Light-induced production of singlet oxygen and superoxide by the fungal toxin, cercosporin.

Cercosporin, a toxin produced by members of the fungal genus Cercospora, is a photosensitizing compound which rapidly kills plant cells in the light. We have found that cercosporin, when activated by light in the presence of oxygen, is able to generate both singlet oxygen and superoxide ions. Cercosporin, when illuminated in the presence of O(2), reacted with cholesterol to form the 5alpha-hydroperoxide of cholesterol which is only produced by reaction with singlet oxygen. Cercosporin, in the presence of light, O(2), and a reducing substrate, was also able to reduce p-nitro blue tetrazolium chloride, a compound readily reduced by superoxide. Superoxide dismutase, a scavenger of superoxide, inhibited this reaction. Production of both singlet oxygen and superoxide by cercosporin must be considered when studying the possible mechanisms of resistance to cercosporin.