Dodder is a wiry, orange vine that steals water and nutrients from other plants. Scientists have now found that this vine chooses its victim by smell, growing its shoots in the direction of a plant's natural perfume. A seedling of a vine known as dodder attaches to a tomato plant.
When a dodder seed sprouts, it doesn't grow roots to seek its own food. Instead, it grows a shoot that reaches out to other plants, tapping them for food. The baby vine needs to find a host within a week to survive. It then grows into a spaghetti tangle that can even ensnare more than one plant.
Also known as strangleweed and witches' shoelaces, dodders are listed among the 10 worst weeds in the United States. They can cost farmers millions of dollars by stunting their crops.
To figure out how a type of dodder vine known to prefer tomato plants finds a victim, scientists placed dodder sprouts near several possible targets. These targets included pots of moist soil, little jars of dyed water that created colored lights, young tomato plants, and even a cup of perfume made from chemicals that tomato plants give off.
Seedlings grew toward the tomato plant. They also reached out toward the cup of tomato perfume. They tended not to grow toward the moist soil or colored water.
The scientists then used a different setup, hiding the targets in chambers connected to dodder sprouts only by curving pipes, so the vine could find them only by smell. Dodder sprouts still grew toward their favored targets.
By placing dodder sprouts near different plants, the scientists found that the type of dodder that they were studying prefers tomatoes and a flower called impatiens. And when given a choice between tomato and wheat, vine seedlings grow toward tomato.
The researchers then tested seven of the ingredients that make up tomato perfume separately. Dodder sprouts were attracted to three of them.
One of these ingredients turns up in wheat perfume, but the wheat perfume also contains a substance that repels dodder sprouts. This chemical could offer farmers one way to fight the vine and save their crops.—C. Gramling
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 http://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."
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