Plant Physiology Preview
Published on November 17, 2006; 10.1104/pp.106.091330
OPEN ACCESS ARTICLE
Received October 15, 2006
Accepted November 3, 2006
Tobacco isoenzyme 1 of NAD(H)-dependent Glutamate Dehydrogenase Catabolizes Glutamate in vivo
Matthew Peter Purnell and José Ramon Botella *
Department of Botany, School of Integrative Biology, The University of Queensland, QLD, 4072, Australia
* Corresponding author; email: J.Botella{at}uq.edu.au.
Glutamate dehydrogenase (GDH, EC 1.4.1.2-1.4.1.4) catalyses in vitro the reversible amination of 2-oxoglutarate to Glu. The in vivo direction(s) of the GDH reaction in higher plants and hence the role(s) of this enzyme is unclear, a situation confounded by the existence of isoenzymes comprised totally of either GDH  - (isoenzyme 1) or  - (isoenzyme 7) subunits, as well as another five - isoenzyme permutations. To clarify the in vivo direction of the reaction catalyzed by GDH isoenzyme 1, [15N]Glu was supplied to roots of two independent transgenic tobacco (Nicotiana tabacum L.) lines with increased isoenzyme 1 levels (S4-H and S49-H). The [15N]ammonium (NH4+) accumulation rate in these lines was elevated  65% compared with a null segregant control line, indicating that isoenzyme 1 catabolizes Glu in roots. Leaf glutamine synthetase (GS) was inhibited with a GS-specific herbicide to quantify any contribution by GDH towards photorespiratory NH4+ reassimilation. Transgenic line S49-H did not show enhanced resistance to the herbicide, indicating that the large pool of isoenzyme 1 in S49-H leaves was unable to compensate for GS, and suggesting that isoenzyme 1 does not assimilate NH4+ in vivo.
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