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

本研究的目的是阐明调节水稻氮素同化和氮素循环的分子实体，了解水稻氮素代谢的信号转导系统。NADH-谷氨酸合成酶(NADH-GOGAT)是水稻氨同化和氮素循环的关键酶。水稻NADH-GOGAT1基因的表达受谷氨酰胺(Gln)的调控，提示Gln信号转导。细菌Gln传感器GlnD有两个ACT结构域。这些结构域被认为是谷氨酰胺结合所必需的。研究了由4个与GlnD相似的ACT结构域组成的水稻ACT结构域重复蛋白(OSACR)作为谷氨酰胺传感器的候选基因。OsACR7和OsACR9定位于水稻细胞核。并提出了OsACR7与谷氨酰胺结合的潜在能力。在水稻中，OsACR9的细胞定位与NADH-GOGAT1基因产物的细胞定位有很好的重叠。虽然在OsACR9转基因水稻后代的幼叶中，NADH-GOGAT1基因的表达受到抑制，但在供应氨离子后的T1代转基因水稻的根中，没有观察到这种对NADH-GOGAT1基因的抑制作用。这些结果表明，OsACR9可能参与了水稻幼叶的谷氨酰胺信号转导。此外，还试图找到控制水稻产量和氮素利用的特异基因座。利用培育的水稻染色体片段代换系对这些数量性状基因座进行了检测。定位了不同浓度氨离子处理下控制水稻生长发育的QTL。控制水稻种子干重增加的QTL位于第2染色体上的5 cm左右区域，控制水稻种子干重的QTL位于630kb的区域。将需要进一步的工作来确定这些QTL的负责基因。

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.