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Chapter 24

Natural Products
(Secondary Metabolites)


Plants produce a great variety of organic compounds that are not directly involved in primary metabolic processes of growth and development. The roles these natural products or secondary metabolites play in plants have only recently come to be appreciated in an analytical context. Natural products appear to function primarily in defense against predators and pathogens and in providing reproductive advantage as attractants of pollinators and seed dispersers. They may also act to create competitive advantage as poisons of rival species.
      Most natural products can be classified into three major groups: terpenoids, alkaloids, and phenolic compounds (mostly phenylpropanoids). Terpenoids are composed of five-carbon units synthesized by way of the acetate/mevalonate pathway or the glyceraldehyde 3-phosphate/pyruvate pathway. Many plant terpenoids are toxins and feeding deterrents to herbivores or are attractants of various sorts. Alkaloids are synthesized principally from amino acids. These nitrogen-containing compounds protect plants from a variety of herbivorous animals, and many possess pharmacologically important activity. Phenolic compounds, which are synthesized primarily from products of the shikimic acid pathway, have several important roles in plants. Tannins, lignans, flavonoids, and some simple phenolic compounds serve as defenses against herbivores and pathogens. In addition, lignins strengthen cell walls mechanically, and many flavonoid pigments are important attractants for pollinators and seed dispersers. Some phenolic compounds have allelopathic activity and may adversely influence the growth of neighboring plants.
      Throughout the course of evolution, plants have developed defenses against herbivory and microbial attack and produced other natural products to aid competitiveness. The better-defended, more-competitive plants have generated more progeny, and so the capacity to produce and safely store such ecologically useful metabolites has become widely established in the plant kingdom. Pressures from herbivores and pathogens, as well as constant competition, continue to select for new natural products. In cultivated species, however, such chemical defenses have often been artificially selected against.
      Study of the biochemistry of plant natural products has many practical applications. Biotechnological approaches can selectively increase the amounts of defense compounds in crop plants, thereby reducing the need for costly and potentially toxic pesticides. Similarly, genetic engineering can be utilized to increase the yields of pharmaceuticals, flavor and perfumery materials, insecticides, fungicides, and other natural products of commercial value. Although many natural products and their functions have been described in this chapter, the metabolism of natural products in most plant species remains to be elucidated. A great deal of fascinating biochemistry remains to be discovered.

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