Identification and characterization of MYB-bHLH-WD40 regulatory complexes controlling proanthocyanidin biosynthesis in strawberry (Fragaria x ananassa) fruits

摘要

Strawberry (Fragaria x ananassa) fruits contain high concentrations of flavonoids. In unripe strawberries, the flavonoids are mainly represented by proanthocyanidins (PAs), while in ripe fruits the red-coloured anthocyanins also accumulate. Most of the structural genes leading to PA biosynthesis in strawberry have been characterized, but no information is available on their transcriptional regulation. In Arabidopsis thaliana the expression of the PA biosynthetic genes is specifically induced by a ternary protein complex, composed of AtTT2 (AtMYB123), AtTT8 (AtbHLH042) and AtTTGI (WD40-repeat protein). A strategy combining yeast-two-hybrid screening and agglomerative hierarchical clustering of transcriptomic and metabolomic data was undertaken to identify strawberry PA regulators. Among the candidate genes isolated, four were similar to AtTT2, AtTT8 and AtTTGI (FaMYB9/FaMYB11, FabHLH3 and FaTTGI, respectively) and two encode putative negative regulators (FaMYB5 and FabHLH3A). Interestingly, FaMYB9/FaMYB11, FabHLH3 and FaTTGI were found to complement the tt2-1, tt8-3 and ttg1-1 transparent testa mutants, respectively. In addition, they interacted in yeast and activated the Arabidopsis BANYULS (anthocyanidin reductase) gene promoter when coexpressedin Physcomitreiia patens protoplasts. Taken together, these results demonstrated that FaMYB9/FaMYB11, FabHLH3 and FaTTGI are the respective functional homologues of AtTT2, AtTT8 and AtTTGI, providing new tools for modifying PA content and strawberry fruit quality.