Steps of nodule formation
1. Rhizobium bacteria present in soil contact a susceptible root hair.
2. Infection of the root hair cause it to curve and deformed due to chemical secretion.
3. An infection thread is produced carrying the bacteria into the cortex of the root.
4. The bacteria get modified into rod shaped bacteria and cause inner cortical and pericycle cells to divide, plant produce cytokinin and auxin to stimulate cell division and enlarge to form nodules.
5. Division and growth of cortical and pericycle cells lead to nodules formation.