Accumulation of antifungal isoflavonoids and 1a-hydroxyphaseollone, a phaseollin metabolite, in bean tissue infected with Fusarium solani f. sp. phaseoli

Abstract

1a-Hydroxyphaseollone, a metabolic product of phaseollin produced in vitro by Fusarium solani f. sp. phaseoli, was isolated from bean pod “drop diffusates” and infected bean hypocotyls 24 and 48 h, respectively, after inoculation with F. solani f. sp. phaseoli. Bioassays of phaseollin and 1a-hydroxyphaseollone against eight fungal species indicated that the latter was less inhibitory, representing a detoxification product of phaseollin. The occurrence of 1a-hydroxy-phaseollone in F. solani f. sp. phaseoli-infected tissue provided evidence for an in situ interaction between phaseollin and F. solani f. sp. phaseoli. While phaseollin, phaseollinisoflavan and 2′-methoxyphaseollinisoflavan accumulated to high concentrations (as high as 5% of the dry weight) in F. solani f. sp. phaseoli-infected tissue, only trace amounts of phaseollidin occurred and no kievitone was detected. F. solani f. sp. phaseoli metabolized phaseolinsoflavan in vitro, but 2′-methoxyphaseollinisoflavan was not detected as a product. This methylated analogue of phaseollinisoflavan, also isolated from Thielaviopsis basicola-infected bean tissue, may be synthesized by the host.