A Novel NAC Transcription Factor, MdNAC42, Regulates Anthocyanin Accumulation in Red-fleshed Apple by Interacting with MdMYB10

Anthocyanin pigmentation is an important consumption trait of apple (Malus domestica Borkh.). In this study, we focused on the identification of NAC (NAM, ATAF1/2 and CUC2) proteins involved in the regulation of anthocyanin accumulation in apple flesh. A group of MdNACs was selected for comparison of expression patterns between the white-fleshed cultivar 'Granny Smith' and red-fleshed 'Redlove'. Among them, MdNAC42 was screened, which exhibited a higher expression level in red-fleshed than in white-fleshed fruit, and has a positive correlation with anthocyanin content as fruits ripened. Moreover, overexpression of MdNAC42 in apple calli resulted in the up-regulation of flavonoid pathway genes, including MdCHS, MdCHI, MdF3H, MdDFR, MdANS and MdUFGT, thereby increasing the accumulation of anthocyanins, which confirmed the roles of MdNAC42 in anthocyanin biosynthesis. Notably, MdNAC42 was demonstrated to have an obvious interaction with MdMYB10 either in vitro or in vivo by yeast two-hybrid combined with bimolecular fluorescence complementation, further suggesting that MdNAC42 is an important part of the regulatory network controlling the anthocyanin pigmentation of red-fleshed apples. To the best of our knowledge, this is the first report identifying the MdNAC gene as related to anthocyanin accumulation in red-fleshed apples. This study provides valuable information for improving the regulatory model of anthocyanin biosynthesis in apple fruit.