Describe biological nitrogen fixation with reference to Rhizobium and Legume.

Explanation

Symbiotic bacterium like Rhizobium fixes atmospheric nitrogen. Cyanobacteria found in Lichens, Anthocemos, Azolla and coralloid roots of Cycas also fix nitrogen. Non-symbiotic (free living bacteria) like Clostridium also fix nitrogen.
Symbiotic nitrogen fixation:
Nitrogen fixation with nodulation: Rhizobium bacterium is found in leguminous plants and fix atmospheric nitrogen. This kind of symbiotic association is beneficial for both the bacterium and plant. Root nodules are formed due to bacterial infection. Rhizobium enters into the host cell and proliferates, it remains separated from the host cytoplasm by a membrane.
Stages of Root nodule formation:
(i) Legume plants secretes phenolics which attracts Rhizobium.
(ii) Rhizobium reaches the rhizosphere and enters into the root hair, infects the root hair and leads to curling of root hairs.
(iii) Infection thread grows inwards and separates the infected tissue from normal tissue.
(iv) A membrane bound bacterium is formed inside the nodule and is called bacteroid.
(v) Cytokinin from bacteria and auxin from host plant promotes cell division and leads to nodule formation
Non-Legume: Alnus and Casuarina contain the bacterium Frankia. Psychotria contains the bacterium Klebsiella.
Nitrogen fixation without nodulation.
The following plants and prokaryotes are involved in nitrogen fixation.
Lichens - Anabaena and Nostoc
Anthocemos - Nostoc
Azolla - Anabaena azollae
Cycas - Anabaena and Nostoc.