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

Multiple interactions are involved in the formation of root nodules:

1) The Rhizobium bacteria divide and form colonies. These get attached to the root hairs and epidermal cells.

2) The root hairs get curled and are invaded by the bacteria.

3) 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.

4) As a result, the cells in the cortex and pericycle undergo division, leading to the formation of root nodules.

5) The nodules finally get connected with the vascular tissues of the roots for nutrient exchange.