ISOLATION OF MUTANTS OF THE FUNGUS Cercospora nicotianae ALTERED IN THEIR RESPONSE TO SINGLET-OXYGEN-GENERATING PHOTOSENSITIZERS

Fungi in the genus Cercospora produce a singlet-oxygen-generating photosensitizing toxin cercosporin, which plays an important role in the ability of these fungi to parasitize plants. Previous studies demonstrated that resistance to cercosporin was associated with the ability of the fungus to reduce and detoxify cercosporin transiently. In order to characterize the mechanism of cercosporin resistance further, two types of mutants were isolated. One set of mutants was selected directly for sensitivity to cercosporin. Six cercosporin-sensitive mutants were isolated. Five of the mutants were totally inhibited by cercosporin as well as by five other photosensitizers, methylene blue, toluidine blue, eosin Y, rose bengal and hematoporphyrin. These five mutants were also deficient in cercosporin-reducing ability. The sixth cercosporin-sensitive mutant was only partially inhibited by cercosporin. This mutant was unaltered in cercosporin-reducing ability and in its response to the other photosensitizers. In order to test hypotheses concerning the mechanism of cercosporin reduction, a second set of mutants was selected for general deficiency in cell surface-reducing capability by screening for the inability to reduce two tetrazolium dyes (p-tolyltetrazolium red and 2,3,5-triphenyltetrazolium chloride) of similar redox potential to cercosporin. Dye-reduction-deficient mutants were unaltered in their resistance to cercosporin and in cercosporin-reducing ability. We conclude that cercosporin resistance of these fungi is strongly correlated with the ability to reduce cercosporin. However, cercosporin-reducing ability is separable from the general ability to reduce external electron acceptors.