In her article "Mycorrhizae - Optimizing the roots of your plants", LariAnn Garner introduced us to mycorrhizas & told of the significance these plant-fungal relationships have for trees & many crop species. In short, study & use of mycorrhizas are fields with potentially enormous implications for agriculture & ecological management. For those seeking information relating to mycorrhizal applications, LariAnn's article is a good start. This article will take a look at the variant forms of mycorrhiza, the different plant families in which they are found & some of the role these associations play in various natural regimes around the world.
Exactly how many plants are involved in mycorrhizal relationships is not known, but estimates range as high as 90% of all species. Vesicula-arbuscula (VA) mycorrhizas are the most common & oldest. Scientists have dated these back more than 400 million years & believe this form of plant-fungus co-operation may have been integral to the movement of plants from sea to land. VA mycorrhizas represent one of two broad categories of these associations, each of which appears in a number of forms. 
These are cases where fungi penetrate root cells, forming either shrub-like arbuscules or tuber-like vesicles. The fungus does not enter the cell proper but instead, permeates the cell membrane. Through this process of invagination, transfer of nutrients occurs between the plant & fungus. Endomycorrhizal (EdM) arrangements appear in five forms: VA, arbutoid, ericoid, monotropoid, & orchid mycorrhizas.
As mentioned, VA mycorrhizas are the most numerous form of EdM; appearing in more than 80% of plant familes. They are the ancestral form of all other mycorrhizas. Notably, VA associations occur in a large number of crop species such as wheat. Fungi involved in these associations are thought be a major source of soil carbon. 
An atypical example of VA occurence is in the Japanese Clethra or Sweetshrub, Clethra barbinervis. This is an attractive ornamental, not disimilar to Japanese maple in its fiery autumn display. It is hardy to Zone 5 & grows to around 6 metres, making it suitable for many gardens. It is unusual in being both, the only clethra known to form a mycorrhizal association & the only species in the large & diverse order Ericales to form a VA mycorrhiza.
Amongst the many other Ericales genera are familar species like the camellias (which include tea), the fouquierias & azaleas. There is also Ericaceae, the heath family. Heath plants generally represent the more common - ie. ericoid mycorrihzal - associations seen in this order. Spread around many of the world's edaphically poorer regions, the success of heathlands depends upon the aid of fungi. Without mycorrhizal help, many Ericaceae species struggle to garner required nutrients.
Ericoid mycorrhizas vary from other EdMs only in micro-structural terms. Those interested in the details should see this Ericaceous mycorrhiza site. The important points to observe are ...
- That most plants have some kind of mycorrhizal association, making them each the centre of a micro-system; comprised of plant, soil & fungi. In other words, the soil around a plant's root can be considered a living extension of it, in the majority of species.
- Most plants associate with VA mycorrhiza but a handful of families are involved with other forms. It could be worth knowing which is which, especially when planning a garden.
- The degree to which plants depend on mycorrhizas varies according to species but more particularly, soil regimes. Some mycorrhizal plants will thrive without fungal assistance, others will simply perish. In general terms, most gardens, farms & natural situations will do better with the help of mycorrhizal applications.
The beautiful Sun Orchids, (Thelymitra sp.) are an example of terrestrial orchids
Less commonly than ericoid associations, ericales species also form arbutoid & monotropoid mycorrhizas. As with ericoids, these are defined by microscopic variations between cell-penetrating structures. For an example of a monotropoid ericales, we head for Mongolia to find an unusual member of the heath family in Monotropastrum globosum. This is a fully non-photosynthetic mycoheterotroph, ie. it has abandoned photosynthesis & survives by forming a mycorrhiza with other mycorrhizas already formed by trees.
Notably, M. globosum forms both endo & ectomycorrhizal structures. It appears naturally in forests of mycorrhizal species such as beech & grows a beautifully delicate, semi-transparent flower. In Japan, M. globosum essence is sold for medicinal purposes. In its case, research is underway with regard to the nature of its relationship with other mycorrhizal species. Whether it is mutually beneficial, parasitic or somewhere in between, is yet to be determined. 
Three more unusual monotropoids - even as far as non-photosynthetic epiparasites go - constitute the orchid genus, Rhizanthella. Australia's Western, Eastern & Lamington Underground Orchids, (Rhizanthella gardneri, slateri & omissa) were discovered in 1928, 1931 & 2006 respectively. Not unlike M. globosum in some ways, these plants form parasitic, mycotrophic mycorrhizas & have a glassy, fungus-like appearance. R. gardneri feeds on mycorrhizas formed by the Broom Honey Myrtle, (Melaleuca uncinata). M. slateri appears in eucalypt forests, but its exact symbiotic counterpart(s) has not been determined. R. omissa has only been recently distinguished from R. slateri & remains the most obscure. These achlorophyllous orchids grow entirely underground, only breaching the forest floor to flower & allow pollination by insects. What stands the rhizanthellas out amongst terrestrial orchids however, is the characteristic of forming non-orchid mycorrhizas. 
The life-cycles of nearly all terrestrial orchids depend upon the success of orchid mycorrhizas. These vary from other EdMs in the nature of the nutrient exchange. Organic carbon & minerals are passed from the fungus to the orchid whilst it is a seed. This is known as asymbiotic germination. The fungus feeds the protocorm until it is able to photosynthesise but research thus far, has not uncovered what benefit fungi derive from participating in these associations. The mystery is an added dimension to the rare & unlikely beauty that terrestrial orchids embody.
We have looked at the five forms of EdM & seen some of the associated plant families. Notably, Ericales species & terrestrial orchids feature specialised fungal relationships. This Wikipedia list of Ericales genera makes interesting reading & a handy reference. Gardeners may also take interest in the diverse array of terrestrial orchids around the world. A good example is the Eulophia family; just over 200 hardy & beautiful species spread through Africa, Asia, Australia & the Americas. Certain members such as E. macra, E. petersii, E. plantaginea & E. speciosa are genuine desert orchids. They like to be treated as succulents & will thrive in a cactus garden. There are many more examples & a good place to read up is this Terrestrial Orchid Wiki.
Around 10% of plants are involved in ectomycorrhizal (EcM) associations. In these, fungi generally do not penetrate the roots of their counterparts but form a complex of micro-structures between & around them. These are normally comprised of a hyphal sheaf covering the root tip & a mass of hyphae known as a Hartig Net, surrounding the root cortex.*
Whilst fewer plants form EcM associations, the relationships involved can be as notable & unusual as those seen in EdM arrangements. EcM plants include the rose, pine, eucalyptus & oak familes. And amongst EcMs themselves, there is at least one predator.
* Rare cases in which Hartig Net hyphae penetrate root cells are called ectendomycorrhiza.
Of all the EcM fungus species, the most widely known would be members of the genus Tuber. These cultures associate with a variety of deciduous trees such as beech, oak, hazel & poplar. They are ascomycetes or Sac fungi, making them related to Morels but Tubers vary in featuring an ascoma that develops underground. These truffles, as they are more well known, develop down amongst the roots of trees; but like many mushroom-forming fungi, Tubers rely on animals to find & spread their fruiting bodies. Consequently, mature truffles produce chemical compounds that entice animals to unearth & devour them. The spores are then dispersed in droppings.
Tuber species are not unique as there are many EcMs around the world that form truffle-like ascomas. As a general rule they produce attractive compounds & depend on animals for spore dispersal, but not all are hypogeous. Australia has over 300 truffle-like fungi; all EcMs formed by forest trees such as Eucalyptus, Allocasuarina, Leptospermum, Acacia & Nothofagus . In Africa & the Middle East, there are Desert truffles (Terfeziaceae sp.) & in Europe, the Horse Dung Fungus or Dye-makers Puffball, (Pisolithus tinctorius) is eaten as the Bohemian truffle. Also in Europe, ascomas formd by Elaphomyces species are known as Hart's truffles. These are one of the few non-fragrant types. 
Truffle-like ascomas form under trees such as sheoaks & are dispersed by marsupials. Truffles can be found using pigs in Europe & the US but in Australia, trained bandicoots might be needed
As outlined, EcMs are characteristic of many cool climate trees, with the notable exceptions of maples & cedars. A few trees such as willows are capable of forming both, EcMs & VA EdMs. As discussed by LariAnn in her article, mycorrhizas associated with pines tend to form true mushrooms. These appear in an array of colours & often fairytale-like forms. Classic examples include Fly Agarics, (Amanita muscaria) & Milk Caps (Lactarius sp.). An uncommon example is the Bicoloured Deceiver, (Laccaria bicolor).
L. bicolor associates with the usual variety of deciduous trees but is distinguished by an unusual characteristic; the ability to lure & kill insects in order to obtain nitrogen.  In typical mycorrhizal style, the Deceiver shares garnered nutrients with its vegetable counterpart & interestingly, research has shown that trees like the Eastern White Pine, (Pinus strobus) can gain as much as 25% of its nitrogen in-take in this manner. 
Like the orchid-mothering mycorrhizas, this fungus that hunts for trees clearly illustrates the natural wonder of symbiotic systems. More particularly, it exemplifies the complexity & potency of mycorrhizal relationships. These associations may be largely invisible to us, but they are a critical factor in the lives of most plants. We have everything to gain by understanding mycorrhizas & using them to help build naturally healthier growing environments. Now & in the future, they will prove to be a great ally.
As a work of the U.S. federal government, the image is in the public domain.
Find Out More:
Some kind DG members have offered information for this article that deserves adding in. Gloria125 recommended this excellent Sunseed Technology site that talks about making your own mycorrhiza. LariAnn points out that the Mycorrhizal Applications site has a feature that identifies general mycorrhiza types by plant names (down the bottom of the page). Thank you both for the contributions.
- Mycorrhiza & Tree Nutrition
- Arkive: Images of life on Earth - Rhizanthella gardneri
- Terrestrial Orchid Forum, Gallery & Wiki