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JXB Advance Access published online on January 16, 2006
Journal of Experimental Botany, doi:10.1093/jxb/erj054
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Published by Oxford University Press [2006] on behalf of the Society for Experimental Biology.
Received September 26, 2005
Accepted October 11, 2005

RESEARCH PAPER

Physiological and molecular aspects of aspartate-derived amino acid metabolism during germination and post-germination growth in two maize genotypes differing in germination efficiency

Fabiola Anzala 1, Marie-Christine Morère-Le Paven 1, Sylvie Fournier 2, David Rondeau 2, 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 Services Commun Analyses Spectroscopiques, SCAS, University of Angers, 2 Bd Lavoisier, F-49045 Angers cedex 01, France

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


  Abstract

The Asp-derived amino acid pathway has been studied during the early stages of development in two maize genotypes, Io and F2, differing in germination efficiency and post-germination growth. In both genotypes expression of Ask2 (monofunctional Asp-kinase-2), Akh1 and Akh2 (bifunctional Asp-kinase-homo-Ser dehydrogenase-1 and 2), increased throughout germination and post-germination growth, suggesting a developmental regulation, whereas Ask1 (monofunctional Asp-kinase-1) was expressed constitutively. The major difference between Io and F2 concerned genes encoding bifunctional enzymes, particularly Akh2, the expression of which was dramatically low in F2. 15N-Asp labelling showed differences in in vivo Asp-kinase activities between the genotypes studied. Asp flux through the Met/Thr branches was higher in Io than in F2, while the latter exhibited a higher flux of Asp through the Lys branch. Physiological results, together with the higher Akh2 expression in Io, suggest that bifunctional enzyme activity, favourable to Met/Thr, was higher in Io than in F2 and that the monofunctional pathway was boosted in F2 because of the lower competition by the bifunctional pathway, thus allowing for higher flux of Asp through the Lys branch. In conclusion, it is suggested that F2 germination and post-germination growth might have been partially inhibited due to a limitation in Met and Thr availability. A negative physiological effect related to Lys accumulation in F2 is also discussed.

Keywords: AA metabolism; Akh; Ask; aspartate kinase; Asp-family pathway; germination; lysine; methionine; T50; threonine.
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