Advances in Study of Na⁺ Uptake and Transport in Higher Plants and Na⁺ Homeostasis in the Cell

Salt stress is one of the factors that influence agriculture productivity. In this paper, we review transporters and channels such as HKT, LCT1, AKT and NSCC possibly involved in plant Na⁺ uptake, as well as long-distance Na⁺ transport in plants and Na⁺ homeostasis in the cell. HKTs are ubiquitous transporters mediating Na⁺ uptake, whereby the charged amino acid residues play a pivotal role in cation selectivity. LCT1 is a novel transporter found in wheat that can mediate low-affinity cation uptake but does not function in Na⁺ uptake at typical soil Ca²⁺ concentrations. AKTs, another family of transporters, may also be involved in Na⁺ uptake under high salinity. Although no gene encoding NSCC has been cloned, NSCCs are considered the main pathway though which plants take up excessive Na⁺. SOS1 and HKT are involved in long-distance Na⁺ transport between roots and shoots and play important roles in Na⁺ loading and unloading in xylem and phloem, respectively, thus affecting plant salt tolerance. In addition, SOS1, an Na⁺/H⁺ antiporter located in the plasma membrane, together with protein kinase SOS2 and Ca²⁺ binding protein SOS3, forms a complex of SOS, which plays a critical role in Na⁺ homeostasis in cells. This regulatory mechanism is conserved among dicots and monocots both in structure and function. The SOS complex, together with other Na⁺/H⁺ antiporters and H⁺ pumps localizing in plasma membrane or tonoplast, controls cellular Na⁺ homeostasis.