In the early 1900's, it was determined that the southern hemisphere needed more pine trees. At the time, it had very few – pines are native only to the northern hemisphere. But areas in the south had a dearth of good timber trees, and people were determined to fix that with pines. There was one major problem: the trees started from seed in the south were weak and spindly. The solution was quickly uncovered: transplant seedlings along with their associated soil from the northern hemisphere, or else have some northern soil on hand to place in the planting holes. Trees given this treatment tended to do quite well – in fact, a little too well: pines are now habitat-destroying invasive species in South America, South Africa, Australia, and New Zealand.
What was it about this soil treatment that was able to turn southern pines from wusses to bullies? As it turns out, a single tree is actually an elaborate ecosystem – in the words of the army recruiting motto, a “forest of one” – that depends on the cooperation of many different organisms other than the plant itself to function. For 95% of terrestrial plants, the most important members of this community are fungi known as mycorrhizas, and mycorrhizas were what made the difference between success and failure for these pine trees.
What's a Mycorrhiza?
“Mycorrhiza” comes from the Greek roots “myco” meaning “fungus” and “rhizae” meaning “roots.” The function of a mycorrhiza is essentially that – to act as root extensions for the plant. Fungi have an advantage over plants in that they're smaller and thinner than plant cells, and so they can penetrate the nooks and crannies of the soil that plants cannot. They are also capable of growing quickly, forming a netlike mycelium that spreads through the soil well beyond the roots of the plant itself. In exchange for vital and rare nutrients like nitrogen, phosphorous, and trace metals from the mycorrhiza, the plant provides sugars from photosynthesis. In many cases, a mycorrhiza's services aren't cheap and can cost the plant up to 50% of its annual production, but, for the most part, this is an unavoidable cost of doing business. Many mycorrhizal plants grow poorly in the wild if their fungal partners are absent. This was true for the pines – because they weren't native to the southern hemisphere, there weren't any mycorrhizal fungi in the local soil that recognized them. The trees only became successful after the importation of pine-associating fungi.
There are two main types of mycorrhizae: the ectomycorrhizae (“ecto” meaning “outside”) and the endomycorrhizae (“endo” meaning “inside”), so named because the endomycorrhizae will penetrate the host plant cells, and the ectomycorrhizae will grow around them. While the endomycorrhizae are generally small and cryptic, the ectomycorrhizae include many gourmet mushrooms, including porcini (Boletus edulis), truffles (Tuber spp.), morels (Morchellus spp.), and chanterelles (Cantherellus spp.). The fact that these require living trees to associate with is one of the reasons these mushrooms aren't readily available in the grocery store and must be harvested wild.
Why Be Mycorrhizal?
Rather unusually, the ectomycorrhizae consist of two disparate fungal phyla: the Ascomycota, or cup fungi (including truffles and morels) and the Basidiomycota, or mushroom fungi. These two groups are only distantly related, indicating that the complicated mycorrhizal relationship has evolved separately at least twice. Further evidence suggest that many ectomycorrhizal groups within the two phyla also evolved independently, indicating a strong evolutionary pressure to cooperate with plants. The probable reason for this pressure is the fact that fungi don't have a lot of career options; if you were a fungus, you could be a saprotroph (something that feeds on dead and decaying plants and animals), a pathogen or a parasite, a wood-rotter, or a mycorrhiza. Saprotrophs generally have a wide array of available food sources, but they have to compete with bacteria and fungi of the phylum Zygomycota (molds), which are generally smaller, faster-growing, and more efficient with their resources. Pathogens and parasites have to contend with the elaborate immune responses of both plants and animals, and while many are quite successful, they often depend on one or more specific hosts in order to complete their life cycle, making their strategy very risky (see Sara's post on parasites). Wood-rotters, though their food source is abundant, have the most dangerous job of the fungal kingdom – the only way to get rid of the tough lignin binding the edible cellulose together is through a process that generates tons of free radicals, which can damage DNA and shred cell membranes. This is generally not something you want to have in your dinner. Wood is also very low in essential nutrients, especially nitrogen, which is necessary to make protein. This leaves the mycorrhizal relationship as a safer career path – your host isn't actively trying to kill you, and you get an abundant, easy-to-digest, and reliable food source.
Mutualism or Parasitism?
It has been argued, however, that the mycorrhizal relationship isn't so much a win-win relationship for the the plant and the fungus, but a sort of mutual parasitism where each partner is trying to get the most it can out of it's associate, often to its detriment. Past experiments have suggested as much – give the tree enough nitrogen and phosphorus in its soil, and it will starve its fungal partners. Give fungi easy access to sugar, and it will turn on its host and attack the roots. More recent research in natural settings, however, suggests that this may be more of a mutualistic relationship than a parasitic one. First, the two partners are capable of regulating how much sugar or minerals they give up, a sort of “negotiation” which involves an elaborate system of chemical signaling that is still far from fully understood.
Second, the fungi establish “Common Mycorrhizal Networks,” or CMNs, between different trees. Through these networks, the fungus acts as a courier for trees to send nutrients to one another and can give seedlings from host species an advantage by “subsidizing” them with sugars from the parent plant. While this is not an example of pure altruism (more here) – the fungus benefits by having more healthy trees that will support it – it does show that the relationship between plant and fungus can be quite complex.
Andrew Tomes is an MS student at SUNY-ESF in Syracuse whose work focuses on the mycorrhizal partners of the American chestnut. He earned his B.S. in Botany from the University of Maine, where he worked on wetland restoration. When not in the field, the lab, or the classroom, he can often be found in the kitchen baking bread.