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 deprivati on in plants can cause a number of metabolic deficiencies in plants. T hus, the study of the enzymes involved in ammonium assimilation have a n impact on both basic and applied plant research. Ammonium is first a ssimilated into the amino acids glutamine and glutamate by the concert ed actions of glutamine synthetase (GS), glutamine-oxoglutarate aminot ransferase (GOGAT), and glutamate dehydrogenase (GDH). The glutamate a nd glutamine are then channeled into aspartate and asparagine by aspar tate amino transferase (AspAT) and asparagine synthetase (AS). However , the actual biology of the ammonium assimilation pathway has been obs cured by the fact that most reactions are catalyzed by multiple isoenz ymes, located in distinct tissues and/or subcellular compartments. The refore, standard biochemical methods used to define rate-limiting enzy mes in a given pathway may lead to misleading interpretations when emp loyed to study metabolic pathways in plants. Here we discuss how the a vailability of genetic and molecular tools, especially in the model pl ant Arabidopsis thaliana, have made it possible to start delineating t he mechanisms of genetic regulation of the ammonium assimilatory pathw ay, and to destine the in vivo role of each isoenzyme. The basic knowl edge obtained on the genes involved in the process of ammonium assimil ation may be applied in attempts to increase the efficiency with which nitrogen is incorporated into organic form which may have marked effe cts on plant productivity, biomass, and crop yield.