According to Jack Schultz, plants are "just very slow animals." And the fault is not at all a lack of understanding of fundamental biology. Schultz is a professor in the Department of Plant Science at the University of Missouri at Columbia. He spent forty years researching the interactions between plants and insects. He knows his stuff. Instead, he draws attention to the general ideas about our hardwood brothers, which, in his opinion, we consider almost furniture. Plants fight for territory, look for food, elude predators and trap prey. They are alive, like any animal, and - like animals - they exhibit special behavior.
“To be convinced of this, you just have to shoot a growing plant in quick motion - then it will behave like an animal,” enthuses Olivier Hamant, who studies plants at the University of Lyon in France. Indeed, fast motion captures the amazing world of plant behavior in all its glory.
Plants do not move aimlessly at all, which means that they must be aware of what is happening around them. “Plants also need sophisticated sensing devices tuned to changing conditions to respond properly,” says Schultz.
What are sunflowers silent about?
What do plants feel? Daniel Hamowitz of Tel Aviv University in Israel believes their feelings are not so different from our own. When Hamowitz decided to write What a Plant Knows in 2012 - in which he explored the experiences of plants as reflected in the most rigorous and modern scientific research - he was in some degree in awe.
“I was very worried about the response,” he says.
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And his concern was not unreasonable. Descriptions of how plants see, smell, feel, and, indeed, know, echoed in The Secret Life of Plants, which came out in 1973 for the Flower Age generation, but contained very little evidence. In particular, this book completely discredited the idea that plants respond positively to the sounds of classical music.
But plant perception research has come a long way since the 1970s, and there has been a surge in plant sensation research in recent years. The goal of this work is not only to demonstrate that “plants have feelings too”, but also to ask the questions “why” and “how” the plant feels its environment.
Schultz's Missouri colleagues Heidi Appel and Rex Cockcroft are seeking the truth about plant hearing.
“The main contribution of our work is to find the reason why sound affects plants,” says Appel. Beethoven's symphony is unlikely to attract the attention of a plant, but the approach of a hungry caterpillar is another story.
In their experiments, Appel and Cockcroft found that recordings of the chewing sounds produced by caterpillars caused the plants to flood their leaves with chemical defenses designed to fend off attackers. “We have shown that plants respond to ecologically relevant sound with an ecologically relevant response,” says Cockcroft.
Environmental relevance, or fit, is very important. Consuelo de Mores of the Swiss Federal Institute of Technology in Zurich and colleagues have shown that, in addition to the ability to hear approaching insects, some plants can smell them or even the smell of flying signals emitted by nearby plants in response to approaching insects.
Even more ominous is the 2006 demonstration that a parasitic plant - the vine dodder - sniffs out a potential host. Then the dodder wriggles in the air, entwines the unlucky owner and sucks out nutrients from him.
It would seem, how do these actions differ from ours? Plants hear or smell something and then act accordingly, as we do.
But, of course, there is a significant difference. “We don't know how similar the mechanisms of odor perception are in plants and animals, because we don't know much about the mechanisms in plants,” says de Mores.
We have noses and ears. What do plants have?
The lack of clear centers for sensory input makes it difficult to understand plant senses. This is not the case - the photoreceptors that plants use to "see" are fairly well understood - but the field as a whole certainly warrants further study.
For their part, Appel and Cockcroft hope to find the part or parts of the plant that respond to sound. Mechanoreceptor proteins that are found in all plant cells are likely candidates. They convert the microstrains generated by sound waves into electrical or chemical signals.
Scientists are trying to understand if plants with defective mechanoreceptors can still respond to noise from insects. Plants seem to have no need for something as bulky as an ear.
Another ability that we share with plants is proprioception: the "sixth sense" that allows (some of us) to blindly type, juggle and know where different parts of our body are in space.
Since this feeling is not associated with a specific organ in animals, but rather relies on a feedback loop between mechanoreceptors in muscles and the brain, the comparison with plants will be quite accurate. Although the details vary slightly at the molecular level, plants also have mechanoreceptors that detect and respond to changes in their environment.
“The general idea is the same,” says Hamant, who co-authored a 2016 review of proprioception research. "Until now, we knew that in plants it is more due to microtubules (structural components of the cell) that respond to stretching and mechanical deformation."
In fact, a study published in 2015 found similarities that could go even deeper and involve actin - a key component in muscle tissue - as involved in plant proprioception. “There is less support for this,” says Hamant, “but there was evidence that actin fibers were involved; almost like muscles."
These results are not the only ones of their kind. As they investigated plant senses, scientists began to find repetitive patterns hinting at deep parallels with animals.
In 2014, a team of scientists at the University of Lausanne in Switzerland showed that when a caterpillar attacks an Arabidopsis plant, it sets off a wave of electrical activity. The presence of electrical signaling in plants is far from a new idea - physiologist John Burdon-Sanderson proposed it as the mechanism of action of the Venus flytrap back in 1874, but what's really interesting is the role that molecules - glutamate receptors - play.
Glutamate is the most important neurotransmitter in our central nervous system, and it plays exactly the same role in plants, with one major difference: plants do not have a central nervous system.
"Molecular biology and genomics tell us that plants and animals are made up of a surprisingly limited set of molecular 'building blocks' that are quite similar," says Fatima Tsverchkova, a researcher at Charles University in Prague, Czech Republic. Electrical communication evolved in two different ways, each time using a set of building blocks that supposedly triggered the rift between animals and plants 1.5 billion years ago.
“Evolution has led to a number of possible mechanisms for communication, and while you can get them in different ways, the bottom line is the same,” says Hamowitz.
The realization that such similarities exist, and that plants have much more sense of the world than their appearance suggests, has led to a number of notable claims about "plant intelligence" and even initiated a new discipline. Electrical signaling in plants was one of the key factors in the birth of "plant neuroscience" (the term is used even though plants lack neurons), and today there are plant researchers studying non-plant areas such as memory, learning, and problem solving.
This kind of thinking has even led Swiss lawmakers to write a manual on protecting the “dignity of plants” - whatever that means.
While terms such as "plant intelligence" and "plant neuroscience" are viewed by many as more metaphorical, they are met with criticism. “Do I think plants are intelligent? I think plants are complicated,”says Hamowitz. And complexity shouldn't be confused with intelligence.
Thus, while it is very useful to describe plants in anthropomorphic terms, there are limits. The danger is that we might think of plants as inferior versions of animals, which is not at all true.
“We who study plants are happy to talk about the similarities and differences between plant and animal lifestyles when we present the results of the study to the general public,” says Tsverchkova. But she also believes that dependence on animal metaphors when describing plants leads to problems. "I want to avoid such metaphors to avoid the usually fruitless discussions about whether carrots hurt when bitten."
Plants are eminently capable of doing what they do. They may not have a nervous system, brain, or other complex functions, but they surpass us in other areas. For example, even though they don't have eyes, plants like Arabidopsis have at least 11 types of photoreceptors, while we only have 4. This means that their vision is more complex than ours. Plants have different priorities, and their sensory systems reflect this. As Hamowitz notes in his book, “light for a plant is not only a signal; light is food."
Therefore, although plants face the same problems as animals, their sensory abilities are shaped by their main differences. “The rootedness of plants - the fact that they don't move - means that they need to be much more aware of their environment than me or you,” says Hamowitz.
To fully appreciate how plants perceive the world, it is important to change the paradigm of attitudes towards plants. The danger is that if people compare plants to animals, they will miss out on the value of the former. Plants should be considered interesting, exotic and amazing living things, not pieces of furniture. And to a lesser extent - a source of human nutrition and biofuel. This attitude would benefit everyone. Genetics, electrophysiology, and the discovery of transposons are just a few examples of areas that began with plant research, all of which turned out to be pivotal in biology to some degree.
On the other hand, the realization that we may have something in common with plants can be an opportunity to recognize that we are more plants than we thought, just as plants are more animals than we thought.
“We may be more mechanistic than we used to think,” concludes Hamowitz. In his opinion, these similarities should hint at the amazing complexity of plants, as well as common factors that connect all life on Earth. And then we will begin to value unity in biology.
ILYA KHEL
