DETOXIFICATION OF PHYTOANTICIPINS AND PHYTOALEXINS BY PHYTOPATHOGENIC FUNGI

Most plants synthesize antimicrobial compounds as part of normal plant development (i.e., phytoanticipins) or synthesize such compounds de novo when challenged by microorganisms (i.e., phytoalexins). The presumed role of these plant antibiotics is to protect the plant from disease. However, many phytopathogenic fungi have enzymes that can detoxify the phytoanticipins or phytoalexins produced by their host. This may be a means that these pathogens have evolved to circumvent resistance mechanisms based on the production of plant antibiotics. Many of the phytoanticipin- and phytoalexin-detoxifying enzymes produced by phytopathogenic fungi have biochemical and regulatory properties that would indicate the phytoanticipins and phytoalexins produced by their host are their normal substrates. In addition, their activity, enzymatic products, or transcripts can be detected in infected plant tissue suggesting that they are functioning in planta during pathogenesis. Specific mutations have been made by transformation-mediated gene-disruption procedures that eliminate the ability of gaeumannomyces graminis var. avenae, gloeocercospora sorghi, and Nectria haematococca to detoxify the phytoanticipins or phytoalexins produced by their hosts. The effect of these mutations on pathogenicity indicates a requirement for detoxifying enzymes in G. graminis var. avenae but not in G. sorghi or N. haematococca.