Mycorrhizal Fungi and Plant Roots: A Symbiotic Relationship

Mycorrizal fungi help plant roots absorb nutrients and fight off harmful, soil-dwelling predators. In exchange, the fungus receives sugars and nutrients from its host plant.
By Douglas Chadwick
August/September 2014
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Lichens from Antarctica survived 34 days in a laboratory setting designed to simulate the environment on Mars.
Photo by Fotolia/Ifrabanedo

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What we call a mushroom is merely the temporary structure some fungi grow to produce spores. The main body of those species and many others typically consists of fine-branching threads known as hyphae. While you’ll sometimes see them massed together, spread like a web across decomposing wood or detritus, they are usually hidden underground and essentially invisible, for the individual filaments are only a single cell wide. The fungus’s network of hyphae is called a mycelium.

Observations of hyphae bound together with root hairs weren’t reported until the 19th century. No one made much of the findings for decades afterward, because botanists took them to be examples of fungi parasitizing plants. Polish scientist Franciszek Kamienski gets credit for discovering in the 1880’s that the fungus and plant combination was in fact a symbiotic relationship. A contemporary gave it the name mycorrhiza, Latin for fungus-root.  Say it with me: my-core-rise-uh. The plural is mycorrhizae: rise-A.  It’s worth remembering, because as the years went by, researchers discovered mycorrhizae among the roots of more and more trees, shrubs, grasses, herbs, and even non-vascular plants such as ferns and liverworts.

We All Need Somebody to Lean On: Symbiotic Relationships

At least 80 percent of the plant species on the globe, representing more than 90 percent of all the plant families, are known to form mycorrhizae.  In addition to facilitating the transportation of nutrients, at least one kind of mycorrhizal fungus attracts and kills the tiny soil-dwelling arthropods called springtails, a rich source of nitrogen. Other carnivorous fungi capture the superabundant microscopic worms known as nematodes, either with sticky knobs that develop from the hyphae, fine filament meshes, or loops that constrict to snare passing prey — fungal lassoes. Weird, but Yeehaw! A variety of mycorrhizal fungi protect plant associates from root-devouring nematodes by producing chemicals lethal to the worms, nematicides, which have drawn interest from the agricultural pest control industry. Many mycorrhizal fungi secrete antibiotics fatal to bacteria that infect root systems. Not surprisingly, those chemicals have generated close interest among researchers, too.

The more vigorous a plant, the better it can contend with diseases and parasites, compete for space and sunlight, invest extra energy in the production of flowers or cones, successfully reproduce, and replace growth lost to insects, larger grazing animals, storm breakage and seasonal defoliation. That’s the game. Engaging in a symbiotic relationship with fungi is clearly a winning combination for plants, and the connections reach more widely than you might suppose.

Interspecies Communication

Combining old-fashioned shovel work with modern genetic analysis, researchers have traced mycelia that directly connect two or more individuals of the same plant species, allowing them to share resources. They have also found mycelia with hyphae connecting different species. For example, a cluster of conifer saplings arising from a dark forest floor and struggling upward toward the light needs nitrogen to continue building tissues. This element is particularly hard to come by in many woodland soils, and there may be little or none near the saplings’ roots. But if one of the young conifers can get an infusion of that element through hyphae linked to an alder or birch tree, whose roots host symbiotic nitrogen-fixing bacteria, that particular sapling may be good to go. Make that good to grow.

Of course, a physical attachment via a mycelium isn’t necessary for a plant lacking a nutrient to benefit from a surplus associated with a different plant. If hyphae from the impoverished plant only reach the soil near the second plant, this can be enough. People have been planting nitrogen-hungry crops like maize next to legumes like peas and beans for generations, think of the Native American’s Three Sisters Gardens. Some farmers might have guessed that the roots of one plant borrowed good stuff from the soil around another, but nobody was aware of the bacteria in nodes on the legume roots making the nitrogen available or aware of the mycorrhizal hyphae gathering it. They just knew the maize grew better.

These days, orchardists, commercial farmers and dedicated gardeners tend to be keenly aware of the symbiotic relationship between plants’ roots and fungi. A good measure of growers’ interest can be found in the long list of companies currently selling mycorrhizal fungi. They offer packets and jars of inoculants to treat roots or seeds prior to planting and larger quantities for broadcasting onto croplands, especially those whose mycelial structures have been disrupted by chemical treatments, over-tilling or compaction from trampling. To learn more gardening with mycorrhizal fungi in mind, read Mycorrhizal Fungi: The Amazing Underground Secret to a Better Garden from our August/September, 2014 issue.

Lichen: A Biological Alloy

If you ask the general public to name a partnership between a fungus and a plant, those who aren’t at a loss will probably answer “lichens.” Easily found and often strikingly colorful, lichens are indeed a fungus combined with a photosynthesizing species, but that partner isn’t a plant. It will be a microbe, single-celled algae or else cyanobacteria, which can convert sunlight to energy as well. Some fungi partner with both types at once. As in a mycorrhiza, the fungus takes a share of the sugars produced by its solar-powered collaborator. Cyanobacteria also fix nitrogen, making that available to any resident algae as well as to the fungus. The fungus meanwhile shelters the partner cells nested among its filaments and keeps them moist by absorbing water from rain, mists, and dew. In addition, the fungus delivers nutrients from airborne dust caught on its threads and from whatever surface it’s anchored to by the filaments extending from its base.

Swiss botanist Simon Schwendener proposed in 1867 that this combination of creatures represented a symbiotic relationship. It earned him years of scorn from prominent lichenologists. That all organisms are separate, autonomous beings wasn’t just an assumption in those days. It was more like a creed — a projection of the human sense of individual identity in Western culture.

As of 2014, thousands of species of lichens have been identified. By one estimate, they cover as much as six percent of the planet’s land surface. Their nature as a sort of biological alloy makes them tremendously self-sufficient and able to inhabit extreme environments. Often the first to colonize sites destroyed by catastrophic natural events or human disturbance, lichens are also among the last organisms you’ll find standing as you travel from well-watered realms into deserts. It’s the same as you journey from moderate climes toward the barren terrain of alpine crags or polar expanses. Lichens from Antarctica survived 34 days in a laboratory setting designed to simulate the environment on Mars. For that matter, lichens have been shot into orbit and placed outside a spacecraft in a container that was then opened, directly exposing those composite creatures to the flash-freezing temperatures and cosmic radiation of space for 15 days. Upon returning to Mother Earth, they simply resumed growing!

So many of the plants we see in a field or forest are symbiotic with fungi, and the soils underfoot are so saturated with hyphae, it’s not hard to picture such habitats as titanic lichens. You just have to imagine the plants as equivalent to the single cells of symbiotic algae — big algae poking into the air above ground while enwrapped in a mesh of fungal threads below.

I am You, and You Are Me

Perhaps this is where we should shift our gaze from other species to the one calling itself Homo sapiens. The body of people who puzzle over how the living world works — just like the body of people who aren’t all that interested — each contains trillions of human cells and ten times that many microbes. Some are harmless hitchhikers, but most are symbionts that contribute to our well-being. Roughly 30,000 species — primarily bacteria but also archaea, protists, and fungi (mostly in the form of yeasts) — typically inhabit the human stomach and intestinal tract. They carry out much of our digestion, manufacture vitamins, fatty acids and other nutrients often missing in the foods we eat; secrete enzymes and hormones that influence the body’s metabolism, energy storage, and immune system, and they destroy or neutralize harmful microbes. Thousands more species inhabit our mouth and throat, flourishing in those warm, humid environments while helping ensure that harmful varieties of microbes don’t. Still others congregate on our skin and in its pores, in the conjunctiva of our eyes, and in ….. let’s just say any other place you care to imagine.

People are increasingly aware of these facts nowadays. Yet the human-microbe symbiosis goes way deeper. Every cell in every plant and animal, many protists, and all fungi contains organelles known as mitochondria. Commonly described as the power sources of the cell, they build the molecule ATP (adenosine triphosphate), whose complex bonds, when broken, release the energy needed to drive other cellular functions. Mitochondria have their own DNA, different from that in the cell’s nucleus but similar to DNA found in bacteria. These organelles also reproduce on their own by splitting, just as bacteria do. The prevailing opinion among experts is that when you’re looking through a microscope at mitochondria, you’re looking at highly modified bacteria whose ancestor formed a symbiosis with a larger, single-celled creature eons ago. It probably began with the bigger cell engulfing a bacterium to eat it. Somehow the eatee avoided being digested, took up residence inside the eater’s protoplasm, and carved out a niche in the energy production business. That combination became the primordial line that ultimately led to the larger life forms we know today.

Plants have an additional type of organelle in their cells: chloroplasts. These are the photosynthesizing modules, where green pigments in complex proteins convert the sun’s radiation to chemical energy. That in turn fuels the construction of sugars from ordinary carbon dioxide and water, with oxygen given off as a byproduct. Like mitochondria, chloroplasts have their own DNA and reproduce independently. As far as scientists can tell, the chloroplasts are almost certainly a strain of cyanobacteria. Widespread in early seas, those microbes were among the first — and maybe the very first — organisms to develop photosynthesis. Scientists largely credit them with converting earth’s early atmosphere of methane, ammonia and carbon dioxide to a breathable, oxygen-rich one. At some point, like the ancestors of mitochondria, ancient cyanobacteria merged with larger, single-celled organisms. Once again, it may have started when a bigger cell engulfed a smaller one, in this case a cyanobacterium that survived to carry on its sunlight-driven routines. The sugars it contributed led to a better-than-average survival rate for subsequent generations of both species as they reproduced. Their descendants developed into unicellular algae, then multicellular algae, and then — with the help of symbiotic fungi — land plants.

And there you have it. You, I, the rest of humanity, and just about every visible creature we relate to as wildlife, pets, livestock, crops, ornamental plants, and so on, are symbionts, joint ventures in the business of existence, partnered-up from head to toe (or root) with invisible life forms. To me this means that whether you are lost in the wild, mowing a suburban lawn or sitting on the top floor of a skyscraper in an empty, sanitized room, you are never really alone and never truly separate from nature, no matter what you feel or prefer to believe. It’s for others to speculate on the implications for our cherished sense of individuality, not to mention our politics, religious views and environmental consciousness.

Wildlife biologist, author, and longtime contributor to National Geographic, Douglas H. Chadwick has spent much of his career among wild animals — very big wild animals. Yet ever since receiving his first microscope as a child, he has been equally fascinated by miniscule life-forms.

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