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Photosynthetic Nitrogen Assimilation: Inter-Pathway Control and Signaling

  • Christine H Foyer
  • Graham Noctor
Part of the Advances in Photosynthesis and Respiration book series (AIPH, volume 12)

Summary

The leaf is the predominant site of nitrogen assimilation in many crop species, and the stimulation of nitrogen assimilation by light reveals a close dependence on photosynthesis. Light controls the activity of nitrate reductase and, directly or indirectly, provides the reducing power necessary for the reductive incorporation of nitrate into amino groups. Photosynthetic and respiratory carbon metabolism is also required to generate the carbon skeletons necessary for amino acid synthesis. Amino acids represent the hub around which revolve the processes of nitrogen assimilation, associated carbon metabolism, photorespiration, export of organic nitrogen from the leaf, and the synthesis of nitrogenous end-products. Specific major amino acids are modulated differentially by photorespiration and nitrogen assimilation, even though these processes are tightly intermeshed. Minor amino acids show marked diurnal rhythms and their contents fluctuate in a co-ordinated manner. We discuss how regulation of the expression and activity of key enzymes allows co-ordination of carbon and nitrogen assimilation, and we assess the relative roles of key ‘sensors’ of Carbon-Nitrogen status. Analysis reveals a complex network of controls brokered by an interplay of signals emanating from nitrate, carbohydrates, key metabolites such as glutamine, and plant hormones. In particular, abscisic acid is clearly implicated in the sensing of sugars and nitrate and associated signaling in higher plants. These controls act not only to orchestrate the activities of carbon and nitrogen assimilation at the intracellular level, but also influence plant development. The integrated perception of signals from hormones, nitrate, sugars, organic acids, and amino acids permits the plant to tailor its capacity for nitrogen assimilation to nutrient availability and requirements.

Abbreviations

2-OG — 2-oxoglutarate ABA — abscisic acid Ala — alanine Asp — aspartate Fd — ferredoxin GCN — general control non-reversible GDH — glutamate dehydrogenase Gln — glutamine Glu — glutamate GOGAT—glutamine: 2-oxoglutarate aminotransferase (glutamate synthase) GS — glutamine synthetase GS1 — cytosolic glutamine synthetase GS2 — chloroplastic glutamine synthetase ICDH — isocirate dehydrogenase LR — lateral root NR — nitrate reductase OAA — oxaloacetate PDH — pyruvate dehydrogenase PEPc — phosphoenolpyruvate carboxylase PK — pyruvate kinase Ser — serine SNF1 — yeast sucrose non-fermenting control gene encoding a protein kinase SnRK — SNF-related protein kinase SPS — sucrose phosphate synthase TCA — tricarboxylic acid 

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Copyright information

© Kluwer Academic Publishers 2002

Authors and Affiliations

  • Christine H Foyer
    • 1
  • Graham Noctor
    • 2
  1. 1.Crop Performance and Improvement DivisionInstitute of Arable Crops Research, RothamstedHarpendenUK
  2. 2.Institut de Biotechnologie des Plantes, Bâtiment 630Université de Paris XIOrsay cedexFrance

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