Signaling pathways involved in the repression of root nitrate uptake by nitrate inArabidopsis thaliana

AbstractInArabidopsis 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. ForNRT2.1, this regulation is due to the action of (i) feedback downregulation by N metabolites and (ii) repression by NO3-itself mediated by the transceptor NRT1.1(NPF6.3). However, forNRT2.4andNRT2.5the signaling pathway(s) remain unknown along with the molecular elements involved. Here we show that unlikeNRT2.1, NRT2.4andNRT2.5are not induced in a NO3-reductase mutant but are strongly upregulated following replacement of NO3-by ammonium (NH4+) as the N source. Moreover, increasing NO3-concentration in a mixed nutrient solution with constant NH4+concentration results in a gradual repression ofNRT2.4andNRT2.5, which is suppressed in anrt1.1mutant. This indicates thatNRT2.4andNRT2.5are 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 NRT2s transporters, such as HHO1, HRS1, PP2C, LBD39, BT1 and BT2, are also regulated by NRT1.1-mediated NO3-sensing, and that several are involved in NO3-repression ofNRT2.1, 2.4and2.5. 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 to propose a regulatory model for high-affinity NO3-uptake inArabidopsis, highlighting several NO3-transduction cascades downstream the phosphorylated form of the NRT1.1 transceptor.One sentence summaryCharacterisation and identification of molecular elements involved in the signaling pathways repressing NRT2s transporters and root nitrate uptake in response to nitrate.