Which Bacterium Fixes Nitrogen in Plant Root Nodules?
- Stanislav M.
- 11 minutes ago
- 2 min read
The primary bacteria that fix nitrogen in plant root nodules are rhizobia, a group including Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium, and others that form symbiotic partnerships mainly with legumes. These soil microbes invade root cells, create specialized nodules, and use nitrogenase to convert atmospheric N₂ into plant-usable ammonia—supplying 50-300 kg N/ha annually without fertilizers.indogulfbioag+3
For a quick answer on the most common one, see our FAQ: Which bacterium fixes nitrogen in plant root nodules?.[indogulfbioag]
Understanding Root Nodules and Nitrogen Fixation
Root nodules are pinkish growths on legume roots housing bacteroids—differentiated bacteria protected by plant membranes (symbiosomes). Inside, nitrogenase reduces N₂ under microaerobic conditions maintained by leghemoglobin, preventing enzyme inactivation.indogulfbioag+1
This symbiosis evolved for nutrient-poor soils: plants provide carbohydrates (10-15% photosynthates); bacteria deliver fixed N as glutamine/asparagine. Active nodules turn pink/red; ineffective ones stay green/white.
Key Rhizobial Species and Their Plant Partners
Rhizobia show host specificity via Nod factors (lipochitooligosaccharides) from nod genes, ensuring matched partnerships.indogulfbioag+1
Bacterium | Key Crops | Nodule Type | Notes |
|---|---|---|---|
Rhizobium leguminosarum | Peas, beans, lentils, cloverindogulfbioag+1 | Determinate (spherical, fixed size) | Fast-growing; temperate legumes. Fixes 100-200 kg N/ha.[indogulfbioag] |
Bradyrhizobium japonicum | Soybeansindogulfbioag+1 | Indeterminate (elongated, persistent meristem) | Slow-growing; superior stress tolerance. Boosts yield 25%, cuts N fertilizer 50%.[indogulfbioag] |
Bradyrhizobium elkanii | Soybeans, some tropical legumes[indogulfbioag] | Indeterminate | High N-fixing capacity in acidic soils.[indogulfbioag] |
Sinorhizobium meliloti | Alfalfa, sweetclover | Indeterminate | High competitiveness in soils.[indogulfbioag] |
Mesorhizobium spp. | Chickpeas, vetches | Variable | Drought-tolerant strains available.[indogulfbioag] |
Related FAQ: Learn nodule formation steps in Rhizobium species role in plant nutrition.[indogulfbioag]
How Infection and Nodulation Work
Signaling: Legume roots exude flavonoids → activate bacterial nod genes → Nod factors produced.
Root hair curling: Infection thread forms; bacteria enter cortex.[indogulfbioag]
Nodule development: Zoned structure (meristem → infection → fixation → senescence).[indogulfbioag]
Bacteroid formation: Bacteria enlarge, lose division; nitrogenase activates.[indogulfbioag]
Exchange: NH₄⁺ → amino acids to plant; C sources to bacteria.[indogulfbioag]
Monitor: Pink interiors confirm fixation; poor nodulation signals strain mismatch or stress.[indogulfbioag]
Exceptions: Non-Legume Nodules
Rarely, non-legumes form nodules:
Parasponia (trees) with Bradyrhizobium.[indogulfbioag]
Cereals engineered with nif genes (research stage).[nature]
Related FAQ: See symbiotic mechanisms in Nitrogen-fixing bacteria innovations.[indogulfbioag]
Practical Tips for Farmers
Strain matching: Use crop-specific inoculants (e.g., B. japonicum for soy).indogulfbioag+1
Soil factors: pH 6-7; avoid high N (>20 ppm) suppresses nodulation; P/Mo adequate.
Inoculation: Seed treatment; peat/sticker; 10^9 CFU/kg seed.
Troubleshooting: Few/white nodules? Re-inoculate, check soil N/PH.
Benefits: 20-50% yield boost, soil N legacy for rotations.
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