PARA-HYDROXYBENZOYLFORMIC ACID AND (R)-(-)-PARA-HYDROXYMANDELIC ACID, 2 ANTIFUNGAL COMPOUNDS ISOLATED FROM THE LIQUID CULTURE OF THE ECTOMYCORRHIZAL FUNGUS PISOLITHUS-ARHIZUS

Two antifungal compounds isolated from the liquid culture medium of Pisolithus arhizus were identified as p-hydroxybenzoylformic acid and (R)-(-)-p-hydroxymandelic acid and given the trivial names pisolithin A and pisolithin B, respectively. The efficacy of the compounds to inhibit the germination of conidia of Truncatella hartigii was compared with that of commercially available structural analogues, and a comparable range of effectiveness for 50% germination inhibition (GI50) of conidia was recorded. The commercially available synthetic compounds (R)-mandelic acid, benzoylformic acid, and racemic p-hydroxymandelic acid, had GI50 values of 82, 72, and 59-mu-g/mL, respectively, as compared with the natural compounds pisolithin A, 67-mu-g/mL, and pisolithin B, 71-mu-g/mL. Two synthetic S enantiomers of mandelic acid, (S)-mandelic acid and (S)-(+)-p-hydroxymandelic acid, were the most effective compounds, with GI50 values of 31 and 33-mu-g/mL, respectively. A sodium salt of mandelic acid had no activity below 500-mu-g/mL. Pisolithin A and pisolithin B were compared with polyoxin D for inhibition of hyphal growth, as measured by protein estimation. Both pisolithin A and B measured higher levels of putative extractable protein than polyoxin D, but less mycelial wet weight was measured. It is suggested that the pisolithins caused a disruption of cell turgor. A measurement of mycelial dry weights of phytopathogens, incubated with the commercially available analogues, benzoylformic acid and racemic p-hydroxymandelic acid, indicated that benzoylformic acid was either more effective than, or as effective as, racemic p-hydroxymandelic acid or nystatin in arresting fungal growth. For ectomycorrhizal fungi confronted with the pisolithins, both compounds were effective in inhibiting the growth of an equal number of fungi, but benzoylformic acid caused a growth stimulation of some ectomycorrhizal fungi. The result of growth stimulation by the compound is discussed.