Production of cercosporin toxin by the phytopathogenic Cercospora fungi is affected by diverse environmental signals.

Cercosporin is a polyketide phytotoxin produced by many phytopathogenic Cercospora spp. We investigated environmental signals that have elaborate control of cercosporin production. Light is the most critical factor for cercosporin production. Cercospora nicotianae accumulated substantial quantities of cercosporin only when grown on a particular potato dextrose agar under light but produced little cercosporin on other brands of potato dextrose agar or media with defined ingredients. In addition to light regulation, numerous factors including salts, buffers, and ions markedly affected cercosporin production. By contrast, pH had little effect on cercosporin production. Depletion or alteration of the carbon or nitrogen sources also affected cercosporin production. Production of cercosporin was elevated to varying levels by metal ions, such as cobalt, ferric, manganese, and zinc. Significant differences in cercosporin production were observed among various Cercospora species. Further, regulation of cercosporin production by phosphate buffer, ammonium, LiCl, but not metal ions appeared to occur at transcriptional levels. Expression of the genes involved in cercosporin biosynthesis and regulation decreased markedly and was closely concomitant with the amounts of cercosporin reduced as the fungus was grown on medium containing phosphate, LiCl, ammonium, or dimethyl sulfoxide. The results reveal the complexity of cercosporin production at the physiological and genetic levels. A model delineating regulatory controls of cercosporin biosynthesis is proposed and discussed.