ChiIV3 Acts as a Novel Target of WRKY40 to Mediate Pepper Immunity Against Ralstonia solanacearum Infection

Abstract

ChiIV3, a chitinase of pepper (Capsicum annuum), stimulates cell death in pepper plants. However, there are only scarce reports on its role in resistance against bacterial wilt disease such as that caused by Ralstonia solanacearum and their transcriptional regulation. In this study, the silencing of ChiIV3 in pepper plants significantly reduced the resistance to R. solanacearum. The transcript of ChiIV3 was induced by R. solanacearum inoculation (RSI) as well as exogenous application of methyl jasmonate and abscisic acid. The bioinformatics analysis revealed that the ChiIV3 promoter consists of multiple stress-related cis elements, including six W-boxes and one MYB1AT. With the 5′ deletion assay in the ChiIV3 promoter, the W4-box located from −640 to −635 bp was identified as the cis element that is required for the response to RSI. In addition, the W4-box element was shown to be essential for the binding of the ChiIV3 promoter by the WRKY40 transcription factor, which is known to positively regulate the defense response to R. solanacearum. Site-directed mutagenesis in the W4-box sequence impaired the binding of WRKY40 to the ChiIV3 promoter. Subsequently, the transcription of ChiIV3 decreased in WRKY40-silenced pepper plants. These results demonstrated that the expression of the defense gene ChiIV3 is controlled through multiple modes of regulation, and WRKY40 directly binds to the W4-box element of the ChiIV3 promoter region for its transcriptional regulation.