Cucurbitacins, 'attack' enzymes and laccase in Botrytis cinerea

The effect of cucurbitacins on the secretion by Botrytis cinerea of polygalacturonase, cellulase, acid proteinase, carboxypeptidase and pectin methylesterase was studied. Neither the activity of these 'attack' enzymes, nor their appearance in the culture medium of Botrytis was affected by the presence of cucurbitacins. In contrast, the appearance of laccase activity was strongly repressed both in the medium and in the mycelium by the four cucurbitacins studied, B, E, D and I. In the presence of cucurbitacins, the electrophoretic mobility of extra-cellular laccase from Botrytis is altered, indicating that the compounds may cause a change in enzyme structure or conformation. Since cucurbitacin D does not affect laccase in vitro it is probable that the compound acts during enzyme formation. Cucurbitacins were shown to protect carrot tissue and cucumber leaves from infection by Botrytis in addition to the protective effect on cucumber fruit and cabbage leaves. It is suggested that, in Botrytis, laccase is part of the system of 'attack' enzymes involved in the infection process and in fungal virulence.