Nitrogen is one of the most important macronutrients required by the plant. All proteins consist of nitrogen-containing amino acids. Heme component of the chlorophylls, nitrogenous bases of DNA and RNA, and phenylpropanoids (such as flavonoids) are all nitrogen-containing biomolecules (Fig. 11.1). Plants absorb carbon in the form of CO2 fixed by photosynthetic process; hydrogen and oxygen are taken up in the form of H2O. Oxygen is also absorbed from the air. In contrast, plants are not able to utilize molecular nitrogen directly. In spite of the fact that nitrogen constitutes almost 80% of the air, it needs to be provided to plants in the form of fertilizers. There are some prokaryotes, however, which have the ability to fix molecular nitrogen by means of biological fixation leading to enhancement of soil nitrogen as well. Thus, it becomes necessary to study the forms of nitrogen which plants can absorb, mechanisms of their uptake, biological nitrogen fixation, and also the mechanisms involved in their further metabolism. Plants absorb nitrogen through their roots either in the form of ammonium ions or nitrates which are available in the soil. Nitrate uptake can take place through leaves, only in case of epiphytes or only through foliar spray of fertilizers. Molecular nitrogen can be fixed directly by the nitrogen-fixing prokaryotes (diazotrophs), which are either free living or growing in symbiotic associations with plants. When uptake of nitrogen by plants is in the form of nitrate, it needs to be reduced to ammonium before further assimilation for amino acid biosynthesis, followed by biosynthesis of purines and pyrimidines. Within plants nitrogen is transported in the form of nitrates, amino acids, amines, and ureides. This chapter shall focus on nitrogen uptake in the form of ammonium and nitrate ions, mechanisms and the enzymes involved in their reduction, physiology and biochemistry of molecular nitrogen fixation, ammonium assimilation, transport of the assimilated nitrogen, and finally amino acid biosynthesis (Fig. 11.2).
KeywordsAmides Aminotransferases Nitrate reductase Nitrogenase Oxidative deamination Reductive amination Transamination Ureides
Suggested Further Readings
- Browsher C, Steer M, Tobin A (eds) (2008) Plant biochemistry. Garland Science, Tailor & Francis Group, New York, pp 237–300Google Scholar
- Long SR, Kahn M, Seefeldt L, Tsay Y, Kopriva S (2015) Nitrogen and Sulfur. In: Buchanan BB, Gruissem W, Jones RL (eds) Biochemistry and molecular biology of plants. Wiley-Blackwell, Chichester, pp 711–745Google Scholar
- Jones R, Ougham H, Thomas H, Waaland S (2013) The molecular life of plants. Wiley-Blackwell, Chichester, pp 463–475Google Scholar
- Taiz L, Zieger E, Moller IM, Murphy A (2015) Plant physiology and development, 6th edn. Sinauer Associates, Inc. Publishers, Sunderland, pp 354–367Google Scholar