Glutamine synthetase and glutamate dehydrogenase in cadmium-stressed triticale seedlings
The studies were performed on young triticale seedlings grown on a mineral medium containing 5 mM NO3− as the nitrogen source, with the addition of 0.5 mM CdCl2. It was determined that cadmium ions accumulated mainly in the plant roots. Decreases in nitrate concentrations both in the roots and shoots of seedlings, as well as decreases in soluble protein contents with simultaneous increases in endopeptidase activity were also observed. Both in roots and shoots significant decreases in glutamic acid were noted. Toxic cadmium ion accumulation in seedlings significantly modified activity of primary nitrogen assimilating enzymes, i.e. glutamine synthetase (GS, EC 184.108.40.206) and glutamate dehydrogenase (GDH, EC 220.127.116.11). There was a significant decrease in GS activity both in roots and in shoots of the stressed plants, in comparison to plants grown without cadmium. In shoots of the control plants and plants subjected to stress two GS isoforms were discovered: cytoplasmatic (GS1) and chloroplastic (GS2). Substantial decreases in total glutamine synthetase activity in green parts of seedlings, occurring under stress conditions, result from dramatic decrease in GS2 activity (by 60 % in relation to the control plants); despite simultaneous increases in the cytoplasmatic isoform (GS1) activity by approx. 96 %.
Cadmium ions accumulating in roots and shoots of seedlings not only increased GDH activity, but also modified its coenzymatic specificity.