Specialized plant terpenoids have found fortuitous uses in medicine due to their evolutionary and biochemical selection for biological activity in animals. However, these highly functionalized natural products are produced through complex biosynthetic pathways for which we have a complete understanding in only a few cases. Here we review some of the most effective and promising plant terpenoids that are currently used in medicine and medical research and provide updates on their biosynthesis, natural occurrence, and mechanism of action in the body. This includes pharmacologically useful plastidic terpenoids such as p-menthane monoterpenoids, cannabinoids, paclitaxel (taxol®), and ingenol mebutate which are derived from the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway, as well as cytosolic terpenoids such as thapsigargin and artemisinin produced through the mevalonate (MVA) pathway. We further provide a review of the MEP and MVA precursor pathways which supply the carbon skeletons for the downstream transformations yielding these medically significant natural products.

Pelargonium graveolens is a wild predecessor to rose-scented geranium hybrids prized for their essential oils used as fragrances and flavorings. However, little is known about their biosynthesis. Here we present metabolic evidence that at least two distinct monoterpene biosynthetic pathways contribute to their volatile profiles, namely, cyclic p-menthanes such as (−)-isomenthone and acyclic monoterpene alcohols such as geraniol and (−)-citronellol and their derivatives (referred to here as citronelloid monoterpenes). We established their common origin via the 2C-methyl-d-erythritol-4-phosphate pathway but found no indication these pathways share common intermediates beyond geranyl diphosphate. Untargeted volatile profiling of 22 seed-grown P. graveolens lines demonstrated distinct chemotypes that preferentially accumulate (−)-isomenthone, geraniol, or (−)-citronellol along with approximately 85 minor volatile products. Whole plant 13CO2 isotopic labeling performed under physiological conditions permitted us to measure the in vivo rates of monoterpenoid accumulation in these lines and quantify differences in metabolic modes between chemotypes. We further determined that p-menthane monoterpenoids in Pelargonium are likely synthesized from (+)-limonene via (+)-piperitone rather than (+)-pulegone. Exploitation of this natural population enabled a detailed dissection of the relative rates of competing p-menthane and citronelloid pathways in this species, providing real time rates of monoterpene accumulation in glandular trichomes.

Summary Geraniol, citronellol and their esters are high‐value acyclic monoterpenes used in food technology, perfumery and cosmetics. A major source of these compounds is the essential oil of rose‐scented geraniums of the genus Pelargonium. We provide evidence that their biosynthesis mainly takes place in the cytosol of glandular trichomes via geranyl monophosphate (GP) through the action of a Nudix hydrolase. Protein preparations could convert geranyl diphosphate (GDP) to geraniol in in vitro assays, a process which could be blocked by inorganic phosphatase inhibitors, suggesting a two‐step conversion of GDP to geraniol. Pelargonium graveolens chemotypes enriched in either geraniol or (−)‐citronellol accumulate GP or citronellyl monophosphate (CP), respectively, the presumed precursors to their monoterpenoid end products. Geranyl monophosphate was highly enriched in isolated glandular trichomes of lines producing high amounts of geraniol. In contrast, (−)‐isomenthone‐rich lines are depleted in these prenyl monophosphates and monoterpene alcohols and instead feature high levels of GDP, the precursor to plastidic p‐menthane biosynthesis. A Nudix hydrolase cDNA from Pelargonium glandular trichomes, dubbed PgNdx1, encoded a cytosolic protein capable of hydrolyzing GDP to GP with a KM of about 750 nm but is only weakly active towards farnesyl diphosphate. In citronellol‐rich lines, GDP, GP and CP were detected in nearly equimolar amounts, while citronellyl diphosphate was absent, suggesting that citronellol biosynthesis may proceed by reduction of GP to CP in this species. These findings highlight the cytosol as a compartment that supports monoterpene biosynthesis and expands the roles of Nudix hydrolases in the biosynthesis of plant volatiles.

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Understanding the evolution of plant defenses against herbivores requires identifying the benefits and costs of defense. Here, we tested the hypothesis that the benefits and costs of hydrogen cyanide (HCN) defense against herbivory on white clover (Trifolium repens) are temperature dependent. We first tested how temperature affected HCN production in vitro, and then examined how temperature influenced the efficacy of HCN defense of T. repens against a generalist slug (Deroceras reticulatum) herbivore using no-choice and choice feeding trial assays. To understand how temperature affected the costs of defense, plants were exposed to freezing, and HCN production, photosynthetic activity, and ATP concentration were quantified. HCN production increased linearly from 5 °C to 50 °C, and cyanogenic plants experienced reduced herbivory compared to acyanogenic plants only at warmer temperatures when fed upon by young slugs. Freezing temperatures induced cyanogenesis in T. repens and decreased chlorophyll fluorescence. Cyanogenic plants experienced lower ATP levels than acyanogenic plants due to freezing. Our study provides evidence that the benefits of HCN defense against herbivores are temperature dependent, and freezing may inhibit ATP production in cyanogenic plants, but the physiological performance of all plants recovered quickly following short-term freezing. These results contribute to understanding how varying environments alter the benefits and costs of defense in a model system for the study of plant chemical defenses against herbivores.