The role of cytosolic glutamine synthetase in wheat

The role of glutamine synthetase (GS; EC 6.3.1.2) was studied in wheat. GS isoforms were separated by HPLC and the two major leaf isoforms (cytosolic GS1 and chloroplastic GS2) were found to change in content and activity throughout plant development. GS2 dominated activity in green, rapidly photosynthesising leaves compared to GS1 which was a minor component. GS2 remained the main isoform in flag leaves at the early stages of grain filling but GS1 activity increased as the leaves aged. During senescence, there was a decrease in total GS activity which resulted largely from the loss of GS2 and thus GS1 became a greater contributor to total GS activity. The changes in the activities of the GS isoforms were mirrored by the changes in GS proteins measured by western blotting. The changes in GS during plant development reflect major transitions in metabolism from a photosynthetic leaf (high GS2 activity) towards a senescencing leaf (relatively high GS1 activity). It is likely that, during leaf maturation and subsequently senescence, GS1 is central for the efficient reassimilation of ammonium released from catabolic reactions when photosynthesis has declined and remobilisation of nitrogen is occurring. Preliminary analysis of transgenic wheat lines with increased GS1 activity in leaves showed that they develop an enhanced capacity to accumulate nitrogen in the plant, mainly in the grain, and this is accompanied by increases in root and grain dry matter. The possibility that the manipulation of GS may provide a means of enhancing nitrogen use in wheat is discussed.