The bHLH transcription factor AcB2 regulates anthocyanin biosynthesis in onion ( L.).

Onion bulb color is a key breeding trait. The red bulb color is caused by the presence of anthocyanins, which are products of the flavonoid synthesis pathway. Research on flavonoid regulation in onion is lagging compared with that in other crops. encodes a basic helix-loop-helix (bHLH) transcription factor, and its transcription is positively associated with anthocyanin accumulation and correlated with the expression of , which is an activator in the flavonoid biosynthetic pathway in onion. Phylogenetic analysis showed that AcB2 was grouped into the TRANSPARENT TESTA 8 (TT8) clade of the bHLH IIIf subgroup. The AcB2 protein contained an MYB-interacting region and physically interacted with AcMYB1 in yeast and tobacco leaves. AcMYB1 directly bound to the promoters of () and () and activated their expression. The coexpression of with in protoplasts dramatically increased the expression of and compared with that under the expression of alone. Transient co-overexpression of with induced anthocyanin accumulation in the epithelial cells of onion bulbs. Complementation of the mutant with restored pigmentation defects in . In addition, AcB2 physically interacted with AtTT2 in yeast cells and tobacco leaves, indicating that the functions of AcB2 were similar to those of AtTT8. Together, these results demonstrated that enhanced the function of in upregulating anthocyanin biosynthesis in onion, which provides a theoretical basis for breeding onions with higher anthocyanin contents.