On the mode of action of the phytotoxin (8R,16R)-(−)-pyrenophorin

The response of Avena sterilis to the phytotoxic fungal metabolite (8R,16R)-(−)-pyrenophorin was studied under illumination. The phytotoxicity of pyrenophorin at 70 μM was expressed as bleaching of leaf sections which was accompanied by increased electrolyte leakage, elevated superoxide dismutase activity, severe lipid peroxidation, rapid loss of photosynthetic pigments, and a decline in total protein. Proteolytic enzyme activity in the treated tissues was not affected and remained at the same low levels as in the untreated. Pyrenophorin exerts its phytotoxicity through a mechanism operating under both light and dark as indicated by electrolyte leakage measurements. The presence of PSII inhibitor diuron (172 μM) did not reverse electrolyte leakage and bleaching caused by pyrenophorin. The results indicate that the oxidative damage triggered in A. sterilis by the phytotoxin is not light dependent and is exerted through a mechanism which involves electron misdirection and generation of reactive oxygen species.