The R2R3-MYB Factor FhMYB5 From Freesia hybrida Contributes to the Regulation of Anthocyanin and Proanthocyanidin Biosynthesis

Abstract

The flavonoids are important and nourishing compounds for plants and human. The transcription regulation of anthocyanin and proanthocyanidin (PA) biosynthesis was extensively studied in dicot compared with monocot plants. In this study, we characterized the functionality of an R2R3-MYB gene <i>FhMYB5</i> from the monocotyledonous flowering plant of Iridaceae, <i>Freesia hybrida.</i> Multiple sequence alignment and phylogenetic analysis implied that FhMYB5 was clustered into grapevine VvMYB5b subclade. Correlation analysis indicated that the spatio-temporal expression patterns of <i>FhMYB5</i> coincided well with anthocyanin and PA accumulations in <i>Freesia per se</i>. Furthermore, transient transfection assays in <i>Freesia</i> protoplasts revealed that the late flavonoid biosynthetic genes (e.g., <i>DFR</i> and <i>LDOX</i>) were slightly up-regulated by <i>FhMYB5</i> alone, whereas both early and late biosynthetic genes were significantly activated when <i>FhMYB5</i> were co-infected with either of the two IIIf clade bHLH genes, <i>FhTT8L</i> and <i>FhGL3L</i>. Moreover, these results were further confirmed by co-transfection of <i>FhMYB5</i> with either of the bHLH genes aforementioned into protoplasts expressing <i>GUS</i> reporter gene driven by <i>Freesia</i> promoters. In addition, the overexpression of <i>FhMYB5</i> in tobacco and <i>Arabidopsis</i> could also significantly up-regulate the expression of genes participating in the general flavonoid pathway. In conclusion, <i>FhMYB5</i> was proved to function in the general flavonoid pathway in <i>Freesia</i>. The results implied a function conservation of flavonoid biosynthesis related MYB regulators in angiosperm plants.