Understanding phytoalexin detoxification pathways utilized by phytopathogenic fungi and co-occurring microorganisms is of great interest to a bio-rational approach to control plant pathogens. Our studies provide remarkable examples of phytoalexin detoxification by phytopathogenic fungi, suggesting that enzymes involved in such processes may result from a genetic exchange between brassicas and their fungal pathogens. We have shown that  three different rapeseed pathogens convert the phytoalexin cyclobrassinin into three different phytoalexins, brassilexin, dioxibrassinin, and brassicanal A, and ultimately each phytoalexin into nontoxic products. We proposed that such pathogens may have acquired a more effective mechanism for overcoming this plant defense by adopting biosynthetic pathways operating in planta. This strategy appears quite plausible, especially considering that most fungal pathogens have been coevolving with plants for numerous generations (Pedras & Okanga, Journal Agricultural & Food Chemistry, 1999) (Figure 8). As well, the phytoalexin brassinin is detoxified via different pathways, which are dependent on the fungal species (review: Pedras et al., Current Organic Chemistry, 2003, Figure 9).

Figure 8. Enzymatic transformation of the phytoalexin cyclobrassinin  by economically important fungal pathogens of brassicas.

 

Figure 9. Enzymatic transformation of the phytoalexin brassinin  by economically important fungal pathogens of brassicas.

 

Our overall results on the detoxification of phytoalexins by diverse plant pathogenic fungi indicate that each detoxification pathway is (i) specific to a group of pathogens and that (ii) the enzymes involved in these transformations are selective. Thus, we envision that in a not too distant future a new generation of inhibitors, the "PALDOXINS" (Fig. 10), that can prevent fungal detoxification of the natural plant defenses will be available (for reviews see: Pedras et al. Phytochemistry, 2000, 53, 161-176; Pedras, Canadian Journal of Chemistry, 2004; Pedras et al., Current Organic Chemistry, 2003; Pedras et al., Natural Products Communications, 2007).

Figure 10. Paldoxins (designer phytoalexins) to treat plants.