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Published on January 11, 2006; 10.1104/pp.105.071910

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Received September 23, 2005
Returned for revision October 12, 2005
Accepted December 5, 2005

Glutamine synthetase/glutamate synthase pathway and glutamate dehydrogenase play distinct roles for sink/source nitrogen cycle in tobacco (Nicotiana tabacum L.)

Céline Masclaux-Daubresse , Michèle Reisdorf-Cren , Karine Pageau , Maud Lelandais , Olivier Grandjean , Joceline Kronenberger , Marie-Hélène Valadier , Magali Feraud , Tiphaine Jouglet , and Akira Suzuki *

Unité de Nutrition Azotée des Plantes, Institut National de la Recherche Agronomique, Route de St-Cyr, 78026 Versailles cedex, France
Laboratoire Commun de Cytologie, Institut National de la Recherche Agronomique, Route de St-Cyr, 78026 Versailles cedex, France

* Corresponding author; email: suzuki{at}versailles.inra.fr.

Glutamate 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]glutamate tracers. Irrespective of leaf age, [15N]ammonium assimilation occurred via glutamine synthetase (GS, EC 6.1.1.3) and glutamate synthase (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 glutamate 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]glutamate 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 glutamate levels in the phloem sap suggested that glutamate was continuously synthesized and supplied into the phloem regardless of leaf age. Further study using transgenic tobacco lines, harbouring the promoter of the GLU1 gene (encoding Arabidopsis Fd-GOGAT) fused to a GUS reporter gene, revealed that the expression of Fd-GOGAT remained higher in the young leaves, compared to the old leaves, and higher in the veins compared to the mesophyll. Confocal laser-scanning microscopy localised 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 glutamate for the synthesis and transport of the 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.




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