What are the steps involved in formation of a root nodule?

Multiple interactions are involved in the formation of root nodules. The Rhizobium bacteria divide and form colonies. These get attached to the root hairs and epidermal cells. The root hairs get curled and are invaded by the bacteria. This invasion is followed by the formation of an infection thread that carries the bacteria into the cortex of the root. The bacteria get modified into rod-shaped bacteroides. As a result, the cells in the cortex and pericycle undergo division, leading to the formation of root nodules. The nodules finally get connected with the vascular tissues of the roots for nutrient exchange.

Development of root nodules happens in following steps: 

a. Rhizobium bacteria contact a susceptible root hair and divides near it. 

b. Successful infection of the root hair results in curling of the root hair.

c. The infected thread carries the bacteria to the inner cortex. The bacteria get modified into rodshaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation. 

d. A mature nodule is complete with vascular tissues. The vascular tissues of the nodule are continuous with those of the root.

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.