MdMYB66 Is Associated with Anthocyanin Biosynthesis via the Activation of the MdF3H Promoter in the Fruit Skin of an Apple Bud Mutant

Skin color is an important trait that is mainly determined by the content and composition of anthocyanins in apples. In this study, a new bud mutant (RM) from 'Oregon Spur II' (OS) of Red Delicious apple was obtained to reveal the mechanism underlying red color formation. Results showed that the total anthocyanin content in RM was significantly higher than that in OS with the development of fruit. Through widely-targeted metabolomics, we found that cyanidin-3-O-galactoside was significantly accumulated in the fruit skin of RM. Transcriptome analysis revealed that the structural gene MdF3H and MdMYB66 transcription factor were significantly up-regulated in the mutant. Overexpression of MdMYB66 in apple fruit and apple callus significantly promoted anthocyanin accumulation and significantly increased the expression level of MdMYB66 and structural genes related to anthocyanin synthesis. Y1H and LUC analysis verified that MdMYB66 could specifically bind to the promoter of MdF3H. The results of the double luciferase activity test showed that MdMYB66 activated MdF3H 3.8 times, which led to increased anthocyanin contents. This might explain the phenotype of red color in RM at the early stage. Taken together, these results suggested that MdMYB66 was involved in regulating the anthocyanin metabolic pathways through precise regulation of gene expression. The functional characterization of MdMYB66 provides insight into the biosynthesis and regulation of anthocyanins.