WRKY domain-encoding genes of a crop legume chickpea (Cicer arietinum): comparative analysis with Medicago truncatula WRKY family and characterization of group-III gene(s)

Unlocking nature's secrets: The pivotal role of WRKY transcription factors in plant flowering and fruit development

The WRKY transcription factor family is a key player in the regulatory mechanisms of flowering plants, significantly influencing both their biotic and abiotic response systems as well as being vital to numerous physiological and biological functions. Over the past two decades, the functionality of WRKY proteins has been the subject of extensive research in over 50 plant species, with a strong focus on their roles in responding to various stresses. Despite this extensive research, there remains a notable gap in comprehensive studies aimed at understanding how specific WRKY genes directly influence the timing of flowering and fruit development. This review offers an up-to-date look at WRKY family genes and provides insights into the key genes of WRKY to control flowering, enhance fruit ripening and secondary metabolism synthesis, and maintain fruit quality of various plants, including annuals, perennials, medicinal, and crop plants. The WRKY transcription factors serve as critical regulators within the transcriptional regulatory network, playing a crucial role in the precise enhancement of flowering processes. It is also involved in the up-regulation of fruit ripening was strongly demonstrated by combined transcriptomics and metabolomic investigation. Therefore, we speculated that the WRKY family is known to be a key regulator of flowering and fruiting in plants. This detailed insight will enable the identification of the series of molecular occurrences featuring WRKY proteins throughout the stages of flowering and fruiting.

Comprehensive Expression Analysis of the WRKY Gene Family in Phoebe bournei under Drought and Waterlogging Stresses

In response to biotic and abiotic stresses, the WRKY gene family plays a crucial role in plant growth and development. This study focused on Phoebe bournei and involved genome-wide identification of WRKY gene family members, clarification of their molecular evolutionary characteristics, and comprehensive mapping of their expression profiles under diverse abiotic stress conditions. A total of 60 WRKY gene family members were identified, and their phylogenetic classification revealed three distinct groups. A conserved motif analysis underscored the significant conservation of motif 1 and motif 2 among the majority of PbWRKY proteins, with proteins within the same class sharing analogous gene structures. Furthermore, an examination of cis-acting elements and protein interaction networks revealed several genes implicated in abiotic stress responses in P. bournei. Transcriptomic data were utilized to analyze the expression patterns of WRKY family members under drought and waterlogged conditions, with subsequent validation by quantitative real-time PCR (RT-qPCR) experiments. Notably, PbWRKY55 exhibited significant expression modulation under drought stress; PbWRKY36 responded prominently to waterlogging stress; and PbWRKY18, PbWRKY38, and PbWRKY57 demonstrated altered expression under both drought and waterlogging stresses. This study revealed the PbWRKY candidate genes that potentially play a pivotal role in enhancing abiotic stress resilience in P. bournei. The findings have provided valuable insights and knowledge that can guide further research aimed at understanding and addressing the impacts of abiotic stress within this species.

Transcription factor OsWRKY11 induces rice heading at low concentrations but inhibits rice heading at high concentrations

The heading date of rice is a crucial agronomic characteristic that influences its adaptability to different regions and its productivity potential. Despite the involvement of WRKY transcription factors in various biological processes related to development, the precise mechanisms through which these transcription factors regulate the heading date in rice have not been well elucidated. The present study identified OsWRKY11 as a WRKY transcription factor which exhibits a pivotal function in the regulation of the heading date in rice through a comprehensive screening of a clustered regularly interspaced palindromic repeats (CRISPR) ‒ CRISPR-associated nuclease 9 mutant library that specifically targets the WRKY genes in rice. The heading date of oswrky11 mutant plants and OsWRKY11-overexpressing plants was delayed compared with that of the wild-type plants under short-day and long-day conditions. Mechanistic investigation revealed that OsWRKY11 exerts dual effects on transcriptional promotion and suppression through direct and indirect DNA binding, respectively. Under normal conditions, OsWRKY11 facilitates flowering by directly inducing the expression of OsMADS14 and OsMADS15. The presence of elevated levels of OsWRKY11 protein promote formation of a ternary protein complex involving OsWRKY11, Heading date 1 (Hd1), and Days to heading date 8 (DTH8), and this complex then suppresses the expression of Ehd1, which leads to a delay in the heading date. Subsequent investigation revealed that a mild drought condition resulted in a modest increase in OsWRKY11 expression, promoting heading. Conversely, under severe drought conditions, a significant upregulation of OsWRKY11 led to the suppression of Ehd1 expression, ultimately causing a delay in heading date. Our findings uncover a previously unacknowledged mechanism through which the transcription factor OsWRKY11 exerts a dual impact on the heading date by directly and indirectly binding to the promoters of target genes.

WRKY transcription factors modulate flowering time and response to environmental changes

WRKY transcription factors (TFs), originating in green algae, regulate flowering time and responses to environmental changes in plants. However, the molecular mechanisms underlying the role of WRKY TFs in the correlation between flowering time and environmental changes remain unclear. Therefore, this review summarizes the association of WRKY TFs with flowering pathways to accelerate or delay flowering. WRKY TFs are implicated in phytohormone pathways, such as ethylene, auxin, and abscisic acid pathways, to modulate flowering time. WRKY TFs can modulate salt tolerance by regulating flowering time. WRKY TFs exhibit functional divergence in modulating environmental changes and flowering time. In summary, WRKY TFs are involved in complex pathways and modulate response to environmental changes, thus regulating flowering time.

Plant salinity stress, sensing, and its mitigation through WRKY

Salinity or salt stress has deleterious effects on plant growth and development. It imposes osmotic, ionic, and secondary stresses, including oxidative stress on the plants and is responsible for the reduction of overall crop productivity and therefore challenges global food security. Plants respond to salinity, by triggering homoeostatic mechanisms that counter salt-triggered disturbances in the physiology and biochemistry of plants. This involves the activation of many signaling components such as SOS pathway, ABA pathway, and ROS and osmotic stress signaling. These biochemical responses are accompanied by transcriptional modulation of stress-responsive genes, which is mostly mediated by salt-induced transcription factor (TF) activity. Among the TFs, the multifaceted significance of WRKY proteins has been realized in many diverse avenues of plants' life including regulation of plant stress response. Therefore, in this review, we aimed to highlight the significance of salinity in a global perspective, the mechanism of salt sensing in plants, and the contribution of WRKYs in the modulation of plants' response to salinity stress. This review will be a substantial tool to investigate this problem in different perspectives, targeting WRKY and offering directions to better manage salinity stress in the field to ensure food security.

Functions of WRKYs in plant growth and development

As sessile organisms, plants must overcome various stresses. Accordingly, they have evolved several plant-specific growth and developmental processes. These plant processes may be related to the evolution of plant-specific protein families. The WRKY transcription factors originated in eukaryotes and expanded in plants, but are not present in animals. Over the past two decades, there have been many studies on WRKYs in plants, with much of the research concentrated on their roles in stress responses. Nevertheless, recent findings have revealed that WRKYs are also required for seed dormancy and germination, postembryonic morphogenesis, flowering, gametophyte development, and seed production. Thus, WRKYs may be important for plant adaptations to a sessile lifestyle because they simultaneously regulate stress resistance and plant-specific growth and development.

Genome-wide identification and bioinformatics analysis of the WRKY transcription factors and screening of candidate genes for anthocyanin biosynthesis in azalea (Rhododendron simsii)

WRKY transcription factors have been demonstrated to influence the anthocyanin biosynthesis in many plant species. However, there is limited knowledge about the structure and function of WRKY genes in the major ornamental plant azalea (Rhododendron simsii). In this study, we identified 57 RsWRKY genes in the R. simsii genome and classified them into three main groups and several subgroups based on their structural and phylogenetic characteristics. Comparative genomic analysis suggested WRKY gene family has significantly expanded during plant evolution from lower to higher species. Gene duplication analysis indicated that the expansion of the RsWRKY gene family was primarily due to whole-genome duplication (WGD). Additionally, selective pressure analysis (Ka/Ks) suggested that all RsWRKY duplication gene pairs underwent purifying selection. Synteny analysis indicated that 63 and 24 pairs of RsWRKY genes were orthologous to Arabidopsis thaliana and Oryza sativa, respectively. Furthermore, RNA-seq data was used to investigate the expression patterns of RsWRKYs, revealing that 17 and 9 candidate genes may be associated with anthocyanin synthesis at the bud and full bloom stages, respectively. These findings provide valuable insights into the molecular mechanisms underlying anthocyanin biosynthesis in Rhododendron species and lay the foundation for future functional studies of WRKY genes.

The nuclear effector ArPEC25 from the necrotrophic fungus Ascochyta rabiei targets the chickpea transcription factor CaβLIM1a and negatively modulates lignin biosynthesis, increasing host susceptibility

Fungal pathogens deploy a barrage of secreted effectors to subvert host immunity, often by evading, disrupting, or altering key components of transcription, defense signaling, and metabolic pathways. However, the underlying mechanisms of effectors and their host targets are largely unexplored in necrotrophic fungal pathogens. Here, we describe the effector protein Ascochyta rabiei PEXEL-like Effector Candidate 25 (ArPEC25), which is secreted by the necrotroph A. rabiei, the causal agent of Ascochyta blight disease in chickpea (Cicer arietinum), and is indispensable for virulence. After entering host cells, ArPEC25 localizes to the nucleus and targets the host LIM transcription factor CaβLIM1a. CaβLIM1a is a transcriptional regulator of CaPAL1, which encodes phenylalanine ammonia lyase (PAL), the regulatory, gatekeeping enzyme of the phenylpropanoid pathway. ArPEC25 inhibits the transactivation of CaβLIM1a by interfering with its DNA-binding ability, resulting in negative regulation of the phenylpropanoid pathway and decreased levels of intermediates of lignin biosynthesis, thereby suppressing lignin production. Our findings illustrate the role of fungal effectors in enhancing virulence by targeting a key defense pathway that leads to the biosynthesis of various secondary metabolites and antifungal compounds. This study provides a template for the study of less explored necrotrophic effectors and their host target functions.

The WRKY transcription factor family in cowpea: Genomic characterization and transcriptomic profiling under root dehydration

Cowpea [Vigna unguiculata (L.) Walp.] is one of the most tolerant legume crops to drought and salt stresses. WRKY transcription factor (TF) family members stand out among plant transcriptional regulators related to abiotic stress tolerance. However, little information is currently available on the expression of the cowpea WRKY gene family (VuWRKY) in response to water deficit. Thus, we analyzed genomic and transcriptomic data from cowpea to identify VuWRKY members and characterize their structure and transcriptional response under root dehydration stress. Ninety-two complete VuWRKY genes were found in the cowpea genome based on their domain characteristics. They were clustered into three groups: I (15 members), II (58), and III (16), while three genes were unclassified. Domain analysis of the encoded proteins identified four major variants of the conserved heptapeptide motif WRKYGQK. In silico analysis of VuWRKY gene promoters identified eight candidate binding motifs of cis-regulatory elements, regulated mainly by six TF families associated with abiotic stress responses. Ninety-seven VuWRKY modulated splicing variants associated with 55 VuWRKY genes were identified via RNA-Seq analysis available at the Cowpea Genomics Consortium (CpGC) database. qPCR analyses showed that 22 genes are induced under root dehydration, with VuWRKY18, 21, and 75 exhibiting the most significant induction levels. Given their central role in activating signal transduction cascades in abiotic stress response, the data provide a foundation for the targeted modification of specific VuWRKY family members to improve drought tolerance in this important climate-resilient legume in the developing world and beyond.

Characterization of WRKY Gene Family in Whole-Genome and Exploration of Flowering Improvement Genes in Chrysanthemum lavandulifolium

Chrysanthemum is a well-known ornamental plant with numerous uses. WRKY is a large family of transcription factors known for a variety of functions ranging from stress resistance to plant growth and development. Due to the limited research on the WRKY family in chrysanthemums, we examined them for the first time in Chrysanthemum lavandulifolium. A total of 138 ClWRKY genes were identified, which were classified into three groups. Group III in C. lavandulifolium contains 53 members, which is larger than group III of Arabidopsis. The number of introns varied from one to nine in the ClWRKY gene family. The "WRKYGQK" motif is conserved in 118 members, while other members showed slight variations. AuR and GRE responsive cis-acting elements were located in the promoter region of WRKY members, which are important for plant development and flowering induction. In addition, the W box was present in most genes; the recognition site for the WRKY gene may play a role in autoregulation and cross-regulation. The expression of the most variable 19 genes in terms of different parameters was observed at different stages. Among them, 10 genes were selected due to the presence of CpG islands, while nine genes were selected based on their close association with important Arabidopsis genes related to floral traits. ClWRKY36 and ClWRKY45 exhibit differential expression at flowering stages in the capitulum, while methylation is detected in three genes, including ClWRKY31, ClWRKY100, and ClWRKY129. Our results provide a basis for further exploration of WRKY members to find their functions in plant growth and development, especially in flowering traits.

Early wound-responsive cues regulate the expression of WRKY family genes in chickpea differently under wounded and unwounded conditions

Insect wounding activates a large number of signals that function coordinately to modulate gene expression and elicit defense responses. How each signal influences gene expression in absence of wounding is also important since it can shed light on changes occurring during the shift to wound response. Using simulated Helicoverpa armigera herbivory on chickpea, we had identified at least 14 WRKY genes that showed 5-50 fold increase in expression within 5-20 min of wounding. Our studies show that contrary to their collective effects upon wounding, individual chemical cues show distinct and often opposite effects in absence of wounding. In particular, jasmonic acid, a key early defense hormone, reduced transcripts of most WRKY genes by > 50% upon treatment of unwounded chickpea leaves as did salicylic acid. Neomycin (a JA biosynthesis inhibitor) delayed and also reduced early wound expression. H₂O₂ transiently activated several genes within 5-20 min by 5-8 fold while ethylene activated only a few WRKY genes by 2-5 fold. The summation of the individual effects of these chemical cues does not explain the strong increase in transcript levels upon wounding. Detailed studies of a 931 nt region of the CaWRKY41 promoter, show strong wound-responsive GUS expression in Arabidopsis even in presence of neomycin. Surprisingly its expression was lost in the coi1, ein2 and myc2myc3myc4 mutant backgrounds suggesting the requirement of intact ethylene and JA signaling pathways (dependent on MYCs) for wound-responsive expression. The studies highlight the complexity of gene regulation by different chemical cues in the presence and absence of wounding.

SUPPLEMENTARY INFORMATION

The online version contains Supplementary material available at 10.1007/s12298-022-01170-y.

Genome-wide identification and expression analysis of the GRAS gene family in response to drought stress in chickpea (Cicer arietinum L.)

The GRAS (gibberellic acid insensitive, repressor of GAI and scarecrow) transcription factors (TFs) regulate diverse biological processes involved in plant growth and development. These TFs are also known to regulate gene expression in response to various abiotic stress factors like cold, drought, etc. In chickpea one of the most devastating abiotic stress factors is terminal drought. The GRAS TF family has not been characterized in chickpea (Cicer arietinum L.) until now. In this study, we report 46 GRAS TF genes (CaGRAS genes) in the chickpea genome. The CaGRAS proteins were categorized into nine subfamilies based on their phylogenetic relationship with known GRAS members of Arabidopsis and soybean. The PAT subfamily was the largest consisting of ten CaGRAS members whereas the LAS subfamily was the smallest with only one member. Gene duplication analysis revealed that segmental duplication was the primary reason for the expansion of this gene family within the chickpea genome. The gene expression levels of CaGRAS genes were analysed using two different chickpea varieties contrasting for drought tolerance trait, i.e., ICC 4958 (drought tolerant) and ICC 1882 (drought sensitive). On exposure to drought stress, the two chickpea genotypes, exhibited differential drought response, which was quantified and estimated in terms of differences in leaf relative water content (RWC). The well-watered or control plants of the drought tolerant variety were able to maintain a higher leaf RWC by the end of the drought stress period, whereas the control plants of the drought sensitive variety continued to show a decline in leaf RWC. The two genotypes also differed in their root morphologies, under well-watered and drought stress conditions. The gene expression analysis revealed a potential role of PAT, SCR, SCL3 and SHR GRAS members in the regulation of differential response to drought, in the root tissues, for both the genotypes. CaGRAS 12 (SCR) was identified as a drought-responsive GRAS TF gene, which could serve as a potential candidate gene for utilization in developing chickpea varieties with improved drought tolerance. This study demonstrates the drought-responsive expression of CaGRAS genes in chickpea and also describes the morpho-physiological response of chickpea plants to drought stress conditions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03104-z.

Genomics Associated Interventions for Heat Stress Tolerance in Cool Season Adapted Grain Legumes

Cool season grain legumes occupy an important place among the agricultural crops and essentially provide multiple benefits including food supply, nutrition security, soil fertility improvement and revenue for farmers all over the world. However, owing to climate change, the average temperature is steadily rising, which negatively affects crop performance and limits their yield. Terminal heat stress that mainly occurred during grain development phases severely harms grain quality and weight in legumes adapted to the cool season, such as lentils, faba beans, chickpeas, field peas, etc. Although, traditional breeding approaches with advanced screening procedures have been employed to identify heat tolerant legume cultivars. Unfortunately, traditional breeding pipelines alone are no longer enough to meet global demands. Genomics-assisted interventions including new-generation sequencing technologies and genotyping platforms have facilitated the development of high-resolution molecular maps, QTL/gene discovery and marker-assisted introgression, thereby improving the efficiency in legumes breeding to develop stress-resilient varieties. Based on the current scenario, we attempted to review the intervention of genomics to decipher different components of tolerance to heat stress and future possibilities of using newly developed genomics-based interventions in cool season adapted grain legumes.

QTL dissection and mining of candidate genes for Ascochyta fabae and Orobanche crenata resistance in faba bean (Vicia faba L.)

BACKGROUND

Ascochyta blight caused by Ascochyta fabae Speg. and broomrape (Orobanche crenata) are among the economically most significant pathogens of faba bean. Several QTLs conferring resistance against the two pathogens have been identified and validated in different genetic backgrounds. The aim of this study was to saturate the most stable QTLs for ascochyta and broomrape resistance in two Recombinant Inbred Line (RIL) populations, 29H x Vf136 and Vf6 x Vf136, to identify candidate genes conferring resistance against these two pathogens.

RESULTS

We exploited the synteny between faba bean and the model species Medicago truncatula by selecting a set of 219 genes encoding putative WRKY transcription factors and defense related proteins falling within the target QTL intervals, for genotyping and marker saturation in the two RIL populations. Seventy and 50 of the candidate genes could be mapped in 29H x Vf136 and Vf6 x Vf136, respectively. Besides the strong reduction of the QTL intervals, the mapping process allowed replacing previous dominant and pedigree-specific RAPD flanking markers with robust and transferrable SNP markers, revealing promising candidates for resistance against the two pathogens.

CONCLUSIONS

Although further efforts in association mapping and expression studies will be required to corroborate the candidate genes for resistance, the fine-mapping approach proposed here increases the genetic resolution of relevant QTL regions and paves the way for an efficient deployment of useful alleles for faba bean ascochyta and broomrape resistance through marker-assisted breeding.

Genome-wide identification and expression analysis of the WRKY transcription factor family in flax (Linum usitatissimum L.)

BACKGROUND

Members of the WRKY protein family, one of the largest transcription factor families in plants, are involved in plant growth and development, signal transduction, senescence, and stress resistance. However, little information is available about WRKY transcription factors in flax (Linum usitatissimum L.).

RESULTS

In this study, comprehensive genome-wide characterization of the flax WRKY gene family was conducted that led to prediction of 102 LuWRKY genes. Based on bioinformatics-based predictions of structural and phylogenetic features of encoded LuWRKY proteins, 95 LuWRKYs were classified into three main groups (Group I, II, and III); Group II LuWRKYs were further assigned to five subgroups (IIa-e), while seven unique LuWRKYs (LuWRKYs 96-102) could not be assigned to any group. Most LuWRKY proteins within a given subgroup shared similar motif compositions, while a high degree of motif composition variability was apparent between subgroups. Using RNA-seq data, expression patterns of the 102 predicted LuWRKY genes were also investigated. Expression profiling data demonstrated that most genes associated with cellulose, hemicellulose, or lignin content were predominantly expressed in stems, roots, and less in leaves. However, most genes associated with stress responses were predominantly expressed in leaves and exhibited distinctly higher expression levels in developmental stages 1 and 8 than during other stages.

CONCLUSIONS

Ultimately, the present study provides a comprehensive analysis of predicted flax WRKY family genes to guide future investigations to reveal functions of LuWRKY proteins during plant growth, development, and stress responses.

Developing Climate-Resilient Chickpea Involving Physiological and Molecular Approaches With a Focus on Temperature and Drought Stresses

Chickpea is one of the most economically important food legumes, and a significant source of proteins. It is cultivated in more than 50 countries across Asia, Africa, Europe, Australia, North America, and South America. Chickpea production is limited by various abiotic stresses (cold, heat, drought, salt, etc.). Being a winter-season crop in northern south Asia and some parts of the Australia, chickpea faces low-temperature stress (0-15°C) during the reproductive stage that causes substantial loss of flowers, and thus pods, to inhibit its yield potential by 30-40%. The winter-sown chickpea in the Mediterranean, however, faces cold stress at vegetative stage. In late-sown environments, chickpea faces high-temperature stress during reproductive and pod filling stages, causing considerable yield losses. Both the low and the high temperatures reduce pollen viability, pollen germination on the stigma, and pollen tube growth resulting in poor pod set. Chickpea also experiences drought stress at various growth stages; terminal drought, along with heat stress at flowering and seed filling can reduce yields by 40-45%. In southern Australia and northern regions of south Asia, lack of chilling tolerance in cultivars delays flowering and pod set, and the crop is usually exposed to terminal drought. The incidences of temperature extremes (cold and heat) as well as inconsistent rainfall patterns are expected to increase in near future owing to climate change thereby necessitating the development of stress-tolerant and climate-resilient chickpea cultivars having region specific traits, which perform well under drought, heat, and/or low-temperature stress. Different approaches, such as genetic variability, genomic selection, molecular markers involving quantitative trait loci (QTLs), whole genome sequencing, and transcriptomics analysis have been exploited to improve chickpea production in extreme environments. Biotechnological tools have broadened our understanding of genetic basis as well as plants' responses to abiotic stresses in chickpea, and have opened opportunities to develop stress tolerant chickpea.

Arabidopsis WRKY53, a Node of Multi-Layer Regulation in the Network of Senescence

Leaf senescence is an integral part of plant development aiming at the remobilization of nutrients and minerals out of the senescing tissue into developing parts of the plant. Sequential as well as monocarpic senescence maximize the usage of nitrogen, mineral, and carbon resources for plant growth and the sake of the next generation. However, stress-induced premature senescence functions as an exit strategy to guarantee offspring under long-lasting unfavorable conditions. In order to coordinate this complex developmental program with all kinds of environmental input signals, complex regulatory cues have to be in place. Major changes in the transcriptome imply important roles for transcription factors. Among all transcription factor families in plants, the NAC and WRKY factors appear to play central roles in senescence regulation. In this review, we summarize the current knowledge on the role of WRKY factors with a special focus on WRKY53. In contrast to a holistic multi-omics view we want to exemplify the complexity of the network structure by summarizing the multilayer regulation of WRKY53 of Arabidopsis.

Understanding the Effect of Structural Diversity in WRKY Transcription Factors on DNA Binding Efficiency through Molecular Dynamics Simulation

A precise understanding of the molecular mechanism involved in stress conditions has great importance for crop improvement. Biomolecules, such as WRKY proteins, which are the largest transcription factor family that is widely distributed in higher plants, plays a significant role in plant defense response against various biotic and abiotic stressors. In the present study, an extensive homology-based three-dimensional model construction and subsequent interaction study of WRKY DNA-binding domain (DBD) in CcWRKY1 (Type I), CcWRKY51 (Type II), and CcWRKY70 (Type III) belonging to pigeonpea, a highly tolerant crop species, was performed. Evaluation of the generated protein models was done to check their reliability and accuracy based on the quantitative and qualitative parameters. The final model was subjected to investigate the comparative binding analysis of different types of WRKY–DBD with DNA-W-box (a cis-acting element) by protein–DNA docking and molecular dynamics (MD) simulation. The DNA binding specificity with WRKY variants was scrutinized through protein–DNA interaction using the HADDOCK server. The stability, as well as conformational changes of protein–DNA complex, was investigated through molecular dynamics (MD) simulations for 100 ns using GROMACS. Additionally, the comparative stability and dynamic behavior of each residue of the WRKY–DBD type were analyzed in terms of root mean square deviation (RMSD), root mean square fluctuation (RMSF)values of the backbone atoms for each frame taking the minimized structure as a reference. The details of DNA binding activity of three different types of WRKY–DBD provided here will be helpful to better understand the regulation of WRKY gene family members in plants.

Systems Biology Approaches Reveal a Multi-stress Responsive WRKY Transcription Factor and Stress Associated Gene Co-expression Networks in Chickpea

Background:

Chickpea is a nutritional rich premier pulse crop but its production encounters setbacks due to various stresses and understanding of molecular mechanisms can be ascribed foremost importance.

Objective:

The investigation was carried out to identify the differentially expressed WRKY TFs in chickpea in response to herbicide stress and decipher their interacting partners.

Methods:

For this purpose, transcriptome wide identification of WRKY TFs in chickpea was done. Behavior of the differentially expressed TFs was compared between other stress conditions. Orthology based cofunctional gene networks were derived from Arabidopsis. Gene ontology and functional enrichment analysis was performed using Blast2GO and STRING software. Gene Coexpression Network (GCN) was constructed in chickpea using publicly available transcriptome data. Expression pattern of the identified gene network was studied in chickpea-Fusarium interactions.

Results:

A unique WRKY TF (Ca_08086) was found to be significantly (q value = 0.02) upregulated not only under herbicide stress but also in other stresses. Co-functional network of 14 genes, namely Ca_08086, Ca_19657, Ca_01317, Ca_20172, Ca_12226, Ca_15326, Ca_04218, Ca_07256, Ca_14620, Ca_12474, Ca_11595, Ca_15291, Ca_11762 and Ca_03543 were identified. GCN revealed 95 hub genes based on the significant probability scores. Functional annotation indicated role in callose deposition and response to chitin. Interestingly, contrasting expression pattern of the 14 network genes was observed in wilt resistant and susceptible chickpea genotypes, infected with Fusarium.

Conclusion:

This is the first report of identification of a multi-stress responsive WRKY TF and its associated GCN in chickpea.

Understanding the Role of the WRKY Gene Family under Stress Conditions in Pigeonpea (Cajanus Cajan L.)

Pigeonpea (Cajanus cajan L.), a protein-rich legume, is a major food component of the daily diet for residents in semi-arid tropical regions of the word. Pigeonpea is also known for its high level of tolerance against biotic and abiotic stresses. In this regard, understanding the genes involved in stress tolerance has great importance. In the present study, identification, and characterization of WRKY, a large transcription factor gene family involved in numerous biological processes like seed germination, metabolism, plant growth, biotic and abiotic stress responses was performed in pigeonpea. A total of 94 WRKY genes identified in the pigeonpea genome were extensively characterized for gene-structures, localizations, phylogenetic distribution, conserved motif organizations, and functional annotation. Phylogenetic analysis revealed three major groups (I, II, and III) of pigeonpea WRKY genes. Subsequently, expression profiling of 94 CcWRKY genes across different tissues like root, nodule, stem, petiole, petal, sepal, shoot apical meristem (SAM), mature pod, and mature seed retrieved from the available RNAseq data identified tissue-specific WRKY genes with preferential expression in the vegetative and reproductive stages. Gene co-expression networks identified four WRKY genes at the center of maximum interaction which may play a key role in the entire WRKY regulations. Furthermore, quantitative real-time polymerase chain reaction (qRT-PCR) expression analysis of WRKY genes in root and leaf tissue samples from plants under drought and salinity stress identified differentially expressed WRKY genes. The study will be helpful to understand the evolution, regulation, and distribution of the WRKY gene family, and additional exploration for the development of stress tolerance cultivars in pigeonpea and other legumes crops.

CaMPK9 increases the stability of CaWRKY40 transcription factor which triggers defense response in chickpea upon Fusarium oxysporum f. sp. ciceri Race1 infection

Physical interaction and phosphorylation by CaMPK9 protects the degradation of CaWRKY40 that induces resistance response in chickpea to Fusarium wilt disease by modulating the transcription of defense responsive genes. WRKY transcription factors (TFs) are the global regulators of plant defense signaling that modulate immune responses in host plants by regulating transcription of downstream target genes upon challenged by pathogens. However, very little is known about immune responsive role of Cicer arietinum L. (Ca) WRKY TFs particularly. Using two contrasting chickpea genotypes with respect to resistance against Fusarium oxysporum f. sp. ciceri Race1 (Foc1), we demonstrate transcript accumulation of different CaWRKYs under multiple stresses and establish that CaWRKY40 triggers defense. CaWRKY40 overexpressing chickpea mounts resistance to Foc1 by positively modulating the defense related gene expression. EMSA, ChIP assay and real-time PCR analyses suggest CaWRKY40 binds at the promoters and positively regulates transcription of CaDefensin and CaWRKY33. Further studies revealed that mitogen Activated Protein Kinase9 (CaMPK9) phosphorylates CaWRKY40 by directly interacting with its two canonical serine residues. Interestingly, CaMPK9 is unable to interact with CaWRKY40 when the relevant two serine residues were replaced by alanine. Overexpression of serine mutated WRKY40 isoform in chickpea fails to provide resistance against Foc1. Mutated WRKY40^{Ser.224/225 to AA} overexpressing chickpea resumes its ability to confer resistance against Foc1 after application of 26S proteasomal inhibitor MG132, suggests that phosphorylation is essential to protect CaWRKY40 from proteasomal degradation. CaMPK9 silencing also led to susceptibility in chickpea to Foc1. Altogether, our results elucidate positive regulatory roles of CaMPK9 and CaWRKY40 in modulating defense response in chickpea upon Foc1 infection.

WRKY transcription factors in legumes

BACKGROUND

WRKY transcription factors, so named because of the WRKYGQK heptapeptide at the N-terminal end, are widely distributed in plants and play an important role in physiological changes and response to biotic and abiotic stressors. Many previous studies have focused on the evolution of WRKY transcription factors in a given plant; however, little is known about WRKY evolution in legumes. The gene expression pattern of duplicated WRKY transcription factors remains unclear.

RESULTS

We first identified the WRKY proteins in 12 legumes. We found that the WRKYGQK heptapeptide tended to mutate into WRKYGKK. The Q site in WRKYGQK preferentially mutated, while W, K, and Y were conserved. The phylogenetic tree shows that the WRKY proteins in legumes have multiple origins, especially group IIc. For example, WRKY64 from Lupinus angustifolius (LaWRKY64) contains three WRKY domains, of which the first two clustered together in the N-terminal WRKY domain of the group I WRKY protein, and the third WRKY domain grouped in the C-terminal WRKY domain of the group I WRKY protein. Orthologous WRKY genes have a faster evolutionary rate and are subject to constrained selective pressure, unlike paralogous WRKY genes. Different gene features were observed between duplicated WRKY genes and singleton WRKY genes. Duplicated Glycine max WRKY genes with similar gene features have gene expression divergence.

CONCLUSIONS

We analyzed the WRKY number and type in 12 legumes, concluding that the WRKY proteins have multiple origins. A novel WRKY protein, LaWRKY64, was found in L. angustifolius. The first two WRKY domains of LaWRKY64 have the same origin. The orthologous and paralogous WRKY proteins have different evolutionary rates. Duplicated WRKY genes have gene expression divergence under normal growth conditions in G. max. These results provide insight into understanding WRKY evolution and expression.

mQTL-seq and classical mapping implicates the role of an AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED (AHL) family gene in Ascochyta blight resistance of chickpea

Ascochyta blight (AB) caused by the fungal pathogen Ascochyta rabiei is a serious foliar disease of chickpea (Cicer arietinum L.). Despite many genetic studies on chickpea-Ascochyta interaction, genome-wide scan of chickpea for the identification of AB-associated quantitative trait loci (QTLs) and their gene(s) has not been accomplished. To elucidate narrow QTLs for AB resistance, here, we report the use of multiple QTL-sequencing approach on 2 sets of extreme AB phenotype bulks derived from Cicer intraspecific and interspecific crosses. Two major QTLs, qABR4.1 and qABR4.2, and a minor QTL, qABR4.3, were identified on assembled chickpea pseudomolecule 4. We narrowed qABR4.1 to a "robust region" at 4.568-4.618 Mb through mapping on a larger intraspecific cross-derived population and comparative analysis. Among 4 genes, the CaAHL18 gene showed higher expression under Ascochyta stress in AB resistant parent suggesting that it is the candidate gene under "robust qABR4.1." Dual-luciferase assay with CaAHL18 polymorphic cis-regulatory sequences showed that allelic variation is associated with higher expression. Thus, our findings on chickpea-Ascochyta interaction have narrowed down AB resistance associated QTLs on chickpea physical map. The narrowed QTLs and gene-associated markers will help in biotechnological and breeding programs for chickpea improvement.

Soil environment is a key driver of adaptation in Medicago truncatula: new insights from landscape genomics

Spatial differences in environmental selective pressures interact with the genomes of organisms, ultimately leading to local adaptation. Landscape genomics is an emergent research area that uncovers genome-environment associations, thus allowing researchers to identify candidate loci for adaptation to specific environmental variables. In the present study, we used latent factor mixed models (LFMMs) and Moran spectral outlier detection/randomization (MSOD-MSR) to identify candidate loci for adaptation to 10 environmental variables (climatic, soil and atmospheric) among 43 515 single nucleotide polymorphisms (SNPs) from 202 accessions of the model legume Medicago truncatula. Soil variables were associated with a large number of candidate loci identified through both LFMMs and MSOD-MSR. Genes tagged by candidate loci associated with drought and salinity are involved in the response to biotic and abiotic stresses, while those tagged by candidates associated with soil nitrogen and atmospheric nitrogen, participate in the legume-rhizobia symbiosis. Candidate SNPs identified through both LFMMs and MSOD-MSR explained up to 56% of variance in flowering traits. Our findings highlight the importance of soil in driving adaptation in the system and elucidate the basis of evolutionary potential of M. truncatula to respond to global climate change and anthropogenic disruption of the nitrogen cycle.

Current advances in chickpea genomics: applications and future perspectives

Chickpea genomics promises to illuminate our understanding of genome organization, structural variations, evolutionary and domestication-related insights and fundamental biology of legume crops. Unprecedented advancements of next generation sequencing (NGS) technologies have enabled in decoding of multiple chickpea genome sequences and generating huge genomic resources in chickpea both at functional and structural level. This review is aimed to update the current progress of chickpea genomics ranging from high density linkage map development, genome-wide association studies (GWAS), functional genomics resources for various traits, emerging role of abiotic stress responsive coding and non-coding RNAs after the completion of draft chickpea genome sequences. Additionally, the current efforts of whole genome re-sequencing (WGRS) approach of global chickpea germplasm to capture the global genetic diversity existing in the historically released varieties across the world and increasing the resolution of the previously identified candidate gene(s) of breeding importance have been discussed. Thus, the outcomes of these genomics resources will assist in genomics-assisted selection and facilitate breeding of climate-resilient chickpea cultivars for sustainable agriculture.

Differential Regulation of Genes Involved in Root Morphogenesis and Cell Wall Modification is Associated with Salinity Tolerance in Chickpea

Salinity is a major constraint for intrinsically salt sensitive grain legume chickpea. Chickpea exhibits large genetic variation amongst cultivars, which show better yields in saline conditions but still need to be improved further for sustainable crop production. Based on previous multi-location physiological screening, JG 11 (salt tolerant) and ICCV 2 (salt sensitive) were subjected to salt stress to evaluate their physiological and transcriptional responses. A total of ~480 million RNA-Seq reads were sequenced from root tissues which resulted in identification of 3,053 differentially expressed genes (DEGs) in response to salt stress. Reproductive stage shows high number of DEGs suggesting major transcriptional reorganization in response to salt to enable tolerance. Importantly, cationic peroxidase, Aspartic ase, NRT1/PTR, phosphatidylinositol phosphate kinase, DREB1E and ERF genes were significantly up-regulated in tolerant genotype. In addition, we identified a suite of important genes involved in cell wall modification and root morphogenesis such as dirigent proteins, expansin and casparian strip membrane proteins that could potentially confer salt tolerance. Further, phytohormonal cross-talk between ERF and PIN-FORMED genes which modulate the root growth was observed. The gene set enrichment analysis and functional annotation of these genes suggests they may be utilised as potential candidates for improving chickpea salt tolerance.

Structural modelling and molecular dynamics of a multi-stress responsive WRKY TF-DNA complex towards elucidating its role in stress signalling mechanisms in chickpea

Chickpea is a premier food legume crop with high nutritional quality and attains prime importance in the current era of 795 million people being undernourished worldwide. Chickpea production encounters setbacks due to various stresses and understanding the role of key transcription factors (TFs) involved in multiple stresses becomes inevitable. We have recently identified a multi-stress responsive WRKY TF in chickpea. The present study was conducted to predict the structure of WRKY TF to identify the DNA-interacting residues and decipher DNA-protein interactions. Comparative modelling approach produced 3D model of the WRKY TF with good stereochemistry, local/global quality and further revealed W19, R20, K21, and Y22 motifs within a vicinity of 5 Å to the DNA amongst R18, G23, Q24, K25, Y36, Y37, R38 and K47 and these positions were equivalent to the 2LEX WRKY domain of Arabidopsis. Molecular simulations analysis of reference protein -PDB ID 2LEX, along with Car-WRKY TF modelled structure with the DNA coordinates derived from PDB ID 2LEX and docked using HADDOCK were executed. Root Mean Square (RMS) Deviation and RMS Fluctuation values yielded consistently stable trajectories over 50 ns simulation. Strengthening the obtained results, neither radius of gyration, distance and total energy showed any signs of DNA-WRKY complex falling apart nor any significant dissociation event over 50 ns run. Therefore, the study provides first insights into the structural properties of multi-stress responsive WRKY TF-DNA complex in chickpea, enabling genome wide identification of TF binding sites and thereby deciphers their gene regulatory networks.

Genome-wide identification of the potato WRKY transcription factor family

WRKY transcription factors play pivotal roles in regulation of stress responses. This study identified 79 WRKY genes in potato (Solanum tuberosum). Based on multiple sequence alignment and phylogenetic relationships, WRKY genes were classified into three major groups. The majority of WRKY genes belonged to Group II (52 StWRKYs), Group III had 14 and Group I consisted of 13. The phylogenetic tree further classified Group II into five sub-groups. All StWRKY genes except StWRKY79 were mapped on potato chromosomes, with eight tandem duplication gene pairs and seven segmental duplication gene pairs found from StWRKY family genes. The expression analysis of 22 StWRKYs showed their differential expression levels under various stress conditions. Cis-element prediction showed that a large number of elements related to drought, heat and salicylic acid were present in the promotor regions of StWRKY genes. The expression analysis indicated that seven StWRKYs seemed to respond to stress (heat, drought and salinity) and salicylic acid treatment. These genes are candidates for abiotic stress signaling for further research.

Phylogenomic analysis of MKKs and MAPKs from 16 legumes and detection of interacting pairs in chickpea divulge MAPK signalling modules

The mitogen-activated protein kinase (MAPK)-mediated phosphorylation cascade is a vital component of plant cellular signalling. Despite this, MAPK signalling cascade is less characterized in crop legumes. To fill this void, we present here a comprehensive phylogeny of MAPK kinases (MKKs) and MAPKs identified from 16 legume species belonging to genistoid (Lupinus angustifolius), dalbergioid (Arachis spp.), phaseoloid (Glycine max, Cajanus cajan, Phaseolus vulgaris, and Vigna spp.), and galegoid (Cicer arietinum, Lotus japonicus, Medicago truncatula, Pisum sativum, Trifolium spp., and Vicia faba) clades. Using the genes of the diploid crop chickpea (C. arietinum), an exhaustive interaction analysis was performed between MKKs and MAPKs by split-ubiquitin based yeast two-hybrid (Y2H). Twenty seven interactions of varying strengths were identified between chickpea MKKs and MAPKs. These interactions were verified in planta by bimolecular fluorescence complementation (BiFC). As a first report in plants, four intra-molecular interactions of weak strength were identified within chickpea MKKs. Additionally; two TEOSINTE-BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factors of class I were identified as novel down-stream interacting partners of seven MAPKs. We propose that this highly reliable MAPK interaction network, presented here for chickpea, can be utilized as a reference for legumes and thus will help in deciphering their role in legume-specific events.

Members of WRKY Group III transcription factors are important in TYLCV defense signaling pathway in tomato (Solanum lycopersicum)

Background

Transmitted by the whitefly Bemisia tabaci, tomato yellow leaf curly virus (TYLCV) has posed serious threats to plant growth and development. Plant innate immune systems against various threats involve WRKY Group III transcription factors (TFs). This group participates as a major component of biological processes in plants.

Results

In this study, 6 WRKY Group III TFs (SolyWRKY41, SolyWRKY42, SolyWRKY53, SolyWRKY54, SolyWRKY80, and SolyWRKY81) were identified, and these TFs responded to TYLCV infection. Subcellular localization analysis indicated that SolyWRKY41 and SolyWRKY54 were nuclear proteins in vivo. Many elements, including W-box, were found in the promoter region of Group III TFs. Interaction network analysis revealed that Group III TFs could interact with other proteins, such as mitogen-activated protein kinase 5 (MAPK) and isochorismate synthase (ICS), to respond to biotic and abiotic stresses. Positive and negative expression patterns showed that WRKY Group III genes could also respond to TYLCV infection in tomato. The DNA content of TYLCV resistant lines after SolyWRKY41 and SolyWRKY54 were subjected to virus-induced gene silencing (VIGS) was lower than that of the control lines.

Conclusions

In the present study, 6 WRKY Group III TFs in tomato were identified to respond to TYLCV infection. Quantitative real-time–polymerase chain reaction (RT-qPCR) and VIGS analyses demonstrated that Group III genes served as positive and negative regulators in tomato–TYLCV interaction. WRKY Group III TFs could interact with other proteins by binding to cis elements existing in the promoter regions of other genes to regulate pathogen-related gene expression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3123-2) contains supplementary material, which is available to authorized users.