Information

Utilising Mycorrhizal Association


We know that mycorrhizal association helps the plant to absorb nutrients. Why can't this association be exploited in agriculture ?

Can't we add spores of the specific fungi to specific plant and enjoy the benefit of this association as we would not have to add many fertilizers ?( We could add organic manure whose main drawback is the time it takes to release nutrients. The fungi could probably solve the problem.)


Mycorrhizal fungi are often used in small-scale agriculture for just that reason. If you search mycorrhizae on amazon they sell it by the tub. I think its especially used in organic gardening for just what you said, the slightly less-processed organic fertilizers are harder to take up by plants without their mycorrhizae partners.

I'm not sure about how widespread the practice of applying mycorrhizae is in large-scale non-organic farming. Judging by how big of a problem fertilizer runoff is, my guess is that a lot of companies have decided that their bottom line is best served by just swamping everything with artificial fertilizers. I'm not sure how mycorrhizae is produced either, it might really be prohibitively expensive to scale production of it up that much.


MYCORRHIZAL FUNGI

L.M. Egerton-Warburton , . S.L. Finkelman , in Encyclopedia of Soils in the Environment , 2005

Games and Models for Mapping Causality

Knowing the structure of water yields no clues as to why water goes down a drain in a vortex. Similarly, in mycorrhizae, the genes are expressed and the metabolic machinery produces parts of the system, but they provide no information on how structure in mycorrhizal systems is generated. Mapping is a way of generalizing dynamics and establishing conceptual illustrations and models using simple input⧸output diagrams. The cognitive system map ( Figure 5 ) draws a causal relationship among components and predicts how complex events might play out. Each arrow defines a causal link where ‘plus’ is an increase or enhanced response, ‘minus’ is a decrease, or negative response, and ‘zero’ is stasis. The final cognitive map defines the nonlinear dynamic system.

Figure 5 . A single-state model of the relationship between the environment and the mycorrhizal system in response to soil nitrogen status.

This single-state map for a one plant–one fungus mycorrhizal system and its response to soil N availability illustrates that the fungal response is enhanced by plant nutrient stress and decreased by an increase in nutrient availability and drought. The plant response is enhanced by P availability and decreased by drought. The overall model is the ‘If condition, then action,’ which has the same meaning as statements in programming language. If the condition portion is satisfied, then the action is triggered and other rules are activated. This is the ‘bucket brigade’ algorithm (sensu J.H. Holland), where signals are passed down the line to reinforce a chain of effect. This is also similar to path analysis. Calculations of the potential outcomes (fungus, plant, system, or 3 n ) from the model ( Figure 5 ), indicate that if soil N is limited, then the fungus transports approximately 64% of N to plant, or strengthens the importance of the fungus to the system. However, if soil N increases then the fungus transports only approximately 34% of plant N.

Adding more participants into the system increases the number of possible outcomes exponentially. A two-fungus system responds quite differently to the environment than a one-fungus system. Fungal respiration is depressed in each of Rhizopogon and Suillus mycorrhizae by 72% and 30%, respectively, after the addition of NH4 ( Figure 6 ). An additive cognitive map, however, indicates that NH4 may have a bigger negative effect on respiration in RhizopogonSuillus in concert than on a single species or the edges where they meet. Using mapping rules as patches also demonstrates the nonlinear nature of the sporocarp δ 15 N:δ 13 C divide between mycorrhizal and saprobic fungi ( Figure 7 ). In particular, when δ 15 N is ±0, δ 13 C in mycorrhizal fungi is depleted, whereas δ 13 C in saprobes is enriched.

Figure 6 . An additive map of the predicted interactions between the ectomycorrhizal fungi, Rhizopogon and Suillus, after media enrichment with ammonia. Ranking of analysis (highest to lowest effect) shows that NH4 influences: two species together in culture (Rhizopogon ∪ Suillus, 17⧸12) single-species cultures (Rhizopogon or Suillus alone, 13⧸12) fungal contaminants (neither Rhizopogon nor Suillus, 11⧸12) patches where each species intersects (Rhizopogon ∩ Suillus, 7⧸12).

Figure 7 . (a) Ratio of δ 15 N:δ 13 C in sporocarps of mycorrhizal and nonmycorrhizal (saprobic) fungi (b) additive patch map of δ 15 N:δ 13 C in mycorrhizal and nonmycorrhizal fungi. EM, ectomycorrhizae.

As more knowledge emerges, so do more rules and a more elaborate model that is connected to actual observations. For example, the N needs of the fungus and plant are determined by the level of internal N reservoirs that are depleted at a rate that keeps the fungus and plant functioning, so that low N implies a high need for the resource. The N level in the reservoir may only be increased by providing the correct signal molecule or transporter. Adding these new parameters into Figure 6 increases the number of potential iterations and outcomes and, in turn, the complexity of the system. Such complex systems are controlled by feedback mechanisms between plants and their mycorrhizal mutualists and soil microbes, and local host-dependent preferences for certain communities of fungi.


Author's Note: This is the first in a series of basic mushroom information in a new section of the NAMA website. We start here by describing the three most common types of mycorrhizal relationships. More to follow!

The first single-celled fungi appeared on planet Earth nearly a billion years ago. By the time plants colonized land in the Silurian (443 to 416 million years ago) and Devonian periods (419 to 359 million years ago), fungi in the phylum Mucoromycota had developed an arbuscule, a tree-like structure capable of colonizing plant cells and exchanging nutrients. This evolutionary step resulted in the first plant-fungal symbiosis, arbuscular mycorrhiza. Arbuscules can be found in fossils from 450 million years ago these ancient structures are very, very similar to arbuscules we see today.

As evolution continued, new mycorrhizal types appeared: ectomycorrhizas (about 200 million years ago) and ericoid mycorrhizas (about 100 million years ago) evolved subsequently as the organic matter content of some ancient soils increased and sclerophyllous vegetation arose as a response to nutrient-poor soils respectively (Cairney, 2000).

Mycorrhizae are present in 92% of plant families (80% of species). Plants allow, and indeed require, mycorrhizal fungi to colonize their roots. In this symbiotic relationship, fungal hyphae greatly expand the ability of plants to obtain nutrients and water. Fungi break down organic matter and weather mineral surfaces, and in so doing collect essential nutrients such as nitrogen, phosphorus, potassium, and nearly a dozen minerals.

Let&rsquos look at three major types of mycorrhizal relationships: Ectomycorrhizal, Arbuscular (often formally referred to as Vesicular-Arbuscular), and Ericoid. There are structural and functional differences between these types: how they colonize roots the resources they provide and the types of plants they partner with.

Three Common Mycorrhizal Types

EctomycorrhizalArbuscularEricoid
Hartig net and mantle, intracellular colonization
Arbuscules and vesicles in root cortical cells
Colonize epidermal cell, forming dense hyphal coils
Connections last 2-4 years or moreArbuscules last 4-15 daysHair root associations are ephemeral
Mainly provides Nitrogen and Phosphorus, and 12 other nutrientsMainly provides Phosphorus and Nitrogen, Sulfur, Copper, Iron and ZincMainly provides Nitrogen, Phosphorus and Iron
Colonizes 2 Gymnosperm lineages and 28 Angiosperm lineagesColonizes vascular plants, mainly found in grasslandsColonizes dense fibrous roots of ericaceaous plants
Basidiomycetes and some AscomycetesGlomeromycotaAscomycetes and some Basidiomycetes
Long branching hyphaeThin hyphaeShort hyphae
Temperate, boreal, Mediterranean, and some tropical forestsTropical and temperate forests, grasslands, agricultural cropsHeathlands, tundra, boreal and temperate forest understory

Ectomycorrhizae

Ectomycorrhizae (ECM) are generally host specific, having evolved over the past 200 million years with their plant associates. Most ECM fungi are Basidiomycetes, forming fleshy fungal bodies. In this mycorrhizal type, hyphae cover the root tip with a mantle structure, and then form a Hartig net which penetrates into the space between the root&rsquos cortical cells. Smith and Read describe the Hartig net as a labyrinthine inward growth of hyphae between the epidermal and cortical cells. As the colonized root hairs grow and harden, the association becomes untenable in 2-5 years&rsquo time. The hyphae constantly need to renew connections on new root growth.

ECM associations are generally with pines and hardwoods (gymnosperms and angiosperms). Even though they only represent 2% of mycorrhizae, ECM fungi colonize commercially valuable trees and produce edible fungi, vastly expanding their importance to humans. In the northern temperate regions, plants such as pine (Pinus), spruce (Picea), fir (Abies), poplar (Populus), willow (Salix), beech (Fagus), birch (Betula) and oak (Quercus) typify the ECM association. In total, 140 genera in 43 plant families have been identified as forming ECM.

Ectendo- and Arbutoid Mycorrhizaes

Two additional types of mycorrhizas with characteristics of Ectomycorrhizae (plus five more minor variations) with a high degree of intracellular penetration, should be noted.

Ectendomycorrhiza: occurs primarily on Pinus and Larix and is distinguished by the fact that, in addition to a usually thin fungal mantle and well-developed Hartig net of the ECM type, the epidermal and cortical cells are occupied by intracellular hyphae (Smith and Read, 2008).

Arbutoid: found in ericaceous genera Arbutus (a genus of 12 accepted species of flowering plants in the family Ericaceae) and Arctostaphylos (a genus of plants comprising the manzanitas and bearberries) and genera of the ericaceous subfamily Pyrolae. It is distinguished from the Ectendomycorrhizal category by the restriction of intracellular penetration to the epidermal layers of the root and by the involvement of a distinct suite of largely basidiomycetous fungi more normally found as ECM symbionts of trees (Smith and Read, 2008).

Arbuscular Mycorrhizae

Fossil evidence from the Devonian Period reveals a structure which looks exactly like modern AM fungi. Although the current theory is that plants and fungi arrived on land about the same time, Dr. Joey Spatafora postulates that arbuscular fungal bodies may have preceded plants. AM is thought to have a monophyletic origin in the Ordovician, approximately 480 million years ago (Redecker et al., 2000 Delaux, 2017), and is found in the majority of land plants and virtually all ecological niches (Read, 2002 Wang and Qiu, 2006).



Mature arbuscule of Glomus. Photo courtesy Mark Brundrett, © 2008.

AM fungi are characterized by the formation of unique structures&mdasharbuscules and vesicles&mdashby fungi of the phylum Glomeromycota. AM fungi help plants to capture nutrients such as phosphorus, sulfur, nitrogen and micronutrients from the soil. AM fungi deliver about 90% of the phosphorus and 50% of the nitrogen needed by the host plant.

Arbuscules are short-lived structures (4-15 days) which provide nutrient transfer. Phosphorus is actively transferred to the plant throughout the life of the arbuscule. One reason for the short life of arbuscules could be that because their host plants grow so quickly, fungi constantly need to re-colonize new roots.

After the first arbuscules degrade, structures called vesicles often form within the colonized root. Vesicles, the second characteristic structure of AM mycorrhizae, are thick-walled structures. Vesicles contain large amounts of lipids, and often numerous nuclei. In dead root fragments, vesicles can act as propagules, and re-grow hyphae to colonize new roots (Smith and Read 1997). The hyphal network is relatively long-lived and is able to colonize new roots as they enter its domain.

No convincing evidence has been presented to suggest that AM associations are specific. Lack of specificity is indicated by the fact that globally only around 300&ndash1,600 AM fungal taxa associate with about 200,000 plant spe­cies. In fact, in nature, plant roots are usually simultaneously colonized by multiple AM, which colonize several plant individuals at the same time, often from different species (Walder, 2015). AM fungi colonize grasslands and cultivated agricultural plants, but also colonize trees such as Maples, Cedar, Redwood and Sequoia.

Ericoid Mycorrhizae

Ericaceous plants are generally found in the understory of a forest. They can also be found in alpine ecosystems and bogs in nutrient poor soils. We know ericaceous plants along the West Coast as huckleberry, manzanita, madrone and native rhododendron. In northern states, they&rsquore represented by cranberries and blueberries. On the East Coast, ericaceous plants include magnolia and catalpa.

Plants in Ericaceae have dense fibrous roots which terminate in a structure called &ldquohair roots&rdquo. Because hair roots are &ldquodelicate structures&rdquo, these mycorrhizal associations are relatively short lived. Ericoid mycorrhizae (ERM) are characterized by the formation of intracellular hyphal coils in the epidermis of hair roots and hyphae extended up to 1 cm from the root surface (Read, 1984). ERM typically lack multilayered hyphal mantles.

Glossary Arbuscule &ndash a highly branched (&ldquolittle tree&rdquo) structure which grows inside the plant cell wall but outside the plasma membrane
Ascomycete - a fungus whose spores develop within asci. The ascomycetes include the fungal component of most lichens, and a few large forms such as morels and truffles. Ascomycota is the largest phylum of Fungi, with over 64,000 species.
Basidiomycete &ndash a fungus whose spores develop on a basidium a group of higher fungi that have septate hyphae, including rusts, smuts, mushrooms, and puffballs
Epiphyte - a plant that grows above the ground, supported non-parasitically by another plant or object, and deriving its nutrients and water from the air
Ericoid &ndash resembling heath. The Ericaceae are a family of flowering plants, commonly known as the heath or heather family, found most commonly in acid and infertile growing conditions.
Hartig net - a network of inward growing hyphae, that extends into the root, penetrating between the epidermis and cortex. The Hartig net is named after Theodor Hartig, a 19th-century German forest biologist and botanist.
Hypha (plural &ndash hyphae) - a long, branching filamentous structure of a fungus. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium
Hydrophyte - a plant that grows in water or very moist ground an aquatic plant
Monophyletic &ndash a group of organisms descended from a common evolutionary ancestor
Mycorrhiza (plural - mycorrhizae) - a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's root system.
Rhizosphere &ndash a plant&rsquos root system, and the. narrow region of soil that is directly influenced by root secretions
Vesicle &ndash a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer

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Diversity and classification of mycorrhizal associations

Most mycorrhizas are‘balanced’mutualistic associations in which the fungus and plant exchange commodities required for their growth and survival. Myco-heterotrophic plants have‘exploitative'mycorrhizas where transfer processes apparently benefit only plants. Exploitative associations are symbiotic (in the broad sense), but are not mutualistic. A new definition of mycorrhizas that encompasses all types of these associations while excluding other plant-fungus interactions is provided. This definition recognises the importance of nutrient transfer at an interface resulting from synchronised plant-fungus development. The diversity of interactions between mycorrhizal fungi and plants is considered. Mycorrhizal fungi also function as endophytes, necrotrophs and antagonists of host or non-host plants, with roles that vary during the lifespan of their associations. It is recommended that mycorrhizal associations are defined and classified primarily by anatomical criteria regulated by the host plant. A revised classification scheme for types and categories of mycorrhizal associations defined by these criteria is proposed. The main categories of vesicular-arbuscular mycorrhizal associations (VAM) are and of ectomycorrhizal associations (ECM) are‘epidermal’epidermal ECM occur in certain host plants. Fungus-controlled features result in categories of VAM and ECM. Arbutoid and monotropoid associations should be considered subcategories of epidermal ECM and ectendomycorrhizas should be relegated to an ECM morphotype. Both arbuscules and vesicles define mycorrhizas formed by glomeromycotan fungi. A new classification scheme for categories, subcategories and morphotypes of mycorrhizal associations is provided.


Tree-mycorrhizal associations detected remotely from canopy spectral properties

A central challenge in global ecology is the identification of key functional processes in ecosystems that scale, but do not require, data for individual species across landscapes. Given that nearly all tree species form symbiotic relationships with one of two types of mycorrhizal fungi – arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi – and that AM- and ECM-dominated forests often have distinct nutrient economies, the detection and mapping of mycorrhizae over large areas could provide valuable insights about fundamental ecosystem processes such as nutrient cycling, species interactions, and overall forest productivity. We explored remotely sensed tree canopy spectral properties to detect underlying mycorrhizal association across a gradient of AM- and ECM-dominated forest plots. Statistical mining of reflectance and reflectance derivatives across moderate/high-resolution Landsat data revealed distinctly unique phenological signals that differentiated AM and ECM associations. This approach was trained and validated against measurements of tree species and mycorrhizal association across

130 000 trees throughout the temperate United States. We were able to predict 77% of the variation in mycorrhizal association distribution within the forest plots (P < 0.001). The implications for this work move us toward mapping mycorrhizal association globally and advancing our understanding of biogeochemical cycling and other ecosystem processes.

Table S1. Tree species composition, abundance (%), and mycorrhizal associations (arbuscular mycorrhizal, AM ectomycorrhizal, ECM) at each site (Lilly-Dickey Woods, LDW Smithsonian Conservation Biology Institute, SCBI Tyson Research Center Plot, TRCP Wabikon Forest Dynamics, WFD). Only species with greater than 0.01% basal area are included in this table. A dash indicates the species was not present at that site.

Table S2. Properties of leaves (n = 110) and soils in AM and ECM dominated plots (n = 15) at the LDW site.

Figure S1. A sensitivity analysis whereby the predictive model was trained on half the data then validated against the other half of the data showed robust predictive power relative to the original model (2% degradation).

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.


Which fungi form mycorrhizas?

The fungi that form mycorrhizas are quite varied. Ectomycorrhizal fungi produce many of the easily seen fruiting bodies that you commonly come across. There are many mushroom-producing ectomycorrhizal fungi and examples of Amanita and Cortinarius were given earlier. These are large mycorrhizal genera, with many species worldwide. While there are many native Amanita species in Australia, there are also some introduced species. For example, Amanita muscaria (commonly called the Fly Agaric) and Amanita phalloides (known as the Deathcap [see DEATHCAP SECTION]) have been introduced unintentionally. You’ll commonly see the first of these near pine or birch trees and the latter near oak trees. However, in Tasmania Amanita muscaria has been found growing in native Nothofagus forests - with not a pine or birch tree anywhere to be seen. Amanita muscaria has also been introduced to New Zealand, where it has been found in native forests with Kunzea and Leptospermum. While some fungi may have preferences for particular plants, the Tasmanian and New Zealand reports show that at least some mycorrhizal fungi can take up new partners. There has also been a report of Amanita phalloides associating with eucalypts in Canberra (but this requires more investigation) and there are records of that species in a eucalypt plantation in Tanzania and near eucalypts in Algeria.

Apart from Amanita and Cortinarius there are many more species of mushroom-producing ectomycorrhizal fungi in various genera - Hebeloma , Inocybe, Lactarius , Paxillus, and Russula to give a few examples of commonly seen genera.

As noted at the beginning of this mycorrhizal section, there are numerous non-mushroom, ectomycorrhizal fungi and you’ve already seen the example of Hydnum repandum, which was pictured right at the start. The majority of truffle-like fungi are mycorrhizal and examples of these are the introduced species Melanogaster ambiguus (which is found growing near introduced oak trees), this native species of Setchelliogaster and Peziza whitei , another endemic Australian species. Some of the coral fungi, many boletes, such as the native Phlebopus marginatus and the introduced Suillus luteus (which is found under non-native pine trees) and a few powdery, superficially puffball-like fungi (such as Pisolithus and Scleroderma ) are mycorrhizal.

The earthstar-like Astraeus hygrometricum (introduced to Australia) is known to be ectomycorrhizal with various northern hemisphere tree species.

Species of Morchella can grow quite well as saprotrophs, but some overseas studies suggest that the species of Morchella may sometimes form ectomycorrhizas.

The corticioid fungi commonly produce their flat, sheet-like fruiting bodies on the underside of dead wood, twigs and similar plant litter on the forest floor. It would be excusable to suppose that these are all saprotrophic fungi, feeding on the dead plant matter on which the fruiting bodies are formed. That is often the case but in the northern hemisphere there are about a hundred species of corticioid fungi that are mycorrhizal, many of them in the genus Tomentella. A probable mycorrhizal Tomentella has been found in Australia recently, but the majority of Australian corticioid fungi are poorly studied.

The mycorrhizal nature of a few of those hundred corticioid species in the northern hemisphere has been known for a long time, but in most cases the mycorrhizal evidence was discovered in the late 1900s. These corticioid genera contain confirmed ectomycorrhizal species: Amphinema, Byssocorticium, Byssoporia, Piloderma, Pseudotomentella, Tomentella, Tylospora. Tylopsora fibrillosa is not known from Australia and the photo was taken in a Sphagnum bog in Estonia. The whitish bloom growing over the Sphagnum plants is the Tylospora fruiting body and is lined with the spore-producing basidia. These corticioid genera are suspected to contain ectomycorrhizal species: Athelia, Lindtneria, Tomentellopsis, Trechispora.

There are more photographs of fruiting bodies of ectomycorrhizal fungi here [http://mycorrhizas.info/ecmf.html].

Many of the VA mycorrhizal fungi do not produce fruiting bodies, the spores often being produced on hyphae in the soil near plant roots. Where fruiting bodies are formed they are usually more-or-less spherical in shape and truffle-like, in that they are hidden in the soil or leaf litter. Many fruiting bodies are small, only a millimetre or so in diameter, though some can get to a couple of centimetres or more in diameter. The fungi are neither ascomycetes nor basidiomycetes and are outside the scope of this website.

Identification of the fungi present in ericoid mycorrhizas is mostly based on the physical, chemical or DNA analyses of the hyphae found in roots, since fruiting bodies are rarely found. Understandably, there is still very much to be learnt about the fungi involved. The best known ericoid mycorrhizal fungus, Hymenoscyphus ericae, is an ascomycete that produces small disc-like fruiting bodies up to 1mm in diameter. However, it appears the fruiting bodies have never been seen in the wild and have been produced in laboratory experiments, only with considerable difficulty. While there is still much ignorance about the species of fungi involved, the various chemical, physical and DNA studies indicate that there is considerable similarity between the northern and southern hemisphere fungi that form ericoid mycorrhizas.

Orchids form mycorrhizas with various macro and micro fungi. The association with the mushroom genus Armillaria was mentioned above, but a number of orchids are known to form associations with macrofungi in the genera Ceratobasidium, Sebacina, Thanatephorus, Thelephora, Tomentella and Tulasnella. Many of these form inconspicuous fruiting bodies, sometimes being no more than powdery or cobwebby coatings on forest litter.


Leaf molecules as markers for mycorrhizal associations

Plants, like this wild tobacco Nicotiana attenuata, produce blumenol C derivates (blue) in their roots when they have established a functional symbiosis with arbuscular mycorrhizal fungi (pink). The substances are transported into the leaves and can be used as foliar markers for successful fungal associations. Credit: Ming Wang, Max Planck Institute for Chemical Ecology

In nature, most plants establish mutual relationships with root fungi, so-called mycorrhiza. Mycorrhizal fungi facilitate the plants' nutrient uptake and help them thrive under extreme conditions. Researchers at the Max Planck Institute for Chemical Ecology in Jena, Germany, discovered that certain leaf metabolites can be used as markers for mycorrhizal associations. The discovery of foliar markers provides scientists with an easy-to-conduct tool to screen large amounts of plants for mycorrhizal associations without having to destroy them. This new tool could contribute to breeding more efficient and stress-tolerant crop varieties for sustainable agriculture.

The relationship between plants and so-called arbuscular mycorrhizal fungi is considered to be one of the most important factors for the evolution of terrestrial plants. More than 70 percent of the higher plants establish an association with these fungi, which are believed to be more than 400 million years old. The mutualistic association allows the plant to better absorb nutrients, such as phosphate. Moreover, the symbiosis makes the plants more tolerant of biotic and abiotic stresses, such as insect attack, pathogens and drought.

For plant breeders, mycorrhizal fungi are very important because global phosphate resources are limited. However, until now analysis of the fungal association was only possible by excavating the plant roots. This is not only time-consuming it also destroys the plant.

Scientists at the Max Planck Institute for Chemical Ecology and their partners have now found substances that accumulate in the leaves when arbuscular mycorrhizal fungi successfully colonize plant roots. It has been known for a while that these substances, so-called blumenol C derivates, are produced in the roots exclusively after colonization with the mutualistic fungi. However, until now, all attempts to find a reliable and specific leaf marker have failed.

For their studies, the researchers analyzed the leaf substances with a highly sensitive technique and compared them to leaf compounds from plants that had not been able to establish fungal associations. "Through targeted and highly sensitive mass spectrometry, we were able to find mycorrhizal-specific changes also in above-ground parts of the plants," Ming Wang from the Jena Max Planck Institute describes the unexpected findings. Further experiments confirmed that the observed changes are related to root colonization mycorrhizal fungi. "The blumenols are most likely produced in the roots and then transported to other parts of the plants," Martin Schäfer explains.

Most ecological interactions are highly species-specific. However, the scientists were able to show blumenol accumulation in the leaf tissues of other plant species, including important crop varieties and vegetables. The ubiquity of markers in the shoot across distant plant families is likely due to the long common history of mycorrhizal fungi and plants, suggesting that theses markers play an important role for plants colonized with arbuscular mycorrhizal fungi.

Regardless of the function of these substances, the approach provides a robust and easy-to-apply tool which has the potential to fundamentally change the future of mycorrhizal research and plant breeding. Ian Baldwin, the head of the Department of Molecular Ecology, summarizes the new possibilities: "Our diagnostic marker for the colonization with arbuscular mycorrhiza fungi can be very useful for studying mycorrhizal associations, not only for breeding programs which rely on high-throughput screenings, but also for basic research into fundamental questions about the information transferred from plant-to-plant through fungal networks." Phosphate is a major component of fertilizers and therefore indispensable for agriculture and food production. However, phosphate deposits are limited and are often located in areas of conflict. Experts are already talking about a pending shortage of phosphate and thus fertilizers which could lead to a global food crisis. The new screening method could help breed plants that are more able to negotiate favorable relationships with mycorrhizal fungi so as to acquire phosphate more efficiently.

In a further step, the researchers want to elucidate the role of blumenol accumulation elicited by fungal colonization and find out whether blumenols may also function as signal molecules between plant roots and leaves. They also plan to use the new method to investigate fundamental questions concerning the communication between different plants of the same species and plants of different species over a joint fungal network.


Contents

A mycorrhiza is a symbiotic association between a green plant and a fungus. The plant makes organic molecules such as sugars by photosynthesis and supplies them to the fungus, and the fungus supplies to the plant water and mineral nutrients, such as phosphorus, taken from the soil. Mycorrhizas are located in the roots of vascular plants, but mycorrhiza-like associations also occur in bryophytes [4] and there is fossil evidence that early land plants that lacked roots formed arbuscular mycorrhizal associations. [5] Most plant species form mycorrhizal associations, though some families like Brassicaceae and Chenopodiaceae cannot. Different forms for the association are detailed in the next section. The most common is the arbuscular type that is present in 70% of plant species, including many crop plants such as wheat and rice. [6]

Mycorrhizas are commonly divided into ectomycorrhizas and endomycorrhizas. The two types are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate individual cells within the root, while the hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate the cell membrane. [7] [8] Endomycorrhiza includes arbuscular, ericoid, and orchid mycorrhiza, while arbutoid mycorrhizas can be classified as ectoendomycorrhizas. Monotropoid mycorrhizas form a special category.

Ectomycorrhiza Edit

Ectomycorrhizas, or EcM, are symbiotic associations between the roots of around 10% of plant families, mostly woody plants including the birch, dipterocarp, eucalyptus, oak, pine, and rose [9] families, orchids, [10] and fungi belonging to the Basidiomycota, Ascomycota, and Zygomycota. Some EcM fungi, such as many Leccinum and Suillus, are symbiotic with only one particular genus of plant, while other fungi, such as the Amanita, are generalists that form mycorrhizas with many different plants. [11] An individual tree may have 15 or more different fungal EcM partners at one time. [12] Thousands of ectomycorrhizal fungal species exist, hosted in over 200 genera. A recent study has conservatively estimated global ectomycorrhizal fungal species richness at approximately 7750 species, although, on the basis of estimates of knowns and unknowns in macromycete diversity, a final estimate of ECM species richness would probably be between 20,000 and 25,000. [13]

Ectomycorrhizas consist of a hyphal sheath, or mantle, covering the root tip and a Hartig net of hyphae surrounding the plant cells within the root cortex. In some cases the hyphae may also penetrate the plant cells, in which case the mycorrhiza is called an ectendomycorrhiza. Outside the root, ectomycorrhizal extramatrical mycelium forms an extensive network within the soil and leaf litter.

Nutrients can be shown to move between different plants through the fungal network. Carbon has been shown to move from paper birch trees into Douglas-fir trees thereby promoting succession in ecosystems. [14] The ectomycorrhizal fungus Laccaria bicolor has been found to lure and kill springtails to obtain nitrogen, some of which may then be transferred to the mycorrhizal host plant. In a study by Klironomos and Hart, Eastern White Pine inoculated with L. bicolor was able to derive up to 25% of its nitrogen from springtails. [15] [16] When compared to non-mycorrhizal fine roots, ectomycorrhizae may contain very high concentrations of trace elements, including toxic metals (cadmium, silver) or chlorine. [17]

The first genomic sequence for a representative of symbiotic fungi, the ectomycorrhizal basidiomycete L. bicolor, was published in 2008. [18] An expansion of several multigene families occurred in this fungus, suggesting that adaptation to symbiosis proceeded by gene duplication. Within lineage-specific genes those coding for symbiosis-regulated secreted proteins showed an up-regulated expression in ectomycorrhizal root tips suggesting a role in the partner communication. L. bicolor is lacking enzymes involved in the degradation of plant cell wall components (cellulose, hemicellulose, pectins and pectates), preventing the symbiont from degrading host cells during the root colonisation. By contrast, L. bicolor possesses expanded multigene families associated with hydrolysis of bacterial and microfauna polysaccharides and proteins. This genome analysis revealed the dual saprotrophic and biotrophic lifestyle of the mycorrhizal fungus that enables it to grow within both soil and living plant roots.

Arbutoid mycorrhiza Edit

This type of mycorrhiza involves plants of the Ericaceae subfamily Arbutoideae. It is however different from ericoid mycorrhiza and resembles ectomycorrhiza, both functionally and in terms of the fungi involved. [ citation needed ] It differs from ectomycorrhiza in that some hyphae actually penetrate into the root cells, making this type of mycorrhiza an ectendomycorrhiza. [19]

Endomycorrhiza Edit

Endomycorrhizas are variable and have been further classified as arbuscular, ericoid, arbutoid, monotropoid, and orchid mycorrhizas. [20]

Arbuscular mycorrhiza Edit

Arbuscular mycorrhizas, or AM (formerly known as vesicular-arbuscular mycorrhizas, or VAM), are mycorrhizas whose hyphae penetrate plant cells, producing structures that are either balloon-like (vesicles) or dichotomously branching invaginations (arbuscules) as a means of nutrient exchange. 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. Fossil evidence [5] and DNA sequence analysis [21] suggest that this mutualism appeared 400-460 million years ago, when the first plants were colonizing land. Arbuscular mycorrhizas are found in 85% of all plant families, and occur in many crop species. [9] The hyphae of arbuscular mycorrhizal fungi produce the glycoprotein glomalin, which may be one of the major stores of carbon in the soil. [22] Arbuscular mycorrhizal fungi have (possibly) been asexual for many millions of years and, unusually, individuals can contain many genetically different nuclei (a phenomenon called heterokaryosis). [23]

Ericoid mycorrhiza Edit

Ericoid mycorrhizas are the third of the three more ecologically important types. They have a simple intraradical (growth in cells) phase, consisting of dense coils of hyphae in the outermost layer of root cells. There is no periradical phase and the extraradical phase consists of sparse hyphae that don't extend very far into the surrounding soil. They might form sporocarps (probably in the form of small cups), but their reproductive biology is poorly understood. [8]

Ericoid mycorrhizas have also been shown to have considerable saprotrophic capabilities, which would enable plants to receive nutrients from not-yet-decomposed materials via the decomposing actions of their ericoid partners. [25]

Orchid mycorrhiza Edit

All orchids are myco-heterotrophic at some stage during their lifecycle and form orchid mycorrhizas with a range of basidiomycete fungi. [ citation needed ] Their hyphae penetrate into the root cells and form pelotons (coils) for nutrient exchange. [ citation needed ]

Monotropoid mycorrhiza Edit

This type of mycorrhiza occurs in the subfamily Monotropoideae of the Ericaceae, as well as several genera in the Orchidaceae. These plants are heterotrophic or mixotrophic and derive their carbon from the fungus partner. This is thus a non-mutualistic, parasitic type of mycorrhizal symbiosis. [ citation needed ]

Mycorrhizal fungi form a mutualistic relationship with the roots of most plant species. In such a relationship, both the plants themselves and those parts of the roots that host the fungi, are said to be mycorrhizal. Relatively few of the mycorrhizal relationships between plant species and fungi have been examined to date, but 95% of the plant families investigated are predominantly mycorrhizal either in the sense that most of their species associate beneficially with mycorrhizae, or are absolutely dependent on mycorrhizae. The Orchidaceae are notorious as a family in which the absence of the correct mycorrhizae is fatal even to germinating seeds. [26]

Recent research into ectomycorrhizal plants in boreal forests has indicated that mycorrhizal fungi and plants have a relationship that may be more complex than simply mutualistic. This relationship was noted when mycorrhizal fungi were unexpectedly found to be hoarding nitrogen from plant roots in times of nitrogen scarcity. Researchers argue that some mycorrhizae distribute nutrients based upon the environment with surrounding plants and other mycorrhizae. They go on to explain how this updated model could explain why mycorrhizae do not alleviate plant nitrogen limitation, and why plants can switch abruptly from a mixed strategy with both mycorrhizal and nonmycorrhizal roots to a purely mycorrhizal strategy as soil nitrogen availability declines. [27] It has also been suggested that evolutionary and phylogenetic relationships can explain much more variation in the strength of mycorrhizal mutualisms than ecological factors. [28]

Sugar-water/mineral exchange Edit

The mycorrhizal mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose. [29] The carbohydrates are translocated from their source (usually leaves) to root tissue and on to the plant's fungal partners. In return, the plant gains the benefits of the mycelium's higher absorptive capacity for water and mineral nutrients, partly because of the large surface area of fungal hyphae, which are much longer and finer than plant root hairs, and partly because some such fungi can mobilize soil minerals unavailable to the plants' roots. The effect is thus to improve the plant's mineral absorption capabilities. [30]

Unaided plant roots may be unable to take up nutrients that are chemically or physically immobilised examples include phosphate ions and micronutrients such as iron. One form of such immobilization occurs in soil with high clay content, or soils with a strongly basic pH. The mycelium of the mycorrhizal fungus can, however, access many such nutrient sources, and make them available to the plants they colonize. [31] Thus, many plants are able to obtain phosphate, without using soil as a source. Another form of immobilisation is when nutrients are locked up in organic matter that is slow to decay, such as wood, and some mycorrhizal fungi act directly as decay organisms, mobilising the nutrients and passing some onto the host plants for example, in some dystrophic forests, large amounts of phosphate and other nutrients are taken up by mycorrhizal hyphae acting directly on leaf litter, bypassing the need for soil uptake. [32] Inga alley cropping, proposed as an alternative to slash and burn rainforest destruction, [33] relies upon mycorrhiza within the root system of species of Inga to prevent the rain from washing phosphorus out of the soil. [34]

In some more complex relationships, mycorrhizal fungi do not just collect immobilised soil nutrients, but connect individual plants together by mycorrhizal networks that transport water, carbon, and other nutrients directly from plant to plant through underground hyphal networks. [35]

Suillus tomentosus, a basidiomycete fungus, produces specialized structures known as tuberculate ectomycorrhizae with its plant host lodgepole pine (Pinus contorta var. latifolia). These structures have been shown to host nitrogen fixing bacteria which contribute a significant amount of nitrogen and allow the pines to colonize nutrient-poor sites. [36]

Mechanisms Edit

The mechanisms by which mycorrhizae increase absorption include some that are physical and some that are chemical. Physically, most mycorrhizal mycelia are much smaller in diameter than the smallest root or root hair, and thus can explore soil material that roots and root hairs cannot reach, and provide a larger surface area for absorption. Chemically, the cell membrane chemistry of fungi differs from that of plants. For example, they may secrete organic acids that dissolve or chelate many ions, or release them from minerals by ion exchange. [37] Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils. [38]

Disease, drought and salinity resistance and its correlation to mycorrhizae Edit

Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens. These associations have been found to assist in plant defense both above and belowground. Mycorrhizas have been found to excrete enzymes that are toxic to soil borne organisms such as nematodes. [39] More recent studies have shown that mycorrhizal associations result in a priming effect of plants that essentially acts as a primary immune response. When this association is formed a defense response is activated similarly to the response that occurs when the plant is under attack. As a result of this inoculation, defense responses are stronger in plants with mycorrhizal associations. [40]

AMF was also significantly correlated with soil biological fertility variables such as soil microbial communities and associated disease suppressiveness. [41] Thus, ecosystem services provided by AMF may depend on the soil microbiome. [41] Furthermore, AMF was significantly correlated with soil physical variable, but only with water level and not with aggregate stability. [42] [43] and are also more resistant to the effects of drought. [44] [45] [46] The significance of arbuscular mycorrhizal fungi includes alleviation of salt stress and its beneficial effects on plant growth and productivity. Although salinity can negatively affect arbuscular mycorrhizal fungi, many reports show improved growth and performance of mycorrhizal plants under salt stress conditions. [47]

Resistance to insects Edit

Research has shown that plants connected by mycorrhizal fungi can use these underground connections to produce and receive warning signals. [48] [49] Specifically, when a host plant is attacked by an aphid, the plant signals surrounding connected plants of its condition. The host plant releases volatile organic compounds (VOCs) that attract the insect's predators. The plants connected by mycorrhizal fungi are also prompted to produce identical VOCs that protect the uninfected plants from being targeted by the insect. [48] Additionally, this assists the mycorrhizal fungi by preventing the plant's carbon relocation which negatively affects the fungi's growth and occurs when the plant is attacked by herbivores. [48]

Colonization of barren soil Edit

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. [50] The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes. [51] The introduction of alien mycorrhizal plants to nutrient-deficient ecosystems puts indigenous non-mycorrhizal plants at a competitive disadvantage. [52] This aptitude to colonize barren soil is defined by the category Oligotroph.

Resistance to toxicity Edit

Fungi have been found to have a protective role for plants rooted in soils with high metal concentrations, such as acidic and contaminated soils. Pine trees inoculated with Pisolithus tinctorius planted in several contaminated sites displayed high tolerance to the prevailing contaminant, survivorship and growth. [53] One study discovered the existence of Suillus luteus strains with varying tolerance of zinc. Another study discovered that zinc-tolerant strains of Suillus bovinus conferred resistance to plants of Pinus sylvestris. This was probably due to binding of the metal to the extramatricial mycelium of the fungus, without affecting the exchange of beneficial substances. [52]

Mycorrhizae and climate change refers to the effects of climate change on mycorrhizae, a fungus which forms an endosymbiotic relationship between with a vascular host plant [54] by colonizing its roots, and the effects brought on by climate change. Climate change is any lasting effect in weather or temperature. It is important to note that a good indicator of climate change is global warming, though the two are not analogous. [55] However, temperature plays a very important role in all ecosystems on Earth, especially those with high counts of mycorrhiza in soil biota.

Mycorrhizae are one of the most widespread symbioses on the planet, as they form a plant-fungal interaction with nearly eighty percent of all terrestrial plants. [56] The resident mycorrhizae benefits from a share of the sugars and carbon produced during photosynthesis, while the plant effectively accesses water and other nutrients, such as nitrogen and phosphorus, crucial to its health. [57] This symbiosis has become so beneficial to terrestrial plants that some depend entirely on the relationship to sustain themselves in their respective environments. The fungi are essential to the planet as most ecosystems, especially those in the Arctic, are filled with plants that survive with the aid of mycorrhizae. Because of their importance to a productive ecosystem, understanding this fungus and its symbioses is currently an active area of scientific research.

At around 400 million years old, the Rhynie chert contains an assemblage of fossil plants preserved in sufficient detail that mycorrhizas have been observed in the stems of Aglaophyton major. [5]

Mycorrhizas are present in 92% of plant families studied (80% of species), [9] with arbuscular mycorrhizas being the ancestral and predominant form, [9] and the most prevalent symbiotic association found in the plant kingdom. [29] The structure of arbuscular mycorrhizas has been highly conserved since their first appearance in the fossil record, [5] with both the development of ectomycorrhizas, and the loss of mycorrhizas, evolving convergently on multiple occasions. [9]

Associations of fungi with the roots of plants have been known since at least the mid-19th century. However early observers simply recorded the fact without investigating the relationships between the two organisms. [58] This symbiosis was studied and described by Franciszek Kamieński in 1879–1882. [59] Further research was carried out by Albert Bernhard Frank, who introduced the term mycorrhiza in 1885. [60]


Nutrient Source and Mycorrhizal Association jointly alters Soil Microbial Communities that shape Plant-Rhizosphere-Soil Carbon-Nutrient Flows

Interactions between plants and microorganisms strongly affect ecosystem functioning as processes of plant productivity, litter decomposition and nutrient cycling are controlled by both organisms. Though two-sided interactions between plants and microorganisms and between microorganisms and litter decomposition are areas of major scientific research, our understanding of the three-sided interactions of plant-derived carbon flow into the soil microbial community and their follow-on effects on ecosystem processes like litter decomposition and plant nutrient uptake remains limited. Therefore, we performed a greenhouse experiment with two plant communities differing in their ability to associate with arbuscular mycorrhizal fungi (AMF). By applying a 13 CO2 pulse label to the plant communities and adding various 15 N labelled substrate types to ingrowth cores, we simultaneously traced the flow of plant-derived carbon into soil microbial communities and the return of mineralized nitrogen back to the plant communities. We observed that net 13 C assimilation by the rhizosphere microbial communities and their community composition not only depended on plant-AMF association but also type of substrate being decomposed. AMF-association resulted in lower net 13 C investment into the decomposer community than absence of the association for similar 15 N uptake. This effect was driven by a reduced carbon flow to fungal and bacterial saprotrophs and a simultaneous increase of carbon flow to AMF. Additionally, in presence of AMF association CN flux also depended on the type of substrate being decomposed. Lower net 13 C assimilation was observed for decomposition of plant-derived and microorganism-derived substrates whereas opposite was true for inorganic nitrogen. Interestingly, the decomposer communities assembled in the rhizosphere were structured by both the plant community and substrate amendments which suggests existence of functional overlap between the two soil contexts. Moreover, we present preliminary evidence that AMF association helps plants access nutrients that are locked in bacterial and plant necromass at a lower carbon cost. Therefore, we conclude that a better understanding of ecosystem processes like decomposition can only be achieved when the whole plant-microorganism-litter context is investigated.


THE LIVING SOIL: FUNGI

Fungi are microscopic cells that usually grow as long threads or strands called hyphae, which push their way between soil particles, roots, and rocks. Hyphae are usually only several thousandths of an inch (a few micrometers) in diameter. A single hyphae can span in length from a few cells to many yards. A few fungi, such as yeast, are single cells.

Hyphae sometimes group into masses called mycelium or thick, cord-like &ldquorhizomorphs&rdquo that look like roots. Fungal fruiting structures (mushrooms) are made of hyphal strands, spores, and some special structures like gills on which spores form. A single individual fungus can include many fruiting bodies scattered across an area as large as a baseball diamond.

Fungi perform important services related to water dynamics, nutrient cycling, and disease suppression. Along with bacteria, fungi are important as decomposers in the soil food web. They convert hard-to-digest organic material into forms that other organisms can use. Fungal hyphae physically bind soil particles together, creating stable aggregates that help increase water infiltration and soil water holding capacity.

Soil fungi can be grouped into three general functional groups based on how they get their energy.

  • Decomposers &ndash saprophytic fungi &ndash convert dead organic material into fungal biomass, carbon dioxide (CO2), and small molecules, such as organic acids. These fungi generally use complex substrates, such as the cellulose and lignin, in wood, and are essential in decomposing the carbon ring structures in some pollutants. A few fungi are called &ldquosugar fungi&rdquo because they use the same simple substrates as do many bacteria. Like bacteria, fungi are important for immobilizing, or retaining, nutrients in the soil. In addition, many of the secondary metabolites of fungi are organic acids, so they help increase the accumulation of humic-acid rich organic matter that is resistant to degradation and may stay in the soil for hundreds of years.
  • Mutualists &ndash the mycorrhizal fungi &ndash colonize plant roots. In exchange for carbon from the plant, mycorrhizal fungi help solubolize phosphorus and bring soil nutrients (phosphorus, nitrogen, micronutrients, and perhaps water) to the plant. One major group of mycorrhizae, the ectomycorrhizae (see third photo below), grow on the surface layers of the roots and are commonly associated with trees. The second major group of mycorrhizae are the endomycorrhizae that grow within the root cells and are commonly associated with grasses, row crops, vegetables, and shrubs. Arbuscular mycorrhizal (AM) fungi are a type of endomycorrhizal fungi. Ericoid mycorrhizal fungi can by either ecto- or endomycorrhizal.
  • The third group of fungi, pathogens or parasites, cause reduced production or death when they colonize roots and other organisms. Root-pathogenic fungi, such as Verticillium, Pythium, and Rhizoctonia, cause major economic losses in agriculture each year. Many fungi help control diseases. For example, nematode-trapping fungi that parasitize disease-causing nematodes, and fungi that feed on insects may be useful as biocontrol agents.

Many plants depend on fungi to help extract nutrients from the soil. Tree roots (brown) are connected to the symbiotic mycorrhizal structure (bright white) and fungal hyphae (thin white strands) radiating into the soil.

Credit: Randy Molina, Oregon State University, Corvallis. P lease contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

Fungus beginning to decompose leaf veins in grass clippings.

Credit: No. 48 from Soil Microbiology and Biochemistry Slide Set. 1976. J.P. Martin, et al., eds. SSSA, Madison WI. P lease contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

Ectomycorrhizae are important for nutrient absorption by tree and grape roots. The fungus does not actually invade root cells but forms a sheath that penetrates between plant cells. The sheath in this photo is white, but they may be black, orange, pink, or yellow.

Credit: USDA, Forest Service, PNW Research Station, Corvallis, Oregon. P lease contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

The dark, round masses inside the cells of this clover root are vesicules for the arbuscular mycorrhizal fungus (AM).

Credit: Elaine R. Ingham. P lease contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

Where Are Fungi?

Saprophytic fungi are commonly active around woody plant residue. Fungal hyphae have advantages over bacteria in some soil environments. Under dry conditions, fungi can bridge gaps between pockets of moisture and continue to survive and grow, even when soil moisture is too low for most bacteria to be active. Fungi are able to use nitrogen up from the soil, allowing them to decompose surface residue which is often low in nitrogen.

Fungi are aerobic organisms. Soil which becomes anaerobic for significant periods generally loses its fungal component. Anaerobic conditions often occur in waterlogged soil and in compacted soils.

Fungi are especially extensive in forested lands. Forests have been observed to increase in productivity as fungal biomass increases.

In arid rangeland systems, such as southwestern deserts, fungi pipe scarce water and nutrients to plants.

Credit: Jerry Barrow, USDA-ARS Jornada Experimental Range, Las Cruces, NM. P lease contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

Mushrooms, common in forest systems, are the fruiting bodies made by a group of fungi called basidiomycetes. Mushrooms are "the tip of the iceberg" of an extensive network of underground hyphae.

Credit: Ann Lewandowski, NRCS Soil Quality Institute. P lease contact the Soil and Water Conservation Society at [email protected] for assistance with copyrighted (credited) images.

Mycorrhizal Fungi in Agriculture

Mycorrhiza is a symbiotic association between fungi and plant roots and is unlike either fungi or roots alone. Most trees and agricultural crops depend on or benefit substantially from mycorrhizae. The exceptions are many members of the Cruciferae family (e.g., broccoli, mustard), and the Chenopodiaceae family (e.g. lambsquarters, spinach, beets), which do not form mycorrhizal associations. The level of dependency on mycorrhizae varies greatly among varieties of some crops, including wheat and corn.

Land management practices affect the formation of mycorrhizae. The number of mycorrhizal fungi in soil will decline in fallowed fields or in those planted to crops that do not form mycorrhizae. Frequent tillage may reduce mycorrhizal associations, and broad spectrum fungicides are toxic to mycorrhizal fungi. Very high levels of nitrogen or phosphorus fertilizer may reduce inoculation of roots. Some inoculums of mycorrhizal fungi are commercially available and can be added to the soil at planting time.

Mycorrhizal fungi link root cells to soil particles sand grains are bound to a root by hyphae from endophytes (fungi similar to mycorrhizae), and by polysaccharides secreted by the plant and the fungi.