Isoprenoid Metabolism and Engineering in Glandular Trichomes of Lamiaceae

Abstract

The isoprenoids play important ecological and physiological roles in plants. They also have a tremendous impact on human lives as food additives, medicines, and industrial raw materials, among others. Though some isoprenoids are highly abundant in nature, plants produce many at extremely low levels. Glandular trichomes (GT), which cover the aerial parts of more than 25% of vascular plants, have been considered as natural biofactories for the mass production of rare industrially important isoprenoids. In several plant genera (e.g., Lavandula and Mentha), GTs produce and store large quantities of the low molecular weight isoprenoids, in particular mono- and sesquiterpenes, as essential oil constituents. Within each trichome, a group of secretory cells is specialized to strongly and specifically express isoprenoid biosynthetic genes, and to synthesize and deposit copious amounts of terpenoids into the trichome's storage reservoir. Despite the abundance of certain metabolites in essential oils and defensive resins, plants, particularly those lacking glandular trichomes, accumulate small quantities of many of the biologically active and industrially important isoprenoids. Therefore, there is a pressing need for technologies to enable the mass production of such metabolites, and to help meet the ever-increasing demand for plant-based bioproducts, including medicines and renewable materials. Considerable contemporary research has focused on engineering isoprenoid metabolism in GTs, with the goal of utilizing them as natural biofactories for the production of valuable phytochemicals. In this review, we summarize recent advances related to the engineering of isoprenoid biosynthetic pathways in glandular trichomes.

Keywords: glandular trichomes; isoprenoids; lavender; metabolic engineering; mint.

Figure 1

Overview of the biosynthetic pathways for major plant isoprenoids. The biosynthesis of isoprenoids starts with a common pool of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). IPP and DAMPP are initially condensed to form either lavandulyl diphosphate (LPP), the precursor for irregular monoterpenes, or geranyl diphosphate (GPP), the precursor to regular monoterpenes. GPP can also be condensed with one or two IPP units to form FPP for sesquiterpene synthesis, or geranylgeranyl diphosphate (GGPP) for diterpene production. Two FPP units can be condensed to form squalene, the immediate precursor for triterpene metabolism. The isoprenyl diphosphate synthases (IDSs) that catalyzed the precursor(s) into specific prenyl diphosphate include LPP synthase (LPPS), GPP synthase (GPPS), geranylgeranyl diphosphate synthase (GGPPS), FPP synthase (FPPS), and squalene synthase (SS).

Figure 2

Schematic representation of peltate glandular trichome found in Lamiaceae plants. The secretory cells produce and secret essential oil constituents into the trichome’s secretory cavity.

Figure 3

The biosynthetic pathways for monoterpene metabolism in lavender, peppermint, and spearmint. Representative monoterpenes derived from GPP are shown, and the corresponding enzymes involved in the process are previously reported for each species (Croteau et al., 2005; Ringer et al., 2005; Landmann et al., 2007; Demissie et al., 2012; Adal et al., 2019).