Rhizobium is a gram-negative bacteria that resides in the roots of leguminous plants.
Rhizobium forms an endo-symbiotic relationship for the process of nitrogen fixation with the roots of legumes and some other flowering plants.
Leguminous plants have the capability of fixing the atmospheric nitrogen in the soil.
Some leguminous plants are- beans, peas, soybeans, etc.
Importance of Rhizobium:
Rhizobium is present in the root nodules of legumes and helps in nitrogen-fixing.
Plants need nitrogen to form amino acids and proteins but they are incapable of fixing the nitrogen so they live in symbiotic association with bacteria like Rhizobium.
In the root cortex cells of legumes the Rhizobium reproduces and nodules are formed.
When the process of nitrogen fixation begins the root nodules start to turn pinkish in color.
The nitrogen when fixed gets absorbed by the plants, and thus, the plants meet their nitrogen requirement.
Figure showing process of nitrogen fixation by Rhizobium: