Deciphering heat-enhanced resistance of grapevine berries to Botrytis cinerea highlights differential cuticular and secondary metabolite accumulations between Merlot and Cabernet Sauvignon

In the context of climate change, temperature is a key abiotic driver of bunch microclimate, which, in order to reduce Botrytis cinerea development, is often managed in vineyards via practices such as leaf removal. The heat-dependent mechanisms of pathogen resistance in grapevines nevertheless remain to be fully elucidated. In this study, the effect of heat stress (HS) applied specifically to green bunches on infections caused by B. cinerea on ripe berries inoculated 23 days later was assessed for two years in a greenhouse. Bunches of the Cabernet Sauvignon (CS) and Merlot (M) cultivars were heated 6 days 8 h daily with a 10 °C increased temperature. In vitro bio tests highlighted a significant heat-enhanced resistance only in CS berries, whereas a stable constitutive resistance characterized the M berries. Bunch veraison and total sugar content were not affected by HS, rejecting its effect on maturation dynamics. Therefore, berry preformed barriers at the time of inoculation, which can hinder fungal colonization, were investigated. While HS had nearly no effect on waxes, it significantly affected the cutin content in both varieties, and more significantly its composition in CS. Similarly, the antifungal skin condensed tannins overaccumulated following HS in both cultivars, and their basal level was greater in CS than in M. Otherwise, M accumulated more stilbene and flavonoid compounds, which may have contributed to the observed varietal resistance. Finally, untargeted metabolomic data revealed a range of compounds modulated by HS in CS as potential candidates involved in resistance.