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JXB Advance Access originally published online on May 28, 2008
Journal of Experimental Botany 2008 59(9):2325-2335; doi:10.1093/jxb/ern102
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© 2008 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)

RESEARCH PAPER

Concerted modulation of alanine and glutamate metabolism in young Medicago truncatula seedlings under hypoxic stress

Anis M. Limami1,*, Gaëlle Glévarec1 {dagger}, Claudie Ricoult1, Jean-Bernard Cliquet2 and Elisabeth Planchet1

1University of Angers, UMR 1191, Physiologie Moléculaire des Semences, 2 Bd Lavoisier, F-49045, Angers, France
2University of Caen, UMR 950, Physiologie et Biochimie Végétales, F-14032 Caen, France

* To whom correspondence should be addressed. E-mail: anis.limami{at}univ-angers.fr

The modulation of primary nitrogen metabolism by hypoxic stress was studied in young Medicago truncatula seedlings. Hypoxic seedlings were characterized by the up-regulation of glutamate dehydrogenase 1 (GDH1) and mitochondrial alanine aminotransferase (mAlaAT), and down-regulation of glutamine synthetase 1b (GS1b), NADH-glutamate synthase (NADH-GOGAT), glutamate dehydrogenase 3 (GDH3), and isocitrate dehydrogenase (ICDH) gene expression. Hypoxic stress severely inhibited GS activity and stimulated NADH-GOGAT activity. GDH activity was lower in hypoxic seedlings than in the control, however, under either normoxia or hypoxia, the in vivo activity was directed towards glutamate deamination. 15NH4 labelling showed for the first time that the adaptive reaction of the plant to hypoxia consisted of a concerted modulation of nitrogen flux through the pathways of both alanine and glutamate synthesis. In hypoxic seedlings, newly synthesized 15N-alanine increased and accumulated as the major amino acid, asparagine synthesis was inhibited, while 15N-glutamate was synthesized at a similar rate to that in the control. A discrepancy between the up-regulation of GDH1 expression and the down-regulation of GDH activity by hypoxic stress highlighted for the first time the complex regulation of this enzyme by hypoxia. Higher rates of glycolysis and ethanol fermentation are known to cause the fast depletion of sugar stores and carbon stress. It is proposed that the expression of GDH1 was stimulated by hypoxia-induced carbon stress, while the enzyme protein might be involved during post-hypoxic stress contributing to the regeneration of 2-oxoglutarate via the GDH shunt.

Key words: Alanine, alanine aminotransferase, ammonia, glutamate, glutamate dehydrogenase, glutamate synthase, hyperammonia, hypoxia, nitrogen

{dagger} Present address: E.A. 216, laboratoire Biomolécules et Biotechnologies Végétales (BBV), University of Tours, 31 Avenue Monge, F-37200 Tours, France

Received 7 February 2008; Revised 11 March 2008 Accepted 11 March 2008


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