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.


| August/September 2014



Yellow Lichen on Rock

Lichens from Antarctica survived 34 days in a laboratory setting designed to simulate the environment on Mars.


Photo by Fotolia/Ifrabanedo

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.





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