The cardenolides produced by milkweeds are only one example from the vast chemical arsenal available to plants for protection. Some plants produce alkaloids, nitrogen-bearing compounds that damage the nervous system of animals. Commonly bitter tasting, alkaloids include such well-known compounds as nicotine, caffeine, morphine, theobromine (found in chocolate), quinine (a treatment for malaria), strychnine, and atropine. Alkaloids are a costly defense because they are rich in nitrogen, an essential and often limiting element that plants need to build proteins for photosynthesis. However, because alkaloids affect specific aspects of metabolism, even very small concentrations can deter herbivores. One of the first chemotherapy drugs (Table 32.1), vincristine, is an alkaloid extracted from the Madagascar periwinkle. It inhibits microtubule polymerization and thus prevents cell division (Chapter 11). Vincristine does not harm the plant’s own cells because it is stored within an extracellular system of latex-containing canals.

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A second group of defensive compounds, the terpenes, do not contain nitrogen. As a result, plants can produce them for defense without having to make use of nitrogen that could otherwise be used in protein synthesis. Small terpenes are volatile, vaporizing easily, and so make up many of the essential oils associated with plants. The distinctive smells of lemon peel, mint, sage, menthol, pine resins, and geranium leaves are all due to terpenes. While we find these odors pleasant, they are feeding deterrents to mammals such as squirrels and moose. In addition, terpenes obstruct the growth and metabolism of both fungi and insects. As a result, pyrethrin, a terpene derivative extracted from chrysanthemums, is marketed commercially as an insecticide. Other compounds derived from terpenes interfere with insect development. For example, exposure to compounds that are replicas of insect hormones cause insects to molt prematurely and thereby keep their populations in check. Taxol, which is used in chemotherapy, is a terpene that was first extracted from the bark of the Pacific yew tree.

Phenols form the third main class of defensive compounds, illustrated by the tannins found widely in plant tissues. Tannins bind with proteins, reducing their digestibility. Plants store tannins in cell vacuoles, and so these compounds come into contact with the protein-rich cytoplasm only when cells are damaged. Herbivores attempting to feed on tannin-producing plants obtain a poor reward for their efforts. For this reason, natural selection favors individuals that avoid tannin-rich plants. Many unripe fruits are high in tannins, and the unpleasant experience of biting into an unripe banana illustrates how tannins deter consumers. For thousands of years, humans have taken advantage of the protein-binding properties of tannins, using extracts from tree bark to process animal skins by “tanning” to produce leather.

Some of the chemical defenses used by plants are stored in a separate compartment from the enzymes that activate them. When you bite into a plant belonging to the cabbage family, the chemicals stored in the vacuole comingle with enzymes in the cytosol. The enzymes then catalyze the production of mustard oils from these compounds. Mustard oils give cabbage and its relatives their distinctive smell. They serve as feeding deterrents because they interfere with insect growth.

Cassava roots, an important food crop in Africa and South America, release the toxin hydrogen cyanide when their cells are damaged. Before cassava roots can be safely consumed, the roots must be ground and the cyanide-producing chemicals removed in running water. Although some cassava varieties, so-called sweet cassava, lack these chemicals and can be eaten without any special preparation, the bitter varieties are preferred crops in many cases because they often have higher yields.

Some chemical defenses found in plants are protein based. Plants and animals use the same 20 amino acids to construct proteins, but some plants produce additional amino acids as well. Plants do not incorporate these additional amino acids into their proteins, but herbivores that ingest them do. The resulting proteins can no longer fulfill their function. Thus, insect herbivores that consume nonprotein amino acids grow slowly and often die early. A second protein-based defense is the production of antidigestive proteins called protease inhibitors. These proteins bind to the active site of enzymes that break down proteins in the herbivore’s digestive system. This prevents proteins from being broken down into their individual amino acids and therefore reduces the nutritional value of the plant tissue. Insects that feed on plants that produce protease inhibitors have reduced growth rates.

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