Although a plant cannot flee its herbivorous enemies, it may be able to defend itself chemically. Many plants attract, resist, and inhibit other organisms by producing special chemicals known as secondary metabolites. Primary metabolites are substances, such as proteins, nucleic acids, carbohydrates, and lipids, that are produced and used by all living things. Secondary metabolites are substances that are not used for basic cellular metabolism. Although all organisms use the same kinds of primary metabolites, plants can differ as radically in their secondary metabolites as they do in their external appearance.
The more than 10,000 known secondary plant metabolites range in molecular weight from about 70 to more than 400,000, but most have a low molecular weight. Some are produced by only a single species, while others are characteristic of entire genera or even families. These compounds help plants compensate for being unable to move.
The effects of defensive secondary metabolites on animals are diverse. Some secondary metabolites act on the nervous systems of herbivorous insects, mollusks, or mammals. Others mimic the natural hormones of insects, causing some larvae to fail to develop into adults. Still others damage the digestive tracts of herbivores. Some secondary metabolites are toxic to fungal pests. Humans make commercial use of many secondary plant metabolites as fungicides, insecticides, ro-denticides, and pharmaceuticals. While many secondary metabolites have protective functions, others are essential as attractants for pollinators and seed dispersers. Table 40.1 lists the major classes of defensive secondary plant metabolites and their biological roles.
Let's look at a specific example of an insecticidal secondary metabolite, canavanine.
ological activity. These defects in protein structure and function lead to developmental abnormalities that kill the insect.
A few insect larvae are able to eat canavanine-containing plant tissue and still develop normally. How can this be? In these larvae, the enzyme that charges the arginine tRNA discriminates correctly between arginine and canavanine. The canavanine they ingest is thus not incorporated into the proteins they form, and the larvae are not harmed.
In plants that produce it, canavanine is present whether or not the plant is under attack. Other chemical defenses come into play only when a predator strikes.
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