Plants assimilate nitrogen from the soil mainly in the form of nitrate NO3- and ammonium NH4+ . The process of nitrogen assimilation involves several biochemical pathways and enzymes that help plants convert these inorganic nitrogen sources into organic compounds such as amino acids, nucleotides, and other nitrogen-containing biomolecules.1. Nitrate assimilation:The first step in nitrate assimilation is the uptake of nitrate from the soil by specific transporters present in the root cells. Once inside the plant cell, nitrate undergoes a two-step reduction process to form ammonium.a Nitrate reduction to nitrite: Nitrate is reduced to nitrite NO2- by the enzyme nitrate reductase NR . This reaction requires NADH or NADPH as an electron donor.NO3- + NAD P H + H+ NO2- + NAD P + + H2Ob Nitrite reduction to ammonium: Nitrite is further reduced to ammonium by the enzyme nitrite reductase NiR . This reaction takes place in the chloroplasts and requires reduced ferredoxin as an electron donor.NO2- + 6 Fd_red + 8 H+ NH4+ + 6 Fd_ox + 2 H2O2. Ammonium assimilation:Ammonium can be directly taken up by plant roots or generated through nitrate assimilation. Ammonium is incorporated into organic molecules through two main pathways: the glutamine synthetase-glutamate synthase GS-GOGAT cycle and the glutamate dehydrogenase GDH pathway.a GS-GOGAT cycle: This is the primary pathway for ammonium assimilation in plants. It involves two enzymes: glutamine synthetase GS and glutamate synthase GOGAT .i Glutamine synthetase GS catalyzes the ATP-dependent condensation of ammonium with glutamate to form glutamine.Glutamate + NH4+ + ATP Glutamine + ADP + Piii Glutamate synthase GOGAT transfers the amide group from glutamine to 2-oxoglutarate, generating two molecules of glutamate. This reaction requires reduced ferredoxin or NADH as an electron donor.Glutamine + 2-oxoglutarate + NAD P H + H+ 2 Glutamate + NAD P +b Glutamate dehydrogenase GDH pathway: This pathway plays a minor role in ammonium assimilation and mainly functions in the catabolism of amino acids. GDH catalyzes the reversible reaction between glutamate and 2-oxoglutarate, with ammonium as a substrate.Glutamate + NAD P + 2-oxoglutarate + NH4+ + NAD P HOnce ammonium is incorporated into glutamate and glutamine, it can be further transferred to other amino acids and nitrogen-containing biomolecules through various transamination and biosynthetic reactions.