Detoxification of the phytoalexin brassinin by isolates of Leptosphaeria maculans pathogenic on brown mustard involves an inducible hydrolase

Brassinin is a phytoalexin produced by plants from the family Brassicaceae that displays antifungal activity against a number of pathogens of Brassica species, including Leptosphaeria maculans (Desm.) Ces. et de Not. asexual stage Phoma lingam (Tode ex Fr.) Desm.] and L. biglobosa. The interaction of a group of isolates of L. maculans virulent on brown mustard (Brassica juncea) with brassinin was investigated. The metabolic pathway for degradation of brassinin, the substrate selectivity of the putative detoxifying hydrolase, as well as the antifungal activity of metabolites and analogs of brassinin are reported. Brassinin hydrolase activity was detectable only in cell-free homogenates resulting from cultures induced with brassinin, N'-methylbrassinin, or camalexin. The phytoalexin camalexin was a substantially stronger inhibitor of these isolates than brassinin, causing complete growth inhibition at 0.5mM.