The symbiotic relationship implies a signal exchange between the rhizobia and the legume that leads to the development of symbiotic structures.
The roots of the host legume plant secrete flavonoids which are recognized by the Rhizobia. In response, the rhizobia secrete nod factors recognized by the plant causing root hair deformation and cellular processes such as ion fluxes.
The rhizobia enter through the deformed root hair and form intracellular tubes called the infection thread. The infection triggers cell division in the root cortex where a new organ, the nodule, appears as a result of successive processes.
Infection threads grow to the nodule, infect its central tissue and release the rhizobia in these cells, where they differentiate morphologically into bacteroids and fix nitrogen from the atmosphere into a plant-usable form, ammoniates, proteins, and sufficient oxygen so as not to interfere with the fixation process. Leghaemoglobins ions (NH4+), using the enzyme nitrogenous. In return, the plant supplies the bacteria with carbohydrates are secreted which are plant proteins similar to human hemoglobin’s that help to provide oxygen for respiration while keeping the free oxygen concentration low enough not to inhibit nitrogenous activity.