Fungal modification of pterocarpan phytoalexins from Melilotus alba and Trifolium pratense

Medicarpin (3-hydroxy-9-methoxypterocarpan), an isoflavonoid phytoalexin characteristic of sweetclover (Melilotus alba), is metabolised by three fungi, Botrytis cinerea, Colletotrichum lindemuthianum (race β) and C. coffeanum (isolates CCA and CCP) to afford products with decreased antifungal activity. Peterocarpan phytoalexins from red clover (Trifolium pratense) are similarly modified. A fourth organism, Helminthosporium carbonum apparently lacks this ability. Fungal-mediated tranformation principally involves hydroxylation, methylation and demethylation although ring fission has also been noted.