How fungi join forces with plants to feed our forests.
Though fungi are often associated with decay, they actually play a vital role in providing forests with the nutrients they need to prosper.
All ecosystems are made up of intricately connected species doing whatever it takes to survive and produce as many offspring as possible. Chasing this goal can produce some very interesting and seemingly strange interactions. Previously antagonistic species can come to rely on each other, taking advantage of each other’s skills to benefit their mutual fitness.
One of the most ubiquitous examples of a mutualistic relationship is mycorrhizal fungi. These relationships involve the exchange of essential nutrients between the roots of plants and the mycelial network of fungi (essentially roots for a mushroom). Fungi occasionally even source their nutrients from insects and share their catches with plants.
Mycorrhizal relationships are so ancient and intricate that they are deeply ingrained and necessary for the functioning of many species of both plants and fungi.
Exactly when this strategy arose is unclear, though recent evidence shows that this strategy may have been so successful that it evolved multiple times in distinct groups of fungi. This includes groups that were once purely pathogenic, stealing nutrients from plants and animals and giving nothing in exchange. The recurrence of mycorrhizal relationships shows that even in the seemingly harsh and cruel world of nature, working with, rather than against, your neighbors may be the best way to survive.
The Nitrogen Problem
Although our atmosphere is made of 78% gaseous nitrogen, this essential nutrient used in protein formation and other essential biological processes is often the limiting factor for plant growth. This is because N2, the form of nitrogen that primarily exists in the atmosphere, is useless to plants which have no mechanism for absorbing it and incorporating it into the many proteins they need to survive. Instead they rely on nitrogen fixing bacteria, fungi and occasionally even animals to sequester and provide them with this essential nutrient. Through time plants have evolved complex relationships with these species to fill their needs.
Plants have evolved many strategies to deal with their nitrogen struggles. Bacteria are the only organisms that are capable of “fixing” atmospheric nitrogen into ammonia (NH3) which is the simplest biologically useful form. Legumes (such as soy and peas) form mutually beneficial relationships with soil bacteria. These bacteria invade the plants roots, taking carbon and other resources from the plant in order to grow. In exchange they provide the plant with the NH3 it needs to live.
Fungal nitrogen exchange works quite similarly, however the fungi scavenge bio-available nitrogen from decomposing organic matter in the soil rather than pulling it from the air. The massive surface area provided by mycelia, which are made up of extremely small strands of fungal cells, can access and consolidate more nitrogen than plant roots. In exchange for this nitrogen, the plants provide the fungi with carbon and other necessary nutrients. Around half of the plants on earth rely on these mycorrhizal relationships for their nitrogen and around 90% of known plant species have been observed benefiting from this relationship.
Not all plants are vegans
A more unusual way for plants to access nitrogen is through carnivory. The most explored example of animal based nitrogen sourcing in plants is carnivorous plants such as tropical pitcher plants, venus flytraps and sundews. These showy carnivorous plants inspire people to imagine a plant based rebellion, giant venus fly traps swinging through the streets of New York, eating people whole in their strange toothless mouths. Alternatively, plant carnivory can manifest far less conspicuously.
Certain fungal species have developed the ability to collect nutrients from insects through a pathogenic infection, just like human diseases which feed on us. This infection generally starts in larval insects and kills them, giving the fungus the nitrogen stored and collected by the insect. Some of these species are generalists, capable of feeding from multiple sources, and actually can gain nitrogen by infecting insects or from decomposing matter. These fungi can even trade this nitrogen for the resources of plants.
A recent study on these species revealed that some mycorrhizae were actually transferring insect derived nitrogen into host plants in exchange for carbon. This relationship allows plants to regain nutrients previously lost to insects through herbivory, reversing the normal flow of the food web and improving reproductive success for both species. This complex ecological relationship is particularly interesting due to its appearance within groups of fungi that split on their evolutionary paths long ago.
Two heads are better than one
A recent paper published in Communicative and Integrative Biology describes how these groups of fungi appear to have independently evolved this symbiotic strategy of nutrient exchange with plant roots. The fungi that attack insects and provide their nitrogen to plants are more closely related to species that are exclusively plant diseases than to other mycorrhizal fungi.
These plant predators evolved a symbiotic relationship independently, judging by their genetic separation and differing structures. This indicates that fungal pathogens have modified their strategy and become symbiotic multiple times in evolutionary history, a testament to the power and effectiveness of this symbiotic strategy. Examples like this reveal how evolution can repeatedly find the same solution when species encounter the same scenario.
Mutualistic ecological relationships show the kinder side of a natural world that often appears harsh and unforgiving. Evolution doesn’t necessarily force competition between species, it forces species to maximize their reproduction. Sometimes this is most easily achieved through mutually beneficial relationships. In cases like this we can see how ecological systems treat their constituent parts similarly to organs in a living organism.
Scientists hypothesize that over the course of evolutionary history, organisms have joined together to take advantage of each others skills. Both mitochondria and chloroplasts (the power plants of animal and plant cells) were first incorporated into living cells as a result of this history of mutualistic relationships.
Mycorrhizae are so important to and interconnected with their host plants that they may appear to be the same organism. Some fungi are so closely linked to their plant hosts that they are passed on in their seeds, appearing to be one organism. This evolutionary strategy is a powerful example of how ecological systems can behave like organisms or at least work in a coordinated way for mutual fitness.
Relationships like these occur extremely often in nature and reveal the intricate interrelatedness of all species in an ecosystem. Without the plant’s root system fungi would have a much harder time collecting carbon, a task that plant leaves have adapted perfectly for through billions of years of optimizing the process of photosynthesis. Similarly, the plants would have extremely stunted growth in a world without mycorrhizal fungi because they are not as well adapted for nitrogen scavenging as the fungi.
The usefulness of mycorrhizal relationships is made obvious by its repeated occurrence in fungal evolution. Clearly this strategy provides a simpler solution for the problems of both species than evolving unique mechanisms within the species, a process that could take quite a long time or require costs too great to be beneficial.
Ecological processes tend to move along the path of least resistance, seeking out a simpler solution, even if that means working alongside an old enemy. Evolution has repeatedly created the same solutions to similar problems across the tree of life, teaching those who look much about the best way to get things done.
Behie, S.W., Padilla-Guerrero, I.E., and Bidochka, M.J. (2013). Nutrient transfer to plants by phylogenetically diverse fungi suggests convergent evolutionary strategies in rhizospheric symbionts. Communicative & Integrative Biology 6, e22321. http://www.tandfonline.com/doi/pdf/10.4161/cib.22321
Behie, S.W., Zelisko, P.M., and Bidochka, M.J. (2012). Endophytic Insect-Parasitic Fungi Translocate Nitrogen Directly from Insects to Plants. Science 336, 1576–1577. http://science.sciencemag.org/content/336/6088/1576.full
Photos taken by me, Casey Hofford