Overexpression of OsGS1;2 for improved nitrogen use efficiency and grain yield of rice: A field test

In rice grain production, low nitrogen (N) use efficiency (NUE) is a persistent problem that impairs the balance between higher grain yield and the environmental risk caused by overuse of N fertilizer. Efficient N utilization is achieved through close coordination between N acquisition by roots and assimilation in the plant. In this study, we engineered the N assimilation process by the generation of transgenic rice overexpressing OsGS1;2—the gene coding for glutamine synthetase (GS) for primary NH4+ assimilation in rice roots. The NUE and grain yield of these genetically modified rice lines were then assessed under field conditions with naturally fluctuating environment for two successive growth seasons. Three levels of N application corresponding respectively to N deficiency (LN, no N applied), moderate or reduced N input (MN, 200 kg N hm−2) and N overuse (HN, 350 kg N hm−2) were designed for the experiment. Under either levels of N application, overexpression of OsGS1;2 led to increases in plant biomass, tiller numbers and flag leaf N contents, and consequently greater recovery N use efficiency (REN). A significant improvement of the physiological (PEN) and agronomic (AEN) N use efficiencies with increased grain yield can be achieved with the OsGS1;2 overexpressors under reduced N input (MN). However, such advantages of the overexpressors are diminished under N overuse (HN) conditions and even causes significant grain yield loss of rice. Conclusively, our results suggest a potentially useful approach that coordinates improved N use efficiencies with higher grain yield under conditions of reduced cost of N application by overexpression of OsGS1;2.