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BIOENERGETICS AND PHOTOSYNTHESIS

Glutamine Synthetase-Glutamate Synthase Pathway and Glutamate Dehydrogenase Play Distinct Roles in the Sink-Source Nitrogen Cycle in Tobacco¹

Unité de Nutrition Azotée des Plantes (C.M.-D., M.R.-C., K.P., M.L., M.-H.V., M.F., T.J., A.S.) and Laboratoire Commun de Cytologie (O.G., J.K.), Institut National de la Recherche Agronomique, 78026 Versailles cedex, France

Glutamate (Glu) metabolism and amino acid translocation were investigated in the young and old leaves of tobacco (Nicotiana tabacum L. cv Xanthi) using [¹⁵N]ammonium and [2-¹⁵N]Glu tracers. Regardless of leaf age, [¹⁵N]ammonium assimilation occurred via glutamine synthetase (GS; EC 6.1.1.3) and Glu synthase (ferredoxin [Fd]-GOGAT; EC 1.4.7.1; NADH-GOGAT; EC 1.4.1.14), both in the light and darkness, and it did not depend on Glu dehydrogenase (GDH; EC 1.4.1.2). The [¹⁵N]ammonium and ammonium accumulation patterns support the role of GDH in the deamination of [2-¹⁵N]Glu to provide 2-oxoglutarate and [¹⁵N]ammonium. In the dark, excess [¹⁵N]ammonium was incorporated into asparagine that served as an additional detoxification molecule. The constant Glu levels in the phloem sap suggested that Glu was continuously synthesized and supplied into the phloem regardless of leaf age. Further study using transgenic tobacco lines, harboring the promoter of the GLU1 gene (encoding Arabidopsis [Arabidopsis thaliana] Fd-GOGAT) fused to a GUS reporter gene, revealed that the expression of Fd-GOGAT remained higher in young leaves compared to old leaves, and higher in the veins compared to the mesophyll. Confocal laser-scanning microscopy localized the Fd-GOGAT protein to the phloem companion cells-sieve element complex in the leaf veins. The results are consistent with a role of Fd-GOGAT in supplying Glu for the synthesis and transport of amino acids. Taken together, the data provide evidence that the GS-GOGAT pathway and GDH play distinct roles in the source-sink nitrogen cycle of tobacco leaves.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Akira Suzuki (suzuki{at}versailles.inra.fr).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.071910.

^* Corresponding author; e-mail suzuki{at}versailles.inra.fr; fax 33–1–30–83–30–96.

Received September 23, 2005; returned for revision October 31, 2005; accepted December 5, 2005.

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