Structure—activity relationship of some bean phytoalexins and related isoflavonoids

The antifungal activity of the five structurally related isoflavonoids phaseollin, phaseollinisoflavan, 7-o-methylphaseollinisoflavan, 2′-o-methylphaseollinisoflavan, and 7,2′-di-o-methylphaseollinisoflavan was tested in vitro using the bean pathogens Colletotrichum lindemuthianum, Fusarium oxysporum f. sp. phaseoli, Rhizoctonia solani and Thanatephorus cucumeris, as well as pathogens which do not attack Phaseolus beans such as Aphanomyces euteiches, Phytophthora megasperma f.sp. glycinea, Pythium paroecandrum and Fusarium solani f.sp. pisi. Colony growth on solid medium and growth of sporelings in liquid medium were used to measure the effect of the isoflavonoids. The fungi differed in their sensitivity to these compounds. With the exception of C. lindemuthianum, however, the order of effectiveness of the five isoflavonoids was virtually the same in all target organisms. There appeared to be no clear relationship between lipophilicity and anti-fungal activity of these compounds. The substituent at position 7 of the isoflavans may chiefly affect biological activity. The basis for antifungal activity of this series of isoflavonoids is discussed.