Mechanisms of Resistance to Fenpropimorph and Terbinafine, Two Sterol Biosynthesis Inhibitors, in Nectria haematococca, a Phytopathogenic Fungus

The mechanisms of resistance to terbinafine, a squalene epoxidase inhibitor, and to fenpropimorph, a sterol Δ14-reductase and/or Δ8 → Δ7-isomerase inhibitor, were investigated in laboratory mutants of the phytopathogenic fungus Nectria haematococca. Neither modified fungicide uptake nor fungicide metabolism could explain resistance in the mutants studied. The terbinafine-resistant mutants contained at least 10 times more squalene than the wild-type strain, and when cultivated in presence of terbinafine, they required a 30-times higher fungicide concentration to obtain a very high level of accumulated squalene similar to that of the wild-type strain. Thus, a reduced affinity of the squalene epoxidase toward both the substrate and the fungicide could explain terbinafine resistance in these mutants. In some fenpropimorph-resistant mutants, the effects of fenpropimorph on sterol biosynthesis were similar to those in the wild-type strain, suggesting that resistance could be due to tolerance to sterol Δ8,14-sterol accumulation and to Δ5,7-sterol decrease. In one of them, the major sterol in absence of fungicide was not ergosterol, but ergosta-5,7,22,24(241)-tetraenol, indicating reduced Δ24(241)-reductase activity. The potential role of a modified Δ24(241) reductase in fenpropimorph resistance has yet to be elucidated. In the remaining fenpropimorph-resistant mutants, much higher fenpropimorph concentrations were required to cause ergosterol decreases similar to that in the wild-type strain, suggesting a Δ14-reductase modification. Among them, some mutants accumulated Δ8-sterols as well as Δ8,14-sterols, indicating that the mechanism of resistance may be due to reduced affinity of the Δ14-reductase toward fenpropimorph, whereas the other mutants accumulated only Δ8,14-sterols. Those last mutants contained much larger amounts of 4,4-dimethyl and 4α-methyl sterols than the wild-type strain, in the absence of fungicide. The most abundant methylated sterol was 4,4-dimethylfecosterol, the presumed product of the Δ14-reductase, suggesting that resistance may be due to overproduction of the Δ14-reductase in these mutants.