Summary: Most isolates of mating population (MP) VI detoxify pisatin, the major phytoalexin produced by pea. However, there is evidence of another pisatin tolerance mechanism that does not depend on pisatin degradation. To facilitate further studies on nondegradative tolerance, a bioassay was developed in which culture turbidity was used as a measure of mycelial growth. Under conditions of this assay, four MP VI isolates degraded 0.4 mM-pisatin within 15 h, but two other isolates required more than 24 h. A remaining group of four isolates did not degrade pisatin. These three groups were typified by the isolates 126-71. T-77 and 126-80, respectively. Regardless of their ability to degrade pisatin, however, all 10 isolates grew equally well after addition of 0.4 mM-pisatin, suggesting that detoxification may not be needed for pisatin tolerance during the short time interval of this assay. The existence of a nondegradative tolerance in isolate 126-71 was confirmed when additional experiments showed that growth could occur (i) before degradation had reduced pisatin below initially noninhibitory levels, and (ii) when pisatin degradation was inhibited by 2% ethanol. When tested in growth medium, pretreatment with a noninhibitory concentration of pisatin for 3 h enhanced equally the ability of isolates 126-71 and 126-80 to tolerate pisatin. After cultures were pretreated with pisatin while temporarily suspended in buffer, isolates 126-71 and T-77 were stimulated to degrade pisatin, and they grew better than did isolate 126-80. Again, however, growth began before there was significant detoxification, especially for isolate T-77. Thus, these three isolates all appeared to have an inducible, nondegradative tolerance to pisatin. MP I isolate T-145 lacked an adaptive tolerance to pisatin and was more sensitive to pisatin than any of the MP VI isolates.


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