Scent stalking: parasitic vine grows toward tomato odor.

A wiry orange vine finds plants to raid for nutrients by growing toward their smell, researchers report.

One of the parasitic plants called dodders responds to volatile compounds wafting off nearby plants and shows preferences for certain species, says Consuelo De Moraes of Pennsylvania State University in University Park. They say that their new work marks the first time that anyone has shown that a plant will grow toward airborne chemicals from other plants.

The experiment finally identifies a cue-scent--that draws dodder to its victims, adds Mark C. Mescher, also of Penn State.

The U.S. Department of Agriculture lists dodder among the country's 10 worst weeds. When a dodder seed sprouts, it doesn't grow roots. All its energy goes into a tendril that shoots out in search of plants to tap for water and nutrients. If it's going to survive, it must latch on to a victim within about a week. The vine grows into a spaghetti tangle and can attack multiple plants, stunting their growth but not killing them.

Of the 150 species of dodder, the researchers selected Cuscuta pentagona, says coauthor Justin Runyon, also of Penn State. This species bedevils tomato growers in California, where it costs them an estimated $4 million a year in reduced yields.

De Moraes' team and other researchers have studied the volatile compounds released by plants that are mauled by caterpillars or other pests. In the new study, reported in the Sept. 29 Science, the team took a different point of view, looking at how an attacker, the dodder, takes advantage of volatiles to target its prey.

At first, the researchers set various possible targets several centimeters from dodder sprouts. A pot of moist soil alone didn't attract the seedlings, nor did vials of dyed water that created colored light. But a pot with a young tomato plant, and even a cup of perfume made of tomato volatiles, did attract the seedlings (see movie at www.sciencenews.org/articles/ 20060930/tomato.mov).

To minimize any confounding cues, such as shading or light, the researchers then set the possible attractants in chambers connected to the plant by curving pipes. Again, the seedlings grew toward the scent.

Testing various victim species, the researchers found that dodder grows toward impatiens and tomatoes. Wheat won't sustain dodder well, and given a choice, parasite seedlings shunned it and grew toward tomatoes.

When researchers tested seven ingredients in the tomato perfume individually, three of them proved attractive to the dodder. One of those attractants showed up in wheat, but the wheat perfume also contained a substance that repelled the seedlings. Such a repellent might offer a new route for fighting dodder, Mescher speculates.

An insect ecologist who has also studied plant volatiles, Rick Karban of the University of California, Davis comments, "The significance of this [study] to me is that it indicates that without a central nervous system, plants are capable of behaving in ways that appear fairly sophisticated."