Novel Proteins of the High-Affinity Nitrate Transporter Family NRT2, SaNRT2.1 and SaNRT2.5, from the Euhalophyte : Molecular Cloning and Expression Analysis.

Two genes of nitrate transporters and , putative orthologs of high-affinity nitrate transporter genes and from , were cloned from the euhalophyte . Phylogenetic bioinformatic analysis demonstrated that the proteins SaNRT2.1 and SaNRT2.5 exhibited higher levels of homology to the corresponding proteins from the plants of family Amaranthaceae; the similarity of amino acid sequences between proteins SaNRT2.1 and SaNRT2.5 was lower (54%). Both SaNRT2.1 and SaNRT2.5 are integral membrane proteins forming 12 transmembrane helices as predicted by topological modeling. An attempt to demonstrate nitrate transporting activity of or by heterologous expression of the genes in the yeast () mutant strain Δ lacking the only yeast nitrate transporter was not successful. The expression patterns of and were studied in plants that were grown in hydroponics under either low (0.5 mM) or high (15 mM) nitrate and salinity from 0 to 750 mM NaCl. The growth of the plants was strongly inhibited by low nitrogen supply while stimulated by NaCl; it peaked at 250 mM NaCl for high nitrate and at 500 mM NaCl for low nitrate. Under low nitrate supply, nitrate contents in roots, leaves and stems were reduced but increased in leaves and stems as salinity in the medium increased. Potassium contents remained stable under salinity treatment from 250 to 750 mM NaCl. Quantitative real-time PCR demonstrated that without salinity, was expressed in all organs, its expression was not influenced by nitrate supply, while was expressed exclusively in roots-its expression rose about 10-fold under low nitrate. Salinity increased expression of both and under low nitrate. peaked in roots at 500 mM NaCl with 15-fold increase; peaked in roots at 500 mM NaCl with 150-fold increase. It is suggested that SaNRT2.5 ensures effective nitrate uptake by roots and functions as an essential high-affinity nitrate transporter to support growth of adult plants under nitrogen deficiency.