Parasitic Plants Sniff Out Hosts

Parasitic plants do not haphazardly flail about looking for a host but sense volatile chemicals produced by other plants and identify potential hosts by their emissions, according to a team of chemical ecologists in Penn State’s College of Agricultural Sciences.

“We are interested in how plants respond to their environment and have looked at plantinsect interactions,” says Consuelo De Moraes, assistant professor of entomology. “It was surprising to see how little was known about how aboveground parasitic plants find their hosts from far-off.”

The researchers looked at Cuscuta pentagona, field dodder or five-angle dodder—a plant that infests a variety of crops including tomatoes, carrots, onions, citrus trees, cranberries, and alfalfa—and reported their finding in a recent issue of Science. Dodder grows throughout the world and is a difficult pest to eliminate because chemicals that kill the parasite also often kill the host plant.

"There is currently no reliable way to get rid of these pests,” says Justin Runyon, graduate student in entomology. “It is estimated that in California each year, a 20 percent infestation of the tomato crop reduces yield by 25 percent and causes a loss of $4 million.”

The researchers used a variety of experiments to determine how newly emerging dodder shoots find a host. The length of time these parasites can live without a host is determined by the amount of food stored in the seed, but they can only grow about four inches before they die.

“These plants have no roots and barely have leaves, and the flowers are very tiny,” says Mark Mescher, assistant professor of entomology.

First, the researchers placed dodder seedlings in a water vial at the center of a filter-paper disk. A tomato plant was placed near the edge of the disk, and the dodder plant was allowed to grow and attempt to locate its host. Dodder plants search for hosts by growing and moving in a circular pattern. In the past, many researchers assumed that the search was random and the location of a host simply a chance encounter. After four days, when the plant was growing flat on the filter paper, the scientists measured the direction of the shoot.

The researchers report that 80 percent of the dodder plants grew onto the side of the filter paper nearest the tomato, with many growing directly towards the tomato plant. Statistical analysis provided strong evidence for directed growth by dodder but did not indicate what causes the directionality.

The Penn State researchers then challenged dodder seedlings with artificial tomato plants, pots of moist dirt, and vials of green or red water. None of these objects elicited any directional growth. Then, to narrow down the possible cues being used, they tested the seedlings’ response to tomato plants slightly separated from the dodder seedlings—out of view, so to speak— in a setup designed to block possible light cues. The researchers observed a growth response toward the tomato plants similar to that in their first experiment.

Finally, to firmly establish that volatile chemicals from the host plant were causing this response, the researchers used the same setup to examine the response to extracted host volatiles, using a solvent- only sample as a control in the opposite direction. They again observed a strong growth response toward the tomato volatiles.

“This showed that host volatiles elicit a growth response in the absence of any other plant-derived clues,” says Mescher. “However, while volatile chemicals might be key, our results do not rule out the possibility that other cues, such as light or shade, may play a role.”

After establishing the role of volatiles in leading the parasite plants to their tomato hosts, the researchers looked at other potential hosts, including wild impatiens, and showed that the parasites were attracted to a wide variety of plants. They even found attraction to wheat plants, a poor host on which the dodder seedlings do not survive. However, when the researchers offered the seedlings a choice between wheat and tomato plants, the tomato won out, indicating that the parasites have some way of deciding between a good host and a bad host.

The researchers also examined responses to some of the individual compounds released by host plants. Of seven compounds tested from tomato, three caused a directional growth response in dodder. One of these chemicals is also released by wheat, which might explain why wheat is somewhat attractive despite being a poor host. However, another chemical compound from wheat actually repels the dodder seedlings, perhaps explaining why the odor of wheat is less attractive than that of the preferred host tomato.

The Penn State researchers note that the identification of at least one repellent compound raises the possibility of eventually using airborne chemicals to deter plant parasites. Looking forward, they also would like to determine exactly how the parasites are able to sense and respond to host volatiles. They hope to identify the specific chemical receptors involved. In addition, they are examining the defensive mechanisms by which host plants respond to attack by parasitic plants.

—A’ndrea Messer