Role of a mitogen-activated protein kinase cascade in ion flux-mediated turgor regulation in fungi

Fungi normally maintain a high internal hydrostatic pressure (turgor) of about 500 kPa. In response to hyperosmotic shock, there are immediate electrical changes: a transient depolarization (1 to 2 min) followed by a sustained hyperpolarization (5 to 10 min) prior to turgor recovery (10 to 60 min). Using ion-selective vibrating probes, we established that the transient depolarization is due to Ca²⁺ influx and the sustained hyperpolarization is due to H⁺ efflux by activation of the plasma membrane H⁺-ATPase. Protein synthesis is not required for H⁺-ATPase activation. Net K⁺ and Cl⁻ uptake occurs at the same time as turgor recovery. The magnitude of the ion uptake is more than sufficient to account for the osmotic gradients required for turgor to return to its original level. Two osmotic mutants, os-1 and os-2, homologs of a two-component histidine kinase sensor and the yeast high osmotic glycerol mitogen-activated protein (MAP) kinase, respectively, have lower turgor than the wild type and do not exhibit the sustained hyperpolarization after hyperosmotic treatment. The os-1 mutant does not exhibit all of the wild-type turgor-adaptive ion fluxes (Cl⁻ uptake increases, but net K⁺ flux barely changes and net H⁺ efflux declines) (os-2 was not examined). Both os mutants are able to regulate turgor but at a lower level than the wild type. Our results demonstrate that a MAP kinase cascade regulates ion transport, activation of the H⁺-ATPase, and net K⁺ and Cl⁻ uptake during turgor regulation. Other pathways regulating turgor must also exist.