Mycorrhizal associations have been described in virtually all economically important plant groups. Investigators in Europe detected fungal associations in most European species of flowering plants, all gymnosperms, ferns, and some bryophytes, especially the liverworts.
Similar patterns are predicted in other ecosystems. Continuing studies of ecosystems, from boreal forests to temperate grasslands to tropical rain forests and agroecosystems, also suggest that most plant groups are intimately linked to one ormore species of fungus.
It is theorized that most of the plants in stable habitats where competition for resources is common probably have some form of mycorrhizal association. Species from all of the major taxonomic groups of fungi, including the Ascomycotina, Basidiomycotina, Deuteromycotina,and Zygomycotina, have been found as partners with plants in mycorrhizae.
Considering the prevalence of mycorrhizae in the world today, botanists theorize that mycorrhizae probably arose early in the development of land plants. Some suggest that mycorrhizae may have been an important factor in the colonization of land.
The fungal partner (or mycobiont) in a mycorrhizal relationship benefits by gaining a source of carbon. Often these mycobionts are poor competitors in the soil environment. Some mycobionts have apparently coevolved to the point that they can no longer live independently of a plant host.
The plant partner in the mycorrhizal relationship benefits from improved nutrient absorption. This may occur in different ways; for example, the mycobiont may directly transfer nutrients to the root. Infected roots experience more branching, thus increasing the volume of soil that the plant can penetrate and exploit.
Evidence also suggests that mycorrhizal roots may live longer than roots without these associations. Comparison of the growth of plants without mycorrhizae to those with fungal partners suggests that mycorrhizae enhance overall plant growth.
Types of Mycorrhizae
Mycorrhizae may be classified into two broad groups: endomycorrhizae and ectomycorrhizae. Endomycorrhizae enter the cells of the root cortex. Ectomycorrhizae colonize plant roots but do not invade root cortex cells.
The most common form of endomycorrhizae are the vesicular-arbuscular mycorrhizae. The fungi involved are zygomycetes. These mycorrhizae have internal structures called arbuscules, which are highly branched, thin-walled tubules inside the root cortex cells near the vascular cylinder.
It is estimated that 80 percent of all plant species may have vesicular-arbuscular mycorrhizae. This type of mycorrhiza is especially important in tropical trees.
There are several other subtypes of endomycorrhizae. Ericoid mycorrhizae, found in the family Ericaceae and closely related families, supply the host plants with nitrogen. These are usually restricted to nutrient-poor, highly acidic conditions, such as heath lands.
Arbutoid mycorrhizae, found in members of the Arbutoideae and related families, share some similarities with ectomycorrhizae in that they form more developed structures called the sheath and Hartig net (described below).
Monotropoid mycorrhizae, found in the plant family Monotropaceae, are associated with plants that lack chlorophyll. The host plant is completely dependent on the mycobiont, which also has connections to the roots of a nearby tree.
Thus the host, such as Monotropa, indirectly parasitizes another plant by using the mycobiont as an intermediate. Orchidaceous mycorrhizae are essential for orchid seed germination.
Ectomycorrhizae are common in forest trees and shrubs in the temperate and subarctic zones. Well-developed fungal sheaths characterize these mycorrhizae, along with special structures called Hartig nets.
Basidiomycetes are the usual mycobionts and often form mushrooms or truffles. Ectomycorrhizae help protect the host plant from diseases by forming a physical fungal barrier to infection.
Anatomy and Development
Individual filaments of a fungal body are called hyphae. The entire fungal body is called a mycelium. Root infection may occur from fungal spores that germinate in the soil or from fungal hyphae growing from the body of a nearby mycorrhiza. When infection occurs, hyphae are drawn toward certain chemical secretions from a plant root.
In ectomycorrhizae, root hairs do not develop in roots after infection occurs. Infected roots have a fungal sheath, or mantle, that ranges from 20 to 40 micrometers thick. Fungal hyphae penetrate the root by entering between epidermal cells. These hyphae push cells of the outer root cortex apart and continue to grow outside of the cells.
This association of hyphal cells and root cortex cells is called a Hartig net. In ectomycorrhizae, the mycobionts never invade plant cells, nor do they penetrate the endodermis or enter the vascular cylinder. The root tip may be ensheathed by fungi, but the apical meristemis never invaded.
Main roots experience fewer anatomical changes than lateral roots after infection. Lateral roots become thickened, may show the development of characteristic pigments, and grow very slowly. Infected roots also show different branching patterns than those of uninfected roots.
Endomycorrhizae are highly variable in structure. Many endomycorrhizae do not have sheaths or Hartig nets. In all endomycorrhizae, hyphae penetrate into root cortex cells, while portions of the mycelium remain in contact with the soil.
The hyphae that remain in the soil are important in fungal reproduction and produce large numbers of haploid spores. Fungi do not invade root meristems, vascular cylinders, or chloroplast-containing cells in the plant.
Some of the host cells contain fungal extensions called vesicles that are filled with lipids. Vesicles are specialized structures that are often thick-walled and may serve as storage sites or possibly in reproduction.
Vesicles are also produced on the hyphae that grow in the soil. Near the vascular cylinder, the hyphae branch dichotomously and form large numbers of thin-walled tubules called arbuscules that invade host cells.
The arbuscules cause the host membranes to fold inward, creating a plant-fungus interface that has a very large surface area. The arbuscules last for about fourteen days before they break down on their own or are digested by the host cell. Host cells whose fungal arbuscules have broken down may be reinvaded by other hyphae.