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.