Phomic acid, a toxic metabolic product ofPhoma medicaginis, has been isolated from culture filtrates of the fungus. The toxin crystallizes as colorless rectangular plates having a refractive index of 0.56 and melting at 259ndash;260 deg;C. It has absorption peaks at 208 and 260ensp;mmu;. The infrared spectrum of the toxin suggests that it may be an unsaturated aliphatic hydroxy acid.The toxic principle causes discoloration and drying of the leaf tip and leaf margin and a general stunting of the flax plant. It inhibits germination of seeds of flax, alfalfa, rape, and cress. The effect of phomic acid on seed germination can be reversed by glutathione and cysteine. Germination of the conidia of a few fungi is completely inhibited by the toxin, whereas it only retards the growth of the germ tube ofHelminthosporium sativum. It inhibits the activity of triosephosphate dehydrogenase but not that of malic dehydrogenase. It is suggested that the toxin might act by inhibiting enzyme systems requiring sulphydryl groups for activity.
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