Bearcorn: A Mysterious Parasite from Eastern North America

June 14, 2021

Bearcorn (Conopholis americana) is one of those plants that really challenges mainstream assumptions of what a plant should look like. It produces no leaves, no chlorophyll, and all you ever see of it are its strange reproductive structures. One can easily be forgiven for thinking they had encountered some type of fungus.

Bearcorn is an obligate parasite on oak trees. It simply can’t exist without access to oak roots. From what I have been able to gather, the preferred hosts of bearcorn are the red oaks (section Lobatae). That is not to say the exceptions have not been documented. At least one author claims to have found bearcorn attached to the roots of a white oak (Quercus alba) and even earlier work suggests that American chestnut (Castanea dentata) may have served as an occasional host as well. Regardless, if you want to find bearcorn in the woods, you would do well to search out red oaks first.

According to those who have run germination trials, bearcorn seeds must be in close proximity to oak roots in order to germinate. Some sources say that direct contact is needed whereas others claim that seeds have to be close enough to detect root presence. It is likely that some sort of chemical cue is what initiates the process and this makes sense. For a plant that relies completely on another plant for its water and nutritional needs, it doesn’t make much sense for bearcorn seeds to germinate anywhere but near oak roots.

Upon germinating, the tiny seedling needs to act fast before its meager energy reserves are exhausted. If lucky, the growing seedling will come into contact with an oak root and begin developing a strange organ referred to as the nodule or tubercle. Thus begins its parasitic lifestyle. The tubercle continues to grow throughout the life of the plant, developing into an amorphous, woody blob that continues to envelope more and more oak roots. Its within the tubercle that all of the parasitism takes place.

Cells within the bearcorn tubercle penetrate into the vascular tissues of the oak root, stealing all the water and nutrients the plant will ever need. Over time, the bearcorn tubercle coaxes the roots of the oak to fan outward like the crown of a tiny tree. In doing so, bearcorn is effectively increasing the amount of surface area available to make more parasitic connections. Apparently this all comes at great cost to the oak roots. Over time, oak root size within the tubercle greatly diminishes until some completely perish. Considering the size of some bearcorn populations, one could expect the oak host to fight back.

Indeed, it would appear that oaks are not helpless against bearcorn infestations. Examination of the cells within bearcorn tubercles revealed that as the parasite grows, the oak will begin flooding the infected cells with tannin-rich compounds. Apparently this serves to slow the flow of water and nutrients into the tubercle. There is even evidence that some of those tannins are transferred into the bearcorn tubercle, leading some to suggest that the oak is literally poisoning its bearcorn parasites, albeit slowly.

There is a strong possibility that such oak defenses lend to the relatively short lifespan of bearcorn plants. In at least one study I read, no bearcorn individuals over 13 years of age were found and the average age is estimate to be about 10 years. Perhaps just over a decade is about all a bearcorn can hope for once the its oak host begins to fight back. Good thing bearcorn populations can be surprisingly fecund.

Bearcorn plants reach reproductive maturity at after about 3 years of growth. They flower in the spring and that is when they are at their most obvious. Numerous thick, finger-like stems emerge from the ground covered in whirls of cream-colored, tubular flowers. Though a dense population of flowering bearcorn may look like a bonanza for pollinators, they don’t seem to attract a lot of attention. From what I was able to find, bumblebees are pretty much the only insects to visit the flowers, and even then, visitation rates are low. Apparently bearcorn flowers do not produce any detectable scent nor are they full of nectar. I guess the only real reward is a meager helping of pollen.

Photo by Joshua Mayer licensed under CC BY-SA 2.0

No matter, bearcorn has a nice reproductive trick to ensure plenty of seeds are produced each year - it selfs. The anatomy of the flowers is such that, at maturity, the anthers are in direct contact with the stigma. Even if nothing visits a bloom, it will still go on to clone itself year after year. Once fertilized, each flower gives way to a large fruit chock full of seed. This is where the corn part of the name bearcorn comes from. A stem thick with fruits does resemble a strange, albeit juicy ear of corn sitting on the forest floor. The bear part of the name likely has to do with the fact that bear readily consume bearcorn fruits, stem and all. Working in the southern Appalachian Mountains, I can’t tell you how many times I came across bear scat absolutely loaded with bearcorn fruits and seeds. It’s not just bear either, deer are also very interested in bearcorn fruits.

Lucky for bearcorn, its seeds pass through the guts of these animals unharmed. Hopefully, with a bit of luck, at least one of these animals will make a deposit in an oak-rich region of the forest. With even more luck, some of those seeds might even find themselves nestled in near an oak root to begin the process anew.

Further Reading: [1] [2] [3]

When Trillium Flowers Go Green

June 1, 2021

The first time I encountered a white trillium (Trillium grandiflorum) with green stripes on its flowers, I thought I had found a new variant. I excitedly took a bunch of pictures and, upon returning home, shared them among friends. It didn’t take long for someone far more informed than me to point out that this was not a new variant of this beloved plant. What I had found was signs of an infection.

The green stripes on the petals are the result of a very specific bacterial infection. The bacteria responsible belongs to a group of bacterial parasites collectively referred to as phytoplasmas. Phytoplasmas are not unique to trillium. In fact, these bacteria can be found around the world and infect many different kinds of plants from coconuts to sugarcane. Indeed, most of the research on phytoplasmas is motivated by their impacts on agriculture. Despite the damage they can cause, their natural history is absolutely fascinating.

Phytoplasmas are obligate parasites. They can only live long-term inside the phloem of their preferred host plants. Once inside the plant, phytoplasmas begin tinkering with cell expression, causing an array of different symptoms that (to the best of my knowledge) depend on their botanical host. In the case of trillium, phytoplasma infection causes a change in the flower petals. By altering gene expression, petal cells becoming increasingly leaf-like, resulting in the green striping I had observed. That isn’t all the phytoplasma does either. Infections usually result in complete sterilization of the flower. I have even heard some reports that the infected plants are also weakened to the point that they eventually die.

Why the phytoplasma do this has to do with their bizarre life cycle. Now, to be fair, much of what I have been able to gather on the subject comes from research done on other plant species. Still, there are enough commonalities among phytoplasma infections that I strongly suspect they apply to the trillium system as well. Nevertheless, take what I am saying here with a grain of salt.

As mentioned, phytoplasma can only exist long-term within the phloem of their plant host. They don’t produce any sort of fruiting bodies, nor are they transferred by air or contact with tissues. This creates a bit of an issue when it comes to finding new hosts, especially if infection inevitably results in the death of the plant. This is the point in which a vector must enter the picture.

The vector in question in many cases are sap-feeding insects like leafhoppers. Leafhoppers use their needle-like proboscis to pierce the phloem and suck out sap. It’s this feed behavior that phytoplasma capitalize on to complete their lifecycle. Moreover, the phytoplasma don’t do so passively. Just as the phytoplasma alter the gene expression in the petal cells, they can also alter the expression of genes involved in plant defenses.

Research on infected Arabidopsis plants has shown that phytoplasma cause the plant to decrease production of a hormone called jasmonate. This is fascinating because jasmonate is involved in defending plants against herbivory. It was found that when plants produced less jesmonate, leafhoppers were 30%-60% more likely to lay eggs on those plants. Essentially, the phytoplasma are reducing the plants’ defenses in such a way that there is a greater chance that they will be fed on by a greater number of sap-suckers.

As leafhoppers feed on the sap of infected plants, they inevitably suck up plenty of phytoplasma in the process. Through a complex series of events, the ingested phytoplasma eventually make their way into the salivary glands of the leafhopper. Then, as the leafhopper moves from plant to plant, piercing the phloem to feed, it inevitably transfers some of the phytoplasma in its saliva into a new host, thus completing the lifecycle of these plant parasites.

To bring it back to those green stripes on the trillium flowers, I suspect that by altering the petal cells to look more like leaves, the phytoplasma may be “encouraging” leafhoppers to concentrate their feeding on infected tissues. However, this is purely speculation on my part. The lack of data outside the agricultural realm represents an important scientific void that needs filling.

Further Reading: [1] [2] [3] [4] [5] [6]

How a Giant Parasitic Orchid Makes a Living

February 13, 2018

Photo by mutolisp licensed under CC BY-NC-SA 2.0

Imagine a giant vine with no leaves and no chlorophyll scrambling over decaying wood and branches of a warm tropical forest. As remarkable as that may seem, that is exactly what Erythrorchis altissima is. With stems that can grow to upwards of 10 meters in length, this bizarre orchid from tropical Asia is the largest mycoheterotrophic plant known to science.

Mycoheterotrophs are plants that obtain all of their energy needs by parasitizing fungi. As you can probably imagine, this is an extremely indirect way for a plant to make a living. In most instances, this means the parasitic plants are stealing nutrients from the fungi that were obtained via a partnership with photosynthetic plants in the area. In other words, mycoheterotrophic plants are indirectly stealing from photosynthetic plants.

In the case of E. altissima, this begs the question of where does all of the carbon needed to build a surprising amount of plant come from? Is it parasitizing the mycorrhizal network associated with its photosynthetic neighbors or is it up to something else? These are exactly the sorts of questions a team from Saga University in Japan wanted to answer.

All orchids require fungal partners for germination and survival. That is one of the main reasons why orchids can be so finicky about where they will grow. Without the fungi, especially in the early years of growth, you simply don't have orchids. The first step in figuring out how this massive parasitic orchid makes its living was to identify what types of fungi it partners with. To do this, the team took root samples and isolated the fungi living within.

By looking at their DNA, the team was able to identify 37 unique fungal taxa associated with this species. Most surprising was that a majority of those fungi were not considered mycorrhizal (though at least one mycorrhizal species was identified). Instead, the vast majority of the fungi associated with with this orchid are involved in wood decay.

Stems climbing on fallen dead wood (a) or on standing living trees (b). A thick and densely branched root clump (c) and thin and elongate roots (d) [Source]

To ensure that these wood decay fungi weren't simply partnering with adult plants, the team decided to test whether or not the wood decay fungi were able to induce germination of E. altissima seeds. In vitro germination trials revealed that not only do these fungi induce seed germination in this orchid, they also fuel the early growth stages of the plant. Further tests also revealed that all of the carbon and nitrogen needs of E. altissima are met by these wood decay fungi.

These results are amazing. It shows that the largest mycoheterotrophic plant we know of lives entirely off of a generalized group of fungi responsible for the breakdown of wood. By parasitizing these fungi, the orchid has gained access to one of the largest pools of carbon (and other nutrients) without having to give anything back in return. It is no wonder then that this orchid is able to reach such epic proportions without having to do any photosynthesizing of its own. What an incredible world we live in!

Photo Credits: [1] [2]