How does a plant sound the alarm?

18 September 2018

Interview with 

Dr Phil Wigge, Sainsbury Laboratory, Cambridge University

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If something starts to eat us humans, we can register our displeasure by either making a fast getaway or beating off the guilty party. But plants don’t have this luxury – they’re literally rooted to the spot so they need to resort to other means to discourage things from eating them. One deterrent at their disposal is to make themselves taste unappealing. But they don’t want to waste resources on tasting bad until they need to, so how do they send a rapid ‘I’m being eaten’ message all around the plant? A recent paper in the journal Science, reveals the answer. Georgia Mills paid a visit to the University of Cambridge’s Sainsbury lab, to speak to Philip Wigge, who wasn’t involved directly in the study, but he does work on how plants can sense their environment.

Georgia - It looks like a sort of bank vault in here.

Philip - This is where we grow our plants you see. This is a plant called Arabidopsis and it’s just a small mustard plant. And this was the same plant that was used in the study which is described in this paper. You can see it’s quite a modest looking plant - it’s got small white leaves but for research it’s been an absolute boon. This is really sort of the lab mouse for plant research because it has a fully sequenced genome and it’s very well understood.

Georgia - But, unfortunately for us sequenced genome or no, Arabidopsis also lives in an incredibly noisy home. So we went to find a quieter part of the greenhouse…

Philip - This is what I thought was a quiet area.

Georgia - Which was harder than it sounds.

Philip - This is quite noisy too. There are a few ambient noises that you don’t think about when you’re working here.

Georgia - Ah, sweet silence. We settled down to chat amongst some other plant specimens who, believe it or not, may have felt our presence. And they need to…

Philip - You can imagine, if you are a delicious plant sitting outside trying to grow in the sunshine, and if an army of caterpillars comes along you really want to be able to respond very quickly and defend yourself as best as you can. And that’s quite complex because obviously you can’t move away, so plants have to be very resourceful and very perceptive when dealing with pests like caterpillars that want to have them for lunch.

So if you imagine this plant here, you could imagine this leaf, and if we just pull the leaf apart. And as I do that what I’m doing is I’m pulling apart millions of cells, so millions of cells are being crushed and broken open. Now it turns out, although we can’t see it, as we tear this leaf the plant is actually responding inside within seconds.

Georgia - We can’t see it, but we have known that plants are able to do something like this for a long time. But, unlike you and me, they don’t have the luxury of a central nervous system, so this group, among many others, were wondering how are they sending these messages?

Philip - What they show is that the plant is using a small amino acid called glutamate. Now glutamate is also used in humans as a neurotransmitter remarkably enough. So what happens when a cell is damaged is that the cell releases glutamate into the open, and that glutamate is then sensed and picked up by channels. And when these channels become activated they release calcium, and it’s the calcium signal that this paper shows, is a mobile signal that travels within seconds throughout the plant. And then what the plant does is it activates the expression of genes that control a response to pathogens. So one way for a plant to protect against the caterpillar is to make itself taste very bad.

Georgia - How did they find this out? How did they work out this is what was going on inside the plant?

Philip - They used a number of tools from molecular biology to identify the actual receptors. They had a hypothesis that calcium could be involved and that glutamate could be the signal and then they were able to find particular plants that lack the channels that respond to glutamate. What they were then able to show was that if you take away just these channels within the plant, the plant no longer transmits this signal. That’s kind of definitive proof, that you need these channels in order to respond to herbivory.

Georgia - I feel kind of bad now for pulling up daisies to make daisy chains when they’re sending out these ‘bah help me’ signals.

Philip - That’s a good point actually. We know plants respond very much to any kind of perturbation and they have these large scale changes. Whether or not they’re aware of this or whether they feel pain is probably unlikely, so I wouldn’t feel too guilty about cutting the lawn. But it’s interesting to be aware  when you do but the lawn you’re causing these large scale changes in how every single blade of grass is responding to being cut.

Georgia - And you get that lovely smell. Is that their death throes?

Philip - That’s one way of looking at it but I try not to think about that.

Georgia - As someone who works on this, in this field how important and how much of a change has this paper brought would you say?

Philip - Oh, I think it’s a great study and it’ll go into the textbooks I think. There’s a whole range of very fundamental questions that we still don’t know about plants. This is quite remarkable in a way because we are so dependent on farm grown plants to sustain the entire human population. And so until relatively recently farming has been largely a sort of trial and error process and just in the last few decades we’ve started to understand the molecular basis by which plants grow and how they develop. And what this means is that it enables us to potentially improve crop yields and food security.

There’s no question that herbivory is a major problem in many areas of agriculture, so if we can create smarter plants that are more perceptive and more resilient to feeding insects then that’s going to be, potentially, a huge advantage.

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