Mode of action of terbinafine in Ustilago maydis and characterization of resistant mutants

The colony-forming ability of Ustilago maydis was completely inhibited on agar medium containing 2 μg/ml of the allylamine antimycotic, terbinafine. The colony-forming ability was only 6% of that of the untreated control when sporidia (2.4 × 10 6/ml) were grown for 12 hr in liquid medium with 1 μg/ml of the fungicide prior to plating. A marked accumulation of squalene, no net synthesis of ergosterol, and a 96% inhibition of phospholipid fatty acid synthesis occurred in sporidia incubated for 8 hr with 2 μg/ml of the fungicide. The primary action of terbinafine in U. maydis, as in other fungi, appears to be inhibition of squalene epoxidase activity. Among 152 terbinafine-resistant laboratory isolates selected, only AR217 had a growth rate as high as that of the wild type (WT). AR217 and AR477 showed inhibition of growth similar to that of the WT on agar medium containing low concentrations of sterol C-14 demethylation inhibitors, but the mutants were able to form colonies at higher inhibitor concentrations than the WT. AR212 was hypersensitive to these inhibitors, possibly due to an inability to produce 14α-methylfecosterol, an inefficient substitute for ergosterol.