The mevalonate pathway contributes to monoterpene production in peppermint

Peppermint produces monoterpenes which are of great commercial value in different traditional and modern pharmaceutical and cosmetic industries. In the classical view, monoterpenes are synthesized via the plastidic 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, while the cytosolic mevalonate (MVA) pathway produces sesquiterpenes. Interactions between both pathways have been documented in several other plant species, however, a quantitative understanding of the metabolic network involved in monoterpene biosynthesis is still lacking. Isotopic tracer analysis, steady state 13C metabolic flux analysis (MFA) and pathway inhibition studies were applied in this study to quantify metabolic fluxes of primary and isoprenoid metabolism of peppermint glandular trichomes (GT). Our results offer new insights into peppermint GT metabolism by confirming and quantifying the crosstalk between the two isoprenoid pathways towards monoterpene biosynthesis. In addition, a quantitative description of precursor pathways involved in isoprenoid metabolism is given. While glycolysis was shown to provide precursors for the MVA pathway, the oxidative bypass of glycolysis fueled the MEP pathway, indicating prominent roles for the oxidative branch of the pentose phosphate pathway and RuBisCO. This study reveals the potential of 13C-MFA to ascertain previously unquantified metabolic routes of the trichomes and thus advancing insights on metabolic engineering of this organ.