2.A.1.8.12
The root cortical and epidermal cell, high affinity, plasma membrane, NO₃^- uptake transporter, NRT2.1 (Wirth et al., 2007). Also functions in nitrate sensing and signaling (Miller et al., 2007; Girin et al., 2010). Activity only occurs when NRT2.1 is complexed with NAR2.1 (WR3; 8.A.20.1.1) in a 2:2 tetrameric complex (Yong et al., 2010). NAR2.1 has an N-terminal and a C-terminal TMS and has been annotated as a calcineurin-like phosphoesterase family member (Yong et al., 2010).  Ntr transporters may also play a role in gaseous NO₂ uptake by leaves (Hu et al. 2014).  The Medicago truncatula orthologue has been characterized (Pellizzaro et al. 2014). An NRT2 homologue in wheat has been identifed and partially characterized (Kumar et al. 2022). Nitrate is the main form of inorganic nitrogen that crops absorb, and nitrate transporters 2 (NRT2) are high affinity nitrate uptake porters. When the available nitrate is limiting, the high affinity transport systems are activated. Most NRT2s cannot transport nitrates alone and require the assistance of helper proteins belonging to nitrate assimilation related family (NAR2; TC# 8.A.20.1.1) to complete the transport of nitrate (Zhao et al. 2023). Crop nitrogen utilization efficiency is affected by environmental conditions, and there are differences between different plant varieties.  Sorghum bicolor has high stress tolerance and is efficient in soil nitrogen utilization. The S. bicolor genome database was scanned for gene structures, chromosomal localizations, physicochemical properties, secondary structures and transmembrane domains, signal peptides and subcellular localizations, promoter region cis-acting elements, phylogenetic evolution, SNP recognition and annotation, and selection pressure of gene family members (Zhao et al. 2023). Through bioinformatics analysis, 5 NRT2 gene members (designated as SbNRT2-1a, SbNRT2-1b, SbNRT2-2, SbNRT2-3, and SbNRT2-4) and 2 NAR2 gene members (designated SbNRT3-1 and SbNRT3-2) were identified, the number of which was less than that of foxtail millet. SbNRT2/3 could be divided into four subfamilies. All were present in the plasma membrane; SbNRT2 proteins lacked signal peptides, but SbNRT3 proteins contained them. Expression was responsive to plant hormones and stress response elements (Zhao et al. 2023).