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ENVIRONMENTAL STRESS AND ADAPTATION

Iron-Regulated Expression of a Cytosolic Ascorbate Peroxidase Encoded by the APX1 Gene in Arabidopsis Seedlings

Biochimie et Physiologie Moléculaire des Plantes, Centre National de la Recherche Scientifique (Unité Mixte de Recherche 5004), Université Montpellier 2, Institut National de la Recherche Agronomique et Ecole Nationale Supérieure Agronomique, Montpellier, France (P.F., G.V., J.-F.B.); and Department of Plant Systems Biology, Flanders Interuniversity Institute for Biotechnology, Ghent University, K.L. Ledeganckstraat 35, B–9000 Ghent, Belgium (S.K., D.I.)

Iron (Fe) is an essential element for living organisms. However, under aerobic conditions, its use is complicated because of its high insolubility and its potential toxicity through reactivity with reduced forms of oxygen. In plants, Fe overload can lead to intracellular concentrations beyond the storage and detoxification capacities of cells. Such a displacement toward a pro-oxidant state can activate antioxidant defenses, including Fe-mediated expression of ascorbate peroxidase genes. In this work, we demonstrate that Fe overload specifically induces the AtAPX1 gene encoding a cytosolic ascorbate peroxidase in Arabidopsis leaves. The strong constitutive expression of the AtAPX1 gene in roots is unaffected by Fe and depends on the first 5'-untranslated region intron. Presence of an AtAPX1 expressed sequence tag in the Arabidopsis database, longer in its 5' region than what could be predicted from the published AtAPX1transcription initiation site, leads to define a new transcription initiation region for this gene. A minimal promoter sequence enabling Fe-induced expression of the AtAPX1 gene is defined by following expression of various AtAPX1::-glucuronidase constructs in transformed Arabidopsis plantlets. This 118-bp minimal promoter sequence contains an Fe-dependent regulatory sequence-like cis-element known to be necessary for maize (Zea mays) and Arabidopsis ferritin gene derepression in response to Fe overload. Site-directed mutagenesis of this element within the AtAPX1 promoter sequence does not abolish the Fe-dependent activation of a reporter gene, indicating that it is likely not involved in the Fe-regulated expression of the AtAPX1 gene.

^* Corresponding author; e-mail fourcroy{at}ensam.inra.fr; fax 33–0–467–525–737.

Received July 11, 2003; returned for revision August 13, 2003; accepted November 1, 2003.

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