Mycorrhizal fungi

Fungi and plant anatomy

FUNGI

Fungi can consist of one cell e.g. a yeast cell or they can be multicellular e.g. a toadstool.
They cannot make their own food.
Multicellular fungi on land consist of thread like structures called hyphae. A mass of interconnected hyphae is a mycelium. The mycelium is generally not visually obvious as it grows under the ground.
The fruiting body (technically known as the sporangia) is what most people think of as a mushroom or toadstool. It is made by the hyphae for the purpose of reproduction and dispersal, but it is only a transient small part of the entire fungus.

PLANTS

A. leaves where photosynthesis occurs
B. stem holding leaves up to sunlight; specialised cells to transport water and nutrients around the plant body are found here
C. underground root system anchors the plant to the soil and is the site of nutrient uptake Note: Roots have fine projections on their surface (root hairs) to greatly extend their surface area in contact with the soil.

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What does the term mycorrhiza mean?

fungal hyphae and tree roots — mutual benefits of plant/fungus connection made by mycorrhiza. By Nefronus - Own work, CC BY-SA 4.0

Mycorrhiza is a term indicating a relationship between the roots of a plant and the hyphae of a fungus. Generally it is a mutualistic symbiosis, ie both organisms benefit from its occurrence. The fungi facilitate water and nutrient uptake in the plant, and the plant provides nutrients created by photosynthesis to the fungi.

According to an 2015 article, in the journal New Phytologist¹

Estimates suggest that there are about 50,000 fungal species that form mycorrhizal associations with about 250,000 plant species.

More recent estimates in 2020 put the number of land plant species at 340,000.

Further estimates suggest that about 80% to 90% of plants have at least one mycorrhizal partner. These partnerships can be found in terrestrial ecosystems all over the planet.

The only visual indication of mycorrhizal presence is usually the fungal fruiting body (technically known as the sporangia) of the fungus, if the fungus being considered reproduces in this way.

Examples of mycorrhizal fungi

cross section through soil of plant roots and hyphae

Ways in which the fungus and plant actually connect

There are two types of mycorrhizal connection between plant and fungi - endorrhizal and ectorrhizal.

endo-and-ectomycorrhizal-association-between-plant

Endomycorrhizal - hyphae penetrate and embed them selves inside the cells of the root. Commonly found in 85% to 90% of all plant families.

There are several types of endomycorrhizal patterns of connection. The most common is the arbuscular arrangement.

The hyphae penetrate the cell wall of the root cells and form vesicles and arbuscules, the difference being the shape of the formation.

Ectomycorrhizal - hyphae penetrate the spaces between the cells of the roots, but do not do not enter the cells. Exchange of materials is dependent on the proximity of the root cells and hyphae. A sheath of fungal threads encases the root to form a sheath. Common in woody plants, such as birch, beech, willow, pine, oak, spruce, and fir.

Services

Mycorrhizal services - a matter of life or death?

experimental demonstration of the effects of mycorrhizal fungi

Mycorrhizal fungi quietly go about providing a number of services that benefit plants and through them the human population. It is predicted that 80% to 90% of land plants including most crop plants would die without the assistance provided by mycorrhizal fungi.

Two plants of the same species are shown in the image on the left. Both grown in the same soil and provided with the same amount of water. A benefits from the presence of the fungi; B dies in its absence.

Note: given the number of possible combinations of plant and fungi is astronomically high, this will not necessarily be the case in every pairing and in all circumstances. We make some generalisations based on our current knowledge e.g.

Up to 80% of plant N and P is provided by mycorrhizal fungi and many plant species depend on these symbionts for growth and survival.²

Note: N stands for nitrogen; P stands for phosphorus.

Nutrition

Increasing absorption of nitrogen, phosphorus, and potassium
Reduced nutritional deficiencies. Endomycorrhizae mine out the growing medium to efficiently bring nutrients (particularly phosphorus, copper, manganese and zinc) to the plant where plant roots are not present. This delays nutrient deficiencies and their visual symptoms from appearing.
Improved growth. Efficient acquisition of nutrients helps the plant maintain its optimal growing rate longer, so top growth and root growth are not compromised.
They help to increase the quality and yield of crops.

Resilience

Resistance to salt toxicity. Endomycorrhizae fungi have been found to protect plants from high salt and micronutrient toxicities.
Improving water absorption rate and reduce wilting episodes, this improving drought tolerance.
Reduced root disease attack. Endomycorrhizal fungi help to reduce the effects of stress on plants, making them less susceptible to attack by root rot pathogens. Endomycorrhizal fungi help to reduce the effects of stress on plants, making them less susceptible to attack by root rot pathogens. Not only do endomycorrhizae serve as competition to root rot pathogens by being present on plant roots and consuming root exudates (such as carbohydrates), but they cause the cell walls of the cortex to thicken, making pathogen penetration more difficult. Studies on mycorrhizae fungi have found that plants with healthy colonies of these fungi were less likely to have disease-causing organisms.

Soil health

They help to control soil erosion.

Help in maintaining nutrient cycling and soil quality.

Participation in the food web

Mycorrhizal fungi are eaten by mites and nematodes which are, in turn, eaten by higher level consumers including birds and mammals.
Details may vary from location to location and species to species, but these fungi are are important members of the food web in all locations where they are found. Removal of them will destabilise the entire ecosystem,

Participation in the evolution of land plants

Plant-fungi mutualism is thought to have played a critical role in the establishment of land plants more than 500 million years ago. This is supported by fossil evidence and the prevalence of mycorrhizal associations involving so many species across the planet.

Human health pharmaceutical benefits

According to a 2023 article in the journal Plants, People, Planet

Evidence has emerged that the antioxidant ergothioneine may be important in preventing many inflammatory diseases in humans. However, ergothioneine is not produced by humans or plants and is only made by fungi and some bacteria in soils. As such, humans get ergothioneine from eating fungi (mushrooms) or plants that take it up from the soil. In this study, we found that growing plants with beneficial fungi called arbuscular mycorrhizal fungi increased the amount of ergothioneine in plant tissues.³

Communication and transport between plants

Mycorrhizal fungi were first recognised in the 1880s, but it was not until about 100 years later that we started to really get a handle on their full potential. The following video is an excellent explanation of how and why trees communicate and co-operate.

Threats to services

the clearing of land for the expansion of agriculture and increasing urbanisation -
changes to soil due to agricultural practices, mechanical barriers due to building density
pollution of both water and air due to the release of chemicals from industrial/farming practices -
gases such as SO2, NO-x, and O3 produced by human activity may harm mycorrhiza, causing reduction the colonization of roots, degradation in connections between trees, reduction in the mycorrhizal incidence in trees, and reduction in the enzyme activity of ectomycorrhizal roots.
ploughing practices and indiscriminate use of fungicides
changes to the climate caused by human activity -
this may include changes in temperatures, water availability and changed fire regimes

What can we do to retain these services?

Policies and actions to reduce the climate crisis and pollution
Mycorrhizal networks contribute greatly to plant resilience in dealing with changed environmental conditions due to human induced climate change. If the mycorrhizal fungi are themselves damaged by the changed conditions, it will make the plants less able to cope.
Support initiatives such as SPUN (Society for the Protection of Underground Networks) and other organisations doing valuable research work
Reduce indiscriminate use of fungicides in working with the soil
Soil management practices e.g. ploughing and use of fertilisers have been shown to have a detrimental effect on many mycorrhizal associations. People interested in a more organic approach to agriculture and gardening often recommend inoculation of soil with mycorrhizal fungi in order to make plants healthier and more resilient. This is paired with a no-tillage approach to soil treatment. There is a thriving market in mycorrhizal fungal inoculants. The global market in 2022 was USD 580 million, and the forecast for 2027 is over USD 1 billion, so it's a growth industry in more ways than one.

sample of brands of mycorrhizal inoculants

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