The role of MrbHLH1 and MrMYB1 in regulating anthocyanin biosynthetic genes in tobacco and Chinese bayberry (Myrica rubra) during anthocyanin biosynthesis

Abstract

Anthocyanins, being important for both plant functions and human health, were transcriptionally regulated by the MYB–bHLH–WD40 transcription complex. The key MYB regulator for Chinese bayberry (Myrica rubra), MrMYB1, has been characterized in previous studies, while the specific bHLH partner(s) are unknown. In this study, MrbHLH1 and MrbHLH2 were isolated based on their homology to known plant bHLHs involved in anthocyanin biosynthesis regulation. Coordinate expression of MrbHLH1 with MrMYB1 and the anthocyanin biosynthetic genes was observed during fruit development, while MrbHLH2 showed a weaker correlation. Further transient assays in tobacco leaves suggested that MrbHLH1, but not MrbHLH2, was associated with MrMYB1 and triggered significant anthocyanin production. The lack of function of the MrbHLH2 in anthocyanin biosynthesis regulation suggested that different MrbHLH genes within the same phylogenic subfamily have different functions. Overexpression of MrMYB1 and MrbHLH1 in tobacco confirmed the crucial role of MrMYB1–MrbHLH1 in anthocyanin biosynthesis and all of the structural genes from NtCHS were up-regulated by the complex. Dual luciferase assays, however, indicated that MrMYB1 and MrbHLH1 selectively activated five of the eight promoters of biosynthetic genes from bayberry (MrCHI, MrF3′H, MrDFR1, MrANS, MrUFGT), although expression levels of all eight biosynthetic genes including MrCHS and downstream genes were coordinately increased during fruit ripening. Moreover, the interaction between MrbHLH1 and MrMYB1 was confirmed by yeast two-hybrid assay. In conclusion, MrbHLH1, but not MrbHLH2, was the essential partner of MrMYB1 during anthocyanin biosynthesis regulation in tobacco and bayberry, however, the biosynthetic genes in these two species responded differently to the MrMYB1–MrbHLH1 complex.