Fungi-Plant Mutualism 
Fungi-Plant Mutualism Fungi and plants have always been closely tied in human culture. Most of this is due to their superficial similarities, such as growing from the soil and being (mostly) immobile. But centuries of research has discovered that while not taxonomically related, these two groups are deeply linked by their complex relationships, especially mutualistic ones. While plants and fungi may seem similar on the surface, both taxa have developed entirely different abilities, and those differences are key to how they can work togther so well.
Image Source: Ellen Larsson, CC BY 2.5, via Wikimedia Commons
This shows the hyphae an ectomycorrhizal fungus, specifically an amanita species.
The relationship between mycorrhizal fungi and plants is a contender for the most widespread and successful mutualism across all terrestrial ecosystems. Fungi have been forming mycorrhizal associations for over 400 million years, and modern mycorrhizal species includes famous mushrooms like the fly amanita and chanterelles. As for plants, its estimated that 80% of all plants species benefit from mycorrhizal relationships, and some taxa, such as orchids, cannot grow at all without the right fungi symbionts. These relationships are common across nearly every taxa of plants, from trees and grasses to mosses and liverworts. If its lives on land and isn't extinct, there's probably a fungi that can associate with it. Not all plants necessarily need fungi, but almost all have far better odds of survival with a friend to help them out.
And these associations certainly provide a great advantage. While plants may be able to absorb water, minerals, and other nutrients though their roots, their efficiency tends to be pretty poor. Fungi, however, excell at making the most of what they have, and are more than capable of absorbing enough for both themselves and nearby plants. There is plenty of variation in exactly how these fungi get the necessary nutrients, with Laccaria bicolor known to lure in and kill springtails for their nitrogen, but most simply absorb it from the soil. But because fungi are heterotrophs and thus incapable of photosynthesis, they need the plant to provide food, which they are more than capable of doing. Because their symbiont is their main source of food, many types of mycorrhizal fungi lack the adaptations to survive without their plant associate, and are exclusively symbiotic. These aren't the only benefits to mycorrhizal assocations. Fungi have been found to contribute to defense against herbivores, protection from disease, and can even help the plant filter out heavy metals and other pollutants. But the exchange of minerals for food is the most lasting and fundemental benefit this symbiosis provides, and the trait that defines their interactions.
There are 4 distinct categories of mycorrhizal fungi. Each one is defined by where they're found, what plants they interact with, how they interact with their symbiont, and the impact they have. Of these, Arbuscular and Ectomycorrhizal are the most common and widespread, with Orchid and Ericoid being comparitively niche categories. But all types are distinct, and all have unique advantages to their plant associates. After all, different regions have different limiting factors, and the fungi must be different to adapt to it.
Image Source: Rajarshi Rit(https://orcid.org/0000-0003-3122-5926), CC BY-SA 4.0, via Wikimedia Commons
This shows an arbuscular fungi and its symbiont. Clusers of hyphae are visible growing in the plant's cells.
Arbuscular Mycorrhizal fungi, or AMF, are the most common type of mycorrhizal fungi worldwide. They form relationships with the most diverse set of plants, with around 70% of all terrestrial plants, including everything from trees to liverworts, assocating with AMF. They're most dominant in the tropics and southern hemisphere, as well as in grasslands and rainforests. They're likely the most ancient, too, with fossils dating back over 400 million years old. AM fungi are monophyletic, meaning that all species are part of the same taxonomic group, rather than the term being a descriptor applied to multiple taxonomic groups, as is the case with ectomycorrhizal fungi. All AM fungi are part of Glomeromycota, consisting of 230 described species, most of them mycorrhizal. One unique trait that sets them apart is the rarity of fruiting bodies, as many species produce few or even no mushrooms, instead relying on other, less visible methods of reproduction. Between their unique taxonomy, ancient history, and far-reaching connections with plants, there is strong evidence that they had a metaphorical hand in the very evolution of plants themselves, being assocates of the first green plants to grow on the surface. They work by forming connections between the cells of the root and the soil, with long hyphae threads extending far from the root in order to absorb the most nutrients possible. While plants struggle to extend root hairs past a certain point, AM fungi are able to reach past the depletion zone, where the plant has already absorbed nutrients, into the more valuable areas beyond. It's estimated that AM fungi alone lead to a 20% increase in photosynthesis within the plants they help.
Ectomycorrhizal fungi are unique for a few reasons, but most notable is how they interact with plants. All other type of mycorrhizal fungi physically grow into their host, with hyphae entering into the plant cells, but ectomycorrhizal species do not. They may enter the root tissues, but the hyphae remain in clusters and threats along the exterior of the cell walls, and extending into the soil. These fungi eventually form thick mantles of hyphae all around the roots, completely surrounding the root. This restricts the plant's access to the environment, but it also gives it a layer of protection from disease and other threats. Ectomycorrhizal fungi are polyphyletic, meaning the various species are part of many seperate taxa, and that the term is a descriptor rather than a taxonomic group. There are between 7,000 and 10,000 known species of fungi in association with around 8,000 plant species, which is only around 3% of all plants. However, theses are plants that cover disproportionately large surface areas, primarily trees. These fungi are most common in the northern hemisphere and boreal regions, and many of them are known for their highly visible fruiting bodies. Amanita, truffles, and porcini mushrooms are some examples. While AM fungi are commonly studied for their ability to contribute to crop yield, ectomycorrhizal fungi are often known for their impact on forest, and for their delicious (or poisonous) fruiting bodies.
Ericoid fungi, on the other hand, are defined not by how they grow, but the plants they associate with: the Ericaceae. Often called the heath or heather family, this group includes cranberries, blueberries, azalea, heath, and heather. These plants are known for growing in acidic or nutrient-poor soil, such as bogs and heathlands, so ericoid fungi have evolved to be much more tolerant of acidic conditions than other fungi. Certain ericoid fungi can even act as saprotrophs, breaking down rotting matter in the enviroment to make vital nutrients accessible immediately. Their association with Ericaceae has given them a wide distribution but a narrow niche, with fungal habitats limited by the soil and plants of specific regions. Some species get past this limitation by having the potential to be either ericoid and ectomycorrhizal, with the same fungi forming different structures depending on what plant they're associating with. Most ericoid fungi are from the phylum Ascomycota, though examples from Basidiomycota have been discovered. Overall, ericoid fungi tend to form densly coiled bundles of hyphae within plant cells, rather than the looser growths found in arbuscular fungi.
As you may expect, orchid mycorrhizal fungi associate with orchids. But just like the delicate and bizarre flowers they interact with, their ordinary surface hides a strange pattern of symbiosis. Like their plant symbionts, these fungi are primarily found in tropical regions, although they do have a worldwide distribution. Their hyphae grow inside the plant cells, like other fungi. But that's where the ordinary aspects of orchid mycorrhizal fungi end. The symbiosis begins when the orchid is a seed, as small as a piece of dust. These tiny seeds have almost no nutrient stores of their own, and rely on fungi (typically basidiomycota) to feed it with sugar absorbed from its surroundings and help the plant develop its root system. At this stage in its life, the orchid is myco-heterotrophic, as it gets all of its nutrients from the fungi rather than photosynthesis. Of all mycorrhizal plants, orchids tend to be the most reliant on their symbiont, and many species of orchids being unable to germinate without fungal help. However, that changes as the plant grows. Some plants never develop photosynthesis and remain reliant on their fungi, becoming parasites. In other cases, the fungi will contribute less and less as the plant grows, eventually become a parasite itself.
While most mutualistic fungi interact with plant roots, some species live in the stems or leaves of their symbiont, and these fungi are called endophytes. Like mycorrhizal fungi, they form webs of hyphae between or within plant cells, even forming clusters inside the plant tissue. They're similarly widespread, being found in nearly every plant taxa. But unlike mycorrhizal fungi, they rarely produce mushrooms, making them not only hard to find, but difficult to identify. Between the rarity of fruiting bodies and the obscure location of hyphae, endophytes have gone unnoticed for centuries, but ongoing research is currently learning more about them, and the benefits they may provide to their symbionts.
While I'm only going to cover mutualistic endophytes, there are plenty of parasitic and commensalistic examples. Endophytes are defined as any type of fungi or bacteria that lives within the body of a plant without causing visible illness. Mutualist fungi are far from the only type of endophyte, but they are the only one that fits in the scope of this article. Unlike mycorrhizal fungi, endophytes can't do much to absorb nutrients or water. Rather, the benefit they provide is defense. Mutualistic endophytes are great at producing poisons that leave the host plant unharmed while sickening any herbivores that try to eat them, protecting both symbionts from predation. While most of these chemical defenses are targetted at insects, as they are the most widespread and diverse taxa of herbivores, they can negatively impact mammals as well. Endophytes in grass, for example, have been found to occasionally poison or even kill grazing cattle. Despite their near-invisibility, endophytic fungus have a major impact on plant survival and success.
For some particularly interesting examples of endophytes affecting animals, you can look at their interactions with Elm Bark Beetles and Attini ants. Elm bark beetles are themselves mutualists of certain types of wood rotting fungi, Ceratocystis ulmi, which infects trees with Dutch Elm disease. The fungi gets an easy mode of transit, the beetle gets a more nutritious source of food, and the tree gets Dutch Elm Disease. But the tree has mutualistic fungi of its own, specifically Phomopsis oblonga. This fungi, while otherwise parastic, has been found to be a significant deterrent to bark beetles, and even outright poisonous to the insects. Similarly, the presence of endophytic fungi in leaves collected by leaf-cutter ants will slow the growth of the mutualistic fungi the ants cultivate, making those leaves less useful to both of them. For more information, check out the page on animal-fungi interactions.
Endophytic fungi are a broad group, containing many unrelated taxa. Many of them are close relatives of dangerous plant pathogens, or can become parasitic themselves in the right circumstances. Some of them reproduce like pathogens, by producing fruiting bodies, while others extend their hyphae to the plant's reproductive structures, ensuring that the seeds will contain fungul associates of their own. While far less visible than mycorrhizal fungi, they are just as fascinating, and provide a unique window into a space between mutualism and parasitism.
But this is not the full story. Mycorrhizal fungi not only interact with many types of plants, but with other types of fungi, forming an underground network of sugars and water. The impact and complexity of these network can, and likely has, filled entire books. I hope to give you an overview, and encourage you to look deeper and see what interests you. This is only an introduction.
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