EFFECTS OF PISATIN ON DICTYOSTELIUM-DISCOIDEUM - ITS RELATIONSHIP TO INDUCIBLE RESISTANCE TO NYSTATIN AND EXTENSION TO OTHER ISOFLAVONOID PHYTOALEXINS

Dictyostelium discoideum amoebae can acquire resistance to otherwise i nhibitory concentrations of pisatin, an isoflavonoid phytoalexin of pe a, and nystatin, a polyene antibiotic, following pretreatment with sub lethal concentrations of these compounds. Additionally, growth on medi um containing pisatin can induce nystatin resistance. We show here tha t distinct mechanisms mediate the inducible resistance to these two co mpounds because it is possible to isolate mutations that specifically block the induction of nystatin resistance but do not affect the induc tion of pisatin resistance. Pisatin did not affect wild-type sterol bi osynthesis; therefore, the induction of nystatin resistance by pisatin is probably not via an alteration of membrane sterols. The inducible pisatin resistance phenotype was shown to extend to the isoflavonoid p hytoalexins maackiain and biochanin A, and all three compounds inhibit ed the aggregation of amoebae that is normally triggered by starvation .