在盐胁迫下，检测了耐盐性不同的水稻（Oryza sativa L.）品种根部氨同化酶及其相关参数的变化。结果表明，根的可溶性蛋白、谷氨酰胺合成酶（GS）及依赖于NADH的谷氨酸合酶（NADH-GOGAT）活性在高盐浓度下不同程度地降低，其影响大小依次为早花二号（盐敏感品种）、金珠一号（正常栽培品种）、津稻779（耐盐品种），与其耐盐性相一致。在盐胁迫条件下，在耐盐性较高的水稻品种中，GS和GOGAT活性比盐敏感品种高，NH4+浓度维持在较低的水平。Native-PAGE和活性染色结果表明，GSrb更容易受到外界环境的影响。在高浓度盐的胁迫下，早花二号、金珠一号的依赖于NADH的谷氨酸脱氢酶（NADH-GDH）活性都有较显著的升高，津稻779却无明显的变化，这和NH4+含量的变化相一致。盐不同程度地导致可溶性糖（TSS）在金珠一号和津稻779根部积累，而在早花2号的根部，可溶性糖的水平则随盐浓度的不同而表现出不同的变化。在所检测的品种中，脯氨酸的含量均有不同程度的升高，但在高盐浓度下，盐敏感品种的含量较低。这些结果提示，不同的水稻品种对盐胁迫的敏感程度与该品种GS以及G O G A T 活性的高低有关。

The top three leaves play important roles in biomass production and grain yield of rice (Oryza sativa L.) crop since the three leaves not only assimilate majority of carbon for grain filling during ripening phase, but also provide large proportion of remobilized-nitrogen (N) for grain development during their senescence. The objectives of this study were to (a) compare senescence of the top three leaves and (b) compare the changes in N, chlorophyll, and ribulose-1,5-bisphosphate carboxylase=oxygenase (Rubisco) contents of the top three leaves after their full expansion in field-grown rice plants. When the basis of comparison among the top three leaves was plant age in terms of days after transplanting (DAT), senescence generally started earliest in-3rd leaf, intermediate in-2nd leaf, and latest in flag leaf. If the basis of comparison among the top three leaves was leaf age in terms of days after full leaf expansion (DAFE), it was not clear which leaf senesced earlier. Senescence rate was generally greatest in flag leaf, intermediate in 2nd leaf, and smallest in-3rd leaf. Ribulose-1,5-bisphosphate carboxylase=oxygenase content declined earlier, and at a faster rate than N and chlorophyll contents during the senescence of all top three leaves. Correlation analysis indicated a close relationship between N and chlorophyll contents. Ribulose-1,5-bisphosphate carboxylase=oxygenase content correlated with N content better than with chlorophyll content. The suitability of N, chlorophyll, and Rubisco contents for quantifying the leaf senescence of field-grown rice plants is discussed.

测定了水稻种子不同萌发时期胚乳、胚芽鞘和幼根的谷氨酰胺合成酶（GS）和依赖于NADH的谷氨酸合酶（NADH2GOGAT）活性变化。胚乳和胚芽鞘的GS活性在萌发过程中升高，幼根的GS活性则有所降低。NADH2 GOGAT的活性变化趋势与GS相同。Native-PAGE活性染色表明，在萌发阶段的水稻种子胚乳和幼根里，始终只观察到一种GS活性带。但是，在水稻种子萌发3d后，在胚芽鞘中除继续检测到GS1的活性外，还可以观察到GS2的活性。蛋白质印迹显示，水稻种子胚乳中的GS（Gse）与GS1和GSra一样是一种胞质型GS。实验结果提示，这些不同组织中的GS与NADH2GOGAT构成的循环途径也许是水稻种子萌发时氨同化的主要途径。

以前的研究表明，高等植物叶绿体谷氨酰胺合成酶（GS2）受光调节，但叶片胞液GS（GS1）和非光合作用组织中的GS很少受光的影响。在本报道中，笔者运用GS活性染色和Westernblotting研究了光对非光合作用组织水稻根GS同工酶表达的影响。在阳光的直接照射下以及在室内不同光照强度下，可以很清楚地观察到GSra和GSrb的活性带及其蛋白质带。但是当用尼龙网挡住阳光的直接照射时，GSrb的活性带和蛋白质带消失；当阳光被尼龙网遮挡住后，其光强度仍然比室内光照强度大得多，表明光照强度不是影响GSrb表达的主要因素。当分析生长在暗处以及生长在光/暗转换下的水稻幼苗根GS 同工酶变化时，仍然可以观察到GSrb的存在。在所有实验条件下，GSra都未发生明显变化。这些结果提示，光对GSrb表达的影响可能是由某些光谱相互作用所产生的未知因素造成的。

用纯化的水稻（Oryza sativa L.）根部存在的两种谷氨酰胺合成酶（GS）同工酶GSra和GSrb分别免疫兔子，得到相应的抗体。免疫扩散和免疫印迹实验表明，GSra或GSrb的抗体对GS及其同工酶是专一的。免疫沉淀试验表明，GSra或GSrb不仅识别它的相应的抗原，而且也能很好地识别彼此的抗原。这两种抗体也能较好地识别水稻叶片胞液型的GS1，但对水稻叶片和菠菜（Spinacia oleracea Mill.）叶片叶绿体型的GS2的识别较弱。这些结果表明，水稻根的GSra和GSrb有很相似的抗原性，他们是非常相似的蛋白质。揭示GSra和GSrb像GS1一样都是胞液型GS。

Glutamine synthetase (GS) and its isoforms have been studied extensively in rice (Oryza sativa L.) leaves, but little is known about GS in the spikelets and rachis of rice panicles. In this study, GS activity and its isoforms were determined in the spikelets, rachis, and flag leaves during grain development. The GS activities per unit fresh weight of these tissues reached maximum levels around the milk stage of the endosperm and declined thereafter as grain filling progressed. Flag leaves had 5-fold higher GS activity per unit fresh weight than rachis and 7-fold higher than spikelets at the milk stage. Native-PAGE and activity staining indicated that there were two GS isoforms in the rachis and they were the same as the cytosolic form (GS1) and the chloroplastic form (GS2) in leaves, respectively. The GS2 was the dominant isoform in flag leaves, whereas GS1 was the dominant isoform in the rachis. However, only GS1 activity was detectable in spikelets including hull and kernel. The lack of GS2 activity in the spikelets was consistent throughout the entire grain development stage and was confirmed by protein immunoblotting.

Root biomass of rice seedlings was increased at lower concentration of exogenous NH+4, but it was decreased at higher concentration of exogenous NH+4. The level of free NH+4 in the roots was accumulated gradually with the increase of NH+4 concentration in the nutrient solution. The content of the soluble proteins was essentially constant at higher NH+4. The activities of glutamine synthetase(GS), NADH-dependent glutamate synthase(NADH-GOGAT), and NADH-dependent glutamate dehydrogenase (NADH-GDH) were risen with exogenous NH+4 concentration at the lower NH+4 concentration range. But the activities of GS and NADH-GOGAT were declined, and the level of NADH-GDH activity was kept constant under higher NH+4 concentration. The GS/GDH ratio suggested that NH+4 was assimilated by GS-GOGAT cycle under lower NH+4 concentration, but NADH-GDH was more important for NH+4 assimilation and detoxifying NH+4 to the tissue cells at the higher NH+4 level. According to the growth and the activity changes of these ammonium-assimilating enzymes of rice seedling roots, 10.0μg/mL NH+4-N in nutrient solution was more suitable to the rice growth.

Many factors, including exogenous carbon metabolites influence the expression of enzymes involved in nitrogen metabolism, but little is known about the effect of low temperature on the expression of those enzymes. To investigate that rice (Oryza sativa L.) seedlings grown at the optimal temperature (30 8C) were subjected to low temperature (20 8C) stress, and several key enzymes involved in ammonium assimilation and carbon metabolism, including glutamine synthetase (GS; EC 6.3.1.2), NADP-dependent isocitrate dehydrogenase (NADP-ICDH; EC 1.1.1.42), and NAD(H)-dependent glutamate dehydrogenase (GDH; EC 1.4.1.2) from rice roots were assessed. At low temperature, GS and ICDH activities were increased by 80 and 75%, respectively; thus, a positive parallel correlation was observed between the two enzymes. Native-PAGE analysis, together with activity staining and Western blot assays, showed that both GSrb activity and GSrb protein level were enhanced under low temperature. Meanwhile, NADH-GDH and NAD-GDH activity were both reduced, though to different extents. Biomass, NH4+, pH, and proline were measured as well. At low temperature, ammonium absorption was stimulated by the elevation of GS activity. Proline content was increased two-fold by low temperature, and this accumulation was in good agreement with the induction of GS activity. Similar observations were made in the rice roots fed with sucrose, indicating that at least partially low temperature has the similar effect as carbon compounds on the modulation of nitrogen metabolism. The possibility of the regulation of this metabolic pathway by 2-oxoglurate (2-OG) is discussed.

Previous studies, using ion-exchange chromatography, reported that one isoform of glutamine synthetase (GS) existed in the cytosol of rice (Oryza sativa L.) roots. We report that two isoforms of GS were observed in rice roots using native polyacrylamide gel electrophoresis (PAGE), GS activity staining and immunoblotting. One isoform (GSra) had a mobility similar to that of cytosolic GS in rice leaves (GSI). This isoform existed in seminal roots of germinating seeds and in the roots of seedlings grown in N-free and N-containing culture solutions. Another isoform (GSrb) moved more slowly than GSra and occurred only in the presence of external N. Both ammonium-N and nitrate-N induced GSrb, but the activity of GSrb was higher under ammonium-N than nitrate-N. After the removal of N source from the solution, GSrb activity and GSrb protein disappeared as judged by native-PAGE and immunoblotting but GSra remained relatively constant. It was estimated that GSrb contributed about 80% of total GS activity of the rice roots grown in the solution containing NH4+. SDS-PAGE and immunoblotting test showed that subunits of GSra, GSrb, and GS1 had the same molecular weight. Our results suggest that GSra was a constitutive enzyme and GSrb was an enzyme induced by external N.