Volatile compounds act as a language that plants use for their communication and interaction with the surrounding environment. To date, a total of 1700 volatile compounds have been isolated from more than 90 plant families. These volatiles, released from leaves, flowers, and fruits into the atmosphere and from roots into the soil, defend plants against herbivores and pathogens or provide a reproductive advantage by attracting pollinators and seed dispersers. Plant volatiles constitute about 1% of plant secondary metabolites and are mainly represented by terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives, and amino acid derivatives. In this review we focus on the functions of plant volatiles, their biosynthesis and regulation, and the metabolic engineering of the volatile spectrum, which results in plant defense improvement and changes of scent and aroma properties of flowers and fruits.

Plant Volatiles: Recent Advances and Future Perspectives

http://www.medicinalgenomics.com/wp-content/uploads/2011/12/Monoterpene-and-sesquiterpene-synthases-and-the-origin-of-terpene-skeletal.pdf

Monoterpene and sesquiterpene synthases and the origin of terpene skeletal diversity in plants.

Plants have the capacity to synthesize, accumulate and emit volatiles that may act as aroma and flavor molecules due to interactions with human receptors. These low-molecular-weight substances derived from the fatty acid, amino acid and carbohydrate pools constitute a heterogenous group of molecules with saturated and unsaturated, straight-chain, branched-chain and cyclic structures bearing various functional groups (e.g. alcohols, aldehydes, ketones, esters and ethers) and also nitrogen and sulfur. They are commercially important for the food, pharmaceutical, agricultural and chemical industries as flavorants, drugs, pesticides and industrial feedstocks. Due to the low abundance of the volatiles in their plant sources, many of the natural products had been replaced by their synthetic analogues by the end of the last century. However, the foreseeable shortage of the crude oil that is the source for many of the artificial flavors and fragrances has prompted recent interest in understanding the formation of these compounds and engineering their biosynthesis. Although many of the volatile constituents of flavors and aromas have been identified, many of the enzymes and genes involved in their biosynthesis are still not known. However, modification of flavor by genetic engineering is dependent on the knowledge and availability of genes that encode enzymes of key reactions that influence or divert the biosynthetic pathways of plant-derived volatiles. Major progress has resulted from the use of molecular and biochemical techniques, and a large number of genes encoding enzymes of volatile biosynthesis have recently been reported.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2008.03446.x

Biosynthesis of plant-derived flavor compounds.

Terpene synthases and the regulation, diversity and biological roles of terpene metabolism.

Structural biology and chemistry of the terpenoid cyclases.

Terpene synthases catalyze the first committed steps in the biosynthesis of monoterpenes, sesquiterpenes, and diterpenes. An overview is presented of the enzymology and mechanism of these terpene synthases, and their molecular cloning, expression, and sequence analysis. Detailed structural and functional evaluation of four representative monoterpene, sesquiterpene, and diterpene synthases is also presented.

Cyclization Enzymes in the Biosynthesis of Monoterpenes, Sesquiterpenes, and Diterpenes

(−)-Menthol is the most familiar of the monoterpenes as both a pure natural product and as the principal and characteristic constituent of the essential oil of peppermint (Mentha x piperita). In this paper, we review the biosynthesis and molecular genetics of (−)-menthol production in peppermint. In Mentha species, essential oil biosynthesis and storage is restricted to the peltate glandular trichomes (oil glands) on the aerial surfaces of the plant. A mechanical method for the isolation of metabolically functional oil glands, has provided a system for precursor feeding studies to elucidate pathway steps, as well as a highly enriched source of the relevant biosynthetic enzymes and of their corresponding transcripts with which cDNA libraries have been constructed to permit cloning and characterization of key structural genes. The biosynthesis of (−)-menthol from primary metabolism requires eight enzymatic steps, and involves the formation and subsequent cyclization of the universal monoterpene precursor geranyl diphosphate to the parent olefin (−)-(4S)-limonene as the first committed reaction of the sequence. Following hydroxylation at C3, a series of four redox transformations and an isomerization occur in a general “allylic oxidation–conjugate reduction” scheme that installs three chiral centers on the substituted cyclohexanoid ring to yield (−)-(1R, 3R, 4S)-menthol. The properties of each enzyme and gene of menthol biosynthesis are described, as are their probable evolutionary origins in primary metabolism. The organization of menthol biosynthesis is complex in involving four subcellular compartments, and regulation of the pathway appears to reside largely at the level of gene expression. Genetic engineering to up-regulate a flux-limiting step and down-regulate a side route reaction has led to improvement in the composition and yield of peppermint oil.

Menthol biosynthesis and molecular genetics

Monoterpene double-bond reductases of the (-)-menthol biosynthetic pathway: isolation and characterization of cDNAs encoding (-)-isopiperitenone reductase and (+)-pulegone reductase of peppermint.

The essential oils of peppermint ( Mentha x piperita ) and spearmint ( Mentha spicata ) are distinguished by the oxygenation position on the p -menthane ring of the constitutive monoterpenes that is conferred by two regiospecific cytochrome P450 limonene-3- and limonene-6-hydroxylases. Following hydroxylation of limonene, an apparently similar dehydrogenase oxidizes (−)-trans-isopiperitenol to (−)-isopiperitenone in peppermint and (−)-trans-carveol to (−)-carvone in spearmint. Random sequencing of a peppermint oil gland secretory cell cDNA library revealed a large number of clones that specified redox-type enzymes, including dehydrogenases. Full-length dehydrogenase clones were screened by functional expression in Escherichia coli using a recently developed in situ assay. A single full-length acquisition encoding ( − )-trans-isopiperitenol dehydrogenase (ISPD) was isolated. The (−)-ISPD cDNA has an open reading frame of 795 bp that encodes a 265-residue enzyme with a calculated molecular mass of 27,191. Nondegenerate primers were designed based on the (−)-trans-ISPD cDNA sequence and employed to screen a spearmint oil gland secretory cell cDNA library from which a 5′-truncated cDNA encoding the spearmint homolog, (−)-trans-carveol-dehydrogenase, was isolated. Reverse transcription-PCR amplification and RACE were used to acquire the remaining 5′-sequence from RNA isolated from oil gland secretory cells of spearmint leaf. The full-length spearmint dehydrogenase shares >99% amino acid identity with its peppermint homolog and both dehydrogenases are capable of utilizing (−)-trans-isopiperitenol and (−)-trans-carveol. These isopiperitenol/carveol dehydrogenases are members of the short-chain dehydrogenase/reductase superfamily and are related to other plant short-chain dehydrogenases/reductases involved in secondary metabolism (lignan biosynthesis), stress responses, and phytosteroid biosynthesis, but they are quite dissimilar (approximately 13% identity) to the monoterpene reductases of mint involved in (−)-menthol biosynthesis. The isolation of the genes specifying redox enzymes of monoterpene biosynthesis in mint indicates that these genes arose from different ancestors and not by simple duplication and differentiation of a common progenitor, as might have been anticipated based on the common reaction chemistry and structural similarity of the substrate monoterpenes.

http://www.plantphysiol.org/content/plantphysiol/137/3/863.full.pdf

Monoterpene Metabolism. Cloning, Expression, and Characterization of (−)-Isopiperitenol/(−)-Carveol Dehydrogenase of Peppermint and Spearmint

Peppermint (Mentha × piperita L.) was transformed with various gene constructs to evaluate the utility of metabolic engineering for improving essential oil yield and composition. Oil yield increases were achieved by overexpressing genes involved in the supply of precursors through the 2C-methyl-D-erythritol 4-phosphate (MEP) pathway. Two-gene combinations to enhance both oil yield and composition in a single transgenic line were assessed as well. The most promising results were obtained by transforming plants expressing an antisense version of (+)-menthofuran synthase, which is critical for adjusting the levels of specific undesirable oil constituents, with a construct for the overexpression of the MEP pathway gene 1-deoxy-D-xylulose 5-phosphate reductoisomerase (up to 61% oil yield increase over wild-type controls with low levels of the undesirable side-product (+)-menthofuran and its intermediate (+)-pulegone). Elite transgenic lines were advanced to multiyear field trials, which demonstrated consistent oil yield increases of up to 78% over wild-type controls and desirable effects on oil composition under commercial growth conditions. The transgenic expression of a gene encoding (+)-limonene synthase was used to accumulate elevated levels of (+)-limonene, which allows oil derived from transgenic plants to be recognized during the processing of commercial formulations containing peppermint oil. Our study illustrates the utility of metabolic engineering for the sustainable agricultural production of high quality essential oils at a competitive cost.

https://www.pnas.org/content/pnas/108/41/16944.full.pdf

Edward M. Davis

Papers

Cyclization Enzymes in the Biosynthesis of Monoterpenes, Sesquiterpenes, and Diterpenes

Menthol biosynthesis and molecular genetics

Monoterpene double-bond reductases of the (-)-menthol biosynthetic pathway: isolation and characterization of cDNAs encoding (-)-isopiperitenone reductase and (+)-pulegone reductase of peppermint.

Monoterpene Metabolism. Cloning, Expression, and Characterization of (−)-Isopiperitenol/(−)-Carveol Dehydrogenase of Peppermint and Spearmint

Improving peppermint essential oil yield and composition by metabolic engineering