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...