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Utah biologists working deep in tropical rain forests have found a new way to identify which plants are rich in exotic chemical compounds - the potential sources of medicines that someday may be used to combat cancer, AIDS or other diseases.

The therapeutic value of plant compounds has been recognized for centuries. Until now, finding which among tens of thousands of plant species have pharmaceutical value has been hit-or-miss. But the Utahns - biology professors Phyllis D. Coley and her husband, Thomas A. Kursar, along with their colleague Todd L. Capson - were able to improve the odds.In one test, they chemically analyzed 105 species spotlighted by their technique. "Forty-four percent of ours came out as active," Coley said in a talk Thursday. That is, 44 percent were rated as having chemicals so potent that they were effective against such disorders as colon cancer.

By comparison, only four percent of plants that the National Cancer Institute studied had "active" compound. The Utahns' record was 10 times as good as that of the NCI.

Coley presented a seminar at the University of Utah's chemistry department Thursday, speaking in the Skaggs Hall on the topic, "Chemical Defenses of Rain Forest Leaves: Can Ecological Insight Help in Drug Discovery?"

"Tom and I have spent the last 20 years trying to figure out why basically the insects haven't eaten up all the plants," she said.

They found that rain forest plants' leaves may live for many years, but they are most vulnerable to ravages by bugs when they are young and tender. "It's a general pattern that young leaves are hard hit," she said.

Of plants whose leaves may live for eight or 15 years, 80 percent of the damage they ever suffer happens in their first few weeks, according to Coley.

One plant defense is to grow leaves so rapidly that they reach maturity too fast to be destroyed by insects - some leaves double in size every day, she said. Other plants put out a huge number of new leaves at once, gambling that some will survive.

One species' stems are covered with extra pollen. Ants are lured aboard to eat the pollen, and when insect predators show up hoping to ravage the leaves, the ants eat them, too.

Another defense is to produce new leaves that are low in chlorophyll, so they aren't rich enough in nutrients to attract bugs.

The Utahns found other plants had a more active defense: chemical warfare. Many slow-growing leaves produce toxins that fight fungus or that will poison grasshoppers, caterpillars and other bugs. These chemicals are also potential pharmaceuticals.

Using their method of "ecological insight," the team came up with ways to spot plants that produce the chemicals. The new leaves must mature slowly and show a good green color (they aren't lacking chlorophyll).

"Basically, we're primarily looking at the chemistry of young leaves now," she said. Previous researchers would go through the forest gathering mature leaves because they are most abundant. But when leaves reach maturity they become tough and less palatable, no longer needing chemical arms.

The team is concentrating on a rain forest in Panama. They work at a laboratory sponsored by the Smithsonian Institution, "bioprospecting" for new compounds. They are also working to engage Panamanians in the effort.

They hope that by demonstrating the tremendous value of the rain forest to medicine, they can help preserve the jungle. "The reason we're doing this is to make money from and for the forest," Coley said in an interview.