The Arabidopsis thaliana salt tolerance gene SOS1 encodes a putative Na+/H+ antiporter

In Arabidopsis thaliana, the SOS1 (Salt Overly Sensitive 1) locus is essent ial for Na+ and K+ homeostasis, and sos1 mutations render plants more sensi tive to growth inhibition by high Na+ and low K+ environments. SOS1 is clon ed and predicted to encode a 127-kDa protein with 12 transmembrane domains in the N-terminal part and a long hydrophilic cytoplasmic tail in the C-ter minal part. The transmembrane region of SOS1 has significant sequence simil arities to plasma membrane Na+/H+ antiporters from bacteria and fungi. Sequ ence analysis of various sos1 mutant alleles reveals several residues and r egions in the transmembrane as well as the tail parts that are critical for SOS1 function in plant salt tolerance. SOS1 gene expression in plants is u p-regulated in response to NaCl stress, This up-regulation is abated in sos 3 or sos2 mutant plants, suggesting that it is controlled by the SOS3/SOS2 regulatory pathway.