Anthocyanins, a group of secondary metabolites synthesized in the phenylpropanoid pathway, largely determine fruit peel color of fleshy fruits, but it is not known if its synthesis is linked to vacuolar malate accumulation that determines fruit acidity. Here, we show that when the coding sequence of Ma1, the major gene controlling apple fruit acidity, is overexpressed in ‘Royal Gala’ (RG), anthocyanin biosynthesis in the fruit peel is enhanced, corresponding to the downregulation of the expression of MYB73, an R2R3-MYB transcription factor. RNAi suppression of MYB73 expression via virus-induced gene silencing increases anthocyanin biosynthesis whereas its transient overexpression decreases anthocyanin biosynthesis in apple fruit peel. MYB73 binds to the promoter of the gene encoding UDP-glycose: flavonoid-3-O-glycosyltransferase (UFGT), the enzyme that catalyzes the last step in anthocyanin synthesis, to repress its expression. When MYB73 expression is suppressed by RNAi, UFGT expression is enhanced, leading to more anthocyanin synthesis, but this effect is blocked by RNAi suppression of UFGT expression. RNAi suppression of MYB73 enhances anthocyanin synthesis in wild-type RG apples whereas its overexpression decreases anthocyanin synthesis in Ma1-OE fruit. In the meantime, MYB73 competes with MYB1, one of the key activators of anthocyanin biosynthesis, binding to the promoter of UFGT and regulating its expression. These results indicate that MYB73 negatively regulates anthocyanin biosynthesis via repressing UFGT expression in apple peel. In Ma1-OE fruit, down-regulation of MYB73 releases UFGT from MYB73 repression and allows more MYB1 binding to UFGT promoter, leading to enhanced anthocyanin biosynthesis.
