Mycorrhiza: Symbiotic Fungus-Root Association Explained

Mycorrhiza is a symbiotic partnership between a fungus and the roots of a plant. From Greek mykes (fungus) and rhiza (root). In this association the fungal threads, called hyphae, either wrap around the root or grow inside the root cells, growing into the surrounding soil.

This partnership matters because it is one of the most widespread and important relationships on land. The fungus extends the root system deep into the soil, while the plant feeds it sugars made in photosynthesis. Both partners gain from the exchange.

Key Concepts

There are two main types of mycorrhiza that students must know. In ectomycorrhiza, the fungus forms a sheath or mantle around the outside of the root and grows between the root's outer cells, forming a network called the Hartig net. Ectomycorrhiza is common in forest trees such as pine, oak and eucalyptus. In endomycorrhiza, also called arbuscular mycorrhiza, the fungal hyphae actually penetrate the root's cortical cells and form tree-shaped structures called arbuscules, where nutrients are exchanged. A common example is the fungus Glomus associating with crop plants like wheat and maize.

• Hyphae: thin fungal threads, only a few micrometres wide, that explore the soil.
• Arbuscule: branched fungal structure inside a root cell; the site of nutrient exchange.
• Hartig net: network of hyphae between root cells in ectomycorrhiza.
• Mutualism: a symbiosis in which both partners benefit.

How It Works

The exchange works step by step. First, the plant releases chemical signals from its roots that attract the fungus. Second, fungal hyphae grow toward the root and make contact, forming a mantle or arbuscules. Third, the hyphae spread into the soil, absorbing water and minerals, especially phosphorus, zinc and copper. Fourth, the fungus delivers these minerals to the root in exchange for sugars and amino acids produced by the plant's photosynthesis.

Worked Example

A root can only absorb phosphorus from a thin zone of soil around it, because phosphorus barely moves in soil. Suppose this depletion zone extends 1 mm from the root surface, but mycorrhizal hyphae reach 100 mm into the soil. Comparing the cross-sectional area of soil explored:

1. Area explored by the root alone is proportional to 1 squared = 1 square unit.
2. Area explored with hyphae is proportional to 100 squared = 10,000 square units.
3. Ratio = 10,000 / 1 = 10,000.

Answer: the mycorrhizal fungus lets the plant explore roughly 10,000 times more soil area per unit root length. This is why mycorrhizal plants grow far better in phosphorus-poor soils.

Common Misconceptions

• Misconception: The fungus is a parasite harming the plant. Correction: In mycorrhiza both partners benefit; the plant grows better, not worse. Parasitic fungi cause disease, but mycorrhizal fungi are mutualists.
• Misconception: Only a few plants form mycorrhiza. Correction: About 80 to 90 per cent of land plant species form mycorrhizal associations; it is the rule, not the exception.
• Misconception: Mycorrhiza only helps with water. Correction: Its biggest contribution is mineral nutrition, especially phosphorus, plus protection against some root pathogens.

Real-World Applications

• Farmers add mycorrhizal inoculants to soil to reduce chemical fertilizer use in crops like maize, wheat and pulses.
• Forest nurseries inoculate tree seedlings with ectomycorrhizal fungi so young pines and oaks survive better after transplanting.
• Mycorrhiza helps reclaim degraded land, such as mine sites, by letting plants establish in poor, dry soils.

Mycorrhiza is a mutualistic fungus-root partnership in which hyphae extend the plant's reach into soil, supplying phosphorus and water in exchange for sugars. Ectomycorrhiza forms a Hartig net around roots of trees, while endomycorrhiza forms arbuscules inside crop roots. This ancient symbiosis feeds most of the world's plants.