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

As a crucial nitrogen source, nitrate (NO3?)is a key nutrient for plants. Accordingly, root systems adapt to maximize NO3? 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-NO3? conditions. lonr2 is defective in the high-affinity NO3? transporter NRT2.1. lonr2 (nrt2.1)mutants exhibit defects in polar auxin transport, and their low-NO3?-induced root phenotype depends on the PIN7 auxin exporter activity. NRT2.1 directly associates with PIN7 and antagonizes PIN7-mediated auxin efflux depending on NO3? levels. These results reveal a mechanism by which NRT2.1 in response to NO3? 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 NO3? availability.