Distinct roles for SOS1 in the convergent evolution of salt tolerance in eutrema salsugineum and schrenkiella parvula

Eutrema salsugineum and Schrenkiella parvula are salt-tolerant relatives of the salt-sensitive species Arabidopsis thaliana. An important component of salt tolerance is the regulation of Na⁺ ion homeostasis, which occurs in part through proteins encoded by the Cation/Proton Antiporter-1 (CPA1) gene family. We used a combination of evolutionary and functional analyses to examine the role of CPA1 genes in the salt tolerance of E. salsugineum and Sc. parvula, and found evidence that changes in CPA1-mediated Na⁺ extrusion may contribute to the salt tolerance of both species. Specifically, we found that a member of the CPA1 family, the Na⁺/H⁺ antiporter gene Salt Overly Sensitive 1 (SOS1), evolved under positive selection in E. salsugineum. In the absence of activation by the SOS2 kinase/SOS3 calcium-binding protein complex, SOS1 from E. salsugineum (EsSOS1) confers greater salt tolerance than SOS1 from Sc. parvula (SpSOS1) and Ar. thaliana (AtSOS1) when expressed in a salt-sensitive strain of Saccharomyces cerevisiae. A single amino acid change in the putative autoinhibitory domain is required but not sufficient for the enhanced salt tolerance conferred by EsSOS1. When activated by SOS2 and SOS3, both EsSOS1 and SpSOS1 confer greater salt tolerance than AtSOS1. Enhanced SOS1-mediated Na⁺ extrusion therefore appears to contribute to the salt tolerance of both E. salsugineum and Sc. parvula, although through apparently different mechanisms.