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. [ABSTRACT FROM AUTHOR]