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Index >> Nitrogen Fixation Free Living and Associative Symbiotic Bacteria >> Nitrogen Fixation by Rhizobium in a Free-living State

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Nitrogen Fixation in Free-Living and Associative Symbiotic Bacteria Nitrogen Fixing Bacteria Nitrogenase Genetics Mechanism of Nitrogen Fixation Response of Plants to Azotobacter Inoculation Nitrogen Fixation in the Root Zone of Rice

Nitrogen Fixation by Rhizobium in a Free-living State

	Nitrogen Fixation by6 Rhizobium in a Free-living State It was generally agreed upon that Rhizobium can fix atmospheric nitrogen only in the root nodules of legumes and that too when it is in the bacteroid stage of its life cycle. All attempts to obtain nitrogen fixation by pure cultures of Rhizobium with or without extracts of host plants failed in the past and it was postulated that some of the genes determining Rhizobium's ability to fix atmospheric nitrogen (nif genes) reside in the host plant and hence the need for symbiosis between the host and the bacterium in the bacteroid tissue of the root nodule.
Interesting experimental findings have emerged some years ago which show that Rhizobium possesses the entire complement of genes for nitrogen-fixation which is normally latent and become active only under special condition. One of the first evidences came from the successful transfer of nif genes from Rhizobium trifolii to a non-nitrogen-fixing strain of Klebsiella aerogenes. Secondly, a strain of Rhizobium sp. cowpea group was induced to fix atmospheric nitrogen in the presence of diffusible substances from callus tissue of leguminous as well as non-leguminous as well as non-leguminous plants, thereby indicating that plant tissues contain some substances which stimulate bacteria to fix nitrogen.
The third set of evidences have come forth simultaneously from two groups of Australian and one group of Canadian workers. A simple synthetic medium was evolved which contains. Besides other ingredients, certain key metabolites such as a pentose sugar (arabinose or xylose), a dicarboxylic acid (such as succinate) and a .relatively small amount of fixed nitrogen (such as glutamine, glutamate or nitrate) to induce nitrogen fixation by a strain of cowpea type of Rhizobium in a free-state on a solid agar medium. The ability of fix nitrogen has been verified by C₂H₂ reduction test as well as by 15N enrichment procedures.
These results lead us to believe that we may have to reconsider the recognition of legume-Rhizobium association within the nodules as a true instance of symbiosis. In a more restricted way, the term symbiosis is valid since the specialised structure of nodule may be meant only to restrict the access of oxygen to rhizobia for proper functioning of the enzyme nitrogenase.

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Nitrogen Fixing Activity of Paddy Soil in Pots under Various Treatments Nitrogen Fixation in the Rhizosphere of Grasses and Weeds Associative Symbiosis Some New Nitrogen Fixing Bacterial Associations Several Nitrogen Fixing Bacteria Response of Plants Azospirillum Inoculation The Concept of C4 Dicarboxylic Acid Pathway Nitrogen Fixation by Rhizobium in a Free Living State

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