Plants That Do Weird Things: Part 1 (Myco-heterotrophy)

Plants are a very diverse group of organisms, so as one could expect, there are a number of them that do "weird" things. There is no shortage of fascinating techniques that plants employ in order to survive in their environment.

Today, I want to look at some of the myco-heterotrophs. There are a number of these species worldwide, especially in more tropical regions, but I will be focusing on species found in Ontario.

"Myco-heterotroph" sure looks like a scary word, but once you break it down, it becomes much more digestible."Myco" comes from the Greek mýkēs, which means "fungus". "Heterotroph" means an organism that gets its nutrients from another organism, but can be further broken down into "hetero", meaning different, and "troph", coming from the Greek trophḗ, meaning "nutrition".

Putting it all together, you get an organism (in this case, a plant) that gets nutrients from parasitism upon a fungus rather than through the conventional method of photosynthesis. Recall that in essence, photosynthesis is the process in which a plant with chlorophyll (the pigment which makes plants green) makes its own energy using the sun's energy. Photosynthetic plants are for the most part autotrophs (the opposite of heterotroph, organisms that produce their own energy)

This is a somewhat complicated process, but in the simplest terms, it goes a little something like this:

First, to set the scene: A fungus colonizes the root tissue of a host plant, to which it has a symbiotic relationship (long-term interaction between two organisms). The host plant gives the fungi nutrients from photosynthesis (e.g., sugars), and in return, the fungi gives the host plant nutrients from the soil (e.g., phosphorus). This relationship is known as a mycorrhizal association. Mycorrhiza can be broken down into "Myco" (fungus) and "rhiza", Greek for "root". 

In most myco-heterophic relationships, the myco-heterotophs will capitalize off this mycorrhizal association (which would occur regardless of whether a myco-heterotroph is present or not). The myco-heterotroph's roots interact with the fungus' mycellium (the thread-like, vegetative part of the fungus, for further reading click here), and this is where the flow of nutrients takes place. Note that unlike a mycorrhizal association, there are no nutrients leaving the myco-heterotroph and going to the fungus—it is a true parasite.

So bottom line, the flow of nutrients is as follows:

Host autotrophic plant ↔ Mycorrhizal fungus → Myco-heterotroph 

In the past, myco-heterotrophs were mistakenly classified as saprotrophs. Saprotrophy is the process in which an organism breaks down organic material and turns it into usable nutrients ("sapro" means "rotten material"). It is now largely accepted that no vascular plants are saprotrophs, and that this is a process largely restricted to fungi. 

There are both partial and full myco-heterotrophs. Partial myco-heterotrophs (also called mixotrophs) are able both to utilize nutrients from mycorrhizal fungi, as well as complete photosynthetic processes. Full myco-heterotrophs are unable to complete photosynthesis due to a lack of chlorophyll, so are dependant on nutrients from fungi. It is important to keep in mind, however, that just because a plant lacks chlorophyll, does not automatically make it a myco-heterotroph! While I do not believe this is an Ontario example of this, it is possible for a species to have individuals that are both partial and full myco-heterotrophs.  

There are several examples of each type from Ontario. Perhaps the most well known is Ghost Pipes (Monotropa uniflora). A common misconception is that this is a fungus, but it is a plant! This is a full myco-heterotroph. This plant is well adapted to life in shady places, such as sugar maple forests (a whole post in itself), as it does not require sunlight, since it does not undergo photosynthesis. Ghost Pipes gets its name from his ghostly appearance, as a result of it not having chlorophyll. Ghost Pipes is parasitic of the fungi family Russulaceae. 

A similar plant is Pinesap (Monotropa hypopitys syn. Hypopitys monotropa). It is, in my experience, nowhere near as common as Ghost Pipes. I have only ever seen this plant in fruit, so I have used a photo off of Wikimedia. The main difference between this plant and Ghost Pipes is that it has multiple flowers on the same stem, whereas the latter only has one. Pinesap parasitizes the family Tricholomataceae, which may have something to do with its abundance, but that is something I will have to look into more. 

From Wikimedia

A somewhat surprising group of myco-heterotrophs are the coralroots, a type of orchid. Spotted Coralroot (Corallorhiza maculata) is a species found throughout Ontario, and like Ghost Pipes, it is a parasite of Russulaceae. There are two varieties found in Ontario, Western (var. occidentalis) and Eastern (var. maculata). The Western variety tends to flower several weeks earlier than the Eastern variety.

var. occidentalis f. flavida

var. maculata

There is an exception however. Early Coralroot (Corallorhiza trifida) is considered partially myco-heterotrophic, as it contains some chlorophyll and is able to produce its own nutrients through autotrophic processes. Early Coralroot's choice of fungus to parasitize is the genus Tomentella (Thelephoraceae). 

There are other examples of partial myco-heterotrophs as well, perhaps species that one would not immediately associate with such an unusual process! First, take a look at some of the taxonomy. The Ghost Pipes and Pinesap mentioned above are part of the heath family (Ericaceae). This contains many familiar plants, such as blueberries, cranberries, and laurels. These two species are part of the subfamily Monotropoideae, which are myco-heterotrophs. Within this subfamily, there are three tribes; Pyroleae, Montropeae, and Pterosporae. The latter two are full myco-heterotropic, whereas Pyroleae is partial myco-heterotrophic. Ghost Pipes and Pinesap, as you may have guessed, are part of Monotropeae. There is one species of Pterosporae in Ontario, the rare Woodland Pinedrops (Pterospora andromedea). (NOTE: some treatments list Pyroleae as its own subfamily, Pyroloideae)

Monotropoideae have very small seeds, known as "dust seeds". These dust seeds require a mycorrhizal fungi to germinate (to begin to grow), however in the case of plants in the tribe Pyroleae, they develop the ability to photosynthesize as they mature. 

There are four genera in Pyroleae, all of which are found in Ontario.

Pipsissewa (Chimaphila umbellata) is the more widespread member of its genus. Striped Wintergreen (C. maculata) is limited to only two or three areas of occurrence in Ontario. 

Pipsissewa

Pyrola is the largest genus in Pyroleae in Ontario. They are commonly referred to as "wintergreens" or "shinleaf". 

Shinleaf (P. elliptica) 

Moneses is represented by a single species, known as Single Delight or One-flowered Wintergreen (M. uniflora). 

And finally, Orthilia. One-sided Wintergreen (O. secunda) is the monotypic member of this genus. 

Myco-heterotrophy is just one of the many unique and crazy things that plants do to both survive and thrive. This is one of the several weird things that plants do that I hope to highlight in this series in an effort to shine some light on these incredible organisms!