The mystery of the mimic plant

More than a decade ago, a plant ecologist named Ernesto Gianoli went for a walk in the woods, where he came across something that defies explanation.

During his afternoon jaunt in the rainforest of southern Chile, he found a familiar shrub known as arrayán. Its leaves were small, lip-shaped, and apiculate — meaning they came to a point at the end. That’s typical.

But when Gianoli took a closer look, he noticed something peculiar: Not all of those arrayán-shaped leaves came from the arrayán plant. Some of them were attached to a different and much thinner stem. As he traced it to the forest floor, he noticed this other plant was a species of vine called Boquila trifoliolata.

This took Gianoli by surprise. Commonly, the leaves of B. trifoliolata are stubby with three blunt lobes, but here, they looked just like those of the arrayán plant. It was as if B. trifoliolata was trying to camouflage itself in the foliage.

He walked around the forest looking for other B. trifoliolata plants, thinking this could be a fluke. Remarkably, roughly half of the other vines he encountered that afternoon had leaves that looked like other plants — not just other plants but the very plants growing next to them.

What Gianoli discovered that day is a remarkable ability that is perhaps unique to B. trifoliolata: As the vine grows, it can mimic other nearby plants. “My mind was blown,” said Gianoli, a researcher at Chile’s Universidad de La Serena. “It was kind of a magical moment.”

Since then, Gianoli’s research has shown that B. trifoliolata, native to southern South America, can mimic the leaf shape, size, and even color of more than a dozen plants. More incredible still is that two different parts of the same individual can mimic the leaves of two distinct plants, even if they look dramatically different, he says. (Although there are few studies on this plant, scientists generally acknowledge that it can shape-shift.)

Scientists have yet to figure out why, exactly, the vine mimics other plants, though it may give them some protection against herbivores like snails and beetles (assuming B. trifoliolata mimics less appetizing plants). The more exciting question, however, is how they do it. Plants have no brains or eyes. So how do they sense the shape of leaves around them and then copy it?

Searching for answers has intensified a fiery debate in the plant world. On one side are mainstream botanists, whose work is rooted in rigorous, repeatable studies, and on the other is a small group of researchers who believe plants share a number of attributes with animals, including humans. To the latter group, B. trifoliolata supports the idea that plants possess a form of vision and perhaps even a brain-like structure to process it.

No matter the mechanism, this one woody vine — and its mimicry schtick — is pushing the boundary of what we know about plants.

Plants are not animals; they’re stuck in place. And that’s one reason some people perceive them as a bit dull, relative to other life on Earth.

But it’s actually the fact that plants are fixed that makes them so interesting. They can’t run from predators or chase after prey, so they’ve evolved all kinds of clever tactics to survive. “Plants are way more complicated than people give them credit for,” said Elizabeth Haswell, a professor and plant biologist at Washington University in St. Louis.

When some plants are attacked by caterpillars or lawnmowers, for example, they release a suite of chemical compounds that can repel pests or even attract insect-killing wasps. In fact, much of the appealing scent of freshly cut grass is made up of those chemicals; you’re literally smelling the grass’s defense system in action. (Don’t worry, most scientists agree that grass doesn’t feel pain.)

Plants also have remarkable ways of finding food. The telegraph plant, for example, will adjust its leaves so fast to find sunlight that you can see it move in real time. And there are several hundred carnivorous plants, including sundews and Venus flytraps, which have elaborate mechanisms for capturing and digesting insects. (Flytraps can apparently count; they only close on prey that bump trigger hairs inside them within about 20 seconds of each other.)

All of these behaviors help plants survive life immobile and, uh, fly in the face of any notion that plants are insipid.

Mimicry is another one, and it crops up all over the plant world. A kind of orchid, for example, convincingly mimics the scent and shape of a female thynnid wasp. As the wasp attempts to mate with the flower, it gets covered in pollen, helping the plant reproduce with other orchids far away.

No mimicry, however, is quite like that of Boquila trifoliolata, as far as scientists understand. It’s one thing to evolve to look like another organism, like the orchid. It’s another to evolve the ability to mimic several different organisms, changing as you grow.

When Gianoli first published his discovery, back in 2014, he could only guess how the vine does its thing. He suggested that Boquila trifoliolata might be picking up chemicals in the air emitted by nearby plants or even obtaining some genetic material from the plants themselves that contain information about leaf shape and size.

Earlier this year, however, a study in the journal Plant Signaling & Behavior presented an alternative, eyebrow-raising theory: B. trifoliolata uses a primitive form of vision to mimic other plants. The plant can, in a sense, see, the authors claimed.

Scientists have long known that plants have photoreceptors and can detect the presence of light, often in highly sophisticated ways. They can, for example, sense the color and direction of a beam, according to Simon Gilroy, a professor of botany at the University of Wisconsin Madison. That’s what the telegraph plant is doing when it swivels its leaves toward the light.

But the new study, led by Felipe Yamashita, a doctoral student at the University of Bonn, takes this a giant step further. It suggests that B. trifoliolata uses lens-like cells in its leaves, or “ocelli,” to detect the shape and other attributes of nearby plants. Then, it somehow processes that information and uses it to form new leaves in their image.

Yamashita and his co-author, Jacob White (who’s not a scientist but raises plants at home), tried to test this by growing the mimic vine next to plastic plants. Their thinking was that if B. trifoliolata uses a form of vision for its mimicry, it should be able to copy any old plastic plant. And sure enough, it could, according to the study. “Leaves of B. trifoliolata mimicked leaves of the artificial plant,” it notes plainly.

There’s just one problem with that conclusion: Mainstream plant biologists don’t buy it.

Several scientists told Vox that there are significant issues with the study design. The authors didn’t adequately control variables that can influence leaf shape, such as the age of the leaves, for example, they said. Mainstream researchers also challenge the underlying theory of plant vision. Certain plant cells may be able to act like lenses that can focus light, but they likely can’t create any sort of detailed pictures.

“I am very skeptical of this work, to say the least,” said Lincoln Taiz, a professor emeritus at the University of California Santa Cruz and co-editor of the textbook Plant Physiology and Development. “The idea that the ‘ocelli’ behave like little eyes that can construct an image of a leaf that would enable a plant to mimic that leaf is far-fetched.” (Yamashita said he’s used to criticism because his work challenges mainstream science. He added that his recent study was just one experiment and plant vision remains only a theory that needs more support.)

Taiz and other mainstream scientists also dispute the broader way of thinking about plant intelligence that vision fits into. A small group of researchers known as plant neurobiologists believe that plants have a form of consciousness and self-awareness. Some of them — including Yamashita’s academic adviser, Frantisek Baluska — have also proposed that plants have bundles of neuron-like cells in parts of their roots that function as a “brain-like command centre.” It’s there that plants could process visual information, Baluska told me.

To support this idea, Baluska, an editor-in-chief of the journal Plant Signaling & Behavior, where the new study was published, points to several features of plant cells that he says are similar to neurons. He claims, for example, that they use the same neurotransmitters for communication as animals like glutamate and GABA, and communicate with synapses. He also shared studies showing how plants are susceptible to anesthesia — Venus flytraps, for example, won’t close when they’re anesthetized — to suggest that certain plant cells may be similar to human neurons.

But according to Taiz, “it’s no surprise” to find these neurotransmitters in plants. Their evolutionary presence in life predates animals and nervous systems. There’s also no clear evidence of synapses in plant cells, according to a recent review he co-authored, which includes several other reasons why such a brain-like structure couldn’t exist.

(As for the effect of anesthetics: These chemicals can inhibit plant movement simply by disrupting the flow of ions and water in plants, Taiz said, so their effects don’t prove that plants have animal-like nervous systems. Baluska said researchers including Taiz have been “attacking” his theories for years now and pointed Vox to his responses to their critiques.)

A common thread in the theories of plant neurobiology is that plants are sentient beings that share certain features with higher-order organisms like humans. And from that perspective, it’s easy to imagine that plants might see their world the way that we do. But according to Taiz, “the vast majority of mainstream plant scientists do not give the work of ‘plant neurobiologists’ much credence.”

The idea of vines with eyes is fun, yet the more mainstream theory behind mimicry is arguably just as exciting. Gianoli and some other researchers believe that B. trifoliolata’s mimicry might be rooted in the rich communities of bacteria and other microbes that live in and around plants.

You can think of plants as mini ecosystems that harbor a wide diversity of microbial life, not unlike an old tree in a forest that’s home to monkeys and lizards and bugs. These microbes — which scientists know little about — play a large role in the life of a plant, affecting its ability to feed, grow, and fight pathogens. Remarkably, research suggests that microbes can even control which genes in a plant’s DNA are turned on, in some cases helping them survive in harsh conditions, such as salty soil.

Normally, Gianoli theorizes, those microbes mainly influence the life and genes of the plants they live on. It’s as though each plant and its bacterial community are a family that share a unique language that other plants can’t understand, he said. Boquila trifoliolata, however, may be able to speak multiple languages — in other words, the vine may be affected by the microbes that live on multiple other plants. Key to his theory, those microbes may control the appearance of leaves.

Put simply: This hypothesis does not imply that B. trifoliolata is “seeing” but rather suggests that microbes are helping to shape the plant’s leaves.

If that’s not strange enough, Gianoli says those microbes may also be transferring some of the plant’s own DNA to the mimic vine in the process, through a complicated mechanism known as horizontal gene transfer, or HGT. (HGT is a widespread and utterly fascinating phenomenon, whereby genes are passed between unrelated organisms. Some bacteria, for example, give plants genes that make them form galls, or tumors, that they use.)

This is only a theory, and a pretty fringe one — we’re talking about invisible, microbial forces affecting the shape of leaves, perhaps to help a plant hide. In a study published last year, however, Gianoli did find some evidence that microbes are involved in mimicry. The paper, published in the Nature journal Scientific Reports, revealed similarities between the communities of bacteria living inside the foliage of a tree and the leaves of B. trifoliolata that mimicked that tree.

“Our results suggest the involvement of bacterial agents in leaf mimicry,” Gianoli and his coauthors wrote in the paper. “Yet we are still far from proving the HGT hypothesis.”

So scientists have yet to figure out how plants mimic other plants. In the coming months, Gianoli will carry out new studies to pinpoint which genes are activated in B. trifoliolata’s genome when it mimics another plant, he said. Yamashita is also planning more experiments and is trying to propagate the plant in the lab. There’s nothing wrong with laying out big or far-fetched hypotheses, as long as you test them, said Haswell of Washington University in St. Louis. That’s what moves science forward.

What can be a problem, however, is to start with the idea that plants possess animal-like intelligence, according to Haswell and other researchers. While humans have a tendency to see ourselves in other organisms — whether they’re octopuses or trees — that perspective doesn’t get us closer to actually understanding those creatures or how they see the world. (Maybe trees aren’t actually talking underground.)

“We are scientists, and we have to admit when we’re wrong,” Gianoli said. “But we don’t have to be too anthropocentric or soul-centric, expecting that plants have to be like animals or like humans. They don’t have to have eyes because they don’t need eyes. They have other sensory systems.”

Those sensory systems are pretty incredible on their own. Boquila trifoliolata can literally shape-shift! Orchids can trick wasps into thinking they’re wasps! Grass can call in flying reinforcements! So perhaps plants aren’t neat because they’re like us, but because they’re so different.