#plant memory

By Sarah Laskow, Atlas Obscura, September 5, 2017

Monica Gagliano began to study plant behavior because she was tired of killing animals. Now an evolutionary ecologist at the University of Western Australia in Perth, when she was a student and postdoc, she had been offing her research subjects at the end of experiments, the standard protocol for many animals studies. If she was to work on plants, she could just sample a leaf or a piece of root. When she switched her professional allegiance to plants, though, she brought with her some ideas from the animal world and soon began exploring questions few plant specialists probe--the possibilities of plant behavior, learning, and memory.

“You start a project, and as you open up the box there are lots of other questions inside it, so then you follow the trail,” Gagliano says. “Sometimes if you track the trail, you end up in places like Pavlovian plants.”

In her first experiments with plant learning, Gagliano decided to test her new subjects the same way she would animals. She started with habituation, the simplest form of learning. If the plants encountered the same innocuous stimulus over and over again, would their response to it change?

At the center of the experiment was the plant Mimosa pudica, which has a dramatic response to unfamiliar mechanical stimuli: Its leaves fold closed, perhaps to scare away eager herbivores. Using a specially designed rail, Gagliano introduced her M. pudica to a new experience. She dropped them, as if they were on a thrill ride in an amusement park for plants. The mimosa plants reacted. Their leaves shut tight. But as Gagliano repeated the stimulus--seven sets of 60 drops each, all in one day--the plants’ response changed. Soon, when they were dropped, they didn’t react at all. It wasn’t that they were worn out: When she shook them, they still shut their leaves tight. It was as if they knew that being dropped was nothing to freak out about.

Three days later, Gagliano came back to the lab and tested the same plants again. Down they went, and … nothing. The plants were just as stoic as before.

This was a surprise. In studies of animals such as bees, a memory that sticks for 24 hours is considered long-term. Gagliano wasn’t expecting the plants to keep hold of the training days later. “Then I went back six days later, and did it again, thinking surely now they forgot,” she says. “Instead, they remembered, exactly as if they had just received the training.”

She waited a month and dropped them again. Their leaves stayed open. According to the rules that scientists routinely apply to animals, the mimosa plants had demonstrated that they could learn.

In the study of the plant kingdom, a slow revolution is underway. Scientists are beginning to understand that plants have abilities, previously unnoticed and unimagined, that we’ve only ever associated with animals. In their own ways, plants can see, smell, feel, hear, and know where they are in the world. One recent study found that clusters of cells in plant embryos act a lot like brain cells and help the embryo to decide when to start growing.

Of the possible plant talents that have gone under-recognized, memory is one of the most intriguing. Some plants live their whole lives in one season, while others grow for hundreds of years. Either way, it has not been obvious to us that any of them hold on to past events in ways that change how they react to new challenges. But biologists have shown that certain plants in certain situations can store information about their experiences and use that information to guide how they grow, develop, or behave. Functionally, at least, they appear to be creating memories.

When most people look at a plant, it’s hard to imagine that it’s waiting for anything. Plants don’t seem to have long-term plans. If they lack water, they droop. If it rains, they perk up. If they sense sunlight, they grow toward it. To our human way of thinking, it doesn’t seem like plants are doing very much at all. But we don’t recognize memories in people or dogs just by looking at them, but rather by their behavior. The dog comes when called by name; the person smiles in recognition. For the mimosa plants, something from the past changed how they reacted in the future--even if we don’t notice or understand why.

Scientists first started talking about “plant memory” explicitly in the 1980s. A team in France, for example, happened upon a type of memory in which a plant recalled a history of damage to a leaf on one side of its stem and therefore dedicated its energy to growing in the other direction. Since then, scientists have found that certain plants can remember experiences of drought and dehydration, cold and heat, excess light, acidic soil, exposure to short-wave radiation, and a simulation of insects eating their leaves. Faced with the same stress again, the plants modify their responses. They might retain more water, become more sensitive to light, or improve their tolerance to salt or cold. In some cases, these memories are even passed down to the next generation. We now know that plants are capable of much more than they’re given credit for. They can “hear” vibrations, which might help them recognize insect attacks. They share information by broadcasting chemicals through the air or from their roots.

In a famous 2013 New Yorker article by Michael Pollan, The Intelligent Plant, Gagliano was introduced to readers as someone whose experiments are extending the concept of cognition to the plant world.

The problem she is addressing is that if plants have brain-like functions and make sentient-like decisions, our existing perception of nature and ourselves must change.

These implications need further analysis. But, first: the experiments. What Gagliano did with her Mimosa pudica plants – also called 'sensitive' plants – was to custom-build an apparatus whereby the plants could be suddenly dropped a foot or so on a regular basis.

Initially, on dropping, the plant retracted and curled its leaves, but after repeats, it stopped reacting. Not only did it appear to "learn" a behaviour (without a brain, mind you) but it also remembered.

How does this work? According to Gagliano:

"Plants may lack brains and neural tissues but they do possess a sophisticated calcium-based signaling network in their cells similar to animals’ memory processes."

Gagliano has published her findings and edited various scholarly books on plant research, ethical implications and changed perceptions. She has collaborated with environmental lawyer Alessandro Pelizzon and others on the language problems of writing about plant life.

There is no vocabulary that can be used to talk about brain-like plant structures beyond mere vascular and survival processes, nor about decision-making, sentience, intelligence, learning and memory in the plant world.

There is much more work to be done by artists and humanists to develop these vocabularies together.

The seventies classic The Secret Lives of Plants claimed carrots screamed when picked. The “new botany” is making similar claims about plant intelligence. Are those claims just as whacky?

No! [Laughs] I think that seventies generation believed plants were not only intelligent but conscious, which is a dramatically different thing. What the new botany is suggesting is that plants are sensitive and problem-solving but bypass the need for self-consciousness and brain activity that we assume is necessary for intelligence. People who think this are often accused of being anthropocentric, believing that plants are behaving like humans.

The philosopher Daniel Dennett marvelously riposted that critics of this theory are "cerebrocentric," believing intelligent behavior is not possible without the infinitely superior human brain. What the new work shows is that plants, by means we do not yet fully understand, are capable of behaving like intelligent beings. They are capable of storing — and learning from — memories of what happens to them.

Tell us about Monica Gagliano’s experiments with mimosa pudica, aka the "sensitive plant."

Monica Gagliano is a very gifted and off-the-wall plant physiologist, or "plant neurobiologist," as she likes to call herself. She did this famous experiment where she potted up a number of mimosa plants, long known as the "sensitive plant." As far back as the 18th century, it was known to react to any kind of touch or threat by curling up its leaves, in sequence, up the stems.

What Gagliano did was simulate the touch action by dropping these potted mimosas a fixed distance to the ground, so they received a mild physical shock. To start with, all of them closed their leaves in the proscribed fashion. On the second and third drop, rather less did. And by the end of a large number of drops, none of them were closing up.  

Conventional botanists who saw the experiment said, “They’re just tired!” [Laughs] But she repeated the experiment with the same plants a week, and then a month, later. They all responded in the same way: They didn’t react to being dropped by folding up their leaves. But when they were simulated in the conventional way, like being grasped by a hand, they all immediately closed up.

By comparison, bees can only retain memories of places to find honey for three days. But the mimosa plants appeared to be able to "remember" the difference between an apparent and a real threat, and retained this discrimination in their memory.

We all know about the World Wide Web. But you describe something called the “wood wide web.” Tell us about it.

It’s long been known that the trees in a forest are connected by mycorrhizal fungi. This means fungi that live symbiotically with the roots of forest trees. The forest trees can’t grow without them because they haven’t got enough access to the minerals in the soil, and the fungi can’t grow without the trees because they have no chlorophyll and therefore can’t manufacture sugars. It's a beautiful symbiosis in which all trees are involved.

What has now begun to be discovered is that the mycorrhizal fungi don’t surround just one tree. By using radioactive trace elements, the researchers were able to show that nutrients were being passed by the fungi between different species of trees over a large area. The trees that were not so good in the winter, like aspens, were being given food manufactured by the conifers, which do much better in winter, and vice versa. So the fungus is actually helping to distribute food amongst forest trees so that they all benefit at the right time.

Messages about predators are also being sent. If a tree is attacked by insects, pheromonic chemicals are distributed through the mycorrhizal fibers beneath the soil, as well as being blown through the air by the trees, to warn other trees that an insect attack is imminent and to prepare themselves by producing more tannin in their leaves. [Laughs] It’s quite a chatty system.

Plants can teach us a thing or two about dealing with the ups and downs of life. They may have evolved the ability to forget stressful situations, as a way of dealing with highly unpredictable environments.

Some plants have “long-term memory”. For instance, Arrhenatherum elatius, a perennial grass species common in Europe, seems to remember drought and is better able to defend against damage from excessive sunlight than plants that haven’t been through an earlier drought.

Such experience helps prime plants to produce the necessary proteins and chemicals at short notice should stressful conditions recur.

Plants can preserve such memories across generations, at times via epigenetic mechanisms, which influence whether or not genes are expressed.

Better to forget?

But when Peter Crisp at the Australian National University in Canberra and his colleagues scoured the literature for examples of such memory of stressful events, they found that memory is more the exception rather than rule. “Generally plants are good at forgetting,” says Crisp.

The team argues that plants are making a trade-off. While being epigenetically primed against previously experienced stress can be beneficial, it also comes with costs.

“You could have an organism that’s spending way too much energy transcribing genes that really aren’t necessary at a specific time,” says team member Steven Eichten.

What’s more, such memory can be bad for future generations. For example, drought-stressed Polygonum hydropiper, a knotweed, passes on its stress response to seedlings, which become smaller, with slower-growing roots – even if they are grown in a drought-free environment.

Competing processes

Crisp and his team say that whether a plant forms a memory depends on what happens after a stressful experience. During this “recovery phase”, the plant can either consolidate its stress response and remain genetically primed, or reset itself to its prior state.

For a new memory to be formed, a plant has to make a protein that will affect its own DNA, affecting future behaviour.

This memory formation has to contend with a process called RNA decay. In cells, double-stranded DNA is transcribed into single-stranded RNA, before being translated into proteins.

RNA decay regulates the amount of RNA molecules that can be turned into proteins, and it can disrupt the RNA molecules related to the stress response, thus preventing memory formation, say Crisp and his colleagues.

Plants, it seems, would rather forget than hold grudges. “I like this idea very much,” says Frantisek Baluska, who studies plant intelligence and behaviour at the University of Bonn, Germany.

By Joanna Klein, NY Times, March 28, 2016

Plants are reviving after a long winter, helped along by warming temperatures and increased light.

But do plants also “remember” what to do? Maybe so. In 2014, Dr. Monica Gagliano and colleagues at the University of Florence in Italy decided to see if they could train a plant to change behavior.

The researchers chose Mimosa pudica, more commonly known as the touch-me-not, which curls up its leaves in response to physical stimulation. Test plants in their pots were dropped onto foam from a height of about six inches to elicit the flinching response.

After repeated exposure with no major harm, the plants no longer recoiled. Even after a month left alone, the plants “remembered” the falls weren’t harmful and ignored them. Dr. Gagliano, now at the University of Western Australia, concluded from the experiment that plants could “learn” long-lasting behaviors, sort of like memories.