Describe the role of Rhizobium in biological nitrogen fixation and explain how this symbiosis benefits leguminous crops, including the steps of nodule formation.
Answer
Rhizobium fixes atmospheric N₂ into ammonia within root nodules of legumes, providing a usable nitrogen source that enhances plant growth and soil fertility.
Step-by-step solution
- Identify Rhizobium as a nitrogen‑fixing soil bacterium that forms a mutualistic association with legume roots.
- Explain that flavonoids released by legume roots attract Rhizobium and induce nod gene expression in the bacteria.
- State that bacterial Nod factors trigger root hair curling, allowing Rhizobium to enter the root through infection threads.
- Mention that the bacteria travel to cortical cells where they induce cell division, forming a nodule.
- Describe that inside the nodule, Rhizobium differentiates into bacteroids and, using the enzyme nitrogenase, reduces atmospheric N₂ to NH₃ (ammonia).
- Explain that the plant converts NH₃ into amino acids, which support its growth, while the bacteria receive carbohydrates and a protective niche.
- Conclude that this symbiosis reduces the need for synthetic nitrogen fertilizers and improves soil nitrogen content.
Common mistake: Assuming nitrogen fixation occurs directly in plant roots without bacterial involvement.
Concept tested: Biological nitrogen fixation by Rhizobium
Published · How this solution was prepared and checked
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