JXB Advance Access first published online on January 13, 2007
This version published online on January 23, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erl269
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© 2007 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 |
The effect of Glc6P uptake and its subsequent oxidation within pea root plastids on nitrite reduction and glutamate synthesis
1Faculty of Life Sciences, The University of Manchester, 3.614 Stopford Building, Oxford Road, Manchester M13 9PT, UK
2Institute for Protein Research, Osaka University, Osaka 565 0871, Japan
3University of Bristol, Long Ashton Research Station, Bristol BS41 9AF, UK
4University of Groningen, Laboratory of Plant Physiology, PO Box 14, 97500 AA, Haren, The Netherlands
5College of Biological Science, University of Guelph, Guelph N1G 2W1, Canada
* To whom correspondence should be addressed. E-mail: caroline.bowsher{at}manchester.ac.uk
In roots, nitrate assimilation is dependent upon a supply of reductant that is initially generated by oxidative metabolism including the pentose phosphate pathway (OPPP). The uptake of nitrite into the plastids and its subsequent reduction by nitrite reductase (NiR) and glutamate synthase (GOGAT) are potentially important control points that may affect nitrate assimilation. To support the operation of the OPPP there is a need for glucose 6-phosphate (Glc6P) to be imported into the plastids by the glucose phosphate translocator (GPT). Competitive inhibitors of Glc6P uptake had little impact on the rate of Glc6P-dependent nitrite reduction. Nitrite uptake into plastids, using 13N labelled nitrite, was shown to be by passive diffusion. Flux through the OPPP during nitrite reduction and glutamate synthesis in purified plastids was followed by monitoring the release of 14CO2 from [1-14C]-Glc6P. The results suggest that the flux through the OPPP is maximal when NiR operates at maximal capacity and could not respond further to the increased demand for reductant caused by the concurrent operation of NiR and GOGAT. Simultaneous nitrite reduction and glutamate synthesis resulted in decreased rates of both enzymatic reactions. The enzyme activity of glucose 6-phosphate dehydrogenase (G6PDH), the enzyme supporting the first step of the OPPP, was induced by external nitrate supply. The maximum catalytic activity of G6PDH was determined to be more than sufficient to support the reductant requirements of both NiR and GOGAT. These data are discussed in terms of competition between NiR and GOGAT for the provision of reductant generated by the OPPP.
Key words: Glucose-6-phosphate dehydrogenase, glucose-6-phosphate uptake, glutamate synthase, nitrite uptake, nitrite reductase maximum catalytic activity, oxidative pentose phosphate pathway competition
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