Figure 1

K ⁺ /Na ⁺ homeostasis in higher plants. Plant root cells generally absorb Na⁺/K⁺ from soil through different channels (NSCCs, AKT1, LCT1, CCC), transporters (KUP/HAK/KT and HKT) and apoplastic. Channel permeations and apoplastic are the main pathways of Na⁺ influx under salt stress. In the SOS pathway Na⁺ crosses the plasma membrane to the apoplast or soil solution and the NHX1 partitions Na⁺ within vacuole and jointly regulate the cytosol Na⁺ concentrations and play a vital role in response to salt stress. AtHKT1;1, OsHKT1;5, TaHKT1;5 and TmHKT1;4/5 retrieve Na⁺ from the xylem into xylem parenchyma cell and prevent the shoot from damage caused by Na⁺ over-accumulation. It is hypothesized that AtHKT1;1 mediates recycling Na⁺ from the shoot to root through removal of Na⁺ from the xylem and loading Na⁺ into the phloem sieves. These processes assure a normal K⁺/Na⁺ homeostasis and maintain a high K⁺/Na⁺ ratio to rescue plants when suffering from salt stress. NSCC, nonselective cation channels; CCC, cation-Cl − co-transporter; LCT, low-affinity cation transporter; SOS1, salt overly sensitive 1; NHX1, Na⁺/H⁺ antiporter 1.