Post-genomic analysis for the molecular basis of regulation mechanisms on nitrogen-assimilation and -recycling among organs and cells in rice.

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The objective of this study was to elucidate molecular entities to regulate ammonium assimilation and nitrogen recycling and to understand the signal transduction system on nitrogen metabolism in rice. NADH-glutamate synthase (NADH-GOGAT) is a key enzyme on ammonium assimilation and nitrogen recycling in rice. Expression of rice NADH-GOGAT1 gene is regulated by glutamine (Gln), suggesting the Gln-signal transduction. Bacterial Gln sensor GlnD has two ACT domains. These domains were suggested to be essential for Gln binding. Rice ACT domain repeat proteins (OsACRs) composed of four ACT domains similar to those of GlnD were studied as a candidate for the Gln sensor. OsACR7 and OsACR9 localized in nuclei of rice cells. The potential ability of OsACR7 to bind Gln was also suggested. In rice, cellular localization of OsACR9 overlapped well with those of NADH-GOGAT1 gene products. Although in young leaves of TO-generations of OsACR9-knockdown rice, expression of NADH-GOGAT1 gene was suppressed, this suppression effect for the NADH-GOGAT1 gene was not observed in roots of T1-generations of the knockdown rice following supply of ammonium ions. These results suggest that OsACR9 may be involved in Gln-signal transduction in young rice leaves. By the way, attempt to identify specific gene loci to control productivity and nitrogen utilization in rice was performed. Chromosome segment substitution lines developed between Japonica and Indica rice were employed to detect these quantitative trait loci (QTLs). QTLs controlling development of rice plants under treatment of various concentrations of ammonium ions were mapped. A QTL controlling increase in dry weights of shoots under supply of low concentration of ammonium ions was narrowed down to approximately 5 cM-region on chromosome 2. A QTL on chromosome 11 controlling weighs of rice seeds was also delimited in a 630 kb-region. Further works to identify responsible genes of these QTLs will be needed.