First published online January 21, 2005; 10.1104/pp.104.051268
Plant Physiology 137:671-680 (2005)
© 2005 American Society of Plant Biologists
ENVIRONMENTAL STRESS AND ADAPTATION
Tonoplast Intrinsic Proteins AtTIP2;1 and AtTIP2;3 Facilitate NH3 Transport into the Vacuole1
Dominique Loqué,
Uwe Ludewig,
Lixing Yuan and
Nicolaus von Wirén*
Institut für Pflanzenernährung, Universität Hohenheim, D–70593 Stuttgart, Germany (D.L., L.Y., N.v.W.); and Zentrum für Molekularbiologie der Pflanzen, Pflanzenphysiologie, Universität Tübingen, D–72076 Tuebingen, Germany (U.L.)
While membrane transporters mediating ammonium uptake across the plasma membrane have been well described at the molecular level, little is known about compartmentation and cellular export of ammonium. (The term ammonium is used to denote both NH3 and NH4+ and chemical symbols are used when specificity is required.) We therefore developed a yeast (Saccharomyces cerevisiae) complementation approach and isolated two Arabidopsis (Arabidopsis thaliana) genes that conferred tolerance to the toxic ammonium analog methylammonium in yeast. Both genes, AtTIP2;1 and AtTIP2;3, encode aquaporins of the tonoplast intrinsic protein subfamily and transported methylammonium or ammonium in yeast preferentially at high medium pH. AtTIP2;1 expression in Xenopus oocytes increased 14C-methylammonium accumulation with increasing pH. AtTIP2;1- and AtTIP2;3-mediated methylammonium detoxification in yeast depended on a functional vacuole, which was in agreement with the subcellular localization of green fluorescent protein-fusion proteins on the tonoplast in planta. Transcript levels of both AtTIPs were influenced by nitrogen supply but did not follow those of the nitrogen-derepressed ammonium transporter gene AtAMT1;1. Transgenic Arabidopsis plants overexpressing AtTIP2;1 did not show altered ammonium accumulation in roots after ammonium supply, although AtTIP2;1 mRNA levels in wild-type plants were up-regulated under these conditions. This study shows that AtTIP2;1 and AtTIP2;3 can mediate the extracytosolic transport of methyl-NH2 and NH3 across the tonoplast membrane and may thus participate in vacuolar ammonium compartmentation.
1 This work was supported by the Deutsche Forschungsgemeinschaft, Bonn (grant no. WI1728/4–2 to N.v.W.).
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.051268.
* Corresponding author; e-mail vonwiren{at}uni-hohenheim.de; fax (+49)–7114593295.
Received August 5, 2004;
returned for revision November 12, 2004;
accepted November 12, 2004.
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