Decoding plant specialized metabolism: new mechanistic insights

Secondary metabolite (SM) production provides biotic and abiotic stress resistance and enables plants to adapt to the environment. Biosynthesis of these metabolites involves a complex interplay between transcription factors (TFs) and regulatory elements, with emerging evidence suggesting an integral role for chromatin dynamics. Here we review key TFs and epigenetic regulators that govern SM biosynthesis in different contexts. We summarize relevant emerging technologies and results from the model species arabidopsis (Arabidopsis thaliana) and outline aspects of regulation that may also function in food, feed, fiber, oil, or industrial crop plants. Finally, we highlight how effective translation of fundamental knowledge from model to non-model species can benefit understanding of SM production in a variety of ecological, agricultural, and pharmaceutical contexts.

UGT76F1 glycosylates an isomer of the C7-necic acid component of pyrrolizidine alkaloids in Arabidopsis thaliana

Identification of unknown metabolites and their biosynthetic genes is an active research area in plant specialized metabolism. By following a gene-metabolite association from a genome-wide association study of Arabidopsis stem metabolites, we report a previously unknown metabolite, 2-hydroxy-2-(1-hydroxyethyl)pentanoic acid glucoside, and demonstrated that UGT76F1 is responsible for its production in Arabidopsis. The chemical structure of the glucoside was determined by a series of analyses, including tandem MS, acid and base hydrolysis, and NMR spectrometry. T-DNA knockout mutants of UGT76F1 are devoid of the glucoside but accumulate increased levels of the aglycone. 2-hydroxy-2-(1-hydroxyethyl)pentanoic acid is structurally related to the C7-necic acid component of lycopsamine-type pyrrolizidine alkaloids such as trachelantic acid and viridifloric acid. Feeding norvaline greatly enhances the accumulation of 2-hydroxy-2-(1-hydroxyethyl)pentanoic acid glucoside in wild-type but not the UGT76F1 knockout mutant plants, providing evidence for an orthologous C7-necic acid biosynthetic pathway in Arabidopsis despite the apparent lack of pyrrolizidine alkaloids.

Diversity and divergence: evolution of secondary metabolism in the tropical tree genus Inga

Plants are widely recognized as chemical factories, with each species producing dozens to hundreds of unique secondary metabolites. These compounds shape the interactions between plants and their natural enemies. We explore the evolutionary patterns and processes by which plants generate chemical diversity, from evolving novel compounds to unique chemical profiles. We characterized the chemical profile of one-third of the species of tropical rainforest trees in the genus Inga (c. 100, Fabaceae) using ultraperformance liquid chromatography-mass spectrometry-based metabolomics and applied phylogenetic comparative methods to understand the mode of chemical evolution. We show: each Inga species contain structurally unrelated compounds and high levels of phytochemical diversity; closely related species have divergent chemical profiles, with individual compounds, compound classes, and chemical profiles showing little-to-no phylogenetic signal; at the evolutionary time scale, a species' chemical profile shows a signature of divergent adaptation. At the ecological time scale, sympatric species were the most divergent, implying it is also advantageous to maintain a unique chemical profile from community members; finally, we integrate these patterns with a model for how chemical diversity evolves. Taken together, these results show that phytochemical diversity and divergence are fundamental to the ecology and evolution of plants.

Comparative transcriptomics and metabolomics reveal specialized metabolite drought stress responses in switchgrass (Panicum virgatum)

Switchgrass (Panicum virgatum) is a bioenergy model crop valued for its energy efficiency and drought tolerance. The related monocot species rice (Oryza sativa) and maize (Zea mays) deploy species-specific, specialized metabolites as core stress defenses. By contrast, specialized chemical defenses in switchgrass are largely unknown. To investigate specialized metabolic drought responses in switchgrass, we integrated tissue-specific transcriptome and metabolite analyses of the genotypes Alamo and Cave-in-Rock that feature different drought tolerance. The more drought-susceptible Cave-in-Rock featured an earlier onset of transcriptomic changes and significantly more differentially expressed genes in response to drought compared to Alamo. Specialized pathways showed moderate differential expression compared to pronounced transcriptomic alterations in carbohydrate and amino acid metabolism. However, diterpenoid-biosynthetic genes showed drought-inducible expression in Alamo roots, contrasting largely unaltered triterpenoid and phenylpropanoid pathways. Metabolomic analyses identified common and genotype-specific flavonoids and terpenoids. Consistent with transcriptomic alterations, several root diterpenoids showed significant drought-induced accumulation, whereas triterpenoid abundance remained predominantly unchanged. Structural analysis verified select drought-responsive diterpenoids as oxygenated furanoditerpenoids. Drought-dependent transcriptome and metabolite profiles provide the foundation to understand the molecular mechanisms underlying switchgrass drought responses. Accumulation of specialized root diterpenoids and corresponding pathway transcripts supports a role in drought stress tolerance.

Cytochrome P450-catalyzed biosynthesis of furanoditerpenoids in the bioenergy crop switchgrass (Panicum virgatum L.)

Specialized diterpenoid metabolites are important mediators of plant-environment interactions in monocot crops. To understand metabolite functions in plant environmental adaptation that ultimately can enable crop improvement strategies, a deeper knowledge of the underlying species-specific biosynthetic pathways is required. Here, we report the genomics-enabled discovery of five cytochrome P450 monooxygenases (CYP71Z25-CYP71Z29) that form previously unknown furanoditerpenoids in the monocot bioenergy crop Panicum virgatum (switchgrass). Combinatorial pathway reconstruction showed that CYP71Z25-CYP71Z29 catalyze furan ring addition directly to primary diterpene alcohol intermediates derived from distinct class II diterpene synthase products. Transcriptional co-expression patterns and the presence of select diterpenoids in switchgrass roots support the occurrence of P450-derived furanoditerpenoids in planta. Integrating molecular dynamics, structural analysis and targeted mutagenesis identified active site determinants that contribute to the distinct catalytic specificities underlying the broad substrate promiscuity of CYP71Z25-CYP71Z29 for native and non-native diterpenoids.

Next-Generation Mass Spectrometry Metabolomics Revives the Functional Analysis of Plant Metabolic Diversity

The remarkable diversity of specialized metabolites produced by plants has inspired several decades of research and nucleated a long list of theories to guide empirical ecological studies. However, analytical constraints and the lack of untargeted processing workflows have long precluded comprehensive metabolite profiling and, consequently, the collection of the critical currencies to test theory predictions for the ecological functions of plant metabolic diversity. Developments in mass spectrometry (MS) metabolomics have revolutionized the large-scale inventory and annotation of chemicals from biospecimens. Hence, the next generation of MS metabolomics propelled by new bioinformatics developments provides a long-awaited framework to revisit metabolism-centered ecological questions, much like the advances in next-generation sequencing of the last two decades impacted all research horizons in genomics. Here, we review advances in plant (computational) metabolomics to foster hypothesis formulation from complex metabolome data. Additionally, we reflect on how next-generation metabolomics could reinvigorate the testing of long-standing theories on plant metabolic diversity.

Managing enzyme promiscuity in plant specialized metabolism: A lesson from flavonoid biosynthesis: Mission of a "body double" protein clarified

Specificities of enzymes involved in plant specialized metabolism, including flavonoid biosynthesis, are generally promiscuous. This enzyme promiscuity has served as an evolutionary basis for new enzyme functions and metabolic pathways in land plants adapting to environmental challenges. This phenomenon may lead, however, to inefficiency in specialized metabolism and adversely affect metabolite-mediated plant survival. How plants manage enzyme promiscuity for efficient specialized metabolism is, thus, an open question. Recent studies of flavonoid biosynthesis addressing this issue have revealed a conserved strategy, namely, a homolog of chalcone isomerase with no catalytic activity binds to chalcone synthase, a key flavonoid pathway enzyme, to narrow (or rectify) the enzyme's highly promiscuous product specificity. Reducing promiscuity via specific protein-protein interactions among metabolic enzymes and proteins may be a solution adopted by land plants to achieve efficient operation of specialized metabolism, while the intrinsic promiscuity of enzymes has likely been retained incidentally.

Developmental Methylome of the Medicinal Plant Catharanthus roseus Unravels the Tissue-Specific Control of the Monoterpene Indole Alkaloid Pathway by DNA Methylation

Catharanthus roseus produces a wide spectrum of monoterpene indole alkaloids (MIAs). MIA biosynthesis requires a tightly coordinated pathway involving more than 30 enzymatic steps that are spatio-temporally and environmentally regulated so that some MIAs specifically accumulate in restricted plant parts. The first regulatory layer involves a complex network of transcription factors from the basic Helix Loop Helix (bHLH) or AP2 families. In the present manuscript, we investigated whether an additional epigenetic layer could control the organ-, developmental- and environmental-specificity of MIA accumulation. We used Whole-Genome Bisulfite Sequencing (WGBS) together with RNA-seq to identify differentially methylated and expressed genes among nine samples reflecting different plant organs and experimental conditions. Tissue specific gene expression was associated with specific methylation signatures depending on cytosine contexts and gene parts. Some genes encoding key enzymatic steps from the MIA pathway were found to be simultaneously differentially expressed and methylated in agreement with the corresponding MIA accumulation. In addition, we found that transcription factors were strikingly concerned by DNA methylation variations. Altogether, our integrative analysis supports an epigenetic regulation of specialized metabolisms in plants and more likely targeting transcription factors which in turn may control the expression of enzyme-encoding genes.

The foxtail millet (Setaria italica) terpene synthase gene family

Terpenoid metabolism plays vital roles in stress defense and the environmental adaptation of monocot crops. Here, we describe the identification of the terpene synthase (TPS) gene family of the panicoid food and bioenergy model crop foxtail millet (Setaria italica). The diploid S. italica genome contains 32 TPS genes, 17 of which were biochemically characterized in this study. Unlike other thus far investigated grasses, S. italica contains TPSs producing all three ent-, (+)- and syn-copalyl pyrophosphate stereoisomers that naturally occur as central building blocks in the biosynthesis of distinct monocot diterpenoids. Conversion of these intermediates by the promiscuous TPS SiTPS8 yielded different diterpenoid scaffolds. Additionally, a cytochrome P450 monooxygenase (CYP99A17), which genomically clustered with SiTPS8, catalyzes the C19 hydroxylation of SiTPS8 products to generate the corresponding diterpene alcohols. The presence of syntenic orthologs to about 19% of the S. italica TPSs in related grasses supports a common ancestry of selected pathway branches. Among the identified enzyme products, abietadien-19-ol, syn-pimara-7,15-dien-19-ol and germacrene-d-4-ol were detectable in planta, and gene expression analysis of the biosynthetic TPSs showed distinct and, albeit moderately, inducible expression patterns in response to biotic and abiotic stress. In vitro growth-inhibiting activity of abietadien-19-ol and syn-pimara-7,15-dien-19-ol against Fusarium verticillioides and Fusarium subglutinans may indicate pathogen defensive functions, whereas the low antifungal efficacy of tested sesquiterpenoids supports other bioactivities. Together, these findings expand the known chemical space of monocot terpenoid metabolism to enable further investigations of terpenoid-mediated stress resilience in these agriculturally important species.

Genomics-enabled analysis of specialized metabolism in bioenergy crops: current progress and challenges

Plants produce a staggering diversity of specialized small molecule metabolites that play vital roles in mediating environmental interactions and stress adaptation. This chemical diversity derives from dynamic biosynthetic pathway networks that are often species-specific and operate under tight spatiotemporal and environmental control. A growing divide between demand and environmental challenges in food and bioenergy crop production has intensified research on these complex metabolite networks and their contribution to crop fitness. High-throughput omics technologies provide access to ever-increasing data resources for investigating plant metabolism. However, the efficiency of using such system-wide data to decode the gene and enzyme functions controlling specialized metabolism has remained limited; due largely to the recalcitrance of many plants to genetic approaches and the lack of 'user-friendly' biochemical tools for studying the diverse enzyme classes involved in specialized metabolism. With emphasis on terpenoid metabolism in the bioenergy crop switchgrass as an example, this review aims to illustrate current advances and challenges in the application of DNA synthesis and synthetic biology tools for accelerating the functional discovery of genes, enzymes and pathways in plant specialized metabolism. These technologies have accelerated knowledge development on the biosynthesis and physiological roles of diverse metabolite networks across many ecologically and economically important plant species and can provide resources for application to precision breeding and natural product metabolic engineering.

Co-Regulation of Clustered and Neo-Functionalized Genes in Plant-Specialized Metabolism

Current findings of neighboring genes involved in plant specialized metabolism provide the genomic signatures of metabolic evolution. Two such genomic features, namely, (i) metabolic gene cluster and (ii) neo-functionalization of tandem gene duplications, represent key factors corresponding to the creation of metabolic diversity of plant specialized metabolism. So far, several terpenoid and alkaloid biosynthetic genes have been characterized with gene clusters in some plants. On the other hand, some modification genes involved in flavonoid and glucosinolate biosynthesis were found to arise via gene neo-functionalization. Although the occurrence of both types of metabolic evolution are different, the neighboring genes are generally regulated by the same or related regulation factors. Therefore, the translation-based approaches associated with genomics, and transcriptomics are able to be employed for functional genomics focusing on plant secondary metabolism. Here, we present a survey of the current understanding of neighboring genes involved in plant secondary metabolism. Additionally, a genomic overview of neighboring genes of four model plants and transcriptional co-expression network neighboring genes to detect metabolic gene clusters in Arabidopsis is provided. Finally, the insights functional genomics have provided concerning the evolution and mechanistic regulation of both the formation and operation of metabolic neighboring clusters is discussed.

Evolution of Structural Diversity of Triterpenoids

Plants have evolved to produce a blend of specialized metabolites that serve functional roles in plant adaptation. Among them, triterpenoids are one of the largest subclasses of such specialized metabolites, with more than 14,000 known structures. They play a role in plant defense and development and have potential applications within food and pharma. Triterpenoids are cyclized from oxidized squalene precursors by oxidosqualene cyclases, creating more than 100 different cyclical triterpene scaffolds. This limited number of scaffolds is the first step towards creating the vast structural diversity of triterpenoids followed by extensive diversification, in particular, by oxygenation and glycosylation. Gene duplication, divergence, and selection are major forces that drive triterpenoid structural diversification. The triterpenoid biosynthetic genes can be organized in non-homologous gene clusters, such as in Avena spp., Cucurbitaceae and Solanum spp., or scattered along plant chromosomes as in Barbarea vulgaris. Paralogous genes organized as tandem repeats reflect the extended gene duplication activities in the evolutionary history of the triterpenoid saponin pathways, as seen in B. vulgaris. We review and discuss examples of convergent and divergent evolution in triterpenoid biosynthesis, and the apparent mechanisms occurring in plants that drive their increasing structural diversity within and across species. Using B. vulgaris' saponins as examples, we discuss the impact a single structural modification can have on the structure of a triterpenoid and how this affect its biological properties. These examples provide insight into how plants continuously evolve their specialized metabolome, opening the way to study uncharacterized triterpenoid biosynthetic pathways.

Biosynthesis of the anti-diabetic metabolite montbretin A: glucosylation of the central intermediate mini-MbA

Type 2 diabetes (T2D) affects over 320 million people worldwide. Healthy lifestyles, improved drugs and effective nutraceuticals are different components of a response against the growing T2D epidemic. The specialized metabolite montbretin A (MbA) is being developed for treatment of T2D and obesity due to its unique pharmacological activity as a highly effective and selective inhibitor of the human pancreatic α-amylase. MbA is an acylated flavonol glycoside found in small amounts in montbretia (Crocosmia × crocosmiiflora) corms. MbA cannot be obtained in sufficient quantities for drug development from its natural source or by chemical synthesis. To overcome these limitations through metabolic engineering, we are investigating the genes and enzymes of MbA biosynthesis. We previously reported the first three steps of MbA biosynthesis from myricetin to myricetin 3-O-(6'-O-caffeoyl)-glucosyl rhamnoside (mini-MbA). Here, we describe the sequence of reactions from mini-MbA to MbA, and the discovery and characterization of the gene and enzyme responsible for the glucosylation of mini-MbA. The UDP-dependent glucosyltransferase CcUGT3 (UGT703E1) catalyzes the 1,2-glucosylation of mini-MbA to produce myricetin 3-O-(glucosyl-6'-O-caffeoyl)-glucosyl rhamnoside. Co-expression of CcUGT3 with genes for myricetin and mini-MbA biosynthesis in Nicotiana benthamiana validated its biological function and expanded the set of genes available for metabolic engineering of MbA.

Biochemical characterization of diterpene synthases of Taiwania cryptomerioides expands the known functional space of specialized diterpene metabolism in gymnosperms

Taiwania cryptomerioides is a monotypic gymnosperm species, valued for the high decay resistance of its wood. This durability has been attributed to the abundance of terpenoids, especially the major diterpenoid metabolite ferruginol, with antifungal and antitermite activity. Specialized diterpenoid metabolism in gymnosperms primarily recruits bifunctional class-I/II diterpene synthases (diTPSs), whereas monofunctional class-II and class-I enzymes operate in angiosperms. In this study, we identified a previously unrecognized group of monofunctional diTPSs in T. cryptomerioides, which suggests a distinct evolutionary divergence of the diTPS family in this species. Specifically, five monofunctional diTPS functions not previously observed in gymnosperms were characterized, including monofunctional class-II enzymes forming labda-13-en-8-ol diphosphate (LPP, TcCPS2) and (+)-copalyl diphosphate (CPP, TcCPS4), and three class-I diTPSs producing biformene (TcKSL1), levopimaradiene (TcKSL3) and phyllocladanol (TcKSL5), respectively. Methyl jasmonate (MeJA) elicited the accumulation of levopimaradiene and the corresponding biosynthetic diTPS genes, TcCPS4 and TcKSL3, is consistent with a possible role in plant defense. Furthermore, TcCPS4 and TcKSL3 are likely to contribute to abietatriene biosynthesis via levopimaradiene as an intermediate in ferruginol biosynthesis in Taiwania. In conclusion, this study provides deeper insight into the functional landscape and molecular evolution of specialized diterpenoid metabolism in gymnosperms as a basis to better understand the role of these metabolites in tree chemical defense.

Plant-derived coumarins shape the composition of an Arabidopsis synthetic root microbiome

The factors that contribute to the composition of the root microbiome and, in turn, affect plant fitness are not well understood. Recent work has highlighted a major contribution of the soil inoculum in determining the composition of the root microbiome. However, plants are known to conditionally exude a diverse array of unique secondary metabolites, that vary among species and environmental conditions and can interact with the surrounding biota. Here, we explore the role of specialized metabolites in dictating which bacteria reside in the rhizosphere. We employed a reduced synthetic community (SynCom) of Arabidopsis thaliana root-isolated bacteria to detect community shifts that occur in the absence of the secreted small-molecule phytoalexins, flavonoids, and coumarins. We find that lack of coumarin biosynthesis in f6'h1 mutant plant lines causes a shift in the root microbial community specifically under iron deficiency. We demonstrate a potential role for iron-mobilizing coumarins in sculpting the A. thaliana root bacterial community by inhibiting the proliferation of a relatively abundant Pseudomonas species via a redox-mediated mechanism. This work establishes a systematic approach enabling elucidation of specific mechanisms by which plant-derived molecules mediate microbial community composition. Our findings expand on the function of conditionally exuded specialized metabolites and suggest avenues to effectively engineer the rhizosphere with the aim of improving crop growth in iron-limited alkaline soils, which make up a third of the world's arable soils.

Terpene Synthases as Metabolic Gatekeepers in the Evolution of Plant Terpenoid Chemical Diversity

Terpenoids comprise tens of thousands of small molecule natural products that are widely distributed across all domains of life. Plants produce by far the largest array of terpenoids with various roles in development and chemical ecology. Driven by selective pressure to adapt to their specific ecological niche, individual species form only a fraction of the myriad plant terpenoids, typically representing unique metabolite blends. Terpene synthase (TPS) enzymes are the gatekeepers in generating terpenoid diversity by catalyzing complex carbocation-driven cyclization, rearrangement, and elimination reactions that enable the transformation of a few acyclic prenyl diphosphate substrates into a vast chemical library of hydrocarbon and, for a few enzymes, oxygenated terpene scaffolds. The seven currently defined clades (a-h) forming the plant TPS family evolved from ancestral triterpene synthase- and prenyl transferase-type enzymes through repeated events of gene duplication and subsequent loss, gain, or fusion of protein domains and further functional diversification. Lineage-specific expansion of these TPS clades led to variable family sizes that may range from a single TPS gene to families of more than 100 members that may further function as part of modular metabolic networks to maximize the number of possible products. Accompanying gene family expansion, the TPS family shows a profound functional plasticity, where minor active site alterations can dramatically impact product outcome, thus enabling the emergence of new functions with minimal investment in evolving new enzymes. This article reviews current knowledge on the functional diversity and molecular evolution of the plant TPS family that underlies the chemical diversity of bioactive terpenoids across the plant kingdom.

Functional Characterization of Two Class II Diterpene Synthases Indicates Additional Specialized Diterpenoid Pathways in Maize (Zea mays)

As a major staple food, maize (Zea mays) is critical to food security. Shifting environmental pressures increasingly hamper crop defense capacities, causing expanded harvest loss. Specialized labdane-type diterpenoids are key components of maize chemical defense and ecological adaptation. Labdane diterpenoid biosynthesis most commonly requires the pairwise activity of class II and class I diterpene synthases (diTPSs) that convert the central precursor geranylgeranyl diphosphate into distinct diterpenoid scaffolds. Two maize class II diTPSs, ANTHER EAR 1 and 2 (ZmAN1/2), have been previously identified as catalytically redundant ent-copalyl diphosphate (CPP) synthases. ZmAN1 is essential for gibberellin phytohormone biosynthesis, whereas ZmAN2 is stress-inducible and governs the formation of defensive kauralexin and dolabralexin diterpenoids. Here, we report the biochemical characterization of the two remaining class II diTPSs present in the maize genome, COPALYL DIPHOSPHATE SYNTHASE 3 (ZmCPS3) and COPALYL DIPHOSPHATE SYNTHASE 4 (ZmCPS4). Functional analysis via microbial co-expression assays identified ZmCPS3 as a (+)-CPP synthase, with functionally conserved orthologs occurring in wheat (Triticum aestivum) and numerous dicot species. ZmCPS4 formed the unusual prenyl diphosphate, 8,13-CPP (labda-8,13-dien-15-yl diphosphate), as verified by mass spectrometry and nuclear magnetic resonance. As a minor product, ZmCPS4 also produced labda-13-en-8-ol diphosphate (LPP). Root gene expression profiles did not indicate an inducible role of ZmCPS3 in maize stress responses. By contrast, ZmCPS4 showed a pattern of inducible gene expression in roots exposed to oxidative stress, supporting a possible role in abiotic stress responses. Identification of the catalytic activities of ZmCPS3 and ZmCPS4 clarifies the first committed reactions controlling the diversity of defensive diterpenoids in maize, and suggests the existence of additional yet undiscovered diterpenoid pathways.

Analysis and Quantification of Glucosinolates

Recent advances in liquid chromatography and mass spectrometry have made it possible to increase the throughput of phytochemical analyses at high sensitivity. These improvements have made it more feasible to monitor metabolic processes at the metabolite level. Glucosinolates, the primary defense compounds in the model plant Arabidopsis thaliana, in particular, have gained increasing attention as model compounds for quantitative genetics and metabolic regulation. Depending on the plant species, tissue, glucosinolate content, complexity of the glucosinolate profile, and, most importantly, the overall purpose of the experiment, different choices need to be made regarding the methods of extraction and analysis. In this chapter, we describe different approaches for the analysis of glucosinolates and highlight advantages, disadvantages, and technical pitfalls. © 2016 by John Wiley & Sons, Inc.

Differential metabolic and coexpression networks of plant metabolism

Recent analyses have demonstrated that plant metabolic networks do not differ in their structural properties and that genes involved in basic metabolic processes show smaller coexpression than genes involved in specialized metabolism. By contrast, our analysis reveals differences in the structure of plant metabolic networks and patterns of coexpression for genes in (non)specialized metabolism. Here we caution that conclusions concerning the organization of plant metabolism based on network-driven analyses strongly depend on the computational approaches used.

Diterpene synthases of the biosynthetic system of medicinally active diterpenoids in Marrubium vulgare

Marrubium vulgare (Lamiaceae) is a medicinal plant whose major bioactive compounds, marrubiin and other labdane-related furanoid diterpenoids, have potential applications as anti-diabetics, analgesics or vasorelaxants. Metabolite and transcriptome profiling of M. vulgare leaves identified five different candidate diterpene synthases (diTPSs) of the TPS-c and TPS-e/f clades. We describe the in vitro and in vivo functional characterization of the M. vulgare diTPS family. In addition to MvEKS ent-kaurene synthase of general metabolism, we identified three diTPSs of specialized metabolism: MvCPS3 (+)-copalyl diphosphate synthase, and the functional diTPS pair MvCPS1 and MvELS. In a sequential reaction, MvCPS1 and MvELS produce a unique oxygenated diterpene scaffold 9,13-epoxy-labd-14-ene en route to marrubiin and an array of related compounds. In contrast with previously known diTPSs that introduce a hydroxyl group at carbon C-8 of the labdane backbone, the MvCPS1-catalyzed reaction proceeds via oxygenation of an intermediate carbocation at C-9, yielding the bicyclic peregrinol diphosphate. MvELS belongs to a subgroup of the diTPS TPS-e/f clade with unusual βα-domain architecture. MvELS is active in vitro and in vivo with three different prenyl diphosphate substrates forming the marrubiin precursor 9,13-epoxy-labd-14-ene, as identified by nuclear magnetic resonance (NMR) analysis, manoyl oxide and miltiradiene. MvELS fills a central position in the biosynthetic system that forms the foundation for the diverse repertoire of Marrubium diterpenoids. Co-expression of MvCPS1 and MvELS in engineered E. coli and Nicotiana benthamiana offers opportunities for producing precursors for an array of biologically active diterpenoids.