HARVARD GAZETTE ARCHIVES

Kristin Lewis, a junior fellow at the Rowland Institute at Harvard, is studying plant parasitism. Parasitic plants attach themselves to host plants' roots or stems and suck nutrients from them. (Staff photos Stephanie Mitchell/Harvard News Office)

Kristin Lewis pulled open the door of the large green, refrigeratorlike box, one of six in her Rowland Institute lab, releasing a puff of warm, humid air and a beam of white light that seemed blinding in the cavernous lab's dimness.

Arranged in neat rows inside are potted plants, some standing 15 inches tall and topped by pink flowers.

But a closer look shows that there are two plants in each pot, not one. The second plant, low-growing and hugging the pot's soil, would be easy to miss as one's eye is caught by the showy flowers on the first plant's spike.

Though the low-growing second plant seems an afterthought, Lewis explained that without it, there wouldn't be a first plant.

Lewis, a junior fellow at the Rowland Institute at Harvard, is studying plant parasitism. Parasitic plants attach themselves to host plants' roots or stems and suck nutrients from them, often, as in the case of the showy pink flowering parasite and the low-growing host, to the host plant's detriment.

Kristin Lewis

Parasitic plants are not just a biological curiosity. Every year, parasitic plants damage farmers' fields, particularly in Africa. Seeds of parasitic plants blow in from surrounding environments or are deposited in bird droppings. The plants that grow from those seeds attach to roots and stems, sucking vital nutrients, stunting the crop plants' growth, and reducing yields.

To foil the parasites, African farmers have adopted the practice of planting fields twice, first with plants that are resistant to the parasites and then, after the parasites germinate and die, with their desired crop, such as sorghum.

"Most of them are very problematic plants," Lewis said.

Lewis became interested in parasitic plants while studying plant defenses. Though they can't fight back as an animal would, plants can generate a wide variety of substances that are toxic or distasteful to insects and other browsers.

In fact, some of our best-known commercial plants are popular exactly because of the unique qualities of the plant's chemical defenses. Caffeine and nicotine, for example, as well as the hot spices contained in pepper plants, are all intended by the plant to discourage herbivores.

Members of Lewis' research team at the Rowland Institute Leslie Wright (left) and Janice Alers-Garcia put caterpillars in potted plants as part of the experiment on parasitism.

After studying plants' reactions to insects eating their leaves, Lewis became interested in the interaction of a plant and its parasite.

She already knew that in some cases a parasitic plant shares the host plant's defensive chemicals, as well as nutrients and carbohydrates. What she wants to find out is how much the two plants communicate.

If an insect attacks a parasitic plant, that insect is just a short distance from the host plant. Though normally the host wouldn't benefit from helping its parasite, it's possible the parasite could manipulate the host into shifting more defensive chemicals into the parasite to keep the leaf-eating insect at bay - over there. That would require some sort of communication from the parasite to the host saying, in essence: "Send help fast. It's BITING me!"

Lewis said she's still gathering data, but she remains fascinated by the active responses to the environment of plants, which on the surface seem so passive.

"I've always been interested in the active responses that plants have to environmental conditions," Lewis said. "We think of plants as the green stuff that's out there, but they're really active."