HOST PROTEIN(S) INDUCES ACCUMULATION OF THE TOXIN CERCOSPORIN AND MESSENGER-RNA IN A PHYTOPATHOGENIC STRAIN OF CERCOSPORA-KIKUCHII

Cerwspora kikuchii strain S2 produces the important pathogenicity factor cercosporin in infected soybeans and in potato dextrose broth. In contrast, S2 accumulates virtually no cercosporin in a complete medium (CM) broth, a medium containing salts, yeast extract and casamino acids. In this study we show that CM cultures supplemented with aqueous extracts of soybean leaves or meal accumulated between 5 to 65 fold higher levels of cercosporin than unsupplemented cultures. Autoclaving or treating extracts with proteinase K severely attenuated their stimulatory properties, indicating soybean protein was necessary for cercosporin production in CM. Other proteins tested failed to stimulate cercosporin synthesis. The inducing effect of leaf extracts increased as plants underwent reproductive development, but fell sharply at seed set. Conditioning experiments demonstrated that stimulation of rcrcosporin accumulation was not due to alteration of the medium by the extract. Accumulation of a transcript whose expression is strongly correlated with cercosporin synthesis was enhanced in the presence of native soybean meal extract, but decreased when extracts were autoclaved or digested. Accumulation of other cercosporin-rclated transcripts also increased in the presence of native extracts, but were more strongly induced by autoclavcd or digested extracts. Our data suggest that C. kikuchii responds to soybean protein(s) with increased ccrcosporin production and changes in transcript accumulation. © 1993 Academic Press.