CpMYB14, a novel R2R3-MYB transcription factor, interacts with Cp1-SST to negatively regulate inulin-type fructan accumulation and modulates plant growth in Codonopsis pilosula

Codonopsis polysaccharides (CPPs), the primary bioactive constituents of the edible Chinese medicinal plant Codonopsis pilosula (Franch.) Nannf., are mainly composed of inulin-type fructans. The biosynthesis of these fructans is catalyzed by Cp1-SST (sucrose:sucrose 1-fructosyltransferase), a key enzyme whose gene expression can be regulated by MYB transcription factors. However, the regulatory mechanism of Cp1-SST remained unclear. Here, two MYB cis-elements were found in Cp1-SST promoter which exhibited transcriptional activation activity and responded to cold stress. subsequently a R2R3-MYB transcription factor, CpMYB14, was isolated and functionally characterized. CpMYB14 was most responsive to cold, temperature difference, iron overload stress and was localized in the nucleus. Silencing of CpMYB14 resulted in a significant upregulation of Cp1-SST expression and inulin-type fructan content but a decreased expression of the genes associated with CPPs hydrolysis metabolism, including Cp1-FEH , Cp6-FEH , CpSuSy and CpNI . Concurrently, the plant stem height and root length were markedly increased in CpMYB14 gene-silenced lines. Oppositely, overexpression of CpMYB14 significantly decreased the Cp1-SST expression in C. pilosula but with significantly increased Cp1-FEH , Cp6-FEH and CpSuSy expression. Yeast one-hybrid and chromatin immunoprecipitation assays confirmed that CpMYB14 directly binds to the promoter of Cp1-SST . Therefore, CpMYB14 is a negative regulator of Cp1-SST and inulin-type fructan biosynthesis. These findings provide a theoretical foundation for the molecular breeding aimed at developing high-quality varieties of C. pilosula .