Plant NHX cation/proton antiporters

Abstract

Although physiological and biochemical data since long suggested that Na(+)/H(+) and K(+)/H(+) antiporters are involved in intracellular ion and pH regulation in plants, it has taken a long time to identify genes encoding antiporters that could fulfil these roles. Genome sequencing projects have now shown that plants contain a very large number of putative Cation/Proton antiporters, the function of which is only beginning to be studied. The intracellular NHX transporters constitute the first Cation/Proton exchanger family studied in plants. The founding member, AtNHX1, was identified as an important salt tolerance determinant and suggested to catalyze Na(+) accumulation in vacuoles. It is, however, becoming increasingly clear, that this gene and other members of the family also play crucial roles in pH regulation and K(+) homeostasis, regulating processes from vesicle trafficking and cell expansion to plant development.

Keywords: NHX-type ion transporters; pH regulation; plant membrane vesicles; potassium homeostasis; salt tolerance.

Figure 1

Phylogenetic tree of 79 proteins of the monovalent cation proton antiporter CPA1 family. Phylogeneitic relationships were inferred using the Neighbor-Joining method [Saitou N, Nei M. The neighbor-joining method: A new method for reconstructing phylogenetic trees. Mol Biol Evol 1987; 4:406–25]. The bootstrap consensus tree inferred from 500 replicates [Felsenstein J. Confidence limits on phylogenies: An approach using the bootstrap. Evolution 1985; 39:783–91], is taken to represent the evolutionary history of the proteins analyzed [Felsenstein J. Confidence limits on phylogenies: An approach using the bootstrap. Evolution 1985; 39:783–91]. Branches corresponding to partitions reproduced in less than 50% bootstrap replicates are collapsed. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the Poisson correction method [Zuckerkandl E, Pauling L. Evolutionary divergence and convergence in proteins. In: Bryson V and Vogel HJ, eds. Evolving Genes and Proteins. New York: Academic Press 1965; 97–166] and are in the units of the number of amino acid substitutions per site. All positions containing gaps and missing data were eliminated from the dataset. There were a total of 93 positions in the final dataset. Phylogenetic analyses were conducted in MEGA4 [Tamura K, Dudley J, Nei M, Kumar S. MEGA4: Molecular Evolutionary Genetics Analysis (MEGA) software version 4.0. Mol Biol Evol 2007; 24:1596–9]. A list of included sequences is provided in Table 1.