Detoxification of cruciferous phytoalexins in Botrytis cinerea: spontaneous dimerization of a camalexin metabolite.

Phytopathogenic fungi are able to overcome plant chemical defenses through detoxification reactions that are enzyme mediated. As a result of such detoxifications, the plant is quickly depleted of its most important antifungal metabolites and can succumb to pathogen attack. Understanding and predicting such detoxification pathways utilized by phytopathogenic fungi could lead to approaches to control plant pathogens. Towards this end, the inhibitory activities and metabolism of the cruciferous phytoalexins camalexin, brassinin, cyclobrassinin, and brassilexin by the phytopathogenic fungus Botrytis cinerea Pers. (teleomorph: Botryotinia fuckeliana) was investigated. Brassilexin was the most antifungal of the phytoalexins, followed by camalexin, cyclobrassinin and brassinin. Although B. cinerea is a species phylogenetically related to the phytopathogenic fungus Sclerotinia sclerotiorum (Lib) de Bary, contrary to S. sclerotiorum, detoxification of strongly antifungal phytoalexins occurred via either oxidative degradation or hydrolysis but not through glucosylation, suggesting that glucosyl transferases are not involved. A strongly antifungal bisindolylthiadiazole that B. cinerea could not detoxify was discovered, which resulted from spontaneous oxidative dimerization of 3-indolethiocarboxamide, a camalexin detoxification product.