Enzyme redundancy and the importance of 2-oxoglutarate in higher plant ammonium assimilation.

The assimilation of inorganic N, in the form of ammonia, onto C skeletons for the production of amino acids is one of the most important biochemical processes in plants. In an actively growing plant, N is taken up as nitrate and to a lesser extent as ammonia. Nitrate is reduced in the cytoplasm by nitrate reductase and nitrite enters the chloroplasts where it is further reduced before incorporation into amino acids. In addition to this steady influx of N, there is recycling of N compounds that also produces ammonia that needs to be assimilated. This recycling results from the operation of the photorespiratory cycle with the oxidation of Gly in the mitochondria (Fig. 1), the breakdown of storage proteins in seeds, leaf, and bark tissues, and the breakdown of proteins and nucleic acids associated with cellular senescence and apoptotic phenomena. Some plants, especially the leguminous plants, are able to convert atmospheric dinitrogen into ammonia in their symbiotic root nodules and this ammonia is rapidly assimilated into nitrogenous compounds for export out of the nodules. Because ammonia is toxic it needs to be rapidly assimilated, and in all of these processes, assimilation is carried out by the concerted action of two enzymes: Gln synthetase (GS) and Glu synthase (GOGAT) (Table I; Ireland and Lea, 1999). For the GS/GOGAT cycle to work, N metabolism must interact with C metabolism, since GS activity requires energy in the form of ATP and the GOGAT uses C skeletons and reductant in the form of 2-oxoglutarate (2OG) and reduced ferredoxin or NADH, respectively. This pathway is of crucial importance, since the Gln and Glu produced are donors for the biosynthesis of major N-containing compounds, including amino acids, nucleotides, chlorophylls, polyamines, and alkaloids (Lea and Ireland, 1999). Although many biochemical studies have improved our understanding of plant N metabolism, the exact enzymes involved and the factors controlling this process are still often lacking. Many enzymes involved in ammonium assimilation are found as multiple enzyme/isoenzyme forms that are often located in distinct subcellular compartments or within different organs and tissues. Whether these enzymes play overlapping (redundant) or distinct (non-redundant) roles is an important question. Recent data have begun to shed some light on this problem. This is relevant for different GS/GOGAT cycle enzymes but also when considering the enzymes producing 2OG, the major organic acid located at the interface between C and N metabolisms and essential for Glu synthesis. This key C compound is mainly derived from sugar respiration and amino acid transamination reactions and its production can be brought about by a number of different enzymes such as isocitrate dehydrogenases, aminotransaminases, and Glu dehydrogenases (GDHs; Table I). This Update gives a review of our current knowledge concerning the enzymes involved in plant ammonium assimilation and 2OG production for primary amino acid synthesis. It highlights the fact that the intermingling of C and N metabolisms is complex, involves the presence of several isoenzyme families, and the integrated functioning of both cytosolic and organelle-associated enzymes. Studies involving molecular genetics and plant engineering have produced much of the recent data presented in this Update that argue in favor of specific non-redundant roles for different enzymes. Furthermore, over the years, it has become evident that plant C and N metabolism are tightly coordinated and that certain signaling molecules like hormones and key C and N metabolites play an important role. Since 2OG levels can reflect C/N status within the cell, this metabolite could be involved in the monitoring and signaling of C/N balance to the plant regulatory machinery, and emerging evidence for this role will be discussed.