(i) Rhizobium is nitrogen fixing bacterial symbiont of legumes, Rhizobia increase in number and attach with epidermis of roots. Root hairs curls and bacteria invade it. An infection thread is formed carrying the bacteria into the cortex of root.
(ii) Nodule formation starts in cortex of root. Bacteria is released from the thread into the cells which leads to formation of specialized nitrogen fixing cells.
(iii) Nodules establish direct vascular connection with host for exchange of nutrients.
(iv) Nodule contain all necessary biochemical components like enzyme nitrogenase and leghaemoglobin.
(v) Enzyme nitrogenase is a Mo-Fe protein and catalyses the conversion of atmospheric nitrogen into ammonia.
(vi) The reaction is as follows :
N2 + 8e- + 8H+ + 16ATP → 2NH3 + H2 + 16ADP +16Pi