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J. Biol. Chem., Vol. 281, Issue 35, 25532-25540, September 1, 2006

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AtIREG2 Encodes a Tonoplast Transport Protein Involved in Iron-dependent Nickel Detoxification in Arabidopsis thaliana Roots^*

Gabriel Schaaf¹², Annegret Honsbein¹, Anderson R. Meda, Silvia Kirchner, Daniel Wipf, and Nicolaus von Wirén³

From the Institut für Pflanzenernährung, Universität Hohenheim, 70593 Stuttgart, Germany and the Institut für Zelluläre und Molekulare Botanik, Universität Bonn, 53115 Bonn, Germany

Iron acquisition in Arabidopsis depends mainly on AtIRT1, a Fe²⁺ transporter in the plasma membrane of root cells. However, substrate specificity of AtIRT1 is low, leading to an excess accumulation of other transition metals in iron-deficient plants. In the present study we describe AtIREG2 as a nickel transporter at the vacuolar membrane that counterbalances the low substrate specificity of AtIRT1 and possibly other iron transport systems in iron-deficient root cells. AtIREG2 is co-regulated with AtIRT1 by the transcription factor FRU/FIT1, encodes a membrane protein, which has 10 putative transmembrane domains and shares homology with vertebrate Fe²⁺ exporters. Heterologous expression of AtIREG2 in various yeast mutants, however, did not demonstrate an iron transport function. Instead, expression in wild-type and nickel-sensitive cot1 yeast cells conferred enhanced tolerance to elevated concentrations of nickel at acidic pH. A role in vacuolar substrate transport was further supported by localization of AtIREG2-GFP fusion proteins to the tonoplast in Arabidopsis suspension cells and root cells of intact plants. Transgenic plants overexpressing AtIREG2 showed an increased tolerance to elevated concentrations of nickel, whereas T-DNA insertion lines lacking AtIREG2 expression were more sensitive to nickel, particularly under iron deficiency, and accumulated less nickel in roots. We therefore propose a role of AtIREG2 in vacuolar loading of nickel under iron deficiency and thus identify it as a novel component in the iron deficiency stress response.

Received for publication, February 3, 2006 , and in revised form, June 21, 2006.

^* This work was supported by the Deutsche Forschungsgemeinschaft and Deutscher Akademischer Austauschdienst, Bonn, Germany. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1-S3.

¹ These authors contributed equally to this work.

² Present address: Dept. of Cell and Developmental Biology, School of Medicine, University of North Carolina, Chapel Hill, NC 27599.

³ To whom correspondence should be addressed: Institut für Pflanzenernährung, Universität Hohenheim, 70593 Stuttgart, Germany. Tel.: 49-711-459-2344; Fax: 49-711-459-3295; E-mail: vonwiren{at}uni-hohenheim.de.

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