ROLE OF CUTINASE IN THE PENETRATION OF APPLE LEAVES BY VENTURIA-INAEQUALIS

Cutin hydrolysate induced the production of extracellular cutinase by mycelium and germinating conidia of Venturia inaequalis. The level of production was dose-dependent up to 0.25 mg ml-1 of hydrolysate added. Glucose was found to act as a repressor of cutinase production, but basal levels of the enzyme could be induced at high concentrations of glucose. Induction of cutinase was inhibited by cycloheximide, but not actinomycin D. O-Methyl-O-butyl-O-(3,5,6-trichloro-2-pyridyl)phosphate was shown to act as an almost quantitative inhibitor of cutinase purified from V. inaequalis. In the presence of the inhibitor on the surface of apple leaves inoculated with conidia of the pathogen, the formation of subcuticular mycelium beneath appressoria, and thus, cuticle penetration, was prevented. The results suggest that cutinase is induced by cutin monomers liberated from cuticles upon contact with conidia, and that the enzyme is crucially involved in cuticle penetration. The induction of basal levels of cutinase in the presence of glucose indicates a leakiness of repression and might be related to a more permanent role of cutinase during subcuticular growth of the pathogen.