Dual affinity nitrate transporter. Involved in proton-dependent nitrate uptake and in the regulation of the nitrate transporter NRT2.1. Acts also as a nitrate sensor that trigger a specific signaling pathway stimulating lateral root growth and seed germination. The uptake activity is not required for sensor function. Displays an auxin transport facilitation inhibited by high nitrate concentration. Required to prevent auxin accumulation in preemerged lateral root primordia and young lateral roots when external nitrate concentration is low or null. May be involved in the basipetal transport of auxin out of the lateral root tips. Acts as a bidirectional transporter involved in root-to-shoot nitrate translocation. Recognizes specifically nitrate and chlorate, but not nitrite, alanine, sulfate, phosphate or the di-peptide Ala-Ala. Monomer and homodimer. The dimer has the 2 monomers in the same orientation. Interacts with CIPK23. Expressed in the stele in lateral root primordia before emergence and in the tip of primary and emerged lateral roots. Detected in emerging and immature leaves, guard cells, flower buds, style, stigma, anthers and pollen grains. Not detected in the shoot apical meristem. Expressed in the columella root cap at day 1 after germination. At day 3, detected in the root meristematic region and at day 5, expressed throughout the root tip. By nitrate and auxin. Acts as a high-affinity nitrate transporter when phosphorylated and as a low-affinity transporter when dephosphorylated. Forms homodimer when unphosphorylated and monomer when phosphorylated. Low nitrogen concentration in the medium stimulates phosphorylation. Phosphorylation also regulates the nitrate signaling. Altered development of nascent organs. Reduced stomatal opening and reduced transpiration rates in the light resulting in enhanced drought tolerance. Slower translocation of nitrate to the leaves. When mutated confers resistance to the herbicide chlorate. The kinase CIPK23 is a negative regulator of the high-affinity response, while the kinase CIPK8 is a positive regulator of the low-affinity response. Thr-101 is not the direct target of CIPK8. Belongs to the major facilitator superfamily. Proton-dependent oligopeptide transporter (POT/PTR) (TC 2.A.17) family.