MdbHLH51 plays a positive role in anthocyanin accumulation in ‘Red Delicious’ apples

Key message MdbHLH51 played a positive regulatory role in the coloration and the accumulation of anthocyanins. Red coloring of fruit skin is an important trait of apple, mainly attributed to the accumulation of anthocyanins, which is a class of plant flavonoid metabolites widely found in plants. Our previous study carried out transcriptome sequencing in fruit skin of apple ( Malus domestica Borkh.) cv. ‘Red Delicious’ and corroborated that bHLH members bHLH51 showed a notable positive correlation with anthocyanin content at color break stage. To our knowledge, the regulation of anthocyanin biosynthesis by MdbHLH51 in apple fruits has not been reported so far. Therefore, it is of great significance to isolate this gene and perform further functional verification. The function of MdbHLH51 was investigated based on the Agrobacterium -mediated method. In this study, MdbHLH51 was cloned from ‘Red Delicious’ fruit skin. MdbHLH51 protein was localized to the nucleus of onion epidermal cells. Transient expression of the bHLH51-GFP in fruit skin and ‘Gala’ apple callus indicated that MdbHLH51 promoted anthocyanin accumulation. In addition, the overexpression of MdbHLH51 in Arabidopsis thaliana also could enhance the anthocyanins content and up-regulate the expression levels of the structural genes related to anthocyanins biosynthesis. Overall, these results suggested that MdbHLH51 played a positive regulatory role in the coloration and the accumulation of anthocyanins. Our results provide a useful information for understanding the function of MdbHLH51 .