Functional and Evolutionary Analysis of Cation/Proton Antiporter-1 Genes in Brassicaceae Adaptation to Salinity

Abstract

The accumulation of salts in soil is an important agricultural problem that limits crop productivity. Salts containing sodium (Na⁺) are particularly problematic, as cytosolic Na⁺ can interfere with cellular metabolism and lead to cell death. Maintaining low levels of cytosolic Na⁺, therefore, is critical for plant survival during growth in salt. Mechanisms to regulate Na⁺ accumulation in plant cells include extrusion of Na⁺ from the cell and sequestration of Na⁺ into intracellular compartments. Both of these processes are controlled in part through the action of Na⁺/H⁺ exchangers belonging to the Cation/Proton Antiporter-1 (CPA1) gene family. Genes belonging to this family have been identified in both salt-sensitive and salt-tolerant species, suggesting that salt-tolerant species may have evolved salt tolerance through modification of these existing pathways. The research presented here has focused on understanding how salt tolerance has evolved in Brassicaceae species, and particularly on the role that CPA1 genes have played in the adaptation to salinity of Eutrema salsugineum. Specific projects have sought to understand 1) how copy number variation and changes in coding sequences of CPA1 genes contribute to salt tolerance in E. salsugineum and its salt-tolerant relative Schrenkiella parvula, 2) whether functional or regulatory changes in Salt Overly Sensitive 1 (SOS1) from E. salsugineum (EsSOS1) contribute to its enhanced salt tolerance, and 3) whether accessions of Arabidopsis thaliana differ significantly in their response to salt stress.The results indicate that EsSOS1 and SOS1 from S. parvula (SpSOS1) both confer greater salt tolerance in yeast than SOS1 from A. thaliana (AtSOS1) when activated by the complex of the SOS2 kinase and SOS3 calcium-binding protein, whereas only EsSOS1 confers enhanced salt tolerance in the absence of activation. When expressed in A. thaliana, EsSOS1 also confers greater salt tolerance than AtSOS1 through regulatory changes that likely involve differences in expression pattern. Together, the results presented here suggest that mechanisms regulating cellular Na⁺ accumulation that exist in salt-sensitive crop species could be altered to enhance growth in salty soils. In addition, the 19 A. thaliana accessions used to create the MAGIC population were shown to differ significantly in their response to salt stress.