Publisher Summary
This chapter focuses on ammonia assimilation and discusses the mechanisms by which the ammonia produced is incorporated into organic compounds. Nitrate and dinitrogen gas, two of the major sources of inorganic nitrogen available to plants, are reduced to ammonia before the nitrogen is incorporated into organic matter. Ammonia is released and reassimilated in large amounts at different stages of the plant's metabolism. Nitrate is the major source of inorganic N available to the leaves of most land plants; ammonia is not present in the xylem stream leaving the roots. Thus, the ammonia assimilated is normally generated in situ from the reduction of nitrate through nitrite. Ammonia is not normally transported into a leaf but is rather a product of nitrate reduction; however, it can also be generated within the leaf by two other ways: (1) by the breakdown of asparagine through transaminase or asparaginase and (2) in photorespiration.

4 – Ammonia Assimilation

The CO2 released in photorespiration is believed to result from a complex mitochondrial reaction in which glycine is converted to equimolar amounts of CO2, NH3 and the C-1 group of N5,N10-methylene-tetrahydrofolate 1–3. Because photorespiratory CO2 production may amount to 80 μmol per h per g fresh weight4,5 the stoichiometry of the decarboxylation reaction indicates that NH3 release from glycine exceeds primary NO3− reduction6,7. Leaf cells must therefore be capable of rapid NH3 reassimilation in photorespiratory conditions. It has been suggested8 that NH3 could be refixed directly into glutamate by mitochondrial glutamate dehydrogenase (GDH), utilizing the NADH generated during glycine decarboxylation1,9,10. This seems unlikely11,12 because of the high Km for NH3 exhibited by GDH in vitro. An alternative suggestion11,12, that photorespiratory NH3 could be re-assimilated into glutamate by the sequential action of cytoplasmic glutamine synthetase (GS) and the chloroplast enzyme glutamate synthase (GOGAT)13,14, is supported by circumstantial evidence from in vitro studies12,15. Here we provide direct evidence for such a pathway, based on the results of experiments with mutants of Arabidopsis thaliana (L.) Heynh which are deficient in leaf GOGAT activity.

Inhibition of photosynthesis in Arabidopsis mutants lacking leaf glutamate synthase activity

Revue des resultats recents obtenus sur le role des nitrates en tant que compose osmotique intracellulaire, sur le bilan energetique de l'assimilation des nitrates par les tiges ou les racines et sur le cout energetique comparatif en tant que facteur de l'ecophysiologie des vegetaux

Nitrate assimilation and translocation by higher plants: Comparative physiology and ecological consequences

Role and regulation of L-glutamate dehydrogenase activity in higher plants

Chloroplasts of maize (Zea mays) leaves differentiate into specific bundle sheath (BS) and mesophyll (M) types to accommodate C4 photosynthesis. Consequences for other plastid functions are not well understood but are addressed here through a quantitative comparative proteome analysis of purified M and BS chloroplast stroma. Three independent techniques were used, including cleavable stable isotope coded affinity tags. Enzymes involved in lipid biosynthesis, nitrogen import, and tetrapyrrole and isoprenoid biosynthesis are preferentially located in the M chloroplasts. By contrast, enzymes involved in starch synthesis and sulfur import preferentially accumulate in BS chloroplasts. The different soluble antioxidative systems, in particular peroxiredoxins, accumulate at higher levels in M chloroplasts. We also observed differential accumulation of proteins involved in expression of plastid-encoded proteins (e.g., EF-Tu, EF-G, and mRNA binding proteins) and thylakoid formation (VIPP1), whereas others were equally distributed. Enzymes related to the C4 shuttle, the carboxylation and regeneration phase of the Calvin cycle, and several regulators (e.g., CP12) distributed as expected. However, enzymes involved in triose phosphate reduction and triose phosphate isomerase are primarily located in the M chloroplasts, indicating that the M-localized triose phosphate shuttle should be viewed as part of the BS-localized Calvin cycle, rather than a parallel pathway.

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Functional Differentiation of Bundle Sheath and Mesophyll Maize Chloroplasts Determined by Comparative Proteomics

Studies on Glutamate Synthase from the Leaves of Higher Plants

The activities of nitrate reductase (EC1.6.6.1), nitrite reductase (EC 1.6.6.4), glutamine synthetase (EC6.3.1.2), glutamate synthase (EC1.4.7.1) and NAD(P)H-dependent glutamate dehydrogenase (EC 1.4.1.3) were investigated in mesophyll and bundle sheath cells of maize leaves (Zea mays L.). Whereas nitrate and nitrite reductase appear to be restricted to the mesophyll and GDH to the bundle sheath, glutamine synthetase and glutamate synthase are active in both tissues. During the greening process, the activities of nitrate and nitrite reductase increased markedly, but glutamine synthetase, glutamate synthase and glutamate dehydrogenase changed little.

E. Harel

Papers

Studies on Glutamate Synthase from the Leaves of Higher Plants

The localisation of enzymes of nitrogen assimilation in maize leaves and their activities during greening.