Oliveira Igor C, Brears Timothy, Knight Thomas J, Clark Alexandra, Coruzzi Gloria M
Department of Biology, New York University, NY 10003, USA.
Plant Physiol. 2002 Jul;129(3):1170-80. doi: 10.1104/pp.020013.
In plants, ammonium released during photorespiration exceeds primary nitrogen assimilation by as much as 10-fold. Analysis of photorespiratory mutants indicates that photorespiratory ammonium released in mitochondria is reassimilated in the chloroplast by a chloroplastic isoenzyme of glutamine synthetase (GS2), the predominant GS isoform in leaves of Solanaceous species including tobacco (Nicotiana tabacum). By contrast, cytosolic GS1 is expressed in the vasculature of several species including tobacco. Here, we report the effects on growth and photorespiration of overexpressing a cytosolic GS1 isoenzyme in leaf mesophyll cells of tobacco. The plants, which ectopically overexpress cytosolic GS1 in leaves, display a light-dependent improved growth phenotype under nitrogen-limiting and nitrogen-non-limiting conditions. Improved growth was evidenced by increases in fresh weight, dry weight, and leaf soluble protein. Because the improved growth phenotype was dependent on light, this suggested that the ectopic expression of cytosolic GS1 in leaves may act via photosynthetic/photorespiratory process. The ectopic overexpression of cytosolic GS1 in tobacco leaves resulted in a 6- to 7-fold decrease in levels of free ammonium in leaves. Thus, the overexpression of cytosolic GS1 in leaf mesophyll cells seems to provide an alternate route to chloroplastic GS2 for the assimilation of photorespiratory ammonium. The cytosolic GS1 transgenic plants also exhibit an increase in the CO(2) photorespiratory burst and an increase in levels of photorespiratory intermediates, suggesting changes in photorespiration. Because the GS1 transgenic plants have an unaltered CO(2) compensation point, this may reflect an accompanying increase in photosynthetic capacity. Together, these results provide new insights into the possible mechanisms responsible for the improved growth phenotype of cytosolic GS1 overexpressing plants. Our studies provide further support for the notion that the ectopic overexpression of genes for cytosolic GS1 can potentially be used to affect increases in nitrogen use efficiency in transgenic crop plants.
在植物中，光呼吸过程中释放的铵超过初级氮同化多达10倍。对光呼吸突变体的分析表明，线粒体中释放的光呼吸铵通过谷氨酰胺合成酶（GS2）的叶绿体同工酶在叶绿体中重新同化，GS2是包括烟草（Nicotiana tabacum）在内的茄科植物叶片中的主要GS同工型。相比之下，胞质GS1在包括烟草在内的几种植物的维管系统中表达。在此，我们报告了在烟草叶肉细胞中过表达胞质GS1同工酶对生长和光呼吸的影响。在叶片中异位过表达胞质GS1的植株，在氮限制和非氮限制条件下均表现出光依赖性的生长表型改善。鲜重、干重和叶片可溶性蛋白的增加证明了生长得到改善。由于生长表型的改善依赖于光照，这表明叶片中胞质GS1的异位表达可能通过光合/光呼吸过程起作用。烟草叶片中胞质GS1的异位过表达导致叶片中游离铵水平降低6至7倍。因此，叶肉细胞中胞质GS1的过表达似乎为叶绿体GS2同化光呼吸铵提供了一条替代途径。胞质GS1转基因植株还表现出CO₂光呼吸猝发增加和光呼吸中间产物水平升高，表明光呼吸发生了变化。由于GS1转基因植株的CO₂补偿点未改变，这可能反映了光合能力的相应增加。总之，这些结果为胞质GS1过表达植株生长表型改善的可能机制提供了新的见解。我们的研究进一步支持了这样一种观点，即胞质GS1基因的异位过表达有可能用于提高转基因作物的氮利用效率。
