Phosphorylation at Ser28 stabilizes the Arabidopsis nitrate transporter NRT2.1 in response to nitrate limitation

Nitrate is one of the main inorganic nitrogen sources for plants. Nitrate absorption from soils is achieved through the combined activities of specific nitrate transporters. Nitrate transporter 2.1 (NRT2.1) is the major component of the root high-affinity nitrate transport system in Arabidopsis thaliana. Studies to date have mainly focused on transcriptional control of NRT2.1. Here, we show that NRT2.1 protein stability is also regulated in response to nitrogen nutrition availability. When seedlings were transferred to nitrate-limited conditions, the apparent half-life of NRT2.1 in roots increased from 3 to 9 h. This stabilization of NRT2.1 protein occurred rapidly, even prior to the transcriptional stimulation of NRT2.1. Furthermore, we revealed that phosphorylation at serine 28 (Ser28) of NRT2.1 is involved in regulating the stability of this protein. Substitution of Ser28 by alanine resulted in unstable NRT2.1, and this loss-of-phosphorylation mutant (NRT2.1^S28A ) failed to complement the growth-restricted phenotype of the nrt2.1 mutant when a low concentration of nitrate was the sole nitrogen source. These results demonstrate that phosphorylation at Ser28 is crucial for NRT2.1 protein stabilization and accumulation in response to nitrate limitation.