Plants are easy to overlook, and have long been considered one of the simplest forms of multi-cellular life. Stationary, silent and without neurons, they are obviously far removed from our own mobile and intelligent mammalian species. But this does not mean that they stopped evolving adaptations that allow them to process information, and use it in their favour. This is the practical, if not philosophical, definition of “thinking”.
The recent media announcement that plants can think may have some reaching for their triffid guns. However, you can put down your flamethrower and your sci-fi book. Plants are not about to start outsmarting humans or staging a ‘Green Revolution’. But they can sense electric signals in a manner analogous to our own nervous system, are hyper-aware of their environment, and can learn to respond to stimuli.
In fact, the idea that plants can “think” was first proposed by none other than Charles Darwin. You’ve probably heard about “On the Origin of Species”, but perhaps less popular is the page turning thriller titled “The Power of Movement in Plants”, published in 1880, in which Darwin suggests that the roots of plants perform a similar function to the brain of “lower” (less complex) animals.
For many decades it was assumed that plants co-ordinated their responses to stimuli (releasing chemicals in response to wounding for example) by using chemical signals which travelled through the phloem. It wasn’t until 1992 that anyone thought to time the responses, and found they were far faster than chemicals could travel around a plant. Their subject, the humble tomato plant, was using electric signalling to co-ordinate itself. Electric signalling also underpins the human nervous system.
Plants can use this signalling to process information from their environment and regulate their growth accordingly. For example, plants famously always grow towards the light. This is the equivalent to you walking towards the fridge. Some plants are capable of more complex behaviours, such as the Venus flytrap, which responds to only the second touch of its insect prey, thereby increasing its accuracy. Mimosa pudica, also called the touch-me-not plant, defensively closes its leaflets when touched. If dropped harmlessly from a short height onto foam, the leaflets close. But after around five drops in a day they stopped doing this – almost as if they realised that the impact wasn’t dangerous. However, they continued to act defensively when physically touched, which would normally be followed by predation. Plants ‘remembered’ the drop for at least a month.
How do plants co-ordinate this information without neurons? And what of Darwin’s ‘rootbrain’ theory? It is well known that the very tip of the root, called the root cap, is capable of sensing a huge range of properties, including nutrients, gravity, and light. Further behind this lies an electrically active region called the transition zone, which was originally deemed to have no purpose, but is now thought to be the ‘nerve centre’ of the plant. Furthermore, the transition zone also gobbles oxygen at high rate, much like our own energy-demanding brains. In addition to this metabolic and electrical activity, a plant hormone called auxin is moved around this region in membrane bubbles called vesicles, in a similar process to the transportation of neurotransmitters around our brains. If you feel happy, for example, a vesicle has just released a load of serotonin across your brain synapses. These activities make the transition zone a likely candidate region for where the sensory information picked up by the root cap is translated into behavioural commands for the plant.
Plants also produce some of our key neurotransmitters and hormones, including serotonin, melatonin and glutamate. Researchers are not entirely sure of the significance of these chemicals. It is possible that their occurrence is a coincidence – these usefully shaped molecules could have been co-opted into another role. However, give a plant drugs to alter these chemical pathways, such as Prozac, Ritalin and methamphetamines, and their roots will not develop properly.
Susan Murch at the University of British Columbia in Kelowna, Canada, pointed out to the New Scientist that, “If you really mess with a plant’s ability to either transport or make melatonin or serotonin, root development is very strange – they are malformed and disjointed.”
Not convinced? Consider the following: adult humans have around 100-500 trillion neurons, which last for a lifetime and form the central nervous system. But in a plant, every single cell is capable of behaving like a neuron, being able to create and convey electrical impulses. This capability is amplified in the electrically active phloem, which acts as an information highway down the centre of the plant, transmitting rapid electric signals in addition to transporting chemicals and water.
So, if plants can think, what are they thinking about? Not exactly helping the evil plant image established by John Wynhdam, researchers found that creeping vines prefer to select the juiciest prey. The parasitic vine Cuscuta appears to sniff the air looking for a host, then pounces and wraps itself around its victim. It prefers tomato over wheat. No humans were offered by the researchers to the vine. On second thoughts, keep your flamethrower close!
Despite the accumulating evidence, the term “plant neurobiology” is controversial. In 2007 thirty six scientists published a letter in the journal Trends in Plant Science claiming “plant neurobiology is a metaphor” alone. This escalated to a 2009 science brawl (or maybe more of a catfight), in which the Society for Plant Neurobiology was forced to change its name to the Society of Plant Signalling and Behaviour. A win for the anti-sensationalists; but perhaps a wake-up call to redefine intelligence beyond the neuron. The Society of Plant Signalling and Behaviour continue to state on their website; “Plants accurately compute inputs from the environment, use sophisticated cost-benefit analysis, and take action to mitigate diverse environmental insults. Plants are also capable of refined recognition of self and non-self, and are territorial in behaviour.”
Whether their brains are metaphorical or not, it is clear that plants already have a far superior feeding system (autotrophic, from sunlight), more flexible reproductive biology, and a wider range of adaptations and habitats than the big-headed animal kingdom. If we could stop underestimating (and destroying) plants, and instead start using their abilities to make our species more sustainable; well that would be thinking smart. Like a plant.
Isobel Steer is a researcher developing disease biosensors at Imperial College London. She recently graduated in Biological Sciences from the University of Oxford, where she enjoyed communicating science in several student magazines. Occasionally she tries to catch frogs and tracks dung beetles through rainforests in the name of conservation.