The onset of grapevine berry ripening is characterized by ros accumulation and lipoxygenase-derived galactolipid peroxides

The ripening of fleshy fruits is a complex developmental program characterized by extensive transcriptomic and metabolic remodeling in the pericarp tissues (pulp and skin). The onset of ripening is modulated by a network of external and endogenous signals. Previous studies reported the accumulation of hydrogen peroxide (H2O2) and extensive modulation of reactive oxygen species (ROS) scavenging enzymes at véraison in grape berries, suggesting ROS may participate to fruit development control. In order to better characterize the oxidative events occurring at ripening onset, Vitis vinifera cv. Pinot Noir berries sampled during seven weeks centered on véraison were analyzed for ROS identification and quantification. We show that H2O2 and singlet oxygen (1O2) accumulate in berry skin cells at softening, in the cytosol and plastids, respectively. H2O2 peak at véraison is followed immediately by a peak of catalase activity. Also a specific class of membrane lipids, i.e. galactolipids, shows the accumulation of oxidized species at véraison. Monogalactosyl diacylglycerols (MGDGs) and digalactosyl diacylglycerol (DGDGs) are peroxidized on one or both -linolenic fatty acid chains, with a 13(S) absolute configuration implying the action of a specific enzyme. We identified a lipoxygenase (PnLOXA) which is expressed at véraison and localizes at the plastid thylakoid membranes. This enzyme is indeed able to catalyze membrane galactolipid peroxidation in tobacco leaves overexpressing PnLOXA, strongly supporting its role in grapevine berry lipid peroxidation. This reaction could represent the first step of oxylipins biosynthesis or a mechanism to regulate membrane proteins redox state, possibly involved in ripening control.