Shuilin He

It has been previously found that pepper (Capsicum annuum) activates high-temperature specific immunity against R. solanacearum in cytokinin signalling dependent manner at high temperature (37°C), while how this high-temperature specific immunity is activated remain unclear. Herein, we demonstrate that both CaARR2 and CaLBD24 acted positively in pepper immunity against R. solanacearum infection at 37°C but not at 28°C. CaLBD24 was found to interact in the nuclei with CaARR2, which is otherwise present in plastids upon R. solanacearum infection at 28°C but translocated into the nuclei upon 37°C exposure alone, R. solanacearum infection at 37°C or the exogenous application of ABA or R. solanacearum infection at 28°C combined with exogenous application of tZ. By ChIP-seq combined with RNA-seq assay, the CaARR2-CaLBD24 module in the nuclei was further found to activate high temperature specific immunity by directly co-targeting and activating genes including CaRPT2 via binding their corresponding cis-elements, and CaRPT2 was further found by virus gene silencing to act positively in pepper immunity against R. solanacearum at 37°C. In addition, CaARR2, CaLBD24 and CaRPT2 ware also found to act positively in defense response of tomato R. solanacearum infection at 37°C. The results uncover that high-temperature specific immunity is activated in pepper and tomato in cytokinin signalling dependent manner using ARR2-LBD24 module to activate genes including RPT2, providing new insights into mechanism underlying bacterial wilt resistance in pepper and tomato at high temperatures.

Plant immunity against pathogens is largely regulated at the transcriptional level by various transcription factors. In pepper, CaHDZ27 positively regulates immunity against Ralstonia solanacearum . However, the mechanisms that fine‐tune this CaHDZ27‐mediated immunity remain elusive. Here, we report that CaHDZ27 interacted with CaZAT11, a C2H2‐type zinc finger transcription factor harbouring an EAR repression motif. Silencing of CaZAT11 significantly reduced susceptibility to R. solanacearum without impairing plant growth, whereas its overexpression promoted growth of Nicotiana benthamiana but compromised its immunity against R. solanacearum . We further found that CaZAT11 acted as a transcriptional repressor suppressing the transcriptional activity of CaHDZ27 under basal conditions. Upon infection with R. solanacearum , however, CaHDZ27 dissociated from CaZAT11 and instead interacted with the 14‐3‐3 protein CaTFT7. This interaction released CaHDZ27 from repression, which in turn enabled the activation of downstream immune responses. Collectively, our study identified CaZAT11 as a central regulatory node in the plant immunity network, modulating the critical trade‐off between growth and defence.

It has previously been found that CaWRKY40 is employed by pepper to activate immunity against Ralstonia solanacearum and to activate thermotolerance context-specifically, but the underlying mechanisms are not fully understood. Here, CaGβ, a subunit in the heterotrimeric G protein complex that was originally found to probably interact with CaWRKY40, was expressional and functionally characterized; the results showed that CaGβ was upregulated by R. solanacearum infection; its silencing by virus-induced silencing impaired pepper immunity against R. solanacearum infection, accompanied by downregulation of immunity-related marker genes, including CaPR1, CaDEF1, CaNPR1, and CaPR-STH2. In addition, CaGβ–CaWRKY40 interaction was confirmed by BiFC and pull-down assay using prokaryotically expressed proteins, and activations of immunity-related CaPR1, CaPR-STH2, and CaNPR1 by CaWRKY40 were all promoted, but the activation of thermotolerance-related CaHSP24 by CaWRKY40 blocked CaGβ through its interaction with CaWRKY40. All these data indicate that immunity against R. solanacearum and its antagonism to thermotolerance in pathogen-infected pepper plants are mediated by CaWRKY40 through physical interaction with CaGβ.

Pepper (Capsicum annuum), a widely cultivated vegetable of significant economic importance globally, is frequently subjected to attacks from pathogens such as Ralstonia solanacearum, as well as high-temperature stress. However, the mechanisms by which pepper combats these stresses remain poorly understood. Herein, we reported that the expression of the leucine-rich repeat protein CaLRR13, which lacks a nucleotide-binding site (NBS), kinase domains, and a transmembrane region, was transcriptionally activated by both R. solanacearum inoculation and high-temperature stress. Through transient overexpression in the epidermal cells of Nicotiana benthamiana leaves, we found that CaLRR13 localized in both the cytoplasm and the nuclei. Reducing the expression of CaLRR13 via virus-induced gene silencing (VIGS) increased the sensitivity of pepper to R. solanacearum infection and high-temperature exposure, accompanied by reduced expression of immunity- and thermotolerance-related genes, including CaWRKY40, CaPR1, CaNPR1, CaDEF1, and CaHSP24. In contrast, transient overexpression of CaLRR13 in pepper leaves induced a like-hypersensitive response (HR) and enhanced the expression of the aforementioned immunity- and thermotolerance-related genes. Thus, we conclude that CaLRR13 plays a positive role in pepper immunity against R. solanacearum and thermotolerance, providing a new perspective on the crosstalk and management of plant responses to these two stresses.

The pyrabactin resistance 1-like (PYR/PYL) proteins are abscisic acid receptors that perform multiple functions in various plant growth and development processes. However, the PYR/PYL gene family in luffa (Luffa cylindrica L.) has not been well-explored. In this study, we analysed the effects of whole-genome member identification, endogenous soluble sugars (SS), soluble proteins (SP), abscisic acid (ABA), indole-3-acetic acid (IAA, auxin) and the gene expression pattern of PYR/PYL influenced by exogenous abscisic acid (ABA) during the fruit development of luffa through the use of physiological and biochemical analyses, bioinformatics, and RT-qPCR techniques. We conducted a comprehensive genome-wide identification and characterisation of the PYR/PYL gene family in luffa fruit development. Four LcPYR and 10 LcPYL genes were identified in the luffa reference genome via bioinformatics analyses. A chromosomal mapping of the identified LcPYR/PYL genes showed that they were distributed on 9 of the 13 chromosomes in the luffa genome. Conserved structural domain analyses of the 14 proteins encoded by the LcPYR/PYL genes identified the PYR_PYL_RCAR_like structural domains typical of this family; however, no regulatory component of abscisic acid receptor (RCAR)-type genes was found. At six luffa fruit development stages (i.e., 0, 3, 6, 9, 12, and 15 days after pollination), the contents of soluble sugars, soluble proteins, and endogenous hormones ABA and IAA in the fruit significantly increased. Under the exogenous ABA treatments, the contents of these four endogenous substances in the fruits were significantly higher than they were in the control group at the same time period, and ABA and IAA seemed to be synergistically involved in the luffa fruit-ripening process. An analysis of the luffa transcriptome data and real-time fluorescence quantitative PCR (RT-qPCR) experiments showed that multiple LcPYR/PYLs (e.g., LcPYL10 and LcPYR4) had differential expression levels in the seven different tissues and exogenous ABA-treated fruits that were analysed, suggesting their roles in ABA hormone-mediated ripening of luffa fruit. Together, the results provide basic information about the LcPYR/PYL family in L. cylindrica and their involvement in fruit development.

Bacterial wilt, caused by Ralstonia solanacearum, is a devastating disease affecting plants in the Solanaceae family. In our previous study, CaHDZ27 was shown to act crucially in the pepper defense response to R. solanacearum. However, the molecular basis underlying CaHDZ27 function remains unexplored. In this study, we demonstrate that CaHDZ27 is post-translationally regulated by the 14-3-3 protein CaTFT7, which functions as a positive regulator in pepper immunity against R. solanacearum. RT-qPCR analysis revealed that CaTFT7 is transcriptionally induced by R. solanacearum infection. The data from virus-induced gene silencing revealed that CaTFT7 positively affects pepper immunity, which was further confirmed by the data of CaTFT7-overexpressing Nicotiana benthamiana. CaTFT7 interacted with CaHDZ27, thereby promoting the stability of CaHDZ27 and enhancing CaHDZ27 binding to the promoter of cysteine-rich receptor-like protein kinase 5 (CaCRK5), a gene that positively affects pepper defense against R. solanacearum. The above data indicated that CaTFT7 enhanced CaHDZ27 stability and promoted its ability to activate pepper immunity, shedding light on the mechanisms underlying pepper resistance to bacterial wilt.

Pepper (Capsicum annuum L.) suffers severe quality and yield loss from oomycete diseases caused by Phytophthora capsici. CaSGT1 was previously determined to positively regulate the immune response of pepper plants against P. capsici, but by which mechanism remains elusive. In the present study, the potential interacting proteins of CaSGT1 were isolated from pepper using a yeast two-hybrid system, among which CaARP1 was determined to interact with CaSGT1 via bimolecular fluorescence complementation (BiFC) and microscale thermophoresis (MST) assays. CaARP1 belongs to the auxin-repressed protein family, which is well-known to function in modulating plant growth. The transcriptional and protein levels of CaARP1 were both significantly induced by infection with P. capsici. Silencing of CaARP1 promotes the vegetative growth of pepper plants and attenuates its disease resistance to P. capsici, as well as compromising the hypersensitive response-like cell death in pepper leaves induced by PcINF1, a well-characterized typical PAMP from P. capsici. Chitin-induced transient expression of CaARP1 in pepper leaves enhanced its disease resistance to P. capsici, which is amplified by CaSGT1 co-expression as a positive regulator. Taken together, our result revealed that CaARP1 plays a dual role in the pepper, negatively regulating the vegetative growth and positively regulating plant immunity against P. capsici in a manner associated with CaSGT1.

Bacterial wilt caused by Ralstonia solanacearum is one of the most important diseases in solanaceous plants, including peppers. It generally tends to be more serious under warm−temperature and moist (WM) conditions than at moist room−temperature (RM) conditions. Although immunity mechanisms at room temperature have been intensively studied, the mechanisms underlying WM conditions remain poorly understood. Herein, the pepper cysteine protease CaZingipain2 was expressed and functionally characterized in pepper immunity against R. solanacearum at WM conditions and at room temperature. The results showed that CaZingipain2 localized to the nucleus and was upregulated at the transcript level in pepper plants upon R. solanacearum infection under WM conditions (RSWM). Virus−induced gene silencing of CaZingipain2 significantly increased the susceptibility of pepper plants to RSWM, and was coupled with the downregulation of CaPRP1 and CaMgst3, which are specifically related to pepper immunity against RSWM, according to our previous studies, while its overexpression significantly reduced the susceptibility of N. benethamiana plants to RSWM compared to that of wild−type plants. In addition, our data showed that CaZingipain2 also acts positively in pepper immunity against R. solanacearum infection at room temperature by upregulating the SA− and JA−responsive PR genes, including CaNPR1 and CaDEF1. All these results indicate that CaZingipain2 improves pepper immunity against R. solanacearum under WM conditions and at room temperature by regulating different PR genes.

Summary: To challenge the invasion of various pathogens, plants re‐direct their resources from plant growth to an innate immune defence system. However, the underlying mechanism that coordinates the induction of the host immune response and the suppression of plant growth remains unclear. Here we demonstrate that an auxin response factor, CaARF9, has dual roles in enhancing the immune resistance to Ralstonia solanacearum infection and in retarding plant growth by repressing the expression of its target genes as exemplified by Casmc4, CaLBD37, CaAPK1b and CaRROP1. The expression of these target genes not only stimulates plant growth but also negatively impacts pepper resistance to R. solanacearum. Under normal conditions, the expression of Casmc4, CaLBD37, CaAPK1b and CaRROP1 is active when promoter‐bound CaARF9 is complexed with CaIAA2. Under R. solanacearum infection, however, degradation of CaIAA2 is triggered by SA and JA‐mediated signalling defence by the ubiquitin‐proteasome system, which enables CaARF9 in the absence of CaIAA2 to repress the expression of Casmc4, CaLBD37, CaAPK1b and CaRROP1 and, in turn, impeding plant growth while facilitating plant defence to R. solanacearum infection. Our findings uncover an exquisite mechanism underlying the trade‐off between plant growth and immunity mediated by the transcriptional repressor CaARF9 and its deactivation when complexed with CaIAA2.