In japonica type rice (Oryza sativa L. cv.
Sasanishiki) plants, approximately 80% of total nitrogen in the ear is the nitrogen which is remobilized through the phloem from older, senescing organs. Senescing leaf blades contribute about 50% of the total nitrogen in the ear. Thus, the process of nitrogen recycling is very important in determining both the productivity and quality of rice. However, little attention has been paid to the mechanisms of nitrogen remobilization from the senescing organs or to the re-utilization of the remobi-lized nitrogen for biosynthetic reactions in developing organs. Major forms of nitrogen in the phloem sap of rice plants are glutamine and asparagine. The asparagine is probably synthesized from glutamine. Thus, the synthesis of glutamine in senescing organs, as well as the utilization of glutamine in developing organs, are the key steps for nitrogen recycling in rice plants.
Our immunocytological studies with a japonica type rice at the reproductive stage show that cytosolic glutamine synthetase (GS1; EC 184.108.40.206) is important for the export of leaf nitrogen from senescing leaves, because the GS1 protein was detected in companion cells, which are important for phloem loading of solutes, and vascular parenchyma cells. NADH-dependent glutamate synthase (NADH-GOGAT; EC 220.127.116.11) in developing organs, such as expanding leaves and developing grains, is involved in the utilization of glutamine that is transported through the vascular system, because the protein was located in the cells which are important for solute transport from the phloem and xylem elements. Transgenic rice plants expressing antisense RNA for NADH-GOGAT at T0 generation markedly reduced the weight of 1,000 grains, indicating that NADH-GOGAT is indeed a key step for nitrogen recycling and a possible target for the improvement of plant productivity. The content of NADH-GOGAT protein in the developing organs alters dramatically and its expression is regulated in a cell type-specific manner. We recently isolated both genomic and cDNA clones for rice NADH-GOGAT. Regulation of NADH-GOGAT gene expression is currently being studied more precisely using these molecular tools. When various cultivars of japonica, indica, and javanica type rice plants were tested to determine the contents of NADH-GOGAT in expanding leaves and those of GS1 in senears were found to contain less NADH-GOGAT protein and more GS1 protein than a typical japonica, Sasanishiki. If the functions of GS1 and NADH-GOGAT also hold true in indica type cultivars, the content differences may be related to the morphological characteristics different from japonica. Molecular physiology, together with the use of genetic resources, could provide targets in nitrogen recycling to improve productivity in rice plants.
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