Secondary metabolism contributes to the adaptation of a plant to its environment. In wine grapes, fruit secondary metabolism largely determines wine quality. Climate change is predicted to exacerbate drought events in several viticultural areas, potentially affecting the wine quality. In red grapes, water deficit modulates flavonoid accumulation, leading to major quantitative and compositional changes in the profile of the anthocyanin pigments; in white grapes, the effect of water deficit on secondary metabolism is still largely unknown. In this study we investigated the impact of water deficit on the secondary metabolism of white grapes using a large scale metabolite and transcript profiling approach in a season characterized by prolonged drought. Irrigated grapevines were compared to non-irrigated grapevines that suffered from water deficit from early stages of berry development to harvest. A large effect of water deficit on fruit secondary metabolism was observed. Increased concentrations of phenylpropanoids, monoterpenes, and tocopherols were detected, while carotenoid and flavonoid accumulations were differentially modulated by water deficit according to the berry developmental stage. The RNA-sequencing analysis carried out on berries collected at three developmental stages—before, at the onset, and at late ripening—indicated that water deficit affected the expression of 4,889 genes. The Gene Ontology category secondary metabolic process was overrepresented within up-regulated genes at all the stages of fruit development considered, and within down-regulated genes before ripening. Eighteen phenylpropanoid, 16 flavonoid, 9 carotenoid, and 16 terpenoid structural genes were modulated by water deficit, indicating the transcriptional regulation of these metabolic pathways in fruit exposed to water deficit. An integrated network and promoter analyses identified a transcriptional regulatory module that encompasses terpenoid genes, transcription factors, and enriched drought-responsive elements in the promoter regions of those genes as part of the grapes response to drought. Our study reveals that grapevine berries respond to drought by modulating several secondary metabolic pathways, and particularly, by stimulating the production of phenylpropanoids, the carotenoid zeaxanthin, and of volatile organic compounds such as monoterpenes, with potential effects on grape and wine antioxidant potential, composition, and sensory features.

Background: Structural genes of the phenyl-propanoid pathway which encode flavonoid 3'- and 3',5'-hydroxylases (F3'H and F3'5'H) have long been invoked to explain the biosynthesis of cyanidin- and delphinidin-based anthocyanin pigments in the so-called red cultivars of grapevine. The relative proportion of the two types of anthocyanins is largely under genetic control and determines the colour variation among red/purple/blue berry grape varieties and their corresponding wines.Results: Gene fragments ofVvF3'HandVvF3'5'H, that were isolated fromVitis vinifera'Cabernet Sauvignon' using degenerate primers designed on plant homologous genes, translated into 313 and 239 amino acid protein fragments, respectively, with up to 76% and 82% identity to plant CYP75 cytochrome P450 monooxygenases. Putative function was assigned on the basis of sequence homology, expression profiling and its correlation with metabolite accumulation at ten different ripening stages. At the onset of colour transition, transcriptional induction ofVvF3'HandVvF3'5'Hwas temporally coordinated with the beginning of anthocyanin biosynthesis, the expression being 2-fold and 50-fold higher, respectively, in red berries versus green berries. The peak ofVvF3'5'Hexpression was observed two weeks later concomitantly with the increase of the ratio of delphinidin-/cyanidin-derivatives. The analysis of structural genomics revealed that two copies ofVvF3'Hare physically linked on linkage group no. 17 and several copies ofVvF3'5'Hare tightly clustered and embedded into a segmental duplication on linkage group no. 6, unveiling a high complexity when compared to other plant flavonoid hydroxylase genes known so far, mostly in ornamentals.Conclusion: We have shown that genes encoding flavonoid 3'- and 3',5'-hydroxylases are expressed in any tissues of the grape plant that accumulate flavonoids and, particularly, in skin of ripening red berries that synthesise mostly anthocyanins. The correlation between transcript profiles and the kinetics of accumulation of red/cyanidin- and blue/delphinidin-based anthocyanins indicated thatVvF3'HandVvF3'5'Hexpression is consistent with the chromatic evolution of ripening bunches. Local physical maps constructed around theVvF3'HandVvF3'5'Hloci should help facilitate the identification of the regulatory elements of each isoform and the future manipulation of grapevine and wine colour through agronomical, environmental and biotechnological tools.

Grape leaves provide the biochemical substrates for berry development. Thus, understanding the regulation of grapevine leaf metabolism can aid in discerning processes fundamental to fruit development and berry quality. Here, the temporal alterations in leaf metabolism in Merlot grapevine grown under sufficient irrigation and water deficit were monitored from veraison until harvest. The vines mediated water stress gradually and involving multiple strategies: osmotic adjustment, transcript-metabolite alteration and leaf shedding. Initially stomatal conductance and leaf water potential showed a steep decrease together with the induction of stress related metabolism, e.g. up-regulation of proline and GABA metabolism and stress related sugars, and the down-regulation of developmental processes. Later, progressive soil drying was associated with an incremental contribution of Ca2+ and sucrose to the osmotic adjustment concomitant with the initiation of leaf shedding. Last, towards harvest under progressive stress conditions following leaf shedding, incremental changes in leaf water potential were measured, while the magnitude of perturbation in leaf metabolism lessened. The data present evidence that over time grapevine acclimation to water stress diversifies in temporal responses encompassing the alteration of central metabolism and gene expression, osmotic adjustments and reduction in leaf area. Together these processes mitigate leaf water stress and aid in maintaining the berry-ripening program.

In grapevine, the anatomy of xylem conduits and the non-structural carbohydrates (NSCs) content of the associated living parenchyma are expected to influence water transport under water limitation. In fact, both NSC and xylem features play a role in plant recovery from drought stress. We evaluated these traits in petioles of Cabernet Sauvignon (CS) and Syrah (SY) cultivars during water stress (WS) and recovery. In CS, the stress response was associated to NSC consumption, supporting the hypothesis that starch mobilization is related to an increased supply of maltose and sucrose, putatively involved in drought stress responses at the xylem level. In contrast, in SY, the WS-induced increase in the latter soluble NSCs was maintained even 2 days after re-watering, suggesting a different pattern of utilization of NSC resources. Interestingly, the anatomical analysis revealed that conduits are constitutively wider in SY in well-watered (WW) plants, and that water stress led to the production of narrower conduits only in this cultivar.

Grapevine breeding has successfully developed mildew-resistant varieties, yet their susceptibility to arthropod pests remains largely unexplored. This review synthesizes current knowledge of arthropod pest interactions with mildew-resistant grapevines and compares them with those of conventional and parental varieties. Only a few studies have reported detailed information on arthropod pest susceptibility, covering 23 resistant varieties tested against seven species/groups, including Daktulosphaira vitifoliae, Lobesia botrana, Drosophila suzukii and eriophyid mites. About half of the available studies focused on berry pests—particularly the invasive D. suzukii—while leaf and root pests received little attention. In several cases, resistant varieties exhibited susceptibility levels comparable to those of their parental varieties, although the limited number of studies makes it difficult to draw clear conclusions. Overall, major knowledge gaps remain, highlighting the need for breeding programs that integrate mildew resistance with improved assessment of arthropod pest susceptibility, without compromising agronomic performance and wine quality.