JXB Advance Access originally published online on March 8, 2007
Journal of Experimental Botany 2007 58(9):2319-2327; doi:10.1093/jxb/erm016
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RESEARCH PAPER |
Assimilation of ammonium ions and reutilization of nitrogen in rice (Oryza sativa L.)*
Graduate School of Agricultural Science, Tohoku University, 1-1 Tsutsumidori-Amamiyamachi, Sendai 981-8555, Japan
To whom correspondence should be addressed. E-mail: tyamaya{at}biochem.tohoku.ac.jp
A major source of inorganic nitrogen for rice plants grown in paddy soil is ammonium ions. The ammonium ions are actively taken up by the roots via ammonium transporters and subsequently assimilated into the amide residue of glutamine (Gln) by the reaction of glutamine synthetase (GS) in the roots. The Gln is converted into glutamate (Glu), which is a central amino acid for the synthesis of a number of amino acids, by the reaction of glutamate synthase (GOGAT). Although a small gene family for both GS and GOGAT is present in rice, ammonium-dependent and cell type-specific expression suggest that cytosolic GS1;2 and plastidic NADH-GOGAT1 are responsible for the primary assimilation of ammonium ions in the roots. In the plant top, approximately 80% of the total nitrogen in the panicle is remobilized through the phloem from senescing organs. Since the major form of nitrogen in the phloem sap is Gln, GS in the senescing organs and GOGAT in developing organs are important for nitrogen remobilization and reutilization, respectively. Recent work with a knock-out mutant of rice clearly showed that GS1;1 is responsible for this process. Overexpression studies together with age- and cell type-specific expression strongly suggest that NADH-GOGAT1 is important for the reutilization of transported Gln in developing organs. The overall process of nitrogen utilization within the plant is discussed.
Key words: Ammonia assimilation, cytosolic glutamine synthetase, NADH-glutamate synthase, nitrogen remobilization, rice
* This paper is dedicated to Dr Ann Oaks, Professor Emeritus of McMaster University, Canada, who passed away on 13 January 2006.
These two authors contributed equally to this work.
Received 17 October 2006; Revised 15 January 2007 Accepted 18 January 2007