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 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 provided a theoretical basis for breeding onions with higher anthocyanin contents.

中文翻译：

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bHLH 转录因子 AcB2 调节洋葱 (Allium cepa L.) 中的花青素生物合成

洋葱鳞茎颜色是一个关键的育种性状。红色球茎颜色是由花青素的存在引起的，花青素是类黄酮合成途径的产物。与其他作物相比，洋葱中黄酮类化合物调控的研究滞后。AcB2 编码一个 bHLH 转录因子，其转录与花青素积累呈正相关，并与 AcMYB1 的表达相关，AcMYB1 是洋葱中黄酮类生物合成途径的激活剂。系统发育分析表明，AcB2 被归入 bHLH IIIf 亚组的 TRANSPARENT TESTA 8 (TT8) 进化枝。AcB2 蛋白含有一个 MYB 相互作用区域，并与酵母和烟草叶中的 AcMYB1 物理相互作用。AcMYB1 直接与花青素合酶 (AcANS) 和类黄酮 3-羟化酶 1 (AcF3H1) 的启动子结合并激活它们的表达。与单独表达AcMYB1 相比，拟南芥原生质体中AcB2 与AcMYB1 的共表达显着增加了AcANS 和AcF3H1 的表达。AcB2 与 AcMYB1 的瞬时共过表达诱导花青素在洋葱鳞茎上皮细胞中的积累。拟南芥 tt8-1 突变体与 AcB2 的互补恢复了 tt8-1 中的色素沉着缺陷。此外，AcB2在酵母细胞和烟叶中与AtTT2发生物理相互作用，表明AcB2的功能与AtTT8相似。总之，这些结果表明AcB2增强了AcMYB1在上调洋葱花青素生物合成中的功能，