Inhibition mechanisms of indole derivatives against Fusarium oxysporum f. sp. spinaciae and different sensitivity of formae speciales of F. oxysporum to the compounds

The inhibition mechanisms of antifungal indole derivatives, 5-fluoroindole, 4-fluoroindole and 5-bromoindole, were examined using Fusarium oxysporum f. sp. spinaciae. The suppression of mycelial colony growth of the phytopathogenic fungus by 5-fluoroindole and 4-fluoroindole was derepressed by the addition of tryptophan and anthranilic acid, an intermediary metabolite of tryptophan biosynthetic pathway. These results suggest that 5-fluoroindole and 4-fluoroindole inhibit function of anthranilate synthase catalyzing conversion of chorismic acid to anthranilic acid. On the other hand, the addition of these metabolites did not release the suppression of colony growth by 5-bromoindole, suggesting that the inhibition mechanism for 5-bromoindole is different from those for 5-fluoroindole and 4-fluoroindole. In addition, the inhibition efficiency of these indole derivatives was compared among different formae speciales (lycopersici, melonis, fragariae, and spinaciae) of F. oxysporum. The result indicated that the colony growth of these fungi was suppressed at the different levels, and that these fungi could be distinguished each other by their colony sizes, especially when treated with 25μg/ml of 4-fluoroindole. The characteristic responses of the fungi to the compound could be effectively integrated to a monitoring system for marker gene-integrated transformants of different formae speciales of F. oxysporum.