Sorry Mark Wahlberg, this is not a prequel to the 2008 sci-fi thriller “The Happening,” but maybe M. Night Shyamalan was onto something. Could plants learn that large groups of people are threatening, then kill them with an aerosol neurotoxin? Probably not. But they do learn, remember, forget, and even make decisions in some capacity.
If you’ve ever touched a mimosa pudica, the so-called sensitive plant, you know that plants can freak out. They sense a disturbance and react, possibly to protect themselves from hungry insects. Could they learn to tell the difference between threatening and non-threatening stimuli?
To find out, Dr. Monica Gangliano, associate professor in the school of animal sciences at the University of Western Australia, stimulated mimosa plants repeatedly with water droplets. After several showers, the plants stopped responding. Remarkably, months later, the same plants still didn’t recoil from a water droplet. The plants seemingly learned the water was not a threat, and remembered, even in the absence of stimuli.
These results are striking, but discoveries of learning and memory in plants are not new. Plants are exposed to a variety of stresses. Some are enduring like the cold of winter. Others are transient like heat stress in the late afternoon. Responding to stress requires energy, and often stalls growth. Thus it is beneficial for plants to remember and respond quickly to some stresses while learning to ignore others.
Plants have several strategies for recording memories and passing them on to future generations. In response to stress, plants can reorganize their DNA to activate or inactivate certain genes in a semi-permanent and heritable way. Plants can also store molecules of RNA, which act like molecular messages to enable a rapid response to future threats. A recent discovery by Dr. Susan Lindquist’s group at Massachusetts Institute of Technology suggests that plants may also store and transmit memories using proteins called prions
Prions are best known as the causal agent for mad cow disease (scrapies in sheep and Creutxfeldt-Jakob Disease in humans), but they are also associated with long-term memory storage in animals. Prions are able to adopt unique folding patterns. These patterns can be inherited and transmitted to neighboring proteins like a molecular switch that causes cascading and permanent changes.
Lindquist’s group used computational methods to look for genes in plants that resembled those encoding prion proteins in yeast. They then expressed the functional domain of what appeared to be a plant prion in yeast. The plant domain was able to functionally replace an analogous domain in a yeast prion protein. This strongly suggests that plants have prions or prion-like proteins.
Does this mean plants have brains, or plant neurobiology is the next big field of study? Doubtful. What some have called plant neurobiology is remarkably similar to what has always been recognized as stress response. To say that plants are making decisions probably gives them a bit too much credit. Decision-making is a more-or-less voluntary response. Plant reflexes might be a more appropriate term to describe the changes that occur in a plant in response to stress.
Nonetheless, similarities between plants and animals should not be ignored. Gangliano and Lindquist’s studies demonstrate the value of breaking down organism barriers in science. In the current structure, plant scientists go to plant science meetings, veterinary/livestock researchers go to animal meetings, and everyone else goes to meetings focused primarily on human health. Plants, humans, and other animals coexist in an interdependent web, so should our approaches to studying them.