MONOTERPENE AND SESQUITERPENE BIOSYNTHESIS IN GLANDULAR TRICHOMES OF PEPPERMINT (MENTHA X PIPERITA) RELY EXCLUSIVELY ON PLASTID-DERIVED ISOPENTENYL DIPHOSPHATE

The subcellular compartmentation of isopentenyl diphosphate (IPP) synt hesis was examined in secretory cells isolated from glandular trichome s of peppermint (Mentha x piperita L. cv. Black Mitcham). As a consequ ence of their anatomy and the conditions of their isolation, the isola ted secretory cells are non-specifically permeable to low-molecular-we ight water-soluble metabolites. Thus, the cytoplasm is readily accessi ble to the exogenous buffer whereas the selective permeability of subc ellular organelles is maintained. With the appropriate choice of exoge nous substrates, this feature allows the assessment of cytoplasmic and organellar (e.g. plastidic) metabolism in situ. Glycolytic substrates such as [C-14]glucose-6-phosphate and [C-14]pyruvic acid are incorpor ated into both monoterpenes and sesquiterpenes with a monoterpene:sesq uiterpene ratio that closely mimics that observed in vivo, indicating that the correct subcellular partitioning of these substrates is maint ained in this model system. Additionally, exogenous [C-14]mevalonic ac id and [C-14]IPP, which are both intitially metabolized in the cytopla sm, produce an abnormally high proportion of sesquiterpenes. In contra st, incubation with either [C-14]citrate or [C-14]acetyl-CoA results i n the accumulation of 3-hydroxy-3-methyl-glutaryl-CoA. (HMG-CoA) with no detectable isoprenoids formed. Taken together, these results indica te that the cytoplasmic mevalonic acid pathway is blocked at HMG-CoA r eductase and that the IPP utilized for both monoterpene and sesquiterp ene biosynthesis is synthesized exclusively in the plastids.