Stimulation of secondary metabolism in grape berry exocarps by a nature-based strategy of foliar application of polyols

In grapes under drought stress, polyols accumulate through tight coordination at the molecular level between increased membrane transport of polyols and inhibition of polyol oxidation. Here, the effects on grape metabolism of an exogenous foliar application of polyols as a potential sustainable viticultural practice to increase grapevine performance and berry quality were thoroughly assessed. Grapevines were pulverised with a polyol solution containing 2 mM mannitol and 2 mM sorbitol, and the metabolome of grape berry exocarps and important metabolic pathways associated with berry quality and polyol metabolism were analysed at véraison and mature stages. By combining metabolomics analysis using UPLC-MS, enzyme activity assays and targeted transcriptional analyses, it was demonstrated that foliar application of polyols stimulated by 3.5-fold and 6-fold abscisic acid (ABA) biosynthesis at véraison and mature grape berries, respectively. It also stimulated the concentration of anthocyanins, stilbenes and total phenolics in exocarps through the upregulation of phenylpropanoid, stilbenoid and anthocyanin biosynthetic pathways shown by increases in phenylalanine-ammonia lyase (PAL) activity (3-fold) and VviPAL1 expression, stilbene synthase 1 (VviSTS1) transcripts (ca. 5-fold), UDP-glucose:flavonoid 3-O-glucosyltransferase (UFGT) activity and VviUFGT1 expression, among other results, at the mature stage, when these changes were most noticeable. Many secondary metabolites synthesised in these pathways identified by UPLC-MS analysis were present at higher quantities in exocarps from polyol-treated plants such as fertaric acid, E-resveratrol, E-piceatannol, piceid, pallidol, E-ε-viniferin, myricetin-hexoside 1, cyanidin-3-O-glucoside and malvidin-3-O-(6-p-coumaroyl)-glucoside. Foliar application of low-concentration polyols is, therefore, a promising biostimulant-based strategy to improve grape berry quality and nutritional value in the current context of climate change.