TARGETED GENE DISRUPTION OF CAROTENOID BIOSYNTHESIS IN CERCOSPORA-NICOTIANAE REVEALS NO ROLE FOR CAROTENOIDS IN PHOTOSENSITIZER RESISTANCE

Many phytopathogenic Cercospora species synthesize the photoactivated phytotoxin cercosporin which is correlated with the ability of these fungi to cause plant disease, Cercosporin is a photosensitizer which upon illumination transfers energy to molecular oxygen, generating the universally toxic, activated oxygen species, singlet oxygen, Like other photosensitizers, cercosporin exhibits extensive toxicity. Cercospora spp,, however, not only synthesize but are resistant to high concentrations of cercosporin. Because carotenoids are known to be potent quenchers of singlet oxygen, we tested the possible role of carotenoids in resistance of Cercospora nicotianae to autotoxicity, Targeted gene disruption was used to create carotenoid-minus derivatives of a wild-type and two cercosporin-sensitive C. nicotianae strains, These carotenoid-minus disruption mutants were no more sensitive to either cercosporin or five other photosensitizers than the parent strains from which they were derived, Pathogenicity tests of one of the carotenoid-minus disruption mutants indicated that it was also unaltered in its ability to infect plants, From these data, we conclude that carotenoids are not involved in cercosporin resistance nor are they required for plant infection, These results suggest that Cercospora have a distinct and highly effective mechanism for photosensitizer resistance which has the potential for widespread applicability in other organisms.