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NRT2.1, the major high affinity nitrate transporter in roots, can be phosphorylated at five different sites within the N- and the C-terminus. Here, we characterized the functional relationship of two N-terminal phosphorylation sites, S21 and S28, in Arabidopsis. Based on a site-specific correlation network, we identified a receptor kinase (HPCAL1, AT5G49770), phosphorylating NRT2.1 at S21 and resulting in active nitrate uptake. HPCAL1 itself was regulated by phosphorylation at S839 and S870 within its kinase domain. In the active state, when S839 was dephosphorylated and S870 was phosphorylated, HPCAL1 was found to interact with the N-terminus of NRT2.1, mainly when S28 was dephosphorylated. Phosphorylation of NRT2.1 at S21 resulted in a reduced interaction of NRT2.1 with its activator NAR2.1, but nitrate transport activity remained. By contrast, phosphorylated NRT2.1 at S28 enhanced the interaction with NAR2.1, but reduced the interaction with HPCAL1. Here we identified HPCAL1 as the kinase affecting this phospho-switch through phosphorylation of NRT2.1 at S21. [ABSTRACT FROM AUTHOR]