Plant growth and development depends heavily on availability of nitrogen. Nitrogen is available in rhizosphere in different forms depending on the soil condition. Nitrate in aerobic soil and ammonium in acidic soils or flooded wetlands. Root exudates modify local chemical environment and microbial activity. Roots can release oxygen and other exudates which modify local redox potential and the activity of microbial population. These modifications interconvert various soil nitrogen forms. Plants have high and low affinity nitrate transporters to deal with different concentrations of nitrogen forms.
These systems are constitutive and nitrate inducible. Various membrane proteins are involved in nitrate uptake, translocation, compartmentalization and remobilization. Nitrate uptake is affected by nitrate, plant nitrogen status and plant carbohydrate status. Major part of absorbed nitrogen is transported to shoot and the remaining is assimilated in the roots itself. The ammonium ions are actively taken up by the roots via ammonium transporters. Ammonium is then assimilated into the amide residue of glutamine (Gln) by the reaction of glutamine synthetase (GS) in the roots. In roots, nitrate is first reduced to nitrite by nitrate reductase in cytoplasm then to ammonium by nitrite reductase in plastids. Glutamine synthetase (GS) and Asparagines synthetase (AS) play important role nitrogen metabolism. GS reduces nitrite to ammonium in plastids and cytoplasm. Ammonium is then assimilated into amino acids by GS/glutamine-2-oxoglutarate amino-transferase (GOGAT) cycle. Various amino acids are synthesized by transferring the amino group of glutamate by aminotransferases. AS synthesizes asparagine and glutamine from aspartate and glutamate respectively. In addition C skeletons synthesized in photosynthesis are required to assimilate nitrogen. Nitrate transporters (NRT1s) play important role in the loading and unloading from root to shoot vascular system.
In Arabidopsis thaliana, AtNRT1.5 and AtNRT1.8 low affinity nitrate transporters are involved in loading and unloading in root from shoot. Nitrogen stored in leaves during vegetative stage is remobilized to developing seeds as amino acids during senescence. About 95 % of seed protein is derived from remobilized amino acids from seed after degradation of proteins in leaves. Asn and Gln play key role N remobilization from senescing leaves. Glutamate has a central role in amino acid metabolism in plants. The imbalance between nitrogen accumulation and nitrogen assimilation in plants is the important reason for N loss from aerial parts of plants.
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