what the steps involved in nodule formation?
The formation of root nodules involves multiple steps. The process may be summarized as follows:
1) Rhizobium, a bacteria divides itself to form colonies which gets attached to the root hair and the epidermal cells of the plant.
2) The bacteria invades the root hair making the roots hair to curl.
3) An infection thread is then formed which carries the bacteria into the cortex of the root.
4) The bacteria gets itself modified into rod shaped bacteroids.
5) Then the cells present inside the cortex and pericycle divide resulting in the formation of root nodules.
6) The root nodules then formed gets connected with the vascular tissues of the plant leading to nutrient exchange by the roots.
It is well explained through animation in the study material at the following link