Model of Na+ fluxes in plant cells. Sodium ions enter root cells through HKT proteins and non-selective voltage-independent cation channels, some of which (labeled CNGC) are inactivated by cyclic nucleotides (cAMP and cGMP). Although there is no direct experimental evidence for this suggestion, the transport activity or ion selectivity of HKT proteins could be regulated by a process dependent on the Ca2+ sensor SOS3 to prevent excessive Na+ uptake. SOS3 associated with the protein kinase SOS2 positively regulates the activity of the plasma membrane Na+/H+antiporter SOS1, which in turn mediates Na+ extrusion and possibly long-distance Na+ transport from roots to shoots [11,21]. HAK is a K+/H+ symporter that can transport Na+ at low affinity. Cytoplasmic Na+ is compartmentalized into vacuoles within cells by the tonoplast (vacuolar membrane) Na+/H+ antiporter NHX1, dissipating the H+ gradient generated by the V-ATPase (not shown) and the pyrophosphatase AVP1 (which hydrolyzes pyrophosphate, PPi).