MYB12 and MYB22 play essential roles in proanthocyanidin and flavonol synthesis in red-fleshed apple (Malus sieversii f. niedzwetzkyana).

Flavonoids are major polyphenol compounds in plant secondary metabolism. Wild red-fleshed apples (Malus sieversii f. niedzwetzkyana) are an excellent resource because of their much high flavonoid content than cultivated apples. In this work, R6R6, R6R1 and R1R1 genotypes were identified in an F1 segregating population of M. sieversii f. niedzwetzkyana. Significant differences in flavonoid composition and content were detected among the three genotypes by ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry analysis. Furthermore, two putative flavonoid-related genes encoding R2R3-MYB transcription factors, designated MYB12 and MYB22, were cloned and characterized. The expression patterns of MYB12 and MYB22 directly correlated with those of leucoanthocyanidin reductase and flavonol synthase, respectively. Their roles in flavonoid biosynthesis were identified by overexpression in apple callus and ectopic expression in Arabidopsis. MYB12 expression in the Arabidopsis TT2 mutant complemented its proanthocyanidin-deficient phenotype. Likewise, MYB22 expression in an Arabidopsis triple mutant complemented its flavonol-deficient phenotype. MYB12 could interact with bHLH3 and bHLH33 and played an essential role in proanthocyanidin synthesis. MYB22 was found to activate flavonol pathways by combining directly with the flavonol synthase promoter. Our findings provide a valuable perspective on flavonoid synthesis and provide a basis for breeding elite functional apples with a high flavonoid content.