Multiple Modes of Photodynamic Action by Cercosporin

Abstract—Cercosporin, one of a number of 4,9‐dihydroxyperylene‐3,10‐quinones synthesized by plant pathogenic fungi, abundantly produces singlet oxygen when illuminated in solution. Singlet oxygen production is substantially decreased and superoxide production is greatly enhanced in the presence of the reducing agents ergothioneine (2‐thiol‐L‐histidine betaine) or urate. Both ergothioneine and urate enhance superoxide production at a rate approximately 25‐fold that elicited by an equimolar amount of methionine, the agent that is traditionally used in such assays. Such ‘switching’ in production of different active oxygen species under different environmental conditions may be of significance in biological processes, in this case host cell killing by the plant pathogen.