Even though Botrytis cinerea, the causal agent of gray mold, is a highly variable fungus with strains displaying very different degrees of virulence toward one given host plant species, no study has yet shown any correlation between the lack of aggressiveness of one given strain and its ability to stimulate a defense response from its host. Strains of B. cinerea collected from different host plant species were screened for their pathogenicity on grapevine to select two strains with similar morphological characteristics but different levels of virulence. In grapevine leaves, the less aggressive strain, T4, enhanced the accumulation of many defense products including secondary metabolites and the pathogenesis-related proteins, chitinase and beta-1,3-glucanase. Interestingly, secondary metabolites were formed in cells around a small group of dead cells. When compared with T4, the more aggressive strain, T8, had larger necrotic spots, no secondary metabolite biosynthesis, and accumulations of chitinases and beta-1,3-glucanases that were more delayed, yet only slightly weaker. The culture fluids of both strains mimicked the differential effect of each isolate in stimulating chitinase activity when infiltrated into grapevine leaves.
In order to better understand the defense strategy of grape berries (Vitis vinifera L. cv. Pinot noir) as they mature, the activities of the defense‐related proteins, chitinase (CHV, EC 3.2.1.14) and β‐1,3‐glucanase (laminarinase, EC 3.2.1.39) were first estimated in berries at different maturation stages. Chitinase levels rose proportionally to the berry reducing sugar content, an indicator of the berry ripening degree, up to values 10 times higher than the ones seen in resting grapevine leaves. This rise in activity was due to the accumulation of two isoforms, CHV 5 and CHV 11. One more chitinase isoform, CHV 12, appeared in senescent berries. Conversely, no glucanase activity could be detected in berries at any maturation stage. Accumulation of chitinases and (β‐1,3‐glucanases could be stimulated by wounding the berry peduncle. Adding salicylic acid to the wounded berries only potentiated the wounding effect on the berry chitinase activity. The most active chitinase isoform, CHV 5, was purified to homogeneity. It represented about 40% of the total extractable protein content of a ripe berry. Its molecular mass was estimated to be 31 kDa. The peptide sequencing of four of its tryptic fragments revealed strong homologies to several class IV chitinases. Finally, it was shown to inhibit the germination of conidia of Botrytis cinerea by 50% at a concentration of 7.5 µg ml−1.
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