While these results clearly show that one tree or one plant can be affected by volatile chemicals given off by another, does this really prove though that trees talk to each other? Or could there be another hypothesis, could it be that the neighboring trees are simply eavesdropping? In other words, that what's happening is that the attack tree is giving off the gas in order to help itself. For example, you could imagine where one branch is attacked, and it wants to signal to its own sister branch watch out. On the other hand, so it gives off the gas, and then it helps its own branches further up the tree. The neighboring tree then is just listening in. It absorbs the gas, whereas the intent wasn't necessarily for communication between the trees. Now in lieu of plant psychologists, where you can't really ask this question, what type of experiment can we do to check if the internal gas being given off helps also its own plant? I wanna describe one more experiment, and this was done by actually one of Baldwin's students, Martin Heil, who's in Mexico, who's asking the question, are we talking about volatile signaling between plants? Or are the plants simply, excuse the expression, simply passing gas? So, what Heil already knew, in his experimental system were wild lima beans, he knew that wild lima beans that were attacked by herbivores, give off volatile gases. And also, when they are attacked by herbivores, they would also make more nectar in their flowers as a response to this damage. What he wanted to know, again, could you find, first of all, the same signalling between plants? And is it for signalling between plants, or you're also getting response within the same plant? So, here's his experimental set up, and we're gonna go through a couple of his figures from his article that was published. So, in the first figure, what we're gonna see is that he could actually repeat Baldwin's experiments from the early 1980s. What we have here are two branches, one which was taken from a lima bean that had been attacked by insects and another from a lima bean that had not been attacked. When he put these two branches next to a third lima bean plant, the one that was put next to the attacked branch, this is gonna get a little confusing, I think, started reacting as in Baldwin's experiment. In other words, if this branch was neighbor to a branch that itself had been attacked, its own leaves started making chemicals as if it was attacked. On the other hand in the control branch, which was put next to another branch which had never been attacked, we didn't see these chemicals being made, and they were actually checking the chemicals in the air around the leaves. Next what he saw, though, was that it wasn't only that the chemicals were being given off, this gave the lima bean an advantage. For example, if he then took the control branch and had bugs put on it, it was more likely to be eaten. It was less likely to survive than the branch that had been next to one that had already been induced, one that had already been induced by other insects. So, in other words, these chemicals from the air, he showed that they're helping its neighboring plants survive, very similar to what Baldwin showed, so then he asked the question. Are the volatile chemicals that are being given off by the attacked branch being used by the neighboring? Or are they also being used by that same branch itself? And to check this, he did the following experimental setup. In one control, he then took two or three leaves and caused them to release the volatile chemicals, sort of similar to what he had in the first experiment. In the second grouping, he isolated these leaves with plastic bags and only then added the chemical, which would let the leaves respond. So in the first one, he's attacking two leaves, seeing how the neighboring leaves respond. In the second one, he's attacking two leaves, but isolating them from the rest of the leaves on the same plant. What he found was that when he didn't isolate the leaves, its neighboring leaves on the same branch were also resistant to the pathogens. Excuse me, they were also resistant to attack. In terms of nectar, they were making nectar as if they themselves had been attacked. On the other hand, when the leaves were isolated in bags, its neighboring leaves on the same branch responded just like the controls, it had no knowledge that its neighboring leaves had been damaged. Then he did one more thing. He opened those bags and using a very, very small motor, a very small fan, blew the air from one of the leaves on the same branch to the neighboring leaves further up the branch. And when the leaves further up the branch were exposed to the air coming off those leaves below the branch, they themselves started to make the chemicals as if they had been attacked. In other words, at least in lima beans, the air around an attacked leaf gives off a chemical, which is then taken up by its neighboring leaf on the exact same branch, and it becomes resistant. So then, of course, the next question is, what is the active volatile chemical that is released by the attacked leaves? Of course, the answer's not so simple, it's a mixture of chemicals, and it depends on what the damage is. Is it being torn? Is it being eaten by a bug? Is it being attacked by a bacteria? If it's insects, for example, the leaves are primarily giving off a chemical that's called methyl jasmonate. And if it's bacteria or viruses, the leaves give off a chemical that's called methyl-salicylic acid. Now why is it important for me to say what the name of the chemical is? Because methyl-salicylic acid is very similar to another chemical in the plant called salicylic acid. The only difference is the addition of this methyl group. Methyl-salicylic acid is volatile if dissolved in the air, salicylic acid is not. It's dissolved in water. In other words, salicylic acid is tasted by the plant, it's dissolved in the water. Whereas methyl-salicylic acid is smelled, it's dissolved in the air. Why is this important for me to emphasize? Because salicylic acid has long been known to be what's called a defense hormone in the plants. It's a potentiator of a plant's immune system. When plants are attacked by viruses, they release salicylic acid into the leaves, and this causes a number of responses that protects the plant from the viruses, or from bacteria. Now, some of you may have actually have heard of salicylic acid, because the first physician, the ancient Greek physician Hippocrates, used salicylic acid to treat fever and aches in humans. Salicylic acid is found in the bark of willow trees, and salicylic acid is used to make aspirin. So here we see that the plant's chemical, salicylic acid, affects us by reducing fever, by protecting us against sickness. But it also affects the plant, and primarily it affects the plant and protects it against attack from pathogens. How can information be transferred from one leaf to another, whether it being a neighboring leaf on the same branch or a leaf on another branch, rather far away from the one that's sending the information? How can one leaf communicate that it's being eaten by an insect? Or that it's being attacked by a virus and have that information go throughout the plant? One of the paradigms in plant biology, and we've taught this actually for several years, was that this information is always transferred through the vascular system, from the attacked leaf, through the vascular system of the plant, to the neighboring leaves. But what the results from Heil's lab and from other laboratories has shown us is this signal can also be transferred through the air, through volatile signals. And it actually may be that this is the pathway that's most often taken. That the attacked leaf, for example, gives off methyl-salicylic, the methyl-salicylic wafts through the air, is taken up by a leaf far away, and then once it goes inside, it's transferred back into salicylic acid, signaling the defense response. But this doesn't answer the question, do plants talk to each other? This does show that the communication from volatiles is utilized by one plant to help itself. But this doesn't prove that plants aren't communicating with each other. And there actually are evolutionary models, what can show some kind of advantage for coevolution of a communication of volatiles between communicating plants. Until we come up, though, with some type of plant psychologist which can ask the plant, well, what did you really mean to do, we can just keep debating this question. So to summarize today's lecture, what we've seen is that plants can smell quite a bit. They can smell small, highly volatile compounds, such as ethylene, and then there's also evidence that they can smell other compounds, such as isoprene or even acetone. Now these chemicals readily diffuse through the atmosphere, and so they might only be able to signal close by. But we do know that plants give off other chemicals, which are less volatile, such as methyl jasmonate or other things which are called green leaf volatiles. This is the smell of cut grass. We're not sure yet how these smells are integrated into plant biology. What is the mechanism by which they notice these volatiles? And so, there's obviously a huge amount of work still to go. So that concludes today's lecture. Join me next week, and we'll continue on studying what a plant knows.
