Glycosylation and oxidative dimerization of resveratrol are respectively associated to sensitivity and resistance of grapevine cultivars to downy mildew

Levels of resistance to Plasmopara viticola , from susceptible to highly resistant, of different grapevine cultivars were observed in vineyards and confirmed by the symptoms developed after inoculations. On the abaxial surface of infected leaves, P. viticola developed abundant sporangiophores on susceptible cultivars (Chasselas, Gamay, Gamaret and Pinot Noir), whereas on less susceptible cultivars (Seyval Blanc and Johanniter) the parasite produced few sporangiophores and some necrotic spots at the site of infection. On resistant cultivars (Bronner, Solaris, IRAC 2091), P. viticola induced a hypersensitive response and only necrotic spots were visible and the disease ceased to develop. Stilbenes were analyzed 4, 7, 24, 48 h post-infection (hpi) on small leaf samples cut from the site of infection. Large differences were observed between the cultivars at 24 and 48 hpi. Susceptible grapevines produced resveratrol and its glycoside, piceide. In contrast, resistant plants produced high concentration of e- and δ-viniferin. Resveratrol and piceide have little or no toxicity activity against P. viticola , whereas δ-viniferin is highly toxic and can be considered an important marker for resistance of grapevine to downy mildew. The importance of oxidative dimerization of resveratrol in comparison to the extent of its glycosylation in defense reaction of grapevines against P. viticola is discussed.