Plant Responses to the Environment — Six Graduate Student Projects

Why Insects Dine at Night

You may think a plant just sits there when an insect chews on it. But plants can change the composition of their flesh at will, making themselves unpalatable or even toxic to a predator. "This occurs whether plants are being attacked by insects, bacteria, fungi, or viruses," says graduate student Anne Walton. To some plants, however, such tricks may be costly. "Plants growing in low light are more susceptible to insects and disease," Walton says. "I believe this may have to do with limited resources."

Imagine that plants have a budget. They gain their "currency," or sugar, in the chloroplast the site in the cell that converts sunlight, water, and carbon dioxide into sugar and energy. Under normal conditions, the sugars are either sent to growing leaves or stored as starch to be accessed later. Because plants growing in bright light have more access to sunlight, they are richer in sugars. They may always have enough sugar to spend on weaponry. But the same plants, growing in low light, may have to budget their resources. "Plants growing in shade or darkness have to make a choice," Walton says. "Will they continue to grow, or will they defend themselves?"

Entomologist Jack Schultz, Walton's adviser, found this idea interesting because insects feed primarily at night. "Insects may do this to strike plants when their guard is down, but the biological mechanism has not been studied," she says.

Walton is using her background in plant biochemistry to test hypotheses about which biochemical pathways in the plant cell determine when, how fast, and how well a plant can respond to a challenge. Because many of the biochemical pathways that make defensive chemicals are found in the chloroplast, she believes that's where the trade-off between growth and defense occurs.

To figure out how plants allocate resources during an attack, Walton will observe the behavior of genetically- engineered tobacco plants, which were purposefully created with defects in their sugar/starch pathways. Her goal is to find the point in the pathway where resources are rerouted from growth into defenses. "Are the resources rerouted from chloroplast sugars, or do the resources come from the chloroplast's starch reserves?" Walton says. "I'll assess the defensive response by measuring concentrations of phenylpropanoids common defensive compounds produced from the chloroplast sugars and one of the types of chemicals that gives smoking tobacco its rich brown color."

"Anne's work exemplifies the goals of the training program," says Schultz. "She'll start with an experimental system where she has a lot of control, then eventually carry that work back to a natural system. She'll figure out the fundamental ways in which a plant does something, then take it back into an ecologically realistic setting to assess the ecological consequences for a real plant growing in a real environment."