Agricultural productivity is severely affected by soil salinity. One possible mechanism by which plants could survive salt stress is to compartmentalize sodium ions away from the cytosol. Overexpression of a vacuolar Na+/H+ antiport from Arabidopsis thaliana in Arabidopsis plants promotes sustained growth and development in soil watered with up to 200 millimolar sodium chloride. This salinity tolerance was correlated with higher-than-normal levels of AtNHX1 transcripts, protein, and vacuolar Na+/H+ (sodium/proton) antiport activity. These results demonstrate the feasibility of engineering salt tolerance in plants.

Salinity limits plant growth and impairs agricultural productivity. There is a wide spectrum of plant responses to salinity that are defined by a range of adaptations at the cellular and the whole-plant levels, however, the mechanisms of sodium transport appear to be fundamentally similar. At the cellular level, sodium ions gain entry via several plasma membrane channels. As cytoplasmic sodium is toxic above threshold levels, it is extruded by plasma membrane Na(+)/H(+) antiports that are energized by the proton gradient generated by the plasma membrane ATPase. Cytoplasmic Na(+) may also be compartmentalized by vacuolar Na(+)/H(+) antiports. These transporters are energized by the proton gradient generated by the vacuolar H(+)-ATPase and H(+)-PPiase. Here, the mechanisms of sodium entry, extrusion, and compartmentation are reviewed, with a discussion of recent progress on the cloning and characterization, directly in planta and in yeast, of some of the proteins involved in sodium transport.

The ability of plants to grow in high NaCl concentrations is associated with the ability of the plants to transport, compartmentalize, extrude, and mobilize Na + ions. While the influx and efflux at the roots establish the steady state rate of entry of Na + into the plant, the compartmentation of Na + into the cell vacuoles and the radial transport of Na + to the stele and its loading into the xylem establish the homeostatic control of Na + in the cytosol of the root cells. Removal of Na + from the transpirational stream, its distribution within the plant and its progressive accumulation in the leaf vacuoles, will determine the ability to deal with the toxic effects of Na + . The aim of this review is to highlight and discuss the recent progress in understanding of Na + transport in plants.

SummaryThe function of vacuolar Na /H antiporter(s) in plants has been studied primarily in the context of salinity tolerance. By facilitating the accumulation of Na away from the cytosol, plant cells can avert ion toxicity and also utilize vacuolar Na as osmoticum to maintain turgor. As many genes encoding these antiporters have been cloned from salt-sensitive plants, it is likely that they function in some capacity other than salinity tolerance. The wide expression pattern of Arabidopsis thaliana sodium proton exchanger 1 (AtNHX1) in this study supports this hypothesis. Here, we report the isolation of a T-DNA insertional mutant of AtNHX1, a vacuolar Na /H antiporter in Arabidopsis. Vacuoles isolated from leaves of the nhx1 plants had a much lower Na /H and K /H exchange activity. nhx1 plants also showed an altered leaf development, with reduction in the frequency of large epidermal cells and a reduction in overall leaf area compared to wild-type plants. The overexpression of AtNHX1 in the nhx1 background complemented these phenotypes. In the presence of NaCl, nhx1 seedling establishment was impaired. These results place AtNHX1 as the dominant K and Na /H antiporter in leaf vacuoles in Arabidopsis and also suggest that its contribution to ion homeostasis is important for not only salinity tolerance but development as well.

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