Towards a deep understanding of the function of grape regulators VvMYB5a and VvMYB5b

Few members of the large family of grapevine R2R3-MYB transcriptional regulators have been isolated and unequivocally assigned to specific metabolic and/or developmental processes. In previous studies, VvMYB5a and VvMYB5b have been partially characterized and proposed to regulate the early flavonoid structural genes in different stages of berry development. However several indications such as the expression profile in several plant organs/tissues, the results of genome-wide coexpression analyses and the comparative analysis with other plant systems, point out that these two regulators may play additional roles, not strictly related to the flavonoid biosynthesis. To gain information about the pathways controlled by these two MYB proteins and to unravel the regulatory network they belong to, we produced grapevine transgenic plants with an altered expression of VvMYB5a and VvMYB5b, and transgenic plants silenced for the TTG2-like WRKY gene putatively acting downstream these two MYBs in the regulatory cascade. All transgenic lines displayed severe phenotypic alterations affecting leaves and sometimes the whole plant. Transcriptomic analysis of young leaves revealed that a small set of modulated genes was shared among different transgenic lines. These genes seem not directly involved in the flavonoid pathway and, more likely, are related to lipid metabolism and ion transport across membranes. Altogether these results indicate that VvMYB5a and VvMYB5b may act in a transcriptional regulatory network conserved among plant species.