Terpenoid metabolism.

The terpenoids constitute the largest family of natural products; over 22,000 individual compounds of this class have been described (Connolly and Hill, 1991), and the number of defined structures has doubled every decade since the 1970s (Devon and Scott, 1972; Glasby, 1982). The terpenoids play diverse functional roles in plants as hormones (gibberellins, abscisic acid), photosynthetic pigments (phytol, carotenoids), electron carriers (u biquinone, plastoquinone), mediators of polysaccharide assembly (polyprenyl phosphates), and structural components of membranes (phytosterols). In addition to these universal physiological, metabolic, and structural functions, many specific terpenoid compounds (commonly in the CIO, C15, and C20 families) serve in communication and defense, for example, as attractants for pollinators and seed dispersers, competitive phytoxins, antibiotics, and herbivore repellents and toxins (Harborne, 1991). Terpenoids available in relatively large amounts as essential oils, resins, and waxes are important renewable resources and provide a range of commercially useful products, including solvents, flavorings and fragrances, adhesives, coatings, and synthetic intermediates (Zinkel and Russell, 1989; Dawson, 1994). Members of the terpenoid group also include industrially useful polymers (rubber, chicle) and a number of pharmaceuticals (artemisinin, taxol) and agrochemicals (pyrethrins, azadirachtin). In spite of the economic significance of the terpenoids and their many essential functions, relatively little is known about terpenoid metabolism and its regulation in plants. There are severa1 reasons for this. An overwhelming problem with the terpenoids is their sheer number. A given plant may synthesize (and catabolize) many different terpenoid types (from C5 to C40 and higher) at different times and locations for many different purposes throughout the course of development. Because all terpenoids are produced by a common biosynthetic pathway, sophisticated control mechanisms must exist to ensure the production of appropriate levels of these often structurally complex compounds in the proper metabolic, developmental, and environmental context. Pathway elucidation for highly functionalized terpenoid metabolites is not trivial, and determining the enzymology of terpenoid metabolism has proven very challenging, both because relatively little enzymatic machinery is dedicated to terpenoid metabolism and because many of the reaction types involved (carbonium ion