Mycorrhiza

(French) diagram of Ectomycorrhizae and Endomycorrhizae. The hyphae of ectomycorrhizal fungi do not penetrate the root cells, the Endomycorrhiza do.

Topic in Gardening courses

When we understand the intricate relationships that exist between plants on the one side and the air that we breathe, the soil on that we walk, we realize that the only good way of gardening is to do it organically. Almost all plants live in symbiotic harmony with organisms in the soil, such as bacteria and fungi. This article is about the association between plant roots and fungi, also called 'mycorrhiza.' Fungicides and chemical fertilizers can damage this natural association. Read this article and you understand why. --JE 01:43, 14 June 2012 (PDT)

A mycorrhiza is a symbiotic (generally mutualistic, but occasionally weakly pathogenic) association between a fungus and the roots of a higher plant. In a mycorrhizal association, the fungus colonizes the host plant's roots, either intracellularly or extracellularly. They are an important component of soil life.

Mycorrhiza literally means “fungus-root”. The hyphae or the microscopic filaments of a fungus penetrate the root of a plant. The hyphae form a network or mycelium in the soul around the root. This greatly increases the plant's ability to absorb water and minerals. The fungus shares water and minerals with the plant and the plant shares carbohydrates produced by photosynthesis with the fungus. Some plants are very dependent on the minerals they receive from the fungus, for example some plants are entirely dependent to receive phosphorous in this way.

Definitions of mycorrhiza

• "Mycorrhizas are symbiotic associations essential for one or both partners, between a fungus (specialised for life in soils and plants) and a root (or other substrate-contacting organ) of a living plant, that is primarily responsible for nutrient transfer. Mycorrhizas occur in a specialised plant organ where intimate contact results from synchronised plant-fungus development."
• "A mycorrhiza is a symbiotic (generally mutualistic, but occasionally weakly pathogenic) association between a fungus and the roots of a vascular plant. In a mycorrhizal association, the fungus colonizes the host plant's roots, either intracellularly as in arbuscular mycorrhizal fungi (AMF), or extracellularly as in ectomycorrhizal fungi. They are an important component of soil life and soil chemistry.
• "The symbiotic association of the mycelium of a fungus with the roots of a seed plant"

How many plants form mycorrhiza

• "While it is clear that the majority of plants form mycorrhizas, the exact percentage is uncertain, but it is likely to lie somewhere between 80% and 90%. In many of these associations between 10% and 30% of the food produced by the plant moves through to the fungi."
• "At least 95% of all plants have the mycorrhizal relationship. (One big exception to the mycorrhizal rule is the mustard family (the Brassicaceae). This family of plants with about 3000 species is mostly devoid of the helpful fungus and, in the odd case where mycorrhizae do exist in this family, it is at best a weak relationship.)"

The nutrients that are exchanged

The plants provide the fungus with carbohydrates, such as glucose and sucrose. These carbohydrates are produced through photosynthesis mainly in the leaves of plants and flow to the roots and from there to the fungi.

The fungi extend the root system of the plant and give access to more water and to several minerals.

Mycorrhizal fungi degrade organic matter directly and some pass on nitrogen, given to them by nitrogen fixing bacteria that live on these fungi, to their plant hosts in a form that these plants can use. This is how nutrient-poor sites can be colonized, for example by pines.

Also minerals such as phosphorus and potassium can be passed on from the fungi to the plants. Ectomycorrhizal fungi are able to dissolve rock to extract such minerals.

In forests the hyphae of ectomycorrhizal fungi form an extensive network linking together several trees. This highly branched network largely increases the surface of the plant roots and it also allows for exchanges of nutrients and water between trees. Water can travel from moist soil to dry areas to be absorbed by mycorrhizal plants therein.

Moreover, and this is the case with all mycorrhizal plants, the closure of stomata is faster and this gives these plants better resistance to water stress.

Resistance to toxicity

In acid soils, aluminium, iron and manganese can become soluble and some of these elements are toxic to plants. Some ectomycorrhizal fungi secrete siderophores, molecules with the ability to chelate certain metals, including the three mentioned above. This prevents the plants to be poisoned by toxic metals and would also prevent leaching of iron and manganese, which are essential to plant growth.

Studies discovered that certain fungi protect trees and plants against zinc in the soil.

Types of mycorrhiza

Endomycorrhiza

They are the most abundant type of mycorrhiza and the most ancient. It is likely that these fungi originated between 350 and 450 million years ago and probably played an essential role in the colonization of land by the plants. The hyphae of endomycorrhizal fungi penetrate directly into the starch-storing cortex cells of the plant root.

The most common type of endomycorrhiza and also overall type of mycorrhiza is Vesicular Arbuscular Mycorrhiza (VAM). This fungi forms vesicles (balloon-like structures) and arbuscles (tree-like structures) within the cortex of the plant roots it lives on. Plants like mosses and ferns, lilies and tulips, strawberries and violets, apples and oranges all have VAM.

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  Arbuscular mycorrhiza seen under microscope. Arbuscular mycorrhiza are those where the fungus penetrates the outer layer of the cells of the roots of a plant, in this case flax.

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  Cross section Arbuscular mycorrhiza (endomycorrhiza)

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  Wheat is an example of an endomycorrhizal plant

Ectomycorrhiza

Leccinum aurantiacum or red-capped scaber stalk is an example of an ectomycorrhizal fungus

The hyphae of ectomycorrhizal fungi form a mantle around the root and also grow into the spaces between root cells - but do not penetrate the root cells. The hyphae form a net-like covering, called a Hartig net, around the cells.

Ectomycorrhizal associations are formed mostly on the fine root tips of the host plant. They are more abundant in topsoil layers that contain humus.

Ectomycorrhizas are typically formed between the roots of around 10% of plant families, mostly woody plants including for example the birch, eucalyptus, oak, pine, and rose families, and certain types of fungi. Some of these fungi are symbiotic with only one particular genus of plant, while other form mycorrhizas with many different plants. An individual tree may have 15 or more different fungal ectomycorrhizal partners at one time. Thousands of ectomycorrhizal fungal species exist.

For all these trees, they could not compete or survive without the the help of their fungi symbionts. Some of these fungi are famous, such as the truffles, underground fruiting bodies of fungi of the genus Tuber. Other of these fungi are poisonous mushrooms.

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  The (white) mycelium of a fungi associated with the roots of a white spruce.

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  Douglas-fir and Cortinarius spp. ectomycorrhizae

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  Roots of pine colonized by Amanita muscaria, ectomycorrhiza.

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  The mycelium of a fungus surrounds the rootlets of a tree

Ericoid mycorrhiza

An ericoid mycorrhizal fungus isolated from Woollsia pungens

Ericoid mycorrhiza are a symbiotic relationship between fungi and the roots of plants from the order Ericales (for example tea, blueberry, azalea).

Cranberry harvest in New Jersey. Cranberries are an ericoid mycorrhizal crop.

Ericaceous plants commonly grow in low-nutrient soils. The ericoid mycorrhizal fungi enable their host plants to obtain nutrients in these depauperate soils.

The effect of chemical fertilizers and herbicides on mycorrhizae

Pesticides applied to the foliage of plants generally do not have any adverse effects on mycorrhizae formation or function. Some fungicides applied to soil do inhibit mycorrhizae; soil fumigants can kill mycorrhizal fungi. High rates of fertilizers, especially phosphorus, inhibit the formation of mycorrhizae; organic forms of fertilizers seems to have less inhibitory effect on mycorrhizae than inorganic, soluble fertilizers.

Additional interesting information

• Do you know how long all the root segments of the entire root system of a single rye plant can become? According to Brian Capon in his book 'Botany for gardeners' it was once measured. The figure is unbelievable: 380 miles or 630 km. It is estimated that every kilogram of soil contains at least 200 km of fungi strands.
• Mycorrhizal fungi themselves do not fix nitrogen, but bacteria that may associate with them can.
• Mycorrhizal fungi are not present in composts unless the compost contained plant roots. If the compost has been heated from microbial activity, then the mycorrhizal fungi may have been killed. Some composts may be compatible with mycorrhizal fungi, but others may have high salt or nutrient content that can inhibit mycorrhizal fungi.

See also

• Balance of nature
• Micro-organisms in the soil
• Research page:Mycorrhiza

External links

• A plant's best friend, by Gerry Barclay pdf
• An online textbook to provide current information about mycorrhizal associations
• International Mycorrhiza Society
• Mycorrhiza Literature Exchange "Posted here are the abstracts of almost all journal articles dealing with mycorrhizae from 1991 through 2009 (over 11,000 citations), as well as citations to some 260 mycorrhizal reviews, over 70 mycorrhizal books, many mycorrhizal theses and dissertations. Also included is almost every article ever published dealing with water relations and mycorrhizae".
• SpringerLink: Online magazine about mycorrhiza

Videos

• Arbuscular mycorrhiza A collection of videos in the Mashpedia Life Encyclopedia.

"Ever wonder how mycorrhizal fungi improve plant nutrition? This video shows how mycorrhizal hyphae access, unlock and transport soil nutrients."

"Michael Melendrez talks about the role of Mycorrhiza: endo/ecto." Part of the content: Grass roots of any kinds of grass grow with many mycorrhiza, which is why lawns and grass land still keeps much of their fertility despite otherwise wrong agricultural methods.