How do vacuolar NHX exchangers function in plant salt tolerance?
Potassium (K+) is a major osmoticum of plant cells, and the vacuolar accumulation of this element is a especially crucial feature for plants under high-salt conditions. Emerging evidence indicates that cation/proton transporters of the NHX family are instrumental in the H+-linked K+ transport that mediate active K+ uptake at the tonoplast for the unequal partitioning of K+ between vacuole and cytosol. However, and in spite of tenuous supporting evidence, NHX proteins are widely regarded as key players in the sequestration of sodium (Na+) into vacuoles to avert ion toxicity in the cytosol of plants under salinity stress. Here, we propose an updated model positing that NHX proteins fulfill a protective function to minimize salt-related stress mainly through the vacuolar compartmentalization of K+ and, in some cases, of Na+ as well thereby preventing toxic Na+-K+ ratios in the cytosol while accruing solutes for osmotic balance.
