Assimilation of Nitrates in Plants

Abstract

Nitrogen is one of the soil macronutrient elements commonly required by higher plants. Plant growth and development are predominantly regulated by nitrogen assimilation. Most plants acquire their nitrogen from the soil in the form of nitrates, which are derived from either fertilizers or the mineralization of indigenous organic matter. The activities of micro-organisms make the nitrogen accessible to plants and because of these soils may show a striking seasonal discrepancy in the supply and form of readily available nitrogen. The primary route for the introduction of nitrogen into the biosphere is the assimilation of ammonia by a wide variety of organisms (diazotrophs). The nitrogen first gets converted into nitrate and then into NH4 +, by the sequential reductive action of the plant enzymes nitrate reductase and nitrite reductase, respectively. For many plants, some nitrate is taken up and assimilated via roots, but the larger part is transported to the shoot, where it is first reduced to nitrite in the cytoplasm by the activity of nitrate reductase and then further to ammonium by nitrite reductase in the plastids. Inorganic nitrogen is assimilated into the various amino acids like glutamine, glutamate, asparagine, and aspartate, which serve as important nitrogen carriers in plants. The enzymes glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), aspartate aminotransferase (AspAT), and asparagine synthetase (AS) catalyze the process and are responsible for the biosynthesis of these nitrogen carrying amino acids. Nitrogen is sequestered from organic compounds like ammonia and reassimilated during the movement of nitrogen around the plant, from seed reserves, through transport to vegetative organs, to eventual redeposition in a new crop of seeds. Recent molecular analyses depict that each enzyme is encoded by a particular gene family and every enzyme is differentially regulated by various factors environmental stimuli, metabolic control, developmental control, and tissue or cell-type specificity.