The Expression of Resistance ofUstilago avenaeto the Sterol Demethylation Inhibitor Triadimenol is an Induced Response

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Abstract
A triadimenol-sensitive wild-type strain and a triadimenol-resistant mutant of Ustilago avenae were investigated with regard to their responses to the sterol demethylation inhibitor triadimenol. A fungicide concentration of 2 .mu.g ml-1 was fully inhibitory to the sensitive but not the resistant strain after long-term treatments for 48 hr. The initial response to triadimenol treatment was clearly different from the long-term effects. Reproduction of the sensitive strain was halted within 6 hr, and the few cells that continued to emerge over a period of 24 hr were morphologically altered and remained in cell aggregates. The initial pattern of growth inhibition was similar for the resistant mutant. Reproduction of sporidia was blocked subsequent to a 4-hr lag-phase, and new cells remained in aggregates. In contrast to the sensitive strain, the inhibitory effect on both sporidial reproduction and segregation of daughter cells was transient for the resistant mutant, and undisturbed growth resumed 12 hr after treatment with triadimenol. The alteration of sterol profiles during the course of inhibitor treatment reflected the different patterns of morphological responses. During the initial inhibitory phase, C-14 methyl sterols accumulated under the action of triadimenol in both strains. The extent of target saturation, as revealed by lipid labeling with [14C]-acetate, also was similar for both the sensitive and resistant strains. In the sensitive strain, the sterol precursors continued to comprise the pool of free sterols. In contrast, the accumulation of precursors was only transient for the triadimenol-resistant mutant. When growth resumed, authentic desmethyl sterols were synthesized, and the content of precursor sterols declined to levels similar to the nontreated control. The expression of resistance of U. avenae to triadimenol appears to involve an inducible sytem that relieves the cells from an initially pronounced inhibitory action of the sterol demethylation inhibitor.