Nitrate uptake by roots represents the critical first step of nitrogen acquisition in plants,however, its detailed regulation mechanism still remains foggy.Transporter CHL1 of plant cells plays a central role in the process of nitrate uptake, as Arabidopsis thaliana-CHL1 has been identified as a dual-affinity nitrate transporter.Subsequent in vitro and in vivo studies showed that the transition of CHL1 from low affinity state to high affinity state is controlled by the phosphorylation of its Thr101catalyzed by CBL-interacting protein kinase 23 (CIPK23).Although the structures of CIPK23 and CHL1 from Arabidopsis thaliana have been reported recently, the molecular mechanism of how CIPK23 phosphorylates Thr101 of CHL1 has not been thoroughly elucidated.In order to uncover the exact molecular mechanism of CHL1 phosphorylation by CIPK23, we are currently performing structural studies on the complex of CIPK23 and its substrate peptide derived from CHL1 (CHL1P).We have successfully expressed a CIPK23 fragment comprising residues 24-340(CIPK23△C) and solved its crystal structure.Subsequently,CIPK23ACand its substrate peptide CHL1Pwas subjected to co-crystallization trials, and complex crystals were obtained with poor diffraction quality.Further optimization of crystallization conditions is under way.We believe that our results will shed light on the mechanism of how CIPK23 regulates the affinity of CHL1 via the phosphorylation of its Thr101 and will provide structural information for the agricultural engineering aiming to improve the nitrate uptake capabilities of industrial crop plants.