Metabolism of the phytoalexin, phaseollinisoflavan, by Fusarium solani f. sp. phaseoli

Abstract

Phascollinisoflavan, an isoflavonoid phytoalexin produced by Phaseolus vulgaris, was metabolized by Fusarium solani f. sp. phaseoli. A major product of this reaction, designated metabolite-1 (M-1), was isolated from fungal cultures; whereas phaseollinisoflavan could not be recovered from the fungal culture medium 21 h after its addition, the level of M-1 rose steadily between nine and 24 h. Bioassays of radial mycelial growth or of dry matter accumulation involving F. solani f. sp. phaseoli, Pythium myrioylum, Phytophthora cryptogea and/or Rhizoctonia solani indicated that M-1 was less fungitoxic than phaseollinisoflavan. In addition to its production in liquid cultures treated with phaseollinisolflavan, M-1 was also isolated from Fusarium-infected bean tissues; trace amounts of M-1 were detected two days after inoculation and levels rose to at least I mg g⁻¹ dry weight 8 days later. Purification of M-1 involved solvent partitioning, TLC, gel filtration and gas chromatography. Partial characterization indicated that the molecular weight of this metabolite was 342, 18 mass units greater than that of phaseollinisoflavan, suggesting the addition of the elements of water to the phytoalexin.