A group of grapevine MYBA transcription factors located in chromosome 14 control anthocyanin synthesis in vegetative organs with different specificities compared with the berry color locus.

Grapevine organs accumulate anthocyanins in a cultivar-specific and environmentally induced manner. The MYBA1-A2 genes within the berry color locus in chromosome 2 represent the major genetic determinants of fruit color. The simultaneous occurrence of transposon insertions and point mutations in these genes is responsible for most white-skinned phenotypes; however, the red pigmentation found in vegetative organs suggests the presence of additional regulators. This work describes a genomic region of chromosome 14 containing three closely related R2R3-MYB genes, named MYBA5, MYBA6 and MYBA7. Ectopic expression of the latter two genes in grapevine hairy roots promoted anthocyanin accumulation without affecting other phenylpropanoids. Transcriptomic profiling of hairy roots expressing MYBA1, MYBA6 and MYBA7 showed that these regulators share the activation of late biosynthetic and modification/transport-related genes, but differ in the activation of the FLAVONOID-3'5'-HYDROXYLASE (F3'5'H) family. An alternatively spliced MYBA6 variant was incapable of activating anthocyanin synthesis, however, because of the lack of an MYC1 interaction domain. MYBA1, MYBA6.1 and MYBA7 activated the promoters of UDP-GLUCOSE:FLAVONOID 3-O-GLUCOSYLTRANSFERASE (UFGT) and ANTHOCYANIN 3-O-GLUCOSIDE-6″-O-ACYLTRANSFERASE (3AT), but only MYBA1 induced F3'5'H in concordance with the low proportion of tri-hydroxylated anthocyanins found in MYBA6-A7 hairy roots. This putative new color locus is related to the red/cyanidic pigmentation of vegetative organs in black- and white-skinned cultivars, and forms part of the UV-B radiation response pathway orchestrated by ELONGATED HYPOCOTYL 5 (HY5). These results demonstrate the involvement of additional anthocyanin regulators in grapevine and suggest an evolutionary divergence between the two grape color loci for controlling additional targets of the flavonoid pathway.

Characterization of a multifunctional caffeoyl-CoA O-methyltransferase activated in grape berries upon drought stress.

Drought stress affects anthocyanin accumulation and modification in vegetative and reproductive plant tissues. Anthocyanins are the most abundant flavonoids in grape (Vitis vinifera L.) coloured berry genotypes and are essential markers of grape winemaking quality. They are mostly mono- and di-methylated, such modifications increase their stability and improve berry quality for winemaking. Anthocyanin methylation in grape berries is induced by drought stress. A few caffeoyl-CoA O-methyltransferases (CCoAOMTs) active on anthocyanins have been described in grape. However, no drought-activated O-methyltransferases have been described in grape berries yet. In this study, we characterized VvCCoAOMT, a grapevine gene known to induce methylation of CoA esters in cultured grape cells. Transcript accumulation of VvCCoAOMT was detected in berry skins, and increased during berry ripening on the plant, and in cultured berries treated with ABA, concomitantly with accumulation of methylated anthocyanins, suggesting that anthocyanins may be substrates of this enzyme. Contrary as previously observed in cell cultures, biotic stress (Botrytis cinerea inoculation) did not affect VvCCoAOMT gene expression in leaves or berries, while drought stress increased VvCCoAOMT transcript in berries. The recombinant VvCCoAOMT protein showed in vitro methylating activity on cyanidin 3-O-glucoside. We conclude that VvCCoAOMT is a multifunctional O-methyltransferase that may contribute to anthocyanin methylation activity in grape berries, in particular under drought stress conditions.

Recovery from water stress affects grape leaf petiole transcriptome.

Fast and efficient recovery from water stress is a key determinant of plant adaptation to changing meteorological conditions modulating transpiration, i.e. air temperature and humidity. We analysed transcriptomic responses during rehydration after water stress in grapevine leaf petioles, where embolism formation and repair commonly take place, and where metabolic changes related to embolism recovery are expected to be particularly important. We compared gene expression of recovering plants with irrigated controls, upon high and low transpiration conditions, using cDNA microarrays. In parallel, we assessed the daily dynamics of water relations, embolism formation and repair, and leaf abscisic acid concentration. In recovering plants, the most affected gene categories were secondary metabolism, including genes linked to flavonoid biosynthesis; sugar metabolism and transport, and several aquaporin genes. The physiological dynamics of recovery were lower and the number of differentially expressed probes was much lower upon low transpiration than found in actively transpiring grapevines, suggesting the existence of a more intense metabolic reorganization upon high transpiration conditions and of a signal eliciting these responses. In plants recovering under high transpiration, abscisic acid concentrations significantly increased, and, in parallel, transcripts linked to abscisic acid metabolism and signalling (ABA-8'-hydroxylase, serine-threonine kinases, RD22 proteins) were upregulated; a trend that was not observed upon low transpiration. Our results show that recovery from water stress elicits complex transcriptomic responses in grapevine. The increase observed in abscisic acid cellular levels could represent a signal triggering the activation of responses to rehydration after stress.