A Critical Role of Sodium Flux via the Plasma Membrane Na⁺/H⁺ Exchanger SOS1 in the Salt Tolerance of Rice

Rice (<i>Oryza sativa</i>) stands among the world's most important crop species. Rice is salt sensitive, and the undue accumulation of sodium ions (Na⁺) in shoots has the strongest negative correlation with rice productivity under long-term salinity. The plasma membrane Na⁺/H⁺ exchanger protein Salt Overly Sensitive 1 (SOS1) is the sole Na⁺ efflux transporter that has been genetically characterized to date. Here, the importance of SOS1-facilitated Na⁺ flux in the salt tolerance of rice was analyzed in a reverse-genetics approach. A <i>sos1</i> loss-of-function mutant displayed exceptional salt sensitivity that was correlated with excessive Na⁺ intake and impaired Na⁺ loading into the xylem, thus indicating that SOS1 controls net root Na⁺ uptake and long-distance Na⁺ transport to shoots. The acute Na⁺ sensitivity of <i>sos1</i> plants at low NaCl concentrations allowed analysis of the transcriptional response to sodicity stress without effects of the osmotic stress intrinsic to high-salinity treatments. In contrast with that in the wild type, <i>sos1</i> mutant roots displayed preferential down-regulation of stress-related genes in response to salt treatment, despite the greater intensity of stress experienced by the mutant. These results suggest there is impaired stress detection or an inability to mount a comprehensive response to salinity in <i>sos1</i> In summary, the plasma membrane Na⁺/H⁺ exchanger SOS1 plays a major role in the salt tolerance of rice by controlling Na⁺ homeostasis and possibly contributing to the sensing of sodicity stress.