FtMYB18 acts as a negative regulator of anthocyanin/proanthocyanidin biosynthesis in Tartary buckwheat

Key message FtMYB18 plays a role in the repression of anthocyanins and proanthocyanidins accumulation by strongly down-regulating theCHSandDFRgenes in Tartary buckwheat, and the C5 motif plays an important role in this process. Anthocyanins and proanthocyanidins (PAs) are important flavonoids in Tartary buckwheat (Fagopyrum tataricumGaertn.), which provides various vibrant color and stronge abiotic stress resistance. Their synthesis is generally regulated by MYB transcription factors at transcription level. However, the negative regulations of MYB and their effects on flavonol metabolism are poorly understood. A SG4-like MYB subfamily TF, FtMYB18, containing C5 motif was identified from Tartary buckwheat. The expression ofFtMYB18was not only showed a negative correlation with anthocyanins and PAs content but also strongly respond to MeJA and ABA. As far as the transgenic lines withFtMYB18overexpression, anthocyanins and PAs accumulations were decreased through down-regulating expression levels ofNtCHSandNtDFRin tobacco,AtDFRandAtTT12in Arabidopsis,FtCHS,FtDFRandFtANSin Tartary buckwheat hairy roots, respectively. However,FtMYB18showed no effect on theFLSgene expression and the metabolites content in flavonol synthesis branch. The further molecular interaction analysis indicated FtMYB18 could mediate the inhibition of anthocyanins and PAs synthesis by forming MBW transcriptional complex with FtTT8 and FtTTG1, or MYB-JAZ complex with FtJAZ1/-3/-4/-7. Importantly, in FtMYB18 mutant lines with C5 motif deletion (FtMYB18-C), both of anthocyanins and PAs accumulations had recovered to the similar level as that in wild type, which was attributed to the weakened MBW complex activity or the deficient molecular interaction between FtMYB18 Delta C5 with FtJAZ3/-4. The results showed that FtMYB18 could suppress anthocyanins and PAs synthesis at transcription level through the specific interaction of C5 motif with other proteins in Tartary buckwheat.