Let’s enter a very strange world. Dark, silent, microscopic, underground. The soil – on which our crops and forages depend – is a veritable Amazon jungle teeming with thousands of species of animals and plants and something that’s not quite either: fungi. Over the past few years, we’ve heard a lot about things such as soil health, regenerative agriculture, carbon sinks, mycorrhizae, etc. This month, let’s focus on soil health and, literally, get under the surface. Let’s talk about the astonishing group of soil fungi called arbuscular mycorrhizae.
You may have heard about mycorrhizae, as in some type of soil fungus that somehow helps plant roots. Let’s get past the “somehow” and examine the details.
Starting basics
First, however, we’ll review some facts about plants, animals and fungi. The tissues of all these organisms are composed of cells, and the interior protoplasm of every cell is encased by a cell membrane, kind of like a water-filled balloon. This cell membrane is a flexible film, a few molecules thick, composed primarily of fat molecules (lipids). But it’s more than just a protective sac. Embedded in every cell membrane is a huge array of proteins that perform critical tasks for that cell, including nutrient transport, immune function, communication and much more.
In contrast to animal cells, plant cells also have an additional outer layer: the cell wall. This is a solid capsule composed primarily of fiber molecules such as cellulose, hemicellulose, and lignin. Does this sound like the neutral detergent fiber (NDF) number that you see on forage test reports? Well, yes – because it is exactly that. The cell wall is analyzed as the NDF value. The cell wall is the fibrous hard capsule that protects the cell membrane and the cell interior, and it provides the plant with a firm physical structure. Imagine a tree without cell walls. That’s a joke that would probably fall flat.
Fungi, on the other hand, march to the beat of a very different drummer. They are neither plants nor animals. Fungi cells also have cell walls, kind of like plant cells, but fungi cell walls do not contain the standard plant fiber compounds of cellulose or lignin. Instead, they contain chitin, which interestingly is the same molecule found in the hard exterior skeleton of insects. In addition, fungi grow by extending long thin filaments called hyphae. The next time you are in the woods, look at a log rotting on the forest floor. You’ll see long white filaments throughout the decaying wood. Those filaments are the hyphae of the fungus. Botanists have classified fungi together with yeasts, molds, rusts, mildews and mushrooms in a special kingdom of their own – not surprisingly called the fungi.
Going even deeper
Now let’s focus on the arbuscular mycorrhizae. These are a very specialized type of microscopic soil fungi associated with the roots of higher plants. Specifically, these fungi live around and inside the root hairs – the tiny protuberances of plant roots that are the actual sites where the nutrients are absorbed. Sharp-eyed botanists with their microscopes first observed these tiny fungi in the 1870s, and in 1885 the German scientist A. B. Frank coined the word mycorrhizae. He derived it from two sources: myco meaning fungus, as in mycotoxin, mycology (biology of fungi) and mycosis (fungus infection); and rrhizae, from the Greek rhiza meaning “root” and the word rhizosphere, which is the soil region immediately surrounding plant roots. (For the grammarians among us, “mycorrhizae” is plural for “mycorrhiza,” but I think mycorrhizae is easier to say. And to say it, think “micor-eye-zae” with the accent on the “eye.”)
Mycorrhizae fungi are tiny and thin, much thinner than root hairs. They begin life on the root surface, but then – and this is their defining characteristic – they penetrate the cells of the root hairs. They send hyphae through the cell walls into the cell membrane. Then, within the cell membrane, they develop a branched network of filaments, like a fishing net. This filament network is the arbuscule – hence the name arbuscular mycorrhizae.
But so what? Isn’t this just another type of infection that the plant should worry about? No, not at all. This complex arrangement benefits everybody. Arbuscular mycorrhizae represent an extreme version of symbiosis: a mutually beneficial relationship where each species helps the other species and gains from the association. In this case, the fungus gains nutrients synthesized by photosynthesis in the plant leaves, and the host plant gains nutrients that the fungus absorbs from the soil.
Once the arbuscular mycorrhizae develop its arbuscule inside the root hair cells, the fungus sends additional hyphae outward from the plant into the soil, extending up to 10 inches into the soil around the roots. These hyphae are extremely thin, much thinner than the root hairs. Like tentacles, they can work their way into soil particles and crevices that root hairs can’t. In addition, because they are so thin, their surface-to-mass ratio is extremely high, meaning there is lots and lots of surface area that can absorb nutrients from the soil. In effect, healthy arbuscular mycorrhizae greatly amplify the nutrient absorption capacity of the host plant. This translates to more nutrients being taken up from the soil, including from places that the plant roots cannot reach.
Researchers have demonstrated this conclusively with phosphorus, nitrogen, sulfur and some trace minerals like copper and zinc. Especially phosphorus, because arbuscular mycorrhizae are much more efficient at extracting phosphorus from the soil than higher plants. In turn, the host plant provides the arbuscular mycorrhizae with nutrients it cannot synthesize on its own (fungi are not capable of photosynthesis), such as sugars and some fats. The fungi’s arbuscule network in the plant’s cell membrane is a busy two-way street. Inorganic nutrients like phosphorus, iron and nitrogen flow from the arbuscular mycorrhizae to the host plant while organic nutrients flow from the host plant into the arbuscular mycorrhizae. Of course, there’s a biochemical cost to this: The host plant transfers as much as 25% of its photosynthetic carbon to support the arbuscular mycorrhizae. From a biological perspective, this is a good deal. The price is clearly worth it, and everyone is happy.
Arbuscular mycorrhizae also convey additional benefits to their hosts. Things get very complex here, so I’ll just mention a couple. Healthy arbuscular mycorrhizae can help plants resist stresses like drought and infections, or attacks by other organisms such as bacteria, nematodes and other fungi. Arbuscular mycorrhizae also can help plants survive in soils with high levels of toxic elements, such as aluminum and arsenic, by reducing their uptake. And in legumes, arbuscular mycorrhizae may increase nodulation of the roots by the nitrogen-fixing Rhizobia bacteria and increase plant yields and seed production.
Applying it all
But let’s get real. Arbuscular mycorrhizae is certainly a fascinating topic, especially if you talk about it at the dinner table where people will stare at you intently. But seriously, what does this all mean to us who raise animals and grow forages, corn, wheat and other crops?
First, we know that arbuscular mycorrhizae are natural fungi that are good for their host plants. They can reduce the need for phosphorus fertilizer and help plants resist drought, stress and infection. We also know, however, that some standard cropping practices disrupt or suppress arbuscular mycorrhizae, like any tillage of the soil – plowing, disking, etc. Tillage destroys the soil structure, and with it, the delicate three-dimensional hyphae networks of these fungi. Also, using fungicides on the soil (e.g., to kill specific fungi that invade crops) can kill arbuscular mycorrhizae. And applying high rates of commercial fertilizers can suppress them.
These concepts are a bit unnerving because they undermine some of the basic agricultural methods farmers have used for centuries. But these concepts also reinforce some basic principles of good soil management: minimize tillage and keep the soil covered with green plants, preferably perennials. Platitudes, to be sure, but they clearly apply to arbuscular mycorrhizae.
Second, there is so much that we don’t know. We are just scratching the surface, or rather, scratching microscopically under the surface. This is really new information. Much of the solid science about arbuscular mycorrhizae is quite recent, as are the molecular techniques and precise laboratory equipment required to study these fungi. But we are gaining. Many laboratories around the world are conducting research on arbuscular mycorrhizae. We’ll learn much more about this story in the next few years.
And one thing is clear: Arbuscular mycorrhizae is more than just a strange academic oddity; it’s a fundamental part of the huge topic that we call soil health.
We should definitely stay tuned.
