Question

……bacterium is found in the root of the leguminous plants.

Answer 1

Hint: The bacterium which is found in the root of the leguminous plants is a diazotrophic bacteria that helps in fixing nitrogen after it gets attached to the root nodules of leguminous plants.

Complete answer:
Rhizobium is found in the root of the leguminous plants. Leguminous plants have a symbiotic association with the bacteria Rhizobium which is present in the soil. It helps in fixing atmospheric nitrogen.
A legume is a plant in the family Fabaceae, for example, all the varieties of beans, peas, and groundnuts. Symbiotic nitrogen fixation is an element of a mutualistic relationship during which plants provide a place to live in. Plants fix carbon to bacteria in exchange for fixed nitrogen.
These bacteria colonize plant cells within the root, forming nodules. Here they convert atmospheric nitrogen into ammonia using the enzyme nitrogenase complex, which is the primary enzyme encoded by the nif genes and then provides organic nitrogenous compounds such as glutamine or ureides to the plant. The plants, in turn, provide the bacteria with organic compounds made by photosynthesis.
There are many other methods of fixing nitrogen. For example, it can be fixed by lightning. It comes under non-biological factors. Due to lightning, ${ N }{ O }_{ 2 }$ is produced. ${ N }{ O }_{ 2 }$ cannot be used by plants, so ${ N }{ O }_{ 2 }$ reacts with water to form ${ HN }{ O }_{ 3 }$which is usable by plants.
Note: The Nif genes are found in both free-living nitrogen-fixing bacteria and the symbiotic bacteria.  Apart from the nitrogenase enzyme, the nif genes also encode several regulatory proteins involved in nitrogen fixation.