Fungicide Resistance and Osmotic Stress Sensitivity in os Mutants of Neurospora crassa

Abstract

The os-1 (osmotic sensitive-1) mutants in Neurospora crassa were classified into two groups based on their resistance to various fungicides (e.g., fludioxonil, iprodione, and quintozene) and osmotic stress. The first group of strains, represented by NM233(t) and Y256M209, grew on medium containing fludioxonil even at a concentration of 25 μg/ml but their growth was inhibited only when a high concentration of NaCl was used for osmotic stress. The second group of strains, represented by M155-1, M16, P3282, and P5990, was less resistant to fludioxonil than the first group, but their growth was inhibited even under low osmotic stress. Both groups of the os-1 mutant strains transformed by the wild-type os-1 gene restored fungicide sensitivity and osmotic tolerance, suggesting that the os-1 gene mutations are responsible for phenotypes in both groups of os-1 mutants. The results indicated that the level of resistance to fludioxonil is not related to that of osmotic sensitivity in the os-1 mutant strains. To investigate the correlation between fungicide resistance and osmotic sensitivity in other os mutant strains, we isolated revertant strains from the os-5 mutant strain on medium containing 6% NaCl. The revertant os-5R-18 strain reversed its osmotic sensitivity to the level of the wild-type strain but still remained resistant to fludioxonil. Genetic analysis suggested that an additional mutation, named su(os-5), in the revertant os-5R-18 strain suppressed the osmotic sensitivity of the os-5 mutant strain but did not do so with those of the os-1 and os-2 mutant strains. The os-5 mutant strain accumulated less glycerol by osmotic stress, while glycerol synthesis in the revertant os-5/su(os5) strain was induced by high osmolarity to the level of the wild-type strain. However, glycerol accumulation by fludioxonil observed in the wild-type strain did not occur in either strain with os-5 and os-5/su(os5). These results suggested that phenylpyrroles affect glycerol synthesis in the osmotic signal transduction pathway, but that high osmotic sensitivity is not directly linked with resistance to dicarboximides and phenylpyrroles.