Isolation and functional analysis of SrMYB1, a direct transcriptional repressor of SrUGT76G1 in Stevia rebaudiana

SrUGT76G1, the most well-studied diterpene glycosyltransferase in Stevia rebaudiana, is key to the biosynthesis of economically important steviol glycosides (SGs). However, the molecular regulatory mechanism of SrUGT76G1 has rarely been explored. In this study, we identified a MYB transcription factor, SrMYB1, using a yeast one-hybrid screening assay. SrMYB1 belongs to the typical R2R3-type MYB protein and is specifically localized in the nucleus with strong transactivation activity. The transcript of SrMYB1 is predominantly accumulated in flowers, but is also present at a lower level in leaves. Yeast one-hybrid and electrophoretic mobility shift assays verified that SrMYB1 binds directly to the MYB binding sites in the F4-3 fragment (+50–(–141)) of the SrUGT76G1 promoter. Furthermore, we found that SrMYB1 could significantly repress the expression of SrUGT76G1 in both epidermal cells of tobacco leaves and stevia callus. Taken together, our results demonstrate that SrMYB1 is an essential upstream regulator of SrUGT76G1 and provide novel insight into the regulatory network for the SGs metabolic pathway in S. rebaudiana.