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ENVIRONMENTAL STRESS AND ADAPTATION

Urea Transport by Nitrogen-Regulated Tonoplast Intrinsic Proteins in Arabidopsis¹

Institut für Pflanzenernährung, Universität Hohenheim, D–70593 Stuttgart, Germany (L.-H.L., B.G., N.v.W.); Zentrum für Molekularbiologie der Pflanzen (ZMBP), Pflanzenphysiologie, Universität Tübingen, Auf der Morgenstelle 1, D–72076 Tübingen, Germany (U.L., W.B.F.); Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, United Kingdom (L.-H.L.); and Carnegie Institution, 260 Panama Street, Stanford, California 94305 (W.B.F.)

Urea is the major nitrogen (N) form supplied as fertilizer in agricultural plant production and also an important N metabolite in plants. Because urea transport in plants is not well understood, the aim of the present study was to isolate urea transporter genes from the model plant Arabidopsis. Using heterologous complementation of a urea uptake-defective yeast (Saccharomyces cerevisiae) mutant allowed to isolate AtTIP1;1, AtTIP1;2, AtTIP2;1, and AtTIP4;1 from a cDNA library of Arabidopsis. These cDNAs encode channel-like tonoplast intrinsic proteins (TIPs) that belong to the superfamily of major intrinsic proteins (or aquaporins). All four genes conferred growth of a urea uptake-defective yeast mutant on 2 mM urea in a phloretin-sensitive and pH-independent manner. Uptake studies using ¹⁴C-labeled urea into AtTIP2;1-expressing Xenopus laevis oocytes demonstrated that AtTIP2;1 facilitated urea transport also in a pH-independent manner and with linear concentration dependency. Expression studies showed that AtTIP1;2, AtTIP2;1, and AtTIP4;1 genes were up-regulated during early germination and under N deficiency in roots but constitutively expressed in shoots. Subcellular localization of green fluorescent protein-fused AtTIPs indicated that AtTIP1;2, AtTIP2;1, and AtTIP4;1 were targeted mainly to the tonoplast and other endomembranes. Thus, in addition to their role as water channels, TIP transporters may play a role in equilibrating urea concentrations between different cellular compartments.

¹ This work was supported by the Deutsche Forschungsgemeinschaft (Bonn; grant no. Wi 1728/2 to N.v.W.).

^* Corresponding author; e-mail vonwiren{at}uni-hohenheim.de; fax 49–711–459–3295.

Received May 23, 2003; returned for revision June 30, 2003; accepted August 14, 2003.

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