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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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