Résumé
In Arabidopsis the NRT2.1 gene encodes a main component of the root high-affinity nitrateuptake system (HATS). However, despite its central role in plant nutrition, little is knownconcerning the molecular mechanisms involved in its regulation. By combining animmunological approach and the use of transgenic lines expressing a functional 35S::NRT2.1transgene in an atnrt2.1 mutant background, we were able to show the occurrence ofposttranslational regulatory mechanisms. Since one mechanism could correspond to NRT2.1C-terminus processing we further investigate this hypothesis by producing transgenic plantswith truncated forms of NRT2.1. This revealed an essential sequence for NRT2.1 activity,located between the residues 494 and 513. Using a phospho-proteomic approach, we foundthat this sequence contains one phosphorylation site, at serine 501, which can inactivateNRT2.1 function when mimicking the constitutive phosphorylation of this residue intransgenic plants. This phenotype could neither be explained by changes in abundance ofNRT2.1 and NAR2.1, a partner protein of NRT2.1, nor by a lack of interaction between thesetwo proteins. Finally, the relative level of serine 501 phosphorylation was found to beincreased by ammonium nitrate in wild-type plants, leading to the inactivation of NRT2.1and to a decrease in high affinity nitrate transport into roots. Altogether, these observationsreveal a new and essential mechanism for the regulation of NRT2.1 activity.