What is Mycorrhizae

Mycorrhizae

Let’s start with some basics. Not all species need to compete with each other to survive and evolve. Most plant species form a mutually beneficial living relationship, or a “symbiosis”, with beneficial fungi. It’s a 1+1=3 relationship.

An estimated 85% of the world’s plant species’ roots are colonized by symbiotic fungi. We call this root/fungus combination a mycorrhiza, and the plural is mycorrhizae. Mycorrhiza literally translates into fungus root, and mycorrhizal is the adjective describing the relationship.

Mycorrhizal fungi are a network of fungal filaments that permeate the soil from their home to the plant root.

The body of the mycorrhizal fungus consists of microscopic filaments called hyphae. An individual hypha (singular) is approximately 1/25th the diameter of a human hair and can grow up to 15 to 25 inches.

Hyphal strands grow from within and around the root cells of the “host” plant, spreading out into the surrounding soil, greatly increasing the root system’s effective surface area.

Mycorrhizae are so fundamental to plant nutrition that most plant species could not survive without the mycorrhizal relationship without artificial inputs. Mycorrhizae are as common to plants’ roots as chloroplasts, the photosynthetic factories, are to plants’ leaves. We all need to know more about them because they play key roles in the health and productivity of our planet Earth.

Learn The Important Types of Mycorrhizae

The fossil evidence indicates that the specialized mycorrhizal plant relationship dates back over 460 million years and is crucial in allowing aquatic plants to invade and utilize land habitats.

Aquatic plants could not survive in the harsh soil conditions on land until the fortuitous marriage of plant and mycorrhizal fungus. The root structures of these prehistoric plants evolved specialized attachment sites to better accommodate these fungi.

Trading soil water and nutrients captured by the mycorrhiza for sugars produced by plant photosynthesis was the foundation for this ancient relationship. Mycorrhiza, in nature, is the rule, not the exception. You don’t need to be a rocket scientist to understand the significance of the relationship.

Ecto’s

The group of mycorrhizae associated with most conifers, oaks, pecans, hazelnuts, beeches, eucalyptus, and some tropical hardwoods is called “ectomycorrhiza” (Figure 1). The ectomycorrhizal fungi do not enter the root cells but grow around the outer cortical cells of
the root forming what is called the “Hartig net”.

Ectomycorrhizae (plural for ectomycorrhiza and shortened to echoes to conserve on syllables) exist most often as a “mantle” or covering of interwoven fungal hyphae on the surface of the fine roots of trees. The mantle makes the roots look swollen and can be visible to the unaided eye or viewed closely with a low-power dissecting microscope.

Well, over 4000 species of ectomycorrhizal fungi occur in our forests across the globe. When you walk through a forest and see a mushroom or puffball, you may see an ectomycorrhizal fungal’s fruiting body. Left unpicked, these structures will eventually decompose into spores or “fungal seeds” that disperse and colonize the sites of new tree roots.

Some of these species, including boletes, chanterelles, and several types of morels, are prized by mushroom hunters. You may also be walking over a group of underground mushrooms called truffles. These “small potato-looking” fruiting bodies can be an epicurean delicacy, and they contain billions of spores waiting for a tree root to colonize.

Endo’s

Endo mycorrhizae (plural for endomycorrhiza and shortened to “endo’s”) (Figure 2) form a symbiotic relationship with a much broader array of plants. Like ectomycorrhizal fungi, the filaments of endomycorrhizal fungi expand into the soil matrix, creating a feeding network that provides soil nutrients and water to the plant in exchange for energy and other compounds supplied by the plant.

But unlike the ectos, the endos penetrate the plant root cells and lack the thick mantle over the root’s surface common to ectos. They also reproduce very differently by not producing fruiting bodies like mushrooms, puffballs, or truffles; they form spores individually or in clusters in the soil.

There are several types of endo mycorrhizae. The largest endo group is the arbuscular mycorrhizal fungi (AMF), which also claims the most mycorrhizal species in the plant kingdom.

As the name implies, arbuscular mycorrhizae are composed of arbuscular – structures that are shaped like “little trees” inside the plant root system (Figure 2). Many AMFs contain vesicles which are oil storage organs in the root cell. Other specialized and less common endo mycorrhizae form with orchids, rhododendrons, azaleas, blueberries, cranberries, and a few other plant groups.

Most plants, including grains, vegetables, orchard trees, vines, turf grasses, and horticulturally important plants, evolved with AMF and are naturally “designed” to achieve optimum growth and vigor by forming arbuscular fungal relationships.

There are approximately 200 AMF species (with many more undiscovered) on the planet, forming with more than 300,000 plant species. Nearly all of the AMF species are generalists who associate with a wide variety of plants in a wide assortment of soil types, geologies, topographies, and climates.

In this arbuscular symbiosis, the plant has better access to and uptake of nutrients and water from the soil. In return, the fungus, which cannot synthesize its own nourishment, receives its energy source through carbohydrates donated by the plant.

Although the arbuscular mycorrhizae cannot be seen with the naked eye, your rocket science degree will not be necessary to observe their occurrence. A dissecting microscope of 30x power will do the trick. Fine plant roots are soaked in potassium hydroxide solution to clear out the root tannins, then rinsed and placed in an ink solution. The ink turns the hyphae, arbuscules, vesicles, and spores blue inside the roots for easy viewing.

What They Do

The effect of mycorrhizae on the root system of a colonized plant is mind-boggling. What we typically think of as a plant root system is in most cases a web of fungal hyphae doing most
of the work feeding the plant. A teaspoon of healthy soil can contain up to several miles
of hyphae!

This is because hyphae are far thinner than roots or root hairs and can penetrate the tiniest pores and fissures in the soil. Mycorrhizal fungi can be viewed as the plant’s “stomach,” producing enzymes that digest and absorb food in the soil.

And like the bacteria and fungi in our gut, mycorrhizal fungi increase the availability of the “pools” of nutrients that may otherwise be limited or unavailable. Research confirms that mycorrhizal fungi are particularly important in accessing phosphorus, nitrogen, zinc, iron, calcium, magnesium, manganese, sulfur, and other important soil nutrients by breaking “tightly held” soil chemical bonds that bind nutrients to the soil and transporting them back to the plant. Plant uptake and fertilizer utilization likewise become far more efficient, often leading to significant savings in fertilizer costs in a mycorrhizal-plant system.

Mycorrhizal benefits do not stop there. These fungi also play a definitive role in a plant’s natural defense against widespread fungal root diseases, which include Phytophthora, Fusarium, Pythium, and Rhizoctonia. (St-Arnaoud et al. 2007)

Mycorrhizal fungi release suppressive exudates, such as antibiotics, that inhibit infection by these and other fungal root pathogens. Studies have documented that mycorrhizae also defend root systems by forming a physical barrier to deter invasion by soil pathogens. This barrier is made of “chitin” (the same tough material in mammal claws and insect shells) and forms a tough, protective layer protecting root cells.

Drought Tolerance

Anyone who grows plants recognizes the need for fresh water is not always in sync with Nature’s inclination to provide it. We often see abundant, lush vegetation without irrigation in natural and wild plant habitats. How do natural areas provide for such luxuriant plant growth without irrigation?

One key factor is that mycorrhizal fungi find water where plant roots can not (Auge 2004). They absorb water during adequate soil moisture, then retain and slowly release it to the plant during drought.

Plants in natural areas generally achieve levels of drought tolerance far exceeding those found in agriculture or horticulture, partly due to the enormous web of mycorrhizal hyphae, which act like giant sponges to protect the plant communities from extreme soil moisture deficits.

The mycorrhizal filaments can penetrate the smallest of soil pores and fissures to access microscopic sources of water that are unavailable to the thicker roots. An extensive body of research documents the importance of the mycorrhizal relationship for efficient water use
and drought protection among many important plant species (Al Karaki 1998).

The declining water availability and its ever-increasing cost are formidable issues facing today’s growers. Mycorrhizal fungi can be a powerful tool to enhance water-use efficiencies.

Climate Change

In the last 50 years, atmospheric carbon dioxide (CO2) levels have risen from 320 ppm to 415 ppm (Lindsey 2018). These kinds of changes in the atmosphere usually occur over geologic time scales. The effects of climate change are indisputable. Added CO2 has contributed to hotter and more extreme changes in weather and the environment and threatens the health and safety of future generations.

Concerning stabilizing our increasingly unruly climate, mycorrhizal fungi have been taking CO2 out of the atmosphere for millions of years and storing it in a sticky compound called glomalin. This super glue-like substance is composed of 40% carbon and binds soil particles into stable aggregates. Mycorrhizal fungi produce this substance in their vast web of root-like hyphal threads. It just may be the most important soil component you have never heard of.

Mycorrhizal fungal activity has helped create a huge carbon pool in our soils. Glomalin
may account for as much as one-third of the world’s soil carbon, and the soil contains more carbon than all plants and the atmosphere combined! Scientists are discovering the activity
of mycorrhizal fungi to deposit carbon in the soil, which is a valuable tool to mitigate global warming.

Mycorrhizal connections may also allow for the successful transition from one plant community to another in response to a changing climate (Perry and Amaranthus, 1990). As a result of climate change, fire, heat, and drought are causing plant communities to migrate. Rapid mycorrhizal colonization allows plants to survive and adapt to new growing conditions. Keeping mycorrhizal populations healthy during periods of transition will help plant communities establish. Mycorrhizal plant communities prevent capture by non-mycorrhizal weeds and rapid degradation in the productive capacity of soils.

How do host plants become colonized?

Most plant roots become readily colonized by indigenous mycorrhizal fungi in natural and undisturbed settings. Before 1970, scientific papers used the term “infected” to describe the mycorrhizal relationship.

The infection sounded pathological in describing the relationship, especially since both the plant and fungus benefited. Scientists now use the word “colonized” to describe the mycorrhizal root. Host plants can differ in the degree to which they depend on mycorrhizal relations.

A few plant families, such as the Amaranthaceae, Chenopodiaceae, Brassicaceae, and Cyperaceae, do not form mycorrhizae. Many weed species gain little from the relationship, even though they can form mycorrhiza. The vast majority of plants, however, benefit and will not grow well without their fungal partners unless heavily watered and fertilized.

Dialing Into The Network To Become Colonized

Plants germinating or planted in soil with an existing mycorrhizal network can quickly become mycorrhizal (Figure 4). They plug into the existing mycorrhizal network of hyphae and mycelium. When a mycorrhizal hypha supported by an adjacent plant comes in contact with a non-mycorrhizal root, it penetrates the root forming the mycorrhiza. The fungal hyphae effectively link one plant root system to the next.

This is possible because the mycorrhizal fungi, especially the arbuscular mycorrhizal fungi, can colonize almost any plant species. The network spreads using energy gained from previously colonized roots linking to nearby plants. Plants germinating or planted in soil without a mycorrhizal network become colonized much more slowly and sometimes not at all.

Much like the Internet, mycorrhizae can link all plant species into an underground network known as the “common mycelial network”. New scientific evidence indicates that this network does more than transport water and nutrients – it can act as a communication system by sharing chemical information between plants. Communication between plants via linked mycelium has demonstrated benefits to the plant recipients. For example, signals between plants can stimulate a common defense against soil pathogens, inhibit the growth of neighboring plants, and warn of insect attacks.

Examples in the scientific literature are compelling. Investigators demonstrated the transfer
of defensive molecules between tomato plants linked by mycorrhizal fungi to protect against the root pathogen Alternaria solani (Song et al. 2010).

These shared chemicals made recipient plants more resistant to the pathogen. Researchers have found that marigolds inhibit the growth of neighboring plants by producing phytotoxic chemicals delivered through their mycorrhizal network (Barto et al. 2006).

In another recent study, beans attacked by aphids sent chemical messages via the common mycelial network, alerting neighboring bean plants of the threat and activating a defense system that protected them from attack (Babikova et al., 2013). Interplant communication via the common mycelial network is now well-established in the scientific community.

Inoculating To Become Colonized

Where soils are low in mycorrhizae due to disturbance or poor management, colonization can occur by reintroducing mycorrhizal fungi through a commercial product (Amaranthus et al. 2003). These products contain mycorrhizal propagules or “fungal seeds” to create new mycorrhizae.

Mycorrhizal propagules are commonly the spores or mycorrhizal colonized root fragments containing spores, arbuscules, or vesicles. Colonized root fragments such as inoculum are as effective and, in many cases, more effective than spores.

The inoculum is often a powder or clay granule with spores or colonized root fragments attached. As a plant root grows into the soil and encounters a mycorrhizal fungus propagule, it sends a signal to the propagule to germinate.

This starts a symbiotic process in which the germinating mycorrhizal propagule grows a hypha to the root and, upon contact, penetrates the root surface with specialized cells called an appressorium. These flattened cells press through the infection site and enter the root.

Once inside, the endomycorrhiza branches and forms arbuscular within the cells of the root cortex. Inoculation is most successful, and the relationship happens more quickly when propagules are adjacent to the growing plant root system. Once the connections have formed inside the root, the external portion of

the fungus starts to grow and branch. At this point, the plant begins to feed the mycorrhizal fungus with carbohydrates obtained through photosynthesis. This fuels the fungus to continue to expand into the soil, absorbing nutrients and water and contacting new roots.

Conclusions

Before World War II, scientific and technological advances in plant science focused primarily on developing chemical and mechanical approaches to improving plant performance. Synthetic fertilizers addressed nutrient needs, tillage and herbicides accomplished weed suppression, and various chemical pesticides controlling plant diseases.

More recently, modern plant science has begun to understand that in natural habitats, plant roots are a complex interaction between the fungus and plant and are fundamental to life on our planet. Reintroducing the mycorrhizal relationship on disturbed lands is a “growing” opportunity.

Because of jargon and complicated presentations, it took a long time for people to recognize the importance of mycorrhizal fungi. Learning the basics of the living soil, like mycorrhiza, can be
a rewarding undertaking and leave one with a vast respect for this hidden world.

This includes
a greater inclination to treat soil with thoughtful consideration. It’s time to get the message out regarding mycorrhiza. The mycorrhizal interactions of the soil don’t have to be presented as “rocket science”.

References

Al-Karaki, G. 1998. Benefit, cost, and water-use efficiency of arbuscular mycorrhizal durum wheat grown under drought stress. Mycorrhiza 8:41-45.

Amaranthus, M. D. Steinfeld and E. Cazares 2003. Survival of Ponderosa Pine (Pinus ponderosa Dougl. ex Laws.) seedlings out-planted with mycorrhizae inoculated with spores at the nursery. Journal of Arboriculture 29(4): pp 11-15.

Auge, R. 2004. Arbuscular mycorrhizae and soil/plant relationships. Can. J. Soil Sci. 84:373-381.

Babikova, Z., T. Bruce, M. Birchett, J. Cauldfield, and D. Johnson. 2013. Underground signals carried through common mycelial networks warn neighboring plants of aphid attack. Ecology letters 16 (7) 835-843.

Barto E., M. Hilker, F. Müller,. Mohney, J. Weidenhamer, and M. Rillig . 2011. The Fungal Fast Lane: Common Mycorrhizal Networks Extend Bioactive Zones of Allelochemicals in Soils. PLOS ONE, 6 (11) PMID: 22110615.

Lindsey, R. 2018. Climate change: atmospheric carbon dioxide. Climate.gov https://www. climate.gov/news-features/understanding-climate /climate-change-atmospheric-carbon-dioxide.

Perry D., and M. Amaranthus. 1990. Species migrations and ecosystem stability during climate change: the below-ground connection. Conservation Biology 4 (30) 266-274.

Song Y., R. Zeng, J. Xu , J. Li , X. Shen , and W. Yihdego . 2010. Interplant communication of tomato plants through underground common mycorrhizal networks. PLOS ONE, 5(10) PMID: 20967206.

St-Arnaud M., V. Vujanovic, C. Hamel and C. Plenchette, editors. 2007. Effect of the arbuscular mycorrhizal symbiosis on plant diseases and pests. Mycorrhizae in Crop Production: Applying Knowledge Binghampton, New York: Haworth Press. 67–122.

ABOUT

Mike Amaranthus is a retired research soil scientist for the USDA and an associate adjunct professor at Oregon State University. He received the USDA Highest Honors award for scientific achievement

and has several mushrooms and truffles named in his honor. He was the founder of Mycorrrhizal Applications, Inc. He is president of Myco Analytics L.L.C. in Grants Pass, Oregon.

drmikeamaranthus@gmail.com

The author would like to thank soil scientist David Steinfeld for reviewing this article and Linda Woodrow-Gray for the illustrations.