Glutamine Synthetase and its Isoforms in Rice Spikelets and Rachis during Grain Development

Glutamine synthetase (GS) and its isoforms have been studied extensively in rice (Oryza sativa L.) leaves, but little is known about GS in the spikelets and rachis of rice panicles. In this study, GS activity and its isoforms were determined in the spikelets, rachis, and flag leaves during grain development. The GS activities per unit fresh weight of these tissues reached maximum levels around the milk stage of the endosperm and declined thereafter as grain filling progressed. Flag leaves had 5-fold higher GS activity per unit fresh weight than rachis and 7-fold higher than spikelets at the milk stage. Native-PAGE and activity staining indicated that there were two GS isoforms in the rachis and they were the same as the eytosolic form (GS 1) and the chloroplastic form (GS2) in leaves, respectively. The GS2 was the dominant isoform in flag leaves, whereas GS 1 was the dominant isoform in the rachis. However, only GS 1 activity was detectable in spikelets including hull and kernel. The lack of GS2 activity in the spikelets was consistent throughout the entire grain development stage and was confirmed by protein immunoblotting.