We live on top of a hidden world. Beneath the soil surface is a fascinating combination of life forms interacting in an abundance of ways. But life in the soil does not have to be a mystery. It does not have to be so technical, so “un” understandable, so full of jargon that we cringe at the sound of certain terms. Terms like mycorrhizae … “Myco what?” Mycorrhiza are probably the best studied of all plant-microbial relationships. There are over 100,000 peer reviewed scientific studies in the technical literature. The problem is the scientific literature is difficult to read without expertise in technical papers. Access to the literature is also buried in journals that are obscure to the general public. The purpose of this article is to explain the basic workings and benefits of this remarkable group of fungi.
Let’s start with some basics. Not all species need to compete with each other to survive and evolve. The vast majority of plant species form a mutually beneficial living relationship, or a “symbiosis”, with beneficial fungi. It’s a 1+1=3 relationship. The roots of an estimated 85% of the world’s plant species are colonized by symbiotic fungi. We call this root/fungus combination a mycorrhiza and the plural is mycorrhizae. Mycor-rhiza literally translates into fungus-root. Mycorrhizal is the adjective describing the relationship. The mycorrhizal fungi are the network of fungal filaments that permeate into the soil from their home on 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 in length. Hyphal strands grow from within and around the root cells of the “host” plant, spreading out into the surrounding soil, greatly increasing the effective surface area of the root system. Mycorrhizae are so fundamental to plant nutrition that most plant species could not survive without the mycorrhizal relationship in the absence of artificial inputs. Mycorrhizae are as common to the roots of plants as chloroplasts, the photosynthetic factories, are to the leaves of plants. We all need to know more about them because they play key roles in the health and productivity of our planet earth.
The Types of Mycorrhizae
The fossil evidence indicates that the specialized mycorrhizal plant relationship dates back over 460 million years and played a key role in allowing aquatic plants to invade and utilize land habitats. Aquatic plants did not have the ability to 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.
The trading of 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.
The group of mycorrhizae that are associated with most conifers, oaks, pecan, hazelnut, beeches, eucalyptus, 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 call the “Hartig net”. Ectomycorrhizae (plural for ectomycorrhiza and shortened to ecto’s 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 be seeing the fruiting body of an ectomycorrhizal fungal. Left unpicked, these structures will eventually decompose and 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 mushrooms that live underground 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.
Endomycorrhizae (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 endomycorrhizae fungi expand into the soil matrix forming a feeding network that provide soil nutrients and water to the plant in exchange for energy and other compounds provided by the plant. But unlike the ecto’s, the endo’s penetrate into the plant root cells and lack the thick mantle over the surface of the root common to ecto’s. They also reproduce very differently by not producing fruiting bodies like mushrooms, puffballs, or truffles; but forming spores individually or in clusters in the soil.
There are several types of endomycorrhizae. By far the largest endo group is the arbuscular mycorrhizae fungi (AMF), which also claims the most mycorrhizal species in the plant kingdom. As the name implies, arbuscular mycorrhizae are composed of arbuscules – structures that are shaped like “little trees” inside the plant root system (Figure 2). Many AMF contain vesicles which are oil storage organs in the root cell. Other specialized and less common types of endomycorrhizae form with orchids, rhododendron, azalea, blueberries, cranberries and a few other plant groups.
Most plants, including most 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 which associate with a wide variety of plants in a broad assortment of soil types, geologies, topographies, and climates. In this arbuscular symbiosis, the plant is provided better access and uptake of nutrients and water from the soil. In return, the fungus, which cannot synthesize its own nourishment, receives its energy source in the form of 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. Just 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 are able to penetrate the tiniest pores and fissures in the soil. The mycorrhizal fungi can be viewed as the “stomach” of the plant, producing enzymes that digest and absorb food in the soil (Figure 3). 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 not available. 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 utilization of fertilizer likewise becomes 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, phythium 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 that is in mammal claws and insect shells) and forms a tough, protective layer protecting root cells.
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 in natural and wild plant habitats without the benefit of irrigation. 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 periods of adequate soil moisture, then retain and slowly release it to the plant during periods of 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 a giant sponge to protect the plant communities from extreme soil moisture deficits.
The mycorrhizal filaments can penetrate into 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 a wide array of important plant species (Al Karaki 1998). The declining availability of water and its ever-increasing cost are formidable issues facing today’s growers. Mycorrhizal fungi can be a powerful tool to enhance water-use efficiencies.
In the last 50 years atmospheric carbon dioxide (CO2) levels have risen from 320 ppm to 415 ppm (Lindsey 2018). Usually these kinds of changes in the atmosphere occur over geologic time scales. The effects on climate change are indisputable. Added CO2 has contributed to hotter and extreme changes in weather, the environment, and is threatening the health and safety of generations to come.
With regard to stabilizing our increasingly unruly climate, mycorrhizal fungi have been busy taking CO2 out of the atmosphere for millions of years and storing it in a sticky compound called glomalin. This superglue-like substance is composed of 40% carbon and binds soil particles together 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 pool of carbon 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 is a valuable tool to mitigate global warming. Mycorrhizal connections may also allow for the successful transition from one plant community to another in responses to a changing climate (Perry and Amaranthus, 1990). Fire, heat, and drought, as a result of climate change, 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 the 1970’s scientific papers used the term “infected” to describe the mycorrhizal relationship. Infected sounded so pathological to describe 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 in which they depend on the mycorrhizal relations. A few plant families such as the Amaranthaceae, Chenopodiaceae, Brassicaaceae, Cyperaceae do not form mycorrhizae at all. Many weed species gain little from the relationship even though they are capable of forming 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 that germinate or are 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 into the root forming the mycorrhiza. The fungal hyphae effectively link one plant root system to the next (Figure 5). This is possible because the mycorrhizal fungi, especially the arbuscular mycorrhizal fungi, can colonize most any plant species. The network spreads using energy gained from previously colonized roots linking to nearby plants. Plants that germinate or are 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 together into an underground network known as the “common mycelial network”. New scientific evidence indicates that this network does more than just 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 allowed the recipient plants to be 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 that were 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” used to create new mycorrhizae. Mycorrhizal propagules are commonly the spores or mycorrhizal colonized root fragments containing spores, arbuscules, or vesicles. Colonized root fragments as inoculum have been shown to be as effective and, in many cases, more effective than spores themselves.
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 propagule of a mycorrhizal fungus, it sends a signal to the propagule to germinate (Figure 6a). This starts a symbiotic process where the germinating mycorrhizal propagule grows
a hypha to the root and upon contact, penetrates the root surface with specialized cells, called appressorium. These flattened cells press through the infection site and enter the root.
Once inside, the endomycorrhiza branches and forms arbuscules within the cells of the root cortex (Figure 6b). 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, water and contacting new roots.
Since before World War II, scientific and technological advances in plant science focused primarily on the development of chemical and mechanical approaches to improving plant performance. Nutrient needs have been addressed using synthetic fertilizers while weed suppression have been accomplished through tillage and herbicides, and plant diseases controlled using an array of chemical pesticides. More recently, modern plant science has begun to understand that in natural habitats plant roots are a complex interaction between fungus and plant and fundamental to life on our planet. Reintroducing the mycorrhizal relationship on disturbed lands is a “growing” opportunity.
It took a long time for people to recognize the importance of mycorrhizal fungi because of jargon and complicated presentations. 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”.
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 Dr. Mike
Mike Amaranthus is a retired research soil scientist for the USDA and associate adjunct professor at Oregon State University. He was the recipient of 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.
The author would like to thank soil scientist David Steinfeld for review of this article and Linda Woodrow-Gray for the illustrations.