Conclusion

To increase crop yield per unit of land area, an efficient nitrogen use within the plants is very important under conditions of sufficient irradiation. Because most of the nitrogen in rice grains is transported from senescing organs, the process of nitrogen recycling is very important in determining both the productivity and the quality of rice. Our immunolocal- ization studies clearly show that these GS isoenzymes or GOGAT species are individually accumulated in different cell types of tissues, and hence each enzyme has a distinct function in nitrogen metabolism. The localization of GS2 and Fd-GOGAT in mesophyll cells of rice leaves is in good agreement with the excellent studies using mutants which show that the major function of these enzymes is in the reassimilation of ammonium ions released during photorespira-tion.8-10 On the other hand, GS1 could be important in the synthesis of glutamine, which is a major form of nitrogen exported from the senescing leaves. In association with this, NADH-GOGAT has a role in re-utilizing the glutamine transported from senescing organs. The fact that some lines of the antisense transformants reduce the weight of seeds further supports the important role of NADH-GOGAT in nitrogen recycling in rice plants. Analyses of the promoter for the NADH-GOGAT gene will provide information on the regulatory mechanisms for the cell type-specific, age-specific, and nitrogen-responsive expression of the NADH-GOGAT gene in rice plants.

As far as genetic resources are concerned, more than half of the indica cultivars tested possess approximately two-fold greater content of the GS1 protein in senescing leaves than most japonica and javanica cultivars, while most of the indica cultivars contain lower amounts of NADH-GOGAT protein in unexpanded young leaf blades. Most cultivars of indica and javanica show a greater biomass productivity than japonica, even when cultivated in northern Japan. The great production of biomass of indica might be related to the enhanced system for exporting leaf nitrogen to the developing organs. If our hypothesis were true, enhancement of GS1 protein in senescing leaves of japonica and that of NADH-GOGAT in developing leaves of indica cultivars would possibly provide more nitrogen in the grains. At the same time, to increase the source capacity, genetic approaches to enhance the amounts of total nitrogen in leaves and also to delay the senescing period would be targets for genetic manipulation to improve the productivity of rice plants. Because the expression of NADH-GOGAT protein, as well as GS1 protein, is regulated in an age-specific, cell type-specific and nitrogen-responsive manner, mechanisms for the fine control of NADH-GOGAT gene expression should be studied in detail in the near future. The mapping of genes related to nitrogen recycling in chromosomes and analysis of quantitative trait loci (QTL) related to these characteristics will be tested by using recombinant inbred lines between a japonica and an indica. Information from these experiments will provide direction for genetic manipulation to improve the productivity and quality of rice plants.

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