Regulation of Nodule Glutamine Synthetase by CO(2) Levels in Bean (Phaseolus vulgaris L.)

Nodulated bean (Phaseolus vulgaris) plants were grown for 17 days after infection in normal (0.02%) CO₂ and from day 8 to 17 in high (0.1%) CO₂ in order to increase nitrogen fixation and define how nodule glutamine synthetase (GS) isoforms are regulated by the ammonia derived from the bacteroid. Nitrogenase activity was detected by day 10, and by day 17 activity was over twofold higher in 0.1% of CO₂ compared with plants grown in 0.02% CO₂ and inoculated with Rhizobium wild-type strain CE3. Likewise, plant fresh weight increased in response to increased CO₂, particularly in plants inoculated with the Rhizobium phaseoli mutant strain CFN037. Glutamine synthetase specific activity increased 2.5- to 6.5-fold from day 11 to 17. However, increased CO₂ did not appear to have an effect on GS specific activity. Analysis of the nodule GS polypeptide composition revealed that the γ polypeptide was significantly reduced in response to high CO₂, whereas the β polypeptide was not affected. The significance of this result in relation to the regulation of GS isoforms and their role in the assimilation of ammonia in the nodule is discussed in this paper.