Cloning and characterization of a wheat vacuolar cation/proton antiporter and pyrophosphatase proton pump

Sodium at high millimolar levels in the cytoplasm is toxic to plant and yeast cells. Sequestration of Na⁺ ions into the vacuole through the action of tonoplast proton pumps (an H⁺-ATPase in the case of yeast, and either a H⁺-pyrophosphatase (H⁺-PPase) or H ⁺-ATPase in the case of plants) and a Na⁺/H⁺ antiporter is one mechanism that confers salt tolerance to these organisms. The cloning and characterization of genes encoding these tonoplast transport proteins from crop plants may contribute to our understanding of how to enhance crop plant response to saline stress. We cloned wheat orthologs of the Arabidopsis genes AtNHX1 and AVP1 using the polymerase chain reaction and primers corresponding to conserved regions of the respective coding sequences, and a wheat cDNA library as template. The wheat NHX cDNA cloned by this approach was a variant of the previously reported TNHX1 gene. The vacuolar H ⁺-PPase pump we cloned (TVP1) is the first member of this gene family cloned from wheat; it is deduced translation product is homologous to proteins encoded by genes in barley, rice, and Arabidopsis. Function of TNHX1 as a cation/proton antiporter was demonstrated using the nhx1 yeast mutant. TNHX1 was capable of suppressing the hyg sensitivity of nhx1. Functional characterization of the wheat H⁺-PPase TVP1 was demonstrated using the yeast ena1 (plasma membrane Na⁺-efflux transporter) mutant. Expression of TVP1 in ena1 suppressed its Na⁺ hypersensitivity. Expression analysis of salt-stressed wheat plants showed substantial up-regulation of TNHX1 transcript levels as compared to control plants, while transcript accumulation for TVP1 was not greatly affected by exposure of plants to salt stress.