Functional and Molecular Characterization of Plant Nitrate Transporters Belonging to NPF (NRT1/PTR) 6 Subfamily.

Plant nitrate transporters in the NPF (NRT1) family are characterized by multifunctionality and their involvement in a number of physiological processes. The proteins in this family have been identified in many monocotyledonous and dicotyledonous species: a bioinformatic analysis predicts from 20 to 139 members in the plant genomes sequenced so far, including mosses. Plant NPFs are phylogenetically related to proton-coupled oligopeptide transporters, which are evolutionally conserved in all kingdoms of life apart from Archaea. The phylogenetic analysis of the plant NPF family is based on the amino acid sequences present in databases; an analysis identified a separate NPF6 clade (subfamily) with the first plant nitrate transporters studied at the molecular level. The available information proves that proteins of the NPF6 clade play key roles not only in the supply of nitrate and its allocation within different parts of plants but also in the transport of chloride, amino acids, ammonium, and plant hormones such as auxins and ABA. Moreover, members of the NPF6 family participate in the perception of nitrate and ammonium, signaling, plant responses to different abiotic stresses, and the development of tolerance to these stresses and contribute to the structure of the root-soil microbiome composition. The available information allows us to conclude that genes are among the promising targets for engineering/editing to increase the productivity of crops and their tolerance to stresses. The present review summarizes the available published data and our own results on members of the NPF6 clade of nitrate transporters, especially under salinity; we outline their molecular, structural, and functional characteristics and suggest potential lines for future research.