In planta sequential hydroxylation and glycosylation of a fungal phytotoxin: Avoiding cell death and overcoming the fungal invader

To facilitate plant colonization, some pathogenic fungi produce phytotoxic metabolites that damage tissues; plants may be resistant to a particular pa thogen if they produce an enzyme(s) that catalyzes detoxification of this m etabolite(s). Alternaria blackspot is one of the most damaging and signific ant fungal diseases of brassica crops, with no source of resistance known w ithin the Brassica species. Destruxin B is the major phytotoxin produced by the blackspot-causing fungus, Alternaria brassicae (Berkley) Saccardo. We have established that a blackspot-resistant species (Sinapis alba) metaboli zed C-14-labeled destruxin B to a less toxic product substantially faster t han any of the susceptible species. The first metabolite, hydroxydestruxin B (C-14-labeled), was further biotransformed to the beta -D-glucosyl deriva tive at a slower rate. The structures of hydroxydestruxin B and beta -D-glu cosyl hydroxydestruxin B were deduced from their spectroscopic data [NMR, h igh resolution (HR)-MS, Fourier transform infrared (FTIR)] and confirmed by total chemical synthesis. Although these hydroxylation and glucosylation r eactions occurred in both resistant (S. alba) and susceptible (Brassica nap us, Brassica juncea, and Brassica rapa) species, hydroxylation was the rate limiting step in the susceptible species, whereas glucosylation was the ra te limiting step in the resistant species. Remarkably. it was observed that the hydroxydestruxin B induced the biosynthesis of phytoalexins in blacksp ot-resistant species but not in susceptible species. This appears to be a u nique example of phytotoxin detoxification and simultaneous phytoalexin eli citation by the detoxification product. Our studies suggest that S. alba ca n overcome the fungal invader through detoxification of destruxin B coupled with production of phytoalexins.