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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Regulation of the High-Affinity NO₃^– Uptake System by NRT1.1-Mediated NO₃^– Demand Signaling in Arabidopsis^[W]

Biochimie et Physiologie Moléculaire des Plantes, Unité Mixte de Recherche 5004, Agro-M, Centre National de la Recherche Scientifique, Institut National de la Recherche Agronomique, Université Montpellier 2, 34060 Montpellier cedex 1, France

The NRT2.1 gene of Arabidopsis thaliana encodes a major component of the root high-affinity transport system (HATS) that plays a crucial role in uptake by the plant. Although NRT2.1 was known to be induced by and feedback repressed by reduced nitrogen (N) metabolites, NRT2.1 is surprisingly up-regulated when concentration decreases to a low level (<0.5 mM) in media containing a high concentration of or Gln (1 mM). The NRT3.1 gene, encoding another key component of the HATS, displays the same response pattern. This revealed that both NRT2.1 and NRT3.1 are coordinately down-regulated by high external availability through a mechanism independent from that involving N metabolites. We show here that repression of both genes by high is specifically mediated by the NRT1.1 transporter. This mechanism warrants that either NRT1.1 or NRT2.1 is active in taking up in the presence of a reduced N source. Under low provision, NRT1.1-mediated repression of NRT2.1/NRT3.1 is relieved, which allows reactivation of the HATS. Analysis of atnrt2.1 mutants showed that this constitutes a crucial adaptive response against toxicity because taken up by the HATS in this situation prevents the detrimental effects of pure nutrition. It is thus hypothesized that NRT1.1-mediated regulation of NRT2.1/NRT3.1 is a mechanism aiming to satisfy a specific demand of the plant in relation to the various specific roles that plays, in addition to being a N source. A new model is proposed for regulation of the HATS, involving both feedback repression by N metabolites and NRT1.1-mediated repression by high .

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.087510

^* Corresponding author; e-mail gojon{at}ensam.inra.fr; fax 33–4–67–52–57–37.

Received July 27, 2006; accepted September 8, 2006; published September 22, 2006.

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