Plants muster defenses when they 'hear' an attack

In the popular Plants vs. Zombies game, there are many ingenious ways that the plants defend your home from the brain-eaters. But they kind of have an unfair advantage. They can see. And shoot peas. And launch cabbages.

In the real world, plants have to rely on other mechanisms to combat the forces that would do them harm. At the University of Missouri (MU), researchers have just discovered that one of these defense mechanisms can be triggered when plants sense the vibrations from caterpillars munching on their leaves.

Heidi Appel, senior research scientist, and Rex Cocroft, professor in the Division of Biological Sciences at MU, placed caterpillars on a plant known as Arabidopsis that's similar to cabbage and mustard. They then recorded the sound of the insects' munching using a special laser microphone. Next, they divided plants into two groups: one was played the sound of the munching and the other was presented with silence.

What the researchers discovered was that the plants that "heard" the sounds of caterpillars munching on leaves produced more mustard oils, a compound that's toxic to caterpillars. In effect, the plants primed themselves for an attack simply by hearing the sounds of one.

"Caterpillars react to this chemical defense by crawling away, so using vibrations to enhance plant defenses could be useful to agriculture," Appel said in a statement. "This research also opens the window of plant behavior a little wider, showing that plants have many of the same responses to outside influences that animals do, even though the responses look different."

What's even more interesting is that the studies showed that the plants could tell the difference between vibrations caused by wind or those that "share some acoustic features with caterpillar feeding vibrations," according to Cocroft. They only increased the mustard oils when the specific sound of caterpillar munching was played.

"Previous research has investigated how plants respond to acoustic energy, including music," said Appel. "However, our work is the first example of how plants respond to an ecologically relevant vibration."

But just how the plants sense the vibrations from marauding insects is still a bit of a mystery to the scientists -- and it's one that they'll be working to unravel during the next steps of their research.