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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

A New Substrate Cycle in Plants. Evidence for a High Glucose-Phosphate-to-Glucose Turnover from in Vivo Steady-State and Pulse-Labeling Experiments with [¹³C]Glucose and [¹⁴C]Glucose¹

Institut National de la Recherche Agronomique, Université Bordeaux 1, Université Victor Segalen Bordeaux 2, Unité Mixte de Recherche 619 Physiologie et Biotechnologie Végétales, 33883 Villenave d'Ornon cedex, France

Substrate (futile) cycling involving carbohydrate turnover has been widely reported in plant tissues, although its extent, mechanisms, and functions are not well known. In this study, two complementary approaches, short and steady-state labeling experiments, were used to analyze glucose metabolism in maize (Zea mays) root tips. Unidirectional rates of synthesis for storage compounds (starch, Suc, and cell wall polysaccharides) were determined by short labeling experiments using [U-¹⁴C]glucose and compared with net synthesis fluxes to determine the rate of glucose production from these storage compounds. Steady-state labeling with [1-¹³C]glucose and [U-¹³C]glucose showed that the redistribution of label between carbon C-1 and C-6 in glucose is close to that in cytosolic hexose-P. These results indicate a high resynthesis flux of glucose from hexose-P that is not accounted for by glucose recycling from storage compounds, thus suggesting the occurrence of a direct glucose-P-to-glucose conversion. An enzyme assay confirmed the presence of substantial glucose-6-phosphatase activity in maize root tips. This new glucose-P-to-glucose cycle was shown to consume around 40% of the ATP generated in the cell, whereas Suc cycling consumes at most 3% to 6% of the ATP produced. The rate of glucose-P cycling differs by a factor of 3 between a maize W22 line and the hybrid maize cv Dea, and is significantly decreased by a carbohydrate starvation pretreatment.

² Present address: Department of Plant Biology, Michigan State University, 166 Plant Biology Building, East Lansing, MI 48824.

³ Present address: Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, 14476 Golm, Germany.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.062083.

^* Corresponding author; e-mail dieuaide{at}bordeaux.inra.fr; fax 33–5–57–12–25–41.

Received March 1, 2005; returned for revision April 29, 2005; accepted May 1, 2005.

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