Inhibition of ergosterol biosynthesis by triadimenol in Ustilago avenae

In Ustilago avenae sporidia, following the first doubling period of about 4 h, triadimenol (2 μg ml−1) affected sporidial multiplication more severely than other growth processes; daughter cells failed to separate from the parent sporidia resulting in chains of interconnected cells. Triadimenol incubated with the fungus for 8 h interfered neither with respiration nor with protein and nucleic acid synthesis but after 6 h the toxicant had induced a higher content of free fatty acids. Triadimenol markedly altered, both quantitatively and qualitatively, the sterols in sporidia of U. avenae. Incorporation of [14C]acetate (in the form of sodium acetate) into lipid fractions for a period of 2 h revealed that the toxicant powerfully inhibited the synthesis of the 4‐demethyl sterol fraction (predominantly ergosterol), whilst the 4,4‐dimethyl sterol fraction rapidly accumulated. This was confirmed by g.1.c. analysis of the sterols after 6 and 8 h incubation which showed that the amount of ergosterol, the major sterol in untreated sporidia, was diminished while simultaneously 4,4‐dimethyl, 4‐methyl and 14‐methyl sterols increased. The accumulation of 14‐methyl sterols suggests that triadimenol acts as a potent inhibitor of one of the metabolic steps involved in the demethylation at the 14‐position during ergosterol biosynthesis.