Strategies of cruciferous pathogenic fungi: detoxification of the phytoalexin cyclobrassinin by mimicry.

The remarkable metabolism of the cruciferous phytoalexin cyclobrassinin by the phytopathogenic root rot (Rhizoctonia solani Kuhn) and blackleg [Phoma lingam (Tode ex Fr.) Desm., asexual stage of Leptosphaeria maculans (Desm.) Ces. et de Not.] fungi is reported. It was established that R. solani metabolized and detoxified cyclobrassinin via the phytoalexin brassicanal A, which was further transformed into nontoxic products. Detoxification of cyclobrassinin in P. lingam avirulent isolate Unity occurred via the phytoalexin brassilexin, whereas the detoxification in P. lingam virulent isolate BJ 125 occurred via the phytoalexin dioxibrassinin. The chemistry involved in the structure determination of the intermediates of these three apparently different pathways and their antifungal activities are described. In addition, efficient syntheses of both phytoalexins brassicanal A and brassilexin by mimicry of the fungal biotransformation route are reported. Implications of these unprecedented transformations are discussed.