Walk in most any wild setting and beneath your feet lies miles upon miles of filamentous fungal networks called mycorrhizae, connecting plants through a complex underground web.
What Are Mycorrhizae?
Mycorrhiza is the symbiotic relationship between a plant and fungus, with the plural form being mycorrhizae.
Mycor-rhiza translates as fungus-root.
Mycorrhizal is the adjective used to describe the relationship. About 90% of plants on Earth form mycorrhizal associations.
Fossil evidence dating back 460 million years shows that mycorrhizae played a key role in allowing aquatic plants to migrate into – and survive – land habitats.
Mycorrhizal fungi connect with a plant’s roots, working as an extension of the roots and growing out further into the soil.
Consequently, plants are able to obtain even more nutrients, minerals, and water from beyond their root zone. Fungi provide these resources in exchange for food in the form of carbohydrates, which they cannot synthesize themselves.
Thus, fungi rely on their host for sustenance.
Mycorrhizal fungi are obligate symbionts, meaning that they require a living host in order to survive. Spores will remain dormant within the soil, only germinating when they are close enough to living roots to detect and receive chemical signals.
Mycorrhizal fungi are able to offer a number of functions to a plant’s health including supplying nutrients, disease protection, a means of communication, and soil building.
Mycorrhizae: The Ultimate Multi-Taskers
Mycorrhizae work as miners, chemists, communication experts, food delivery services, builders, security, physicians, and more.
Tiny filaments of fungi called hyphae are smaller in diameter than root hairs. Hyphae allow fungi to go places to find resources where plants can’t.
They work to supply plants with food by transferring nutrients and micronutrients cell-to-cell (like a bucket brigade) to the roots of their host plant.
Mycorrhizal fungi also use chemistry to unlock nutrients and minerals within the soil.
They release enzymes and acids which break down organic materials, even stones, in order for them to absorb these foods. Nutrients and minerals are shared with the host plant in exchange for carbohydrates produced by the plant.
If not for mycorrhizae, plants would need constant watering and added fertilizers.
Communication Experts in the Soil
Plants are connected underground through the mycorrhizal network, often referred to as the “common mycelial network” or “wood wide web”.
This network shares chemical signals between trees, allowing literal communication between them.
Dr. Suzanne Simard, Professor of Forest Ecology at the University of British Columbia, researches mycorrhizae in forest systems. She notes presence of “Mother Trees” in forests, where an elder tree acts as a hub, connecting and communicating with other trees in the system, much like a server in a computer network.
What’s even crazier is that mycorrhizal connections allow plants to transfer defense protection to other plants in the network, protecting them against pathogens.
Healthier plants are able to share nutrients and other resources to less fortunate ones within the network. It’s rumored that a mother tree may even detect disease within a sector and cut off foods to that area, knowing that the plants will not survive.
As hyphae from mycorrhizal fungi move through the soil, they excrete enzymes to break down substances they encounter.
Acting as glues, these enzymes hold soil particles together, forming aggregates and improving soil structure. These porous aggregates allow more water and air movement, essentially making the soil into a giant sponge.
Torrential downpours have become the norm with our changing climate.
The ability to store excess water in the soil for future use can make a big difference for farms (and even those of you who love your lawn).
Security Against Soil Pathogens
Mycorrhizae provide at least two forms of protection for plants.
For one, they discharge suppressive chemicals, like antibiotics, inhibiting infection from pathogens.
Secondly, mycorrhizal fungi form a protective barrier on root systems.
Picture armed guards covering every square inch of the outside of a building. These barriers are made of chitin, the same substance that is in crustacean and insect shells.
Pathogens have no effect on the impenetrable shield of fungi.
Plants are more drought-tolerant through the benefit of mycorrhizae.
Wild habitats are full of green, vibrant vegetation with no need for irrigation, while our backyard tomato plants droop with the slightest delay in the watering schedule.
Mycorrhizal fungi find water where plant roots cannot. Absorbing and retaining water during times with sufficient soil moisture, they slowly release it to the plant during a drought.
Want to Learn More About the Soil Food Web?
Fungi are crucial “characters” in what we know as the soil food web.
Check out what other characters do in our comprehensive post about the living soil under your feet.
Mycorrhizal Fungi Differ From Other Soil Fungi
Soil features 3 types of fungi: mycorrhizal fungi, saprophytic fungi, and pathogenic fungi.
Saprophytic fungi break down dead and decaying carbon-rich organic matter like logs, limbs, sticks, leaves, stalks, even cardboard and paper.
If you have ever observed thick, white threads on wood chips in a compost pile or under leaf litter in the forest, THAT is saprophytic fungi.
Don’t mistake all fungal hyphae observed in the soil or compost as mycorrhizal fungi.
Any observed white strands you find in your compost are saprophytic fungi.
As mentioned earlier, mycorrhizal fungi are obligate symbionts and require a living plant host to survive. But compost piles are full of dead and decaying material, just what a saphrophyte loves!
Ectomycorrhizal vs Endomycorrhizal Fungi
Mycorrhizae can be ectomycorrhizal or endomycorrhizal.
What’s the difference?
All mycorrhizae perform the same functions as far as soils and plants are concerned.
But there are slight variations between the two, mainly in the way that the fungi connect to a plant’s roots and the manner with which they release spores to multiply.
There are more than 4000 species of ectomycorrhizal fungi, but they only work with trees, so you’ll solely find them in forests systems.
Oak, pecan, almost all conifers, beeches, eucalyptus, and a portion of tropical hardwoods will make ready associations with ectomycorrhizal fungi.
These fungi form connections with roots by entering the root and growing around the cortical cells (outermost cells) but they do not enter a root’s cells.
This forms what is called the “Hartig net”. This illustration taken from Wikipedia shows the exact structure of the Hartig net.
Ectomycorrhizal fungi form a sheath of interwoven hyphae, referred to as a “mantle”, all over the surfaces of root hairs of trees.
This mantle helps to provide a protective barrier from pathogenic fungi that may try to move in on the host tree. It will look like a white covering over the roots and is visible to the naked eye.
Almost all fungi procreate by producing spores.
Ectomycorrhizae reproduce by releasing spores through mushrooms. Morels, chanterelles, truffles, and boletes are all popular mycorrhizal – and edible! – mushrooms.
For this reason, expert mushroom hunters will examine trees species and can sniff out their favorite fungi while foraging.
Endomycorrhizal fungi associate with a much larger variety of plants in the world.
These fungi provide all the same benefits as ectomycorrhizal fungi, but endomycorrhizal fungi actually penetrate and enter into a plant’s root cells. They also do not produce the thick mantle, or covering, on the exterior of plant roots.
See the illustration below for the difference between the way ectomycorrhizae and endomycorrhizae make connections inside the roots.
On the left you can see ectomycorrhizae growing around the cells with a think mantle on the exterior.
On the right you can see the fungal hyphae of endomycorrhizal fungi have entered inside the cells but lack the mantle on the exterior.
Unlike their ecto counterparts, endomycorrhizae do not produce fruiting bodies, i.e. mushrooms.
Instead, they form spores, or clusters of spores, within the soil as their means of reproduction.
AMF: Arbuscular Mycorrhizal Fungi
By far, the most prominent endomycorrhizae are arbuscular mycorrhizal fungi, aka “AMF”.
AMF associate with the majority of Earth’s plant species. And their name comes from the tree-shaped arbuscules that form once the hyphae enter the root cells (see previous illustration).
About 200 species of AMF have been identified so far, forming associations with more than 300,000 plant species. Most are generalists, meaning that they work with a wide variety of plants in all kinds of climates and environments.
Other, less-common endomycorrhizal fungi exist to perform very specialized functions within plants. One well-known species is ericoid mycorrhizae. It benefits a limited number of plant groups including blueberries, cranberries, orchids, azaleas, and rhododendrons.
A Word About Phosphorous
Of all the nutrients and minerals that mycorrhizae provide to plants, phosphorous is at the top of the list.
Phosphorus is essential to all life forms as it is a component of nucleic acids (RNA and DNA) and phospholipids. This mineral is important for plants as it is involved in enzymatic reactions and metabolic pathways.
Plants use it to convert nutrients into important building blocks for growth.
Phosphorous concentrations in soils control mycorrhizal associations. Concentrations in the soil of more than 50 ppm reduces or eliminates colonization of mycorrhizal fungi.
At the current rate that humans are mining and using rock phosphate for agriculture, many experts estimate that we only have about a century before it is all used up. Moving towards a sustainable form of agriculture will mean taking advantage of the benefits that mycorrhizae offer in regards to providing plants with phosphorus.
Should I Add Mycorrhizae to my Vermicompost?
You can purchase mycorrhizae in the form of spores mixed with colonized root fragments. But spores need to be within a couple millimeters of a root to detect chemical signals.
If you add spores to vermicompost, compost, or a soil mix, you are taking a chance that all of them will come close to a root at some point soon after use.
This is probably a waste of money.
Instead, purchase the spores and use them in a powder form or slurry to directly inoculate seeds, plant starts, or any type of plant that is already in the ground. That way, the spores are coming in contact with the seed or root systems where they are much more likely to germinate and benefit the plants.
Younger plants mean smaller plants. Therefore, inoculating plants when they are seeds or seedlings means that much less inoculant you will need.
I even add spores to compost teas or extracts which I then apply to turf grasses or by injecting the solution into the root zones of trees and larger plants.
Mycorrhizae are the key to keeping almost all the plant species in the world healthy and robust. These plants would not grow well without the benefit of mycorrhizal associations (unless they are constantly watered and fertilized artificially).
We owe a lot to these microscopic networkers.
If you enjoy hiking in the woods like I do, we have much gratitude to offer towards these wonders of the natural world. The next time you are walking in a forest, consider what lies beneath, and remember….everything is connected.
This article was co-written by the Urban Worm Soil Biologist Troy Hinke, a Soil Food Web trained expert in soil microbes. Check out his What’s Brewing Podcast on Apple Podcasts and Spotify.
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