A novel leucine-rich repeat protein, CaLRR51, acts as a positive regulator in the response of pepper to Ralstonia solanacearum infection.

The leucine-rich repeat (LRR) proteins play important roles in the recognition of corresponding ligands and signal transduction networks in plant defence responses. Herein, a novel LRR protein from Capsicum annuum, CaLRR51, was identified and characterized. It was localized to the plasma membrane and transcriptionally up-regulated by Ralstonia solanacearum infection (RSI), as well as the exogenous application of salicylic acid (SA), jasmonic acid (JA) and ethephon (ETH). Virus-induced gene silencing of CaLRR51 significantly increased the susceptibility of pepper to RSI. By contrast, transient overexpression of CaLRR51 in pepper plants activated hypersensitive response (HR)-like cell death, and up-regulated the defence-related marker genes, including PO2, HIR1, PR1, DEF1 and ACO1. Moreover, ectopic overexpression of CaLRR51 in transgenic tobacco plants significantly enhanced the resistance to RSI. Transcriptional expression of the corresponding defence-related marker genes in transgenic tobacco plants was also found to be enhanced by the overexpression of CaLRR51, which was potentiated by RSI. These loss- and gain-of-function assays suggest that CaLRR51 acts as a positive regulator in the response of pepper to RSI. In addition, the putative signal peptide and transmembrane region were found to be required for plasma membrane targeting of CaLRR51, which is indispensable for the role of CaLRR51 in plant immunity.

Genome-wide identification and expression analysis of calcium-dependent protein kinase and its closely related kinase genes in Capsicum annuum.

As Ca2+ sensors and effectors, calcium-dependent protein kinases (CDPKs) play important roles in plant growth, development, and response to environmental cues. However, no CDPKs have been characterized in Capsicum annuum thus far. Herein, a genome wide comprehensive analysis of genes encoding CDPKs and CDPK-related protein kinases (CRKs) was performed in pepper, a total of 31 CDPK genes and five closely related kinase genes were identified, which were phylogenetically divided into four distinct subfamilies and unevenly distributed across nine chromosomes. Conserved sequence and exon-intron structures were found to be shared by pepper CDPKs within the same subfamily, and the expansion of the CDPK family in pepper was found to be due to segmental duplication events. Five CDPKs in the C. annuum variety CM334 were found to be mutated in the Chiltepin variety, and one CDPK present in CM334 was lost in Chiltepin. The majority of CDPK and CRK genes were expressed in different pepper tissues and developmental stages, and 10, 12, and 8 CDPK genes were transcriptionally modified by salt, heat, and Ralstonia solanacearum stresses, respectively. Furthermore, these genes were found to respond specifically to one stress as well as respond synergistically to two stresses or three stresses, suggesting that these CDPK genes might be involved in the specific or synergistic response of pepper to salt, heat, and R. solanacearum. Our results lay the foundation for future functional characterization of pepper CDPK and its closely related gene families.

CaWRKY6 transcriptionally activates CaWRKY40, regulates Ralstonia solanacearum resistance, and confers high-temperature and high-humidity tolerance in pepper.

High temperature (HT), high humidity (HH), and pathogen infection often co-occur and negatively affect plant growth. However, these stress factors and plant responses are generally studied in isolation. The mechanisms of synergistic responses to combined stresses are poorly understood. We isolated the subgroup IIb WRKY family member CaWRKY6 from Capsicum annuum and performed quantitative real-time PCR analysis. CaWRKY6 expression was upregulated by individual or simultaneous treatment with HT, HH, combined HT and HH (HTHH), and Ralstonia solanacearum inoculation, and responded to exogenous application of jasmonic acid (JA), ethephon, and abscisic acid (ABA). Virus-induced gene silencing of CaWRKY6 enhanced pepper plant susceptibility to R. solanacearum and HTHH, and downregulated the hypersensitive response (HR), JA-, ethylene (ET)-, and ABA-induced marker gene expression, and thermotolerance-associated expression of CaHSP24, ER-small CaSHP, and Chl-small CaHSP. CaWRKY6 overexpression in pepper attenuated the HTHH-induced suppression of resistance to R. solanacearum infection. CaWRKY6 bound to and activated the CaWRKY40 promoter in planta, which is a pepper WRKY that regulates heat-stress tolerance and R. solanacearum resistance. CaWRKY40 silencing significantly blocked HR-induced cell death and reduced transcriptional expression of CaWRKY40. These data suggest that CaWRKY6 is a positive regulator of R. solanacearum resistance and heat-stress tolerance, which occurs in part by activating CaWRKY40.