Metabolism of crucifer phytoalexins in Sclerotinia sclerotiorum: detoxification of strongly antifungal compounds involves glucosylation

The strongly antifungal phytoalexins brassilexin and sinalexin were metabolized by the stem rot fungus Sclerotinia sclerotiorum to glucosyl derivatives, whereas the phytoalexins brassicanal A, spirobrassinin and 1-methoxyspirobrassinin, displaying lower antifungal activity, were transformed via non-glucosylating pathways. Significantly, these transformations led to metabolites displaying no detectable antifungal activity. The chemical characterization of all new metabolites as well as the chemistry of these processes and a facile chemical synthesis of 1-beta-D-glucopyranosylbrassilexin are reported. Overall, our results indicate that phytoalexins, strongly antifungal against S. sclerotiorum, are detoxified via glucosylation, which in turn suggests that S. sclerotiorum has acquired efficient glucosyltransferase(s) that can disarm some of the most active plant chemical defenses. Consequently, we suggest that these glucosylation reactions are potential metabolic targets to control S. sclerotiorum.