| Na+/H+ antiporters play a major role in pH and ion homeostasis of cells throughout the biological kingdom. At the plant vacuolar membrane, these antiports use the energy available in the electrochemical gradient of protons to regulate the steady-state ion distribution between the cytosol and the endomembranal compartments. This ion distribution and the concomitant ion homeostasis are essential for the regulation of key physiological processes such as cell expansion and the regulation of cytosolic pH. This thesis describes the cloning and initial characterization of a family of Na +/H+ antiporters in Arabidopsis thaliana. Experiments using overexpression and T-DNA insertional mutants of one of the members of this family, AtNHX1, confirmed the paramount role of vacuolar Na+ compartmentation in the tolerance of plants to salinity and enabled the in planta characterization of AtNHX1 activity. For example, the accumulation of Na+ in plants overexpressing vacuolar Na+/H+ antiporters was greater than that in wild-type plants, which was in turn, greater than that shown in nhx1 plants. The severe reduction of AtNHX1 function appears to have an effect on leaf development, which is also related to cell expansion, the maintenance of turgor, and K+ homeostasis. Moreover, the dramatic reduction in K+-dependent H+ efflux in vacuoles isolated from nhx1 leaves confirmed the role of AtNHX1 as the dominant cation/H+ antiport in leaves. |
