Rhizobium and Mycorrhiza — Revision Notes

⚡ 30-Second Revision

Rhizobium: — Bacteria, legumes, root nodules, nitrogen fixation ($N_2 \rightarrow NH_3$), nitrogenase, leghemoglobin (O$_2$ protection).
Mycorrhiza: — Fungi, most plants, enhanced P & water uptake.

- Ectomycorrhiza: Mantle, Hartig net (between cells). - Endomycorrhiza (AMF): Arbuscules, vesicles (within cells).

Both are mutualistic biofertilisers.

2-Minute Revision

Rhizobium and Mycorrhiza are key biofertilisers, forming mutualistic relationships with plants. Rhizobium are bacteria that associate with leguminous plants, forming root nodules. Inside these nodules, they fix atmospheric nitrogen into ammonia, a usable form for the plant.

This process is catalyzed by the oxygen-sensitive enzyme nitrogenase, which is protected by leghemoglobin. In return, the plant provides carbohydrates. Mycorrhiza are fungi that form symbiotic associations with the roots of most vascular plants.

Their extensive hyphal networks significantly enhance the plant's uptake of water and mineral nutrients, especially phosphorus, from the soil. There are two main types: Ectomycorrhiza, which forms a fungal sheath (mantle) around the root and a network between root cells (Hartig net), and Endomycorrhiza (Arbuscular Mycorrhizal Fungi or AMF), which penetrates into root cells to form arbuscules (for nutrient exchange) and vesicles (for storage).

Both associations are crucial for plant growth, soil fertility, and sustainable agriculture.

5-Minute Revision

Let's quickly review the essential aspects of Rhizobium and Mycorrhiza for NEET. Both are vital biofertilisers, meaning they are living organisms that enrich soil nutrient quality. Their core principle is mutualistic symbiosis, where both the microbe and the plant benefit.

Rhizobium: These are Gram-negative soil bacteria. Their primary role is biological nitrogen fixation. They specifically associate with leguminous plants (e.g., peas, beans). The interaction starts with chemical signals from the plant, leading to root hair curling, infection thread formation, and finally, the development of root nodules on the plant roots.

Inside these nodules, Rhizobium differentiates into bacteroids which house the nitrogenase enzyme complex. Nitrogenase converts atmospheric nitrogen ($N_2$) into ammonia ($NH_3$), which the plant can use.

A critical aspect is the presence of leghemoglobin, a plant-synthesized protein that binds oxygen, creating the anaerobic environment necessary for nitrogenase to function without being inactivated.

The plant provides carbohydrates to the bacteria, completing the mutualistic exchange.

Mycorrhiza: This term describes a symbiotic association between fungi and plant roots, found in over 90% of plant species. The main benefit for the plant is enhanced uptake of water and mineral nutrients, particularly phosphorus, which is often immobile in soil. The fungal hyphae extend far beyond the root hairs, effectively increasing the plant's absorptive surface area. There are two main types:

1
Ectomycorrhiza (ECM): — The fungal hyphae form a dense sheath called a mantle around the root tip and penetrate *between* the cortical cells, forming a Hartig net. Common in forest trees (oaks, pines).
2
Endomycorrhiza (Arbuscular Mycorrhizal Fungi - AMF): — The fungal hyphae penetrate *into* the cortical cells, forming highly branched structures called arbuscules (sites of nutrient exchange) and swollen storage structures called vesicles. This is the most widespread type, found in most agricultural crops. The plant provides carbohydrates to the fungus in return for nutrients.

Key Differences to Remember: Rhizobium are bacteria, fix nitrogen, form nodules, and associate with legumes. Mycorrhiza are fungi, enhance P/water uptake, form hyphal networks (mantle/Hartig net or arbuscules/vesicles), and associate with most plants. Understanding these distinctions and the specific structures/enzymes involved is crucial for NEET.

Prelims Revision Notes

Rhizobium

Organism: — Gram-negative, rod-shaped bacteria.
Symbiosis: — Mutualistic with leguminous plants (Fabaceae family).
Location: — Forms root nodules on host roots.
Process: — Biological Nitrogen Fixation.

* Input: Atmospheric Nitrogen ($N_2$). * Output: Ammonia ($NH_3$). * Enzyme: Nitrogenase (highly oxygen-sensitive). * Protection: Leghemoglobin (plant-synthesized, binds O$_2$, maintains anaerobic conditions).

Mechanism of Nodule Formation:

1. Plant releases flavonoids $\rightarrow$ attracts Rhizobium, activates bacterial Nod genes. 2. Rhizobium produces Nod factors $\rightarrow$ root hair curling. 3. Bacteria penetrate root hair $\rightarrow$ infection thread formation. 4. Infection thread grows into cortex $\rightarrow$ cortical cell division $\rightarrow$ nodule initiation. 5. Bacteria released into plant cells $\rightarrow$ differentiate into bacteroids (nitrogen-fixing form).

Benefit to Plant: — Provides usable nitrogen for protein synthesis, nucleic acids.
Benefit to Bacteria: — Receives carbohydrates from plant photosynthesis.

Mycorrhiza

Organism: — Fungi.
Symbiosis: — Mutualistic with roots of most vascular plants (over 90%).
Primary Benefit to Plant: — Enhanced uptake of phosphorus (P), water, and other immobile nutrients (Zn, Cu).
Benefit to Fungi: — Receives carbohydrates from plant photosynthesis.
Types:

1. Ectomycorrhiza (ECM): * Fungal hyphae form a dense sheath (mantle) around the root tip. * Hyphae penetrate *between* cortical cells, forming a network called the Hartig net. * Do NOT penetrate *into* plant cells.

* Common in temperate forest trees (e.g., pine, oak, birch). 2. Endomycorrhiza (Arbuscular Mycorrhizal Fungi - AMF): * Most common type, found in most crops. * Fungal hyphae penetrate *into* the cortical cells.

* Form highly branched structures called arbuscules (site of nutrient exchange). * Form swollen, lipid-rich structures called vesicles (storage, propagules).

Additional Benefits: — Increased drought tolerance, disease resistance, improved soil structure.

Key Distinctions for NEET

Rhizobium: — Bacteria, Nitrogen, Legumes, Nodules, Nitrogenase, Leghemoglobin.
Mycorrhiza: — Fungi, Phosphorus/Water, Most Plants, Hyphae, Arbuscules/Vesicles (AMF), Mantle/Hartig net (ECM).

Vyyuha Quick Recall

To remember the key features of Rhizobium and Mycorrhiza:

Rhizobium: Root nodules, Red (leghemoglobin), Really fixes Nitrogen for Legumes.

Mycorrhiza: Many plants, Mainly Phosphorus, More water, Mycelial network (hyphae), Arbuscules & Vesicles (AMF).