Mycorrhiza: Definition, Types and Significances

Introduction

The term “mycorrhiza” comes from the Greek words “mykes” (fungus) and “rhiza” (root), literally meaning “fungus-root.” This name perfectly captures the essence of one of nature’s most remarkable symbiotic relationships—a partnership between fungi and plant roots that has existed for over 400 million years. These relationships are not just fascinating biological curiosities; they are fundamental to the health of terrestrial ecosystems worldwide, influencing everything from individual plant growth to global carbon cycles.

What Are Mycorrhizae?

Mycorrhizae represent a mutualistic symbiotic relationship between soil fungi and plant roots. In this partnership, the fungus colonizes the root system of a host plant, creating an extension of the plant’s root network through its mycelium (the network of fungal filaments or hyphae). This expanded network dramatically increases the plant’s ability to absorb water and nutrients from the soil.

The relationship is mutually beneficial:

• The plant provides carbohydrates (sugars) produced through photosynthesis to the fungus
• The fungus provides enhanced nutrient and water uptake to the plant, along with improved stress resistance and soil structure

It’s estimated that approximately 90% of land plants form mycorrhizal associations, highlighting just how prevalent and important these relationships are in natural ecosystems.

Types of Mycorrhizae

Mycorrhizal associations are diverse and have evolved multiple times throughout evolutionary history. They can be broadly categorized into several main types, each with distinct structural characteristics and host preferences.

The Major Types of Mycorrhizal Associations

Arbuscular Mycorrhizae (AM)

Arbuscular mycorrhizae are the most common type, forming associations with approximately 80% of terrestrial plant species. These fungi penetrate the cortical cells of roots to form highly branched structures called arbuscules, which serve as the primary site of nutrient exchange between the plant and fungus.

AM fungi belong to the phylum Glomeromycota and are obligate symbionts, meaning they cannot complete their life cycle without a plant host. They are particularly important for enhancing phosphorus uptake in plants, as this nutrient is often poorly mobile in soil.

Ectomycorrhizae (EM)

Ectomycorrhizal fungi do not penetrate individual root cells but instead form a dense sheath or mantle around the root surface and grow between root cells to create a network called the Hartig net. They are common in forest ecosystems, particularly with trees of the Pinaceae, Fagaceae, and Betulaceae families.

Many edible mushrooms, including truffles, porcini, and chanterelles, are the fruiting bodies of ectomycorrhizal fungi. These fungi are especially effective at mobilizing nutrients from organic matter and protecting roots from pathogens.

Significance of Mycorrhizae

The importance of mycorrhizal associations extends far beyond individual plants, influencing entire ecosystems and even human activities. Here are some of the key areas where mycorrhizae play crucial roles:

Plant Nutrition and Growth

Mycorrhizal fungi dramatically enhance a plant’s ability to access and absorb soil nutrients, particularly:

1. Phosphorus acquisition: Mycorrhizal hyphae can access phosphorus from soil pores too small for plant roots and from sources unavailable to non-mycorrhizal plants.
2. Nitrogen uptake: Some mycorrhizal fungi can break down organic nitrogen sources, making this essential nutrient more available to plants.
3. Micronutrient access: Zinc, copper, and other micronutrients are more efficiently absorbed by mycorrhizal plants.
4. Water relations: The extensive hyphal network increases the surface area for water absorption, improving drought tolerance.

Research shows that mycorrhizal plants often grow 30-50% larger than their non-mycorrhizal counterparts under similar conditions.

Soil Structure and Health

Mycorrhizal fungi contribute significantly to soil formation and stability:

1. Aggregate formation: Fungal hyphae physically bind soil particles together, while also producing glomalin, a glycoprotein that acts as a natural soil glue.
2. Carbon sequestration: Mycorrhizal networks store significant amounts of carbon in soil, contributing to long-term carbon storage.
3. Soil biodiversity: These fungi create habitat niches for other beneficial soil organisms, enhancing overall soil health.

Ecosystem Functioning

At the ecosystem level, mycorrhizal networks create connections between plants that influence community dynamics:

1. Common mycorrhizal networks (CMNs): Also called “wood wide webs,” these underground networks connect different plants through shared fungal partners, allowing for resource sharing and communication.
2. Successional processes: Mycorrhizal fungi can influence which plant species establish and thrive in a given area.
3. Forest resilience: Mature trees can support seedlings through mycorrhizal networks, particularly in stressful environments.

Plant Protection

Mycorrhizal associations provide multiple protective benefits:

1. Pathogen resistance: Fungi can produce antibiotics, compete with pathogens for space and resources, and trigger plant defense mechanisms.
2. Heavy metal tolerance: Some mycorrhizal fungi can bind toxic metals, preventing their uptake by plants.
3. Salt and pH tolerance: Enhanced ability to withstand soil chemical stresses.

Applications in Human Systems

The beneficial properties of mycorrhizae have numerous practical applications:

Factors Affecting Mycorrhizal Colonization

Several environmental and management factors influence the development and functioning of mycorrhizal associations:

1. Soil disturbance: Practices like tilling and excavation can disrupt fungal networks.
2. Fertilization: Excessive phosphorus fertilization can reduce mycorrhizal colonization.
3. Fungicides: Many agricultural fungicides negatively impact beneficial mycorrhizal fungi.
4. Plant diversity: More diverse plant communities typically support more diverse and resilient mycorrhizal networks.
5. Soil organic matter: Higher organic matter generally supports more abundant mycorrhizal communities.
6. pH and soil chemistry: Different mycorrhizal types have different preferences for soil conditions.

Understanding these factors is essential for managing systems to promote healthy mycorrhizal associations.

FAQs About Mycorrhizae

Q: Can all plants form mycorrhizal associations?

A: No. While approximately 90% of land plants form mycorrhizal relationships, some plant families are typically non-mycorrhizal, including Brassicaceae (cabbage family), Chenopodiaceae (goosefoot family), and Amaranthaceae. These plants have evolved alternative strategies for nutrient acquisition.

Q: How can I promote mycorrhizal fungi in my garden?

A: Several practices support mycorrhizal development:

• Minimize soil disturbance (reduce tilling)
• Avoid over-fertilization, especially with phosphorus
• Add organic matter through compost or mulch
• Reduce fungicide use
• Consider using commercial mycorrhizal inoculants when planting

Q: How long does it take for mycorrhizal associations to form?

A: The time varies by plant and fungal species, but initial colonization can begin within days of root contact with fungal propagules. Substantial colonization typically develops within 2-4 weeks under favorable conditions.

Q: Can mycorrhizal fungi help plants in contaminated soils?

A: Yes. Many mycorrhizal fungi can help plants tolerate heavy metals and other soil contaminants by either preventing their uptake or assisting plants in managing absorbed contaminants. This property makes them valuable for phytoremediation projects.

Q: Are mushrooms I see in my garden mycorrhizal fungi?

A: Some may be, but many garden mushrooms are saprophytic (decomposers) rather than mycorrhizal. Ectomycorrhizal fungi produce visible mushrooms (like boletes and amanitas), but the more common arbuscular mycorrhizal fungi do not produce visible fruiting bodies.

Q: How do mycorrhizal fungi survive when there are no host plants?

A: Different types have different strategies. Ectomycorrhizal fungi can sometimes live saprophytically for periods without hosts. Arbuscular mycorrhizal fungi produce resistant spores that can remain viable in soil for years until suitable host plants are available.

References

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