Although physiological and biochemical data since long suggested that Na(+)/H(+) and K(+)/H(+) antiporters are involved in intracellular ion and pH regulation in plants, it has taken a long time to identify genes encoding antiporters that could fulfil these roles. Genome sequencing projects have now shown that plants contain a very large number of putative Cation/Proton antiporters, the function of which is only beginning to be studied. The intracellular NHX transporters constitute the first Cation/Proton exchanger family studied in plants. The founding member, AtNHX1, was identified as an important salt tolerance determinant and suggested to catalyze Na(+) accumulation in vacuoles. It is, however, becoming increasingly clear, that this gene and other members of the family also play crucial roles in pH regulation and K(+) homeostasis, regulating processes from vesicle trafficking and cell expansion to plant development.

Summary Here, the function of the tomato (Solanum lycopersicon) K+/H+ antiporter LeNHX2 was studied using 35S‐driven gene overexpression of a histagged LeNHX2 protein in Arabidopsis thaliana and LeNHX2 gene silencing in tomato. Transgenic A. thaliana plants expressed the histagged LeNHX2 both in shoots and in roots, as assayed by western blotting. Transitory expression of a green fluorescent protein (GFP) tagged protein showed that the antiporter is present in small vesicles. Internal membrane vesicles from transgenic plants displayed enhanced K+/H+ exchange activity, confirming the K+/H+ antiporter function of this enzyme. Transgenic A. thaliana plants overexpressing the histagged tomato antiporter LeNHX2 exhibited inhibited growth in the absence of K+ in the growth medium, but were more tolerant to high concentrations of Na+ than untransformed controls. When grown in the presence of NaCl, transgenic plants contained lower concentrations of intracellular Na+, but more K+, as compared with untransformed controls. Silencing of LeNHX2 in S. lycopersicon plants produced significant inhibition of plant growth and fruit and seed production as well as increased sensitivity to NaCl. The data indicate that regulation of K+ homeostasis by LeNHX2 is essential for normal plant growth and development, and plays an important role in the response to salt stress by improving K+ accumulation.

With concentrations between 0.1 and 0.2 M, potassium is the most abundant cation in plant cells. The main pool of potassium inside the plant cell is in the vacuole. The function of potassium in this organelle is thought to be purely biophysical; to generate cell turgor to drive cell expansion (1). Under conditions that limit K ϩ availability, the role of K ϩ in the vacuole can be replaced by other ions like Na ϩ , as has been reported to occur under conditions of salt stress (2). Vacuolar concentrations thus vary from high (200 mM) to low (20 mM), suggesting the existence of active K ϩ import and export mechanisms at the vacuolar membrane (3).The role and concentration of K ϩ in other endomembrane organelles in plants are largely unknown. Apart from a specific K ϩ requirement, secondary ion transporters might rely on K ϩ for pH regulation in these organelles. It was shown that apart from the vacuole, V-type H ϩ -transporting ATPase is found in various membranes of the secretory system where vesicle acidification is important for ligand-receptor binding and protein modification, trafficking, and sorting (4, 5). In animal cells, the pH gradient from neutral to acidic along organelles from both the secretory and endocytic pathways is suggested to be under tight control by the operation of secondary ion transporters providing proton leak pathways (6,7,8). Second, solute uptake energized by the pH gradient might be required to generate the osmotic pressure needed for vesicle fusion (5). In view of the abundance of K ϩ in the cell, K ϩ /H ϩ exchangers are likely candidates to be involved in pH and osmoregulation of intracellular compartments as well as active uptake of K ϩ into vacuoles (3) although the biochemical evidence for the existence of such antiporters is scarce (9, 10).In contrast to the limited information available on K ϩ /H ϩ antiporters, many reports have indicated the existence of vacuolar Na ϩ /H ϩ antiporters in plants (11). The first vacuolar Na ϩ /H ϩ exchanger AtNHX1 was identified recently (12), and it was shown that overexpression of this gene in plants enhances salt tolerance (13,14,15). A family of six genes was identified in Arabidopsis (AtNHX1 to AtNHX6) that shows sequence homology to mammalian and yeast NHE or NHX exchangers (16,17). It was demonstrated however that AtNHX1 could catalyze both Na ϩ /H ϩ and K ϩ /H ϩ exchange (14, 18). A similar ion specificity was reported for the human NHE7 isoform. It was shown that this isoform is expressed in the trans-Golgi network, indicating that regulation of pH and ionic composition of intracellular compartments by K ϩ /H ϩ or Na ϩ /H ϩ exchange is an important task of these antiporters (7). This notion was strengthened by the observation that the NHX1 protein is essential for osmotolerance and endosomal protein trafficking in yeast (19,20). Indeed, plant NHX genes were shown not only to be involved in salinity tolerance, but also in vacuolar pH regulation (21,22), and to be induced by NaCl, KCl, and osmotic stress (12,16,(23)(24)(25)(26), or even h...

The Ca 2+ -dependent SOS pathway has emerged as a key mechanism in the homeostasis of Na + and K + under saline conditions. We have identified and functionally characterized the gene encoding the calcineurin-interacting protein kinase of the SOS pathway in tomato, SlSOS2. On the basis of protein sequence similarity and complementation studies in yeast and Arabidopsis, it can be concluded that SlSOS2 is the functional tomato homolog of Arabidopsis AtSOS2 and that SlSOS2 operates in a tomato SOS signal transduction pathway. The biotechnological potential of SlSOS2 to provide salt tolerance was evaluated by gene overexpression in tomato (Solanum lycopersicum L. cv. MicroTom). The better salt tolerance of transgenic plants relative to non-transformed tomato was shown by their faster relative growth rate, earlier flowering and higher fruit production when grown with NaCl. The increased salinity tolerance of SlSOS2-overexpressing plants was associated with higher sodium content in stems and leaves and with the induction and up-regulation of the plasma membrane Na /H+ (LeNHX2 and LeNHX4) antiporters, responsible for Na + extrusion out of the root, active loading of Na + into the xylem, and Na + and K + compartmentalization.