Even when you can’t see them, fungi are all around you. Most plants are host to many different species of fungi, living within and between their cells. These fungi are called endophytes, meaning “inside plant,” and they can form parasitic or symbiotic relationships with their host plant. Most of the time, they live inside the plant without ever producing symptoms.
Though mushrooms superficially resemble plants, they do not photosynthesize. Instead, they collect most of their energy from decaying plants. Mycorrhizal fungi have found another way around this problem. These fungi happily share their nutrients with plants in exchange for something they don’t come by so easy: sugar!
One of the best-known relationships between plants and fungi are mycorrhizae. These networks of mycelium (fungal roots) provide essential nutrients to forests.
Mycelial threads are much slimmer than plant roots. This increased surface area makes fungi great at absorbing nutrients from the soil. In addition, fungi secrete enzymes to digest plants and animals.
Nitrogen is often a limiting factor in plant growth. This nutrient is essential for making proteins. Though the atmosphere is 78% nitrogen, plants cannot “fix” this gas. Instead, they rely on bacteria or fungi to provide nitrogen to them. Many plants are happy to share some of their energy to take advantage of nitrogen from mycorrhizae.
Over half of all plant rely on mycorrhizae to survive. 90% of known plant species have been observed benefiting from this relationship.
Fungi produce many interesting chemicals that provide protection against bacteria or herbivores. Many endophytes also produce toxins. By producing these secondary metabolites inside of a plant, they provide protection against herbivores.
Natural toxins are often useful to humans. Though they evolved as a defense, they can be a source of medicine. Both traditional and modern medicine lean heavily on natural products. Many modern drugs are based upon compounds found in plants, bacteria, and fungi. Evolution has had a long time to discover uniquely useful compounds.
A prime example of this is penicillin. This antibiotic was discovered by accident. A fungus, Penicillium, contaminated a petri dish of bacteria. The fungus naturally produced penicillin to fight the bacteria. When Alexander Fleming saw this, he realized its significance for human health. Purified penicillin brought about a revolution in modern medicine.
Endophytic fungi in plants also produce interesting compounds. The anticancer drug Taxol is produced by a fungus in the bark of the yew tree. Though this drug is currently expensive and difficult to produce, culturing the fungus may provide a cost-effective solution.
Another endophyte with interesting effects grows within the related morning glory and Hawaiian baby woodrose vines. This fungus produces lysergic acid amide, or LSA as well as other ergoline alkaloids. These chemicals are closely related to the psychedelic drug LSD. Related chemicals are also used as vasoconstrictors to treat migraines.
But not all endophytes are good. Many of the toxins produced by endophytes are toxic to cattle. When these grow in grazing areas they can have huge economic consequences. Because endophytes often show no signs in the plant, this problem is very difficult to combat.
Endophytes produce many other interesting compounds. Many of these are valuable to human and animal health.
Fungi reside in their own kingdom of life, outside or plants, animals, and bacteria. Their sedentary life leads many to think of them as a type of plant. In reality, they are far more closely related to animals. Animals and fungi are both heterotrophs, meaning they get their energy from other life. The cellular structure of fungi also shares characteristics with human cells. However, fungi are extremely diverse and unique.
It is difficult to say how many species of fungi exist. Though there have only been 120,000 species of fungi identified, scientists estimate that there are millions of unidentified species.
This question is made particularly difficult by the unique characteristics of fungi. Though we often think of the red and white Amanita muscaria toadstool as an archetype of fungi, these fruiting bodies are actually quite rare.
Most of a mushroom’s life is completely hidden. They grow within their “medium”. This can be soil, a dead log or, in the case of endophytes, a living plant. They can cover large areas without being apparent. One of the largest organisms in the world is a fungus, called the humongous fungus. Its mycelial mass covers over 3 square miles in Malheur National Forest in Oregon. It may weigh as much as 200 gray whales and be 2,000 – 8,000 years old.
This network of mycelium is the common form of most fungi. Mycelium all looks more or less the same to the naked eye. Though there are differences on the microscopic level, these are difficult to identify.
What we are familiar with is a fruiting body. Toadstools appear in the fall when conditions are favorable for reproduction. They only come above the surface to spread their spores.
These fruiting bodies are the easiest way to determine a fungus’ species. However, many species of fungi only produce microscopic fruiting bodies, called sporangia. These are tiny organs that are difficult to identify.
Endophytes can be even harder to classify. Many of them don’t reproduce through traditional means. Instead, they are passed between generations of their host plants by growing in the seeds. This is called vertical transmission. These species may never form spores. This makes it incredibly difficult to define species.
Because it is so difficult to identify a fungal species by morphology, genetic techniques are preferred. However, even with genetics, things are still unclear. There is still uncertainty about what should be called a species. How different the genes should be and even which genes should be compared is debated.
There is no doubt that fungi are extremely diverse. With millions of unidentified species, there are sure to be many evolutionary treasure troves. Fungi have provided medical marvels such as penicillin.
Fungi take advantage of many unique niches. Endophytes are a prime example of how fungi are uniquely adaptable. This flexibility and ubiquity make fungi a great source of genetic diversity.
The number of unidentified species represents a huge bank of evolutionary experience for humans to draw upon. As we face widespread antibiotic resistance and new diseases, discovering new sources of drugs is essential. By cataloging and exploring the diversity of fungi we will be more prepared to face new challenges.