Ammonium transport and CitAMT1 expression are regulated by N in Citrus plants

Citrus seedlings (Citrus sinensis L. Osbeck × Poncirus trifoliata Blanco) were used to describe the effects of different N treatments on the NH4 + influx mediated by high- and low-affinity transport systems (HATS and LATS, respectively) and CitAMT1 gene expression. Results show that Citrus plants favor NH4 + over NO3 − influx mediated by HATS and LATS when both N sources are present in the nutrient solution and Citrus plants display a much higher capacity to take up NH4 + than NO3 −. Furthermore, NH4 + exerts a regulatory effect on NH4 + HATS activity and CitAMT1 expression, both are down-regulated by high N status of the plant, but specifically stimulated by NH4 + and the balance between these two opposite effects depends on the prior nutrition regime of the plant. On the other hand, supply of NO3 − inhibitsCitAMT1 expression but doesn’t affect NH4 + HATS activity on the roots. To explain this discrepancy, it is possible that other CitAMT1 transporters, up-regulated by N limitation, but not repressed by NO3 − could be involved in the stimulation of NH4 + HATS activity under pure NO3 − nutrition or CitAMT1 transporter could be regulated at the post-transcriptional leve

NRT2.1 phosphorylation prevents root high affinity nitrate uptake activity in Arabidopsis thaliana

In Arabidopsis thaliana, NRT2.1 codes for a main component of the root nitrate high-affinity transport system. Previous studies revealed that post-translational regulation of NRT2.1 plays an important role in the control of root nitrate uptake and that one mechanism could correspond to NRT2.1 C-terminus processing. To further investigate this hypothesis, we produced transgenic plants with truncated forms of NRT2.1. It revealed an essential sequence for NRT2.1 activity, located between the residues 494-513. Using a phospho-proteomic approach, we found that this sequence contains one phosphorylation site, at serine 501, which can inactivate NRT2.1 function when mimicking the constitutive phosphorylation of this residue in transgenic plants. This phenotype could neither be explained by changes in abundance of NRT2.1 and NAR2.1, a partner protein of NRT2.1, nor by a lack of interaction between these two proteins. Finally, the relative level of serine 501 phosphorylation was found to be modulated by nitrate in wildtype plants. Altogether, these observations allowed us to propose a model for a new and essential mechanism for the regulation of NRT2.1 activity

Modulation of plant root growth by nitrogen source-defined regulation of polar auxin transport

Availability of the essential macronutrient nitrogen in soil plays a critical role in plant growth, development, and impacts agricultural productivity. Plants have evolved different strategies for sensing and responding to heterogeneous nitrogen distribution. Modulation of root system architecture, including primary root growth and branching, is among the most essential plant adaptions to ensure adequate nitrogen acquisition. However, the immediate molecular pathways coordinating the adjustment of root growth in response to distinct nitrogen sources, such as nitrate or ammonium, are poorly understood. Here, we show that growth as manifested by cell division and elongation is synchronized by coordinated auxin flux between two adjacent outer tissue layers of the root. This coordination is achieved by nitrate‐dependent dephosphorylation of the PIN2 auxin efflux carrier at a previously uncharacterized phosphorylation site, leading to subsequent PIN2 lateralization and thereby regulating auxin flow between adjacent tissues. A dynamic computer model based on our experimental data successfully recapitulates experimental observations. Our study provides mechanistic insights broadening our understanding of root growth mechanisms in dynamic environments

Nitrate regulation of root development : a signalling role for nitrate transporters ?

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Nitrate signaling mechanisms in Arabidopsis roots

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Nitrate transceptor(s) in plants

International audienceNitrogen (N) availability in soils is highly variable in both time and space. To cope with this constraint, plants develop a wide range of adaptive responses, which modulate their N use efficiency according to external N availability and internal N status. Many of these responses are triggered by the action of nitrate as a signal molecule. Plants are able to sense nitrate in the environment, and increasing evidence indicate that this is at least in part due to plasma membrane nitrate transceptors which fulfil a dual transport/sensing function. The best known example is the nitrate transporter NRT1.1(CHL1) of Arabidopsis thaliana. NRT1.1 has been shown to govern several responses of the plant to nitrate, including regulation of other nitrate transporters and of enzymes of nitrate metabolism, relief of seed dormancy, and changes in root system architecture. Nitrate signalling mediated by NRT1.1 is independent of its nitrate transport activity, and the two functions can be uncoupled genetically. The mechanisms of nitrate sensing by NRT1.1 are still not clearly understood. However, it has been proposed that NRT1.1 is responsible for the nitrate-induced changes in lateral root growth because it is able to facilitate transport of auxin in addition to nitrate. As a consequence, NRT1.1 modulates auxin accumulation in lateral roots as a function of nitrate availability, thereby coupling nitrate sensing with the hormonal control of organ development. Besides NRT1.1, other nitrate/nitrogen transporters with a signalling role probably exist. Thus, transceptor-mediated nutrient signalling may be of more general occurrence in plants

N sensing and signalling mechanisms triggering adaptive responses to N limitation in Arabidopsis

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