CaWRKY30 Positively Regulates Pepper Immunity by Targeting CaWRKY40 against Ralstonia solanacearum Inoculation through Modulating Defense-Related Genes

Comparative identification of WRKY transcription factors and transcriptional response to Ralstonia solanacearum in tomato

In order to study the responses of tomato (Solanum lycopersicum) WRKY TFs to bacterial wilt caused by Ralstonia solanacearum, the most up-to-date genomes and transcriptional profiles were used to identify WRKY TFs in control and infected inbred lines. In total, 85 tomato WRKY TFs were identified and categorized into groups I, IIa + b, IIc, IId + e, and III. These WRKYs, especially those from group IIe, were mainly distributed at chromosome ends and in clusters. More than 45 % and 70 % of tomato WRKYs exhibited intraspecific and interspecific synteny, respectively. Nearly 60 % of tomato WRKYs (mainly in groups I and IIc) formed 73 pairs of orthologs with WRKYs in Arabidopsis and pepper, with Ka/Ks less than 1. Sixteen tomato WRKYs (mainly in groups IIa + b and IIc) responded strongly to biotic stress, and 12 differentially expressed WRKYs (mainly in groups III and IIb) were identified. RT-qPCR revealed that tomato WRKYs could respond to bacterial wilt through positive (predominant) or negative regulation. In particular, the interaction between Solyc03g095770.3 (group III) and Solyc09g014990.4 (group I) may play an important role. In brief, WRKY TFs were comprehensively identified in tomato and several bacterial wilt responsive genes were screened.

Pepper immunity against Ralstonia solanacearum is positively regulated by CaWRKY3 through modulation of different WRKY transcription factors

BACKGROUND

Several WRKY transcription factors (TFs), including CaWRKY6, CaWRKY22, CaWRKY27, and CaWRKY40 are known to govern the resistance of pepper (Capsicum annuum L.) plants to Ralstonia solanacearum infestation (RSI) and other abiotic stresses. However, the molecular mechanisms underlying these processes remain elusive.

METHODS

This study functionally described CaWRKY3 for its role in pepper immunity against RSI. The roles of phytohormones in mediating the expression levels of CaWRKY3 were investigated by subjecting pepper plants to 1 mM salicylic acid (SA), 100 µM methyl jasmonate (MeJA), and 100 µM ethylene (ETH) at 4-leaf stage. A virus-induced gene silencing (VIGS) approach based on the Tobacco Rattle Virus (TRV) was used to silence CaWRKY3 in pepper, and transiently over-expressed to infer its role against RSI.

RESULTS

Phytohormones and RSI increased CaWRKY3 transcription. The transcriptions of defense-associated marker genes, including CaNPR1, CaPR1, CaDEF1, and CaHIR1 were decreased in VIGS experiment, which made pepper less resistant to RSI. Significant hypersensitive (HR)-like cell death, H2O2 buildup, and transcriptional up-regulation of immunological marker genes were noticed in pepper when CaWRKY3 was transiently overexpressed. Transcriptional activity of CaWRKY3 was increased with overexpression of CaWRKY6, CaWRKY22, CaWRKY27, and CaWRKY40, and vice versa. In contrast, Pseudomonas syringae pv tomato DC3000 (Pst DC3000) was easily repelled by the innate immune system of transgenic Arabidopsis thaliana that overexpressed CaWRKY3. The transcriptions of defense-related marker genes like AtPR1, AtPR2, and AtNPR1 were increased in CaWRKY3-overexpressing transgenic A. thaliana plants.

CONCLUSION

It is concluded that CaWRKY3 favorably regulates phytohormone-mediated synergistic signaling, which controls cell death in plant and immunity of pepper plant against bacterial infections.

CaSTH2 disables CaWRKY40 from activating pepper thermotolerance and immunity against Ralstonia solanacearum via physical interaction

CaWRKY40 coordinately activates pepper immunity against Ralstonia solanacearum infection (RSI) and high temperature stress (HTS), forms positive feedback loops with other positive regulators and is promoted by CaWRKY27b/CaWRKY28 through physical interactions; however, whether and how it is regulated by negative regulators to function appropriately remain unclear. Herein, we provide evidence that CaWRKY40 is repressed by a SALT TOLERANCE HOMOLOG2 in pepper (CaSTH2). Our data from gene silencing and transient overexpression in pepper and epoptic overexpression in Nicotiana benthamiana plants showed that CaSTH2 acted as negative regulator in immunity against RSI and thermotolerance. Our data from BiFC, CoIP, pull down, and MST indicate that CaSTH2 interacted with CaWRKY40, by which CaWRKY40 was prevented from activating immunity or thermotolerance-related genes. It was also found that CaSTH2 repressed CaWRKY40 at least partially through blocking interaction of CaWRKY40 with CaWRKY27b/CaWRKY28, but not through directly repressing binding of CaWRKY40 to its target genes. The results of study provide new insight into the mechanisms underlying the coordination of pepper immunity and thermotolerance.

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

Comparative Transcriptomics Analysis Reveals the Differences in Transcription between Resistant and Susceptible Pepper (Capsicum annuum L.) Varieties in Response to Anthracnose

Pepper (Capsicum annuum L.) is a herbaceous plant species in the family Solanaceae. Capsicum anthracnose is caused by the genus Colletotrichum. spp., which decreases pepper production by about 50% each year due to anthracnose. In this study, we evaluated the resistance of red ripe fruits from 17 pepper varieties against anthracnose fungus Colletotrichum capsici. We assessed the size of the lesion diameter and conducted significance analysis to identify the resistant variety of B158 and susceptible variety of B161. We selected a resistant cultivar B158 and a susceptible cultivar B161 of pepper and used a transcription to investigate the molecular mechanisms underlying the plant's resistance to C. capsici, of which little is known. The inoculated fruit from these two varieties were used for the comparative transcription analysis, which revealed the anthracnose-induced differential transcription in the resistant and susceptible pepper samples. In the environment of an anthrax infection, we found that there were more differentially expressed genes in resistant varieties compared to susceptible varieties. Moreover, the response to stimulus and stress ability was stronger in the KANG. The transcription analysis revealed the activation of plant hormone signaling pathways, phenylpropanoid synthesis, and metabolic processes in the defense response of peppers against anthracnose. In addition, ARR-B, AP2-EREBP, bHLH, WRKY, and NAC are associated with disease resistance to anthracnose. Notably, WRKY and NAC were found to have a potentially positive regulatory role in the defense response against anthracnose. These findings contribute to a more comprehensive understanding of the resistance mechanisms of red pepper fruit to anthracnose infection, providing valuable molecular insights for further research on the resistance mechanisms and genetic regulations during this developmental stage of pepper.

WRKY transcription factors in plant defense

Environmental stressors caused by climate change are fundamental barriers to agricultural sustainability. Enhancing the stress resilience of crops is a key strategy in achieving global food security. Plants perceive adverse environmental conditions and initiate signaling pathways to activate precise responses that contribute to their survival. WRKY transcription factors (TFs) are essential players in several signaling cascades and regulatory networks that have crucial implications for defense responses in plants. This review summarizes advances in research concerning how WRKY TFs mediate various signaling cascades and metabolic adjustments as well as how epigenetic modifications involved in environmental stress responses in plants can modulate WRKYs and/or their downstream genes. Emerging research shows that clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas)-mediated genome editing of WRKYs could be used to improve crop resilience.

CaWRKY50 Acts as a Negative Regulator in Response to Colletotrichum scovillei Infection in Pepper

Chili anthracnose is one of the most common and destructive fungal pathogens that affects the yield and quality of pepper. Although WRKY proteins play crucial roles in pepper resistance to a variety of pathogens, the mechanism of their resistance to anthracnose is still unknown. In this study, we found that CaWRKY50 expression was obviously induced by Colletotrichum scovillei infection and salicylic acid (SA) treatments. CaWRKY50-silencing enhanced pepper resistance to C. scovillei, while transient overexpression of CaWRKY50 in pepper increased susceptibility to C. scovillei. We further found that overexpression of CaWRKY50 in tomatoes significantly decreased resistance to C. scovillei by SA and reactive oxygen species (ROS) signaling pathways. Moreover, CaWRKY50 suppressed the expression of two SA-related genes, CaEDS1 (enhanced disease susceptibility 1) and CaSAMT1 (salicylate carboxymethyltransferase 1), by directly binding to the W-box motif in their promoters. Additionally, we demonstrated that CaWRKY50 interacts with CaWRKY42 and CaMIEL1 in the nucleus. Thus, our findings revealed that CaWRKY50 plays a negative role in pepper resistance to C. scovillei through the SA-mediated signaling pathway and the antioxidant defense system. These results provide a theoretical foundation for molecular breeding of pepper varieties resistant to anthracnose.

WRKY transcription factor family in lettuce plant (Lactuca sativa): Genome-wide characterization, chromosome location, phylogeny structures, and expression patterns

WRKY transcription factors (TF) have been identified in many plant species and play critical roles in multiple stages of growth and development and under various stress conditions. As one of the most popular vegetable crops, asparagus lettuce has important medicinal and nutritional value. However, study of WRKY TFs family in asparagus lettuce is limited. With the lettuce (Lactuca sativa L.) genome publication, we identified 76 WRKY TFs and analyzed structural characteristics, phylogenetic relationships, chromosomal distribution, interaction network, and expression profiles. The 76 LsWRKY TFs were phylogenetically classified as Groups I, II (IIa-IIe), and III. Cis element analysis revealed complex regulatory relationships of LsWRKY genes in response to different biological progresses. Interaction network analysis indicated that LsWRKY TFs could interact with other proteins, such as SIB (sigma factor binding protein), WRKY TFs, and MPK. The WRKYIII subfamily genes showed different expression patterns during the progress of asparagus lettuce stem enlargement. According to qRT-PCR analysis, abiotic stresses (drought, salt, low temperature, and high temperature) and phytohormone treatment could induce specific LsWRKYIII gene expression. These results will provide systematic and comprehensive information on LsWRKY TFs and lay the foundation for further clarification of the regulatory mechanism of LsWRKY, especially LsWRKYIII TFs, involved in stress response and the progress of plant growth and development.

WRKY genes provide novel insights into their role against Ralstonia solanacearum infection in cultivated peanut (Arachis hypogaea L.)

As one of the most important and largest transcription factors, WRKY plays a critical role in plant disease resistance. However, little is known regarding the functions of the WRKY family in cultivated peanuts (Arachis hypogaea L.). In this study, a total of 174 WRKY genes (AhWRKY) were identified from the genome of cultivated peanuts. Phylogenetic analysis revealed that AhWRKY proteins could be divided into four groups, including 35 (20.12%) in group I, 107 (61.49%) in group II, 31 (17.82%) in group III, and 1 (0.57%) in group IV. This division is further supported by the conserved motif compositions and intron/exon structures. All AhWRKY genes were unevenly located on all 20 chromosomes, among which 132 pairs of fragment duplication and seven pairs of tandem duplications existed. Eighteen miRNAs were found to be targeting 50 AhWRKY genes. Most AhWRKY genes from some groups showed tissue-specific expression. AhWRKY46, AhWRKY94, AhWRKY156, AhWRKY68, AhWRKY41, AhWRKY128, AhWRKY104, AhWRKY19, AhWRKY62, AhWRKY155, AhWRKY170, AhWRKY78, AhWRKY34, AhWRKY12, AhWRKY95, and AhWRKY76 were upregulated in ganhua18 and kainong313 genotypes after Ralstonia solanacearum infection. Ten AhWRKY genes (AhWRKY34, AhWRKY76, AhWRKY78, AhWRKY120, AhWRKY153, AhWRKY155, AhWRKY159, AhWRKY160, AhWRKY161, and AhWRKY162) from group III displayed different expression patterns in R. solanacearum sensitive and resistant peanut genotypes infected with the R. solanacearum. Two AhWRKY genes (AhWRKY76 and AhWRKY77) from group III obtained the LRR domain. AhWRKY77 downregulated in both genotypes; AhWRKY76 showed lower-higher expression in ganhua18 and higher expression in kainong313. Both AhWRKY76 and AhWRKY77 are targeted by ahy-miR3512, which may have an important function in peanut disease resistance. This study identified candidate WRKY genes with possible roles in peanut resistance against R. solanacearum infection. These findings not only contribute to our understanding of the novel role of WRKY family genes but also provide valuable information for disease resistance in A. hypogaea.

Genome-wide identification of Cymbidium sinense WRKY gene family and the importance of its Group III members in response to abiotic stress

Transcription factors (TFs) of the WRKY family play pivotal roles in defense responses and secondary metabolism of plants. Although WRKY TFs are well documented in numerous plant species, no study has performed a genome-wide investigation of the WRKY gene family in Cymbidium sinense. In the present work, we found 64 C. sinense WRKY (CsWRKY) TFs, and they were further divided into eight subgroups. Chromosomal distribution of CsWRKYs revealed that the majority of these genes were localized on 16 chromosomes, especially on Chromosome 2. Syntenic analysis implied that 13 (20.31%) genes were derived from segmental duplication events, and 17 orthologous gene pairs were identified between Arabidopsis thaliana WRKY (AtWRKY) and CsWRKY genes. Moreover, 55 of the 64 CsWRKYs were detectable in different plant tissues in response to exposure to plant hormones. Among them, Group III members were strongly induced in response to various hormone treatments, indicating their potential essential roles in hormone signaling. We subsequently analyzed the function of CsWRKY18 in Group III. The CsWRKY18 was localized in the nucleus. The constitutive expression of CsWRKY18 in Arabidopsis led to enhanced sensitivity to ABA-mediated seed germination and root growth and elevated plant tolerance to abiotic stress within the ABA-dependent pathway. Overall, our study represented the first genome-wide characterization and functional analysis of WRKY TFs in C. sinense, which could provide useful clues about the evolution and functional description of CsWRKY genes.

Plant growth promoting rhizobacteria improve growth and yield related attributes of chili under low nitrogen availability

Nitrogen (N) is a macronutrient desired by crop plants in large quantities. However, hiking fertilizer prices need alternative N sources for reducing its requirements through appropriate management practices. Plant growth promoting rhizobacteria (PGPR) are well-known for their role in lowering N requirements of crop plants. This study assessed the impact of PGPR inoculation on growth, allometry and biochemical traits of chili under different N doses. Two PGPR, i.e., Azospirillum 'Er-20' (nitrogen fixing) and Agrobacterium 'Ca-18' (phosphorous solubilizing) were used for inoculation, while control treatment had no PGPR inoculation. Six N doses, i.e., 100, 80, 75, 70, 60 and 50% of the N required by chili were included in the study. Data relating to growth traits, biochemical attributes and yield related traits were recorded. Interaction among N doses and PGPR inoculation significantly altered all growth traits, biochemical attributes and yield related traits. The highest values of the recorded traits were observed for 100% N with and without PGPR inoculation and 75% N with PGPR inoculation. The lowest values of the recorded traits were noted for 50% N without PGPR inoculation. The PGPR inoculation improved the measured traits compared to the traits recorded noted in same N dose without PGPR inoculation. Results revealed that PGPR had the potential to lower 25% N requirement for chili. Therefore, it is recommended that PGPR must be used in chili cultivation to lower N requirements.