Question

why rhizobium categorised as symbiotic association

Answer 1

Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium forms an endosymbiotic nitrogen fixing association with roots of legumes and Parasponia.

The bacteria colonize plant cells within root nodules where they convert atmospheric nitrogen into ammonia and then provide organic nitrogenous compounds such as glutamine or ureides to the plant. The plant in turn provides the bacteria with organic compounds made by photosynthesis.[2] This mutually beneficial relationship is true of all of the rhizobia, of which the Rhizobium genus is a typical example.