The nitrate transporter NRT2.1 directly antagonizes PIN7-mediated auxin transport for root growth adaptation.

As a crucial nitrogen source, nitrate (NO) is a key nutrient for plants. Accordingly, root systems adapt to maximize NO availability, a developmental regulation also involving the phytohormone auxin. Nonetheless, the molecular mechanisms underlying this regulation remain poorly understood. Here, we identify () in Arabidopsis (), whose root growth fails to adapt to low-NO conditions. is defective in the high-affinity NO transporter NRT2.1. () mutants exhibit defects in polar auxin transport, and their low-NO-induced root phenotype depends on the PIN7 auxin exporter activity. NRT2.1 directly associates with PIN7 and antagonizes PIN7-mediated auxin efflux depending on NO levels. These results reveal a mechanism by which NRT2.1 in response to NO limitation directly regulates auxin transport activity and, thus, root growth. This adaptive mechanism contributes to the root developmental plasticity to help plants cope with changes in NO availability.