Most of us believe we know certain things about plants. One of those indisputable facts is that the roots of plants are the organs that take up water and nutrients from the soil. We’re not entirely sure where we came by this wisdom, but there it is.
This is an excerpt from the Wild Ones Journal
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It turns out that this is not strictly true. It is true, to some extent, for some plants, but our understanding is undergoing an injection of new information that is increasing our appreciation of underground life. We have learned that, for the vast majority (probably more than 95 percent) of plant life, the actual organ of nutrient uptake from the soil is not the root of the plant, but rather a fungus that forms a very special, symbiotic relationship with the plant. This relationship is in fact symbiotic because both partners in the relationship benefit.
Because these relationships are so prevalent, especially in old, stable, native soils, it is very likely that they play a significant role in determining native plant communities and plant succession. The presence of these relationships also plays an enormous role in the survivability of native plants in undisturbed soil – another reason we can say that our native plants can take care of themselves.
The Fungal Partner in this Symbiotic Relationship
We are accustomed to thinking of fungi as mushrooms or toadstools – yeasts, mildews, the multicolored stuff that grows on food left too long in the fridge, the yuckum on shower curtains – but this is a very small part of the picture.
The organisms called fungi produce spores, which, when they have food, water, and the necessary environmental conditions, will produce hyphae – hair-like threads that are tubules with rigid external walls. A very large proportion of a fungus are its hyphae, which may be thought of as analogous to roots of plants, though that is not strictly correct. Fungi make up a separate taxonomic kingdom of life, separate from plants. Their body parts are quite different, and work differently from those of plants. Hyphae are about as thick as a human hair – they take up nutrient-carrying fluids from the soil, and transport these fluids. A mass of hyphae will intertwine to form a mycelium, which is the body of the fungus.
Fungi are capable of both asexual and sexual reproduction – through spores. The mushrooms that we see are actually the fruiting body of that particular species of mycelium. Other fungi produce no fruiting body that is visible above ground. Some fungi have lost their ability to reproduce sexually, and exist as clumps of mycelia. Undisturbed, these mycelia may live for thousands of years, and occupy hundreds of cubic acres of soil.
Unlike plants, fungi cannot produce their own food. Like mammals (such as humans) they are heterotrophs, and must depend on other organisms for their carbon source. If the food is not in readily absorbable form, like wood (lignin, cellulose and pectin), then it must be digested. Various enzymes, that can be both substrate and species-specific, are produced by the hyphae, and the food material is broken down until it can be absorbed through the hyphal cell wall.
The ability of fungi to produce external digestive enzymes makes them major decomposers, breaking down refuse. Think of fallen trees in the forest, the pile of wood chips waiting to be spread in your garden, or any other vegetable matter that slowly breaks down and disappears into the soil. A large part of this conversion happens courtesy of fungi. The meat that turns green after a couple weeks in your fridge also is experiencing this process of decomposition.
If the available molecules are already small they will be absorbed directly through the cell wall of the hypha. Hyphae have access to all the dissolved minerals and nutrients that are suspended among the particles that make up the soil.
As for life supporting carbon molecules, the hyphae of some species of fungi have found an easily accessible carbon source – plants. Indeed, 95 percent of plants.
The Plant Partner in this Symbiotic Relationship
The plant partner has roots that pass life-supporting nutrients on to the rest of the plant that is green and capable of photosynthesis. Another, more important function of roots is the storage of the products of photosynthesis. These carbon-based molecules are stored in specialized cells (cortical parenchyma) within the roots proper and within the root hairs.
Thin, fibrous root hairs that grow out of the main roots are the parts that are actually capable of absorbing water that bears nutrients. But, as thin and extensive as root hairs may be, they are no match for fungal hyphae.
The hyphae extend far beyond the region of access of a plant’s roots, and are probably 20 times thinner than a root hair. This difference in diameter is important because the hypha can absorb the tiniest droplets of fluid and bring it back to the root. Think of the very final droplets of a soda at the bottom of a glass being sucked up by a big, fat straw – it can’t be done. But consider what happens when you use a very fine, thin straw – you don’t have the same problem.
Most of the absorption of nutrient-rich fluids is done by the fungal hyphae, and transported to the plant root hairs.
Being much finer than the root hairs of plants, some hyphae can enter the structure of the root, passing among the cells of root hairs, in search of the carbon-rich molecules that are stored in the specialized cells in the root hairs.
The hyphae of some fungal species can go a step farther and actually enter into the cells of root hairs. The plant maintains its own identity, as do the hyphae. There is tissue between them – the respective cytoplasms do not intermix. The point of this incursion is friendly (in the great majority of cases). The plants cooperate. Everyone is there for a good reason – to exchange goods. The roots make their carbon-rich sugars and carbohydrates available to the hyphae, in exchange for the nutrient-rich fluids and water that the hyphae bring from afar.
Mycorrhizae
Mycorrhiza (plural mycorrhizal, adjective mycorrhizal) is a word that started appearing in the vocabulary of gardeners about 30 years ago, but there wasn’t much information to be had. We understood that somehow mushrooms or fungi were involved, but weren’t sure if this word, that was impossible to spell, was just another word for a kind of fungus. The word has come back into use recently in reference to the organic fertilizing of plants.
Mycorrhiza is the word that covers the entire mechanism of a working relationship between a plant root and fungal hyphae. It refers to the structure, the function, and the members of this symbiotic relationship.
There are seven or eight types of mycorrhizae. They differ structurally and functionally – and arose at different times in evolutionary history.
Many of the trees that dominate temperate forests, members of the oak (Fagaceae) and pine (Pinaceae) families especially are mycorrhizal. Most grassland species form mycorrhizae with a different set of fungal species. These various fungi may play a role in determining the number of plant species that occupy a particular site, contributing to the high species diversity seen in some grasslands and forb-lands.
Members of the blueberry family (Ericaceae) often occur in bogs and other nutrient-poor soils. One reason for this preference may be that the fungi that form their mycorrhizal are capable of extracting mineral nutrients from peat and other organic materials. Conversely, plants with woodland and grassland mycorrhizae are rare in bogs. The specificity of mycorrhizal relationships is likely related to the specificity of the enzymes produced by fungi.
Mycorrhizal relationships are anything but straightforward. A plant may form one type of mycorrhizae in the upper reaches of the soil, and different mycorrhizae deeper in the soil. And hyphae from one mycelium can form mycorrhizae with several plants at the same time. In fact, we have evidence that the hyphae can transport sugars and carbohydrates from one plant to another plant that is under stress.
As a bottom line, it is largely because of mycorrhizal relationships that we can say that our native plants can take care of themselves. Our native plants have worked within these relationships since time immemorial. This underground system serves to bring nutrients and water to our native plants, while adding to and maintaining the underground stores of carbon. Every time we do something to strengthen this relationship, or to help it survive, we are supporting soil biodiversity, helping stabilize self-sustaining systems, while decreasing the size of our own carbon footprint on the Earth.
In the next article we’ll explore further various types of mycorrhizae and how they work with and within our native plants.
By Maryann Whitman
My sincere thanks to Sandy Scheine of the Oakland (MI) Chapter, who checked the accuracy of my statements and made suggestions for this article. She is the Education Committee Chair for the North American Mycological Association.