Under sufficient light conditions, the assimilation of inorganic nitrogen and efficient recycling of the assimilates within plants are the most important processes for determining the productivity and quality of many crops. The major source of nitrogen for developing leaves and spikelets in rice plants is the nitrogen remobilized, via the phloem, from older, senescing organs.1 In particular, senescing leaf blades are the major source of nitrogen; and they contribute about half of the nitrogen in the developing spikelets (Fig. 17.1). This intricate process of nitrogen recycling, from the senescing organs to the developing organs, is very important in determining the productivity and the quality of rice plants. Nitrogen recycling consists of at least the following four major steps:
1. Degradation of nitrogen-containing macromolecules, such as RuBisCO and chlorophyll, during senescence;
2. Conversion of the hydrolyzed nitrogen to compounds for transport in the senescing organs;
3. Long distance transport of the nitrogen via phloem; and
4. Re-utilization of the transported nitrogen in developing organs for many biosynthetic reactions.
The first step, mechanisms for the degradation of RuBisCO for example, is largely unknown at this moment. During natural senescence of rice leaves2 or wheat leaves,3 degradation of RuBisCO occurs prior to the breakdown of chlorophyll, indicating that RuBisCO is possibly hydrolyzed in chloroplasts. Although the presence of proteolytic enzymes in chloro-plasts is not clearly reported, recent findings in which active oxygen splits the RuBisCO large subunit into two polypeptides4 could be a clue to understanding the mechanisms for RuBisCO degradation in chloroplasts. In the phloem sap of rice plants, glutamine and as-paragine, which is synthesized from glutamine,5,6 are the major forms of nitrogen.7 Therefore, synthesis of glutamine is important in senescing organs, whereas the re-utilization of the transported glutamine is necessary in developing organs. Glutamine synthetase (GS;
EC 188.8.131.52) and glutamate synthase (GOGAT) are candidates for performing the synthesis and utilization of glutamine in plants. There are two isoenzymes of GS in many plants:5,6 a minor isoenzyme located in the cytosol (GS1) and the main isoenzyme in the chloroplast (plastid) stroma (GS2). As for GS, two molecular species of GOGAT are found in both green and non- green tissues,5,6 one requiring NADH as reductant (NADH-GOGAT; EC 184.108.40.206) and the other requiring reduced ferredoxin (Fd- GOGAT; EC 220.127.116.11). In leaves, these two GS isoenzymes and two GOGAT species have distinct functions. Elegant studies with mutants lacking either GS28 or Fd-GOGAT9,10 show that a major role of GS2 and of Fd-GOGAT, both located in the chloroplast stroma, is the reassimilation of ammonium ions released from photorespiration. Because the mutants are able to grow normally under nonphotorespiratory conditions,8-10 GS1 and NADH-GOGAT in leaves could be important in the synthesis of glutamine and glutamate for normal growth and development.
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