A new report argues that the way plant cells signal isn’t something we can easily compare with nervous systems in animals, an idea that goes directly against the little-known discipline of plant neurobiology.
If we’ve learned one thing about the chemistry of greenery over recent decades, it’s that plants are far more dynamic than we ever took them for.
They can sense and respond to sounds, from the sexy buzz of a pollinator to the horrid chomping of a predator. Touch elicits a reaction, some of which they really hate. They can communicate with one another. They can be numbed. And they might even have the capacity to remember.
Yet each of these remarkable observations is complicated by emotional language. Even in less poetic jargon, turning measurements of growth responses and fluctuating botanical hormones into a meaningful description can be a challenge.
Since the early 2000s, if not longer, a debate has raged over where we might draw the line between plant responses to environmental cues and something we might even sympathise with, like some kind of analogy for neurology.
That comparison has its limits, critics argue. Worse still, overstepping the mark could come with a serious downside.
“The biggest danger of anthropomorphising plants in research is that it undermines the objectivity of the researcher,” says plant biologist Lincoln Taiz from the University of California Santa Cruz.
“What we’ve seen is that plants and animals evolved very different life strategies. The brain is a very expensive organ, and there’s absolutely no advantage to the plant to have a highly developed nervous system.”
Strictly speaking, only animals have a web of neurons responsible for processing stimuli and calculating a reaction. Nobody is suggesting plants have clumps of grey matter and peripheral nerves.
But as scientists have discovered more about the intricate signalling between plant cells and also the communication between individual plants, there comes the temptation to link them with comparable pathways in animals.
Take the idea of applying the term ‘swarm intelligence’ to plants, for example.
“According to this idea, plant behaviour emerges from the coordination of individual cells and tissues, analogous to the problem-solving that emerges from the communication and cooperation between the members of a bee hive,” the researchers describe in their report.
“However, this analogy has several problems.”
For one thing, they argue bees are free to come and go, unlike cells inside individual plants. There is also a larger degree of genetic conflict between individual members of an insect swarm that make a key difference when it comes to reproduction.
But one of the more problematic analogies, it seems, is the idea that at least some plants might be capable of consciousness.
Philosophers have had a hard time coming up with clear boundaries on this one, with centuries of a head start. Even modern neurologists have struggled to pinpoint the anatomy responsible for the phenomenon in humans.
Suggestions that nervous-like activity identified in the tips of plant roots might be ‘brain-like‘ can go too far when we translate tissue responses in animal-like terms. Especially when there are so many animals with actual nervous systems that we don’t think of in that way.
“And if there are animals that don’t have consciousness, then you can be pretty confident that plants, which don’t even have neurons – let alone brains – don’t have it either,” says Taiz.
In some respects, it’s possible that plant neurology is simply before its time.
Further research just might one day force a reconsideration, demonstrating the kinds of calculations carried out as plants go about their business are comparable with animals we might think of as conscious.
But Taiz and his colleagues argue the kinds of experiments plant neurologists are basing their cases on lack the rigour needed to make such an audacious claim.
“I know a lot of people in the plant neurobiology community would like to see their field in the textbooks, but so far, there are just too many unanswered questions,” says Taiz.
This research was published in Trends in Plant Science.