Schematic summaries of Na+ influx pathways into saline roots and primary protective mechanisms mediated by Na+ transporters on important biological membranes. (a) Schematic representations of several entries for Na+ influx into roots including cell-to-cell and apoplastic pathways. Thick red arrows represent hypothetical Na+ entry sites for the apoplastic bypass flow (see text). (b) Schematic representations of Na+ extrusion at the plasma membrane (PM) of the soil-cell interface (left), Na+ reabsorption at the PM of the xylem parenchyma cell (XPC: middle), and Na+ sequestration into the vacuole at the tonoplast (right). K+ transporting proteins in PMs of the soil-cell interface and the XPC respectively represent high-affinity K+ uptake channels/transporters and K+ efflux channels, which could couple with the each Na+ transporting mechanism in the PM.