Metabolism of the crucifer phytoalexins wasalexin A and B in the plant pathogenic fungus Leptosphaeria maculans

Abstract

Wasalexins A and B are crucifer phytoalexins produced by two substantially different plant species, a wild species abundant in the Canadian prairies and a condiment plant widely cultivated in Japan. Interestingly, both plant species are resistant to an economically important fungal plant pathogen, the blackleg fungus [Leptosphaeria maculans (Desm.) Ces. et de Not., asexual stage Phoma lingam (Tode ex Fr.) Desm.]. The transformation of wasalexins A and B in cultures of isolates of L. maculans, an isolate highly virulent towards canola (BJ 125) and a less common isolate which is virulent towards wasabi (Laird 2/Mayfair 2) was investigated. It was established that both fungal isolates are able to efficiently metabolize and detoxify wasalexins A and B through reduction in the case of wasalexin A or through hydrolysis followed by reduction in the case of wasalexin B. Moreover, a close structural analogue of wasalexins, which does not occur naturally, was also found to be reduced in cultures of L. maculans. The structures of the new metabolic products were elucidated using spectroscopic methods and were confirmed by synthesis. Bioassays indicated that the biotransformation of wasalexins is a detoxification process that may contribute to the aggressive nature of these fungal isolates towards plants that produce wasalexins.