The activity of the Botrytis cinerea endo-polygalacturonase PG1 is detected in berry skins and is required for full virulence during grape infection.

The necrotrophic fungal pathogen Botrytis cinerea is the causal agent of grey mould or Botrytis bunch rot in grapes. During the infection process this fungus secretes several cell-wall degrading enzymes, in particular endo-polygalacturonases (PGs) which are involved in the depolymerization of pectin, the main constituent of primary cell wall and middle lamella. The genome of B. cinerea contains six endo-PG encoding genes. Aim of the present work was to characterize the role of these enzymes during infection of grape berries. First, we studied the expression of the corresponding genes on berries of cv. Pinot blanc and Pinot noir. Among the genes analyzed by qPCR, only Bcpg2 was not expressed, while Bcpg1, encoding a basic isoform, showed the highest transcript levels at all time points analyzed and in both cvs. We also analyzed the PG activity produced by B. cinerea during grape berries infection by loading on a gel activity assay grinded berry skins: the BcPG1 isoform was detected but only as a weak band compared to other PGs produced by the fungus. However, since the BcPG1 has been previously demonstrated to be an important virulence factor in several host tissues although its role has never been investigated on grape berries, we performed infection experiments with a ΔBcPG1 knock-out mutant. A 20% reduction of symptoms caused by the ΔBcPG1 mutant was observed on both Pinot cv. and also on cv. Italia table grape, thus indicating that BcPG1 is required for full virulence on grape berries.