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Critter Chemistry

April 2, 2001


Plants Signal Friends, Foes with Scents that Change as Day Turns to Night

Maureen Rouhi

Plants under attack release a mixture of signaling compounds that varies with time of day, a recent study shows [Nature, 410, 577 (2001)]. Consuelo M. De Moraes and James H. Tumlinson at the USDA Agricultural Research Service, in Gainesville, Fla., and Mark C. Mescher at the University of Georgia discovered this new angle of plant-pest interaction.

NO VACANCY Release of nocturnal compounds induced by grazing caterpillars prevents female moths from laying eggs on infested plants.

"We still lose about 10% of the plant food we produce to insects," notes May R. Berenbaum, an entomology and plant biology professor at the University of Illinois, Urbana-Champaign. "Any insight into the subtleties of those interactions can lead to better ways to reduce losses."

During the day, tobacco plants infested by moth caterpillars (Heliothis virescens) emit significant amounts of the terpenes (E)--ocimene and -caryophyllene. At night, the terpene levels fall dramatically, while four esters and an unidentified compound are added to the mix, the study shows.

The nocturnal compounds repel pregnant moths looking for nests--which they do at night--by emitting compounds that signal that the plant is full of caterpillars, so that the moth will search elsewhere. The plant wards off further infestation, and the moth avoids a place where its offspring will meet stiff competition.

The diurnal terpenes attract wasps, which prey on caterpillars. Pregnant wasps sting the caterpillars and inject them with their eggs. The caterpillars die as wasp larvae eat their way out. The plant defends itself by calling on its enemy's enemy.

"We're beginning to recognize that volatile signals have multiple functions," says Ian T. Baldwin, founding director of the Max Planck Institute for Chemical Ecology in Jena, Germany. For example, his recent work shows that, when attacked, natural populations of coyote tobacco plants emit compounds that attract insects that eat the herbivores' eggs and other substances that prevent egg laying by female herbivores [Science, 291, 2141 (2001)]. However, the time-dependent variation of signals to friends and foes described by De Moraes and coworkers has not been recognized before, Baldwin says.

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