Defence responses of grapevine cultivars to powdery mildew: Ontogenic resistance versus genetic resistance

The most sustainable management of resistant varieties involves understanding which defence mechanisms the plant uses according to its resistance genes and stage of development and how effective these mechanisms are. Three grapevine varieties were compared for their susceptibility versus resistance to powdery mildew: Vitis vinifera 'Cabernet Sauvignon' (CS) and two hybrids, 'Artaban' and 'Prior'. Four different leaf ages were collected from 2-year-old grafted grapevines planted in pot or in vineyard, and several variables were assessed for each leaf: (a) morphological and physiological indicators, (b) pathogenicity traits: spore germination, infection ef?ciency, mycelial growth and sporulation, and (c) gene expression using a high-throughput quantitative reverse transcription PCR method to inform about plant defence status and functioning of primary metabolism. A significant decrease in germination and sporulation with increasing leaf age were observed for the three varieties: Prior showed a susceptibility similar to that of CS, whereas Artaban was fully resistant with no sporulation. Genes highlighted as markers of the variety were involved in defence (VvPR3, VvPR4 overexpressed in Artaban, VvPR4bis, VvAlli2 in Prior) or in primary metabolism (repression of VvCitS in Artaban, VvCHI in Prior, overexpression of VvCAD2, VvGST3 in Prior). After inoculation, more defence genes, especially PR proteins, were overexpressed in the oldest leaves as potential components of ontogenic resistance. PR proteins accounted for 25% of total disease variation. Phenylpropanoid and ethylene pathways were leaf age-dependent. The statistical method used highlighted the contributions of the genotypic expression involved in genetic (cultivar) and ontogenic (leaf age) resistances and of the metabolic pathways to the disease.