Molecular characterization and expression analysis of the Na+/H+ exchanger gene family in Medicago truncatula

One important mechanism plants use to cope with salinity is keeping the cytosolic Na + concentration low by sequestering Na + in vacuoles, a process facilitated by Na + /H + exchangers (NHX). There are eight NHX genes ( NHX1 through NHX8 ) identified and characterized in Arabidopsis thaliana . Bioinformatics analyses of the known Arabidopsis genes enabled us to identify six Medicago truncatula NHX genes ( MtNHX1 , MtNHX2 , MtNHX3 , MtNHX4 , MtNHX6 , and MtNHX7 ). Twelve transmembrane domains and an amiloride binding site were conserved in five out of six MtNHX proteins. Phylogenetic analysis involving A. thaliana , Glycine max , Phaseolus vulgaris , and M. truncatula revealed that each individual MtNHX class (class I: MtNHX1 through 4; class II: MtNHX6; class III: MtNHX7) falls under a separate clade. In a salinity-stress experiment, M. truncatula exhibited ~ 20% reduction in biomass. In the salinity treatment, sodium contents increased by 178 and 75% in leaves and roots, respectively, and Cl − contents increased by 152 and 162%, respectively. Na + exclusion may be responsible for the relatively smaller increase in Na + concentration in roots under salt stress as compared to Cl − . Decline in tissue K + concentration under salinity was not surprising as some antiporters play an important role in transporting both Na + and K + . MtNHX1 , MtNHX6 , and MtNHX7 display high expression in roots and leaves. MtNHX3 , MtNHX6 , and MtNHX7 were induced in roots under salinity stress . Expression analysis results indicate that sequestering Na + into vacuoles may not be the principal component trait of the salt tolerance mechanism in M. truncatula and other component traits may be pivotal.