09/14/2018

When Plants Sense Danger, They Cry Out With Calcium

17:34 minutes

a gif of the veins of a leaf glowing and dimming
Calcium signals move through a leaf in response to wounding of a different, distant leaf. Credit: Masatsugu Toyota

Plants have a unique challenge in staying alive long enough to produce offspring. Unable to move and at the mercy of their surroundings, they present a tempting source of nutrition for bacteria and animals alike. But they’re not helpless. Botanists have long known plants are capable of sensing their environments and responding to them. They can grow differently in response to shade or drought, or release noxious chemicals to fend off predators, even as a caterpillar is mid-way through chewing on a leaf.

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But how does that information travel? New research published in the journal Science shows a first glimpse, in real time, of distress signals traveling from one leaf, snipped, crushed, or chewed, to other healthy leaves in the same plant. The signal, a wave of calcium ions, seems linked to the amino acid glutamate, which in animals acts as a neurotransmitter.

University of Wisconsin-Madison botany professor Simon Gilroy, a co-author on the new research, explains this chemical signaling pathway and other advances in how we understand plant communication. Plus, why research on plants in zero gravity is still a barely explored but vital frontier in how we understand their biology.

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Segment Guests

Simon Gilroy

Simon Gilroy is a professor of botany at the University of Wisconsin-Madison.

Segment Transcript

IRA FLATOW: This is Science Friday. I am Ira Flatow. It’s not easy being green. I’m talking about plants. You’re dependent on very specific environmental conditions. You can’t relocate in search of water. And oh, yeah. Everybody wants to eat you, right? What’s a motionless organism to do?

Well, if you’re a plant, you do have some options. You can slow your growth to consume fewer resources. You can develop a deeper root system to get at the water table better. And if a caterpillar is nibbling on your leaf or you as a leaf, you can produce noxious substances– poisons– bad tasting chemicals to protect yourself.

But one mystery that has puzzled botanists is how a leaf that’s been bitten into can tell the rest of the plant to amp up this chemical warfare. Now, new research published in science points with startling video to a calcium ion signal that travels like electricity to the rest of the plant.

You can see it move like a wave from the wounded leaf through the capillaries to other leaves. And you can see it on our website at sciencefriday.com/plants. It might make you think twice about picking a flower. Here to talk more about this leafy distress signal and more from the secret lives of greenery is Simon Gilroy, professor of botany at the University of Wisconsin in Madison. He’s co-author on the new research. Welcome, Dr. Gilroy.

SIMON GILROY: Good afternoon.

IRA FLATOW: What got you looking for this signal in the first place?

SIMON GILROY: Well, so the researchers who work with me– the team– we’re all interested in one set of questions, which is how do plants know what’s going on in the world around them? They have to be really good at it, because they’re literally rooted to the spot. They have to know what’s attacking them, what’s happening to them.

They have to know things like up from down. A lot of signals being processed. But there’s that just fascinating question. They are obviously really good at doing it. But how do they– like using the words of how we interact with the world, how do they know? And how do they think?

How do they turn information into understanding? So we’ve been trying to piece together the cellular machinery that lets them do that for many, many years. And one of the other things that we’re very interested in is this universal role for the calcium ion.

And you can think of it as it’s just a signal biology worked out a long time ago how to use and uses it everywhere. So the reason your heart is beating at the moment is because flashes of calcium are being released into cells, and they’re causing contractions. It’s the same thing inside plants that calcium carries information. So we were just interested in putting those two things together.

IRA FLATOW: So what happens when the plant gets bitten into? It releases a spike and it tells the other parts of the plant that somebody’s eating me?

SIMON GILROY: Yeah, yeah. So that was sort of the miraculous thing that we managed to visualize. So imagine a caterpillar chewing on the end of one leaf and think about seeing the electrical charges and this calcium ion. We generated some plants that let us visualize that.

And when you chomp on the leaf, you trigger a really quick spike in calcium. And then it shoots through the rest of the plant. And for a plant biologist, it is really, really booking. It’s going about as fast as we can imagine. So it’s going in the range of millimeters a second, which for a plant signal, it’s a fantastically fast signal.

IRA FLATOW: It almost sounds like it has its own nervous system like animals do.

SIMON GILROY: Yeah, so we have– so inevitably, we start talking in the language of sort of nervous system, because that’s the context that we can really understand. But plants don’t have nerves. We should just set the goalposts of that. They do not have nerves, and they don’t have the anatomical structure that we would call a brain. But they still have to accomplish the same kind of things.

If somebody starts chewing on the tip of your arm, you generate a signal that moves through the rest of your body. And your body then responds to it. And usually, it’s by going like, ow, and moving away. Plants have to do the same kind of thing. They can’t respond by movement, but they got to have the same kinds of systems in there. It’s just that the cells that they’re built around are going to be very, very different from a mammalian nervous system.

IRA FLATOW: And the point is to what? To tweak up the defense system of the plant? Telling the rest of the plant this is happening?

SIMON GILROY: Oh, yeah. Plants are also very– they have a set of defenses that are sitting there. If those defenses were on all the time, the plant would be spending a lot of its resources just defending itself for no reason. So it waits for a signal of like, oh, part of me is being eaten.

I would really like the rest of me not to be eaten. Signals course through the plant body. And they switch on a whole bunch of defenses, things like toxic chemicals, things like proteins that prevent insects from digesting food. But they’re inducible defenses, because that way you only switch them on when you need them.

IRA FLATOW: So is the plant– when I chomp into a carrot, is it still sending out that alarm signal?

SIMON GILROY: Absolutely everyone is asking me that. And the answer to it is absolutely yes. The carrot is alive and when you’re eating it, it is sending off those signals. Then the other analogous question that always comes along is, well, then do I think about that when I’m eating a carrot? No, I think the carrot just tastes really good. It’s– yeah.

IRA FLATOW: Well, we know we talk about this communication system underground between plants. It’s called the worldwide woods system.

[LAUGHTER]

Are the plants talking to each other also?

SIMON GILROY: Yeah, I think there is some very good data on that. Some very good researchers have looked at how information could be exchanged not within an individual, which is kind of where our research goes, but between individuals and things like volatile chemicals.

So if you chew up a leaf, it will release chemicals. Those chemicals are volatile. They waft around in the air. And other plants can detect them and switch on defenses. So plants have these mechanisms to pass information between them. Absolutely. Yeah.

IRA FLATOW: Are there other kinds of signals we might be able to find in plants based on another kind of stress? I mean, what does a plant do of it’s too hot, for example?

SIMON GILROY: Yeah, those are fantastic questions, because we are in the discovery phase of science here. We really don’t know. We know a lot of the signals that plants respond to. And we can kind of describe how it changes their growth. And in a lot of cases, we can describe things like genes which get switched on and off.

But how the information is passed throughout the plant and processed– we don’t know. Things like temperature shock. One that I am particularly interested in is how the plants sense up from down. All of those kinds of signals. We are just at the beginning of finding it out.

IRA FLATOW: Our number– 844-724-8255. You can also tweet us at Scifri. 844-SCI-TALK on the phones. But I’m interested in hearing that you’re just beginning to learn this stuff. I mean, we’ve been studying plants for eons.

SIMON GILROY: Oh, yeah. It’s not that we don’t know a lot about plants. And it’s not that the researchers up until now have been sort of like sleeping on the job. We know a tremendous amount about them, but it’s that technology that keeps on advancing. And so now, we have technology where we can do things like measure the level of every single gene in a plant.

And now, the technology that we’ve been capitalizing on is fairly recent technology, which allows us to put proteins into engineered plants to make these proteins, that let us actually see changes in real time inside the plant. So we’re layering on more and more of the details and the phenomena onto this enormously rich background, which has come from all of the previous research.

IRA FLATOW: Now, OK, let’s about what you’d like to know now. What would you like to know most?

SIMON GILROY: So part of the kinds of questions that I’m interested in are at the nuts and bolts level of the machines that make it work. Inside each of the cells within a plant are a bunch of molecular machines– proteins that make the system work. And our research has just chipped away at one little part of it, which is a group of proteins called glutamate-like receptors, that give us an inkling about how the machinery is working.

One of the things we’d like to do is be able to put the details in there, to be able to build the map of how it works with the resolution that we have, for instance, about how a nerve works. Because then we can begin to tinker with the system. And you can imagine that if we understand how the information is being propagated through the plant, we may be able to preemptively switch that system on in a very intelligent way, to protect the plant when we want it to be protected.

IRA FLATOW: It’s funny you mentioned glutamate, because glutamate is what’s all through the brain, too– the nervous system, too. Isn’t it?

SIMON GILROY: Yeah, this one is great and sort of amazing parallels, that plants are using glutamate and the proteins that perceive it, which are very, very similar to the proteins that perceive glutamate in the nervous system. So there aren’t any nerves, but they kind of have the same theme behind what’s going on.

IRA FLATOW: Wow. I understand you’re also doing research on how plants respond to things in spaceflight in space. What fascinates you about that?

SIMON GILROY: Yeah. So I’m actually doing the recording of this in the Kennedy Space Center. And we’re down here getting ready to send some plants up onto the International Space Station. Space is just a fantastic place to put biology, because if you think of the evolutionary history of all biology, it’s been on the surface of the Earth at 1 times gravity. And you can’t get away from it.

Nothing has evaded 1 times gravity, as far as its history, until we went into space. And so we can now remove gravity from the equation– go into the weightless environment of, for instance, the space station and ask, what was gravity contributing to biology? Because when we remove it, things that suddenly don’t work– they become a big deal as far as like, oh, that’s what gravity is regulating on Earth.

And also, then, if we’re going to spend any time in space– and I mean, an extended period of time in space– we’re going to have to have a life support system that comes with us. And plants and microbes keep humans alive on Earth. And so the question is, can we take that machine with us, that biology of plants and microbes and make them work in space?

IRA FLATOW: So it’s just fundamental things about growing a plant in zero gravity that not only have to do with how the plant stalk grows, but how the water, I imagine, right?

SIMON GILROY: Yeah.

IRA FLATOW: Water is influential and it needs gravity for it to work and sort of seep down into the soil. It’s not seeping anymore.

SIMON GILROY: Yeah. I always go, if you think of the simplest thing that you can imagine you do with your potted plant, which is that you water it. So you have a pot. You have a watering can. You turn the watering can on the side. Gravity pulls the water out into the soil and pulls that water down through the soil.

And now, go into space. None of that is going to work. You’ve got your watering can. You turn on its side. Nothing comes out. So how do you physically get the water into the soil? Well, we use things like syringes and pumps. And they work really well. You can push the water into the soil.

And you don’t use soil. You use a bunch of different matrices, things like clay particles. But then weird things start happening that you’re not prepared for, unless you sort of understand the physics of what’s going on. But basically, once you’ve removed gravity, water becomes really sticky and really creepy.

So it’ll stick to surfaces very well. And it will also creep along those surfaces. So now, imagine that you’ve watered your plant and you’ve added just a little bit too much water. And so that water now sticks to the– let’s say that you have it in some kind of soil. It’ll stick to the soil.

But it will also start creeping along the surface of the soil, and it’ll start creeping up over the surface of the plant. And eventually– and this has happened during spaceflight. The water will encase the plant. It’ll be like a watery glove over the surface of the plant. And that is equivalent to flooding the plant. That’s like trying to grow your plant under water and that does not work.

IRA FLATOW: I hate it when that happens. This is Science Friday.

[LAUGHTER]

I’m Ira Flatow. This is Science Friday from WNYC Studios. Wow. Those are the kinds of things– I’m a gardener. I think about gardening all the time, but would never think that that water was going to do that. But also, plants need the right mixture, right? Don’t they– carbon dioxide, and respiration, and all that kind of stuff in a different environment?

SIMON GILROY: Yeah.

IRA FLATOW: That’s all part of the equation you have to figure out?

SIMON GILROY: Yeah. And so I like the Earth. I think it’s a great place to live. I’m happy with how it works. And once you remove something like gravity and move into the realm of spaceflight, a whole bunch of the features which we really, really do take for granted– you go down to your garden and you look at your plants. And they’re growing very happily. A lot of the things which are going on don’t work quite as well in space.

And also, plants– biology is fantastic. Biology copes with going into space. Astronauts– their physiology has changed, but they are able to cope and deal with that new environment. Plants grow OK in space. It’s not that they don’t grow. And biology has this fantastic capacity to sense its environment and adapt and that’s what we’re seeing.

IRA FLATOW: Do plants taste differently when they’re growing in space?

SIMON GILROY: At two levels, yeah. So one, the taste of plants is partly about how they grow, so how quickly they grow, the nutrients they take up, how much water they have, how much light. The plant itself. But also, you know when you’re on a plane and food tastes differently because you’re on the plane? That same phenomenon kind of happens in the space station. So it’s a bit like having a head cold. And so those two things play off with each other. How the plants grow and how you as an astronaut are operating change everything a little bit.

IRA FLATOW: So you’re going to try to figure out how gravity exactly affects how plants grow in space and possibly figure out maybe, will plants grow on Mars, for example? Is the soil fertile enough?

SIMON GILROY: They’re all great questions to which we kind of know the answer. But no one has been to Mars, so we haven’t tried it. But there’s water there. There’s minerals, although we probably would have to bring some fertilizer. But some of the characteristic of Martian soil– the Martian regolith is– we would have to deal with.

So it’s very salty. And it has a bunch of chemicals in there because of the weird chemistry– a bunch of chemicals called perchlorates and those are weed killers. So that might be a small issue, but we can get rid of them. We can wash them away. So, yeah.

IRA FLATOW: OK. But one of the things we think about when we go to Mars is bringing part of Earth to Mars. We don’t want to contaminate Mars. Can you sterilize a seed enough that it wouldn’t contaminate, but still grow if you wanted to test it out on Mars?

SIMON GILROY: So we can absolutely sterilize seeds as far as the outside of the seed is concerned, which does a pretty good job We can get them to be sterile. But the thing is that the way that plants grow on Earth is it– varying in a community and microbes are part of that community. And divorcing plants and microbes is probably not the smartest thing to do. Because there’s a tremendous amount of interactions that make plants grow well that come from the microbial populations in the soil. So that may just not be the correct strategy to go down.

IRA FLATOW: All right. That’s a great point, because I wasn’t thinking about that. We talk a lot on this program about the soil microbiome as being necessary for plants to grow. And you don’t have a soil microbiome in the natural space environment, do you? Do you have to create that in the space station?

SIMON GILROY: No. So you can grow plants under sterile conditions without microbes and they’ll grow. Like I say, biology is fantastic and they will grow but it’s all questions of sustainability and nutrient utilization, how well they’re growing. We’ve only been growing plants in space intensively for a few years. So we’re not really certain that we’re good gardeners yet.

IRA FLATOW: OK. Well, we’ll find out. Well, thank you, Dr. Gilroy, for taking time to be with us today.

SIMON GILROY: My pleasure.

IRA FLATOW: Simon Gilroy, professor of botany at the University of Wisconsin in Madison. When we come back, we’re going to take you inside the grand championship of science competitions– the International Science and Engineering Fair. Stay with us. We’ll be right back.

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