Functional characterization of the Arabidopsis family of intracellular Na+/H+ antiporters
Na+/H+ (NHX-type) antiporters play major roles in pH and K+ (Na+) homeostasis in plant cells. The Arabidopsis NHX gene family comprises a class of six intracellular Na+/H+ antiporters, functionally grouped as vacuolar (NHX1 to NHX4) and endosomal (NHX5 and NHX6). Recent efforts using a multidisciplinary approach combining reverse genetics, biochemistry, cell biology and proteomics uncovered unique roles of several NHX members in plant growth, development and stress responses. Double knockout nhx1nhx2 plantsdisplayed significantly reduced growth, smaller cells, shorter hypocotyls in etiolated seedlings and abnormalities in flower development. Measurements of vacuolar pH and K+ concentrations indicated that nhx1nhx2 vacuoles were more acidic and accumulated less vacuolar K+. Our results indicated that NHX1 and NHX2 play significant roles in the vacuolar K+/H+ exchange and are key determinants in establishing vacuolar K+ homeostasis. nhx1nhx2 knockout plants alsodisplayed severe sensitivity to low (10mM) K+ but not to Na+; and addition of 50mM NaCl partly rescued the growth phenotype. Experiments using multiple knockout lines (nhx1, nhx2, nhx3, nhx4)suggest differential cation/H+ selectivities among the different NHX isoforms with differential contributions to K+ and Na+ homeostasis. Our results raise questions regarding the intracellular ‘compatibility’ of K+ ions, the transport of Na+ into the vacuole, and the role of both ions in generating the vacuolar turgor needed to drive cell expansion, as well as the response to high salinity. In addition, our results showed that the Golgi-, TGN-localized NHX5 and NHX6 are required for vesicular trafficking. The double knockout nhx5/nhx6exhibited reduced growth, with smaller and fewer cells, increased sensitivity to salinity and missorting of vacuolar-destined cargo to the apoplast, highlighting the importance of NHX5 and NHX6 in vesicular trafficking and its function(s) during salt stress. The unique and diverse developmental functions of several NHX members controlling cell volume, ion and pH homeostasis in vacuoles and endosomal compartments will be discussed.
Bassil E., Ohto, M., Esumi, T., Tajima, H., Zhu Z., Belmonte, M., Peleg, Z., Yamaguchi,T.,Blumwald,
E. (2011). The Arabidopsis intracellular Na+/H+ antiporters AtNHX5 and AtNHX6 are
implicated in novel endosomal functions associated with plant growth and development. Plant
Cell.23:224-239.
Bassil E, Tajima H, Liang Y-C, Ohto M, Ushijima K, Nakano R, Esumi T, Coku A, Blumwald E
(2011). The Arabidopsis Na+/H+ Antiporters NHX1 and NHX2 Regulate Vacuolar pH and K+
Homeostasis to Control Growth, Flower Development and Reproduction Plant Cell 23:3482-
3511.