Abstract
The process of nitrate assimilation is a very crucial pathway for the sustainable growth and productivity of higher plants. This process is catalysed by two enzymes, nitrate reductase and nitrite reductase. Both the enzymes differ from each other with respect to their structural organisation, subcellular location, catalytic efficiencies and regulatory mechanisms. Nitrate reductase catalyses the rate limiting step of nitrate assimilation process. The genes and proteins of this enzyme have been isolated and characterised from many higher plants. The additional role of NR in the production of nitric oxide has been also reported in last several years. The reduced ammonium is assimilated into carbon skeleton, ?-ketoglutarate, by the concerted action of glutamine synthetase and glutamate synthase. Glutamine and glutamate are the transportable forms of nitrogen among various tissues and metabolic processes. The rate of nitrate assimilation is regulated by the rate of uptake of nitrate by nitrate transporters, availability of carbon skeleton, accumulation of nitrogenous end products, light and the rate of photosynthesis. The partitioning of metabolites and resources between carbon and nitrogen metabolism is an important factor for the growth and yield of plants. During the last several decades excess use of nitrogen fertiliser has caused environmental pollution. Efforts have been made to increase the nitrogen use efficiency of plants to reduce the cost on fertiliser and nitrate pollution, increase the productivity and protein content of several commonly used crops. This review discusses the process of nitrate assimilation and its interaction with the carbon metabolism.
Highlights
Nitrogen is one of the most essential elements required by all types of organisms because it is a major component of proteins, nucleic acids and other biomolecules
The rate of nitrate assimilation is regulated by the rate of uptake of nitrate by nitrate transporters, availability of carbon skeleton, accumulation of nitrogenous end products, light and the rate of photosynthesis
Nitrogen assimilation depends on carbon metabolism for (i) the electron donors which are required for conversion of nitrate to ammonium [77], (ii) ketoglutarate as the carbon skeleton for ammonia assimilation by GS/GOGAT pathway, (iii) malate as a counter anion to prevent alkalinisation, (iv) carbon precursors for other amino acid pathways and (v) energy in the form of ATP for nitrate transport into the cell and formation of glutamine, the step catalysed by glutamine synthetase in GS/GOGAT cycle [78]
Summary
Nitrogen is one of the most essential elements required by all types of organisms because it is a major component of proteins, nucleic acids and other biomolecules. The major types of nitrogen forms which can be used by plants include ammonia, mineral N (nitrates and ammonium), oxides of nitrogen (Nox) and amino acids and peptides [1]. The assimilatory nitrate reduction pathway is a very important physiological process, as it is one of the major routes by which inorganic nitrogen is assimilated into carbon skeleton in higher plants, algae including cyanobacteria and fungi.
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