If the plant can avoid these costs by taking up N from the soil, it will. Therefore, N fertilization inhibits nodule formation and N fixation. Infection by the Rhizobia bacteria is somewhat similar to an infection in the body. The bacteria enter through a susceptible location travel inward, establish themselves, and multiply. In the case of legumes, the bacteria are welcome as long as they are of a certain species that the plant somehow recognizes.
Infection enters through a root hair, then grows back to the base of the root hair.
Multiplication of the bacteria and enlargement of the root cells form a nodule. Inside the nodule, an enzyme called nitrogenase drives the N fixation reaction. Therefore, availability of this element in the soil is important to the legume. Nitrogenase is peculiar in that contact with oxygen ruins the enzyme. A sophisticated system to accomplish this involves a protein called leghemoglobin that is capable of binding to oxygen and removing it from the presence of nitrogenase.