Cd-stress on nitrogen assimilation

Nitrate assimilation in higher plants is the principal biosynthetic pathway leading to glutamate, required for synthesis of particular metabolites that participate in mechanisms of biochemical adaptation to heavy metal stress such as cadmium. Exposure of maize (Zea mays L.) seedlings to increasing cadmium concentrations for 12 days resulted in a significant decrease in nitrate content coupled to inhibition in activities of the enzymes nitrate reductase (EC 1.6.6.1), nitrite reductase (EC 1.6.6.4), glutamine synthetase (EC 6.3.1.2), ferredoxin-glutamate synthase (EC 1.4.7.1), and NADH-glutamate synthase (EC 1.4.1.14). Patterns of NADH- (aminating) and NAD(+)- (deaminating) glutamate dehydrogenase (EC 1.4.1.2) activities were opposed, with a great increase in the aminating direction. Concurrently, cadmium treatments promoted an increase in protease activity as well as accumulation of ammonia in tissues. These changes in enzyme activities were accompanied by a loss in total amino acids and proteins in both shoots and roots. However, of all amino acids quantified, glutamate, proline, lysine, methionine, and glycine showed a remarkable accumulation in cadmium treated plants, in comparison to control ones. The increased level of glutamate paralleled the elevated contents of gamma-glutamylcysteine and glutathione in response to cadmium treatments. The magnitude of changes in all parameters investigated was concentration-dependent. Considered together, metabolite analysis and enzymatic measurements showed that cadmium induces a substantial shift in the operative pathways of ammonia assimilation, and suggested that the induction of NADH-glutamate dehydrogenase activity under cadmium stress may provide glutamate required for enhancing the synthesis of proline, gamma-glutamylcysteine and glutathione, which is a common response to cadmium stress conditions. The possible association of these findings with cadmium-binding peptides is discussed.