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Agricultural Research Service

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Scientists Help Plants Protect Themselves

By Doris Stanley
September 3, 1998

Like the human body's immune system, plants also have built-in protective mechanisms. Agricultural Research Service scientists are working to manipulate that internal system in plants--triggered by pathogenesis-related (PR) proteins--to increase protection. Plants that are better able to defend themselves require less fungicides and insecticides--a boon to growers as well as the environment.

As insects feed on plants, substances in their saliva can trigger plants to release these defensive proteins. But too much damage from invaders may overwhelm a plant before the defensive proteins reach effective levels.

At the U.S. Horticultural Research Laboratory in Orlando, Fla., ARS scientists have found PR proteins in citrus. The proteins are two classes of enzymes--chitinases and glucanases--in citrus roots, leaves, blossoms and fruit.

Chitinase breaks down chitin, even though the substance does not occur in citrus. However, insect exoskeletons and the cell walls of microbial pathogens are made of chitin, so scientists assume chitinase in citrus acts as a defensive mechanism. Glucanases break down glucans, antifungal compounds found in citrus. The enzyme may help the plant regulate levels of defensive glucan proteins. The next step is to find ways to make the PR proteins more active in citrus.

ARS scientists have isolated citrus genes that produce these protective proteins. They're also experimenting with chemicals that trigger the protective mechanism without the plant actually being attacked by a pest. In tests, leaf miner larvae declined to feed on tomato plants sprayed with BTH (benzothiadiazole). BTH, a nontoxic chemical that doesn't harm humans, plants, or animals, starts an internal chemical reaction in tomato plants that repels the pesky insect.

This research is partially funded by the Florida Citrus Production Research Advisory Council with funds from a self-imposed grower tax. ARS is the principal research arm of the U.S. Department of Agriculture.

Scientific contact: Richard T. Mayer, U.S. Horticultural Research Laboratory, Orlando, Fla., phone (407) 897-7300, fax (407) 897-7309, e-mail

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