The Na(+)/H(+) exchanger SOS1 controls extrusion and distribution of Na(+) in tomato plants under salinity conditions.

Maintaining a high K(+)/Na(+) ratio in the cell cytosol, along with the transport processes implicated in the xylem and phloem loading/unloading of Na(+) in plants (long-distance transport) are key aspects in plant salt tolerance. The Ca(2+)-dependent SOS pathway regulating Na(+) and K(+) homeostasis and long-distance Na(+) transport has been reported in Arabidopsis. However, Arabidopsis might not be the best model to analyze the involvement of the SOS pathway in long-distance Na(+) transport due to the very short stem of these plants which do not allow a precise dissection of the relative content of Na(+) in stem versus leaf. This separation would be critical to assess the role of SOS1 in xylem loading/unloading, Na(+) export by roots, retention in stems and the differential distribution/accumulation in old leaves. Towards this goal, tomato might represent a superior model due to its anatomical structure and agricultural significance. We recently demonstrated the key role played by the plasma membrane Na(+)/H(+) antiporter SlSOS1 in salt tolerance in tomato by maintaining ion homeostasis under salinity stress and in the partitioning of Na(+) in plant organs.