DETOXIFICATION OF PHYTOANTICIPINS AND PHYTOALEXINS BY PHYTOPATHOGENICFUNGI

Most plants synthesize antimicrobial compounds as part of normal plant development (i.e., phytoanticipins) or synthesize such compounds de n ovo when challenged by microorganisms (i.e., phytoalexins). The presum ed role of these plant antibiotics is to protect the plant from diseas e. However, many phytopathogenic fungi have enzymes that can detoxify the phytoanticipins or phytoalexins produced by their host. This may b e a means that these pathogens have evolved to circumvent resistance m echanisms based on the production of plant antibiotics. Many of the ph ytoanticipin- and phytoalexin-detoxifying enzymes produced by phytopat hogenic fungi have biochemical and regulatory properties that would in dicate the phytoanticipins and phytoalexins produced by their host are their normal substrates. In addition, their activity, enzymatic produ cts, or transcripts can be detected in infected plant tissue suggestin g that they are functioning in planta during pathogenesis. Specific mu tations have been made by transformation-mediated gene-disruption proc edures that eliminate the ability of gaeumannomyces graminis var. aven ae, gloeocercospora sorghi, and Nectria haematococca to detoxify the p hytoanticipins or phytoalexins produced by their hosts. The effect of these mutations on pathogenicity indicates a requirement for detoxifyi ng enzymes in G. graminis var. avenae but not in G. sorghi or N. haema tococca.