Molecular-genetic dissection of ammonium assimilation in Arabidopsis thaliana

The process of assimilation of inorganic nitrogen into organic form is essential both for plant growth and development as nitrogen deprivation in plants can cause a number of metabolic deficiencies in plants. Thus, the study of the enzymes involved in ammonium assimilation have an impact on both basic and applied plant research. Ammonium is first assimilated into the amino acids glutamine and glutamate by the concerted actions of glutamine synthetase (GS), glutamine-oxoglutarate aminotransferase (GOGAT), and glutamate dehydrogenase (GDH). The glutamate and glutamine are then channeled into aspartate and asparagine by aspartate amino transferase (AspAT) and asparagine synthetase (AS). However, the actual biology of the ammonium assimilation pathway has been obscured by the fact that most reactions are catalyzed by multiple isoenzymes, located in distinct tissues and/or subcellular compartments. Therefore, standard biochemical methods used to define rate-limiting enzymes in a given pathway may lead to misleading interpretations when employed to study metabolic pathways in plants. Here we discuss how the availability of genetic and molecular tools, especially in the model plant Arabidopsis thaliana, have made it possible to start delineating the mechanisms of genetic regulation of the ammonium assimilatory pathway, and to destine the in vivo role of each isoenzyme. The basic knowledge obtained on the genes involved in the process of ammonium assimilation may be applied in attempts to increase the efficiency with which nitrogen is incorporated into organic form which may have marked effects on plant productivity, biomass, and crop yield.