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Parasitic weed can sniff out its prey

Published September 29, 2006

WASHINGTON - The parasitic dodder plant doesn't have a nose, but it knows how to sniff out its prey.

The dodder attacks such plants as tomatoes, carrots, onions, citrus trees, cranberries, alfalfa and even flowers, and is a problem for farmers because chemicals that kill the pesky weed also damage the crops it feeds on.

So discovering how it finds its prey might help lead to a way to block the weed, or for crops to defend themselves, say researchers at Pennsylvania State University.

The question of how dodder finds a host plant has puzzled researchers. Many thought it grew in a random direction, with discovery of a plant to attack being a chance encounter.

But the researchers led by Consuelo M. De Moraes found that if they placed tomato plants near a germinating dodder, the parasite headed for the tomato 80 percent of the time.

And when they put scent chemicals from a tomato on rubber, 73 percent of the dodder seedlings headed that way.

"It opens a new avenue" for understanding parasitic plants, De Moraes said in a telephone interview.

Co-author Mark C. Mescher said: "One of the interesting things we found was that the plants make choices."

When they gave the dodder seedlings a choice between a tomato plant and a wheat plant, they preferred the tomato.

Dodder will infect wheat if there is no choice, he said, but they discovered that one of the volatile chemicals given off by wheat repels dodder, so it will choose the tomato if allowed to pick.

So, finding one compound that tends to be repellent could lead to ways to either treat crops to resist dodder or even engineer them to produce the compound themselves, Mescher said.

While they were attracted to tomato plants and the chemicals they release, the dodders showed no particular interest when offered a fake tomato plant, a pot of moist soil or vials or red or green colored water.

The plants don't have a nose, of course, so it's not clear how they sense the chemicals given off by potential host plants. When the seedlings start out, they tend to rotate in various directions, and they somehow sense the direction where the chemicals are strongest and then grow toward them, Mescher said.

He said many plants respond to light by tilting to one direction or another.

And, Mescher said, some research has indicated that plants may be able to sense chemicals emitted by other plants that are attacked by insects.

Their research on dodder, listed as one of the top 10 noxious weeds by the Department of Agriculture, appears in the journal Science. Dodder is also known as strangleweed.

[Last modified September 29, 2006, 01:20:55]

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