Q.73 Which of the following is a symbiotic bacterial genus that converts nitrogen into nitrogen
containing compounds that are utilized by legume plants to synthesize amino acids?
(A) Spirillum (B) Azotobacter
(C) Rhizobium (D) Clostridium
Rhizobium is the symbiotic bacterial genus that forms root nodules on legume plants, converting atmospheric nitrogen into ammonia and other compounds for amino acid synthesis.
This process, known as symbiotic nitrogen fixation, supports legume growth without synthetic fertilizers.
Option Analysis
(A) Spirillum: Fixes nitrogen associatively with grasses like rice or Digitaria, not symbiotically with legumes via nodules.
(B) Azotobacter: Free-living aerobic bacterium that fixes nitrogen in soil independently, without symbiosis to legumes.
(C) Rhizobium: Correct; forms mutualistic symbiosis with legumes, infecting roots to create nodules where nitrogenase enzyme reduces N₂ to NH₃ for plant use.
(D) Clostridium: Anaerobic free-living or consortial fixer in some plants, lacks specific legume symbiosis.
Symbiotic nitrogen fixing bacteria legumes partner with, like Rhizobium, enable plants to access atmospheric nitrogen for amino acid synthesis, revolutionizing sustainable agriculture.
This CSIR NET-relevant topic covers the mutualism where bacteria gain carbohydrates and plants gain fixed nitrogen.
Mechanism of Symbiosis
Legume roots release flavonoids attracting Rhizobium, leading to root hair infection and nodule formation.
Inside nodules, bacteroids use nitrogenase to produce ammonia (NH₃), converted to amino acids via glutamine synthetase.
Plants supply energy via photosynthates, excluding oxygen to protect oxygen-sensitive nitrogenase.
Comparison of Options
| Genus | Fixation Type | Plant Association | Legume Symbiosis? |
|---|---|---|---|
| Spirillum | Associative | Grasses (e.g., rice) | No |
| Azotobacter | Free-living | Soil independently | No |
| Rhizobium | Symbiotic (nodules) | Legumes exclusively | Yes |
| Clostridium | Anaerobic free-living | Non-legumes/consortia | No |
Agricultural Importance
Rhizobium-legume symbiosis fixes 50-300 kg N/ha/year, reducing fertilizer needs and enhancing soil fertility.
Inoculants boost yields in crops like soybean and alfalfa for CSIR NET biotechnology topics.
Challenges include strain specificity and environmental stress, addressed via genetic engineering.
