Characterization of the signalling pathways involved in the repression of root nitrate uptake by nitrate in Arabidopsis thaliana.

In Arabidopsis thaliana , root high-affinity nitrate (NO₃^–) uptake depends mainly on NRT2.1, 2.4, and 2.5, which are repressed by high NO₃^– supply at the transcript level. For NRT2.1 , this regulation is due to the action of (i) feedback down-regulation by N metabolites and (ii) repression by NO₃^– itself mediated by the transceptor NRT1.1(NPF6.3). However, for NRT2.4 and NRT2.5 , the signalling pathway(s) remain unknown as do the molecular elements involved. Here we show that unlike NRT2.1 , NRT2.4 and NRT2.5 are not induced in an NO₃^– reductase mutant but are up-regulated following replacement of NO₃^– by ammonium (NH₄⁺) as the N source. Moreover, increasing the NO₃^– concentration in a mixed nutrient solution with constant NH₄⁺ concentration results in a gradual repression of NRT2.4 and NRT2.5 , which is suppressed in an nrt1.1 mutant. This indicates that NRT2.4 and NRT2.5 are subjected to repression by NRT1.1-mediated NO₃^– sensing, and not to feedback repression by reduced N metabolites. We further show that key regulators of NRT2 transporters, such as HHO1, HRS1, PP2C, LBD39, BT1, and BT2, are also regulated by NRT1.1-mediated NO₃^– sensing, and that several of them are involved in NO₃^– repression of NRT2.1 , NRT2.4 , and NRT2.5. Finally, we provide evidence that it is the phosphorylated form of NRT1.1 at the T101 residue, which is most active in triggering the NRT1.1-mediated NO₃^– regulation of all these genes. Altogether, these data led us to propose a regulatory model for high-affinity NO₃^– uptake in Arabidopsis, highlighting several NO₃^– transduction cascades downstream of the phosphorylated form of the NRT1.1 transceptor. [ABSTRACT FROM AUTHOR]