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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]

SUMMARY: Soil water uptake by roots is a key component of plant water homeostasis contributing to plant growth and survival under ever‐changing environmental conditions. The water transport capacity of roots (root hydraulic conductivity; Lpr) is mostly contributed by finely regulated Plasma membrane Intrinsic Protein (PIP) aquaporins. In this study, we used natural variation of Arabidopsis for the identification of quantitative trait loci (QTLs) contributing to Lpr. Using recombinant lines from a biparental cross (Cvi‐0 x Col‐0), we show that the gene encoding class 2 Sucrose‐Non‐Fermenting Protein kinase 2.4 (SnRK2.4) in Col‐0 contributes to >30% of Lpr by enhancing aquaporin‐dependent water transport. At variance with the inactive and possibly unstable Cvi‐0 SnRK2.4 form, the Col‐0 form interacts with and phosphorylates the prototypal PIP2;1 aquaporin at Ser121 and stimulates its water transport activity upon coexpression in Xenopus oocytes and yeast cells. Activation of PIP2;1 by Col‐0 SnRK2.4 in yeast also requires its protein kinase activity and can be counteracted by clade A Protein Phosphatases 2C. SnRK2.4 shows all hallmarks to be part of core abscisic acid (ABA) signaling modules. Yet, long‐term (>3 h) inhibition of Lpr by ABA possibly involves a SnRK2.4‐independent inhibition of PIP2;1. SnRK2.4 also promotes stomatal aperture and ABA‐induced inhibition of primary root growth. The study identifies a key component of Lpr and sheds new light on the functional overlap and specificity of SnRK2.4 with respect to other ABA‐dependent or independent SnRK2s. [ABSTRACT FROM AUTHOR]

Author(s): Liliana Mancio-Silva [1]; Ksenija Slavic [1]; Margarida T. Grilo Ruivo [1]; Ana Rita Grosso [1]; Katarzyna K. Modrzynska [2]; Iset Medina Vera [1]; Joana Sales-Dias [1]; Ana Rita Gomes [...]

Stress impacts negatively on plant growth and crop productivity, caicultural production worldwide. Throughout their life, plants are often confronted with multiple types of stress that affect overall cellular energy status and activate energy-saving responses. The resulting low energy syndrome (LES) includes transcriptional, translational, and metabolic reprogramming and is essential for stress adaptation. The conserved kinases sucrose-non-fermenting-1-related protein kinase-1 (SnRK1) and target of rapamycin (TOR) play central roles in the regulation of LES in response to stress conditions, affecting cellular processes and leading to growth arrest and metabolic reprogramming. We review the current understanding of how TOR and SnRK1 are involved in regulating the response of plants to low energy conditions. The central role in the regulation of cellular processes, the reprogramming of metabolism, and the phenotypic consequences of these two kinases will be discussed in light of current knowledge and potential future developments.