Selective transport capacity for K+ over Na+ is linked to the expression levels of PtSOS1 in halophyte Puccinellia tenuiflora

The plasma membrane Na+/H+ antiporter (SOS1) was shown to be a Na+ efflux protein and also involved in K+ uptake and transport. PtSOS1 was characterised from Puccinellia tenuiflora (Griseb.) Scribn. et Merr., a monocotyledonous halophyte that has a high selectivity for K+ over Na+ by roots under salt stress. To assess the contribution of PtSOS1 to the selectivity for K+ over Na+, the expression levels of PtSOS1 and Na+, K+ accumulations in P. tenuiflora exposed to different concentrations of NaCl, KCl or NaCl plus KCl were analysed. Results showed that the expression levels of PtSOS1 in roots increased significantly with the increase of external NaCl (25-150 mM), accompanied by an increase of selective transport (ST) capacity for K+ over Na+ by roots. Transcription levels of PtSOS1 in roots and ST values increased under 0.1-1 mM KCl, then declined sharply under 5-10 mM KCl. Under 150 mM NaCl, PtSOS1 expression levels in roots and ST values at 0.1 mM KCl was significantly lower than that at 5 mM KCl with the prolonging of treatment time. A significant positive correlation was found between root PtSOS1 expression levels and ST values under various concentrations of NaCl, KCl or 150 mM NaCl plus 0.1 or 5 mM KCl treatments. Therefore, it is proposed that PtSOS1 is the major component of selective transport capacity for K+ over Na+ and hence, salt tolerance of P. tenuiflora. Finally, we hypothesise a function model of SOS1 in regulating K+ and Na+ transport system in the membrane of xylem parenchyma cells by sustaining the membrane integrity; it also appears that this model could reasonably explain the phenomenon of Na+ retrieval from the xylem when plants are exposed to severe salt stress.