Sensitivity to Phenylpyrrole Fungicides and Abnormal Glycerol Accumulation in <i>Os</i> and <i>Cut</i> Mutant Strains of <i>Neurospora crassa</i>

Abstract

Neurospora osmotic sensitive strains with os-1, os-2, os-4 and os-S mutations showed cross-resistance to dicarboximides, aromatic hydrocarbons and fludioxonil. The os-2, os-4, and os-5 mutant strains were highly resistant to them, while the os-1 mutant strain was moderately resistant and its growth was inhibited by fludioxonil (LD50: 0.087 ,ug/ml) and iprodione (LD50:14 ug/ml). Another osmotic sensitive mutation cut did not give resistance to these fungicides. The conidia of wild-type and cut strains swelled and burst without germination on the medium containing fludioxonil, and bursting of conidia was rescued by high osmotic pressure. Stimulation of glycerol biosynthesis by fludioxonil and iprodione observed in the wild-type strain was not induced in os-2, os-4 and os-5 mutant strains. The moderately resistant os-1 mutant strain accumulated substantial quantities of glycerol by 10 pg/ml of fludioxonil, but produced less glycerol by 10ug/ml of iprodione. Thus it was assumed that fungal toxicity of these fungicides might be due to abnormal glycerol accumulation.However, both fludioxonil and iprodione did not induce glycerol biosynthesis in the fungicide-sensitive cut mutant strain. In response to osmotic stress, as mutant strains accumulated less amount of glycerol than the wild-type strain, whereas cut mutant strain did not produce glycerol in the medium containing 4% NaCI. Despite its lack of ability of glycerol biosynthesis in cut mutant strain, wild-type and cut strains were not distinguishable in their modes of fungal toxicity of fludioxonil.These data suggest that dicarboximides, aromatic hydrocarbons and fludioxonij interfere with the osmotic signal transduction pathway resulting in stimulation of glycerol biosynthesis, but abnormal glycerol accumulation is not essential for their fungal toxicity.

Two-Component Signaling System in Filamentous Fungi and the Mode of Action of Dicarboximide and Phenylpyrrole Fungicides

“…In Neurospora crassa, mutants of os-1, a Group III HisK gene, and those of os-2, a HOG1 MAPK gene, exhibit almost the same hyperosmotic sensitivity and fungicide resistance (Fujimura et al, 2000). However, HOG1 MAPK gene disruptants in C. heterostrophus and B. cinerea do not show as strong a hyperosmotic sensitivity and fungicide resistance as the Group III HisK gene ( Fig.…”

“…Colletotrichum lagenarium, C. heterostrophus and B. cinerea, phenylpyrrole, fungicide treatments abnormally induce the phosphorylation of the HOG1 MAPKs of those fungi (Kojima et al, 2004). However, in mutants lacking the Group III HisKs of those filamentous fungi, the phosphorylation of HOG1 MAPK and glycerol accumulation due to exposure to hyperosmotic stress or the fungicides are not observed (Fujimura et al, 2000;Yoshimi et al, 2005). That is, in filamentous fungi, a Group III HisK functions as a responsible upstream factor in an osmotic signaling system, to sense and transduce osmotic signals, in the same way as SLN1 in budding yeast.…”

“…Glycerol accumulation as a result of hyperosmotic stimulus is also observed in filamentous fungal cells, e.g. N. crassa and C. heterostrophus (Fujimura et al, 2000;Tanaka et al, 2006). Moreover, fungicide treatment also induces glycerol accumulation in these filamentous fungal cells.…”

The Response Regulator RRG-1 Functions Upstream of a Mitogen-activated Protein Kinase Pathway Impacting Asexual Development, Female Fertility, Osmotic Stress, and Fungicide Resistance inNeurospora crassa

“…The os-1, os-2, os-4, and os-5 mutants exhibit reduced glycerol accumulation under hyperosmotic stress conditions (Ellis et al, 1991;Fujimura et al, 2000b), which may explain why these mutants are unable to grow on hyperosmotic media. New evidence also suggests that, in addition to osmolyte regulation/accumulation, modulation of turgor pressure plays a role in this hyperosmotic adaptation.…”

“…In contrast to C. albicans, mutation of the C. neoformans skn7 gene leads to reduced virulence in a mouse inhalation model as well as sensitivity to oxidative stress (Wormley et al, 2005;Bahn et al, 2006). Thus, although a response regulator gene is necessary for pathogenesis in C. albicans and C. neoformans, there is currently no clear pattern for predicting whether the ssk1 or skn7 homologue will fulfill this requirement.The os-1, os-2, os-4, and os-5 mutants exhibit reduced glycerol accumulation under hyperosmotic stress conditions (Ellis et al, 1991;Fujimura et al, 2000b), which may explain why these mutants are unable to grow on hyperosmotic media. New evidence also suggests that, in addition to osmolyte regulation/accumulation, modulation of turgor pressure plays a role in this hyperosmotic adaptation.…”