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

Abstract

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.

Multi-omics analysis reveals molecular mechanism of flavonol biosynthesis during the formation of petal color in Camellia nitidissima

“…GtMYBP3 and GtMYBP4 regulate genes involved in early biosynthesis of avonoids and promote avonol biosynthesis [43]. The bHLH transcription factor AcB2 has been shown to interact with AcMYB1 and increase anthocyanin biosynthesis in onion [44]. After our KEGG analysis, there were 20 bHLH and six MYB genes annotated into the avonoid pathway (Fig.…”

MdbHLH162 connects the gibberellin and jasmonic acid signals to regulate anthocyanin biosynthesis in apple

“…A series of repressors of anthocyanin biosynthesis have been identified over the past decade, including the MYB, SPL, miRNA, HD‐ZIP, LBD, NAC, and TCP proteins (Albert et al, 2014; Chen et al, 2019a; LaFountain and Yuan, 2021). Many studies have reported the positive regulatory role of bHLH TFs in anthocyanin biosynthesis in a variety of species, while the exploration of anthocyanin repressors in the bHLH family has been neglected (Nesi et al, 2000; Robbins et al, 2003; Ohno et al, 2011; Xie et al, 2012; Li et al, 2022). Although recent studies have reported that bHLH1 is a repressor of anthocyanin biosynthesis in Chimonanthus praecox and eggplant (Zhao et al, 2020; Duan et al, 2021), the anthocyanin repressors in the bHLH family remain to be investigated.…”

Multi-omics analysis reveals the regulatory mechanism of flavonol biosynthesis during the formation of petal color inCamellia nitidissima

“…GtMYBP3 and GtMYBP4 regulate early biosynthesis genes of flavonoids, FNS and F3’H , and promete flavonol biosynthesis (Nakatsuka et al ., 2012). AcB2, a bHLH transcription factor, had been reported to interact with AcMYB1 and increase anthocyanin biosynthesis in onion (Li et al ., 2022). After our KEGG analysis, there were 20 bHLH and six MYB genes annotated into the flavonoid pathway (Fig 5b, c).…”