Terpenoids: Lower

Abstract

Lower terpenoids (monoterpenes and sesquiterpenes) are volatile components of the scents and fragrances of aromatic plants. They are frequently found in glandular hairs on leaves, in the scent glands of flowers or in specialized resin ducts in conifers. They are synthesized through stepwise condensations of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), the universal intermediates of isoprenoid metabolism. The precursors of monoterpenes are mainly supplied by the 2‐methylerythritol 4‐phosphate (MEP) pathway in plastids, whereas most sesquiterpenes are produced from acetate via the cytosolic mevalonate pathway. Subtle variations of basic terpene carbon skeletons have resulted in the appearance of tens of thousands of unique structures in nature, most of whose functions are currently unknown. As volatile, organic compounds, many participate in plant–plant or plant–insect chemical communications, as pollination vector attractants, feeding deterrents or in direct plant defence. Many have potent biological activities and are used in pharmaceutical and industrial applications. Key concepts Lower terpenoids (monoterpenes and sesquiterpenes) are volatile C 10 and C 15 compounds frequently present in floral emissions. Lower terpenoids emitted from flowers may attract symbiotic insects which assist the plant in pollination. When some plants are attacked by insects or nematodes, they release volatile terpenoids which summon parasitic wasps who indirectly provide plants with protection by preying on their attackers. Most plants have highly specialized structures such as trichomes where terpenoid biosynthesis and storage take place. Lower terpenoids share a common biosynthetic origin with all other terpenoids through IPP and DMAPP, the universal C 5 metabolic precursors of all isoprenoids. Two parallel pathways exist for the formation of IPP and DMAPP in plants, the methylerythritol phosphate pathway in plastids and the mevalonate pathway in the cytosol. Lower terpenoids are frequently chemically modified after their initial biosynthesis to give them highly specific biological activities. In a limited number of plant families, the monoterpene geraniol is used in the biosynthesis of monoterpene indole alkalkoids, a highly specialized family of natural products with potent pharmacological properties in humans. C 15 sesquiterpenes are structurally diverse and are often modified through the addition of a lactone ring, which generally makes them extremely toxic to animals.