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

As a crucial nitrogen source, nitrate (NO 3 - ) is a key nutrient for plants. Accordingly, root systems adapt to maximize NO 3 - availability, a developmental regulation also involving the phytohormone auxin. Nonetheless, the molecular mechanisms underlying this regulation remain poorly understood. Here, we identify low-nitrate-resistant mutant ( lonr ) in Arabidopsis ( Arabidopsis thaliana ), whose root growth fails to adapt to low-NO 3 - conditions. lonr2 is defective in the high-affinity NO 3 - transporter NRT2.1. lonr2 ( nrt2.1 ) mutants exhibit defects in polar auxin transport, and their low-NO 3 - -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 3 - levels. These results reveal a mechanism by which NRT2.1 in response to NO 3 - 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 3 - availability.