Plants, like humans, are plagued by biting insects. But because they can't run inside or spray themselves with repellent, they have evolved an arsenal of nasty compounds to ward off the bugs. Now, it turns out, at least one pest can detect the signals that cue these defense compounds in time to subvert them. That could doom farmers' plans to protect crops by artificially stimulating plant defenses.
Insects and plants have been locked in evolutionary warfare for eons. Plants make toxins to deter attackers, and most toxins are produced on demand, cued by chemical signals. Insects, for their part, have come up with enzymes that can break down the toxins, but nobody knew if they could respond to the handful of signaling compounds to get a head start in countering the toxins.
If any insect is capable of spying on plant signals, it would be the corn earworm, reasoned entomologist May Berenbaum and colleagues at the University of Illinois, Urbana-Champaign. This ubiquitous crop pest can eat more than 100 species of plant (it's also known as the tomato fruitworm and the cotton bollworm), a rarity among herbivorous insects. To find out if it activates its countermeasures in response to plant signals, the team exposed the insect to two such signals--jasmonate and salicylate--and to celery plants that had started to produce them but were not yet producing actual defense compounds. All of the stimuli caused the earworm to activate four cytochrome P450 genes, which are known to detoxify plant defense compounds, the team reports in the 17 October issue of Nature. They concluded that the bugs were "eavesdropping" on plant signals. This could foil plans by some crop-protection experts to protect plants by stimulating them with jasmonate or salicylate.
"The finding makes good adaptive sense," says entomologist Jack Schultz of Pennsylvania State University in University Park. However, he says, it doesn't confirm that insects respond to plant signals. The "eavesdropping" interpretation could be accurate, or something else unexpected could be going on, he says: "Nothing about interactions between insects and plants surprises me."