Because of the vast range of functions that phenylpropanoids possess, their synthesis requires precise spatiotemporal coordination throughout plant development and in response to the environment. The accumulation of these secondary metabolites is transcriptionally controlled by positive and negative regulators from the MYB and basic helix-loop-helix protein families. We characterized four grapevine (Vitis vinifera) R2R3-MYB proteins from the C2 repressor motif clade, all of which harbor the ethylene response factor-associated amphiphilic repression domain but differ in the presence of an additional TLLLFR repression motif found in the strong flavonoid repressor Arabidopsis (Arabidopsis thaliana) AtMYBL2. Constitutive expression of VvMYB4a and VvMYB4b in petunia (Petunia hybrida) repressed general phenylpropanoid biosynthetic genes and selectively reduced the amount of small-weight phenolic compounds. Conversely, transgenic petunia lines expressing VvMYBC2-L1 and VvMYBC2-L3 showed a severe reduction in petal anthocyanins and seed proanthocyanidins together with a higher pH of crude petal extracts. The distinct function of these regulators was further confirmed by transient expression in tobacco (Nicotiana benthamiana) leaves and grapevine plantlets. Finally, VvMYBC2-L3 was ectopically expressed in grapevine hairy roots, showing a reduction in proanthocyanidin content together with the down-regulation of structural and regulatory genes of the flavonoid pathway as revealed by a transcriptomic analysis. The physiological role of these repressors was inferred by combining the results of the functional analyses and their expression patterns in grapevine during development and in response to ultraviolet B radiation. Our results indicate that VvMYB4a and VvMYB4b may play a key role in negatively regulating the synthesis of small-weight phenolic compounds, whereas VvMYBC2-L1 and VvMYBC2-L3 may additionally fine tune flavonoid levels, balancing the inductive effects of transcriptional activators.

The plant WRKY gene family represents an ancient and complex class of zinc-finger transcription factors (TFs) that are involved in the regulation of various physiological processes, such as development and senescence, and in plant response to many biotic and abiotic stresses. Despite the growing number of studies on the genomic organisation of WRKY gene family in different species, little information is available about this family in grapevine (Vitis vinifera L.). In the present study, a total number of 59 putative grapevine WRKY transcription factors (VvWRKYs) were identified based on the analysis of various genomic and proteomic grapevine databases. According to their structural and phylogentic features, the identified grapevine WRKY transcription factors were classified into three main groups. In order to shed light into their regulatory roles in growth and development as well as in response to biotic and abiotic stress in grapevine, the VvWRKYs expression profiles were examined in publicly available microarray data. Bioinformatics analysis of these data revealed distinct temporal and spatial expression patterns of VvWRKYs in various tissues, organs and developmental stages, as well as in response to biotic and abiotic stresses. To also extend our analysis to situations not covered by the arrays and to validate our results, the expression profiles of selected VvWRKYs in response to drought stress, Erysiphe necator (powdery mildew) infection, and hormone treatments (salicilic acid and ethylene), were investigated by quantitative real-time reverse transcription PCR (qRT-PCR). The present study provides a foundation for further comparative genomics and functional studies of this important class of transcriptional regulators in grapevine.

Anthocyanin levels decline in some red grape berry varieties ripened under high-temperature conditions, but the underlying mechanism is not yet clear. Here we studied the effects of two different temperature regimes, representing actual Sangiovese (Vitis vinifera L.) viticulture regions, on the accumulation of mRNAs and enzymes controlling berry skin anthocyanins. Potted uniform plants of Sangiovese were kept from veraison to harvest, in two plastic greenhouses with different temperature conditions. The low temperature (LT) conditions featured average and maximum daily air temperatures of 20 and 29 °C, respectively, whereas the corresponding high temperature (HT) conditions were 22 and 36 °C, respectively. The anthocyanin concentration at harvest was much lower in HT berries than LT berries although their profile was similar under both conditions. Under HT conditions, the biosynthesis of anthocyanins was suppressed at both the transcriptional and enzymatic levels, but peroxidase activity was higher. This suggests that the low anthocyanin content of HT berries reflects the combined impact of reduced biosynthesis and increased degradation, particularly the direct role of peroxidases in anthocyanin catabolism. Overexpression of VviPrx31 decreased anthocyanin contents in Petunia hybrida petals under heat stress condition. These data suggest that high temperature can stimulate peroxidase activity thus anthocyanin degradation in ripening grape berries.

By performing molecular studies coupled to radiation experiments and in silico systems analyses, we have ascertained the role of the grapevine UV-B receptor and two HY5 homologues in regulating flavonol synthesis.

Anthocyanins are flavonoid compounds responsible for red/purple colors in the leaves, fruit, and flowers of many plant species. They are produced through a multistep pathway that is controlled by MYB transcription factors. VvMYBA1 and VvMYBA2 activate anthocyanin biosynthesis in grapevine (Vitis vinifera) and are nonfunctional in white grapevine cultivars. In this study, transgenic grapevines with altered VvMYBA gene expression were developed, and transcript analysis was carried out on berries using a microarray technique. The results showed that VvMYBA is a positive regulator of the later stages of anthocyanin synthesis, modification, and transport in cv Shiraz. show that Vv3AT has a broad anthocyanin substrate specificity and can also utilize both aliphatic and aromatic acyl donors, a novel activity for this enzyme family found in nature. In cv Pinot Noir, a red-berried grapevine mutant lacking acylated anthocyanins, Vv3AT contains a nonsense mutation encoding a truncated protein that lacks two motifs required for BAHD protein activity. Promoter activation assays confirm that Vv3AT transcription is activated by VvMYBA1, which adds to the current understanding of the regulation of the BAHD gene family. The flexibility of Vv3AT to use both classes of acyl donors will be useful in the engineering of anthocyanins in planta or in vitro.