Characterization of mutants of Cercospora nicotianae sensitive to the toxin cercosporin.

Six mutants of Cercospora nicotianae that were isolated for sensitivity to the photoactivated toxin cercosporin were characterized for cercosporin sensitivity and production, presence of protective compounds, and pathogenicity. The mutants fell into two classes. Five of the mutants (CS2, CS6, CS7, CS8, and CS9) were inhibited when grown on medium amended with cercosporin concentrations as low as 0.1 μM. Cercosporin sensitivity of these mutants increased markedly with increasing light intensity from 3 to 40 μE m -2 s -1 . These mutants showed no general light sensitivity in the absence of cercosporin. These mutants were capable of synthesizing cercosporin even though their growth was inhibited when cercosporin was produced. These five mutants were protected against cercosporin toxicity by exogenously added ascorbate, cysteine, and reduced glutathione. The sixth mutant (CS10) was not inhibited by cercosporin concentrations below 10 μM and showed little change in cercosporin sensitivity under increasing light intensity. CS10 also synthesized cercosporin. However, growth of this mutant was unaffected under cercosporin-producing conditions. CS10 was protected against high concentrations of cercosporin by exogenously added cysteine and reduced glutathione, but not by ascorbate. None of the six mutants were altered in endogenous levels of β-carotene, ascorbate, cysteine, reduced glutathione, or total soluble or protein thiols. CS2, CS8, and CS10 were compared to wild-type for ability to produce lesions on tobacco. CS2 and CS8 produced significantly fewer lesions than did wild-type; CS10 was intermediate.