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JXB Advance Access published online on September 10, 2004
Journal of Experimental Botany, doi:10.1093/jxb/erh233
© 2004 by Oxford University Press
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Received February 10, 2004
Accepted June 17, 2004

RESEARCH PAPER

Root phloem-specific expression of the plasma membrane amino acid proton co-transporter AAP3

Sakiko Okumoto 1, Wolfgang Koch 2, Mechthild Tegeder 3, Wolf N. Fischer 4, Alexander Biehl 5, Dario Leister 5, York Dieter Stierhof 2, and Wolf B. Frommer 1*

1 Plant Physiology, Zentrum für Molekularbiologie der Pflanzen (ZMBP), Auf der Morgenstelle 1, D-72076 Tübingen, Germany; Carnegie Institution of Washington, 260 Panama St., Stanford, CA 94305, USA
2 Plant Physiology, Zentrum für Molekularbiologie der Pflanzen (ZMBP), Auf der Morgenstelle 1, D-72076 Tübingen, Germany
3 School of Biological Sciences, Washington State University, Pullman, WA 99164-4236, USA
4 Plant Physiology, Zentrum für Molekularbiologie der Pflanzen (ZMBP), Auf der Morgenstelle 1, D-72076 Tübingen, Germany; Present address: Xenoport Inc., 3410 Central Expressway Santa Clara, CA 95051, USA.
5 Max-Planck-Institut für Züchtungsforschung, Carl von Linné Weg 10, D-50829 Köln, Germany

* To whom correspondence should be addressed. E-mail: wfrommer{at}stanford.edu.


  Abstract

Amino acids are regarded as the nitrogen ‘currency’ of plants. Amino acids can be taken up from the soil directly or synthesized from inorganic nitrogen, and then circulated in the plant via phloem and xylem. AtAAP3, a member of the Amino Acid Permease (AAP) family, is mainly expressed in root tissue, suggesting a potential role in the uptake and distribution of amino acids. To determine the spatial expression pattern of AAP3, promoter-reporter gene fusions were introduced into Arabidopsis. Histochemical analysis of AAP3 promoter-GUS expressing plants revealed that AAP3 is preferentially expressed in root phloem. Expression was also detected in stamens, in cotyledons, and in major veins of some mature leaves. GFP-AAP3 fusions and epitope-tagged AAP3 were used to confirm the tissue specificity and to determine the subcellular localization of AtAAP3. When overexpressed in yeast or plant protoplasts, the functional GFP-AAP3 fusion was localized in subcellular organelle-like structures, nuclear membrane, and plasma membrane. Epitope-tagged AAP3 confirmed its localization to the plasma membrane and nuclear membrane of the phloem, consistent with the promoter-GUS study. In addition, epitope-tagged AAP3 protein was localized in endodermal cells in root tips. The intracellular localization suggests trafficking or cycling of the transporter, similar to many metabolite transporters in yeast or mammals, for example, yeast amino acid permease GAP1. Despite the specific expression pattern, knock-out mutants did not show altered phenotypes under various conditions including N-starvation. Microarray analyses revealed that the expression profile of genes involved in amino acid metabolism did not change drastically, indicating potential compensation by other amino acid transporters.

Keywords: Amino acid transport; Arabidopsis thaliana; long-distance transport; phloem; plasma membrane; root.
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