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Science 29 September 2006:
Vol. 313. no. 5795, p. 1867
DOI: 10.1126/science.313.5795.1867a
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News of the Week

PLANT SCIENCE:
Parasitic Weed Uses Chemical Cues to Find Host Plant

Elizabeth Pennisi

Dodder may be the bloodhound of the plant world. A plant that parasitizes other plants, it sniffs out its victim, Justin Runyon and his colleagues at Pennsylvania State University in State College report on page 1964. "This is a pretty cool example of plants behaving in a way most people think only animals behave," says Richard Karban, a community ecologist at the University of California, Davis. It's also an effective strategy: Dodder ranks among the U.S. Department of Agriculture's top 10 noxious weeds.

Figure 1 Gotcha. A plant that preys on other plants, dodder winds its way up its host.

CREDIT: COURTESY OF THE DE MORAES AND MESCHER LABS

The work bolsters the notion that plants have a chemical language, an idea that's been hotly debated for the past 2 decades. "The results go a long way toward convincing people that plant-plant interaction via volatiles is a real phenomenon," says Eran Pichersky, an evolutionary biologist at the University of Michigan, Ann Arbor.

Barely able to carry out photosynthesis, dodder survives by attaching to the stems and leaves of other plants and robbing them of nutrients. A relative of morning glories, it has many names--goldthread, strangle-weed, witches' shoelaces--that aptly describe the dense, yellowish mats that blanket its hosts, reducing agricultural productivity by as much as 90%.

Runyon, a graduate student working with Pennsylvania State chemical ecologists Consuelo De Moraes and Mark Mescher, wanted to know how dodder found its mark. He placed seedlings of the dodder species Cuscuta pentagona in small vials fitted with a collar of filter paper on which he traced their growth and found that 80% of them headed toward a tomato plant placed nearby. He then put seedlings in an open-air chamber with two 90-degree side tunnels, one of which led to a chamber containing four tomato plants and the other to a chamber with four artificial plants; the seedlings could catch a whiff of the plants through the tunnel. About 77% of the seedlings grew toward the actual plants, the group reports. Runyon then replaced the plants with a vial of plant extract and the fakes with a vial containing only solvent. Again, the dodder seedlings made the right choice.

The seedlings also grew toward touch-me-not plants (Impatiens) and, to a lesser extent, wheat. But when given the choice, the dodder avoided wheat, a poor host, in favor of tomato. Runyon discovered that wheat emits a chemical that somewhat repels the dodder--a finding with possible practical implications, given that dodder is so hard to control. This result, says De Moraes, "suggests the possibility of using volatiles to enhance plant defenses, either by applying repellent compounds or perhaps by engineering plants to produce them."

Past studies have indicated that plants under attack from herbivores emit signals telling nearby plants to boost their chemical defenses. But some researchers have been dubious about this evidence of plant-plant chatter, arguing that the experiments took place in closed chambers where artificially high concentrations of odors built up. Runyon's experiments were "rigorously conducted in an 'open' experimental design, so it's hard to argue that the responses they observe in the greenhouse are not occurring in the real world," says Ian Baldwin of the Max Planck Institute for Chemical Ecology in Jena, Germany.

Many questions remain about how plants perceive the still-unidentified volatile signals. But there will be rapid progress, predicts Andre Kessler, a chemical ecologist at Cornell University. Runyon and his colleagues, he says, have "opened up a new door that can bring us closer to the understanding of airborne plant-plant interactions."

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Science. ISSN 0036-8075 (print), 1095-9203 (online)