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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

AmSUT1, a Sucrose Transporter in Collection and Transport Phloem of the Putative Symplastic Phloem Loader Alonsoa meridionalis¹

Albrecht-von-Haller-Institut für Pflanzenwissenschaften, Abteilung Biochemie der Pflanze, Universität Göttingen, Justus-von-Liebig-Weg 11, 37077 Göttingen, Germany (C.K., G.L.); and Universität Erlangen-Nürnberg, Lehrstuhl Botanik II-Molekulare Pflanzenphysiologie, Staudtstrasse 5, 91058 Erlangen, Germany (R.S., N.S.)

A sucrose (Suc) transporter cDNA has been cloned from Alonsoa meridionalis, a member of the Scrophulariaceae. This plant species has an open minor vein configuration and translocates mainly raffinose and stachyose in addition to Suc in the phloem (C. Knop, O. Voitsekhovskaja, G. Lohaus [2001] Planta 213: 80–91). These are typical properties of symplastic phloem loaders. For functional characterization, AmSUT1 cDNA was expressed in bakers' yeast (Saccharomyces cerevisiae). Substrate and inhibitor specificities, energy dependence, and K_m value of the protein agree well with the properties measured for other Suc transporters of apoplastic phloem loaders. A polyclonal antiserum against the 17 N-terminal amino acids of the A. meridionalis Suc transporter AmSUT1 was used to determine the cellular localization of the AmSUT1 protein. Using fluorescence labeling on sections from A. meridionalis leaves and stems, AmSUT1 was localized exclusively in phloem cells. Further histological characterization identified these cells as companion cells and sieve elements. p-Chloromercuribenzenesulfonic acid affected the sugar exudation of cut leaves in such a way that the exudation rates of Suc and hexoses decreased, whereas those of raffinose and stachyose increased. The data presented indicate that phloem loading of Suc and retrieval of Suc in A. meridionalis are at least partly mediated by the activity of AmSUT1 in addition to symplastic phloem loading.

¹ This work was supported by the Deutsche Forschungsgemeinschaft.

² Present address: Roche Diagnostics GmbH, Nonnenwald 2, 82372 Penzberg, Germany.

^* Corresponding author; e-mail glohaus{at}gwdg.de; fax 49–551–395749.

Received June 27, 2003; returned for revision September 10, 2003; accepted October 5, 2003.

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