Answer: Mimicry in plants?

It's a simple question... 

The question this week was pretty straightforward:  

And you probably also know about some insects that mimic plants: 

 

Leaf insect. P/C Wikipedia.

Mimicry is a fairly common trick in the world of living things--mussels mimic fish, flies mimic spiders, fish mimic their environment... it continues: walking stick insects mimic sticks, mussels can mimic small fish, and Viceroy butterflies mimic Monarch butterflies. This is all well known. But..

1. Can a plant mimic another plant?  Can you find an example of one plant that does this? 

Somewhat unexpectedly, the simple query: 

     [ plant that mimics another plant ] 

teaches us that Boquila trifoliolata, a shrub common in the rain forests of Chile (and much of South America) can somehow mimic the leaves of the plant that it grows on.   It's also called the pilpil, producing edible fruit and stems that can be used for making rope.  

P/C Wikimedia image of B. trifolioata vine mimicking the leaves of the host plant

But knowing THAT leads immediately to a much harder question:  

2. How does the mimicking plant come to be a mimic?  What’s the mechanism by which Plant A comes to look like Plant B?  

This is a bit of a mystery. The pilpil is the only plant species reported to engage not JUST in mimickery, by also in in mimetic polymorphism. That is, the ability to mimic multiple host species simultaneously. This obviously doesn't happen in animals--each animal mimics only one other animal at a time. But somehow, the pilpil manages to mimic multiple species at once.

As Wikipedia tells us, this is a form of Batesian mimicry, when a harmless species mimics a harmful one to ward off predators.

But how does it do the mimicking?

There are a lot of hypotheses about the mechanism include (e.g., microbially mediated horizontal gene transfer, volatile organic compound sensing, and the use of eye-like structures), but nothing seems to have panned out.

On the other hand, looking for:

[ leaf variation on single plant ]

leads us to learn about two concepts new to me: heteroblasty and heterophylly.

Heteroblasty is a significant and abrupt change in form and function, that occurs over the lifespan of certain plants. Like the pilpil changing leaf shape to match the host plant.

Heterophylly is when a plant has multiple leaf shapes on a single plant due to its environment.

As an example, Sassafras (Sassafras albidum) is well-known for having three distinct leaf shapes on the same plant - oval (unlobed), mitten-shaped (two-lobed), and three-lobed all on the same plant. It's the best known example of heterophylly.

More relevant to our discussion, holly leaves (Ilex aquifolium) can also make different types of leaves at the same time, even on the same branch--some with prickles, others without.

P/C after Herrera 

But the mechanism of holly heterophylly is pretty well understood. The prickly variations are a result of deer eating the leaves of the plant. When the leaves are damaged (say, by a passing hungry deer), methylation of the DNA in the leaves happens as a result of tissue damage. (Side note: methylation is the process of adding methyl groups onto pieces of large molecules, like DNA, to modify their behavior. This is the way much of epigenetics works. When an animal chomps on a leaf, methylation happens.)

By comparing the DNA of prickly leaves vs. smooth leaves, it turns out the prickly ones were significantly less methylated than prickless leaves, suggesting that methylation changes are ultimately responsible for leaf shape changes. More methylation = more prickly leaves. What's more, the methylation has an effect on nearby leaves. Other holly leaves nearby will also develop the prickles, with the effect diminishing with distance.

While the variation in leaf shape has been known for a while, it’s now clear that changes in leaf type are associated with differences in DNA methylation patterns, that is, epigenetic changes do not depend on changes in the sequence of DNA, but result from trauma to the plant.

What does this mean for our friend the shape-shifting pilpil? It demonstrates that changes to leaf shape can be epigenetic (that is, the plant doesn't have to modify its DNA, but just tack on a few extra methyl groups here and there).

That doesn't fully explain the way that pilpil leaves can mimic the host plant, but it does suggest a mechanism for changing the leaf structure.

Another intriguing hypothesis is that there is some kind of "visual sensing" that's going on with the vine. What makes this idea particularly interesting is that Boquila can mimic different hosts on the same vine without direct contact with the model leaves, suggesting some form of distance sensing. If the vine is truly using visual cues, it would be amazing--and completely novel. The big problem here is that nobody seems to be able to (pardon the pun) see any such organs!

A more probable hypothesis is that there is some kind of individual plant recognition, perhaps by sensing the release of nearby volatile organic compounds from recognized plants.

Kudu grazing on an acacia tree, causing the tree to put out a cloud of ethylene, telling other
nearby acacias that the browsers are here--increase your tannin load.  

Acacia trees, for instance, can detect ethylene emissions from neighboring damaged trees, triggering increased tannin production in the leaves of the acacia tree as defense against grazing kudu. [Heil, 2010] Other plants do similar things: Arabidopsis thaliana (a small plant in the mustard family) can also detect volatile compounds like methyl jasmonate from neighboring plants that are injured, which triggers its own defensive responses.

So it's not much of a leap to imagine that as a Boquila trifoliolata vine grows from tree to tree, each part of the plant might sense a different host that it's growing on, and invoke different responses--on each different part of the vine--depending on what chemical signals that part of the vine senses. It's also true that mimetic changes appear to be very localized, primarily affecting the leaves within 60 centimeters of the host plant. That's perfect chemical sensing range.

This effect would be mimetic polymorphism at a very fine level of detail.

To broaden my search I asked Claude for:

[ any plant that grows differentially depending on the chemical signals it senses]

I learned about the Centaurea maculosa (spotted knapweed) that detects specific root compounds from competing plants, responding by increasing production of allelopathic compounds (deadly poisons for the competition), essentially tailoring its chemical warfare based on which neighbor it detects.

Obviously, I did a Google Scholar search to verify that claim, and found a wonderfully detailed paper [Kong, et al, 2024] that goes into great detail about how the spotted knapweed senses the competition and then emits specific poisons to kill off the competition!

Just as obviously, I don't know if this hypothesis is correct--we need a good field botanist to do some studies, but it's not crazy.  All of the mechanisms are there and could be the product of evolution.  (And is very similar to the mechanism proposed by [Gianoli, 2014].)  

It's remarkable what you can learn (and hypothesize about) with some desk research!   

SearchResearch Lessons 

1. As with most complex searches, you have to learn as you go.  Note the new terms we had to learn to answer this question (mimetic polymorphism , methylation, heteroblasty, heterophylly).  Learn as you go in order to get more deeply into the topic.  

2. Interleaving "regular search" with LLMs (e.g., Claude, Perplexity, ChatGPT, etc.) can be really useful.  I was able to learn new terms and concepts by working with the AIs.  As always, be sure to CHECK their work. It's like reading an unreliable narrator in a novel--they're useful, but can't be trusted.  

 

Keep searching!

----

Citations:

Gianoli, E., & Carrasco-Urra, F. (2014). Leaf mimicry in a climbing plant protects against herbivory. Curr Biol, 24(9), 984-987.

Heil, Martin, and Richard Karban. "Explaining evolution of plant communication by airborne signals." Trends in ecology & evolution 25.3 (2010): 137-144.

Herrera, C. M., & Bazaga, P. (2013). Epigenetic correlates of plant phenotypic plasticity: DNA methylation differs between prickly and nonprickly leaves in heterophyllous Ilex aquifolium (Aquifoliaceae) trees. Botanical Journal of the Linnean Society, 171(3), 441-452.

Kong, C. H., Li, Z., Li, F. L., Xia, X. X., & Wang, P. (2024). Chemically mediated plant–plant interactions: Allelopathy and allelobiosis. Plants, 13(5), 626.