Ptk2 contributes to osmoadaptation in the filamentous fungus Neurospora crassa

Hyphal tip-growing organisms often rely upon an internal hydrostatic pressure (turgor) to drive localized expansion of the cell. Regulation of the turgor in response to osmotic shock is mediated primarily by an osmotic MAP kinase cascade which activates osmolyte synthesis and ion uptake to effect turgor recovery. We characterized a Neurospora crassa homolog (PTK2) of ser/thr kinase regulators of ion transport in yeast to determine its role in turgor regulation in a filamentous fungi. The ptk2 mutant is osmosensitive, and has lower turgor poise than wildtype. The cause appears to be lower activity of the plasma membrane H+-ATPase. Its role in osmoadaptation is unrelated to the activity of the osmotic MAP kinase cascade. Instead, it acts in an alternative pathway that, like the osmotic MAP kinase cascade, also involves ion transport mediated osmoadaptation.