Ammonia assimilation roots

It is generally agreed that nitrate is the major source of inorganic N available to the leaves of most land plants ammonia is not considered to be present in the xylem stream leaving the roots (Pate, 1971). Thus the ammonia assimilated is normally generated in situ from the reduction of nitrate via nitrite. Consequently experiments in which ammonia is fed to detached leaves or parts of leaves probably do not fully reflect the path of the in vivo metabolism of nitrate to amino acids. 

Finally, what areas remain important for future research Prediction is always a dangerous business but to us the key questions of ammonia assimilation all revolve around carbon metabolism—what are the sources for the carbon skeletons of amino acids that are synthesized in the chloroplast How do the anaplerotic sequences that provide a ready supply of 2-oxoglutarate and oxoloacetate operate and how are they controlled What are the critical steps that ensure, in times of plentiful ammonia supply, that sufficient carbohydrates are transported and oxidised And perhaps most interestingly of all, how are the electrons needed to reduce and assimilate N2 in nodules and NO in roots generated from carbohydrate transported from the shoot ... 

Seeds of pea (Thurman et al., 1965) and Medicago sativa (Hartmann et al., 1973) possess 7 GDH isoenzymes (pattern 1) which decrease in number during germination, while a new pattern of isoenzymes (pattern 2) becomes visible on gel electrophoresis. Pattern 1 is characteristic of the seed, pattern 2 is characteristic of the root, while the shoot possesses some pattern 1 and some pattern 2 isoenzymes. Prior to the discovery of the GS/GOGAT pathway in higher plant tissues, it was postulated that pattern 1 enzymes were involved in deamination reactions, while pattern 2 enzymes were involved in ammonia assimilation. 

The preceding description of nodule initiation, development, and nitrogen-ase clearly demonstrates that the host contribution to symbiosis can be grouped into several functions, including recognition, root hair invasion, infection thread formation, nodule differentiation (e.g., meristems, vascular elements, sclerenchyma), carbon assimilation, organic acid production, ammonia assimilation, maintenance of low O2 by Lb, separation of bacteroids and host cytoplasm by the PBM, senescence, and possibly suppression of host defense responses required for nodule compatibility and/or effectiveness. 

Molecular mechanisms of nitrate accumulation depend not only on the nitrate reductase system, but also on the ability of roots to take from the soil, nitrate or ammonium ions, and on the plant s capacity for their conversion by assimilation processes to higher products. Besides this, the assimilation depends on the ability of a given genotype to transport substances necessary for the synthesis. It was shown that genotype differences of the nitrate reductase level do not depend on the nitrate content in tissues [25]. Nitrates are accumulated in plant organisms at high concentrations when aU the nitrogen accepted cannot be utilized for the production of amino acids and for subsequent protein synthesis [26]. This occurs when the plant, in the course of its metabolism, is unable to reduce the accepted nitrates into the assimilable ammonia form. 

Ultrastructure and Metabolism of the Developing Legume Root Nodule J. G. Robertson and K. j. F. Famden Nitrate and Nitrite Reduction Leonard Beevers and Richard H. Hageman Ammonia Assimulation B. J. Miflin and P. J. Lea Assimilation of Inorganic Sulfate into Cysteine J. W. Anderson... 

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