During the Devonian Period some 400 million years ago, land plants first evolved from their aquatic origins. This required adaptations to the terrestrial environment. Although the plants produced their own photosynthetic nutrients from the sun they lacked the means to readily extract necessary mineral constituents from the land, as they had no true roots. The fungi had preceded them ashore by about 100 million years and had evolved to extract minerals by using root-like tendrils called hyphae to penetrate the primordial soil. However, as the fungi could not produce their own food, they needed carbon-based nutrients to absorb. It is hypothesized that this engendered a necessary relationship between the plants and the fungi to enabled them to live together on the land. This theory is bolstered by the observation that fossils from the Devonian have been discovered that clearly show the commingling of fungal hyphae and plant roots. The ability of plants and fungi to exploit diverse large habitats consequent to the breakup of Pangaea during the Mesozoic Era (225 to 65 million years ago) is considered to have been facilitated by the root-hypha association.
The mutualistic relationship that resulted was the mycorrhiza; the word originated in the late 19th Century when botanists discovered that plant roots, though infested with fungi, were not in any way damaged or dysfunctional. They described the condition quite simply as “fungus root” and gave it a name derived from the Greek words mykos for fungus and rhizon for root. The mycorrhiza is the mutualistic symbiosis of a plant and a fungus: both organisms benefit, and, in some cases, the association is obligatory if the populations of either are to increase. The fungal partner is called the mycobiont and the plant partner the phytobiont, either is referred to as mycorrhizal. In the current Holocene (maybe Anthropocene) Epoch, it is estimated that over 90 percent of all plants are mycorrhizal.
The mutualistic mycorrhizal relationship between a plant and a fungus is in essence a sharing of the resources that are both necessary and sufficient for life. The plant supplies the fungus with hexose (having 6 carbon atoms) sugars. Since it is not “normal” for plants to exude nutrients to their surroundings, it is evident that one of the key mechanisms involved in the symbiosis is for the fungus to stimulate the permeability of the plant’s cell membranes. The fungus converts the sugars into what are generally termed reserve materials, as they are used as a means to store energy. The primary energy repository is glycogen, which is a polymer of glucose molecules; it is sometimes called animal starch as it is one of the primary means of storing carbohydrates. It is in the form of insoluble granules that can constitute as much as 10 percent of the dry weight of the fungus. Mycorrhizal fungi also generally produce the disaccharide trehalose which can be converted directly back to glucose and polyhydric alcohols or polyols; where present, these constituents can comprise an additional 15 percent of the fungal dry weight. Some of the plant’s nutrients are thus essentially stored in their associated “fungus roots,” an energy reservoir with some intriguing implications that are manifest in the behavior of forest ecosystems.
The fungus supplies the plant with minerals from the soil, primarily phosphorous and nitrogen. Phosphorous is one of the key constituents of adenosine triphosphate (ATP), which, when hydrolyzed to adenosine diphosphate (ADP) is the primary mechanism of plant cell energy generation. One mole of hydrolyzed ATP yields about 10,000 calories (10kcal in common parlance) of energy as heat. Nitrogen is needed for nucleic acids and chitin, the primary fungal cell wall material; proteins are about 15 percent nitrogen. Phosphorous and nitrogen are accessed by the fungus through the creation of an extensive branching underground network of filamentous thread-like hyphae. The soil nutrients are scavenged by the hyphae, which are capable of storing soil minerals against a significant concentration gradient. Thus the fungus serves two functions; it searches out critical mineral nutrients over a wide geographic area; and it builds up a reservoir of the minerals for release to the plant when needed. It is this storage and release capability that makes the mycorrhizal relationship critical for plants growing in the middle latitudes which are subject to significant seasonal variations and their concomitant nutrient fluctuations; the fungi provide the surge capacity. A mycorrhizal fungus can store enough phosphorus to provide a reserve for the tree for about ten days.
There are seven types of mycorrhizas of which two predominate: endomychoriza and ectomycorrhiza. The prefixes accentuate the fundamental difference between them: endo is from the Greek endon, meaning within and ecto is from the Greek word ektos, meaning outside or external. In terms of mycorrhizal morphology, this means that endomycorrhizas penetrate within the root and ectomycorrhizas extend outside of the root. They are also distinctly different in their populations. Endomycorrhizas are much more common; there are estimated to be over 300,000 plant species in association with about 130 species of fungi. Ectomycorrhizas only involve about 2,000 mostly arboreal plant species; however, some 5,000 different fungi are involved.
Endomycorrhizas are frequently called vesicular-arbuscular mycorrhizas or VAM due to their structure. When a spore from an endomycorrhizal fungus germinates in the vicinity of a receptive plant root, it sends specialized hyphal tendrils that extend in between the root cells to form an arbuscule, meaning “little tree” to indicate its branching structure. Each arbuscule persists for a period that ranges from several days to about 2 weeks during which time it is believed to actively transfer phosphorus to the plant through its many branches. The fungus also forms vesicles, which are membranous cavities typically filled with lipids. In addition to the arbuscules and vesicles that are internal to the root, the fungus also produces an extensive network of hyphae that extend several inches away from the root. This “fungus-root” provides the plant with a vastly expanded volume of soil from which nutrients can be extracted.
Endomychorrhizal fungi are taxonomically distinct enough from all other fungi to warrant their own family; Glomaceae is in the order Glomales, of the Phylum Zygomycota that belongs to the Kingdom Eumycota (formerly Fungi). They are obligately biotopic, which means that they only survive in association with their mutualistic plant associate and that they cannot be grown in an axenic environment in the laboratory. They do not reproduce like the traditional fruiting mushrooms, but rather produce a large, thick skinned spores that typically form spore agglomerations called sporocarps that can be as large as one inch in diameter. All of these features are intended to promote long term survival of the fungus in a subterranean dormant stage, since they do not create a mushroom-like fruiting body to dispense millions of relatively evanescent airborne spores.
Endomycorrhizal plants are much more ubiquitous, numbering in the hundreds of thousands. It is easier to list the exceptions. Aside from the 2,000 woody plants that are ectomycorrhizal, the majority of those that do not associate with the fungi are what are generally characterized as weeds. That is, they are exploitive pioneer plants that germinate quickly in deficient soils with rapidly spreading, finely branched roots that can absorb adequate nutrients without assistance from VAM fungi. Examples include the cyperaceous sedge family and the juncaceous grasses and rushes. The association of fungi with plants across the broad spectrum of species is an additional insight into the nature of their co-evolution; the diversity is indicative many branches from an early common ancestor some 400 million years ago.
Ectomycorrhizas are not as ubiquitous as endomycorrhizas; however, they have a profound effect on the health of forests as they engage the fungi and the trees in an inter-related network of mutual association. The ectomycorrhizal fungus covers the outside of the roots of its associated photobiont with a mantle of hyphae that is called the Hartig net (Robert Hartig was a 19th-century German plant pathologist). The net consists of the hyphae that penetrate and surround the root, excreting hormones that promote root growth and suppress root hair growth; 30 percent of the root’s volume is actually fungal. The overall effect is that the roots of an ectomycorrhizal plant are thicker and much more branched than the roots of a plant without a mycorrhizal fungus. What this means is the ectomycorrhizal plants have a much better root system that has a surge capacity to provide extra nutrients during periods of adversity and a extended reach to pull in nutrients from a greater volume.
The plants that enter into ectomycorrhizal relationships are limited in number but significant in size and importance. This includes all trees in the families of the Pinaceae (pines, firs, spruces, hemlocks and larches), the Fagaceae (oaks, beeches, and chestnuts), the Betulaceae (birches, alders and hophornbeams) and the Salicaceae (willows and poplars) in addition to most myrtles and legumes. In general, the roughly 2,000 plant species from 130 genera in 43 families that enter into ectomycorrhizal relationships with fungi are perennial and woody trees and shrubs. While some of ectomycorrhizal trees are obligately mycotrophic like the pines, most are facultatively mycotrophic, they can survive without the fungi but assume a mycorrhizal relationship in response to stressful environmental conditions. It is this association that promotes the long-term health of the trees; it is these trees that make up the dense stands of trees that comprise the boreal forests and play a significant role in the ecologically balanced habitats. Fungi provide the anastomosis of the root systems, the interconnections and branches that allow the pure stands of trees to predominate. Laboratory and field experiments have demonstrated the trees share carbon resources through their mycorrhizal root systems.
The fungi that enter into ectomycorrhizal relationships extend across a broad range of species that include 45 genera gilled Basidiomycetes and 18 genera of the Ascomycetes. These include many from the ubiquitous agaric genera such as Russula, Lactarius, Cortinarius and Amanita in addition to the chanterelles and the boletes. Some ascomycetes are also mycorrhizal; the truffles are all thought to rely on tree roots for their sustaining nutrients. Most of the fungi can associate with a number of trees, though there is a preferential relationship between some mushrooms and certain host trees; chanterelles prefer oaks and confers while yellow morels prefer dead elm trees and yellow poplars. Mycorrhizal trees can have many fungal partners; the Douglas fir, among the most studied of the pines due to its importance in the timber industry, is thought to be able to form ectomycorrhizas with over 2,000 different fungi.
Of the other five types of mycorrhizas, three are of some interest, the ericaceous, the monotropoid and orchidaceous mycorrhizas. Ericaceous mycorrhizas are with plants of the family Ericaceae, which includes the heathers, rhododendrons and azaleas. That these plants are able to thrive in marginal acidic soil at high altitudes and colder latitudes is due to the exploitation of these habitats by their associated fungi. The colorless, flowering plants of the genus Monotropa such as the Indian Pipe have an unusual life cycle. As they are achlorophyllus, they cannot make their own food. They get it from a fungus via monotropoid mycorrhizal relationship, though no one knows if the plant provides anything to the fungus in return. The key to this unusual relationship is that it is tripartite; the fungus is in an ectomycorrhizal relationship with a nearby tree. The Monotropa thus gets its nutrients from the fungus which in turn gets it from the tree.
Orchidaceous mycorrhizas are necessary for orchids to survive; they are obligately mycorrhizal. The unusual thing about the relationship is that the fungus provides carbon to the orchid, carbon that it has extracted saprophytically from the soil. So far as is known, the fungus gets nothing in return; like the monotropoid mycorrhiza, it is not a mutualistic association. This is of prime importance when the orchid is a seedling, as the seeds of the orchid are very small and have inadequate resources for development. Without the colonizing fungi, the orchid perishes. The provisioning of the orchid plant with carbon by the fungus can be a long term proposition, as some orchids do not produce their first chlorophyll bearing leaf for over ten years. That this is a successful relationship is manifest in the ubiquity of orchids, there are tens of thousands of species.
Fungi are fundamental to the health of natural ecosystems. The mycorrhizal relationship between the food absorbing species of the Kingdom Eumycota and the food producing species of the Kingdom Plantae is critical to the survival of many of the species of both. The hyphae of mycorrhizal fungi permeate the soil and form extensive networks through which nutrients are shared among the associative trees, a relationship that has been facetiously called the “wood wide web.” Some carry this even further; Paul Stamets in Mycelium Running asserts that the mycelia is “the neurological network of nature.” Perhaps not, but it is abundantly clear that fungi are key to the restoration of healthy ecosystems damaged by human activities associated with timber and mining resource extraction. … To say nothing of the potential gains that could be made in agriculture to feed a hungry planet.