Plant Responses to the Environment — Six Graduate Student Projects

Glowing Green Potatoes

Veterinary science meets plant pathology in graduate student Steve Pechous's research on lipoxygenase, an enzyme common to plants and animals. In animals, lipoxygenases produce hormone-like signaling molecules that stimulate defense responses. In plants, lipoxy-genases produce compounds such as methyl jasmonate, a hormone emitted by wounded plants that tells other tissues and possibly even other plants to start pumping up their defenses.

"Potato tubers contain incredibly high amounts of lipoxygenase, but why is it there?" says Pechous. "One theory is that lipoxygenases play an integral role in tuber resistance to Phytophthora infestans, the fungus that causes late blight." One of the most famous plant diseases in history, late blight caused the Irish potato famine of the 1840s. Late blight also attacks tomatoes. "There's been renewed interest in finding plants with resistance to late blight," Pechous says. "New, highly virulent strains of the fungus have begun to surface. Some farms have even gone under in the past few years as a result."

Researchers have pinpointed a chemical in the fungus, arachidonic acid, that stimulates the potato's defense response. Somehow, arachidonic acid triggers the biochemical events that ultimately tell the genes for defensive proteins to be switched on. "But we don't have definitive proof yet that lipoxygenase is involved," Pechous says.

To prove that lipoxygenase is part of this process, Pechous must show that the product that results from the interaction of arachidonic acid and lipoxygenase a fatty acid with an oxygen added to it is indeed the signal that turns on the genes for the defensive proteins. Thanks to a fluorescent jellyfish, he may be able to put a drop of this oxygenated fatty acid on the potato tissue and literally watch the defensive genes light up.

Pechous will hook a special green fluorescent protein (GFP) "reporter gene"a gene isolated from the jellyfish to the potato's defense gene control elements. The reporter gene will tell him when the natural defense genes become activated. The GFP emits a bright green or blue light when it is exposed to UV light and viewed under a microscope. Since their first use in 1994, GFP's have been used to light up everything from mosses to cancer cells. "If lipoxygenase is involved," Pechous says, "adding the oxygenated fatty acid to these cells should make the potato produce the GFP and glow brightly."

If lipoxygenase turns out to be a key enzyme in the plant's defensive pathway, Pechous's work could be used to breed a more resistant potato. "It also may give us some clues about how these plants can respond to other stresses," he says. "Because many of these defense pathways interact, we could learn a lot about resistance to other pathogens, as well as drought, ozone, and wounding."

"What's interesting about Pechous's work is that it opens up the possibility that plants could have an 'immune response,'" says entomologist Jack Schultz. "We already know that lipoxygenase is involved in the defense response of many plants, but usually it uses a fatty acid from the plant, a by-product of being damaged or attacked, to turn on defenses. But Steve and his colleagues are studying the possibility that lipoxygenase also can use a fatty acid from the blight fungus that is, an acid produced by the attacker. This means that the potato may have the ability to recognize particular enemies and turn on defenses based on who its attacker is, in much the same way the human body responds differently to different agents."