Among life’s most successful evolutionary achievements is a relationship that has developed between two entirely different life forms. It is a type of mutualism more than 400 million years in the making. The phenomenon, involving vascular plants and fungi, can be found virtually anywhere plants grow. It occurs all around us, yet most of us are blithely unaware of it, much less see it.
With photosynthesis going on in leaves and reproductive development taking place among flowers and fruits, much happens in a plant’s aboveground parts. But there is a whole world of action that goes on below ground that we cannot easily see. That unseen action, however, is also critical to plant life — and, by extension, our lives.
In addition to anchoring and supporting plants, roots and other underground structures play key roles in carbohydrate storage and, in some plants, vegetative reproduction. Roots also function vitally in the uptake of oxygen, carbon dioxide, water and nutrients, all of which are delivered to a plant’s aerial parts. In one sense, roots can be thought of as a plant’s mouth, grazing through soil for the food, water and air it needs to survive and thrive.
It has long been recognized that certain types of fungi play a role in assisting plant roots. Research in recent years has revealed that these fungi, known collectively as mycorrhizae, are nearly ubiquitous — and absolutely essential to life for most plant species.
Depending on the species, most plants and fungi form one of two basic mycorrhizal relationships. Ectomycorrhizae form mutualistic pairings with many types of plants, most notably many forest tree species. (See our “Science Spotlight” on Dr. Suzanne Simard for more on that.) These fungi form a kind of sheath around their partner plant’s roots, providing water and essential nutrients to their host in exchange for plant-manufactured food that the fungi needs. Endomycorrhizae work in a similar way, but actually penetrate the cell walls of plant roots to facilitate a more intimate exchange of nutrients and carbohydrates.
The relationships, though often mutually beneficial, should not be thought of as plants and fungi singing “Kumbaya.” Each organism acts in its own interest. In fact, the cost-versus-benefit equation is very often unbalanced, with one of the participants a net winner and the other giving up more than it gains. In some cases — especially an ectomycorrhizal relationship commonly involving nonphotosynthetic orchids and monotropes such as Indian pipe (Monotropa uniflora) or pinesap (Monotropa hypopitys) — the plants are parasitic on their mycorrhizae and provide no advantage at all to the fungi. These plants are often incorrectly referred to as saprophytic, but no plant obtains nutrients through the direct digestion of dead organic matter. Fungi or soil-borne bacteria are always intermediaries.
All plants with chlorophyll manufacture food in the form of carbohydrates through the process of photosynthesis. The food not used for growth and other needs of aerial plant parts is transferred through the vascular system underground for root growth and storage. For most plants, a portion of that transferred food is shared with adjacent mycorrhizae.
For their part, one of the most important functions of mycorrhizal fungi is to greatly enhance their partner-roots’ ability to access water. The other vital function is to provide essential minerals, especially phosphates and some micronutrients that are otherwise poorly taken up or wholly unavailable to the plants.
Some mycorrhizal fungi actually kill and digest minute soil-dwelling insects called springtails and can provide a surprising amount of nutrients from these organisms to their partner plants. If you’re feeling sorry for springtails right about now, please be aware that they make their living mostly by eating other soil-borne fungi. The circle of life.
And you have to admit: A relationship lasting nearly half a billion years? That must be a healthy and beautiful thing.