New Research Indicates Learning Can Take Place Without Neurons

Some long-accepted truths about the brain have a way of eventually being proven false.

For instance, contrary to earlier beliefs, neurogenesis (the growth of new neurons) does occur in adult brains as well as developing brains.(1) And, it turns out that drinking alcohol does not appear to kill off brain cells, as previously supposed.(2) Finally, neuroimaging studies show that we use far more 10 percent of our brains.(3)

But there is one rock-solid 'truth' that—until recently—has never been challenged: that learning occurs only in neurons, specifically through persistent changes in synaptic connections among different neurons.

The intriguing idea that learning can occur without neurons comes not from the animal world (which includes humans), but from the plant kingdom.

Non-neuronal learning

Monica Gagliano and colleagues at the University of Western Australia, writing in the prestigious journal, Nature, demonstrated that associative learning occurs in organisms completely devoid of neurons, ordinary garden peas.(4)

In Gagliano et al.’s study, garden peas were trained in a Y maze, where a conditioned stimulus (CS) in the form of a fan that stimulated the pea’s mechanoreceptors was paired with an unconditioned stimulus (US), illumination with blue light (which stimulates plant chromophore photoreceptors).

Normally, plant seedlings exhibit an innate unconditioned response (UR) to the presence of light, where the seedlings grow towards light sources through a well-documented process called phototropism. But by pairing the CS with the US over a three-day period, the researchers induced most of the plants in the experimental group (random controls received no CS/US pairing) grew towards the arm in the Y maze where CS and US had been paired, even when light was present in the other arm of the maze during “non-training” periods (in other words, during normal daylight).

These results indicate, for the first time, that associative learning can take place outside of neurons (as plants have no neurons). The authors speculate that learned associations in plants are encoded in persistent “epigenetic” changes in signaling among different molecules and cells in plants, but did not point to any specific mechanisms.

Although these findings do upend our beliefs about how learning occurs, and constitute a fun fact for cocktail party conversation, do they have any relevance for learning and memory in humans?

In my opinion, yes.

I say this because nature is extremely elegant, using and re-using the same principles across all living organisms, whether those organisms be bacteria, archaea, plants, animals, or even viruses. Genetic inheritance via DNA/RNA is common in all life forms, as is phenotypic expression via turning on and off of genes. Chemical signaling among all living cells, whether hormonal, pheromonal, neurochemical, or immunological primarily occurs through stereo-chemistry and conformational states (i.e., the relative shapes of signaling molecules and corresponding cellular chemoreceptors).

The ancestors of plants and humans were the same until a billion years ago, so, given that life sprang up on Earth 3.7 billion years ago, the ancestors of humans and bananas were more or less identical for 2.7 billion years. That’s why, today, there is a 60 percent overlap in human and banana genes.(5)

Thus, given nature’s strategy of reusing what works in one organism across many other organisms, it is entirely reasonable to suppose that—as with pea plants—some forms of learning in animals and humans occur outside the nervous system.

Possible support for this idea can be found in Gagliano et al.’s finding that associative learning in plants exhibits a feature also found in both animal and human learning, specifically, that organisms, whether they be plants, animals, or humans, learn better during the daylight portion of their circadian cycle than at night.(4)(6) Although the shared importance of biorhythms in forming new memories could be an example of convergent evolution (it evolved separately in plants and animals in response to a shared environment), it could also stem from a trait that plant and human common ancestors acquired during the 2.7 billion-year period over which those ancestors were exposed to daily light-dark cycles.

The possibility that plant and human learning share at least some non-neuronal underpinnings opens up an entirely new field of neuroscience and could lead to a deeper understanding of memory and learning, and possibly new treatments for learning and memory disorders.

It’s important that we keep an open mind to such possibilities, and to remain humble in the face of nature’s awesome complexity. The comedian Will Rogers summed up why keeping an open mind is so crucial when he observed, "It’s not so much what we don’t know that hurts us, but what we do know that ain’t so!"