Overexpression of cytosolic glutamine synthetase in rice improves utilization nitrogen use efficiency

This thesis tests the hypothesis that overexpression of cytosolic glutamine synthetase (GS1) in rice improves utilization nitrogen use efficiency (UtE). GS1 is responsible for the primary assimilation of ammonia and a role in nitrogen remobilization is implicated from its vascular localization and enhanced expression during senescence. Three GS1 overexpressor lines were identified using activity assays and quantitative real-time polymerase chain reaction. Physiological analysis of the lines was conducted at varying nitrate conditions, growth stages and environments. Overall, overexpressor lines did not differ from azygous controls in shoot biomass or nitrogen accumulation. However, harvest index and UtE were significantly enhanced in some of the transgenic lines relative to their corresponding azygous controls. These characteristics were highly correlated with percent spikelets filled, spikelet number and yield. Thus, nitrogen partitioning in rice during grain filling can be altered by GS1 overexpression, resulting in improved UtE.