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Article|02 Jun 2022|OPEN
The bHLH transcription factor AcB2 regulates anthocyanin biosynthesis in onion (Allium cepa L.)
Xiaojie Li1,2,3 , , Linjiao Cao1 , Bangbang Jiao1 and Haifeng Yang4 , Changsheng Ma5 , , Yi Liang,1,2,3 ,
1Beijing Vegetable Research Center, Beijing Academy of Agriculture and Forestry Sciences, Beijing, 100097, China
2Key Laboratory of Biology and Genetic Improvement of Horticultural Crops (North China), Ministry of Agriculture, Beijing, 100097, China
3Beijing Key Laboratory of Vegetable Germplasm Improvement, Beijing, 100097, China
4Vegetable Research Center, Lianyungang Academy of Agriculture Sciences, Yingbin Boulevard, Haizhou District, Lianyungang, Jiangsu, 222000, China
5Department of Vegetables, College of Horticulture, Henan Agricultural University, Zhengzhou, Henan, 450046, China
*Corresponding author. E-mail:,,

Horticulture Research 9,
Article number: uhac128 (2022)
Views: 129

Received: 12 Apr 2021
Accepted: 26 May 2022
Published online: 02 Jun 2022


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. AcB2 encodes a basic helix–loop–helix (bHLH) transcription factor, and its transcription is positively associated with anthocyanin accumulation and correlated with the expression of AcMYB1, 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 anthocyanidin synthase (AcANS) and flavonoid 3-hydroxylase 1 (AcF3H1) and activated their expression. The coexpression of AcB2 with AcMYB1 in Arabidopsis thaliana protoplasts dramatically increased the expression of AcANS and AcF3H1 compared with that under the expression of AcMYB1 alone. Transient co-overexpression of AcB2 with AcMYB1 induced anthocyanin accumulation in the epithelial cells of onion bulbs. Complementation of the Arabidopsis tt8-1 mutant with AcB2 restored pigmentation defects in tt8-1. 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 AcB2 enhanced the function of AcMYB1 in upregulating anthocyanin biosynthesis in onion, which provides a theoretical basis for breeding onions with higher anthocyanin contents.