalized cDNA library from a cotton mutant, Xiangmian-18, during its pigments gland forming stage. It combined switching mechanism at 5'-end of RNA transcript (SMART) technique and duplex-specific nuclease (DSN) normalization methods. In a model experiment, double-stranded cDNAs were synthesized from mRNAs, processed by normalization and Sfi I restriction endonuclease, and finally the cDNAs were ligated to pDNR-LIB vector. The ligation mixture was transformed into E. coli JM109 by electroporation. Counting the number of colonies, the titer of the original library was 5.86x10(5)cfu/ml in this library. Electrophoresis gel results indicated the fragments ranged from 800bp to 2kb, with the average size of 1400bp. Random picking clones showed that the recombination rate was 94%. The results showed that the cDNA library constructed successfully was a full-length library with high quality, and could be used to screen the genes related to development of pigments gland cottons.

Reduced terpene levels in cottonseed add food to fiber

Using RNA interference (RNAi) technology, the levels of a toxic phytoprotectant have recently been reduced specifically in the seeds of cotton to generate a novel dual-purpose crop. By engineering an endogenous terpene pathway, there is now the exciting potential for an added-value, genetically modified crop with the cash value of the fiber supported by the improved nutritional value and expanded food and feed use for the cottonseed, which is normally a low-value by-product.

Engineering cottonseed for use in human nutrition by tissue-specific reduction of toxic gossypol

Global cottonseed production can potentially provide the protein requirements for half a billion people per year; however, it is woefully underutilized because of the presence of toxic gossypol within seed glands. Therefore, elimination of gossypol from cottonseed has been a long-standing goal of geneticists. Attempts were made to meet this objective by developing so-called "glandless cotton" in the 1950s by conventional breeding techniques; however, the glandless varieties were commercially unviable because of the increased susceptibility of the plant to insect pests due to the systemic absence of glands that contain gossypol and other protective terpenoids. Thus, the promise of cottonseed in contributing to the food requirements of the burgeoning world population remained unfulfilled. We have successfully used RNAi to disrupt gossypol biosynthesis in cottonseed tissue by interfering with the expression of the delta-cadinene synthase gene during seed development. We demonstrate that it is possible to significantly reduce cottonseed-gossypol levels in a stable and heritable manner. Results from enzyme activity and molecular analyses on developing transgenic embryos were consistent with the observed phenotype in the mature seeds. Most relevant, the levels of gossypol and related terpenoids in the foliage and floral parts were not diminished, and thus their potential function in plant defense against insects and diseases remained untouched. These results illustrate that a targeted genetic modification, applied to an underutilized agricultural byproduct, provides a mechanism to open up a new source of nutrition for hundreds of millions of people.

Isoprenoid biosynthesis in Artemisia annua: cloning and heterologous expression of a germacrene A synthase from a glandular trichome cDNA library

Artemisia annua (Asteraceae) is the source of the anti-malarial compound artemisinin. To elucidate the biosynthetic pathway and to isolate and characterize genes involved in the biosynthesis of terpenoids including artemisinin in A. annua, glandular trichomes were used as an enriched source for biochemical and molecular biological studies. The sequencing of 900 randomly selected clones from a glandular trichome plasmid cDNA library revealed the presence of many ESTs involved in isoprenoid biosynthesis such as enzymes from the methylerythritol phosphate pathway and the mevalonate pathway, amorpha-4,11-diene synthase and other sesquiterpene synthases, monoterpene synthases and two cDNAs showing high similarity to germacrene A synthases. Full-length sequencing of the latter two ESTs resulted in a 1686-bp ORF encoding a protein of 562 aa. Upon expression in Escherichia coli, the recombinant protein was inactive with geranyl diphosphate, but catalyzed the cyclization of farnesyl diphosphate to germacrene A. These results demonstrate the potential of the use of A. annua glandular trichomes as a starting material for studying isoprenoid biosynthesis in this plant species.

Antisense suppression of a (+)-delta-cadinene synthase gene in cotton prevents the induction of this defense response gene during bacterial blight infection but not its constitutive expression

In cotton (Gossypium hirsutum) the enzyme (+)-delta-cadinene synthase (CDNS) catalyzes the first committed step in the biosynthesis of cadinane-type sesquiterpenes, such as gossypol, that provide constitutive and inducible protection against pests and diseases. A cotton cDNA clone encoding CDNS (cdn1-C4) was isolated from developing embryos and functionally characterized. Southern analysis showed that CDNS genes belong to a large multigene family, of which five genomic clones were studied, including three pseudogenes and one gene that may represent another subfamily of CDNS. CDNS expression was shown to be induced in cotton infected with either the bacterial blight or verticillium wilt pathogens. Constructs for the constitutive or seed-specific antisense suppression of cdn1-C4 were introduced into cotton by Agrobacterium-mediated transformation. Gossypol levels were not reduced in the seeds of transformants with either construct, nor was the induction of CDNS expression affected in stems of the constitutive antisense plants infected with Verticillium dahliae Kleb. However, the induction of CDNS mRNA and protein in response to bacterial blight infection of cotyledons was completely blocked in the constitutive antisense plants. These results suggest that cdn1-C4 may be involved specifically in the bacterial blight response and that the CDNS multigene family comprises a complex set of genes differing in their temporal and spatial regulation and responsible for different branches of the cotton sesquiterpene pathway.

VdNEP, an elicitor from Verticillium dahliae, induces cotton plant wilting

Verticillium wilt is a vascular disease of cotton. The causal fungus, Verticillium dahliae, secretes elicitors in culture. We have generated approximately 1,000 5'-terminal expressed sequence tags (ESTs) from a cultured mycelium of V. dahliae. A number of ESTs were found to encode proteins harboring putative signal peptides for secretion, and their cDNAs were isolated. Heterologous expression led to the identification of a protein with elicitor activities. This protein, named V. dahliae necrosis- and ethylene-inducing protein (VdNEP), is composed of 233 amino acids and has high sequence identities with fungal necrosis- and ethylene-inducing proteins. Infiltration of the bacterially expressed His-VdNEP into Nicotiana benthamiana leaves resulted in necrotic lesion formation. In Arabidopsis thaliana, the fusion protein also triggered production of reactive oxygen species and induced the expression of PR genes. When added into suspension cultured cells of cotton (Gossypium arboreum), the fusion protein elicited the biosynthesis of gossypol and related sesquiterpene phytoalexins at low concentrations, and it induced cell death at higher concentrations. On cotton cotyledons and leaves, His-VdNEP induced dehydration and wilting, similar to symptoms caused by a crude preparation of V. dahliae elicitors. Northern blotting showed a low level of VdNEP expression in the mycelium during culture. These data suggest that VdNEP is a wilt-inducing factor and that it participates in cotton-V. dahliae interactions.

Characterization of GaWRKY1, a cotton transcription factor that regulates the sesquiterpene synthase gene (+)-delta-cadinene synthase-A

The cotton (+)-delta-cadinene synthase (CAD1), a sesquiterpene cyclase, catalyzes a branch-point step leading to biosynthesis of sesquiterpene phytoalexins, including gossypol. CAD1-A is a member of CAD1 gene family, and its promoter contains a W-box palindrome with two reversely oriented TGAC repeats, which are the proposed binding sites of WRKY transcription factors. We isolated several WRKY cDNAs from Gossypium arboreum. One of them, GaWRKY1, encodes a protein containing a single WRKY domain and a putative N-terminal Leu zipper. Similar to genes encoding enzymes of cotton sesquiterpene pathway, GaWRKY1 was down-regulated in a glandless cotton cultivar that contained much less gossypol. GaWRKY1 showed a temporal and spatial pattern of expression comparable to that of CAD1-A in various aerial organs examined, including sepal, stigma, anther, and developing seeds. In suspension cells, expression of both GaWRKY1 and CAD1-A genes and biosynthesis of sesquiterpene aldehydes were strongly induced by a fungal elicitor preparation and methyl jasmonate. GaWRKY1 interacted with the 3x W-box derived from CAD1-A promoter in yeast (Saccharomyces cerevisiae) one-hybrid system and in vitro. Furthermore, in transgenic Arabidopsis plants, overexpression of GaWRKY1 highly activated the CAD1-A promoter, and transient assay in tobacco (Nicotiana tabacum) leaves demonstrated that W-box was required for this activation. These results suggest that GaWRKY1 participates in regulation of sesquiterpene biosynthesis in cotton, and CAD1-A is a target gene of this transcription factor.

Reduced levels of cadinane sesquiterpenoids in cotton plants expressing antisense (+)-delta-cadinene synthase

Cotton plants were transformed with an antisense construct of cdn1-Cl, a member of a complex gene family of delta-(+)cadinene (CDN) synthase. This synthase catalyzes the cyclization of (E,E)-farnesyl diphosphate to form CDN, and in cotton, it occupies the committed step in the biosynthesis of cadinane sesquiterpenoids and heliocides (sesterterpenoids). Southern analyses of the digestion of leaf DNA from R(o), T(o), and T(1) plants with Hind III, Pst I and Kpn I restriction enzymes show the integration of antisense cdn1-C1 cDNA driven by the CaMV 35S promoter into the cotton genome. Northern blots demonstrate the appearance of cdn synthase mRNA preceding CDN synthase activity and the formation of gossypol in developing cottonseed. T(2) cottonseed show a reduced CDN synthase activity and up to a 70% reduction in gossypol. In T(1) leaves the accumulated amounts of gossypol, hemigossypolone and heliocides are reduced 92.4, 83.3 and 68.4%, respectively. These data demonstrate that the integration of antisense cdn1-C1 cDNA into the cotton genome leads to a reduction of CDN synthase activity and negatively impacts on the biosynthesis of cadinane sesquiterpenoids and heliocides in cotton plants.

Molecular cloning and functional identification of (+)-delta-cadinene-8-hydroxylase, a cytochrome P450 mono-oxygenase (CYP706B1) of cotton sesquiterpene biosynthesis

In cotton, gossypol and related sesquiterpene aldehydes are present in the glands of aerial tissues and in epidermal cells of roots. A cytochrome P450 was found to be expressed in aerial tissues of glanded cotton cultivars, but not or at an extremely low level in the aerial tissues of a glandless cultivar. Its cDNA was then isolated from Gossypium arboreum L. After expression in Saccharomyces cerevisiae, the P450 was found to catalyse the hydroxylation of (+)-delta-cadinene, forming 8-hydroxy-(+)-delta-cadinene. This P450 mono-oxygenase has been classified as CYP706B1, and is the first member of the CYP706 family for which a function has been determined. Sesquiterpene aldehydes and CYP706B1 transcripts were detected in roots of both the glanded and glandless cultivars and in aerial tissues of the glanded cultivar. In suspension cultured cells of G. arboreum, elicitors prepared from the phytopathogenic fungus Verticillium dahliae caused a dramatic induction of CYP706B1 expression. The expression pattern of CYP706B1 and the position at which it hydroxylates (+)-delta-cadinene suggest that it catalyses an early step in gossypol biosynthesis. Southern blotting revealed a single copy of CYP706B1 in the genome of G. arboreum. CYP706B1 holds good potential for manipulation of gossypol levels in cottonseed via genetic engineering.

The cyclization of farnesyl diphosphate and nerolidyl diphosphate by a purified recombinant delta-cadinene synthase

The first step in the conversion of the isoprenoid intermediate, farnesyl diphosphate (FDP), to sesquiterpene phytoalexins in cotton (Gossypium barbadense) plants is catalyzed by delta-cadinene (CDN) synthase. CDN is the precursor of desoxyhemigossypol and hemigossypol defense sesquiterpenes. In this paper we have studied the mechanism for the cyclization of FDP and the putative intermediate, nerolidyl diphosphate, to CDN. A purified recombinant CDN synthase (CDN1-C1) expressed in Escherichia coli from CDN1-C1 cDNA isolated from Gossypium arboreum cyclizes (1RS)-[1-2H](E, E)-FDP to >98% [5-2H]and [11-2H]CDN. Enzyme reaction mixtures cyclize (3RS)-[4,4,13,13,13-2H5]-nerolidyl diphosphate to 62.1% [8,8,15,15,15-2H5]-CDN, 15.8% [6,6,15,15,15-2H5]-alpha-bisabolol, 8.1% [6,6,15,15,15-2H5]-(beta)-bisabolene, 9.8% [4,4,13,13-2H4]-(E)-beta-farnesene, and 4.2% unknowns. Competitive studies show that (3R)-nerolidyl diphosphate is the active enantiomer of (3RS)-nerolidyl diphosphate that cyclized to CDN. The kcat/Km values demonstrate that the synthase uses (E,E)-FDP as effectively as (3R)-nerolidyl diphosphate in the formation of CDN. Cyclization studies with (3R)-nerolidyl diphosphate show that the formation of CDN, (E)-beta-farnesene, and beta-bisabolene are enzyme dependent, but the formation of alpha-bisabolol in the reaction mixtures was a Mg2+-dependent solvolysis of nerolidyl diphosphate. Enzyme mechanisms are proposed for the formation of CDN from (E,E)-FDP and for the formation of CDN, (E)-beta-farnesene, and beta-bisabolene from (3RS)-nerolidyl diphosphate. The primary structures of cotton CDN synthase and tobacco epi-aristolochene synthase show 48% identity, suggesting similar three-dimensional structures. We used the SWISS-MODEL to test this. The two enzymes have the same overall structure consisting of two alpha-helical domains and epi-aristolochene synthase is a good model for the structure of CDN synthase. Several amino acids in the primary structures of both synthases superimpose. The amino acids having catalytic roles in epi-aristochene synthase are substituted in the CDN synthase and may be related to differences in catalytic properties.

Expression pattern of genes encoding farnesyl diphosphate synthase and sesquiterpene cyclase in cotton suspension-cultured cells treated with fungal elicitors

Cotton plants accumulate sesquiterpene aldehydes in pigment glands. The two enzymes farnesyl diphosphate synthase (FPS) and (+)-delta-cadinene synthase (CAD), a sesquiterpene cyclase, are involved in the biosynthesis of these secondary metabolites. A full-length cDNA (garfps) encoding FPS was isolated from Gossypium arboreum and identified by in vitro enzymatic assay of the garfps protein heterologously expressed in Escherichia coli. Treatment of G. arboreum suspension-cultured cells with an elicitor preparation obtained from the phytopathogenic fungus Verticillium dahliae dramatically induced transcription of both FPS and CAD, paralleling the accumulation of the sesquiterpene aldehydes in these cells. For G. australe, a wild species from Australia, the V. dahliae elicitor preparation also caused an induction of FPS but only a low rate of induction of CAD, apparently because of a constitutive expression of the sesquiterpene cyclase gene in suspension-cultured cells. Two transcripts and proteins of FPS were detected in the elicited G. australe cells; the smaller FPS seemed to be de novo synthesized after elicitation. Furthermore, G. australe-cultured cells accumulated the cadinene, instead of sesquiterpene aldehydes, indicating that the biosynthetic pathway leading to sesquiterpene aldehydes was absent or blocked after FPP cyclization.

(3R)-Linalool synthase from Artemisia annua L.: cDNA isolation, characterization, and wound induction

Artemisia annua is an annual herb used in traditional Chinese medicine. A cDNA library was constructed from leaves of A. annua seedlings and target sequences were amplified by PCR using degenerate primers derived from a consensus sequence of angiosperm terpene synthases. Two clones, QH1 and QH5, with high sequence similarity to plant monoterpene synthases were ultimately obtained and expressed in Escherichia coli. These cDNAs encode peptides of 567 aa (65.7 kDa) and 583 aa (67.4 kDa), respectively, and display 88% identity with each other and 42% identity with Mentha spicata limonene synthase. The two recombinant enzymes yielded no detectable activity with isopentenyl diphosphate, dimethylallyl diphosphate, chrysanthemyl diphosphate, farnesyl diphosphate, (+)-copalyl diphosphate, or geranylgeranyl diphosphate, but were active with geranyl diphosphate in yielding (3R)-linalool as the sole product in the presence of divalent metal cation cofactors. QH1-linalool synthase displays a K(m) value of 64 microM for geranyl diphosphate, which is considerably higher than other known monoterpene synthases, and a K(m) value of 4.6 mM for Mg(+2). Transcripts of QH1 and QH5 could be detected by RT-PCR in the leaves and inflorescence of A. annua, but not in the stem stele or roots; transcripts of QH5 could also be detected in stem epidermis. Linalool could not be detected by GC-MS in the essential oil of A. annua, nor in acid or base hydrolysates of aqueous extracts of leaves. RT-PCR demonstrated a wound-inducible increase in QH1 and QH5 transcript abundance in both leaves and stems over a 3-day time course.