Q.50 The uptake of nitrogen (N) and phosphorus (P) by plant roots often involves interaction between
root and some symbiotic organisms. Which of the following associations is most commonly found for the
uptake of these two nutrients?
(A) Bacteria for N , algae for P
(B) Bacteria for N , nematodes for P
(C) Nematodes for N , fungi for P
(D) Bacteria for N , mycorrhizae for P

Plants need essential nutrients like nitrogen (N) and phosphorus (P) for healthy growth, but they often cannot absorb these efficiently from the soil without help. To overcome this limitation, plant roots form symbiotic associations with specific microorganisms. These interactions significantly enhance the uptake of nitrogen and phosphorus.

In many competitive exams, a frequently asked question is which symbiotic organisms are most commonly involved in the uptake of these two key nutrients.

MCQ: Symbiotic Uptake of N and P by Plant Roots

Question:
The uptake of nitrogen (N) and phosphorus (P) by plant roots often involves interaction between root and some symbiotic organisms. Which of the following associations is most commonly found for the uptake of these two nutrients?

(A) Bacteria for N, algae for P
(B) Bacteria for N, nematodes for P
(C) Nematodes for N, fungi for P
(D) Bacteria for N, mycorrhizae for P

Correct Answer: (D) Bacteria for N, mycorrhizae for P

Why Option (D) Is Correct: Bacteria for N, Mycorrhizae for P

• Nitrogen (N): Symbiotic bacteria

  • Many plants, especially legumes (e.g., peas, beans, clover), form symbiotic relationships with nitrogen-fixing bacteria such as Rhizobium.

  • These bacteria live inside root nodules and convert atmospheric nitrogen N2N2, which plants cannot directly use, into ammonia NH3NH3 or related compounds that plants can absorb and assimilate.

  • This process is called biological nitrogen fixation and is a key source of nitrogen for many ecosystems.

• Phosphorus (P): Mycorrhizal fungi

  • Mycorrhizae are mutualistic associations between plant roots and fungi.

  • The fungal hyphae extend far into the soil, increasing the effective surface area of the root system and greatly enhancing the plant’s ability to absorb phosphate ions.

  • Mycorrhizal fungi are particularly important for phosphorus because P is often present in soil in forms that are only slowly available to roots.

  • In return, the plant supplies the fungi with carbohydrates formed during photosynthesis.

Because nitrogen is commonly taken up via symbiotic bacteria and phosphorus via mycorrhizal fungi, the combination given in option (D) exactly matches the most common and well-established symbiotic associations.

Explanation of Incorrect Options

Option (A): Bacteria for N, algae for P

• The first part is partly true:

  • Bacteria for N: Correct for many plants, especially legumes, where nitrogen-fixing bacteria (Rhizobium, Azotobacter, etc.) play a central role.

• The second part is incorrect:

  • Algae for P: Algae are not the primary symbiotic partners for phosphorus uptake in plant roots.

  • While algae can contribute to nutrient cycling in aquatic and some terrestrial ecosystems, they are not known as the main root symbionts for phosphorus absorption.

  • In roots, mycorrhizal fungi are the classic and dominant symbionts that enhance P uptake, not algae.

Therefore, (A) is incorrect because the P part of the association does not match what is most commonly observed in plant root symbiosis.

Option (B): Bacteria for N, nematodes for P

• Bacteria for N: As before, this part is acceptable in principle, since nitrogen-fixing bacteria provide usable nitrogen to many plants.

• Nematodes for P: This part is incorrect and misleading.

  • Nematodes are mostly known as either free-living soil organisms or plant parasites.

  • They do not act as beneficial symbionts for phosphorus uptake. In many cases, plant–nematode interactions are harmful (e.g., root-knot nematodes causing galls and reducing nutrient uptake).

  • They are not involved in enhancing P absorption the way mycorrhizal fungi are.

Because nematodes are not recognized as symbiotic partners for phosphorus uptake, option (B) is incorrect.

Option (C): Nematodes for N, fungi for P

• Nematodes for N:

  • Nematodes do not fix atmospheric nitrogen and are not known as symbiotic nitrogen providers to plant roots.

  • Some soil nematodes feed on bacteria and fungi, indirectly influencing nutrient cycling, but they are not the organisms directly supplying nitrogen in a classical symbiotic sense.

• Fungi for P:

  • This part is essentially correct: mycorrhizal fungi significantly enhance phosphorus uptake in plants.

  • However, the option simply says “fungi” without specifying mycorrhizae, but we still consider this part conceptually correct.

Even though the “fungi for P” part is conceptually right, the “nematodes for N” part is incorrect, so the entire option (C) is wrong.

Key Takeaways for Exams

• Nitrogen (N):

  • Most important symbionts: nitrogen-fixing bacteria

  • Examples: Rhizobium (legumes), Frankia (actinorhizal plants)

  • Role: Convert atmospheric nitrogen N2N2 into ammonia and related compounds usable by plants.

• Phosphorus (P):

  • Most important symbionts: mycorrhizal fungi

  • Types: Arbuscular mycorrhizae (AM), ectomycorrhizae, etc.

  • Role: Increase effective root surface area, solubilize and transport phosphate ions to the plant.

• Correct association for this MCQ:

  • Bacteria for nitrogen

  • Mycorrhizae (fungi) for phosphorus

  • Hence, the correct answer is (D) Bacteria for N, mycorrhizae for P.