Potassium and Sodium Transport Channels Under NaCl Stress

Abstract

A clue for salt tolerance is ability of plants to maintain high cytosolic K⁺/Na⁺ ratio in metabolically active tissues as well as turgor, despite a low water potential in a salinized soil. On the cellular level, this is achieved by Na⁺ extrusion or its vacuolar sequestration, decrease of the permeability of vacuolar and plasma membrane to Na⁺ and, the last but not the least, better K⁺ retention. Active Na⁺ transport from cytosol to exterior or to vacuolar lumen is mediated by Na⁺/H⁺ exchangers and is fueled by H⁺ pumps. On the contrary, Na⁺ entrance to the cytosol is thermodynamically downhill and is mediated by a variety of nonselective channels in plasma membrane and tonoplast. Yet Na⁺ entry, mediated by low- and high-affinity transporters of plasma membrane, may be equally important. Selectivity, gating and regulation, as well as relative expression of these channels and transporters are crucial for salt tolerance. Attention should be paid to the fact that on the onset of the salt stress, membrane potential difference and ion conductance are not anymore the same, but are substantially remodeled by salt itself and stress-related factors. In particular, increases of reactive oxygen species and polyamines levels under stress can inhibit some K⁺ and cation channels, modulate the activity of plasma membrane pumps, and, finally, induce novel conductance in the cell membrane. K⁺ efflux from the cell is governed by Na⁺-induced plasma membrane depolarization. Salt-tolerant plants appear to better control membrane potential difference and/or show lower expression of outwardly rectifying or voltage-independent K⁺ and K⁺-permeable channels in the plasma membrane. On the contrary, sustained activity of vacuolar K⁺ channels under saline conditions may help to hold high cytosolic K⁺/Na⁺ ratio at the expense of vacuolar K⁺ pool. Correct evaluation of cytosolic and vacuolar ionic concentrations is methodologically difficult, but necessary. Few available studies show important differences in ion distributions between different cells along with contrasting patterns for mono- and dicotyledonous plants. In perspective, selective regulation of the expression of K⁺ and cation channels in plasma and vacuolar membranes in a combination with a monitoring of resulted changes in ion relations for cytosol and vacuole in different plant tissues may provide an intelligent strategy to create salt-tolerant crops.