Genome-Wide Identification of WRKY Gene Family and Functional Characterization of CcWRKY25 in Capsicum chinense

Pepper is renowned worldwide for its distinctive spicy flavor. While the gene expression characteristics of the capsaicinoid biosynthesis pathway have been extensively studied, there are already a few reports regarding transcriptional regulation in capsaicin biosynthesis. In this study, 73 WRKYs were identified in the genome of Capsicum chinense, and their physicochemical traits, DNA, and protein sequence characteristics were found to be complex. Combining RNA-seq and qRT-PCR data, the WRKY transcription factor CA06g13580, which was associated with the accumulation tendency of capsaicinoid, was screened and named CcWRKY25. CcWRKY25 was highly expressed in the placenta of spicy peppers. The heterologous expression of CcWRKY25 in Arabidopsis promoted the expression of genes PAL, 4CL1, 4CL2, 4CL3, CCR, and CCoAOMT and led to the accumulation of lignin and flavonoids. Furthermore, the expression of the capsaicinoid biosynthesis pathway genes (CBGs) pAMT, AT3, and KAS was significantly reduced in CcWRKY25-silenced pepper plants, resulting in a decrease in the amount of capsaicin. However, there was no noticeable difference in lignin accumulation. The findings suggested that CcWRKY25 could be involved in regulating capsaicinoid synthesis by promoting the expression of genes upstream of the phenylpropanoid pathway and inhibiting CBGs' expression. Moreover, the results highlighted the role of CcWRKY25 in controlling the pungency of pepper and suggested that the competitive relationship between lignin and capsaicin could also regulate the spiciness of the pepper.