Rodney Croteau
Toni M. Kutchan
Norman G. Lewis

Natural products have primary ecological functions.

Plants produce a vast and diverse assortment of organic compounds, the great majority of which do not appear to participate directly in growth and development. These substances, traditionally referred to as secondary metabolites, often are differentially distributed among limited taxonomic groups within the plant kingdom. Their functions, many of which remain unknown, are being elucidated with increasing frequency. The primary metabolites, in contrast, such as phytosterols, acyl lipids, nucleotides, amino acids, and organic acids, are found in all plants and perform metabolic roles that are essential and usually evident.
      Although noted for the complexity of their chemical structures and biosynthetic pathways, natural products have been widely perceived as biologically insignificant and have historically received little attention from most plant biologists. Organic chemists, however, have long been interested in these novel phytochemicals and have investigated their chemical properties extensively since the 1850s. Studies of natural products stimulated development of the separation techniques, spectroscopic approaches to structure elucidation, and synthetic methodologies that now constitute the foundation of contemporary organic chemistry. Interest in natural products was not purely academic but rather was prompted by their great utility as dyes, polymers, fibers, glues, oils, waxes, flavoring agents, perfumes, and drugs. Recognition of the biological properties of myriad natural products has fueled the current focus of this field, namely, the search for new drugs, antibiotics, insecticides, and herbicides. Importantly, this growing appreciation of the highly diverse biological effects produced by natural products has prompted a reevaluation of the possible roles these compounds play in plants, especially in the context of ecological interactions. As illustrated in this chapter, many of these compounds now have been shown to have important adaptive significance in protection against herbivory and microbial infection, as attractants for pollinators and seed-dispersing animals, and as allelopathic agents (allelochemicals that influence competition among plant species). These ecological functions affect plant survival profoundly, and we think it reasonable to adopt the less pejorative term “plant natural products” to describe secondary plant metabolites that act primarily on other species.

The boundary between primary and secondary metabolism is blurred.

Based on their biosynthetic origins, plant natural products can be divided into three major groups: the terpenoids, the alkaloids, and the phenylpropanoids and allied phenolic compounds. All terpenoids, including both primary metabolites and more than 25,000 secondary compounds, are derived from the five-carbon precursor isopentenyl diphosphate (IPP). The 12,000 or so known alkaloids, which contain one or more nitrogen atoms, are biosynthesized principally from amino acids. The 8000 or so phenolic compounds are formed by way of either the shikimic acid pathway or the malonate/ acetate pathway.
      Primary and secondary metabolites cannot readily be distinguished on the basis of precursor molecules, chemical structures, or biosynthetic origins. For example, both primary and secondary metabolites are found among the diterpenes (C₂₀) and triterpenes (C₃₀). In the diterpene series, both kaurenoic acid and abietic acid are formed by a very similar sequence of related enzymatic reactions (Fig. 24.1); the former is an essential intermediate in the synthesis of gibberellins, i.e., growth hormones found in all plants (see Chapter 17), whereas the latter is a resin component largely restricted to members of the Fabaceae and Pinaceae. Similarly, the essential amino acid proline is classified as a primary metabolite, whereas the C₆ analog pipecolic acid is considered an alkaloid and thus a natural product (Fig. 24.1). Even lignin, the essential structural polymer of wood and second only to cellulose as the most abundant organic substance in plants, is considered a natural product rather than a primary metabolite.
      In the absence of a valid distinction based on either structure or biochemistry, we return to a functional definition, with primary products participating in nutrition and essential metabolic processes inside the plant, and natural (secondary) products influencing ecological interactions between the plant and its environment. In this chapter, we provide an overview of the biosynthesis of the major classes of plant natural products, emphasizing the origins of their structural diversity, as well as their physiological functions, human uses, and potential biotechnological applications.

Figure 24.1
Kaurenoic acid and proline are primary metabolites, whereas the closely related compounds abietic acid and pipecolic acid are considered secondary metabolites.