conference schedule and program with abstracts - Horticulture ...

P-54
Towards a deep understanding of the function of grape flavonoid regulators VvMYB5a
and VvMYB5b
E. Cavallini 1 , M. Walker 2 , S. Zenoni 1 , M. Fasoli* 1 , A. Zamboni 1 , M. Pezzotti 1 , G.B. Tornielli 1
1 Department of Biotecnology, University of Verona, Verona, Italy; 2 CSIRO Plant Industry, Glen
Osmond, South Australia, Australia
*Corresponding author: marianna.fasoli@univr.it
In grapevine, the biosynthesis of flavonoids is highly regulated in order that different compounds
are produced in different organs and in stages of development. Despite the flavonoid biosynthetic
pathway has been well studied, little is known about the transcriptional mechanism that regulates
it. Recent studies suggest that two MYB transcription factors, VvMYB5a and VvMYB5b, are
responsible for the regulation of the early flavonoid structural genes in different stages of berry
development. Until now, the role of VvMYB5a and VvMYB5b has been mainly inferred by
expression analyses and/or ectopic expression in model systems like tobacco and tomato. In
petunia the mutation of the ortholog of VvMYB5a and VvMYB5b (PhPH4) results in the increase
of vacuolar pH and, consequently, in blue shift of the flower color. We used functional
complementation analyses of some well characterized petunia anthocyanin/pH regulatory
mutants to gain information about the role of VvMYB5a and VvMYB5b in the regulatory
network operating in Vitis vinifera. The coding sequence of VvMYB5a and VvMYB5b was
fused to the constitutive promoter 35S and transformed into petunia ph4- mutant. Analyses of
transgenic plants revealed full complementation phenotypes attributable to an activation of target
genes belonging to vacuolar acidification. On the other hand, when the coding sequence of
VvMYB5a and VvMYB5b was overexpressed in an2 (acyanic) and in wild type (cyanic) petunia
backgrounds, the flavonoid pathway was activated, but only in the acyanic line. Overall, specific
and/or overlapping effects of the heterologous expression of the two regulators could be
observed. Functional studies using heterologous systems may give results that do not reflect the
real role of a gene in its homologous background. For this reason, Vitis vinifera cv Shiraz has
been transformed to silence and overexpress both VvMYB5a and VvMYB5b. Analyses of
transgenic plants (underway) will provide more conclusive information about the role of
VvMYB5a and VvMYB5b in the regulation of the flavonoid biosynthetic pathway.
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