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MODULE 3 : PROCESS OF NODULE FORMATION, ROLE OF Nif and Nod GENES IN BIOLOGICAL NITROGEN FIXATION, ENZYME NITROGENASE ACTIVITY  Process of Nodule formation The role of legumes in enriching the fertility of soil by contributing nitrogen through symbiotic nitrogen fixation...

MODULE 3 : PROCESS OF NODULE FORMATION, ROLE OF Nif and Nod GENES IN BIOLOGICAL NITROGEN FIXATION, ENZYME NITROGENASE ACTIVITY  Process of Nodule formation The role of legumes in enriching the fertility of soil by contributing nitrogen through symbiotic nitrogen fixation by Rhizobium is well established since ancient times. Nodulation – The free living Rhizobia infects the host legume through infection thread or at the point of emergence of lateral roots and are transferred into bacteroids which are the site of Nitrogen fixation. The root cap cells secrete large amount of polysaccharides forming mucigel in which Rhizobia are found enmeshed in large numbers. Curling and Branching of root hairs is the first visible plant response to Rhizobia. Rhizobium produced localised auxin at the root hair surface which softens the cell wall for easy entry of infection thread for transfer of bacterial cell into root cortex. Thread grows with growth of root hair itself. Then Rhizobia get trapped with in the deformed root hair, inducing cell wall synthesis and enclose the Rhizobia and finally forms nodule.  The nodule is simply a protective structure and that bacteroids are the seats of N fixation. Nitrogen fixation is anaerobic process. In this process N2 is reduced to NH3 with the help of enzyme nitrogenase. Enzyme nitrogenase is made up of two components one with Fe + Mo and second without Mo. The oxygen supply to bacteroids is excluded due to presence of leghaemoglobin around it. The leghaemoglobin controlled the oxygen supply and helps in low oxygen conditions near the bacteroids thus protect enzyme nitrogenase which is highly sensitive to oxygen. The first intermediate compound of N fixation is ammonia.  Nitrogen fixation The phenomenon of conversion of free nitrogen (molecular and elemental) into nitrogenous compounds (to make it available to the plants for absorption) is called nitrogen fixation. Nitrogen fixation is carried out by physicochemical and biological means. About 10% of natural nitrogen fixation takes place by physicochemical methods and 90% by biological methods. There are two methods via which N2 is fixed A) Physical nitrogen fixation viz... sub divided into two i) Natural Nitrogen Fixation ii) Industrial Nitrogen fixation B) Biological Nitrogen fixation