Some possible mechanisms deployed by fungal pathogens to prevent the effects of plant PR-proteins

Expression and accumulation of PR proteins is part of the defense machinery developed by plant to contrast plant pathogens attack. Different classes of PR-proteins accumulate locally, near the site of infection, and sistemically along the entire plant. It is supposed that plant pathogens have evolved mechanisms to avoid or prevent the harmful effects of PR-proteins. We have studied the effects of PR-proteins from grape on the fungal necrotrophic pathogens Botrytis cinerea and Sclerotium rolfsii. Mature berries and wine were chosen as useful sources of PR-proteins, especially because enriched in chitinase and thaumatin-like proteins. When the fungi were grown on berry juice, PR-proteins precipitated probably because complexes with plant oxidized polyphenols occurred, a process catalyzed by fungal laccase. When berry or wine proteins were added to the medium, B. cinerea produced appreciable proteinase activity and showed a noticeable mycelium fragmentation in comparison when grown in culture containing heat-denatured proteins. Instead, S. rolfsii grew in a normal way in the presence of native PR proteins and secreted only trace amounts of proteinase activity in the medium. The abundant glucan sheath, externally secreted by S. rolfsii, appears as the principle mechanism deployed by this fungus to defend itself from the toxic effects of grape PR proteins.