CHL1 functions as a nitrate sensor in plants. The study shows that CHL1, a nitrate transporter, can sense changes in nitrate concentrations in the soil. When phosphorylated, CHL1 functions as a high-affinity nitrate transporter, while dephosphorylated CHL1 functions as a low-affinity transporter. The phosphorylation state of CHL1 is regulated by protein kinase CIPK23, which phosphorylates T101 of CHL1 in response to low nitrate concentrations. This phosphorylation switch allows CHL1 to sense a wide range of nitrate concentrations and generate different levels of the primary nitrate response. The study also shows that CHL1 can function as a nitrate sensor independent of its uptake activity. The results suggest that CHL1 uses dual-affinity binding and a phosphorylation switch to sense nitrate concentrations and trigger different levels of the primary nitrate response. The study provides evidence that CHL1 is a nitrate sensor responsible for the primary nitrate response and that uptake activity is not required for sensor function. The findings highlight the importance of CHL1 in nitrate sensing and signaling in plants.CHL1 functions as a nitrate sensor in plants. The study shows that CHL1, a nitrate transporter, can sense changes in nitrate concentrations in the soil. When phosphorylated, CHL1 functions as a high-affinity nitrate transporter, while dephosphorylated CHL1 functions as a low-affinity transporter. The phosphorylation state of CHL1 is regulated by protein kinase CIPK23, which phosphorylates T101 of CHL1 in response to low nitrate concentrations. This phosphorylation switch allows CHL1 to sense a wide range of nitrate concentrations and generate different levels of the primary nitrate response. The study also shows that CHL1 can function as a nitrate sensor independent of its uptake activity. The results suggest that CHL1 uses dual-affinity binding and a phosphorylation switch to sense nitrate concentrations and trigger different levels of the primary nitrate response. The study provides evidence that CHL1 is a nitrate sensor responsible for the primary nitrate response and that uptake activity is not required for sensor function. The findings highlight the importance of CHL1 in nitrate sensing and signaling in plants.