Elucidation of the methylerythritol phosphate pathway for isoprenoid biosynthesis in bacteria and plastids. A metabolic milestone achieved through genomics.

Plants synthesize an enormous variety of metabolites that can be classified into two groups based on their function: primary metabolites, which participate in nutrition and essential metabolic processes within the plant, and secondary metabolites (also referred to as natural products), which influence ecological interactions between plants and their environment (Croteau et al., 2000). Isoprenoids (also called terpenoids) are the most functionally and structurally varied group of plant metabolites. Isoprenoids are synthesized in all organisms but are especially abundant and diverse in plants, with tens of thousands of compounds reported to date (Chappell, 1995, 2002; McGarvey and Croteau, 1995; Croteau et al., 2000). Many isoprenoids are present in all plants and act as primary metabolites with roles in respiration, photosynthesis, and regulation of growth and development. However, the highest variety of isoprenoids is secondary metabolites that function in protecting plants against herbivores and pathogens, in attracting pollinators and seed-dispersing animals, and as allelochemicals that influence competition among plant species (Croteau et al., 2000; Chappell, 2002). Many compounds with important commercial value as flavors, pigments, polymers, fibers, glues, waxes, drugs, or agrochemicals are secondary metabolites of isoprenoid origin. Each plant species synthesizes a specific array of isoprenoid secondary metabolites, and most of them (including rubber and the anticancer drug taxol) are produced only in a few wild or semiwild plant species. Although genetic engineering appears to be a powerful tool to direct the production of both primary and secondary isoprenoid products in plants, only a partial knowledge of the pathways involved in the biosynthesis of their precursors was available until very recently. ISOPRENOID BIOSYNTHESIS. A TALE OF TWO PATHWAYS