Changing Activities of Nitrogen-assimilating Enzymes During Growth and Senescence of Dwarf Beans (Phaseolus vulgaris L.)
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Glutamate synthase
Nitrogen Assimilation
The enzymes involved in the assimilation of ammonia by free-living cultures of Rhizobium spp. are glutamine synthetase (EC. 6.o.I.2), glutamate synthase (L-glutamine:2-oxoglutarate amino transferase) and glutamate dehydrogenase (ED I.4.I.4). Under conditions of ammonia or nitrate limitation in a chemostat the assimilation of ammonia by cultures of R. leguminosarum, R. trifolii and R. japonicum proceeded via glutamine synthetase and glutamate synthase. Under glucose limitation and with an excess of inorganic nitrogen, ammonia was assimilated via glutamate dehydrogenase, neither glutamine synthetase nor glutamate synthase activities being detected in extracts. The coenzyme specificity of glutamate synthase varied according to species, being linked to NADP for the fast-growing R. leguminosarum, R. melitoti, R. phaseoli and R. trifolii but to NAD for the slow-growing R. japonicum and R. lupini. Glutamine synthetase, glutamate synthase and glutamate dehydrogenase activities were assayed in sonicated bacteroid preparations and in the nodule supernatants of Glycine max, Vicia faba, Pisum sativum, Lupinus luteus, Medicago sativa, Phaseolus coccineus and P. vulgaris nodules. All bacteroid preparations, except those from M. sativa and P. coccineus, contained glutamate synthase but substantial activities were found only in Glycine max and Lupinus luteus. The glutamine synthetase activities of bacteroids were low, although high activities were found in all the nodule supernatants. Glutamate dehydrogenase activity was present in all bacteroid samples examined. There was no evidence for the operation of the glutamine synthetase/glutamate synthase system in ammonia assimilation in root nodules, suggesting that ammonia produced by nitrogen fixation in the bacteroid is assimilated by enzymes of the plant system.
Glutamate synthase
Nitrogen Assimilation
Rhizobium leguminosarum
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summary In order to investigate the relative contribution of glutamine synthetase and NADP‐glutamate dehydrogenase to the assimilation of ammonium (NH 4 + ) by spruce ectomycorrhizas, changes in free amino acid content and kinetics of 15 N incorporation into free amino acids were measured together with the effect of specific enzyme inhibitors. Exposure of detached ectomycorrhizas to ( 15 NH 4 ) 2 SO 4 showed that the greatest flow of 15 N enters into the amido group of glutamine. Label was also detected in glutamic acid, alanine and γ‐aminobutyric acid. Large amounts of alanine and glutamate accumulated in response to the addition of methionine sulfoximine (MSX) together with a decrease in 15 N incorporation into both amido‐ and ammo‐nitrogen of glutamine. These results are consistent with a major role of glutamate dehydrogenase and glutamine synthetase in nitrogen assimilation in the symbiosis and do not suggest any significant role for glutamate synthase in the synthesis of glutamate. A large accumulation of unlabelled asparagine in response to MSX and albizziine inhibition suggests the occurrence of an unlabelled NH 4 + pool in the host plant. The transfer of nitrogen compounds between the fungal cells and the host tissues is discussed.
Glutamate synthase
Nitrogen Assimilation
Asparagine synthetase
Alanine
Glutamic acid
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Glutamate synthase
Nitrogen Assimilation
Metabolic pathway
Assimilation (phonology)
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Glutamate synthase
Nitrogen Assimilation
Nitrogen Cycle
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Glutamate synthase
Nitrogen Assimilation
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Glutamate synthase
Nitrogen Assimilation
Oxidative deamination
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SUMMARY Naturally occurring grassland plants possess the enzymic potential for assimilation of ammonium via the glutamate dehydrogenase and glutamine synthetase/glutamate synthase pathways. In the majority of species, high exogenous ammonium concentrations enhanced the levels of root and shoot GDH and reduced the levels of root glutamine synthetase. However, the enzyme response to increasing nitrate or ammonium availability was relatively small and suggested that these species are not adapted for major changes in the form or availability of nitrogen.
Glutamate synthase
Nitrogen Assimilation
Assimilation (phonology)
Nitrogen Cycle
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Glutamate synthase
Nitrogen Assimilation
Asparagine synthetase
Assimilation (phonology)
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Nitrogen Assimilation
Glutamate synthase
Assimilation (phonology)
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Glutamate synthase
Nitrogen Assimilation
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Citations (23)