Summary: Isolates of mating population (MP) VI were found previously to possess an inducible tolerance to pisatin, a pea phytoalexin, regardless of their ability to detoxify this compound. Nondegradative tolerance to pisatin was further characterized by studying MP VI isolate 126-80, a strain that does not degrade pisatin. Tolerance was maximally induced in growth medium at 28 °C by a 3 h treatment with 0.1 mM-pisatin. Shorter time intervals or lower pisatin concentrations reduced the degree of induction. Adaptation to pisatin was inhibited if mycelium was temporarily suspended in buffer during the induction period. Enhanced tolerance in growth medium was lost completely within 4 h at 28 °C after pisatin was removed from the medium. However, incubation at 1 °C prevented both the induction and the loss of enhanced pisatin tolerance. These results suggest that the induction of tolerance requires an active metabolic response, and that the expression of tolerance is tightly controlled. Of six isoflavonoid derivatives tested that were structurally related to pisatin, only four were good inducers of pisatin tolerance. Cyanide and the polyene antibiotic amphotericin B also induced some pisatin tolerance. However, tolerance was not a general stress response, because three other antimicrobial compounds failed to induce tolerance. Besides inducing tolerance to itself, pisatin simultaneously induced nondegradative tolerance to the structurally related isoflavonoid phytoalexins medicarpin and phaseollin. A partial tolerance was also induced to the isoflavone biochanin A. Of the non-isoflavonoid toxicants tested, only amphotericin B was tolerated as a result of pisatin pretreatment. Tolerance to amphotericin B was expressed both as the ability to grow and as the decreased leakage of cellular constituents after treatment with normally inhibitory concentrations of amphotericin B. We suggest that the plasma membrane is modified during the induced adaptation to pisatin, and that such modifications could be involved in nondegradative tolerance to phytoalexins.


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