A Critical Role Of Sodium Efflux In The Salt Tolerance Of Rice

Abstract

Rice (Oryza sativa L.) stands among the world's most important and salt-sensitive crop species. 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 SOS1, the only Na+ efflux transporter that has to date been genetically characterized in plants, controls the net Na+ uptake by roots and the long-distance transport to shoots in several species. Hence, we have analyzed the importance of Na+ fluxes governed by the SOS system in the salt tolerance of rice by a reverse-genetics approach. Loss-of-function mutant in the SOS1 gene displayed exceptional salt sensitivity in hydroponic culture and in experimental paddies. Salt sensitivity correlated with excessive Na+ intake and the interruption of Na+ and K+ loading into the xylem, in agreement with the expression pattern of the SOS1 gene and the function of the SOS1 protein in active Na+ efflux.

Contrary to most analyses of the transcriptional response of plants to high salinity in which it is not feasible to separate the response to the osmotic and the ionic components of this multifaceted stress, the extreme Na+-sensitivity of the sos1 mutant plant offered the unique advantage of inspecting the transcriptional response specific to sodicity stress at low external salt concentrations. Whereas the sos1 mutation induced minor transcriptional changes in control conditions, its transcriptional impact was largely specific to the salt-induced response and affected to almost 20% of the expressed genes. Notably, the majority (64%) of these salt-responsive genes were down-regulated in the sos1 root relative to the control. This biased transcriptional response to salinity supports a regulatory role for the SOS1 protein.