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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Gene Expression of the NO₃^– Transporter NRT1.1 and the Nitrate Reductase NIA1 Is Repressed in Arabidopsis Roots by NO₂^–, the Product of NO₃^– Reduction

Biochimie et Physiologie Moléculaire des Plantes, Unité Mixte de Recherche 5004 Institut National de la Recherche Agronomique/Centre National de la Recherche Scientifique/AgroM/UM2, 2 Place Viala, 34060 Montpellier cedex 1, France

NRT1.1 and NIA1 genes, which encode a nitrate (NO₃^–) transporter and the minor isoform of NO₃^– reductase (NR), respectively, are overexpressed in roots of NR-deficient mutants of Arabidopsis grown on nutrient solution containing NO₃^– and reduced N. The overexpression is found only in mutants with reduced NIA2 activity, and disruption of the NIA1 gene alone has no effect on NRT1.1 expression. Because the up-regulation of NRT1.1 and NIA1 is observed in N-sufficient NR mutant plants, it cannot be related to a release of the general feedback repression exerted by the N status of the plant. Our data do not support the hypothesis of overinduction of these genes by an increased concentration of NO₃^– in tissues. Furthermore, although a control by external pH might contribute to the regulation of NRT1.1, changes in external pH due to lack of NR activity cannot alone explain the up-regulation of both genes. The stimulation of NRT1.1 and NIA1 in NR mutants in these conditions suggests that NR activity is able to repress directly the expression of both genes independently of the availability of reduced N metabolites in wild-type plants. Accordingly, nitrite (NO₂^–) strongly represses NRT1.1 and NIA1 transcript accumulation in the roots. This effect is rapid, specific, and reversible. Furthermore, transport studies on plants exposed to NO₂^– show that down-regulation of the NRT1.1 gene is associated with a decrease in NO₃^– influx. These results indicate that feedback regulation of genes of NO₃^– assimilation relies not only on the repression exerted by reduced N metabolites, such as NH₄⁺ or amino acids, but may also involve the action of NO₂^– as a regulatory signal.

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

Received December 2, 2002; returned for revision January 7, 2003; accepted January 20, 2003.

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