Mycorrhiza

A mycorrhiza (Greek for fungus roots; typically seen in the plural forms mycorrhizae or mycorrhizas) is a symbiotic (occasionally weakly pathogenic) association between a fungus and the roots of a plant.^[1] In a mycorrhizal association the fungus may colonize the roots of a host plant either intracellularly or extracellularly.

This mutualistic association provides the fungus with relatively constant and direct access to mono- or dimeric carbohydrates, such as glucose and sucrose produced by the plant in photosynthesis.^[2] The carbohydrates are translocated from their source location (usually leaves) to the root tissues and then to the fungal partners. In return, the plant gains the use of the mycelium's very large surface area to absorb water and mineral nutrients from the soil, thus improving the mineral absorption capabilities of the plant roots.^[3] Plant roots alone may be incapable of taking up phosphate ions that are immobilized, for example, in soils with an basic pH. The mycelium of the mycorrhizal fungus can however access these phosphorus sources, and make them available to the plants they colonize.^[4] The mechanisms of increased absorption are both physical and chemical. Mycorrhizal mycelia are much smaller in diameter than the smallest root, and can explore a greater volume of soil, providing a larger surface area for absorption. Also, the cell membrane chemistry of fungi is different from that of plants. Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils.

Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens, and are also more resistant to the effects of drought. These effects are perhaps due to the improved water and mineral uptake in mycorrhizal plants.

Mycorrhizas form a mutualistic relationship with the roots of most plant species (although only a small proportion of all species have been examined, 95% of all plant families are predominantly mycorrhizal).^[5]

Plants grown in sterile soils and growth media often perform poorly without the addition of spores or hyphae of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil mineral nutrients. The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.^[6]

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Early evidence of mycorrhizal associations

The arbuscular mycorrhizal symbiotic relationship formed between plant roots and fungi is one of the most prevalent symbiotic associations found in plants.^[2] Some of the earliest fossil plants show evidence of mycorrhizas associated with them. Their structure has been highly conserved since they first colonized the soil about 400 million years ago,^[7] and this time point corresponds to the transition from aquatic to terrestrial plant life. Therefore, it is hypothesized that this plant-fungi mutualistic partnership was vital in the colonization of land by early plant species.

Types of mycorrhizae

      The two most common types of mycorrhizas are the ectomycorrhizas and the endomycorrhizas (more commonly known as arbuscular mycorrhizas). The two groups are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate the cell wall of the plant's root cells, while the hyphae of arbuscular mycorrhizal fungi penetrate the cell wall.

Endomycorrhizae

Arbuscular mycorrhizas, or AM (formerly known as vesicular-arbuscular mycorrhizas), are mycorrhiza whose hyphae enter into the plant cell walls, producing structures that are either balloon-like (vesicles) or dichotomously-branching invaginations (arbuscules). The fungal hyphae do not in fact penetrate the protoplast (i.e. the interior of the cell), but invaginate the cell membrane. The structure of the arbuscules greatly increases the contact surface area between the hypha and the cell cytoplasm to facilitate the transfer of nutrients between them.

Arbuscular mycorrhizas are formed only by fungi in the division Glomeromycota, which are typically associated with the roots of herbaceous plants, but may also be associated with woody plants. Fossil evidence^[7]

This type of association is found in 85% of all plant families in the wild, including many crop species such as the grains.^{[citation needed]}

Ectomycorrhizae

Ectomycorrhizas, or EcM, typically form between the roots of woody plants and fungi belonging to the divisions Basidiomycota, Ascomycota, or Zygomycota. They are external mycorrhizae that form a cover on root surfaces and between the root's cortical cells.

Besides the mantle formed by the mycorrhizae, most of the biomass of the fungus is found branching into the soil, with some extending to the apoplast, stopping short of the endodermis.

Ectomycorrhizas are found in 10% of plant families, mostly the woody species, including the oak, pine, eucalyptus, dipterocarp, and olive families.^{[citation needed]}

Other forms of mycorrhizae

Arbuscular and ecto- mycorrhiza associating with plants belonging to the order Ericales form ericoid mycorrhiza, while some Ericales form arbutoid and monotropoid mycorrhiza. All orchids are mycoheterotrophic at some stage during their lifecycle and form orchid mycorrhiza with a range of basidiomycete fungi.

Research by Klironomos and Hart at the University of Guelph, Ontario has found that the mycorrhizal fungus Laccaria bicolor can lure springtails and kill them (probably with a toxin); the fungus utilises the nitrogen from the decaying springtails, and some of this nitrogen also becomes available to the plant that the fungus forms mycorrhizae on (in this study, the Eastern White Pine). Klironomos found that up to 25% of the plant nitrogen came from springtails or insects.^[9][10]

See also

• Glomeromycota
• Basidiomycota
• Ascomycota

References