Root nodule formation in legume

Root nodule formation:-

             Rhizobium bacteria stimulate leguminous plants to develop root nodules which the bacteria infect and inhabit. The nodules develop in a complex series of steps that include root hair penetration and infection thread formation followed by entry of Rhizobia through infection or sides of lateral root emergence.

1) Rhizosphere organisms (Rhizobia) convert Tryptophan in the root exurates into IAA which results in the curling of root hairs.

2) The capsular polysaccharides of Rhizobia play a role in inducing the site of infection.

3) Invagination of the root hair takes place due to the production of polygalactourinase by Rhizobia.

4) After this, infection threads are formed through which Rhizobia enter the host.

5) When the infection thread reaches a cell deep in the cortex, it bursts out and the bacteria are liberated out in the cytoplasm.

6) Each bacterium so liberated out gets surrounded by a membrane pinched off from the infection thread.

7) Mitotic division of the host cell results in the development of the nodule, bacteria multiply rapidly and surrounded by membrane envelop.

8) Motile Rhizobia enlarges in size and loose their flagella and power off division. This state of bacteria is known as bacteroids.

9) The space between the bacteroids and the membrane gets filled with leg haemoglobin. Leg - haemoglobin helps bacteria in synthesis of ATP and nitrogenous activity. N₂ fixation takes place in this nodule and the effective nodules are pink in colour.

Nitrogenous activity:-

            Biological N₂ fixation is represented by the equation in which two moles of NH₃ are produced from 1 mole of N₂ gas at the expense of 16 moles of ATP and a supply of electrons and protons.

N₂ + 8H⁺ + 8e^- + 16ATP    2NH₃ + H₂ + 16ATP + 16Pi

            This reaction is performed exclusively by prokaryotes using an enzyme complex called Nitrogenase. This enzyme consists of two proteins i.e. an iron protein ( nitrogenase reductase ) and a Molybdenum - iron protein. The reaction occur while N₂ is bound to the nitrogenase enzyme. The iron - protein is first reduced by ferredoxin. Then the reduced iron protein binds with ATP and reduces Mo - Fe protein which donates electron to N₂ - producing HN=NH. If further reduced, it produces H₂N-NH₂ is produced and this in turn is reduced to two moles of NH₃.