Ultraviolet B-induced MdWRKY72 expression promotes anthocyanin synthesis in apple

Ultraviolet-B (UV-B) radiation promotes anthocyanin synthesis in many plants. Although several transcription factors promote anthocyanin synthesis in response to UV-B radiation, the underlying mechanism remains unclear. In this study, the MdWRKY72 transcription factor gene was isolated from the 'Taishanzaoxia' apple genome. Quantitative real-time PCR analyses revealed that the genes encoding enzymes and transcription factors involved in the anthocyanin synthesis pathway (MdANS, MdDFR, MdUFGT, and MdMYB1) were more highly expressed in MdWRKY72-overexpressing transgenic calli than in the wild-type 'Orin' apple calli. The results indicated that MdWRKY72 increases anthocyanin synthesis in transgenic calli exposed to UV-B radiation. The results of a gel shift assay and chromatin immunoprecipitation proved that MdWRKY72 promotes MdMYB1 expression indirectly by binding to a W-box element in the MdHY5 promoter and directly by binding to a W-box element in the MdMYB1 promoter. Thus, MdWRKY72 increases anthocyanin synthesis via direct and indirect mechanisms. These findings may be useful for elucidating the molecular mechanism underlying UV-B-induced anthocyanin synthesis mediated by MdWRKY72.