Plants use nitrate and ammonia for nitrogen uptake through a process called nitrate assimilation. Two enzymes, nitrate reductase (NR) and nitrite reductase (NiR), play a crucial role in this process. They regulate how plants assimilate nitrogen. Factors from both inside and outside the plant affect how these enzymes work. Post-translational changes help prevent nitrite buildup. By using molecular biology techniques, scientists can create mutant and transgenic plants to study how NR and NiR are controlled.