Nitrogen Assimilation and its Relevance to Crop Improvement

Abstract

The majority if not all of the organic nitrogen in plants is derived from the assimilation of ammonia into the amide position of glutamine by the enzyme glutamine synthetase (GS). A second enzyme, glutamate synthase, also known as glutamine:2‐oxoglutarate amidotransferase (GOGAT), carries out the transfer of the amide group of glutamine to 2‐oxoglutarate to yield two molecules of glutamate and thus completes the assimilation of ammonia into amino acids. This GS/GOGAT pathway of ammonia assimilation is of crucial importance for crop growth and productivity and ultimately animal and human nutrition. Glutamate dehydrogenase (GDH) is now considered to be only involved in glutamate catabolism to form ammonia, an important role in N recycling within the plant. Nitrogen is also often diverted from glutamine to asparagine as a temporary measure during periods of carbohydrate shortage and excess of reduced nitrogen. This diversion requires the action of asparagine synthetase in the anabolic reaction and asparaginase in the catabolic reaction. This chapter describes the properties of these enzymes in the assimilation and reassimilation of nitrogen and in particular the genes that encode them, their complexity and the time and place they are expressed. Plant transformation has allowed the construction of a range of plants with enhanced and decreased activity of several of these enzymes, some of which have shown improved agronomic performance.

Nitrate and Ammonium Affect the Overall Maize Response to Nitrogen Availability by Triggering Specific and Common Transcriptional Signatures in Roots

“…Furthermore, asparagine has been suggested to play a role in the response to many abiotic stresses, for example sulphur-and phosphate-deficiency, together with drought and salt stress [73,74]. Moreover, N can be redirected from glutamine to asparagine as a temporary measure to control excessive ammonium provision [75]. These results altogether further support the idea that ammonium nutrition could enhance stress tolerance, perhaps as a secondary effect of its slight toxicity, even though the mechanism(s) underlying this toxicity are not still completely understood [76,77].…”

Opposite fates of the purine metabolite allantoin under water and nitrogen limitations in bread wheat

“…Under N deficiency and natural senescence plants translocate available N from source tissues to sink tissues, such as young leaves (Masclaux-Daubresse et al 2010) and developing grains, accounting for 60-92% of total grain N (Barbottin et al 2005). The glutamine synthetase-glutamate synthase (GS-GOGAT) cycle plays an important role in this process since it recycles N liberated from the catabolism of N-rich macromolecules, such as protein and nucleic acids, into lowmolecular-weight organic compounds for long-distance N transport (Lea and Miflin 2010).…”

NADH-GOGAT Overexpression Does Not Improve Maize (Zea mays L.) Performance Even When Pyramiding with NAD-IDH, GDH and GS

“…Whether derived from nitrate, ammonium ions, N fixation, or generated by other reactions within the plant that release ammonium, most, if not all the N in a plant is channeled through the reactions catalyzed by the various cytosolic and plastidic GS isoenzymes. Ammonia, which is the ultimate form of inorganic N available to the plant, is then incorporated into an organic molecule through the reaction catalyzed by the GS enzyme to synthesize glutamine [11,12]. Improvement in NUE increased biomass and higher grain yield was obtained in several species such as rice, maize, barley, pea and poplar, transgenic plants that overexpressed GS1 or GS2, [13,14].…”