Rhizobium and Mycorrhiza — Core Principles

Core Principles

Rhizobium and Mycorrhiza are crucial microbial biofertilisers forming mutualistic symbiotic relationships with plants. Rhizobium are soil bacteria that associate with leguminous plants, forming root nodules.

Within these nodules, they convert atmospheric nitrogen (N$_2$) into usable ammonia (NH$_3$) through a process called nitrogen fixation, mediated by the nitrogenase enzyme and protected by leghemoglobin.

This provides the plant with essential nitrogen for growth, reducing the need for synthetic fertilisers. Mycorrhiza refers to a symbiotic association between fungi and plant roots. These fungi extend the plant's effective root system through their hyphae, significantly enhancing the uptake of water and mineral nutrients, especially phosphorus, from the soil.

There are two main types: Ectomycorrhiza (forming a mantle and Hartig net) and Endomycorrhiza (forming arbuscules and vesicles within root cells). Both Rhizobium and Mycorrhiza receive carbohydrates from the plant in return for their services, making them vital for plant health, soil fertility, and sustainable agriculture by promoting nutrient cycling and reducing environmental impact.

Important Differences

vs Rhizobium vs. Mycorrhiza

Aspect	This Topic	Rhizobium vs. Mycorrhiza
Organism Type	Bacteria (Gram-negative, rod-shaped)	Fungi (various phyla)
Primary Nutrient Provided	Fixed Nitrogen (as ammonia)	Phosphorus, Water, other minerals (Zn, Cu)
Host Plants	Leguminous plants (e.g., peas, beans, clover)	Most vascular plants (over 90%), including many crops and trees
Specialized Structure on Root	Root Nodules	Fungal hyphae forming a network around/within roots (mantle, Hartig net, arbuscules, vesicles)
Key Enzyme/Protein	Nitrogenase, Leghemoglobin	Various fungal enzymes for nutrient solubilization
Mechanism of Nutrient Acquisition	Biological Nitrogen Fixation (converting N$_2$ to NH$_3$)	Extended hyphal network for absorption, solubilization of bound nutrients

Rhizobium and Mycorrhiza are both vital biofertilisers, yet they differ fundamentally in their biological nature and primary contributions to plant nutrition. Rhizobium are bacteria that specifically partner with legumes to fix atmospheric nitrogen, providing the plant with usable nitrogen compounds. Mycorrhiza, on the other hand, are fungi that associate with the roots of most plants, significantly enhancing the uptake of phosphorus, water, and other minerals through their extensive hyphal networks. While both are mutualistic, their host range, the specific nutrient they supply, and the structures they form are distinct, reflecting their specialized evolutionary roles in supporting plant life.