Sending out an SOS

In the cult classic ''Little Shop of Horrors," a talking plant capable of self-defense made for good theater. Turns out, it also makes for good science.

Recent research has shown that a wild tobacco plant can call for help when its leaves are being chewed up by a bug. The plant, Nicotiana attenuata, can send out a chemical signal that attracts a carnivorous insect to eat the offending bug. What's more, neighboring plants appear to eavesdrop on this signal and ready their own defenses in case the plant-eating bugs attack them next.

''Plants live in another temporal rhythm, but they behave just like animals," said Marcel Dicke, an entomologist at Wageningen University in the Netherlands who has made a career of studying plant-insect communication. ''They are not passive in respect to defending their body -- they have to be smart. After all, they cannot run away or punch their attacker in the nose."

The consequences of these smart responses go well beyond entertainment. If scientists could identify the genes responsible for these calls for help, they could perhaps genetically engineer plants -- say, a corn plant -- to send out an insect SOS, saving farmers a fortune on chemical-filled pesticides.

When entomologist Ian Baldwin of the Max Planck Institute for Chemical Ecology in Jena, Germany, first proposed in 1982 the notion that plants could communicate, fellow scientists scoffed. Today, the theory that plants talk to insects is widely accepted, and plant-to-plant communication is gaining ground as well.

When plants are injured, they emit organic compounds called volatiles into the air, ready to be detected by passing bugs or nearby plants. Volatiles are divided into two classes, both of which often act as a plant's first line of defense against insect attack, according to Jack Schultz, an entomologist at Penn State University.

Green leafy volatiles are produced when a plant is injured in any way -- cut by scissors, grazed by a deer, or stomped by the neighborhood kids. Every plant species ever studied produces this class of volatiles, perhaps to make the plant less savory, or even toxic, to insects.

The second class, which scientists are still trying to understand, is more injury-specific. The plant releases chemicals targeted at the specific predator that's attacking.

Take the Nicotiana attenuata example. If a hornworm is nibbling the tobacco plant's leaves, the plant releases a volatile that attracts a Western Big-eye Bug, which eats the hornworm.

''If you are a plant and you are attacked by an herbivore that is going to kill you and you know there are carnivores out there that would eat the herbivores that are killing you, wouldn't you want to give them a call?" Dicke said.

Dicke is searching for the genes involved in the production of volatiles that stimulate these plant-insect chats. The sheer immensity of the plant genome -- some plants, such as Arabidopsis, have more genes than humans -- is a bit daunting, Dicke admitted.

Last fall, his group collaborated with a team at the Weizmann Institute of Science in Israel to create a transgenic Arabidopsis plant that expressed a gene found in strawberry plants, which scientists think is involved in volatile production.

When wounded, the transgenic Arabidopsis released the same volatiles as the strawberry plant, attracting carnivorous mites that help it defend itself, according to a paper published last fall in the journal Science. The next step will be to see if the plant produces the volatiles when herbivorous insects attack. If it does, scientists may be able to insert this gene into other plants, giving them their own pest-control system.

Baldwin is taking a different tack with his research. He discovered some time ago that when a plant sends out that chemical SOS, neighboring plants perceive the call and prepare so they can respond faster if they get attacked, too. Now he's trying to better understand how this eavesdropping works by seeing what happens when plants can't communicate.

To confirm this finding, Baldwin developed ''mute" transgenic plants by turning off a gene the plant needs to emit volatiles. He placed the mute plants next to normal plants and exposed the mute ones to herbivores. The transgenic plant couldn't send out an SOS, and its healthy neighbors took no action.

He did the same experiment in his lab in Germany with plants he'd made ''deaf" by disabling a gene the plant needs to sense volatiles in the air. When exposed to herbivores, the healthy plants began emitting volatiles, while its deaf neighbor did nothing.

Now he's duplicating his lab experiments at his field site in the Great Basin Desert in southwestern Utah. The plants have been placed at varying distances from each other so that Baldwin can measure, based on whether the plants react, how far volatiles can travel in the air. ''We're using the plants as reporters," Baldwin said. ''They'll tell us what's going on." He hopes to have findings from that work in the next few years.

Penn State's Schultz has similar aims, although rather than using plants as reporters, he plans to use insects and computers.

Insects use highly sensitive antennae to sense organic compounds in the air that help them find mates, avoid predators, and locate plants, either for food or as a place to lay their eggs. The information is first picked up by receptors on the antennae, which send electrical impulses to the brain.

To mimic this process, Penn State researchers, led by Tom Baker, cut off the heads of moths -- whose antennae can continue to gather information for up to four hours after the decapitation -- and put them into a device that can decipher the signals the antennae pick up. Researchers hope to use this device to identify the organic compounds the moth can sense in the air, including, in theory, volatiles released by plants.

''If it works, we'll be able to detect signal molecules flying from plant A to plant B," said Schultz, ''which could give us a lot of information we've never been able to collect before."