Biochemical defenses: secondary metabolites:

Plants have developed many substances to act as defenses against herbivores and other organisms that are potentially dangerous to them. Grouped together, these compounds are known as antiherbivory compounds. These compounds can be classified into three groups: Nitrogen Compounds, Terpenoids, and Phenolics.

Antiherbivory compounds are products of secondary metabolism, that is the metabolism of chemicals that occur irregularly and have no known metabolic role. These compounds instead have "ecological roles" that deal with interactions with other species.

The short article below indicates how complex some of these defenses can become:

Plants send out chemicals to fight off predators, study says Science magazine By PAUL RECER,

Some plants send out chemical signals that lure predators to dine on some of the leaf-eating insects that attack them, according to a recent study. Scientists studying a wild tobacco plant growing in the Great Basin desert of southwest Utah said the plant has developed a chemical call for help to protect itself from the five-spotted hawkmoth, its most serious pest. The hawkmoth lays eggs on the plant and when caterpillars hatch they feed on its leaves. Ian T. Baldwin, co-author of a study appearing Friday in the journal Science, said the leaf-chewing by the caterpillars prompts the plant to release a plume of chemicals that distant predators can detect. Predator bugs have the difficult task of finding their favorite prey hidden among many green plants in wide open spaces, Baldwin said. By releasing its chemical signals, he said, the tobacco plant helps predators zero in on their food. In addition to summoning predators, the chemical signals also discourage a continued attack by the egg-laying moth, the researcher said. The study showed that when moths detect the chemical alert, they will avoid the plant and lay their eggs elsewhere. In effect, the signal calls for help and scares off further attack, Baldwin said. "Our study demonstrates that the volatile (airborne chemical) bouquet that is released after attack is very complex," he said. "Predators are attracted and egg-laying moths are repelled."

Baldwin said similar defenses have evolved in the cotton, corn, lima bean, pear and oil-seed plants, among others. In corn, for instance, a chemical signal summons a wasp that deposits eggs inside armyworms that feed on the corn plant. When the eggs hatch, the wasp larvae consume the armyworms.

Lima beans
protect themselves from the spider mite with chemicals that attract predator mites that feed on the pests.

Baldwin said the study suggests that what he calls "indirect defenses" could be engineered into plants and become more successful at controlling agricultural pests than chemical spraying. He notes that insects can develop a resistance to pesticides, but not against natural predators. "The use of these indirect defenses ... will likely be a more sustainable pest control strategy," said Baldwin.

As you can see from the diagram below, these are compounds produced in the process of the synthesis of other necessary metabolites... somewhere along the line the production of these compounds allowed the plant to survive better, and subsequent modifications were selected for.

Evolution of these molecules has occurred over a long period of time....Over 30,000+ compounds are known to be produced by plants; actual nimber is unknown

Some of these alter the hormonal cycle of insects leading to altered life cycles, others act to denature proteins, others alter DNA/CH structure and so on. In bracken ferns these compounds are strong enough deterant to repulse most potential herbivores.

Next page.... the alkaloids.... or go to main defense page