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 spikele ts and rachis of rice panicles. In this study, GS activity and its isoforms were determined in the spikelets, rachis, and flag leaves during grain dev elopment. The GS activities per unit fresh weight of these tissues reached maximum levers around the milk stage of the endosperm and declined thereaft er as grain filling progressed. Flag leaves had 5-fold higher GS activity p er unit fresh weight than rachis and 7-fold higher than spikelets at the mi lk stage. Native-PAGE and activity staining indicated that there were two G S isoforms in the rachis and they were the same as the cytosolic form (GS1) and the chloroplastic form (GS2) in leaves, respectively. The GS2 was the dominant isoform in flag leaves, whereas GSI aas the dominant isoform in th e rachis. However, only GSI activity was detectable in spikelets including hull and kernel. The lack of GS2 activity in the spikelets was consistent t hroughout the entire grain development stage and was confirmed by protein i mmunoblotting.