Regulation of Glutamate Dehydrogenase and Nitrate Reductase Levels in Excised Pea Roots by Exogeneously Supplied Sugar

Abstract

The activities of NADH2 dependent glutamate dehydrogenase (GDH) and nitrate reductase were followed in excised pea roots cultured in liquid medium with or without sucrose. Whilst the persistance of nitrate reductase activity in excised pea roots depends directly on the presence of sugar in the medium and whilst in its absence nitrate reductase activity drops to the initial value before the induction with nitrate, both NADH9 dependent GDH and NADPH2 dependent GDH activities rise in excised pea roots in the absence of sugar in the medium, and vice versa, their activities are maintained at approximately the same level as in corresponding parts of roots on intact seedlings in the presence of sugar in the medium. The increase in NADH2 dependent GDH activity is inhibited by actidione which indicates that de novo synthesis of the enzyme is probably responsible for the increase. Glucose and fructose are in their effects equivalent to sucrose. The omission of sugar from the medium causes also a change in the ratio of the activities of individual isoenzymes of NAD+ dependent GDH. Physiological concentrations of ammonium and nitrate ions neither influence GDH level in the presence of sugar nor in the absence of it. The above mentioned sugars are apparently involved in the regulation of these enzymes directly and not indirectly by means of their metabolites formed during respiration. This is indicated by the results obtained in the experiments with exogenously supplied metabolites of glycolysis, pentose phosphate cycle and Krebs cycle, which did not show any significant effect on GDH and nitrate reductase levels. The increase in GDH level observed after the addition of sublethal concentrations of some respiratory inhibitors to the medium containing sugar was probably caused by decreased sugar uptake and transport. The regulation of GDH level controlled by sugars is in isolated pea roots independent of some other regulatory mechanisms (controlled by acids, nitrite and hydroxylamine), and vice versa. On the other hand, the results of our experiments indicate that the increase in the level of NADH2 dependent GDH observed by other authors in the roots of whole intact plants after their exposure to high concentrations of ammonium salts, may be, at least partly, caused indirectly by changes in the level of sugars in these roots.