Parasitic plants sniff out hosts, according to Penn State research

Consuelo De Moraes, assistant professor of entomology, studies green, leafy things that are, at a glance, lovely and benign. But she will be the first to tell you that even in the most lush and pastoral setting, it's a jungle out there in the plant world. For the past decade, the focus of De Moraes' work has been chemical ecology—specifically, the way that plants and insects communicate with each other. Her latest research, published in the Sept. 29, 2006 issue of Science, examines the parasitic nature of a weed called dodder (of the genusCuscuta) and reveals a plant intelligence not seen before. Dodder has the ability not only to recognize its prey by scent but also to move toward it with a remarkable accuracy and efficiency.

"We tend to think of plants as static," she says. "But then we see dodder, which exhibits an almost animal-like behavior."

The behavior De Moraes describes is dodder's assault on its victim, which in her study was the tomato plant, and it occurs with military precision. Dodder, also known as strangleweed, lovevine, and hellbind, is a viney plant, in the same family as the morning glory. It attacks its quarry by sending out strangling tendrils that wind around the host. Dodder acquires energy and nutrients by sucking out the host's phloem (pronounced "flow-um"), the essential fluid that gives a plant life. Dodder can weakenwithout killing, although its effects are often crippling. But for such a pernicious predator, it has a rather short range—seedlings can only grow about four inches before finding a host. Unlike most plants, dodder can't produce its own food through photosynthesis and does not put down roots, so host plants serve as its only source of nutrition. In order for dodder to survive, it must choose wisely. Tomato plants provide more nutrients than others, and are preferred, but a dodder will latch onto another plant if it is the only thing available.

Dodder relies on its capability to smell the tomato plant, De Moraes explains. During the normal process of transpiration— the process by which a plant releases watery vapors through its membranes—the tomato releases compounds known as volatiles. Adds De Moraes, "Volatiles are made up of alcohol, aldehydes, and terpenes—which is pretty much what you would find in a bottle of perfume."

Hooked on Plant Perfume

De Moraes began studying volatiles when she was a Ph.D. student at the University of Georgia in the early 1990s. At the time, she remembers, she read a paper by entomologist Ted Turlings (then at University of Florida and now at the University of Neuchâtel in Switzerland) that described how plants under attack by certain insects released specific volatiles.

Tomato seedling with parasitic dodder.
Photo by Justin Runyon; Courtesy De Moraes and Mescher Labs

Tomato seedling with parasitic dodder.

"I have been hooked ever since then," she says. "When I started this course of study, it was already known that plants responded to insects. At the time everyone thought the release of volatiles through transpiration was a general response. But if the plant can tell you not only 'someone feeding on me,' but who is feeding, the signal from the plant is very specific." One of De Moraes' first research projects examined how a nicotine plant being attacked by moths sent out a signal to wasps, who in turn, attacked the moths. Calling in the cavalry, as it were.

This early work looked at the insect attack-plant reaction phenomenon, and through it De Moraes discovered that plants release different volatiles when they are stressed. Between transpiration and stress reactions, says De Moraes, "We haven't found any plants yet that don't release volatiles. The most amazing thing for me, though, is that not only can parasitic plants perceive volatiles, they can distinguish among them."

The dodder study, spearheaded by De Moraes and collaborator Mark Mescher, another assistant professor of entomology, was instigated by a proposal from one of their graduate students. Justin Runyon, who is working on a Ph.D. in entomology, was interested in looking at how host plants like the tomato responded to the stress of being strangled by dodder.

"Dodder is well known," says De Moraes. "We asked the question, 'How does dodder find the host plant in the first place?' But when we started searching the literature for the answer, we were surprised that
no one knew. Mark Mescher suggested that maybe they were using volatiles."

Not Easily Tricked

In search of the mechanism that triggered dodder's attraction to the tomato, De Moraes, Mescher, and Runyon followed their collective hunch that the release of volatiles gave away the unsuspecting tomato's location. But to prove their hypothesis, the team gave dodder (specifically Cuscuta pentagona) choices of other potential prey.

"We wanted to know if the plants were using visual cues or something else," says De Moraes. "Our first question was, 'Would it find the host plant?' If you blocked the plant, would the dodder still go to it?"

The selective dodder showed absolutely no interest in an ersatz tomato plant fashioned from felt and pipe cleaners, nor was it drawn toward vials of red—or green—colored water. But when the researchers extracted scent chemicals from the actual tomato and applied them to a piece of rubber, the dodder plant immediately shot out tendrils in the direction of the scent.

Remarkably, dodder plants also displayed an ability to choose among potential hosts. When given the option of tomato or wheat, dodder choose tomato if it is available. If there is no choice, dodder will lean toward wheat, but the team also discovered that one of the volatiles released by wheat repels dodder, which, in the long run, may help researchers discover a way to control dodder infestation.

Just in the state of California, Cuscata pentagona costs farmers an estimated $4 million in reduced tomato crops each year. And there are 150 species of dodder, which attack many other cash crops, including carrots, cranberries, onions, citrus trees, and alfalfa. Farmers have a difficult time eliminating it because the herbicides that kill it also destroy the host plants.

Tomato seedling with dodder wrapping around stem.
Photo by Justin Runyon; Courtesy De Moraes and Mescher Labs

Tomato seedling with dodder wrapping around stem.

A Chemical Language

On the fifth floor of the Ag Sciences and Industries Building, within an artificially-lighted growth chamber, rows of tomato plants grow near dodder plants that already have tenacious tendrils reaching towards succulent, nutrient-rich tomato leaves. The air is humid, fecund even. De Moraes inhales and says that a sensitive nose can discern between the smell of a normal tomato plant and one in distress. Under stress, she says, the tomato emits a sweeter scent. One of the lines of inquiry she and her colleagues plan on pursuing is exactly what kinds of volatiles are being released when a plant is being attacked and what insect the plant is signaling.

In previous research that looked at the relationship between plant attack and insect infestation, De Moraes, playing a hunch, spent several nights staying awake to determine whether or not volatiles released at night were different than those released during the day. She analyzed the chemical structure, and sure enough, they were. "People asked me about staying awake all night, if it was worth it," she remembers. "At the end of the day, when you have seen something that no one has seen before, there is no better reward."

Another ongoing project of the De Moraes/Mescher lab involves the tobacco plant, Nicotiana tabacum, and its nemesis, the Heliothis moth. When the moth attacks the plant, De Moraes explains, the plant releases a volatile that attracts a wasp. The wasp invades the moth chrysalis, killing the moth larva and implanting one of its own, which emerges in the moth's place—"very Alien-like," as De Moraes describes.

"There are all kinds of stories," she says. "How do the insects and plants behave in this arms race? How are they going to sneak up on each other?" One of the lab's post-docs is looking at galls, those bulbous protrusions on plants that are caused by insects laying eggs in plants, to determine whether or not the insects are manipulating the plants or if there is a symbiotic dance occurring.

The controlling factor at the heart of all this research, De Moraes says, is a plant language, chemically communicated, that is driving the plants and insects to do what they do. "I see this as a language for both plants and insects," she says. "They can translate important information for each other."

She says that Penn State is at the forefront of the field with its support of chemical ecology, which has been a formal study here for 20 years. As an entomologist who works primarily with plants, she appreciates the interdisciplinarity afforded by her home in the College of Agricultural Sciences.

De Moraes's work has gained national attention. She recently received funding from the David and Lucille Packard Foundation, the Beckman Foundation, and a five-year career grant for young scientists from the National Science Foundation for $690,000.

"At the end of the day," she says, "we want to know how all of the different systems interact with one another to tell the whole story. But you have to progress, one question at a time." And, she adds, she is grateful that there are enough questions to keep her busy for her entire career. RPS

Consuelo De Moraes, Ph.D., and Mark Mescher, Ph.D., are assistant professors of entomology in the College of Agricultural Sciences. They can be reached at czd10@psu.edu and mcm19@psu.edu.

Gigi Marino is the editor of Bucknell World and a freelance writer and poet living in Elimsport, PA.

Last Updated May 07, 2007