「植物看得見你」公開課筆記／3.7 Inter and Intra Communication

There is another hypothesis, could it be that the neighboring trees are simply eavesdropping? In other words, the attacked tree is giving off the gas in order to help itself (i.e. sister branch) but not for communication to the neighbour trees.

What type of experiment can we do to check if the internal gas being given off helps also its own plant? One experiment was done by one of Baldwin's students, Martin Heil, who's in Mexico. What he wanted to know, could you find the same signalling between plants?

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, started reacting as in Baldwin's experiment.

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.

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.

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.

When he didn't isolate the leaves, its neighboring leaves on the same branch were also resistant to the pathogens.

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.

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. 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.

So then, of course, the next question is, what is the active volatile chemical that is released by the attacked leaves?

If it's insects, for example, the leaves are primarily giving off a chemical that's called methyl jasmonate. If it's bacteria or viruses, the leaves give off a chemical that's called methyl-salicylic acid.

Because methyl-salicylic acid is very similar to another chemical in the plant called salicylic acid. 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.

The first physician, the ancient Greek physician Hippocrates, used salicylic acid to treat fever and aches in humans. It is found in the bark of willow trees, and salicylic acid is used to make aspirin.

But it also affects the plant and protects it against attack from pathogens. 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 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.