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Eavesdropping Plants

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To catch up on the latest gossip, all we humans have to do is pick up the phone. But researchers have found that in nature, news often travels by way of chemicals carried on the wind. In this Science Update, you'll hear how some plants eavesdrop on their neighbors to find out how they're doing.


Transcript

Nosy plants. I'm Bob Hirshon and this is Science Update.

Some people like to eavesdrop on their neighbors to keep up with what's happening next door. Now it appears that some plants do the same.

Rick Karban is an ecologist at the University of California, Davis. He and his colleagues have found that wild tobacco plants in the Sierra mountains are able to sense the condition of neighboring sagebrush.

In a recent study, Karban clipped sagebrush leaves to mimic the effect of insects and animals feeding on the plant.

Karban:

What I found repeatedly was that the tobacco plants that were downwind from sagebrush that I had clipped were eaten less by their herbivores than the tobacco plants that were downwind of unclipped sagebrush. So it looked as if maybe some kind of communication was occurring out in the field under natural conditions.

Karban suspects the tobacco is sensing the plant hormone methyl jasmonate. Sagebrush releases large amounts of the hormone when its leaves are damaged.

Karban:

So it's possible that tobacco plants get sort of an early warning of the kind of environment that they're going to experience a little bit later on.

And that way, they'll have time to mount the chemical defenses they'll need to survive. For the American Association for the Advancement of Science, I'm Bob Hirshon.


Making Sense of the Research

Although plants don't have nearly the same range of behaviors that animals do, they aren't totally passive. This study offers just one example of the many sophisticated ways that a plant can respond to its environment.

This study focuses on plant defense systems, which are kind of like our immune systems. In many cases, when plants get damaged, they produce chemicals that help protect them from further damage. That's what happened to the wild tobacco plants in Karban's experiment. The difference is that the plants weren't reacting to their own damage, but to damage to nearby sagebrush.

After this experiment, Karban's team performed other studies to try and pin down exactly what was going on. First, they wanted to find out if the signal from the sagebrush was actually traveling through the air, and not the roots of the plants (sagebrush and wild tobacco often grow in dense patches with intertwined roots). So they repeated the experiment, except this time they sheathed the plants' roots in plastic so they couldn't touch one another. The results were the same, suggesting that the signal from the sagebrush was indeed airborne.

Next, they tried to replicate the experiment under more naturalistic conditions. So instead of transplanting tobacco and sagebrush for the experiment, they studied plants that were growing together naturally. And instead of clipping the sagebrush with scissors, they let the herbivores actually eat it. Again, they saw the same effect, but only when the plants were within a few inches of each other.

This kind of communication is probably not unique to sagebrush and wild tobacco, but Karban says they do seem to be an especially well-matched pair. Sagebrush produces huge amounts of methyl jasmonate, the chemical that appears to trigger the defenses of many plants, including wild tobacco. When you cut a sagebrush plant, all that methyl jasmonate gets released into the air. In contrast, wild tobacco plants don't seem to react when other wild tobacco plants get chomped on, which makes sense when you consider that wild tobacco produces methyl jasmonate in relatively small quantities.

However, this probably isn't the whole story. Wild tobacco also seems to be more sensitive than most other plants to signals from sagebrush, although this hasn't been extensively studied. In nature, animals and insects that eat sagebrush usually eat wild tobacco as well, but they tend to eat the most sagebrush in the springtime, when wild tobacco is still germinating. So getting an "advance report" on the health of the sagebrush might help wild tobacco build up its defenses before it's ever attacked.

You might ask why the plants just don't crank out lots of defense chemicals automatically, to protect them no matter what. The answer isn't known for sure, but in cases like these scientists usually assume that there must be some kind of cost to producing the chemicals. At the very least, it uses up energy that might otherwise be devoted to making food, growing leaves and roots, and other important business. This principle of energy conservation is seen throughout the plant and animal kingdom: it's the same reason why your muscles get small and flabby if you don't exercise. Muscle takes energy to build, and your body doesn't put in the effort if it thinks you won't need it. For the same reason, these wild tobacco plants may not want to bother boosting their defenses unless they have specific, credible information about a plant-eating threat.

Now try and answer these questions:

  1. How do wild tobacco plants sense when sagebrush is being eaten?
  2. How did the researchers narrow down the way this works?
  3. What are some reasons for wild tobacco and sagebrush to have this relationship?
  4. In the three experiments described above (testing transplanted sagebrush and tobacco, repeating the experiment with sheathed roots, testing in wild plants), what conditions do you think the experimental plants were compared to? Give reasons for your answer.
  5. How might this research be applied to farming technology—for example, in making alternatives to pesticides? What kinds of studies would you need to do if you wanted to apply the research in this way?

For Educators

The University of California Botanical Garden has information about plants from many different ecosystems.

Bug Bodyguards, from the Ontario Science Centre, describes another kind of chemical signal used by tobacco plants.


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