The SOS1 transporter of Physcomitrella patens mediates sodium efflux in planta.

• SOS1 is an Na(+)/H(+) antiporter that plays a central role in Na(+) tolerance in land plants. SOS1 mediation of Na(+) efflux has been studied in plasma-membrane vesicles and deduced from the SOS1 suppression of the Na(+) sensitivity of yeast mutants defective in Na(+) -efflux. However, SOS1-mediated Na(+) efflux has not been characterized in either plant or yeast cells. Here, we use Physcomitrella patens to investigate the function of SOS1 in planta. • In P. patens, a nonvascular plant in which the study of ion cellular fluxes is technically simple, the existence of two SOS1 genes suggests that the Na(+) efflux remaining after the deletion of the ENA1 ATPase is mediated by a SOS1 system. Therefore, we cloned the P. patens SOS1 and SOS1B genes (PpSOS1 and PpSOS1B, respectively) and complementary DNAs, and constructed the PpΔsos1 and PpΔena1/PpΔsos1 deletion lines by gene targeting. • Comparison of wild-type, and PpΔsos1 and PpΔena1/PpΔsos1 mutant lines revealed that PpSOS1 is crucial for Na(+) efflux and that the PpΔsos1 line, and especially the PpΔena1/PpΔsos1 lines, showed excessive Na(+) accumulation and Na(+)-triggered cell death. The PpΔsos1 and PpΔena1/PpΔsos1 lines showed impaired high-affinity K(+) uptake. • Our data support the hypothesis that PpSOS1 mediates cellular Na(+) efflux and that PpSOS1 enhances K(+) uptake by an indirect effect.