Figure 2.

Transceptor mediated nitrate signaling

In low external ${\text{NO}}_3^{-}$ conditions, only one of the NRT1; 1 monomers binds ${\text{NO}}_3^{-}$ with high affinity. This evokes conformational changes in the protein that switches NRT1; 1 to its high-affinity transport mode and provides access for the CIPK23 kinase to phosphorylate NRT1; 1. When external ${\text{NO}}_3^{-}$ is raised, NRT1; 1 is dephosphorylated and both NRT1; 1 monomers bind ${\text{NO}}_3^{-}$. The ensuing conformational change causes the monomers to interact and NRT1; 1 to switch to a low affinity/high capacity transport mode. Simultaneously, a PLC mediated Ca²⁺ signal is generated that is conveyed to the nucleus via CPKs and activates transcription of PNR genes. The PNR includes downregulation of other high-affinity ${\text{NO}}_3^{-}$ uptake transporters such as NRT2; 1 and physiological responses such as upregulated nitrogen assimilation and adapted root architecture.

NRT, nitrate transporter; CIPK, CBL-interacting kinase; PLC, phospholipase; CPK, Ca²⁺-dependent protein kinase; PNR, primary nitrate response. Figure created with BioRender.com.