The proanthocyanidin-specific transcription factor MdMYBPA1 initiates anthocyanin synthesis under low-temperature conditions in red-fleshed apples.

In plants, flavonoids play critical roles in resistance to biotic and abiotic stresses, and contribute substantially to the quality, flavor, and nutritional quality of many fruit crops. In apple (Malus × domestica), inbreeding has resulted in severe decreases in the genetic diversity and flavonoid content. Over the last decade, we have focused on the genetic improvement of apple using wild red-fleshed apple resources (Malus sieversii f. niedzwetzkyana). Here, we found that the MYB transcription factors (TFs) involved in the synthesis of proanthocyanidins can be classified into TT2 and PA1 types. We characterized a PA1-type MYB transcription factor, MdMYBPA1, from red-fleshed apple and identified its role in flavonoid biosynthesis using overexpression and knockdown-expression transgenes in apple calli. We explored the relationship between TT2- and PA1-type MYB TFs, and found that MdMYB9/11/12 bind the MdMYBPA1 promoter. In addition, MdMYBPA1 responded to low temperature by redirecting the flavonoid biosynthetic pathway from proanthocyanidin to anthocyanin production. In binding analyses, MdbHLH33 directly bound to the low-temperature-responsive (LTR) cis-element of the MdMYBPA1 promoter and promotes its activity. In addition, the calli expressing both MdMYBPA1 and MdbHLH33, which together form a complex, produced more anthocyanin under low temperature. Our findings shed light on the essential roles of PA1-type TFs in the metabolic network of proanthocyanidin and anthocyanin synthesis in plants. Studies on red-fleshed wild apple are beneficial for breeding new apple varieties with high flavonoid contents.