SaNAR2.2, A Novel Member of the NAR2 Family from the Euhalophyte Suaeda altissima: Molecular Cloning and Characterization

NAR2 family proteins are involved in the uptake of nitrate by plant roots as partners of high-affinity nitrate transporters of NRT2 family. In this study, we cloned the coding sequence of a gene, which encodes a protein consisting of 196 amino acids with a calculated molecular weight of 21.7 kDa, from the euhalophyte Suaeda altissima. The protein was characterized in terms of structure and phylogeny. On the phylogenetic tree of NAR2 family proteins, the protein from S. altissima is located in a clade with proteins from the NAR2.2 subfamily found in species within the Amaranthaceae. Therefore, on the basis of phylogenetic relationships, we named it SaNAR2.2. According to a prediction of cellular localization and protein topology, SaNAR2.2 is a likely resident of the plasma membrane. We detected the interaction between SaNAR2.2 and putative partner proteins SaNRT2.1 and SaNRT2.5, which are members of the high-affinity nitrate transporter family NRT2 in S. altissima, using the bimolecular fluorescence complementation (BiFC) assay in Nicotiana benthamiana leaves and the yeast mating-based split-ubiquitin system (mbSUS). SaNAR2.2 was expressed predominantly in roots of S. altissima. The level of SaNAR2.2 transcripts significantly increases at low nitrate concentrations in the medium at the background of high NaCl concentrations (up to 0.5 M). These findings point to the important role of SaNAR2.2 as a participant in the high-affinity nitrate transport system under saline conditions.