Concurrent Overexpression of Rice GS1;1 and GS2 Genes to Enhance the Nitrogen Use Efficiency (NUE) in Transgenic Rice

Nitrogen (N) is an essential macronutrient required for crop productivity, which is the limiting resource in most of the agricultural soils. External application of nitrogen fertilizers is necessary to sustain crop productivity. More than 60% of applied nitrogen fertilizer is lost to the environment because of the low nitrogen use efficiency (NUE) of crop plants that results in environmental degradation and also incurs higher input costs to the farmers. In this study, we developed a genetic strategy to improve the nitrogen use efficiency of rice plants by concurrent ectopic expression of OsGS1;1 and OsGS2 encoding genes to enhance the selective glutamine synthetase (GS) isoforms to improve the nitrogen use economy by effectively re–assimilating the internally released ammonia (NH 3 ). We conducted a detailed investigation to establish the physiological impact of the enhanced GS activity in these transgenic rice plants. These transgenic rice plants did not show any fitness cost, moreover displayed an enhanced growth and productivity compared to their corresponding wild type (WT) control/non–transgenic rice plants. The insights of our study demonstrate the increased NUE by effectively re-assimilating the internally released NH 3 by concurrent ectopic expression of OsGS1;1 and OsGS2 genes. The increase in assimilated N in transgenic rice plants positively partitioned into the production of photosynthetic enzymes, chlorophyll pigment accumulation that ultimately improves net photosynthetic efficiency with a significant gain in N and C compared to their corresponding WT control rice plants, suggesting that mutual regulation of C/N balance is crucial for proper plant growth and development. Our study suggests that insufficient C supply may provide a constraint on N assimilation and reduce the positive effect of GS overexpression. To circumvent such metabolic imbalances, more refined strategies are needed for improving both the N and C assimilation simultaneously in the same plant.