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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]

For microorganisms and plants, nitrate and ammonium are the main nitrogen sources and they are also important signaling molecules controlling several aspects of metabolism and development. Over the past decade, numerous studies revealed that nitrogen transporters are strongly regulated at the transcriptional level. However, more and more reports are now showing that nitrate and ammonium transporters are also subjected to post-translational regulations in response to nitrogen availability. Phosphorylation is so far the most well studied post-translational modification for these transporters and it affects both the regulation of nitrogen uptake and nitrogen sensing. For example, in Arabidopsis thaliana, phosphorylation was shown to activate the sensing function of the root nitrate transporter NRT1.1 and to switch the transport affinity. Also, for ammonium transporters, a phosphorylation-dependent activation/inactivation mechanism was elucidated in recent years in both plants and microorganisms. However, despite the fact that these regulatory mechanisms are starting to be thoroughly described, the signaling pathways involved and their action on nitrogen transporters remain largely unknown. In this review, we highlight the inorganic nitrogen transporters regulated at the post-translational level and we compare the known mechanisms in plants and microorganisms. We then discuss how these mechanisms could contribute to the regulation of nitrogen uptake and/or nitrogen sensing. [ABSTRACT FROM AUTHOR]