How Will you Behave?
(May 16th, 2017) Can plants learn, do they think ahead? Many would think these are silly questions. Not so Anthony Trewavas, who recently published a preprint on bioRxiv wondering whether plants are sentient.
If we see a lion chasing a gazelle, the behaviour is obvious: the gazelle is running for its life and the lion for its lunch! But what about when a cow is eating grass? Is the plant capable of reacting in some way? Can it “see” what’s coming? Most would quickly dismiss any sort of plant behaviour but Anthony Trewavas and his team at Edinburgh University, UK, believe it’s all a matter of knowing where to look.
For the researcher, the problem is that we have a very limited view on what constitutes behaviour. Going back to the lion example, the movement we see is the giveaway for behaviour. However, there are a lot of movements that we can’t see - either too fast or too slow - but they’re still classed as behaviour. “We are animals and we impose an animal view on the rest of biology,” says Trewavas. “The behaviours that we recognise in animals are not seen in plants and that’s why we dismiss plant behaviour.”
Plant behaviour may not be in the form of perceptible movement; however, this does not mean it doesn’t exist. One of the most interesting examples is the reaction of plants to being eaten. You may not even notice a cow lazily grazing in the field but there’s a lot of activity going on inside the plant, as it “learns” to react and offset the damage caused by the herbivore. “Being eaten leads to a change in the plant itself”, explains Trewavas, “so, if it’s attacked on another occasion, it responds much more quickly and much more robustly, as a defence mechanism.”
Being eaten is not the only trigger. A disease, weather changes, mechanical damage, contact of toxic materials; all these lead to some kind of priming. The exact mechanisms are still a little foggy but researchers know this priming involves some kind of epigenetic changes, which can still be detected years after the event. “It’s a form of learning, just as the immune system learns and responds accordingly. Exactly the same thing happens,” says Trewavas.
Shade avoidance is another interesting example of plant learning. Plants can actually detect a particular wavelength of light coming from neighbouring vegetation, which triggers a reaction to avoid being shaded. They start putting more resources into growing new leaves, while placing root growth on standby. According to Trewavas, the crucial point is that this new growth is a prediction of a potential future. “It has to be a prediction of the future because plants grow slowly; it would be a great mistake to make a phenotypic change if in the future there was no competition.”
From a physiological point of view, Trewavas believes all these messages are carried around the plant in the phloem. “The phloem is a highly branched, excitable system, which, to my mind, is what you have in the brain,” explains the researcher. “We don’t call it a brain, because we don’t know whether this excitable system is used for reasonable assessment but we do know that, for example, shortage of water or light and dark modifications, initiate some changes.” These are mainly in the form of action potential, which can influence the cells through which it passes and the adjacent cells, causing a transient calcium increase. Researchers don’t really understand these mechanisms yet but there are literally hundreds of proteins that can potentially respond to this calcium signal and initiate a cascade of other changes.
For Trewavas, this is certainly worth further research. “There’s something here that needs investigation about how the whole system functions and whether plants do have an excitable, highly branched system that can assess signals coming in and then determine a response. This is what brains do in most animals: they take in signals, they assess them and they respond,” concludes the researcher.