Which bacterium forms nodules on the roots of leguminous plants throu…

The correct answer is Option (2) → Rhizobium

Some bacteria fix atmospheric nitrogen and so act as biofertilisers. They work in two ways: some live in symbiosis inside root nodules of legumes, and others fix nitrogen while living free in the soil. Only the first kind forms nodules.

Option (1) Azotobacter — Incorrect. Azotobacter fixes atmospheric nitrogen, but it does so while free-living in the soil. It does not form root nodules.

Option (2) Rhizobium — Correct. Rhizobium forms nodules on the roots of leguminous plants through a symbiotic association. Inside the nodules it fixes atmospheric nitrogen into organic forms that the plant uses as a nutrient.

Option (3) Azospirillum — Incorrect. Azospirillum, like Azotobacter, is a free-living nitrogen fixer in the soil. It forms no nodules on legume roots.

Option (4) Acetobacter aceti — Incorrect. Acetobacter aceti is a bacterium used for the commercial production of acetic acid. It does not fix nitrogen.

NCERT Extract:
Microbes as Biofertilisers – “… You have studied about the nodules on the roots of leguminous plants formed by the symbiotic association of Rhizobium. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as nutrient. Other bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.”