Plant Physiology - Musiienko M.M. 2001

Root nutrition of plants
Pathways of ammonia assimilation in plants

An important source of nitrogen Nutrition is ammonium nitrogen (NH₄). It enters the plant even faster than nitrates, as its utilization in the synthesis of Organic compounds bypasses the reduction process characteristic of nitrates. This is the primary and, likely, the only compound directly integrated into Nitrogen METABOLISM processes. Ammonium nitrogen can be of various origins: derived directly from the soil, formed via nitrate reduction, or resulting from the secondary Breakdown of Proteins in Aging Organs and Cells.

Ammonia (ammonium) is incorporated into metabolism through the synthesis of various Amino Acids and amides. The assimilation of ammonium relies heavily on The activity of three Key Enzymes: Glutamate dehydrogenase, Glutamine Synthetase, and glutamate synthase.

Ammonia can enter metabolic pathways through the amination of numerous compounds; however, the primary role in the initial binding of ammonia belongs to the Biosynthesis reactions of glutamic acid and its amide, glutamine. Another potential pathway is the reductive amination of a-ketoglutaric acid, catalyzed by glutamate dehydrogenase:

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Glutamate dehydrogenase is found in virtually all higher plants. In roots and leaves, it is localized predominantly in Cell/35.html">Mitochondria, although small amounts have also been detected in Chloroplasts. It should be noted that, theoretically, this enzyme could play a role in ammonium assimilation, though two factors argue against this. First, the localization of the enzyme in mitochondria suggests that its function here is dissimilation, since mitochondria normally oxidize glutamate. Second, enzyme kinetics research indicates that this enzyme has a low affinity for ammonium, which is inconsistent with an assimilatory function within The Cell, where physiological ammonium concentrations are considerably lower than this value. Consequently, it is now widely accepted that under normal conditions, ammonium is integrated into metabolism by glutamine synthetase and glutamate synthase:

Glutamine synthetase catalyzes the reaction in which glutamate Functions as an ammonia acceptor to form glutamine. Glutamate synthase catalyzes the reductive transfer of the amido-amino group from glutamine to 2-oxoglutarate, resulting in The formation of 2 molecules of glutamate. The electron Donors for this process are reduced ferredoxin or NADPH. Glutamine and glutamate serve as nitrogen sources not only for all protein amino acids but also for Nucleic Acids and amides:

Because ammonia is assimilated in the chloroplasts of leaves—where light energy drives ATP synthesis and ferredoxin reduction—ammonia assimilation is a truly photosynthetic process.

Glutamine synthetase is localized in the chloroplasts and Cytoplasm, catalyzing the formation of glutamine from glutamate with the simultaneous Cleavage of ATP to ADP. The pH optimum for this enzyme's activity depends on the metal ion involved in the reaction: approximately 8.0 for magnesium, and around 5.0 for manganese. The relative concentrations of these Metal Ions in the Active Site of the enzyme remain unknown.

Thus, once converted into an organic form, nitrogen can be utilized in various biosynthetic pathways, transported, or stored in different reserve forms "on demand." This is why Academician D. N. Pryanishnikov referred to ammonia as the "Alpha and Omega" of plant nitrogen metabolism. Secondary assimilation of ammonia into metabolism is also possible via NH₄⁺ released from Glycine during Photorespiration. This plays a crucial role in nitrogen conservation in plants.