Grape berry proteins act as scavengers of grape polyphenols and protect polygalactutonase activity of Botrytis cinerea from inhibition

At maturity grape berries contain a large amount of proteins and polyphenols with potential anti-fungal activity. The protein fraction of grape extract includes mostly thaumatin-like proteins and chitinase while flavonoids predominate among polyphenols. However, Botrytis cinerea grows well in the presence of proteins and polyphenols when these molecules are supplied to the culture in a ratio similar to that measured in the grape fruit extract. In vitro it was observed that a fraction of grape proteins interacts with polyphenols forming complexes with reduced solubility and that B. cinerea laccase enhances this process. Polyphenols are well known inhibitors of fungal polygalacturonases (PGs) activity, and B. cinerea PG activity is inhibited by grape polyphenols in vitro. However, the simultaneous administration of grape proteins diminishes the inhibitory activity of grape polyphenols, and laccase addition restores completely B. cinerea PG activity. Similar results are obtained when the stylbenic phytoalexin resveratrol is used in combination with grape proteins and B. cinerea laccase. The scavenging of polyphenols by grape proteins could favour the berry infection by B. cinerea.