Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant. Principle stages in the nodule formation are summarised as follows. Rhizobia multiply and colonise the surroundings of roots and get attached to epidermal and root hair cells. The root-hairs curl and the bacteria invade the root-hair.
An infection thread is produced carrying the bacteria into the cortex of the root, where they initiate the nodule formation in the cortex of the root. Then the bacteria are released from the thread into the cells which leads to the differentiation of specialised nitrogen fixing cells. The nodule thus formed establishes a direct vascular connection with the host for exchange of nutrients.