XLG2 and CORI3 function additively to regulate plant defense against the necrotrophic pathogen Sclerotinia sclerotiorum

The membrane-bound heterotrimeric G-proteins in plants play a crucial role in defending against a broad range of pathogens. This study emphasizes the significance of Extra-large Gα protein 2 (XLG2), a plant-specific G-protein, in mediating the plant response to Sclerotinia sclerotiorum, which infects over 600 plant species worldwide. Our analysis of Arabidopsis G-protein mutants showed that loss of XLG2 function increased susceptibility to S. sclerotiorum, accompanied by compromised accumulation of jasmonic acid (JA) during pathogen infection. Overexpression of the XLG2 gene in xlg2 mutant plants resulted in higher resistance and increased JA accumulation during S. sclerotiorum infection. Co-immunoprecipitation (co-IP) analysis on S. sclerotiorum infected Col-0 samples, using two different approaches, identified 201 XLG2-interacting proteins. The identified JA-biosynthetic and JA-responsive proteins had compromised transcript expression in the xlg2 mutant during pathogen infection. XLG2 was found to interact physically with a JA-responsive protein, Coronatine induced 1 (CORI3) in Co-IP, and confirmed using split firefly luciferase complementation and bimolecular fluorescent complementation assays. Additionally, genetic analysis revealed an additive effect of XLG2 and CORI3 on resistance against S. sclerotiorum, JA accumulation, and expression of the defense marker genes. Overall, our study reveals two independent pathways involving XLG2 and CORI3 in contributing resistance against S. sclerotiorum.

Flavonols affect the interrelated glucosinolate and camalexin biosynthetic pathways in Arabidopsis thaliana

Flavonols are structurally and functionally diverse biomolecules involved in plant biotic and abiotic stress tolerance, pollen development, and inhibition of auxin transport. However, their effects on global gene expression and signaling pathways are unclear. To explore the roles of flavonol metabolites in signaling, we performed comparative transcriptome and targeted metabolite profiling of seedlings from the flavonol-deficient Arabidopsis loss-of-function mutant flavonol synthase1 (fls1) with and without exogenous supplementation of flavonol derivatives (kaempferol, quercetin, and rutin). RNA-seq results indicated that flavonols modulate various biological and metabolic pathways, with significant alterations in camalexin and aliphatic glucosinolate synthesis. Flavonols negatively regulated camalexin biosynthesis but appeared to promote the accumulation of aliphatic glucosinolates via transcription factor-mediated up-regulation of biosynthesis genes. Interestingly, upstream amino acid biosynthesis genes involved in methionine and tryptophan synthesis were altered under flavonol deficiency and exogenous supplementation. Quercetin treatment significantly up-regulated aliphatic glucosinolate biosynthesis genes compared with kaempferol and rutin. In addition, expression and metabolite analysis of the transparent testa7 mutant, which lacks hydroxylated flavonol derivatives, clarified the role of quercetin in the glucosinolate biosynthesis pathway. This study elucidates the molecular mechanisms by which flavonols interfere with signaling pathways, their molecular targets, and the multiple biological activities of flavonols in plants.

Targeted editing of multiple homologues of GTR1 and GTR2 genes provides the ideal low-seed, high-leaf glucosinolate oilseed mustard with uncompromised defence and yield

Glucosinolate content in the two major oilseed Brassica crops-rapeseed and mustard has been reduced to the globally accepted Canola quality level (<30 μmoles/g of seed dry weight, DW), making the protein-rich seed meal useful as animal feed. However, the overall lower glucosinolate content in seeds as well as in the other parts of such plants renders them vulnerable to biotic challenges. We report CRISPR/Cas9-based editing of glucosinolate transporter (GTR) family genes in mustard (Brassica juncea) to develop ideal lines with the desired low seed glucosinolate content (SGC) while maintaining high glucosinolate levels in the other plant parts for uncompromised plant defence. Use of three gRNAs provided highly efficient and precise editing of four BjuGTR1 and six BjuGTR2 homologues leading to a reduction of SGC from 146.09 μmoles/g DW to as low as 6.21 μmoles/g DW. Detailed analysis of the GTR-edited lines showed higher accumulation and distributional changes of glucosinolates in the foliar parts. However, the changes did not affect the plant defence and yield parameters. When tested against the pathogen Sclerotinia sclerotiorum and generalist pest Spodoptera litura, the GTR-edited lines displayed a defence response at par or better than that of the wild-type line. The GTR-edited lines were equivalent to the wild-type line for various seed yield and seed quality traits. Our results demonstrate that simultaneous editing of multiple GTR1 and GTR2 homologues in mustard can provide the desired low-seed, high-leaf glucosinolate lines with an uncompromised defence and yield.

Defense versus growth trade-offs: Insights from glucosinolates and their catabolites

Specialized metabolites are a structurally diverse group of naturally occurring compounds that facilitate plant-environment interactions. Their synthesis and maintenance in plants is overall a resource-demanding process that occurs at the expense of growth and reproduction and typically incurs several costs. Evidence emerging on different specialized compounds suggests that they serve multiple auxiliary functions to influence and moderate primary metabolism in plants. These new functionalities enable them to mediate trade-offs from defenses to growth and also to offset their production and maintenance costs in plants. Recent research on glucosinolates (GSLs), which are specialized metabolites of Brassicales, demonstrates their emerging multifunctionalities to fine-tune plant growth and development under variable environments. Herein, we present findings from the septennium on individual GSLs and their catabolites (GHPs) per se, that work as mobile signals within plants to mediate precise regulations of their primary physiological functions. Both GSLs and GHPs calibrate growth-defense trade-off interactions either synergistically or directly when they function as storage compounds, abiotic stress alleviators, and one-to-one regulators of growth pathways in plants. We finally summarize the overall lessons learned from GSLs and GHPs as a model and raise the most pressing questions to address the molecular-genetic intricacies of specialized metabolite-based trade-offs in plants.

The mysterious non-arbuscular mycorrhizal status of Brassicaceae species

The Brassicaceae family is unique in not fostering functional symbiosis with arbuscular mycorrhiza (AM). The family is also special in possessing glucosinolates, a class of secondary metabolites predominantly functioning for plant defence. We have reviewed what effect the glucosinolates of this non-symbiotic host have on AM or vice versa. Isothiocyanates, the toxic degradation product of the glucosinolates, particularly the indolic and benzenic glucosinolates, are known to be involved in the inhibition of AM. Interestingly, AM colonization enhances glucosinolate production in two AM-host in the Brassicales family- Moringa oleifera and Tropaeolum spp. PHOSPHATE STARVATION RESPONSE 1 (PHR1), a central transcription factor that controls phosphate starvation response also activates the glucosinolate biosynthesis in AM non-host Arabidopsis thaliana. Recently, the advances in whole-genome sequencing, enabling extensive ecological microbiome studies have helped unravel the Brassicaceae microbiome, identifying new mutualists that compensate for the loss of AM symbiosis, and reporting cues for some influence of glucosinolates on the microbiome structure. We advocate that glucosinolate is an important candidate in determining the mycorrhizal status of Brassicaceae and has played a major role in its symbiosis-defence trade-off. We also identify key open questions in this area that remain to be addressed in the future.

The multifaceted roles of heterotrimeric G-proteins: lessons from models and crops

The review summarizes our advanced understanding of the heterotrimeric G-protein research from model plants and their emerging roles in modulating various plant architecture and agronomical traits in crop species. Heterotrimeric G-proteins (hereafter G-proteins), consisting of G-alpha (Gα), G-beta (Gβ) and G-gamma (Gγ) subunits, are key signal transducers conserved across different forms of life. The discovery of plant lineage-specific G-protein components (extra-large G-proteins and type-C Gγ subunits), inherent polyploidy in angiosperms, and unique modes of G-protein cycle regulation in plants pointed out to a few fundamental differences of plant G-protein signaling from its animal counterpart. Over the last 2 decades, extensive studies in the model plant Arabidopsis thaliana have confirmed the involvement of G-proteins in a wide range of plant growth and development, and stress adaptation processes. The G-protein research in crop species, however, is still in its infancy, and a handful of studies suggest important roles of G-proteins in regulating plant architectural and key agronomical traits including plant's response to abiotic and biotic factors. We propose that the advancement made in plant G-proteins research will facilitate the development of novel approaches to manage plant yield and fitness in changing environments.

Extra-large G-proteins influence plant response to Sclerotinia sclerotiorum by regulating glucosinolate metabolism in Brassica juncea

Heterotrimeric G-proteins are one of the highly conserved signal transducers across phyla. Despite the obvious importance of G-proteins in controlling various plant growth and environmental responses, there is no information describing the regulatory complexity of G-protein networks during pathogen response in a polyploid crop. Here, we investigated the role of extra-large G-proteins (XLGs) in the oilseed crop Brassica juncea, which has inherent susceptibility to the necrotrophic fungal pathogen Sclerotinia sclerotiorum. The allotetraploid B. juncea genome contains multiple homologs of three XLG genes (two BjuXLG1, five BjuXLG2, and three BjuXLG3), sharing a high level of sequence identity, gene structure organization, and phylogenetic relationship with the progenitors' orthologs. Quantitative reverse transcription PCR analysis revealed that BjuXLGs have retained distinct expression patterns across plant developmental stages and on S. sclerotiorum infection. To determine the role of BjuXLG genes in the B. juncea defence response against S. sclerotiorum, RNAi-based suppression was performed. Disease progression analysis showed more rapid lesion expansion and fungal accumulation in BjuXLG-RNAi lines compared to the vector control plants, wherein suppression of BjuXLG3 homologs displayed more compromised defence response at the later time point. Knocking down BjuXLGs caused impairment of the host resistance mechanism to S. sclerotiorum, as indicated by reduced expression of defence marker genes PDF1.2 and WRKY33 on pathogen infection. Furthermore, BjuXLG-RNAi lines showed reduced accumulation of leaf glucosinolates on S. sclerotiorum infection, wherein aliphatic glucosinolates were significantly compromised. Overall, our data suggest that B. juncea XLG genes are important signalling nodes modulating the host defence pathways in response to this necrotrophic pathogen.

A complex interplay of Gβ and Gγ proteins regulates plant growth and defence traits in the allotetraploid Brassica juncea

Gene expression analysis coupled with in-planta studies showed that specific Gβγ combination regulates plant growth and defence traits in the allotetraploid Brassica juncea. Plant heterotrimeric G-proteins regulate a wide range of responses despite their limited repertoire of core components. The roles and functional interactions between different G-protein subunits are quite perplexing, which get further complicated with polyploidy. Here, we show that the allotetraploid Brassica juncea comprises multiple homologs of G-protein genes, encoding six BjuGβ and ten highly divergent BjuGγ subunit proteins, later being classified into type-A1, type-A2 and type-C Gγ proteins. The encoded BjuGβ and BjuGγ proteins shared close evolutionary relationship and have retained distinct spatio-temporal expression patterns during plant developmental stages and in response to the necrotrophic pathogen, Sclerotinia sclerotiorum. RNAi based suppression of BjuGβ and BjuGγ genes suggested functional overlap and selectivity of BjuGβs with three distinct BjuGγ type subunits, to regulate plant height (BjuGβγA2 and BjuGβγC), seed weight (BjuGβGγA1 and BjuGβGγC), silique size (BjuGβGγC) and pathogen response (BjuGβGγA1 and BjuGβGγC). Further, the triplicated BjuGβ genes, formed due to Brassica specific whole-genome-triplication event, showed differential involvement during pathogen response, wherein overexpression of BjuGβ2 displayed higher resistance to Sclerotinia infection. Taken together, our study demonstrates that multiple BjuGβ and BjuGγ proteins have retained distinct spatio-temporal expression and functional selectivity to regulate specific plant growth and defence traits in the oilseed B. juncea.

GTR1 and GTR2 transporters differentially regulate tissue-specific glucosinolate contents and defence responses in the oilseed crop Brassica juncea

GTR1 and GTR2 transporters are components of the source to sink translocation network of glucosinolates, which are major defence metabolites in the Brassicaceae. These transporters can be genetically manipulated for reduction of seed-glucosinolates without inhibiting glucosinolate biosynthesis, thereby maintaining the inherent defence potential of plants. However, the different roles of GTRs in influencing tissue-specific distribution of glucosinolates in agriculturally important Brassica crops are yet unknown. Here, we report functional characterization of two groups of glucosinolate transporters (GTR1 and GTR2) from Brassica juncea based on gene expression data, biochemical analysis, gene-complementation studies in GTR-deficient mutants and RNAi-based knockdown followed by insect feeding experiments. Although both GTRs showed ubiquitous expression patterns and broad substrate specificity, the single-gene knockdown lines displayed different phenotypes. The GTR2-knockdown plants showed a significant reduction of glucosinolates in seeds and a higher accumulation in leaves and pods, while the GTR1-knockdown plants displayed a smaller reduction of glucosinolates in seeds and significantly lower glucosinolate levels in leaves. Consequently, knockdown of GTR2 resulted in higher resistance towards the generalist pest, Spodoptera litura. Overall, our study highlights the distinctive roles of B. juncea GTRs in tissue-specific accumulation of glucosinolates and the potential for manipulating GTR2 for enhanced nutrition and plant defence.

A comprehensive Vis-NIRS equation for rapid quantification of seed glucosinolate content and composition across diverse Brassica oilseed chemotypes

The globally cultivated Brassica crops contain high deliverable concentrations of health-promoting glucosinolates. Development of a Visible-Near InfraRed Spectroscopy (Vis-NIRS) calibration to profile different glucosinolate components from 641 diverse Brassica juncea chemotypes was attempted in this study. Principal component analysis of HPLC-determined glucosinolates established the distinctiveness of four B. juncea populations used. Subsequently, modified partial least square regression based population-specific and combined Vis-NIRS models were developed, wherein the combined model exhibited higher coefficient of determination (R²; 0.81-0.97) for eight glucosinolates and higher ratio of prediction determination (RPD; 2.42-5.35) for seven glucosinolates in B. juncea populations. Furthermore, range error ratio (RER > 4) for twelve and RER > 10 for eight glucosinolates make the combined model acceptable for screening and quality control. The model also provided excellent prediction for aliphatic glucosinolates in four oilseed Brassica species. Overall, our work highlights the potential of Vis-NIR spectroscopy in estimating glucosinolate content in the economically important Brassica oilseeds.

Heterotrimeric Gα subunit regulates plant architecture, organ size and seed weight in the oilseed Brassica juncea

Two BjuGα proteins exhibit conserved GTP-binding and GTP-hydrolysis activities, and function in maintaining overall plant architecture and controlling multiple yield-related traits in the oilseed Brassica juncea. Heterotrimeric G-protein (Gα, Gβ and Gγ) are key signal transducers, well characterized in model plants Arabidopsis and rice. However, our knowledge about the roles played by G-proteins in regulating various growth and developmental traits in polyploid crops, having a complex G-protein signalling network, is quite sparse. In the present study, two Gα encoding genes (BjuA.Gα1 and BjuB.Gα1) were isolated from the allotetraploid Brassica juncea, a globally cultivated oilseed crop of the Brassicaceae family. BjuGα1 genes share a close evolutionary relationship, and the encoded proteins exhibit highly conserved G-protein activities while showing expression differentiation, wherein BjuA.Gα1 was the highly abundant transcript during plant growth and developmental stages. RNAi based suppression of BjuGα1 displayed compromised effects on most of the tested vegetative and reproductive parameters, particularly plant height (32-58%), flower and siliques dimensions, and seed weight (11-13%). Further, over-expression of a constitutively active Gα, lacking the GTPase activity, produced plants with increased height, organ size and seed weight (7-25%), without altering seed quality traits like fatty acid composition, glucosinolates, oil and protein contents. Our study demonstrates that BjuGα1 proteins control overall plant architecture and multiple yield-related traits in the oilseed B. juncea, suggesting that BjuGα1 could be a promising target for crop improvement.

A cell suspension based uptake method to study high affinity glucosinolate transporters

BACKGROUND

Glucosinolates are an important class of secondary metabolites characteristic to the order Brassicales. They are known to play a major role in plant defense and from the human perspective, can be anticarcinogenic or antinutritive. GTRs are plasma-membrane localized high affinity glucosinolate transporters, which are important components of the source (leaf) to sink (seed) translocation of intact glucosinolates in members of Brassicaceae family. GTRs are identified as major candidates for Brassica crop improvement, thus dictating a need for their functional characterization. However, currently there are limitations in availability of heterologous assay systems for functional characterization of plant secondary metabolite transporters. To date, the animal-based Xenopus oocyte system is the best established heterologous system for functional characterization of these transporters. Inherent biochemical and physiological attributes unique to the plant membranes necessitate the need for developing plant-based transporters assay systems as well.

METHODS

In this study, Agrobacterium mediated transformation was used to develop GTR expressing cotton cell lines (CCL-1) for functional characterization of the Arabidopsis high affinity glucosinolate transporters, AtGTR1 and AtGTR2. Following sub-cellular localization of AtGTRs, we standardized the glucosinolate uptake assays using cell suspension cultures of AtGTR expressing CCL-1 its requirement of pH, salt, and time based glucosinolate uptake. Using the GTR expressing CCL-1, we subsequently performed kinetic analysis of AtGTR1 and AtGTR2 for different glucosinolate substrates, sinigrin, gluconapin and sinalbin.

RESULTS

Several clones expressing each of AtGTR1 and AtGTR2 were obtained showing high level of GTR expression and were maintained through regular sub-culturing. Both AtGTR1 and AtGTR2 are predominantly plasma-localized proteins when overexpressed in CCL-1 cells. Uptake assays were standardized, suggesting that glucosinolate uptake of GTR expressing CCL-1 is robust within the physiological pH range 5-6, and at lower concentration of nitrate salts. GTR expressing CCL-1 cells show increasing glucosinolate accumulation in time course experiment. Kinetic studies over a wide glucosinolate concentrations (10-800 µM) revealed that our novel assay system displayed robust GTR-mediated uptake of different glucosinolates and unambiguously helps elucidate the saturable kinetics of GTRs. Our system confirms the high affinity of AtGTRs for both aliphatic and aromatic glucosinolates.

CONCLUSION

The transporter assay system described in this study holds potential for studying sub-functionalization amongst GTR homologs present across Brassicaceae family. The fast growing CCL-1 cells, confer the benefits of an in vitro system for quick assays and is plant based thus enabling optimal expression without sequence modifications. The efficient functioning of the GTR transporters in the heterologous CCL-1 opens the possibility of using this plant cell suspension system for functional characterization of other metabolite transporters.

Molecular Basis of the Evolution of Methylthioalkylmalate Synthase and the Diversity of Methionine-Derived Glucosinolates

The globally cultivated Brassica species possess diverse aliphatic glucosinolates, which are important for plant defense and animal nutrition. The committed step in the side chain elongation of methionine-derived aliphatic glucosinolates is catalyzed by methylthioalkylmalate synthase, which likely evolved from the isopropylmalate synthases of leucine biosynthesis. However, the molecular basis for the evolution of methylthioalkylmalate synthase and its generation of natural product diversity in Brassica is poorly understood. Here, we show that Brassica genomes encode multiple methylthioalkylmalate synthases that have differences in expression profiles and 2-oxo substrate preferences, which account for the diversity of aliphatic glucosinolates across Brassica accessions. Analysis of the 2.1 Å resolution x-ray crystal structure of Brassica juncea methylthioalkylmalate synthase identified key active site residues responsible for controlling the specificity for different 2-oxo substrates and the determinants of side chain length in aliphatic glucosinolates. Overall, these results provide the evolutionary and biochemical foundation for the diversification of glucosinolate profiles across globally cultivated Brassica species, which could be used with ongoing breeding strategies toward the manipulation of beneficial glucosinolate compounds for animal health and plant protection.

Targeted silencing of genes in polyploids: lessons learned from Brassica juncea-glucosinolate system

Intron-spliced hairpin RNAi construct targeting the exonic region of BjuMYB28 driven by the native promoter is the best suited strategy for developing viable low glucosinolate lines in polyploid Brassica juncea. Targeted silencing of specific homolog(s) of a multigene family in polyploids through RNA interference (RNAi) is a challenging effort. Indian oilseed mustard (Brassica juncea), a natural allotetraploid, is expected to have 4-6 copies of every Arabidopsis gene ortholog. In the current study, we have attempted to establish the best gene silencing system suitable for BjuMYB28, a transcription factor gene, with the objective of developing low seed glucosinolate lines in B. juncea. After comparing multiple combinations of BjuMYB28 gene homologs, promoters, target regions (exon and 3' UTR) and silencing strategies (RNAi and antisense), we suggest that the intron-spliced hairpin RNAi construct targeting the specific exonic region of the BjuMYB28 gene under the control of native promoter, whose peak activity synchronises with the highest glucosinolate accumulation phase of the plant, is the best suited strategy for developing viable low glucosinolate lines in polyploid B. juncea.

Pattern of expression and interaction specificity of multiple G-protein beta (Gβ) subunit isoforms with their potential target proteins reveal functional dominance of BjuGβ1 in the allotetraploid Brassica juncea

Heterotrimeric G-protein, consisting Gα, Gβ and Gγ subunits, interacts with various upstream and downstream effector (target) proteins to regulate a large array of conserved and species-specific biological functions. The targets of G-protein components are recently reported in model plant Arabidopsis thaliana; however limited information is available from crop species. In this study, we utilized yeast two-hybrid (Y2H) assay to screen the diversity of interacting partners of multiple Gβ subunit isoforms from allotetraploid Brassica juncea, a globally important oilseed and vegetable crop. The three BjuGβ genes (BjuGβ1-3), resulted from whole genome triplication event in Brassica lineage, showed distinct expression profile during plant developmental stages with maximal transcript abundance during reproductive stages. Protein-protein interaction of three BjuGβ proteins (bait) against the Y2H cDNA library (prey) identified a total of 14 and 1 non-redundant targets for BjuGβ1 and BjuGβ2, whereas BjuGβ3 screening surprisingly did not yield any genuine target, thereby suggesting functional dominance of BjuGβ1. The triplicated BjuGβ isoforms showed a high degree of interaction strength and specificity with the identified target proteins, which are known to be involved in diverse biological functions in plants. qRT-PCR analysis further indicated that the expression of BjuGβ-target genes was developmentally regulated under various tissue types studied and showed a high degree of co-expression pattern with the BjuGβ genes, particularly during flower and silique development in B. juncea. Taken together, our data provides novel insights on pattern of expression and interaction specificity governing functional divergence of multiple Gβ subunit proteins in polyploid B. juncea.

Arsenic affects the production of glucosinolate, thiol and phytochemical compounds: A comparison of two Brassica cultivars

Arsenic (As), a non-essential metalloid, severely affects the normal functioning of plants, animals and humans. Plants play a crucial role in metabolic, physiological and numerous detoxification mechanisms to cope up with As induced stress. This study aimed to examine the differential response in two Brassica juncea cultivars, Varuna and Pusa Jagannath (PJn) exposed to different doses of As (50, 150, 300 μM) for 48 h duration. Change in morphological traits, concentration of individual as well as total GSL, sulfur related thiol proteins, sulfur content, and phytochemicals were analyzed in both cultivars. Accumulation pattern of As showed dose dependent accumulation in both the cultivars, being more in PJn. Our finding revealed that both cultivars were tolerant at low concentrations of As, while at higher concentration Varuna excelled over PJn. The increased tolerance of Varuna cultivar exposed to 150 and 300 μM concentration of As, correlated with its increased thiol related proteins, sulfur content and phytochemicals, which serves as defence strategy in the plant against oxidative stress. Differential pattern of total as well as individual GSLs content was observed in both Varuna and PJn cultivars. Varuna cultivar showed higher level of total and aliphatic GSLs, which serves as defence compound with other detoxification machineries to combat As stress. Our findings provide foundation for developing metalloid tolerant crops by analyzing the role of different genes involved in GSL mechanism and signaling pathways in different organs of plant.

Molecular cloning and expression profiling of multiple Dof genes of Sorghum bicolor (L) Moench

DNA binding with one finger (Dof) proteins represent a family of plant specific transcription factors associated with diverse biological processes, such as seed maturation and germination, phytohormone and light mediated regulation, and plant responses to biotic and abiotic stresses. In present study, a total of 21 Dof genes from Sorghum bicolor were cloned, sequenced and in silico characterized for homology search, revealing their identity to Dof like proteins. The expression profiling of SbDof genes using quantitative RT-PCR in different tissue types and also under drought and salt stresses was attempted. The SbDof genes displayed differential expression either in their transcript abundance or in their expression patterns under normal growth condition. Two of the SbDof genes namely SbDof8 and SbDof12 showed comparatively high level of transcript abundance in all the tissue types tested; whereas some of the SbDof genes showed a distinct tissue specific expression pattern. Further a total of 13 SbDof genes showed differential expression when subjected to either of the abiotic stress i.e. drought or salinity. Three of the SbDof genes namely SbDof12, SbDof19 and SbDof24 were found to be up-regulated in response to drought and salt stress. Comparative analysis of SbDof genes expression revealed existence of a complex transcriptional and functional diversity across plant growth and developmental stages.

Class-Specific Evolution and Transcriptional Differentiation of 14-3-3 Family Members in Mesohexaploid Brassica rapa

14-3-3s are highly conserved, multigene family proteins that have been implicated in modulating various biological processes. The presence of inherent polyploidy and genome complexity has limited the identification and characterization of 14-3-3 proteins from globally important Brassica crops. Through data mining of Brassica rapa, the model Brassica genome, we identified 21 members encoding 14-3-3 proteins namely, BraA.GRF14.a to BraA.GRF14.u. Phylogenetic analysis indicated that B. rapa contains both ε (epsilon) and non-ε 14-3-3 isoforms, having distinct intron-exon structural organization patterns. The non-ε isoforms showed lower divergence rate (Ks < 0.45) compared to ε protein isoforms (Ks > 0.48), suggesting class-specific divergence pattern. Synteny analysis revealed that mesohexaploid B. rapa genome has retained 1-5 orthologs of each Arabidopsis 14-3-3 gene, interspersed across its three fragmented sub-genomes. qRT-PCR analysis showed that 14 of the 21 BraA.GRF14 were expressed, wherein a higher abundance of non-ε transcripts was observed compared to the ε genes, indicating class-specific transcriptional bias. The BraA.GRF14 genes showed distinct expression pattern during plant developmental stages and in response to abiotic stress, phytohormone treatments, and nutrient deprivation conditions. Together, the distinct expression pattern and differential regulation of BraA.GRF14 genes indicated the occurrence of functional divergence of B. rapa 14-3-3 proteins during plant development and stress responses.

Biofortification of oilseed Brassica juncea with the anti-cancer compound glucoraphanin by suppressing GSL-ALK gene family

Glucosinolates are amino acids derived secondary metabolites, invariably present in Brassicales, which have huge health and agricultural benefits. Sulphoraphane, the breakdown product of glucosinolate glucoraphanin is known to posses anti-cancer properties. AOP (2-oxoglutarate-dependent dioxygenases) or GSL-ALK enzyme catalyzes the conversion of desirable glucoraphanin to deleterious gluconapin and progoitrin, which are present in very high amounts in most of the cultivable Brassica species including Brassica juncea. In this study we showed that B. juncea encodes four functional homologs of GSL-ALK gene and constitutive silencing of GSL-ALK homologs resulted in accumulation of glucoraphanin up to 43.11 μmoles g(-1) DW in the seeds with a concomitant reduction in the anti-nutritional glucosinolates. Glucoraphanin content was found remarkably high in leaves as well as sprouts of the transgenic lines. Transcript quantification of high glucoraphanin lines confirmed significant down-regulation of GSL-ALK homologs. Growth and other seed quality parameters of the transgenic lines did not show drastic difference, compared to the untransformed control. High glucoraphanin lines also showed higher resistance towards stem rot pathogen Sclerotinia sclerotiorum. Our results suggest that metabolic engineering of GSL-ALK has huge potential for enriching glucoraphanin content, and improve the oil quality and vegetable value of Brassica crops.

Biotic elicitors and mechanical damage modulate glucosinolate accumulation by co-ordinated interplay of glucosinolate biosynthesis regulators in polyploid Brassica juncea

Glucosinolates are nitrogen and sulfur containing secondary metabolites found mainly in the Brassicaceae. They function as plant defense compounds against a broad spectrum of pathogens and pests. Since these molecules form part of the plant defense mechanism, glucosinolate biosynthesis may be modulated by environmental signals leading to activation of a biological stress response. In the current study, we have mimicked such conditions by exogenously applying biotic elicitors such as methyl jasmonate, salicylic acid, glucose and mechanical injury in Brassica juncea seedling over a time course experiment. We found that total glucosinolates over-accumulated under these stress conditions with maximum accumulation observed 24h post treatment. Indole glucosinolates like 1-methoxy-indol-3-ylmethyl and its precursor indol-3-methyl glucosinolates showed a more significant induction compared to aliphatic glucosinolates thereby suggesting a prominent role of indole glucosinolates during plant defense response in B. juncea seedlings. In contrast, the higher amounts of aliphatic glucosinolates were less regulated by the tested biotic elicitors in B. juncea. Expression profiling of multiple homologs of key transcriptional regulators of glucosinolate biosynthesis further showed that a complex interplay of these regulators exists in polyploid B. juncea where they exert co-ordinated and overlapping effects toward altering glucosinolate accumulation. This study has a significant role toward understanding and augmenting plant defense mechanisms in B. juncea, a globally important oilseed crop of genus Brassica.

Insights into structural and functional diversity of Dof (DNA binding with one finger) transcription factor

The structural, functional and in-silico studies of Dof transcription factor attempted so far reveals immense opportunity to analyze the plant genomes in terms of number of Dof genes and discuss in light of the evolution. The multiple functions of Dof genes needs to explored for crop improvement. Transcription factors play a very vital role in gene regulation at transcriptional level and are being extensively studied across phylas. In recent years, sequencing of plant genomes has led to genome-wide identification and characterizations of diverse types of plant-specific transcription factor gene family providing key insights into their structural and functional diversity. The DNA binding with one finger (Dof), a class belonging to C2H2-type zinc finger family proteins, is a plant-specific transcription factor having multiple roles such as seed maturation and germination, phytohormone and light-mediated regulation and plant responses to biotic and abiotic stresses. Dof proteins are present across plant lineage, from green algae to higher angiosperm, and represent a unique class of transcription factor having bifunctional binding activities, with both DNA and proteins, to regulate the complex transcriptional machinery in plant cells. The structural and functional diversity of the Dof transcription factor family along with the bioinformatics analysis highlighting the phylogeny of Dof families is reviewed in light of its importance in plant biotechnology for crop improvement.

Genomic origin, expression differentiation and regulation of multiple genes encoding CYP83A1, a key enzyme for core glucosinolate biosynthesis, from the allotetraploid Brassica juncea

The multiple BjuCYP83A1 genes formed as a result of polyploidy have retained cell-, tissue-, and condition-specific transcriptional sub-functionalization to control the complex aliphatic glucosinolates biosynthesis in the allotetraploid Brassica juncea. Glucosinolates along with their breakdown products are associated with diverse roles in plant metabolism, plant defense and animal nutrition. CYP83A1 is a key enzyme that oxidizes aliphatic aldoximes to aci-nitro compounds in the complex aliphatic glucosinolate biosynthetic pathway. In this study, we reported the isolation of four CYP83A1 genes named BjuCYP83A1-1, -2, -3, and -4 from allotetraploid Brassica juncea (AABB genome), an economically important oilseed crop of Brassica genus. The deduced BjuCYP83A1 proteins shared 85.7-88.4 % of sequence identity with A. thaliana AtCYP83A1 and 84.2-95.8 % among themselves. Phylogenetic and divergence analysis revealed that the four BjuCYP83A1 proteins are evolutionary conserved and have evolved via duplication and hybridization of two relatively simpler diploid Brassica genomes namely B. rapa (AA genome) and B. nigra (BB genome), and have retained high level of sequence conservation following allopolyploidization. Ectopic over-expression of BjuCYP83A1-1 in A. thaliana showed that it is involved mainly in the synthesis of C4 aliphatic glucosinolates. Detailed expression analysis using real-time qRT-PCR in B. juncea and PromoterBjuCYP83A1-GUS lines in A. thaliana confirmed that the four BjuCYP83A1 genes have retained ubiquitous, overlapping but distinct expression profiles in different tissue and cell types of B. juncea, and in response to various elicitor treatments and environmental conditions. Taken together, this study demonstrated that transcriptional sub-functionalization and coordinated roles of multiple BjuCYP83A1 genes control the biosynthesis of aliphatic glucosinolates in the allotetraploid B. juncea, and provide a framework for metabolic engineering of aliphatic glucosinolates in economically important Brassica species.

Genome wide in silico characterization of Dof gene families of pigeonpea (Cajanus cajan (L) Millsp.)

The DNA binding with One Finger (Dof) protein is a plant specific transcription factor involved in the regulation of wide range of processes. The analysis of whole genome sequence of pigeonpea has identified 38 putative Dof genes (CcDof) distributed on 8 chromosomes. A total of 17 out of 38 CcDof genes were found to be intronless. A comprehensive in silico characterization of CcDof gene family including the gene structure, chromosome location, protein motif, phylogeny, gene duplication and functional divergence has been attempted. The phylogenetic analysis resulted in 3 major clusters with closely related members in phylogenetic tree revealed common motif distribution. The in silico cis-regulatory element analysis revealed functional diversity with predominance of light responsive and stress responsive elements indicating the possibility of these CcDof genes to be associated with photoperiodic control and biotic and abiotic stress. The duplication pattern showed that tandem duplication is predominant over segmental duplication events. The comparative phylogenetic analysis of these Dof proteins along with 78 soybean, 36 Arabidopsis and 30 rice Dof proteins revealed 7 major clusters. Several groups of orthologs and paralogs were identified based on phylogenetic tree constructed. Our study provides useful information for functional characterization of CcDof genes.

Four genes encoding MYB28, a major transcriptional regulator of the aliphatic glucosinolate pathway, are differentially expressed in the allopolyploid Brassica juncea

Glucosinolates are Capparales-specific secondary metabolites that have immense potential in human health and agriculture. Unlike Arabidopsis thaliana, our knowledge about glucosinolate regulators in the Brassica crops is sparse. In the current study, four MYB28 homologues were identified (BjuMYB28-1,-2,-3,-4) from the polyploid Brassica juncea, and the effects of allopolyploidization on the divergence of gene sequence, structure, function, and expression were assessed. The deduced protein sequences of the four BjuMYB28 genes showed 76.1-83.1% identity with the Arabidopsis MYB28. Phylogenetic analysis revealed that the four BjuMYB28 proteins have evolved via the hybridization and duplication processes forming the B. juncea genome (AABB) from B. rapa (AA) and B. nigra (BB), while retaining high levels of sequence conservation. Mutant complementation and over-expression studies in A. thaliana showed that all four BjuMYB28 genes encode functional MYB28 proteins and resulted in similar aliphatic glucosinolate composition and content. Detailed expression analysis using qRT-PCR assays and promoter-GUS lines revealed that the BjuMYB28 genes have both tissue- and cell-specific expression partitioning in B. juncea. The two B-genome origin BjuMYB28 genes had more abundant transcripts during the early stages of plant development than the A-genome origin genes. However, with the onset of the reproductive phase, expression levels of all four BjuMYB28 increased significantly, which may be necessary for producing and maintaining high amounts of aliphatic glucosinolates during the later stages of plant development. Taken together, our results suggest that the four MYB28 genes are differentially expressed and regulated in B. juncea to play discrete though overlapping roles in controlling aliphatic glucosinolate biosynthesis.

Targeted silencing of BjMYB28 transcription factor gene directs development of low glucosinolate lines in oilseed Brassica juncea

Brassica juncea (Indian mustard), a globally important oilseed crop, contains relatively high amount of seed glucosinolates ranging from 80 to 120 μmol/g dry weight (DW). One of the major breeding objectives in oilseed Brassicas is to improve the seed-meal quality through the development of low-seed-glucosinolate lines (<30 μmol/g DW), as high amounts of certain seed glucosinolates are known to be anti-nutritional and reduce the meal palatability. Here, we report the development of transgenic B. juncea lines having seed glucosinolates as low as 11.26 μmol/g DW, through RNAi-based targeted suppression of BjMYB28, a R2R3-MYB transcription factor family gene involved in aliphatic glucosinolate biosynthesis. Targeted silencing of BjMYB28 homologs provided significant reduction in the anti-nutritional aliphatic glucosinolates fractions, without altering the desirable nonaliphatic glucosinolate pool, both in leaves and seeds of transgenic plants. Molecular characterization of single-copy, low glucosinolate homozygous lines confirmed significant down-regulation of BjMYB28 homologs vis-à-vis enhanced accumulation of BjMYB28-specific siRNA pool. Consequently, these low glucosinolate lines also showed significant suppression of genes involved in aliphatic glucosinolate biosynthesis. The low glucosinolate trait was stable in subsequent generations of the transgenic lines with no visible off-target effects on plant growth and development. Various seed quality parameters including fatty acid composition, oil content, protein content and seed weight of the low glucosinolate lines also remained unaltered, when tested under containment conditions in the field. Our results indicate that targeted silencing of a key glucosinolate transcriptional regulator MYB28 has huge potential for reducing the glucosinolates content and improving the seed-meal quality of oilseed Brassica crops.

Two chimeric regulators of G-protein signaling (RGS) proteins differentially modulate soybean heterotrimeric G-protein cycle

Heterotrimeric G-proteins and the regulator of G-protein signaling (RGS) proteins, which accelerate the inherent GTPase activity of Gα proteins, are common in animals and encoded by large gene families; however, in plants G-protein signaling is thought to be more limited in scope. For example, Arabidopsis thaliana contains one Gα, one Gβ, three Gγ, and one RGS protein. Recent examination of the Glycine max (soybean) genome reveals a larger set of G-protein-related genes and raises the possibility of more intricate G-protein networks than previously observed in plants. Stopped-flow analysis of GTP-binding and GDP/GTP exchange for the four soybean Gα proteins (GmGα1-4) reveals differences in their kinetic properties. The soybean genome encodes two chimeric RGS proteins with an N-terminal seven transmembrane domain and a C-terminal RGS box. Both GmRGS interact with each of the four GmGα and regulate their GTPase activity. The GTPase-accelerating activities of GmRGS1 and -2 differ for each GmGα, suggesting more than one possible rate of the G-protein cycle initiated by each of the Gα proteins. The differential effects of GmRGS1 and GmRGS2 on GmGα1-4 result from a single valine versus alanine difference. The emerging picture suggests complex regulation of the G-protein cycle in soybean and in other plants with expanded G-protein networks.

Evaluation of candidate reference genes for gene expression normalization in Brassica juncea using real time quantitative RT-PCR

The real time quantitative reverse transcription PCR (qRT-PCR) is becoming increasingly important to gain insight into function of genes. Given the increased sensitivity, ease and reproducibility of qRT-PCR, the requirement of suitable reference genes for normalization has become important and stringent. It is now known that the expression of internal control genes in living organism vary considerably during developmental stages and under different experimental conditions. For economically important Brassica crops, only a couple of reference genes are reported till date. In this study, expression stability of 12 candidate reference genes including ACT2, ELFA, GAPDH, TUA, UBQ9 (traditional housekeeping genes), ACP, CAC, SNF, TIPS-41, TMD, TSB and ZNF (new candidate reference genes), in a diverse set of 49 tissue samples representing different developmental stages, stress and hormone treated conditions and cultivars of Brassica juncea has been validated. For the normalization of vegetative stages the ELFA, ACT2, CAC and TIPS-41 combination would be appropriate whereas TIPS-41 along with CAC would be suitable for normalization of reproductive stages. A combination of GAPDH, TUA, TIPS-41 and CAC were identified as the most suitable reference genes for total developmental stages. In various stress and hormone treated samples, UBQ9 and TIPS-41 had the most stable expression. Across five cultivars of B. juncea, the expression of CAC and TIPS-41 did not vary significantly and were identified as the most stably expressed reference genes. This study provides comprehensive information that the new reference genes selected herein performed better than the traditional housekeeping genes. The selection of most suitable reference genes depends on the experimental conditions, and is tissue and cultivar-specific. Further, to attain accuracy in the results more than one reference genes are necessary for normalization.

Conventional and novel Gγ protein families constitute the heterotrimeric G-protein signaling network in soybean

Heterotrimeric G-proteins comprised of Gα, Gβ and Gγ proteins are important signal transducers in all eukaryotes. The Gγ protein of the G-protein heterotrimer is crucial for its proper targeting at the plasma membrane and correct functioning. Gγ proteins are significantly smaller and more diverse than the Gα and Gβ proteins. In model plants Arabidopsis and rice that have a single Gα and Gβ protein, the presence of two canonical Gγ proteins provide some diversity to the possible heterotrimeric combinations. Our recent analysis of the latest version of the soybean genome has identified ten Gγ proteins which belong to three distinct families based on their C-termini. We amplified the full length cDNAs, analyzed their detailed expression profile by quantitative PCR, assessed their localization and performed yeast-based interaction analysis to evaluate interaction specificity with different Gβ proteins. Our results show that ten Gγ genes are retained in the soybean genome and have interesting expression profiles across different developmental stages. Six of the newly identified proteins belong to two plant-specific Gγ protein families. Yeast-based interaction analyses predict some degree of interaction specificity between different Gβ and Gγ proteins. This research thus identifies a highly diverse G-protein network from a plant species. Homologs of these novel proteins have been previously identified as QTLs for grain size and yield in rice.

An elaborate heterotrimeric G-protein family from soybean expands the diversity of plant G-protein networks

The repertoire of heterotrimeric G-proteins in plant species analyzed thus far is simple, with the presence of only two possible canonical heterotrimers in Arabidopsis and rice vs hundreds in animal systems. We assessed whether genome duplication events have resulted in the multiplicity of G-protein in plant species like soybean that would increase the complexity of G-protein networks. We identified and amplified four Gα, four Gβ and two Gγ proteins, analyzed their expression profile by quantitative PCR during different developmental stages. We purified the four Gα proteins and analyzed their guanosine-5'-triphosphate (GTP)-binding and GTPase activity. We performed yeast-based interaction analysis to assess the interaction specificity of different G-protein subunits. Our results show that all 10 G-protein genes are retained in the soybean genome and ubiquitously expressed. The four Gα proteins seem to be plasma membrane-localized. The G-protein genes have interesting expression profiles during seed development and germination. The four Gα proteins form two distinct groups based on their GTPase activity. Yeast-based interaction analyses predict that the proteins interact in most of the possible combinations, with some degree of interaction specificity between duplicated gene pairs. This research identifies the most elaborate heterotrimeric G-protein network known to date in the plant kingdom.

Whole genome co-expression analysis of soybean cytochrome P450 genes identifies nodulation-specific P450 monooxygenases

BACKGROUND

Cytochrome P450 monooxygenases (P450s) catalyze oxidation of various substrates using oxygen and NAD(P)H. Plant P450s are involved in the biosynthesis of primary and secondary metabolites performing diverse biological functions. The recent availability of the soybean genome sequence allows us to identify and analyze soybean putative P450s at a genome scale. Co-expression analysis using an available soybean microarray and Illumina sequencing data provides clues for functional annotation of these enzymes. This approach is based on the assumption that genes that have similar expression patterns across a set of conditions may have a functional relationship.

RESULTS

We have identified a total number of 332 full-length P450 genes and 378 pseudogenes from the soybean genome. From the full-length sequences, 195 genes belong to A-type, which could be further divided into 20 families. The remaining 137 genes belong to non-A type P450s and are classified into 28 families. A total of 178 probe sets were found to correspond to P450 genes on the Affymetrix soybean array. Out of these probe sets, 108 represented single genes. Using the 28 publicly available microarray libraries that contain organ-specific information, some tissue-specific P450s were identified. Similarly, stress responsive soybean P450s were retrieved from 99 microarray soybean libraries. We also utilized Illumina transcriptome sequencing technology to analyze the expressions of all 332 soybean P450 genes. This dataset contains total RNAs isolated from nodules, roots, root tips, leaves, flowers, green pods, apical meristem, mock-inoculated and Bradyrhizobium japonicum-infected root hair cells. The tissue-specific expression patterns of these P450 genes were analyzed and the expression of a representative set of genes were confirmed by qRT-PCR. We performed the co-expression analysis on many of the 108 P450 genes on the Affymetrix arrays. First we confirmed that CYP93C5 (an isoflavone synthase gene) is co-expressed with several genes encoding isoflavonoid-related metabolic enzymes. We then focused on nodulation-induced P450s and found that CYP728H1 was co-expressed with the genes involved in phenylpropanoid metabolism. Similarly, CYP736A34 was highly co-expressed with lipoxygenase, lectin and CYP83D1, all of which are involved in root and nodule development.

CONCLUSIONS

The genome scale analysis of P450s in soybean reveals many unique features of these important enzymes in this crop although the functions of most of them are largely unknown. Gene co-expression analysis proves to be a useful tool to infer the function of uncharacterized genes. Our work presented here could provide important leads toward functional genomics studies of soybean P450s and their regulatory network through the integration of reverse genetics, biochemistry, and metabolic profiling tools. The identification of nodule-specific P450s and their further exploitation may help us to better understand the intriguing process of soybean and rhizobium interaction.

Fine mapping of loci involved with glucosinolate biosynthesis in oilseed mustard (Brassica juncea) using genomic information from allied species

Fine mapping of six seed glucosinolate QTL (J2Gsl1, J3Gsl2, J9Gsl3, J16Gsl4, J17Gsl5 and J3Gsl6) (Ramchiary et al. in Theor Appl Genet 116:77-85, 2007a) was undertaken by the candidate gene approach. Based on the DNA sequences from Arabidopsis and Brassica oleracea for the different genes involved in the aliphatic glucosinolate biosynthesis, candidate genes were amplified and sequenced from high to low glucosinolate Brassica juncea lines Varuna and Heera, respectively. Of the 20 paralogues identified, 17 paralogues belonging to six gene families were mapped to 12 of the 18 linkage groups of B. juncea genome. Co-mapping of candidate genes with glucosinolate QTL revealed that the candidate gene BjuA.GSL-ELONG.a mapped to the QTL interval of J2Gsl1, BjuA.GSL-ELONG.c, BjuA.GSL-ELONG.d and BjuA.Myb28.a mapped to the QTL interval of J3Gsl2, BjuA.GSL-ALK.a mapped to the QTL interval of J3Gsl6 and BjuB.Myb28.a mapped to the QTL interval of J17Gsl5. The QTL J9Gsl3 and J16Gsl4 did not correspond to any of the mapped candidate genes. The functionality and contribution of different candidate genes/QTL was assessed by allelic variation study using phenotypic data of 785 BC(4)DH lines. It was observed that BjuA.Myb28.a and J9Gsl3 contributed significantly to the base level glucosinolate production while J16Gsl4, probably GSL-PRO, BjuA.GSL-ELONG.a and BjuA.GSL-ELONG.c contributed to the C3, C4 and C5 elongation pathways, respectively. Three A genome QTL: J2Gsl1harbouring BjuA.GSL-ELONG.a, J3Gsl2 harbouring both BjuA.GSL-ELONG.c and BjuA.Myb28.a and J9Gsl3, possibly the 'Bronowski genes', were identified as most important loci for breeding low glucosinolate B. juncea. We observed two-step genetic control of seed glucosinolate in B. juncea mainly effected by these three A genome QTL. This study, therefore, provides clues to the genetic mechanism of 'Bronowski genes' controlling the glucosinolate trait and also provides efficient markers for marker-assisted introgression of low glucosinolate trait in B. juncea.

Comparative mapping of Brassica juncea and Arabidopsis thaliana using Intron Polymorphism (IP) markers: homoeologous relationships, diversification and evolution of the A, B and C Brassica genomes

Background

Extensive mapping efforts are currently underway for the establishment of comparative genomics between the model plant, Arabidopsis thaliana and various Brassica species. Most of these studies have deployed RFLP markers, the use of which is a laborious and time-consuming process. We therefore tested the efficacy of PCR-based Intron Polymorphism (IP) markers to analyze genome-wide synteny between the oilseed crop, Brassica juncea (AABB genome) and A. thaliana and analyzed the arrangement of 24 (previously described) genomic block segments in the A, B and C Brassica genomes to study the evolutionary events contributing to karyotype variations in the three diploid Brassica genomes.

Results

IP markers were highly efficient and generated easily discernable polymorphisms on agarose gels. Comparative analysis of the segmental organization of the A and B genomes of B. juncea (present study) with the A and B genomes of B. napus and B. nigra respectively (described earlier), revealed a high degree of colinearity suggesting minimal macro-level changes after polyploidization. The ancestral block arrangements that remained unaltered during evolution and the karyotype rearrangements that originated in the Oleracea lineage after its divergence from Rapa lineage were identified. Genomic rearrangements leading to the gain or loss of one chromosome each between the A-B and A-C lineages were deciphered. Complete homoeology in terms of block organization was found between three linkage groups (LG) each for the A-B and A-C genomes. Based on the homoeology shared between the A, B and C genomes, a new nomenclature for the B genome LGs was assigned to establish uniformity in the international Brassica LG nomenclature code.

Conclusion

IP markers were highly effective in generating comparative relationships between Arabidopsis and various Brassica species. Comparative genomics between the three Brassica lineages established the major rearrangements, translocations and fusions pivotal to karyotype diversification between the A, B and C genomes of Brassica species. The inter-relationships established between the Brassica lineages vis-à-vis Arabidopsis would facilitate the identification and isolation of candidate genes contributing to traits of agronomic value in crop Brassicas and the development of unified tools for Brassica genomics.

QTL analysis reveals context-dependent loci for seed glucosinolate trait in the oilseed Brassica juncea: importance of recurrent selection backcross scheme for the identification of 'true' QTL

Seed glucosinolate content in Brassica juncea is a complex quantitative trait. A recurrent selection backcross (RSB) method with a doubled haploid (DH) generation interspersing backcross generations was used for the introgression of low glucosinolate alleles from an east European gene pool B. juncea line, Heera into an Indian gene pool variety, Varuna. Phenotypic comparisons among the DH populations derived from early to advanced backcrosses revealed a shift in the mean values for various glucosinolates with the advancement of backcrossing, indicating a change in the selective values of the alleles with change in the genetic background due to the existence of epistasis and context dependencies. QTL mapping for various seed glucosinolates from early (F(1)DH) and advanced generation (BC(4)DH) populations confirmed the presence of epistasis and context dependency. The common QTL detected in both F(1)DH and BC(4)DH changed their R (2) values from the former to the later generation. Some of the QTL detected in the F(1)DH became irrelevant in the BC(4)DH population. Further, new QTL were detected in the BC(4)DH population for various glucosinolates. A validation study on a population of low glucosinolate DH lines derived from all the backcross generations of the RSB breeding programme revealed that the QTL detected in BC(4)DH were the 'true' QTL. Using glucosinolate as an example, the study provides strong evidence for the importance of the RSB method for the identification of the 'true' QTL which would be significant for marker assisted introgression of a complex quantitative trait whose expression is influenced by epistatic interactions.

Retransformation of a male sterile barnase line with the barstar gene as an efficient alternative method to identify male sterile-restorer combinations for heterosis breeding

We report in this study, an improved method for identifying male sterile-restorer combinations using the barnase-barstar system of pollination control for heterosis breeding in crop plants, as an alternative to the conventional line x tester cross method. In this strategy, a transgenic male sterile barnase line was retransformed with appropriate barstar constructs. Double transformants carrying both the barnase and barstar genes were identified and screened for their male fertility status. Using this strategy, 66-90% of fertile retransformants (restored events) were obtained in Brassica juncea using two different barstar constructs. Restored events were analysed for their pollen viability and copy number of the barstar gene. Around 90% of the restored events showed high pollen viability and approximately 30% contained single copy integrations of the barstar gene. These observations were significantly different from those made in our earlier studies using line (barnase) x tester (barstar) crosses, wherein only two viable male sterile-restorer combinations were identified by screening 88 different cross-combinations. The retransformation strategy not only generated several independent restorers for a given male sterile line from a single transformation experiment but also identified potential restorers in the T(0) generation itself leading to significant savings in time, cost and labour. Single copy restored plants with high pollen viability were selfed to segregate male sterile (barnase) and restorer (barstar) lines in the T(1) progeny which could subsequently be diversified into appropriate combiners for heterosis breeding. This strategy will be particularly useful for crop plants where poor transformation frequencies and/or lengthy transformation protocols are a major limitation.