Differential Response of Fusarium solani Isolates to Pisatin and Phaseollin

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Abstract
One isolate each of F. solani f. sp. pisi, F. solani f. sp. phaseoli and F. solani f. sp. cucurbitae was treated with pisatin and phaseollin under a variety of growth conditions. F. solani f. sp. cucurbitae consistently appeared sensitive to both phytoalexins and F. solani f. sp. pisi consistently appeared tolerant of pisatin. The response of F. solani f. sp. phaseoli to pisatin and the response of both F. solani f. sp. pisi and F. solani f. sp. phaseoli to phaseollin varied; whether an isolate appeared sensitive or tolerant to the phytoalexins depended on the growth conditions employed. Exposure of F. solani f. sp. pisi and F. solani f. sp. phaseoli in shake culture to a low concentration of phaseollin markedly enhanced their tolerance to a subsequent higher concentration of phaseollin. This type of response suggests the presence of an adaptive tolerance mechanism. F. solani f. sp. pisi did not metabolize phaseollin but could metabolize pisatin. However, it grew substantially in shake culture in the presence of pisatin prior to significant metabolism of the phytoalexin. Thus, mechanisms in addition to or instead of metabolic detoxification of pisatin or phaseollin must account for the tolerance of F. solani f. sp. pisi to these phytoalexins.

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