What are Mycorrhizal Fungi and How Do They Benefit Plants and Trees?

mycorrhizal fungi
mycorrhizal fungi
(Source: biostim.com.au, used with permission)

What are Mycorrhizal Fungi?

Mycorrhizal fungi are a type of fungi that have evolved to live in close association with the roots of most plants and trees, in a symbiotic relationship where both benefit from each other.

This connection is described as mutualistic association. The term mutualism refers to an ecological relationship between individuals of different species, in which both benefit from the interaction.

The name “mycorrhiza” means “fungus root”, and is derived from the words myco, a Latinized form of the Greek mykēs for “fungus, mushroom” and the Greek word rhiza meaning “root”.

Fungi spread underground by sending out a network of long fine white filaments known as hyphae throughout the soil. The mass of hyphae is known collectively as a mycelium and is the actual vegetative or growing part of fungi.

The hyphae of mycorrhizal fungi attach themselves to roots, and these long fungal filaments effectively extend the root area of plants and trees, providing them with nutrients, such as phosphorous, nitrogen, and water which they would otherwise be unable to access. In exchange, the plants provide the fungus with energy-rich carbon compounds such as sugars which they produce through photosynthesis.

There are several thousand different species of mycorrhiza fungi, and they’re a fundamental component of nearly all terrestrial (land-based) ecosystems, being associated with the roots of more than 90% of land plants. Their mutually beneficial relationship with plant life has possibly existed as early as 460 million years ago, when plants first began growing on land. During the evolution of plants, the development of root systems coincided with the establishment of symbiotic relationships with mycorrhizal fungi.

These fungi differ from the common saprotrophic mushrooms, such as the commercially produced field mushrooms, button mushrooms and shitake mushrooms that live on and feed off dead organic matter.

In this article we’ll discuss the different types of mycorrhizal fungi, how they benefit plants and trees, and how we can encourage and support mycorrhizal fungi in the garden.

Types of Mycorrhizal Fungi

There are two main types of mycorrhizal fungi, arbuscular mycorrhizal (AM) fungi and ectomycorrhizal (EM) fungi, and they differ in the way they associate themselves with roots.

The difference between how arbuscular mycorrhizal (AM) fungi and ectomycorrhizal (EM) fungi attach themselves to roots to exchange nutrients and water (Source: biostim.com.au, used with permission)

1. Arbuscular Mycorrhizal (AM) Fungi

Arbuscular mycorrhizal fungi are the most common type of mycorrhizal fungi and are found in the roots of about 80% of all plant species. They are the most ancient, likely originating 350 to 450 million years ago and are mainly associated with herbaceous (non-woody) plants.

These mycorrhizal fungi penetrate into the cells of the plant roots (the hyphae extend into the cell membrane of the cortex root cells) and form a highly branched, shrub-like structure inside the root cells known as an arbuscule which provides a large surface area for the exchange of nutrients and other compounds between the fungus and the plant. As such, they are also classified as endomycorrhizal fungi (endo = inside, mycorrhiza living within a plant).

Some of these arbuscular mycorrhizal fungi are also known as vesicular-arbuscular mycorrhizal fungi (or VA mycorrhizal fungi) because the fungal hyphae may also form bladder-like vesicles (storage sacs) every so often along their length.

With fungi in general, the mushroom part we often see growing out from the soil or from rotting trees is really just the fruiting body of the fungi that release spores for reproduction.

Arbuscular mycorrhizal fungi do not produce mushroom-like fruiting bodies like other types of fungi. Instead, they produce spores inside swollen structures located along their hyphae known as vesicles (in VA mycorrhizal fungi) or spores (yes, it’s the same name), which are released into the soil when conditions are favourable, to be carried by water or wind to other locations to form new mycorrhizal associations with the roots of plants.

Due to their small size, arbuscular mycorrhizal fungi are difficult to detect without laboratory facilities, as they’re not visible to the naked eye and their hyphae can only be seen under a microscope.

2. Ectomycorrhizal (EM) Fungi

Ectomycorrhizal fungi are more advanced, but only account for about 3% of mycorrhizae, and are mainly found associated with the roots of trees and woody plants.

Unlike the arbuscular mycorrhizal fungi, of which there are no more than a few hundred species, the number of ectomycorrhizal fungi species are far greater, with over 6,000 species of these fungi involved in mycorrhizal associations.

These fungi do not penetrate inside the cells of plant roots like arbuscular mycorrhizal fungi. The name ectomycorrhizal fungi (ecto = outside, mycorrhiza living around a plant) alludes to this.

Instead, ectomycorrhizal fungi form a thick, mesh-like protective sheath structure known as a Hartig net around the root tips of fine feeder roots, which allows for the exchange of nutrients and other compounds between the fungus and the plant. The hyphae of ectomycorrhizal fungi also grow into the spaces between root cells when attaching themselves to the roots.

These ectomycorrhizal fungi often send up fruiting bodies, forming mushrooms. These include the many toadstools commonly seen around trees, as well as large mycorrhizal mushroom-producing genera (plural of genus) such as Amanita and Cortinarius, with many species worldwide.

Toadstools growing under a tree
Toadstools growing under a tree along the path of the roots

Common Amanita species include the very readily recognisable toxic Amanita muscaria (Fly Agaric) found near pine or birch trees and Amanita phalloides (Deathcap) which grows near oak trees. 

fly agaric mushroom Amanita-muscaria
The fly agaric mushroom (Amanita-muscaria), a poisonous toadstool found beneath pine and birch trees, is an ectomycorrhizal fungus (Source: public domain image from Flickr)

That said, there are also numerous non-mushroom producing ectomycorrhizal fungi. The majority of truffle-like fungi that produce their fruiting bodies below the ground are mycorrhizal, including the highly prized edible truffles.

Black truffle Tuber melanosporum
The Black truffle (Tuber melanosporum) is an ectomycorrhizal fungus produces an underground fruiting body that resembles a rock, and is one of the most expensive mushrooms in the world

The Black truffle (Tuber melanosporum) is associated with the roots of hazelnut and oak trees, and produces its fruiting body underground, which releases an intense, pungent aroma when ripe.

The characteristic aroma is comprised of highly odorous, strong, pungent smelling sulfur-containing compounds such as 2,4-dithiapentane, 2,4-dithiapentene, dimethyl sulfide and dimethyl disulfide; alcohols such as 1-octen-3-ol which has a mushroom-like odor and is commonly found in fungi; aldehydes and ketones which have fruity and floral fragrances, such as the aldehyde 2-methylbutanal which contributes to the truffle’s aroma.

Truffles don’t produce all these aromatic compounds on their own though. What’s really fascinating about truffles is that they rely on another symbiotic relationship with soil microbes to produce parts of their aroma!

  • The aroma of black truffles is primarily attributed to the volatile organic compounds such as 2,4-dithiapentane, 2,4-dithiapentene, 1-octen-3-ol, and 2-methylbutanal which they produce themselves as they grow and mature.
  • There are also other volatile organic compounds such as dimethyl sulfide, dimethyl disulfide, and bis(methylthio)methane that contribute to the aroma but are produced by the microbial community that the black truffles also live in symbiosis with.

The aroma of these truffles serves to attract animals which will dig them up and eat them, depositing the spores along with a pile of manure (which acts as fertiliser) elsewhere, so new mycelium can grow and associate themselves with other tree roots.

The way to detect the presence of ectomycorrhizal fungi is by looking for toadstools that follow the path of the roots growing under trees. On tree and shrub roots, a fungal coating may be visible, and the roots with fungal associations may thicken and display an odd branching structure that appear unnatural, though this is harmless, as the fungal association is beneficial.

How Do Mycorrhizal Fungi Benefit Plants and Trees?

Mycorrhizal fungi play an important role in the growth and health of plants and trees, providing numerous benefits, including:

1. Improved nutrient uptake

The main benefit mycorrhizal fungi provide is that they help plants and trees access large amount of water, and certain essential nutrients, particularly nitrogen, phosphorus, zinc, manganese and copper from the soil.

The mycorrhizal hyphae attached to roots function as long filaments that extend the root system and increase the root surface area that can absorb water and nutrients from the soil.

Additionally, since the mycorrhizal hyphae are much smaller in diameter than plant roots, they can grow through narrower spaces between soil particles to reach areas unavailable to the fine feeder roots.

The ability of mycorrhizal fungi to increase plant access to phosphorus is significant because this nutrient is often in very short supply in natural soils and can be the most critical and limiting nutrient in agriculture in certain environments. Most Australian soils, for example, are ancient and highly weathered, with very low levels of natural phosphorus.

Most plants rely on water-soluble forms of phosphorus, which are more easily accessible for uptake. The problem is that the water-soluble forms of phosphorus are not very stable, and react rapidly with iron, aluminium and calcium in the soil to form more stable insoluble compounds which are not immediately accessible to plants. As a result, only 5 % to 30 % of water-soluble phosphorus that’s applied to the soil is taken up by crops in the year after application. Furthermore, in fairly acidic soils, where the pH is less than 5.0, the soil’s capacity to fix (bind) phosphorus increases significantly, further decreasing the amount of phosphorus available to plants.

When the phosphorus in the soil is in insoluble forms, a very large and extensive root system would be necessary in order for a plant to take up enough of the nutrient to meet its phosphorus requirements without assistance. The mycorrhizal fungi that have a symbiotic relationship with plants play a crucial role in gathering phosphorus in uncultivated (unfertilised) soils and help their host plants access insoluble forms of phosphorus.

By increasing the plant’s access to water and nutrients, mycorrhizal fungi can improve plant growth and health.

2. Increased drought and salinity stress tolerance

Mycorrhizal fungi can help plants and trees tolerate drought conditions by extending their root systems and improving their ability to access water from deeper in the soil profile.

These symbiotic fungi can also increase plant and tree tolerance to salinity stress through several mechanisms:

Improving water uptake – When a plant experiences salinity stress, water moves out of the plant roots (which have a lower salt concentration within) into the surrounding soil that has a higher salt concentration by the passive natural process of osmosis, in order to equalise the salt concentration outside and inside the root, which makes it harder for plants to absorb water from the soil. The ability of mycorrhizal fungi to extend the reach of the plant roots and improve water uptake reduces the effects of salinity stress.

Increasing nutrient uptake – Salinity stress in plants can reduce the uptake of essential macronutrients such as nitrogen, phosphorus, and potassium. Mycorrhizal fungi can help to increase the availability of these nutrients by releasing enzymes that break down organic compounds containing these nutrients, releasing them and making them more available to plants.

Legume (bean and pea family) plants have nodules on their roots which house symbiotic bacteria such as Rhizobium species that can capture nitrogen from the air and convert it to a form that plants can use as a nutrient for growth. In exchange, the plants provide the bacteria with sugars that they produce by photosynthesis, much like they do with mycorrhizal fungi. Studies have shown that inoculation with arbuscular mycorrhizal (AM) fungi enhances nodulation and nitrogen fixation in legume and non-legume plants under different stress conditions, and that dual inoculation of arbuscular mycorrhizal fungi and nitrogen-fixing bacteria increased nitrogen fixation in legume plants.

Production of osmolytes – When plants experience unfavourable environmental conditions such as high salt concentrations, extreme temperatures, or drought, they protect themselves from these environmental stress factors by producing osmolytes, small organic molecules of compatible solutes including proline, betaines, soluble sugars, and polyamines that help maintain their cellular integrity and function under such harsh conditions. These compounds are produced directly in response to these stresses and can accumulate to high levels within cells, where they protect proteins and other cellular components from damage and help osmoregulation (the process of maintaining salt and water balance, or osmotic balance, across membranes within the body) by maintaining the water content of cells.

Mycorrhizal fungi can produce osmolytes such as glycine betaine that can help to protect plant cells from the effects of salinity stress. The compound proline is one of the most important osmolytes produced by plants as a response to salinity stress and studies have reported that arbuscular mycorrhizal (AM) fungi increase the proline concentration in plants under salinity stress.

Modulating plant hormone levels – the compound abscisic acid (ABA) is a well-known plant stress hormone that allows plants to survive and grow under various stress conditions, particularly salinity stress. Under salinity stress conditions, ABA accelerates the accumulation of potassium, calcium, and compatible solutes such as proline and sugars in root cells, preventing the uptake of sodium (Na+) and chloride (Cl–) ions, the charged particles that combine to make up salt (NaCl), to protects plants from the severe impacts of salinity stress.

Mycorrhizal fungi can also modulate plant hormone levels, such as increasing the levels of abscisic acid (ABA), which can help plants to tolerate salinity stress. Studies have shown that treatment of salt-affected plants with arbuscular mycorrhizal (AM) fungi altered the concentration of plant stress hormones such as abscisic acid (ABA), jasmonic acid (JA) and salicylic acid (SA).

Protecting against oxidative stress – When plants are exposed to salinity, they experience salinity stress (salinity-induced osmotic stress and ionic stress), which triggers an overproduction of harmful, highly reactive compounds known as reactive oxygen species (ROS).

These compounds naturally occur in plants, and in low concentrations function as key signaling molecules that enable plants to rapidly respond to stress conditions.

At high concentrations, ROS can cause abnormal cell signaling, and induce a secondary stress in plants, known as oxidative stress, that can damage the internal structures of plant cells, disrupt vital cell functions, and cause oxidative cell death.

To avoid oxidative damage by ROS compounds, plants naturally produce antioxidants to protect themselves. Mycorrhizal fungi can produce antioxidants and other protective compounds that help to protect plants against such oxidative stress.

3. Improved soil structure

To understand how mycorrhizal fungi improve soil structure, and why that matters, we need to first understand what soil aggregates are, why aggregate stability is important and why it’s beneficial for plant health.

Soils are made up of an aggregate (combination) of different sized particles comprised of sand, silt and clay particles (in order of largest to smallest particle size), in various proportions, that are bound together by organic matter to form the soil matrix, a mass of soil consisting of solid particles with empty spaces between them that are filled with water and air.

For plants to remain healthy, they need the movement of air and water to the plant roots, which the porous soil structure of the soil matrix which allows.

Soils that have a stable aggregate structure permit the flow of water though their structure, are able to store water, and also retain their porosity when subjected to external forces such as rainfall and traffic that push the soil particles together, closing up the air spaces between them and compacting the soil.

Stable soil aggregates also provide a favorable environment for soil microbes, earthworms, and other organisms that can help improve soil structure, and as a result of all the beneficial factors discussed, are usually more productive.

Mycorrhizal fungi improve the physical and biological characteristics of the soil by increasing soil porosity, which allows for better water infiltration and retention, and also improves soil aeration.

The long, thin filament-like hyphae of mycorrhizal fungi entangle and bind soil particles like a net (in the same ways as fibrous plant roots do) to form macroaggregates, which are the basic building blocks of soil structure. Additionally, mycorrhizal fungi creating channels through the soil which air can flow, improving soil aeration. This helps to reduce soil compaction and improves the overall structure of the soil.

Mycorrhizal fungi also secrete glomalin, a glycoprotein, onto the outer walls of their hyphae and onto adjacent soil particles, which acts as a rather stable hydrophobic (water repellent) glue that retards water movement into the pores within the aggregate structure, helping hold these macroaggregates together when the soil gets wet or dries out. This glycoprotein also increases the carbon content in the soil and stimulates soil biological activity by serving as additional food for soil microorganisms.

4. Enhanced plant resistance to pathogens

Mycorrhizal fungi can help protect plants and trees from damage by pathogenic fungi and soil bacteria, by:

  • Providing a protective barrier around their root system.
  • Producing antimicrobial compounds that can help protect the plant against harmful pathogens.

Additionally, arbuscular mycorrhizal (AM) fungi help plants deal with toxic heavy metals in the soil which reduce plant growth, such as arsenic, cadmium, chromium, copper, copper, lead and zinc. The mycorrhizal fungi decreasing plants uptake by preventing the heavy metals getting past the roots, or support plants by increase plant tolerance to these heavy metals.

Studies have shown that many plants fail to develop into strong specimens if they’re deprived of their association with mycorrhizal fungi. This shows that plants receive a considerable return for the 10-30% of photosynthesised carbohydrates that they pass on to the mycorrhizal fungi associated with their roots. Overall, the symbiotuc relationship with mycorhhizal fungi leads to improved plant health, and increased resilience to disease and pest damage.

5. Enhanced plant growth and development

Through the mechanisms described previously, mycorrhizal fungi can help provide plants with additional nutrients, reduce stress from extreme environmental conditions, improve access to water, improve soil structure for optimal growth and development, and provide protection from harmful pathogens.

These benefits cumulatively can result in plants that grow larger and healthier, with enhanced flowering, improved yields, higher transplanting success and greater overall performance.

Which Plants Don’t Benefit from Mycorrhizal Fungi?

Some plants don’t form symbiotic associations with mycorrhizal fungi, and therefore don’t gain any benefit from the mycorrhizal fungi in the soil.

The plant groups that do not form associations with mycorrhizal fungi are the Brassicaceae (cabbage, brocolli, mustard, canola) family, the Chenopodiaceae (spinach, silverbeet/chard, beetroot, Malabar spinach, saltbush) family and the Proteaceae (banksia, macadamia) family.

This is a list of plants that do not form associations with mycorrhizal fungi, which includes some those from the above groups:

  • azaleas
  • beets
  • blueberries
  • broccoli
  • Brussels sprouts
  • cabbage
  • carnations
  • cauliflowers
  • collards
  • cranberries
  • heath
  • huckleberries
  • kale
  • mustard
  • proteas
  • rhododendrons
  • sedge
  • spinach

For more information on mycorrhizal fungi, see article – What Are the Best Ways to Increase Beneficial Mycorrhizal Fungi in the Soil?

References

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