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JXB Advance Access published online on August 9, 2006
Journal of Experimental Botany, doi:10.1093/jxb/erl069
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Published by Oxford University Press [2006] on behalf of the Society for Experimental Biology. 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.
Received March 20, 2006
Accepted May 30, 2006

RESEARCH PAPER

Characterization of alanine aminotransferase (AlaAT) multigene family and hypoxic response in young seedlings of the model legume Medicago truncatula

Claudie Ricoult 1, Luis Orcaray Echeverria 2, Jean-Bernard Cliquet 3, and Anis M. Limami 1 *

1 UMR INRA 1191, Physiologie Moléculaire des Semences, University of Angers, 2 Bd Lavoisier, F-49045 Angers cedex 01, France
2 UMR INRA 1191, Physiologie Moléculaire des Semences, University of Angers, 2 Bd Lavoisier, F-49045 Angers cedex 01, France; Department of Ciencias del Medio Natural, Public University of Navarra, Pamplona, Spain
3 UMR INRA 950, Physiologie et Biochimie Végétales, University of Caen, 14032 Caen Cedex, France

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


  Abstract

Four alanine aminotransferases (AlaATs) are expressed in Medicago truncatula. In adult plants, two genes encoding mitochondrial isoforms m-AlaAT and alanine-glyoxylate aminotransferase (AGT), catalysing, respectively, reversible reactions of alanine/oxoglutarate{leftrightarrow}glutamate/pyruvate and alanine/glyoxylate{leftrightarrow}glycine/pyruvate, were expressed in roots, stems, and leaves. A gene encoding a cytosolic (c-AlaAT) isoform, catalysing the same reaction as m-AlaAT, was expressed specifically in leaves, while a gene encoding an isoform involved in branched chain amino acid metabolism was expressed in stems and roots. In young seedlings, only m-AlaAT and AGT were expressed in embryo axes. In hypoxic embryo axes, the amounts of transcript and putative protein of m-AlaAT (EC 2.6.1.2) increased while those of AGT (EC 2.6.1.44) decreased and in vivo enzyme activities changed as revealed by [15N]alanine and [15N]glutamate labelling. Under hypoxia, m-AlaAT catalysed only alanine synthesis while glutamate synthesis using alanine as amino donor was inhibited. As a result, alanine accumulated as the major amino acid in hypoxic seedlings instead of asparagine, in agreement with the involvement of the fermentative AlaAT pathway in hypoxia tolerance. Regulation of m-AlaAT at both the transcriptional and post-translational levels allowed for an increase in gene expression and orientation of the activity of the product of its transcription towards alanine synthesis under hypoxia. Labelling experiments showed that glycine synthesis occurred at the expense of either alanine or glutamate as amino donor, indicating that a glutamate-glyoxylate aminotransferase was operating together with AGT in Medicago truncatula seedlings. Both enzymes seemed to be inhibited by hypoxia, resulting in a very low amount of glycine in hypoxic seedlings.

Keywords: AlaAT; alanine-glyoxylate aminotransferase; alanine-pyruvate aminotransferase; alanine; anoxia/hypoxia; germination; 15N labelling; Medicago truncatula.
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