Abstract
Fusarium oxysporum f. cucumerinum was sensitive to hymexazol (3-hydroxy-5-methylisoxazole, soil fungicide, Tachigaren®), whereas Saccharomyces cerevisiae was tolerant. Growth of F. oxysporum f. cucumerinum in the liquid culture medium was considerably inhibited by 300μg/ml of hymexazol incorporated in the medium, while the growth of S. cerevisiae was not inhibited even at a concentration of 1, 000μg/ml. Differences in toxicity of hymexazol to these microorganisms primarily resulted from difference in uptake. No detectable metabolic conversion of hymexazol occurred in cultures of S. cerevisiae. Oxygen uptake by the intact cells of F. oxysporum f. cucumerinum was not greatly inhibited by concentrations of hymexazol that prevented growth of the fungus. Furthermore, the oxidation of succinate in mitochondrial preparations of the fungus was not significantly sensitive to the toxicant nor was respiration of intact cells. In addition, hymexazol at 500μg/ml did not affect oxygen uptake by mitochondria derived from rat-liver. A concentration of hymexazol giving great inhibition of growth (500μg/ml) did not cause detectable change in the permeability of the cells. Incorporation of phenylalanine into protein and uridine into ribonucleic acid (RNA) were inhibited considerably. Incorporation of thymidine into the deoxyribonucleic acid (DNA) fraction of the mycelial cells was inhibited more than phenylalanine and uridine into protein and RNA respectively. From these results, it appeared that hymexazol inhibits the fungal growth by interfering with the DNA synthesis as primary site of action.
