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Plants that fight back.

Nothing may seem like easier prey for a hungry insect than a succulent plant, which can't pull up its roots and run away or cry out for help. However, plants can and do fight back by synthesizeing toxins that poison attacking insects, repellents that make predators flee, powerful steroid-like hormones that interfere with metamorphosis from larva to pupa to adult, and other chemical weapons.

Biochemist Clarence Ryan and co-workers at Washington State University have isolated, identified, and synthesized the chemical that triggers a plant defense mechanism. The compound discourages predatory insects by giving them a bad case of indigestion.

The discovery caps a 20-year effort by Ryan and other biochemists to identify the signaling agent plants produce when stressed (wounded) by munching insects. The compound, systemin, activates or turns on genes that produce two proteinase inhibitors.

Proteinases are enzymes that animals - insects in this instance - use to digest proteins. The proteinase inhibitors created by systemin's signal interfere with the bugs' digestion of proteins in plant tissue. This gives them a strong incentive to seek nourishment elsewhere and may save a plant from defoliation and death.

The research on systemin could lead to better biological control methods for insects, such as a systemin-based compound that stimulates the defensive mechanisms of plants. Systemin research also has broader implications for the study of the biochemical compounds produced by plants.

Some plants produce defensive chemicals continually, whether or not they are under attack. Many others utilize so-called "inducible" defenses - toxic compounds that are synthesized only in response to a predator's attack. Scientists have known for years that plants such as tomatoes issue "signaling molecules" when attacked by chewing insects. The molecules turn on genes that instruct the plant to begin making proteinase inhibitors. The inhibitors then accumulate in leaves and are consumed by insects.

Ryan's group focused on simpler compounds such as auxins, cytokinins, and bioactive carbohydrates during much of their two-decade effort to identify the compound that activates the proteinase defenses in plants. The search for the molecule proved elusive until, in the late 1980s, they decided to change their focus and look for other potential molecular signals. They systematically searched tomato leaf extracts for a molecule capable of activating the proteinase inhibitor genes. Eventually, Ryan and technicians Gregory Pearce and Scott Johnson identified and one-half years and leaves from more than 30,000 young tomato plants to isolate just one microgram of the polypeptide. That one-millionth of a gram represents all the polypeptide present in 60 pounds of tomato leaves. Researchers were able to use the minute quantity to determine systemin's structure - which consists of 18 amino acids - so that it could be synthesized in larger quantities.
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Publication:USA Today (Magazine)
Date:Jun 1, 1993
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