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

本研究旨在阐明水稻氮素同化和氮素再循环的分子调控机制，了解水稻氮素代谢的信号转导系统。NADH-谷氨酸合成酶（NADH-GOGAT）是水稻铵同化和氮循环的关键酶。水稻NADH-GOGAT 1基因的表达受谷氨酰胺（Gln）的调控，提示存在Gln信号转导。细菌Gln传感器GlnD具有两个ACT结构域。这些结构域被认为是必不可少的谷氨酰胺结合。水稻ACT结构域重复蛋白（OsACRs）是由四个与GlnD相似的ACT结构域组成的，可作为Gln传感器的候选蛋白。OsACR 7和OsACR 9定位于水稻细胞核中。OsACR 7结合Gln的潜在能力也被提出。在水稻中，OsACR 9的细胞定位与NADH-GOGAT 1基因产物的细胞定位有很好的重叠。虽然在OsACR 9敲除水稻的TO代的幼叶中，NADH-G 0 GAT 1基因的表达受到抑制，但是在供应铵离子后的敲除水稻的T1代的根中没有观察到NADH-G 0 GAT 1基因的这种抑制效果。这些结果表明OsACR 9可能参与水稻幼叶Gln信号转导。同时，本研究还试图鉴定控制水稻生产力和氮素利用的特异基因位点。利用粳稻和籼稻间的染色体片段代换系对这些数量性状位点进行了检测。定位了不同浓度铵离子处理下控制水稻生长发育的QTL。一个控制低浓度铵离子供应下芽干重增加的QTL被缩小到第2染色体上约5厘米的区域。在第11染色体上的630 kb区域内还定位了一个控制水稻种子重量的QTL。需要进一步的工作来鉴定这些QTL的相关基因。

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.