Reappraisal of nitrogen use efficiency in rice overexpressing glutamine synthetase1

Cytosolic glutamine synthetase (GS1) is responsible for the primary assimilation of ammonia, and a role in nitrogen (N) remobilization is implicated from its vascular localization and enhanced expression during senescence. This paper tested the hypothesis that overexpression (OX) of GS1 in rice improves utilization N use efficiency (UtE = spikelet yield/shoot N content). Three GS1 OX lines were identified using activity assays and quantitative polymerase chain reaction. Physiological analysis of the OX lines, as well as azygous and wild-type (Wt) controls, was conducted with mature plants after growth under varying nitrate conditions (non-limiting N, limiting N, transfer from non-limiting N to limiting N at panicle emergence) and growth environments (growth chamber vs greenhouse). Overall, OX lines did not differ from azygous controls in vegetative yield or shoot N content. In two of the three growth trials (i.e. the growth chamber trials) harvest index, N harvest index (spikelet N content/shoot N content) and UtE were generally enhanced in the OX lines relative to their azygous controls. These characteristics were highly correlated with percent spikelets filled and spikelet number. Thus, N partitioning in rice during grain filling could be altered by GS1 OX, resulting in improved UtE. Unfortunately, GS OX did not result in more efficient use of N under limiting N than under non-limiting N, and is therefore unlikely to result in the use of less N under field conditions. Transformation effects significantly hindered the productivity of the OX lines, but backcrossing to the Wt should overcome this.