Meccanismi fisiologici e molecolari di resistenza a stress idrico in Vitis vinifera L: aspetti del metabolismo primario e secondario e adattamenti di genotipi diversi

In questa review, affrontiamo i principali aspetti della resistenza della vite (Vitis vinifera L) a stress idrico, soffermandoci sulle risposte ecofisiologiche, fisiologiche e molecolari legate al metabolismo primario e secondario, soprattutto a livello di acino. Esaminiamo i meccanismi di elusione dello stress, relativi al controllo stomatico, al ruolo di segnale dell’acido abscissico, a quello del segnale idraulico (embolismi) e delle acquaporine. Descriviamo i diversi meccanismi di tolleranza, sia dal punto di vista dell’induzione e regolazione delle vie biosintetiche, sia dell’accumulo dei metaboliti derivati. In tutti gli aspetti esaminati, sottolineiamo le peculiarità varietali conosciute. Physiological and molecular mechanisms of resistance to drought in Vitis vinifera L: aspects of primary and secondary metabolisms and adaptation of genotypes. In this review, we address the main aspects of grapevine (Vitis vinifera L) resistance to water stress. We point to examine physiological and molecular answers, involving both primary and secondary metabolisms. The grapevine has been generally classified as drought-avoiding or as "pessimistic" following the ecological classification of plants into "pessimists" and "optimists", even if different grapevine cultivars show different behaviours of defence against drought. This ecological classification is analogous to the physiological classification into isohydric and anisohydric plants and fundamentally linked to stomatal behaviour. Stomatal regulation of grapevine is under hormone (ABA) and hydraulic control, the latter linked to embolism formation and recovery in xylem tissues up-stream the stomata. We review ABA effects on stomata, and their interrelationship with hydraulics of the plant. To this aim, we show that in grapevine, xylem embolism occurs and repairs during diurnal cycles, extreme water potential causing embolism, whereas an almost full recovery of water potential is needed to promote repair mechanisms. In this phase, an ABA-induced transpiration control is proposed. Aquaporins play fundamental role on water stress control, and both early or late contribution (especially in root) are discussed. When drought-avoidance mechanisms are limiting in further plant dehydration avoidance (earlier in anisohydric varieties, but in all grape cultivars during late and/or severe drought stages) metabolic mechanisms of dehydration tolerance arise. In all tissues solutes accumulate to prevent water loss, by minimizing interferences with cellular function. Grapevines accumulate one or more types of compatible solutes (osmotic adjustment). Metabolite profiling reveals that there are higher concentrations of glucose, malate, proline, phenolics and flavour precursors upon water stress. The metabolite differences are linked to differences in transcript abundance of many genes involved in energy metabolism, particularly photosynthesis and photorespiration. Droughted grapevines appear to have a higher demand than non-stressed plants to detoxify free radicals (reactive oxygen species), and to cope with photoinhibition. Results of these defence strategies accumulate in berries, during the ripening period, and are at the base of grape quality. Berry metabolites accumulated in several cultivars, as a response of controlled levels of water stress to the grapevine, are shown. Berry (skin, pulp and seed) phenolic fractions and flavours are reviewed and linked to perturbations of their molecular pathways, when known. Physiological, molecular and metabolic peculiarities genotype-specific are underlined. Future perspectives of research on the reviewed topic are proposed, as focused on the interrelationship between ABA metabolism and molecular aspects of water control (aquaporins).