Figure 4

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.