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PLANT PHYSIOLOGY , Vol 103, Issue 1 73-81, Copyright © 1993 by American Society of Plant Biologists
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MOLECULAR BIOLOGY AND GENE REGULATION |
Differential Expression within the Glutamine Synthetase Gene Family of the Model Legume Medicago truncatula
A. C. Stanford, K. Larsen, D. G. Barker and J. V. Cullimore
Laboratoire de Biologie Moleculaire des Relations Plantes-Microorganismes, Institut National de la Recherche Agronomique, Centre National de la Recherche Scientifique, BP 27, 31326 Castanet-Tolosan Cedex, France
The glutamine synthetase (GS) gene family of Medicago truncatula Gaertn.
contains three genes related to cytosolic GS (MtGSa, MtGSb, and MtGSc),
although one of these (MtGSc) appears not to be expressed. Sequence
analysis suggests that the genes are more highly conserved
interspecifically rather than intraspecifically: MtGSa and MtGSb are more
similar to their homologs in Medicago sativa and Pisum sativum than to each
other. Studies in which gene-specific probes are used show that both MtGSa
and MtGSb are induced during symbiotic root nodule development, although
not coordinately. MtGSa is the most highly expressed GS gene in nodules but
is also expressed to lower extents in a variety of other organs. MtGSb
shows higher levels of expression in roots and the photosynthetic
cotyledons of seedlings than in nodules or other organs. In roots, both
genes are expressed in the absence of an exogenous nitrogen source. However
the addition of nitrate leads to a short-term, 2- to 3-fold increase in the
abundance of both mRNAs, and the addition of ammonium leads to a 2-fold
increase in MtGSb mRNA. The nitrogen supply, therefore, influences the
expression of the two genes in roots, but it is clearly not the major
effector of their expression. In the discussion section, the expression of
the GS gene family of the model legume M. truncatula is compared to those
of other leguminous plants.
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