How do mycorrhizal fungi help the plants harbouring them?

NCERT Exemplar · Q41

Q. How do mycorrhizal fungi help the plants harbouring them?

Telangana TsbieShort· 2mImportance★★★★★

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Concept understanding — Biofertilizers Biopesticides

Imagine you have a garden. You want your plants to grow strong and healthy, and you also want to keep pests away. There are two main ways to do this: one is with harsh chemicals (chemical fertilizers and pesticides), and the other is with living things or natural substances. Biofertilizers and biopesticides are the second way — they are nature’s own tools for farming.

Let’s start with biofertilizers. Think of them as "living manure." A chemical fertilizer is like giving a plant a direct shot of nutrients — it works fast but can burn the soil over time. A biofertilizer, on the other hand, is a preparation containing live microorganisms (like bacteria, fungi, or algae) that help the plant get nutrients from the soil or air. For example, certain bacteria can take nitrogen from the air and convert it into a form the plant can use. The plant doesn’t get fed directly; instead, the biofertilizer helps the soil become richer and more fertile naturally.

Note

The NCERT textbook (Class 12 Biology, Chapter 10) defines biofertilizers as organisms that enrich the nutrient quality of the soil. The main examples are Rhizobium (a bacterium that lives in root nodules of legumes and fixes nitrogen), Azospirillum and Azotobacter (free-living nitrogen-fixing bacteria), and blue-green algae (like Anabaena) which also fix nitrogen.

Now, biopesticides. These are living organisms or natural substances that control pests — insects, fungi, weeds, etc. — without using synthetic chemicals. Instead of spraying a poison that kills everything (good and bad bugs alike), a biopesticide might use a specific bacterium that only harms a particular caterpillar, or a fungus that attacks a weed. The most famous example is Bacillus thuringiensis (often called Bt), a bacterium that produces a protein toxic to certain insect larvae but harmless to humans, animals, and most other insects.

Important

The key difference between chemical and biological agents: Biofertilizers and biopesticides are renewable, eco-friendly, and do not leave toxic residues in the soil or water. They are a cornerstone of sustainable agriculture — farming that can continue for generations without destroying the land.

Why does this matter for a commerce or humanities student? Because agriculture is not just about biology — it’s about economics, policy, and human health. Chemical fertilizers and pesticides are expensive to produce, can pollute groundwater, and their overuse leads to "superpests" that become resistant. Biofertilizers and biopesticides are often cheaper in the long run, safer for farm workers, and help maintain soil health. Governments around the world (including India) promote them through subsidies and organic farming schemes. Understanding them helps you see why "organic" food costs more, why some farmers switch to natural methods, and how environmental regulations work.

Here’s a quick summary of the main types you should know (from NCERT):

Biofertilizers:
- Rhizobium (symbiotic with legumes) …

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Mycorrhizal fungi form a symbiotic relationship with plant roots, primarily enhancing phosphorus absorption and providing resistance to pathogens, drought, and salinity, thereby promoting overall plant growth.

The intricate world beneath our feet holds remarkable partnerships, none more vital to plant life than the symbiotic association known as mycorrhiza. This term, literally meaning "fungus root," describes a mutually beneficial relationship between fungi and the roots of higher plants. Far from being mere parasites, these fungi act as extended arms for the plant's root system, significantly boosting its ability to thrive in various environments.

At its core, the primary way mycorrhizal fungi assist plants is by vastly improving their nutrient uptake, particularly phosphorus. Plant roots, while efficient, have a limited surface area and reach. Mycorrhizal fungi, with their extensive network of hyphae (thread-like structures), can explore a much larger volume of soil than the plant roots alone. They effectively increase the absorptive surface area, allowing the plant to access nutrients that would otherwise be unavailable.

Important

The most significant contribution of mycorrhizal fungi to plants is their ability to absorb phosphorus from the soil and transfer it directly to the plant. Phosphorus is a crucial macronutrient for plant growth, energy transfer, and genetic material.

Beyond nutrient acquisition, mycorrhizal associations confer several other critical advantages to the host plant, enhancing its resilience and overall vigor:

Resistance to Root-borne Pathogens: The presence of mycorrhizal fungi can create a protective barrier around the plant roots, making them less susceptible to infections from harmful soil-borne pathogens. This protective effect can be due to physical exclusion, competition for resources, or even the production of antimicrobial compounds by the fungi.
Tolerance to Salinity: In soils with high salt concentrations, plant growth is often stunted. Mycorrhizal fungi can help plants tolerate these saline conditions, though the exact mechanisms are complex and may involve regulating water uptake or altering ion balance within the plant.
Tolerance to Drought: By extending the reach for water and potentially influencing the plant's physiological responses, mycorrhizal fungi can improve a plant's ability to withstand periods of water scarcity. This enhanced water uptake is crucial for survival in arid or semi-arid regions. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

TG EAPCET 2026Set ap-2026-05-04-AN1 markMCQ
$Q. Free living microbes in soil but as symbionts fix atmospheric nitrogen are (A) Rhizobium and Azotobacter (B) Oscillatoria and Frankia (C) Trichoderma and Azospirillum (D) Rhizobium and Frankia$
›Reveal solutionSolution
The question asks which pair of microbes are free-living in soil but also act as symbionts to fix atmospheric nitrogen. The correct pair is Rhizobium and Frankia, which matches option (D).

The key here is understanding the two distinct lifestyles of nitrogen-fixing bacteria: some live freely in the soil and fix nitrogen on their own, while others form symbiotic relationships with plants (usually in root nodules). But the question specifically asks for microbes that do both — they are free-living in soil and also fix nitrogen as symbionts. That’s a narrower category.

Let’s break down each option.

Option (A): Rhizobium and Azotobacter

Rhizobium is a classic symbiont — it lives inside root nodules of legumes and fixes nitrogen. It is not free-living in soil; it depends on the plant host.

Azotobacter is a free-living nitrogen-fixer in soil, but it does not form symbiotic associations. So this pair fails because Rhizobium is not free-living.

Option (B): Oscillatoria and Frankia

Oscillatoria is a cyanobacterium (blue-green alga) that can fix nitrogen, but it is primarily aquatic or free-living — it does not form symbiotic nitrogen-fixing associations with plants.

Frankia is a bacterium that forms symbiotic root nodules with non-leguminous plants (like alder, casuarina). It is not free-living in soil; it requires a host. So this pair fails because neither does both.

Option (C): Trichoderma and Azospirillum

Trichoderma is a fungus used as a biocontrol agent — it does not fix nitrogen at all.

Azospirillum is a free-living nitrogen-fixer in soil, but it can also associate loosely with grass roots (as a rhizosphere bacterium), though it is not a true symbiont forming nodules. Even if we stretch, Trichoderma is irrelevant. So this pair is wrong.

Option (D): Rhizobium and Frankia …
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TG EAPCET 2022Set ap-2022-07-30-FN1 markMCQ
$Q. Assertion (A): Baculoviruses are used as biological control agents in plants and animals. Reason (R): They have species specific narrow spectrum insecticidal application. The correct option among the following is: (A) (A) is true. (R) is true and (R) is the correct explanation of (A) (B) (A) is true. (R) is true but (R) is not the correct explanation of (A) (C) (A) is true but (R) is false (D) (A) is false but (R) is true$
›Reveal solutionSolution
The assertion is false because baculoviruses are used only as biological control agents in insects (not plants and animals), while the reason is true. Therefore, the correct option is (D).

Concept and Intuition

Baculoviruses are a family of viruses that specifically infect arthropods, particularly insects. Their key feature is species specificity — each baculovirus strain typically attacks only one or a few closely related insect species. This makes them ideal for biological pest control because they kill the target pest without harming beneficial insects, plants, or animals. However, the assertion incorrectly claims they are used in both plants and animals. In reality, they are applied to crops to control insect pests, not directly to the plants themselves or to animals. The reason correctly describes their narrow-spectrum insecticidal nature.

Step-by-step reasoning

Evaluate the Assertion (A):

"Baculoviruses are used as biological control agents in plants and animals."

Baculoviruses are not used directly in plants (they don't infect plant cells) or in animals (they don't infect vertebrates).

They are applied to crops (e.g., as sprays) to infect and kill insect pests. So the phrase "in plants and animals" is misleading — they are used on plants to control insects, not inside plants or animals.

Therefore, the assertion is false as stated.

Evaluate the Reason (R):

"They have species specific narrow spectrum insecticidal application."

This is accurate: baculoviruses are highly host-specific, affecting only certain insect species. …
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Q.How do mycorrhizal fungi help the plants harbouring them?

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