JXB Advance Access originally published online on May 28, 2003
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Journal of Experimental Botany, Vol. 54, No. 388, pp. 1691-1700,
July 1, 2003
© 2003 Oxford University Press
Sugar uptake and proton release by protoplasts from the infected zone of Vicia faba L. nodules: evidence against apoplastic sugar supply of infected cells
Received 21 January 2003; Accepted 10 April 2003
Institute of Plant Nutrition, Interdisciplinary Research Center (IFZ), Justus Liebig University, Heinrich-Buff-Ring 26-32, 35392 Giessen, Germany
* Present address and to whom correspondence should be sent: Biology Department, Area 9, University of York, PO Box 373, York YO10 5YW, UK. Fax: +44 (0)1904 32 8666. E-mail: ep5{at}york.ac.uk
Symbiotic dinitrogen fixation of legume nodules is fuelled by phloem-imported carbohydrates. These have to pass several cell layers to reach cells infected with Rhizobium bacteroids. It is unclear whether apoplastic steps are involved in carbohyd rate translocation within the nodule. Protoplasts were isolated from the infected and uninfected cells of the central tissue of Vicia faba nodules using a recently developed protocol. These protoplasts were used to elucidate pathways for sugar transport in this tissue. Both types of protoplasts released protons into the medium. Acidification was inhibited by vanadate and erythrosin B. However, it was stimulated by fusicoccin only in uninfected cells. A symport of sugars with protons can therefore be energized in both cell types. Uptake of 14C-labelled sugars was determined using a phthalate centrifugation technique. Uninfected protoplasts accumulated glucose through high-affinity H+/glucose-symport that was not competitively inhibited by fructose or sucrose. Uninfected protoplasts also absorbed sucrose with biphasic kinetics. At 0.1, 1, and 10 mM sucrose, uptake was inhibited by CCCP. Fusicoccin did not stimulate the linear phase of sucrose uptake. Glucose inhibited sucrose uptake nearly completely. This was not related to sucrose cleavage in the medium because sucrose was absorbed at a much higher rate than glucose, and glucose concentration did not increase in sucrose-containing protoplast suspensions. By contrast with uninfected protoplasts, infected cells did not show transporter-mediated glucose or sucrose uptake. The findings underline a role of uninfected cells in sugar translocation. Infected cells are not apoplastically supplied with sugars and possibly depend on uninfected cells for carbon supply.
Key words: Faba bean, glucose uptake, membrane transport, metabolite transport, plasma membrane, proton release, root nodule, sucrose uptake.
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