Putative WRKYs associated with regulation of fruit ripening revealed by detailed expression analysis of the WRKY gene family in pepper

WRKY transcription factors play important roles in plant development and stress responses. Here, global expression patterns of pepper CaWRKYs in various tissues as well as response to environmental stresses and plant hormones were systematically analyzed, with an emphasis on fruit ripening. The results showed that most CaWRKYs were expressed in at least two of the tissues tested. Group I, a subfamily of the entire CaWRKY gene family, had a higher expression level in vegetative tissues, whereas groups IIa and III showed relatively lower expression levels. Comparative analysis showed that the constitutively highly expressed WRKY genes were conserved in tomato and pepper, suggesting potential functional similarities. Among the identified 61 CaWRKYs, almost 60% were expressed during pepper fruit maturation, and the group I genes were in higher proportion during the ripening process, indicating an as-yet unknown function of group I in the fruit maturation process. Further analysis suggested that many CaWRKYs expressed during fruit ripening were also regulated by abiotic stresses or plant hormones, indicating that these CaWRKYs play roles in the stress-related signaling pathways during fruit ripening. This study provides new insights to the current research on CaWRKY and contributes to our knowledge about the global regulatory network in pepper fruit ripening.