Question:
____a nitrogen‐fixing actinomycete, forms root nodules with eight non‐leguminous plant families.
____a nitrogen‐fixing actinomycete, forms root nodules with eight non‐leguminous plant families.
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Legume = Rhizobium. Non-legume (like Casuarina/Alder) = Frankia. This distinction is a favorite in agroforestry exams.
Updated On: Aug 7, 2026
- Frankia spp.
- Bacillus spp.
- Rhizobium spp.
- Streptomysis spp.
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Question:
While Rhizobium is the famous nitrogen-fixer for legumes (peas, beans), many important forest trees (non-legumes) also fix nitrogen. The question asks for the specific genus of actinomycete (filamentous bacteria) that performs this task for non-legumes.
Detailed Explanation:
• Actinorhizal Plants: Trees that form symbiotic relationships with nitrogen-fixing actinomycetes are called actinorhizal plants. Examples include Casuarina, Alnus (Alder), and Myrica.
• Frankia: This is the genus of filamentous, Gram-positive actinomycete responsible for forming root nodules in these non-leguminous plants. It can fix atmospheric nitrogen ($N_2$) into a form the plant can use.
• Host Range: Frankia is unique because it can infect and form nodules in plants across eight different families (like Casuarinaceae and Betulaceae), making it vital for soil fertility in forest ecosystems.
• Comparison: Rhizobium (C) is a proteobacterium, not an actinomycete, and is restricted mostly to the Fabaceae (legume) family.
Step 2: Final Answer:
The nitrogen-fixing actinomycete associated with non-legumes is Frankia spp.
While Rhizobium is the famous nitrogen-fixer for legumes (peas, beans), many important forest trees (non-legumes) also fix nitrogen. The question asks for the specific genus of actinomycete (filamentous bacteria) that performs this task for non-legumes.
Detailed Explanation:
• Actinorhizal Plants: Trees that form symbiotic relationships with nitrogen-fixing actinomycetes are called actinorhizal plants. Examples include Casuarina, Alnus (Alder), and Myrica.
• Frankia: This is the genus of filamentous, Gram-positive actinomycete responsible for forming root nodules in these non-leguminous plants. It can fix atmospheric nitrogen ($N_2$) into a form the plant can use.
• Host Range: Frankia is unique because it can infect and form nodules in plants across eight different families (like Casuarinaceae and Betulaceae), making it vital for soil fertility in forest ecosystems.
• Comparison: Rhizobium (C) is a proteobacterium, not an actinomycete, and is restricted mostly to the Fabaceae (legume) family.
Step 2: Final Answer:
The nitrogen-fixing actinomycete associated with non-legumes is Frankia spp.
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