VrMYB90 negatively regulates proanthocyanidin biosynthesis by repressing VrANR in mung bean (Vigna radiata L.)

VrANR and VrMYB90 were identified by transcriptome between 'Sulv1' and 'M0313'. VrMYB90 acted upstream of VrANR by binding to the promoter of VrANR and inhibiting the expression of VrANR, thus regulating negatively the biosynthesis of proanthocyanins. Anthocyanin reductase (ANR), the enzyme responsible for converting anthocyanidins to their corresponding 2,3-cis-flavan-3-ols, which is crucial to balance the anthocyanins and proanthocyanidins (PAs) level. In this study, significant differences were observed in the contents of anthocyanins and PA between the two cultivars. We identified a structural gene VrANR and a MYB transcription factor VrMYB90 that acts upstream of VrANR based on transcriptomic data analysis. Both of the two factors played key roles in PA accumulation in mung bean. Overexpressing VrANR in mung bean hairy roots led to a higher PA accumulation when compared with empty vector. Furthermore, overexpression VrANR in the Arabidopsis ban (anr) mutants increased PA content while reducing anthocyanin levels. Yeast-one-hybridization, β-glucuronidase (GUS) assay and dual-luciferase (LUC) reporter assays revealed that VrMYB90 (a positive regulator in anthocyanin) could bind to the VrANR promoter to repress its expression and then repress PA synthesis. Determination of transcript level of VrANR in VrMYB90 transgenic mung bean also proved that VrMYB90 inhibited the expression of VrANR. Above all, our present results suggested that VrMYB90 can repress the transcription of VrANR to play a negative role in the PAs accumulation in mung bean. These findings enriched our understanding in the regulatory network of PAs in mung bean.