Nodule formation : (a) It includes the reactions between Rhizobium and roots of host plant. Rhizobia multiply, (b) They colonise the surrounding of the roots. They get attached to root epidermal hair cells,
(c) A root hair curls and the bacterium invades into it. An infection thread is formed carrying it into root jcortex: (d) The bacteria are then released from the thread into cells and it causes the differentiation of N2 fixing cells. Nodule formation occurs.