Animals that chomp on young plants may actually be doing them a favor, according to a study in this week's Science. Wild radishes that are nibbled early in their lives respond by beefing up their defenses against later grazers, and as a result live longer and healthier than plants that escaped nibbling. The finding may persuade farmers to begin using chemical treatments that can artificially switch on these defenses in crops.
Researchers already knew that when plants are partially eaten by insect pests or larger grazing animals, they often step up their production of poisonous or distasteful compounds, such as mustard oil, or develop a thicker protective coating on their leaves. In principle, turning on these defenses early in a plant's life would help it thrive. Indeed, salicylic acid, a chemical that activates the natural defenses in certain plants, is already being marketed as a protective treatment. But so far, no one has proven that plants really do better when these defenses are activated early in life.
Anurag Agrawal, a graduate student at the University of California, Davis, studied wild radishes to look for the long-term benefits of these defenses. He allowed caterpillars to munch on one leaf of each of several 4-week-old plants, but protected another group from all hungry herbivores. Two weeks later, he exposed both groups to insects and other pests and compared their fate. The nibbled plants suffered about half as much damage from insects and 30% less colonization by aphids as the control plants. The nibbled female plants also produced 60% more seed mass than controls, and male plants made about 60% more flowers, another sign of health.
"It's a very exciting result," says Robert Marquis, a plant ecologist at the University of Missouri in St. Louis. "We've known about induction of plant defenses for many years, but we've never had actual proof that it was beneficial." Still, says Agrawal, researchers should be cautious about treating crops with chemicals to induce their defenses before the full implications are understood. Inducing plant defenses can stress the plants, he explains, adding, "We've found over and over that one kind of resistance can lead to susceptibility to different attackers."