FLAVONOID BIOSYNTHESIS IN GRAPEVINE: THE ROLE OF NOVEL MYB TRANSCRIPTIONAL REGULATORS

The R2R3 MYB is the largest group of transcriptional regulators in plant. They are involved in different process such as the control of the production of trichomes and root hairs. A part of these MYBs participates to a transcriptional regulatory complex in which includes also a WD40-repeat (WDR) protein and a basic-Helix-Loop-Helix (bHLH) protein. The MYB factor play an important role because it is the major responsible for the specificity of action of the complex. The MYB-bHLH-WDR regulatory complex may act as a transcriptional activator or as a transcriptional repressor: activation function in which MYB activators participate to the complex and activate the transcription of genes by direct binding to their promoters and repression function in which MYB repressors, through different ways, repress the transcription. In Vitis vinifera the MYB-bHLH-WDR complex has been partially studied in relation to the flavonoid pathway, and some MYB factors have been shown to play a role in driving the complex in the activation of specific branches of the pathway. For example, VvMYBA1 and VvMYBA2 are factors responsible for the production of anthocyanin pigments that color the berry skin (Walker et al., 2007), while VvMYBPA1 and VvMYBPA2 are responsible for the production of proanthocyanidins. Thanks to the recently released grape genome we could identify several genes highly homologous to known anthocyanin pathway regulators of other species. In a phylogenetic tree of Vitis vinifera MYB factors there is a clade where VvMYBA1 and VvMYBA2 cluster together with few other MYBs and to date only the MYBA1 and MYBA2 have been characterized. In this clade, three highly homologous genes residing on the same chromosome (Chromosome 2) were named VvMYBA5, VvMYBA6 and VvMYBA7, and chosen for further characterization. Due to the high homology with the characterized VvMYBA1 and VvMYBA2, we hypothesized that they could regulate the anthocyanin synthesis in different organs/stages respect to VvMYBA1. In the same way, we found another clade, called “repressors clade” in witch the proteins are characterized by the presence of the repression motif. The information regarding the MYB repressors in Vitis vinifera is scarce and only one MYB repressor has been identified until now. So another part of my PhD project is the functional characterization of different MYB proteins present in this clade utilizing different approaches as, the heterologous expression in Petunia hybirda, the ectopic overexpression in grapevine hairy roots and the Yeast two hybrid analysis. In details, I choose five different MYB repressors, named VvMYB4a, VvMYB4b, VvMYBC2-L1, VvMYBC2-L2 and VvMYBC2-L3, based on the homology with other repressors belonging to a different species and on particular amino acidic signatures. Moreover, I initiated a study of some MYB proteins belonging to the R2R3 MYB repressor clade, almost completely uncharacterized in grapevine. Finally, I tentatively characterized, a grapevine MYB with a single R3 repeat.