Cellular toxicity of elsinochrome phytotoxins produced by the pathogenic fungus, Elsinoe fawcettii causing citrus scab

Elsinochromes are the red/orange pigments produced by many Elsinoe fungal species and are structurally similar to the phytotoxin, cercosporin. Here, pigments were extracted from cultures of a citrus pathogen, Elsinoe fawcettii and tested for cellular toxicity. On irradiation with light, elsinochromes rapidly killed suspension cultured citrus and tobacco cells. The toxicity was decreased by adding the singlet oxygen ((1)O(2)) quenchers (bixin (carotenoid carboxylic acid), DABCO (1, 4-diazabicyco octane), ascorbate or reduced glutathione). Application of elsinochromes onto rough lemon leaves resulted in necrotic lesions, whereas lesion development was inhibited by the addition of bixin, DABCO or ascorbate, but not a-tocopherol. Incubation of rough lemon leaf discs with elsinochromes in the light induced a steady increase of electrolyte leakage. Compared with two photosensitizing compounds, hematoporphyrin and cercosporin, the accumulation of (1)O(2) induced by elsinochromes after irradiation was indicated by successful detection of the cholesterol oxidation product, 5a-hydroperoxide. Addition of a potent quencher, b-carotene prevented 5a-hydroperoxide production. Elsinochromes generated superoxide ions (O(2)(center dot-)), whereas accumulation of (O(2)(center dot-)) was blocked by addition of the superoxide dismutase, a scavenger of (O(2)(center dot-)) , but not the (1)O(2)-quencher, DABCO. Our study indicated that elsinochromes are functioning as photosensitizing compounds that produce (1)O(2) and (O(2)(center dot-)) , and exert toxicity to plant cells.