Chapter 8

What are Mycorrhizae?

What are Mycorrhizae?

The word mycorrhizae was first used by German researcher A.B. Frank in 1885, and originates from the Greek mycos, meaning 'fungus' and rhiza, meaning 'root'.

Mycorrhiza is a symbiotic mutualistic relationship between special soil fungi and fine plant roots; it is neither the fungus nor the root, but rather the structure formed from these two partners.

Since the association is mutualistic, both organisms benefit from the association.

The fungus receives carbohydrates (sugars) and growth factors from the the plant, which in turn receives many benefits, including increased nutrient absorption.

In this association, the fungus takes over the role of the plant's root hairs and acts as an extension of the root system

Mycorrhizas are highly evolved, mutualistic associations between soil fungi and plant roots. It is commonly known as root fungi.

This association are members of the fungus kingdom (Basidiomycetes, Ascomycetes and Zygomycetes) and most vascular plants.

Host plant receives mineral nutrients while the fungus obtains photosynthetically derived carbon compounds from the plant.

Mycorrhizal associations involve 3-way interactions between host plants, mutualistic fungi and soil factors.

Types of Associations

At least 7 different types of mycorrhizal associations have been recognised, involving different groups of fungi and host plants.

Ectendo-, arbutoid- and monotropoid mycorrhizal associations

Orchid mycorrhizas

Ericoid mycorrhizas

ECM and VAM associations will be considered in detail

Association Occurrence

Vesicular Arbuscular Mycorrhizal (VAM) Plants

Plants with VAM are common in most habitats It is easier to say where they are not found

Ectomycorrhizal (ECM) Plants

Trees with ECM are dominant in coniferous forests, especially in cold boreal or alpine regions

ECM trees and shrubs common in many broad-leaved forests in temperate or mediterranean regions

ECM trees also occur in some tropical or subtropical savanna or rain forests habitats

Ectomycorrhizas (ECM)

The fungus hyphae develop on the root surface.Majority develop fruiting bodiesBasidiomycetes and other fungi form short swollen lateral roots covered by mantle hyphae

(layers of fungal hyphae covering the root surface.)

ECM associations consist of a soil mycelium system, linking mycorrhizal roots and storage or reproductive structures.

Ectomycorrhizal roots are characterised by the presence of a mantle and Hartig net,

Hartig net

Sclerotia

It is a dark, hard and irregular structure

It is the resting bodies formed from the hyphal aggregation

It is a fungal “survival structure”

Formed under unfavorable conditions

Rhizomorphs

Root-like structures, thicker than mycelium.A thick strands of hyphae Extend from the mycelium through the substrateThey help in spreading the fungus from one root system to another root system.

Mycorrhizal roots

Fruiting Bodies

Vesicular-arbuscular mycorrhizas arbuscular mycorrhizas, (VAM or AM)

are associations where Zygomycete fungi in the Glomales produce arbuscules, hyphae and vesicles within roots and outside.

Does not produce fruiting bodies.

Does not form mantle on root surface.

Grow intercellular and intracellular in the root tissue but does not harm the roots

Glomus intraradices

WHY MYCORRHIZAS ARE IMPORTANT

Increased plant nutrient supply by extending the volume of soil accessible to plants.

Increased overall absorption capacity of roots due to morphological and physiological changes in the plant.

There is increased absorption surface area, greater soil area explored (since the fungus acts as an extension of the root) greater longevity of absorbing rootsbetter utilization of low-availability nutrientsbetter retention/storage of soluble nutrients, thus reducing leaching losses.

Increased plant nutrient supply by acquiring nutrient forms that would not normally be available to plants

Increased mobilization and transfer of nutrients (P, N, S, micronutrients Cu, Zn) from the soil to the plant.

Mycorrhizal fungi have been estimated to "substitute" up to 500 lb/a of P for citrus and 170 lb/a for soybeans in tropical areas.

Root colonisation by ECM and VAM fungi can provide olonisation by ECM and VAM fungi can provide protectionprotection from parasitic fungi and nematodes through from parasitic fungi and nematodes through physical and chemical means.physical and chemical means.

Secretion of Secretion of antibiotics antibiotics and support of a community and support of a community that competes or antagonizes pathogenic microorganisms, that competes or antagonizes pathogenic microorganisms, thus aiding in disease suppression;thus aiding in disease suppression;

Increased establishment, nodulation and atmospheric nitrogen fixation capacity in legumes.

Increased production of plant growth hormones such as cytokinins and gibberelins;

Modification of soil-plant-water relations, promoting better adaptation of plant to adverse environment conditions.

At elevated heavy metal concentrations in soils, mycorrhizal fungi have been shown to detoxify the environment for plant growth.