Acts as a dual component transporter with NTR2.1. Required for high-affinity nitrate transport. Acts as a repressor of lateral root initiation. May be involved in targeting NRT2 proteins to the plasma membrane. Heterotetramer composed of two NRT2.1 and two NRT3.1 (PubMed:20561257). Interacts with NRT2.1 and NRT2.3 (PubMed:17012411). Interacts with all other NRT2 transporters, including NRT2.5 (PubMed:22432443). Highly expressed in roots. Detected in shoots. Up-regulated in roots by nitrate. No effect in shoots. Induced by sudden N starvation and by low nitrate concentration. No visible phenotype when grown under normal conditions. Reduced growth on low-nitrate media. Belongs to the NAR2 family.