Evolution of diterpene metabolism: Sitka spruce CYP720B4 catalyzes multiple oxidations in resin acid biosynthesis of conifer defense against insects

Combinatorial biosynthesis in yeast leads to over 200 diterpenoids

Diterpenoids form a diverse group of natural products, many of which are or could become pharmaceuticals or industrial chemicals. The modular character of diterpene biosynthesis and the promiscuity of the enzymes involved make combinatorial biosynthesis a promising approach to generate libraries of diverse diterpenoids. Here, we report on the combinatorial assembly in yeast of ten diterpene synthases producing (+)-copalyl diphosphate-derived backbones and four cytochrome P450 oxygenases (CYPs) in diverse combinations. This resulted in the production of over 200 diterpenoids. Based on literature and chemical database searches, 162 of these compounds can be considered new-to-Nature. The CYPs accepted most substrates they were given but remained regioselective with few exceptions. Our results provide the basis for the systematic exploration of the diterpenoid chemical space in yeast using sequence databases.

Phytotoxicity of weathered petroleum hydrocarbons in soil to boreal plant species

Canada has extensive petroleum hydrocarbon (PHC) contamination in northern areas and the boreal forest region from historical oil and gas activities. Since the 2013 standardization of boreal forest species for plant toxicity testing in Canada, there has been a need to build the primary literature of the toxicity of weathered PHCs to these species. A series of toxicity experiments were carried out using fine-grained (<0.005-0.425 mm) background (100 total mg/kg total PHCs) and weathered contaminated soil (11,900 mg/kg total PHCs) collected from a contaminated site in northern Ontario, Canada. The PHC mixture in the contaminated site soil was characterized through Canadian Council of Ministers of the Environment Fractions, as indicated by the number equivalent normal straight-chain hydrocarbons (nC). The soil was highly contaminated with Fraction 2 (>nC10 to nC16) at 4790 mg/kg and Fraction 3 (>nC16 to nC34) at 4960 mg/kg. Five plant species (Elymus trachycaulus, Achillea millefolium, Picea mariana, Salix bebbiana, and Alnus viridis) were grown from seed in 0%, 25%, 50%, 75%, and 100% relative contamination mixtures of the PHC-contaminated and background soil from the site over 2-6 weeks. All five species showed significant inhibition in shoot length, shoot weight, root length, and/or root weight (Kruskal-Wallis Tests: p < 0.05, df = 4.0). Measurements of 25% inhibitory concentrations (IC25) following PHC toxicity experiments revealed that S. bebbiana was most significantly impaired by the PHC-contaminated soil (410-990 mg/kg total PHCs), where it showed <35% germination. This study indicates that natural weathering of Fraction 2- and Fraction 3-concentrated soil did not eliminate phytotoxicity to boreal plant species. Furthermore, it builds on the limited existing literature for toxicity of PHCs on boreal plants and supports site remediation to existing Canadian provincial PHC guidelines.

Comprehensive and evolutionary analysis of Spodoptera litura-inducible Cytochrome P450 monooxygenase gene family in Glycine max elucidate their role in defense

Plants being sessile organisms and lacking both circulating phagocytic cells and somatic adaptive immune response, have thrived on various defense mechanisms to fend off insect pests and invasion of pathogens. CYP450s are the versatile enzymes, which thwart plants against insect pests by ubiquitous biosynthesis of phytohormones, antioxidants, and secondary metabolites, utilizing them as feeding deterrents and direct toxins. Therefore, a comprehensive analysis of biotic stress-responsive CYPs from Glycine max was performed to ascertain their function against S. litura-infestation. Phylogenetic analysis and evolutionary studies on conserved domains and motifs disclosed the evolutionary correspondence of these GmCYPs with already characterized members of the CYP450 superfamily and close relatedness to Medicago truncatula. These GmCYPs were mapped on 13 chromosomes; they possess 1-8 exons; they have evolved due to duplication and are localized in endoplasmic reticulumn. Further, identification of methyl-jasmonate, salicylic acid, defense responsive and flavonoid biosynthesis regulating cis-acting elements, their interaction with biotic stress regulating proteins and their differential expression in diverse types of tissues, and during herbivory, depicted their responsiveness to biotic stress. Three-dimensional homology modelling of GmCYPs, docking with heme cofactor required for their catalytic activity and enzyme-substrate interactions were performed to understand the functional mechanism of their action. Moreover, to gain insight into their involvement in plant defense, gene expression analysis was evaluated, which revealed differential expression of 11 GmCYPs upon S. litura-infestation, 12 GmCYPs on wounding while foliar spray of ethylene, methyl-jasmonate and salicylic acid differentially regulated 11 GmCYPs, 6 GmCYPs, and 10 GmCYPs respectively. Our study comprehensively analysed the underlying mechanism of GmCYPs function during S. litura-infestation, which can be further utilized for functional characterization to develop new strategies for enhancing soybean resistance to insect pests.

CYP72D19 from Tripterygium wilfordii catalyzes C-2 hydroxylation of abietane-type diterpenoids

KEY MESSAGE

CYP72D19, the first functional gene of the CYP72D subfamily, catalyzes the C-2 hydroxylation of abietane-type diterpenoids. The abietane-type diterpenoids, e.g., triptolide, tripdiolide, and 2-epitripdiolide, are the main natural products for the anti-tumor, anti-inflammatory, and immunosuppressive activities of Tripterygium wilfordii, while their biosynthetic pathways are not resolved. Here, we cloned and characterized the CYP72D19-catalyzed C-2 hydroxylation of dehydroabietic acid, a compound that has been proven to be a biosynthetic intermediate in triptolide biosynthesis. Through molecular docking and site-directed mutagenesis, L386, L387, and I493 near the active pocket were found to have an important effect on the enzyme activity, which also indicates that steric hindrance of residues plays an important role in function. In addition, CYP72D19 also catalyzed a variety of abietane-type diterpenoids with benzene ring, presumably because the benzene ring of the substrate molecule stabilized the C-ring, allowing the protein and the substrate to form a relatively stable spatial structure. This is the first demonstration of CYP72D subfamily gene function. Our research provides important genetic elements for the structural modification of active ingredients and the heterologous production of other 2-hydroxyl abietane-type natural products.

Terpenes modulate bacterial and fungal growth and sorghum rhizobiome communities

Terpenes are among the oldest and largest class of plant-specialized bioproducts that are known to affect plant development, adaptation, and biological interactions. While their biosynthesis, evolution, and function in aboveground interactions with insects and individual microbial species are well studied, how different terpenes impact plant microbiomes belowground is much less understood. Here we designed an experiment to assess how belowground exogenous applications of monoterpenes (1,8-cineole and linalool) and a sesquiterpene (nerolidol) delivered through an artificial root system impacted its belowground bacterial and fungal microbiome. We found that the terpene applications had significant and variable impacts on bacterial and fungal communities, depending on terpene class and concentration; however, these impacts were localized to the artificial root system and the fungal rhizosphere. We complemented this experiment with pure culture bioassays on responsive bacteria and fungi isolated from the sorghum rhizobiome. Overall, higher concentrations (200 µM) of nerolidol were inhibitory to Ferrovibrium and tested Firmicutes. While fungal isolates of Penicillium and Periconia were also more inhibited by higher concentrations (200 µM) of nerolidol, Clonostachys was enhanced at this higher level and together with Humicola was inhibited by the lower concentration tested (100 µM). On the other hand, 1,8-cineole had an inhibitory effect on Orbilia at both tested concentrations but had a promotive effect at 100 µM on Penicillium and Periconia. Similarly, linalool at 100 µM had significant growth promotion in Mortierella, but an inhibitory effect for Orbilia. Together, these results highlight the variable direct effects of terpenes on single microbial isolates and demonstrate the complexity of microbe-terpene interactions in the rhizobiome. IMPORTANCE Terpenes represent one of the largest and oldest classes of plant-specialized metabolism, but their role in the belowground microbiome is poorly understood. Here, we used a "rhizobox" mesocosm experimental set-up to supply different concentrations and classes of terpenes into the soil compartment with growing sorghum for 1 month to assess how these terpenes affect sorghum bacterial and fungal rhizobiome communities. Changes in bacterial and fungal communities between treatments belowground were characterized, followed by bioassays screening on bacterial and fungal isolates from the sorghum rhizosphere against terpenes to validate direct microbial responses. We found that microbial growth stimulatory and inhibitory effects were localized, terpene specific, dose dependent, and transient in time. This work paves the way for engineering terpene metabolisms in plant microbiomes for improved sustainable agriculture and bioenergy crop production.

Physiological, Proteomic, and Resin Yield-Related Genes Expression Analysis Provides Insights into the Mechanisms Regulating Resin Yield in Masson Pine

Masson pine (Pinus massoniana Lamb.) is an important resin-producing conifer species in China. Resin yield is a highly heritable trait and varies greatly among different genotypes. However, the mechanisms regulating the resin yield of masson pine remain largely unknown. In this study, physiological, proteomic, and gene expression analysis was performed on xylem tissues of masson pine with high and low resin yield. Physiological investigation showed that the activity of terpene synthase, as well as the contents of soluble sugar, jasmonic acid (JA), methyl jasmonate (MeJA), gibberellins (GA1, GA4, GA9, GA19, and GA20), indole-3-acetic acid (IAA), and abscisic acid (ABA) were significantly increased in the high yielder, whereas sucrose and salicylic acid (SA) were significantly decreased compared with the low one. A total of 2984 differentially expressed proteins (DEPs) were identified in four groups, which were mainly enriched in the biosynthesis of secondary metabolites, protein processing in the endoplasmic reticulum, carbohydrate metabolism, phytohormone biosynthesis, glutathione metabolism, and plant-pathogen interaction. Integrated physiological and proteomic analysis revealed that carbohydrate metabolism, terpenoid biosynthesis, resistance to stress, as well as JA and GA biosynthesis and signaling, play key roles in regulating resin yield. A series of proteins associated with resin yield, e.g., terpene synthase proteins (TPSs), ATP-binding cassette transporters (ABCs), glutathione S-transferase proteins (GSTs), and heat shock proteins (HSPs), were identified. Resin yield-related gene expression was also associated with resin yield. Our study unveils the implicated molecular mechanisms regulating resin yield and is of pivotal significance to breeding strategies of high resin-yielding masson pine cultivars.

Hydroxylases involved in terpenoid biosynthesis: a review

Terpenoids are pervasive in nature and display an immense structural diversity. As the largest category of plant secondary metabolites, terpenoids have important socioeconomic value in the fields of pharmaceuticals, spices, and food manufacturing. The biosynthesis of terpenoid skeletons has made great progress, but the subsequent modifications of the terpenoid framework are poorly understood, especially for the functionalization of inert carbon skeleton usually catalyzed by hydroxylases. Hydroxylase is a class of enzymes that plays an important role in the modification of terpenoid backbone. This review article outlines the research progress in the identification, molecular modification, and functional expression of this class of enzymes in the past decade, which are profitable for the discovery, engineering, and application of more hydroxylases involved in the plant secondary metabolism.

Plant terpene specialized metabolism: complex networks or simple linear pathways?

From the perspectives of pathway evolution, discovery and engineering of plant specialized metabolism, the nature of the biosynthetic routes represents a critical aspect. Classical models depict biosynthesis typically from an end-point angle and as linear, for example, connecting central and specialized metabolism. As the number of functionally elucidated routes increased, the enzymatic foundation of complex plant chemistries became increasingly well understood. The perception of linear pathway models has been severely challenged. With a focus on plant terpenoid specialized metabolism, we review here illustrative examples supporting that plants have evolved complex networks driving chemical diversification. The completion of several diterpene, sesquiterpene and monoterpene routes shows complex formation of scaffolds and their subsequent functionalization. These networks show that branch points, including multiple sub-routes, mean that metabolic grids are the rule rather than the exception. This concept presents significant implications for biotechnological production.

Identification and functional characterization of three cytochrome P450 genes for the abietane diterpenoid biosynthesis in Isodon lophanthoides

We identify two ferruginol synthases and a 11-hydroxyferruginol synthase from a traditional Chinese medicinal herb Isodon lophanthoides and propose their involvement in two independent abietane diterpenoids biosynthetic pathways. Isodon lophanthoides is a traditional Chinese medicinal herb rich in highly oxidized abietane-type diterpenoids. These compounds exhibit a wide range of pharmaceutical activities, yet the biosynthesis is barely known. Here, we describe the screening and functional characterization of P450s that oxidize the abietane skeleton abietatriene. We mainly focused on CYP76 family and identified 12 CYP76AHs by mining the RNA-seq data of I. lophanthoides. Among the 12 CYP76AHs, 6 exhibited similar transcriptional expression features as upstream diterpene synthases, including root or leaf-preferential expression pattern and highly MeJA inducibility. These six P450s were considered as first-tier candidates and functionally characterized in yeast and plant cells. In yeast assays showed that both CYP76AH42 and CYP76AH43 were ferruginol synthases hydroxylating the C12 position of abietatriene, whereas CYP76AH46 was characterized as a 11-hydroxyferruginol synthase which catalyzes two successive oxidations at C12 and C11 of abietatriene. Heterologous expression of three CYP76AHs in Nicotiana benthamiana resulted in the formation of ferruginol. qPCR analysis showed CYP76AH42 and CYP76AH43 were mainly expressed in the root, which was consistent with the distribution of ferruginol in the root periderms. CYP76AH46 was primarily expressed in the leaves where barely ferruginol or 11-hydroxyferruginol was detected. In addition to distinct organ-specific expression pattern, three CYP76AHs exhibited different genomic structures (w or w/o introns), low protein sequence identities (51-63%) and were placed in separate subclades in the phylogenetic tree. These results suggest that the identified CYP76AHs may be involved in at least two independent abietane biosynthetic pathways in the aerial and underground parts of I. lophanthoides.

Promiscuity, a Driver of Plant Cytochrome P450 Evolution?

Plant cytochrome P450 monooxygenases were long considered to be highly substrate-specific, regioselective and stereoselective enzymes, in this respect differing from their animal counterparts. The functional data that have recently accumulated clearly counter this initial dogma. Highly promiscuous P450 enzymes have now been reported, mainly in terpenoid pathways with functions in plant adaptation, but also some very versatile xenobiotic/herbicide metabolizers. An overlap and predictable interference between endogenous and herbicide metabolism are starting to emerge. Both substrate preference and permissiveness vary between plant P450 families, with high promiscuity seemingly favoring retention of gene duplicates and evolutionary blooms. Yet significant promiscuity can also be observed in the families under high negative selection and with essential functions, usually enhanced after gene duplication. The strategies so far implemented, to systematically explore P450 catalytic capacity, are described and discussed.

Plant Terpenoid Permeability through Biological Membranes Explored via Molecular Simulations

Plants synthesize small molecule diterpenes composed of 20 carbons from precursor isopentenyl diphosphate and dimethylallyl disphosphate, manufacturing diverse compounds used for defense, signaling, and other functions. Industrially, diterpenes are used as natural aromas and flavoring, as pharmaceuticals, and as natural insecticides or repellents. Despite diterpene ubiquity in plant systems, it remains unknown how plants control diterpene localization and transport. For many other small molecules, plant cells maintain transport proteins that control compound compartmentalization. However, for most diterpene compounds, specific transport proteins have not been identified, and so it has been hypothesized that diterpenes may cross biological membranes passively. Through molecular simulation, we study membrane transport for three complex diterpenes from among the many made by members of the Lamiaceae family to determine their permeability coefficient across plasma membrane models. To facilitate accurate simulation, the intermolecular interactions for leubethanol, abietic acid, and sclareol were parametrized through the standard CHARMM methodology for incorporation into molecular simulations. To evaluate the effect of membrane composition on permeability, we simulate the three diterpenes in two membrane models derived from sorghum and yeast lipidomics data. We track permeation events within our unbiased simulations, and compare implied permeation coefficients with those calculated from Replica Exchange Umbrella Sampling calculations using the inhomogeneous solubility diffusion model. The diterpenes are observed to permeate freely through these membranes, indicating that a transport protein may not be needed to export these small molecules from plant cells. Moreover, the permeability is observed to be greater for plant-like membrane compositions when compared against animal-like membrane models. Increased permeability for diterpene molecules in plant membranes suggest that plants have tailored their membranes to facilitate low-energy transport processes for signaling molecules.

Comparative transcriptomic responses of European and Japanese larches to infection by Phytophthora ramorum

BACKGROUND AND OBJECTIVES

Phytophthora ramorum severely affects both European larch (EL) and Japanese larch (JL) trees as indicated by high levels of mortality particularly in the UK. Field observations suggested that EL is less severely affected and so may be less susceptible to P. ramorum than JL; however, controlled inoculations have produced inconsistent or non-statistically significant differences. The present study aimed to compare RNA transcript accumulation profiles in EL and JL in response to inoculation with P. ramorum to improve our understanding of their defence responses.

METHODOLOGY

RNA-sequencing was carried out on bark tissues following the inoculation with P. ramorum of potted saplings in both EL and JL carried out under controlled environment conditions, with sampling at 1, 3, 10, and 25 days post inoculation in infected and control plants.

RESULTS

All of the inoculated trees rapidly developed lesions but no statistically significant differences were found in lesion lengths between EL and JL. RNA-Sequencing comparing control and inoculate saplings identified key differences in differentially expressed genes (DEGs) between the two larch species. European larch had rapid induction of defence genes within 24 hours of infection followed by sustained expression until 25 days after inoculation. Results in JL were more varied; upregulation was stronger but more transient and represented fewer defence pathways. Gene enrichment analyses highlighted differences in jasmonate signalling and regulation including NPR1 upregulation in EL only, and specific aspects of secondary metabolism. Some DEGs were represented by multiple responsive copies including lipoxygenase, chalcone synthase and nucleotide-binding, leucine-rich-repeat genes.

CONCLUSION

The variations between EL and JL in responsive DEGs of interest as potentially related to differences seen in the field and should be considered in the selection of trees for planting and future breeding.

Identification of Genes and Metabolic Pathways Involved in Resin Yield in Masson Pine by Integrative Analysis of Transcriptome, Proteome and Biochemical Characteristics

Masson pine (Pinus massoniana L.) is one of the most important resin-producing tree species in southern China. However, the molecular regulatory mechanisms of resin yield are still unclear in masson pine. In this study, an integrated analysis of transcriptome, proteome, and biochemical characteristics from needles of masson pine with the high and common resin yield was investigated. The results showed that chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl C), carotenoids (Car), glucose (Glu), gibberellin A9 (GA9), gibberellin A15 (GA15), and gibberellin A53 (GA53) were significantly increased, whereas fructose (Fru), jasmonic acid (JA), jasmonoyl-L-isoleucine (JA-ILE), gibberellin A1 (GA1), gibberellin A3 (GA3), gibberellin A19 (GA19), and gibberellin A24 (GA24) were significantly decreased in the high resin yield in comparison with those in the common one. The integrated analysis of transcriptome and proteome showed that chlorophyll synthase (chlG), hexokinase (HXK), sucrose synthase (SUS), phosphoglycerate kinase (PGK), dihydrolipoamide dehydrogenase (PDH), dihydrolipoamide succinyltransferase (DLST), 12-oxophytodienoic acid reductase (OPR), and jasmonate O-methyltransferases (JMT) were consistent at the transcriptomic, proteomic, and biochemical levels. The pathways of carbohydrate metabolism, terpenoid biosynthesis, photosynthesis, and hormone biosynthesis may play crucial roles in the regulation of resin yield, and some key genes involved in these pathways may be candidates that influence the resin yield. These results provide insights into the molecular regulatory mechanisms of resin yield and also provide candidate genes that can be applied for the molecular-assisted selection and breeding of high resin-yielding masson pine.

Comparative transcriptomics of Pinus massoniana organs provides insights on terpene biosynthesis regulation

Terpenoids are the most important natural products collected from conifer species. However, the molecular mechanisms and core factors underlying terpenoid biosynthesis in Pinus massoniana remain unclear. To clarify these mechanisms, this study aimed to identify potential genes that might participate in the terpenoid biosynthesis of P. massoniana. In this study, single molecule real-time (SMRT) sequencing and expression analysis were used to confirm the expression patterns of genes involved in the cones, immature needles, mature needles, immature branches, and mature branches of P. massoniana. A total of 31,331 lncRNAs and 71,240 mRNAs were identified from these organs, and the greatest number of differentially expressed genes (DEGs) was discovered between needles and branches. Weighted gene coexpression network analysis (WGCNA) classified all expressed genes into nine typical modules with 11 kinds of transcription factors (TFs), namely, AP2-ERF, ARF, AUX-IAA, C2H2, Dof, F-box, SBP, WRKY, bHLH, bZIP, and GRAS, and seven kinds of functional genes, namely, ABC transporter, cellulose synthase (CesA), leucine-rich repeats (LRR), cytochrome P450 (CYT P450), pathogenesis-related protein (PR), terpene synthase (TPS), and chlorophyllase enzyme. A molecular network was constructed for hub genes, TFs, and functional genes in three modules. The potential function of eight candidate genes, including PmbHLH2, PmERF1, PmRGA, PmGAI, PmbZIP1, PmLOB1, PmMADS1, and PmMYB1, was validated through correlation analysis between terpenoid contents and expression levels, subcellular localization, and transcriptional activation activity, which provides us with probable regulators of terpenoid biosynthesis in conifers.

A gene cluster in Ginkgo biloba encodes unique multifunctional cytochrome P450s that initiate ginkgolide biosynthesis

The ginkgo tree (Ginkgo biloba) is considered a living fossil due to its 200 million year's history under morphological stasis. Its resilience is partly attributed to its unique set of specialized metabolites, in particular, ginkgolides and bilobalide, which are chemically complex terpene trilactones. Here, we use a gene cluster-guided mining approach in combination with co-expression analysis to reveal the primary steps in ginkgolide biosynthesis. We show that five multifunctional cytochrome P450s with atypical catalytic activities generate the tert-butyl group and one of the lactone rings, characteristic of all G. biloba trilactone terpenoids. The reactions include scarless C-C bond cleavage as well as carbon skeleton rearrangement (NIH shift) occurring on a previously unsuspected intermediate. The cytochrome P450s belong to CYP families that diversifies in pre-seed plants and gymnosperms, but are not preserved in angiosperms. Our work uncovers the early ginkgolide pathway and offers a glance into the biosynthesis of terpenoids of the Mesozoic Era.

Identification and Defensive Characterization of PmCYP720B11v2 from Pinus massoniana

Pinus massoniana is a pioneer species for afforestation timber and oleoresin, while epidemics of pinewood nematode (PWN; Bursaphelenchus xylophilus) are causing a serious biotic disaster for P. massoniana in China. Importantly, resistant P. massoniana could leak copious oleoresin terpenoids to build particular defense fronts for survival when attacked by PWN. However, the defense mechanisms regulating this process remain unknown. Here, PmCYP720B11v2, a cytochrome P450 monooxygenase gene, was first identified and functionally characterized from resistant P. massoniana following PWN inoculation. The tissue-specific expression pattern and localization of PmCYP720B11v2 at the transcript and protein levels in resistant P. massoniana indicated that its upregulation in the stem supported its involvement in the metabolic processes of diterpene biosynthesis as a positive part of the defense against PWN attack. Furthermore, overexpression of PmCYP720B11v2 may enhance the growth and development of plants. In addition, PmCYP720B11v2 activated the metabolic flux of antioxidases and stress-responsive proteins under drought conditions and improved drought stress tolerance. Our results provide new insights into the favorable role of PmCYP720B11v2 in diterpene defense mechanisms in response to PWN attack in resistant P. massoniana and provide a novel metabolic engineering scenario to reform the stress tolerance potential of tobacco.

Mountain Pine Beetle Epidemic: An Interplay of Terpenoids in Host Defense and Insect Pheromones

The mountain pine beetle epidemic has highlighted the complex interactions of bark beetles with conifer host defenses. In these interactions, oleoresin terpenoids and volatiles, produced and released by the host tree, can be both harmful and beneficial to the beetle's success in colonizing a tree and completing its life cycle. The insect spends almost its entire life, from egg to adult, within the bark and phloem of a pine host, exposed to large quantities of complex mixtures of oleoresin terpenoids. Conifer oleoresin comprises mostly monoterpenes and diterpene resin acids as well as many different sesquiterpenes. It functions as a major chemical and physical defense system. However, the insect has evolved host colonization behavior and enzymes for terpenoid metabolism and detoxification that allow it to overcome some of the terpenoid defenses and, importantly, to co-opt pine monoterpenes as cues for host search and as a precursor for its own pheromone system. The insect-associated microbiome also plays a role in the metabolism of conifer terpenoids.

Chromosome-Scale Assembly of the Dendrobium nobile Genome Provides Insights Into the Molecular Mechanism of the Biosynthesis of the Medicinal Active Ingredient of Dendrobium

Orchids constitute approximately 10% of flowering plant species. However, only about 10 orchid genomes have been published. Metabolites are the main way through which orchids respond to their environment. Dendrobium nobile, belonging to Dendrobium, the second largest genus in Orchidaceae, has high ornamental, medicinal, and ecological value. D. nobile is the source of many popular horticultural varieties. Among the Dendrobium species, D. nobile has the highest amount of dendrobine, which is regarded as one of the criteria for evaluating medicinal quality. Due to lack of data and analysis at the genomic level, the biosynthesis pathways of dendrobine and other related medicinal ingredients in D. nobile are unknown. In this paper, we report a chromosome-scale reference genome of D. nobile to facilitate the investigation of its genomic characteristics for comparison with other Dendrobium species. The assembled genome size of D. nobile was 1.19 Gb. Of the sequences, 99.45% were anchored to 19 chromosomes. Furthermore, we identified differences in gene number and gene expression patterns compared with two other Dendrobium species by integrating whole-genome sequencing and transcriptomic analysis [e.g., genes in the polysaccharide biosynthesis pathway and upstream of the alkaloid (dendrobine) biosynthesis pathway]. Differences in the TPS and CYP450 gene families were also found among orchid species. All the above differences might contribute to the species-specific medicinal ingredient biosynthesis pathways. The metabolic pathway-related analysis will provide further insight into orchid responses to the environment. Additionally, the reference genome will provide important insights for further molecular elucidation of the medicinal active ingredients of Dendrobium and enhance the understanding of orchid evolution.

Identification and Tissue-Specific Expression Analysis of CYP720B Subfamily Genes in Slash Pine and Loblolly Pine

Diterpene resin acids (DRAs) are major components of pine oleoresin that can effectively resist the invasion of insects and pathogenic microorganisms. The subfamily of cytochrome P450s, CYP720B, catalyzes diterpene products into DRAs. Identifying CYP720B subfamily members and revealing the characteristics of tissue-specific expression would help understand diterpene-rich structures and diverse types. Slash pine and loblolly pine are important pines that provide oleoresin products. In this study, we identified CYP720B candidate genes based on the Pinus taeda V2.0 genome and full-length transcriptome of slash pine by PacBio. A total of 17 genes in slash pine and 19 in loblolly pine were identified and classified into four main clades by phylogenetic analysis. An analysis of cis-acting elements showed that CYP720B genes were closely related to adversity resistance. The gene expression of these candidates in different tissues was quantified by real-time quantitative PCR (RT–qPCR) analysis. Most of the genes showed relatively higher expression levels in roots and stems than in the other tissues, corresponding with the results of DRA component detection by gas chromatography–mass spectrometry (GC–MS), which indicated that stems and roots might be important tissues in oleoresin biosynthesis. These results provide a valuable resource for a better understanding of the biological role of individual CYP720Bs in slash pine and loblolly pine.

Overexpression of geranyl diphosphate synthase (PmGPPS1) boosts monoterpene and diterpene production involved in the response to pine wood nematode invasion

Outbreaks of pine wood nematode (PWN; Bursaphelenchus xylophilus) represent a severe biotic epidemic for the Pinus massoniana in China. When invaded by the PWN, the resistant P. massoniana might secret abundant oleoresin terpenoid to form certain defensive fronts for survival. However, the regulatory mechanisms of this process remain unclear. Here, the geranyl diphosphate synthase (PmGPPS1) gene was identified from resistant P. massoniana. Tissue-specific expression patterns of PmGPPS1 at transcript and protein level in resistant P. massoniana were determined by quantitative real-time polymerase chain reaction (qRT-PCR) and immunohistochemistry. Functional characteristics analysis of PmGPPS1 was performed on transgenic Nicotiana benthamiana by overexpression, as genetic transformation of P. massoniana is, so far, not possible. In summary, we identified and functionally characterized PmGPPS1 from the resistant P. massoniana following PWN inoculation. Tissue-specific expression patterns and localization of PmGPPS1 indicated that it may play a positive role involved in the metabolic and defensive processes of oleoresin terpenes production in response to PWN attack. Furthermore, overexpression of PmGPPS1 may enhance the production of monoterpene, among which limonene reduced the survival of PWN in vitro. In addition, PmGPPS1 upregulated the expression level of key genes involved in mevalonic acid (MVA) pathway, the methylerythritol phosphate (MEP) pathway and gibberellins (GAs) biosynthesis to boost the growth and development of tobacco through a feedback regulation mechanism. Our results offered new insights into the pivotal role of the PmGPPS1 involved in terpene-based defense mechanisms responding to the PWN invasion in resistant P. massoniana and provided a new metabolic engineering scenario to improve monoterpene production in tobacco.

Diterpene Resin Acids and Olefins in Calabrian Pine (Pinus nigra subsp. laricio (Poiret) Maire) Oleoresin: GC-MS Profiling of Major Diterpenoids in Different Plant Organs, Molecular Identification and Expression Analysis of Diterpene Synthase Genes

A quali-quantitative analysis of diterpenoid composition in tissues obtained from different organs of Pinus nigra subsp. laricio (Poiret) Maire (Calabrian pine) was carried out. Diterpene resin acids were the most abundant diterpenoids across all the examined tissues. The same nine diterpene resin acids were always found, with the abietane type prevailing on the pimarane type, although their quantitative distribution was found to be remarkably tissue-specific. The scrutiny of the available literature revealed species specificity as well. A phylogeny-based approach allowed us to isolate four cDNAs coding for diterpene synthases in Calabrian pine, each of which belonging to one of the four groups into which the d3 clade of the plants' terpene synthases family can be divided. The deduced amino acid sequences allowed predicting that both monofunctional and bifunctional diterpene synthases are involved in the biosynthesis of diterpene resin acids in Calabrian pine. Transcript profiling revealed differential expression across the different tissues and was found to be consistent with the corresponding diterpenoid profiles. The isolation of the complete genomic sequences and the determination of their exon/intron structures allowed us to place the diterpene synthase genes from Calabrian pine on the background of current ideas on the functional evolution of diterpene synthases in Gymnosperms.

A review: biosynthesis of plant-derived labdane-related diterpenoids

Plant-derived labdane-related diterpenoids (LRDs) represent a large group of terpenoids. LRDs possess either a labdane-type bicyclic core structure or more complex ring systems derived from labdane-type skeletons, such as abietane, pimarane, kaurane, etc. Due to their various pharmaceutical activities and unique properties, many of LRDs have been widely used in pharmaceutical, food and perfume industries. Biosynthesis of various LRDs has been extensively studied, leading to characterization of a large number of new biosynthetic enzymes. The biosynthetic pathways of important LRDs and the relevant enzymes (especially diterpene synthases and cytochrome P450 enzymes) were summarized in this review.

Plant cytochrome P450 plasticity and evolution

The superfamily of cytochrome P450 (CYP) enzymes plays key roles in plant evolution and metabolic diversification. This review provides a status on the CYP landscape within green algae and land plants. The 11 conserved CYP clans known from vascular plants are all present in green algae and several green algae-specific clans are recognized. Clan 71, 72, and 85 remain the largest CYP clans and include many taxa-specific CYP (sub)families reflecting emergence of linage-specific pathways. Molecular features and dynamics of CYP plasticity and evolution are discussed and exemplified by selected biosynthetic pathways. High substrate promiscuity is commonly observed for CYPs from large families, favoring retention of gene duplicates and neofunctionalization, thus seeding acquisition of new functions. Elucidation of biosynthetic pathways producing metabolites with sporadic distribution across plant phylogeny reveals multiple examples of convergent evolution where CYPs have been independently recruited from the same or different CYP families, to adapt to similar environmental challenges or ecological niches. Sometimes only a single or a few mutations are required for functional interconversion. A compilation of functionally characterized plant CYPs is provided online through the Plant P450 Database (erda.dk/public/vgrid/PlantP450/).

Identification of Candidate Gene for Internode Length in Rice to Enhance Resistance to Lodging Using QTL Analysis

Internode length and stem diameter are the primary traits affecting the lodging resistance of rice. Traits related to the length of the panicle (LP), uppermost internode (LUI), second internode (LSI), third internode (LTI), fourth internode (LFI), lowest internode (LLI) as well as stem diameter at the uppermost internode (SDUI), second internode (SDSI), third internode (SDTI), fourth internode (SDFI), and lowest internode (SDLI) in 120 Cheongcheong/Nagdong doubled haploid population were investigated using a quantitative trait locus (QTL) analysis. Thirty-four QTL regions affected LP and the length of each internode. Twenty-six QTL regions were associated with the stem diameter of each internode. RM12285-RM212 on chromosome 1 contained 10 QTLs related to the internode length, which have overlapped for over 2 years. Twenty-three candidate genes were screened using mark interval. Among the candidate genes, Os01g0803900, named OsCYPq1, which is in the Cytochrome P450 family, might be involved in gibberellins (GA) synthesis. GA is an essential plant growth regulator that affects plant height. OsCYPq1 catalyzes oxidation steps in the middle part of the GA pathway. OsCYPq1 is expected to provide valuable information to improve the marker assessment for target traits and QTL gene cloning in rice.

Plant cytochrome P450s: Role in stress tolerance and potential applications for human welfare

Cytochrome P450s (CYPs) are a versatile group of enzymes and one of the largest families of proteins, controlling various physiological processes via biosynthetic and detoxification pathways. CYPs perform multiple roles through a critical irreversible enzymatic reaction in which an oxygen atom is inserted within hydrophobic molecules, converting them into the reactive and hydro soluble components. During evolution, plants have acquired significantly more number of CYPs and represent about 1% of the encoded genes . CYPs are highly conserved proteins involved in growth, development and tolerance against biotic and abiotic stresses. Furthermore, CYPs reinforce plants' molecular and chemical defense mechanisms by regulating the biosynthesis of secondary metabolites, enhancing reactive oxygen species (ROS) scavenging and controlling biosynthesis and homeostasis of phytohormones, including abscisic acid (ABA) and jasmonates. Thus, they are the critical targets of metabolic engineering for enhancing plant defense against environmental stresses. Additionally, CYPs are also used as biocatalysts in the fields of pharmacology and phytoremediation. Herein, we highlight the role of CYPs in plant stress tolerance and their applications for human welfare.

Triptolide: reflections on two decades of research and prospects for the future

Covering: 2000 to 2020 Triptolide is a bioactive diterpene triepoxide isolated from Tripterygium wilfordii Hook F, a traditional Chinese medicinal plant whose extracts have been used as anti-inflammatory and immunosuppressive remedies for centuries. Although triptolide and its analogs exhibit potent bioactivities against various cancers, and inflammatory and autoimmune diseases, none of them has been approved to be used in the clinic. This review highlights advances in material sourcing, molecular mechanisms, clinical progress and new drug design strategies for triptolide over the past two decades, along with some prospects for the future course of development of triptolide.

Two terpene synthases in resistant Pinus massoniana contribute to defence against Bursaphelenchus xylophilus

Pine wood nematode (PWN; Bursaphelenchus xylophilus), a destructive pest of Pinus massoniana, is causing a severe epidemic of pine wilt disease in China. When invaded by PWN, resistant P. massoniana secretes an abundance of oleoresin terpenoids as a defensive strategy. However, regulatory mechanisms of this defence in resistant P. massoniana have yet to be elucidated. Here, we characterized two terpene synthase genes, α-pinene synthase (PmTPS4) and longifolene synthase (PmTPS21), identified in resistant P. massoniana and investigate the contribution of these genes to the oleoresin defence strategy in resistant masson pines. Up-regulation of these two genes in the stem supported their involvement in terpene biosynthesis as part of the defence against PWN. Recombinant protein expression revealed catalytic activity for the two PmTPSs, with PmTPS4 primarily producing α-pinene, while PmTPS21 produced α-pinene and longifolene simultaneously. The major enzymatic products of the two terpene synthases had inhibitory effects on PWN in vitro. We demonstrated that PmTPS4 and PmTPS21 played positive roles in terpene-defence mechanisms against PWN infestation. The major products of these terpene synthases could directly inhibit the survival rate of PWN in vitro. We revealed that PmTPS21 was a novel bifunctional enzyme capable of simultaneous production of both monoterpene and sesquiterpene.

The biosynthesis of the anti-microbial diterpenoid leubethanol in Leucophyllum frutescens proceeds via an all-cis prenyl intermediate

Serrulatane diterpenoids are natural products found in plants from a subset of genera within the figwort family (Scrophulariaceae). Many of these compounds have been characterized as having anti-microbial properties and share a common diterpene backbone. One example, leubethanol from Texas sage (Leucophyllum frutescens) has demonstrated activity against multi-drug-resistant tuberculosis. Leubethanol is the only serrulatane diterpenoid identified from this genus; however, a range of such compounds have been found throughout the closely related Eremophila genus. Despite their potential therapeutic relevance, the biosynthesis of serrulatane diterpenoids has not been previously reported. Here we leverage the simple product profile and high accumulation of leubethanol in the roots of L. frutescens and compare tissue-specific transcriptomes with existing data from Eremophila serrulata to decipher the biosynthesis of leubethanol. A short-chain cis-prenyl transferase (LfCPT1) first produces the rare diterpene precursor nerylneryl diphosphate, which is cyclized by an unusual plastidial terpene synthase (LfTPS1) into the characteristic serrulatane diterpene backbone. Final conversion to leubethanol is catalyzed by a cytochrome P450 (CYP71D616) of the CYP71 clan. This pathway documents the presence of a short-chain cis-prenyl diphosphate synthase, previously only found in Solanaceae, which is likely involved in the biosynthesis of other known diterpene backbones in Eremophila. LfTPS1 represents neofunctionalization of a compartment-switching terpene synthase accepting a novel substrate in the plastid. Biosynthetic access to leubethanol will enable pathway discovery to more complex serrulatane diterpenoids which share this common starting structure and provide a platform for the production and diversification of this class of promising anti-microbial therapeutics in heterologous systems.

Genomic basis of white pine blister rust quantitative disease resistance and its relationship with qualitative resistance

The genomic architecture and molecular mechanisms controlling variation in quantitative disease resistance loci are not well understood in plant species and have been barely studied in long-generation trees. Quantitative trait loci mapping and genome-wide association studies were combined to test a large single nucleotide polymorphism (SNP) set for association with quantitative and qualitative white pine blister rust resistance in sugar pine. In the absence of a chromosome-scale reference genome, a high-density consensus linkage map was generated to obtain locations for associated SNPs. Newly discovered associations for white pine blister rust quantitative disease resistance included 453 SNPs involved in wide biological functions, including genes associated with disease resistance and others involved in morphological and developmental processes. In addition, NBS-LRR pathogen recognition genes were found to be involved in quantitative disease resistance, suggesting these newly reported genes are qualitative genes with partial resistance, they are the result of defeated qualitative resistance due to avirulent races, or they have epistatic effects on qualitative disease resistance genes. This study is a step forward in our understanding of the complex genomic architecture of quantitative disease resistance in long-generation trees, and constitutes the first step towards marker-assisted disease resistance breeding in white pine species.

Identification of Three Monofunctional Diterpene Synthases with Specific Enzyme Activities Expressed during Heartwood Formation in Western Redcedar (Thuja plicata) Trees

Upon harvest, Western redcedar (WRC; Thuja plicata) trees have a high incidence and extent of heartwood rot. While monoterpenoids and lignans have been linked to rot resistance in this species, other specialized metabolites, such as diterpenes, are likely to contribute to rot resistance. Here we report the cloning and functional assessment of three putative diterpene synthase (TpdiTPS) genes expressed during heartwood formation in WRC. The predicted proteins of the three genes lack either of the two catalytically independent active sites typical of most diTPS, indicating monofunctional rather than bifunctional activity. To identify potential catalytic activities of these proteins, we expressed them in genetically engineered Escherichia coli strains that produce four potential substrates, geranylgeranyl diphosphate (GGDP), ent, syn, and normal stereoisomers of copalyl diphosphate (CDP). We found that TpdiTPS3 used GGDP to produce CDP. TpdiTPS2 used normal CDP to produce levopimaradiene. TpdiTPS1 showed stereoselectivity as it used normal CDP to produce sandaracopimaradiene and syn-CDP to produce syn-stemod-13(17)-ene. These genes and protein enzymatic activities have not been previously reported in WRC and provide an opportunity to assess their potential roles in heartwood rot resistance in this economically important species.

Gymnosperm glandular trichomes: expanded dimensions of the conifer terpenoid defense system

Glandular trichomes (GTs) are defensive structures that produce and accumulate specialized metabolites and protect plants against herbivores, pathogens, and abiotic stress. GTs have been extensively studied in angiosperms for their roles in defense and biosynthesis of high-value metabolites. In contrast, trichomes of gymnosperms have been described in fossilized samples, but have not been studied in living plants. Here, we describe the characterization of GTs on young stems of a hybrid white spruce. Metabolite and histological analysis of spruce GTs support a glandular function with accumulation of a diverse array of mono-, sesqui- and diterpenes including diterpene methylesters. Methylated diterpenes have previously been associated with insect resistance in white spruce. Headspeace analysis of spruce GTs showed a profile of volatiles dominated by monoterpenes and a highly diverse array of sesquiterpenes. Spruce GTs appear early during shoot growth, prior to the development of a lignified bark and prior to accumulation of terpenes in needles. Spruce GTs may provide an early, terpene-based chemical defense system at a developmental stage when young shoots are particularly vulnerable to foliage and shoot feeding insects, and before the resin duct system characteristic of conifers has fully developed.

Genome assembly of Chiococca alba uncovers key enzymes involved in the biosynthesis of unusual terpenoids

Chiococca alba (L.) Hitchc. (snowberry), a member of the Rubiaceae, has been used as a folk remedy for a range of health issues including inflammation and rheumatism and produces a wealth of specialized metabolites including terpenes, alkaloids, and flavonoids. We generated a 558 Mb draft genome assembly for snowberry which encodes 28,707 high-confidence genes. Comparative analyses with other angiosperm genomes revealed enrichment in snowberry of lineage-specific genes involved in specialized metabolism. Synteny between snowberry and Coffea canephora Pierre ex A. Froehner (coffee) was evident, including the chromosomal region encoding caffeine biosynthesis in coffee, albeit syntelogs of N-methyltransferase were absent in snowberry. A total of 27 putative terpene synthase genes were identified, including 10 that encode diterpene synthases. Functional validation of a subset of putative terpene synthases revealed that combinations of diterpene synthases yielded access to products of both general and specialized metabolism. Specifically, we identified plausible intermediates in the biosynthesis of merilactone and ribenone, structurally unique antimicrobial diterpene natural products. Access to the C. alba genome will enable additional characterization of biosynthetic pathways responsible for health-promoting compounds in this medicinal species.

Grape terpenoids: flavor importance, genetic regulation, and future potential

Terpenes significantly affect the flavor and quality of grapes and wine. This review summarizes recent research on terpenoids with regard to grape wine. Although, the grapevine terpene synthase gene family is the largest identified, genetic modifications involving terpenes to improve wine flavor have received little attention. Key enzyme modulation alters metabolite production. Over the last decade, the heterologous manipulation of grape glycosidase has been used to alter terpenoids, and cytochrome P450s may affect terpene synthesis. Metabolic and genetic engineering can further modify terpenoid metabolism, while using transgenic grapevines (trait transfer to the plant) could yield more flavorful wine. We also discuss traits involved in wine aroma quality, and the strategies that can be used to improve grapevine breeding technology.

Genome of Tripterygium wilfordii and identification of cytochrome P450 involved in triptolide biosynthesis

Triptolide is a trace natural product of Tripterygium wilfordii. It has antitumor activities, particularly against pancreatic cancer cells. Identification of genes and elucidation of the biosynthetic pathway leading to triptolide are the prerequisite for heterologous bioproduction. Here, we report a reference-grade genome of T. wilfordii with a contig N50 of 4.36 Mb. We show that copy numbers of triptolide biosynthetic pathway genes are impacted by a recent whole-genome triplication event. We further integrate genomic, transcriptomic, and metabolomic data to map a gene-to-metabolite network. This leads to the identification of a cytochrome P450 (CYP728B70) that can catalyze oxidation of a methyl to the acid moiety of dehydroabietic acid in triptolide biosynthesis. We think the genomic resource and the candidate genes reported here set the foundation to fully reveal triptolide biosynthetic pathway and consequently the heterologous bioproduction.

Tripterygium wilfordii is a medical plant that can produce antitumor activity compound triptolide. Here, the authors assemble its genome and identify a cytochrome P450 that can catalyze oxidation of a methyl to the acid moiety of dehydroabietic acid in triptolide biosynthesis by integrating multi-omics data.

Functional and morphological evolution in gymnosperms: A portrait of implicated gene families

Gymnosperms diverged from their sister plant clade of flowering plants 300 Mya. Morphological and functional divergence between the two major seed plant clades involved significant changes in their reproductive biology, water-conducting systems, secondary metabolism, stress defense mechanisms, and small RNA-mediated epigenetic silencing. The relatively recent sequencing of several gymnosperm genomes and the development of new genomic resources have enabled whole-genome comparisons within gymnosperms, and between angiosperms and gymnosperms. In this paper, we aim to understand how genes and gene families have contributed to the major functional and morphological differences in gymnosperms, and how this information can be used for applied breeding and biotechnology. In addition, we have analyzed the angiosperm versus gymnosperm evolution of the pleiotropic drug resistance (PDR) gene family with a wide range of functionalities in plants' interaction with their environment including defense mechanisms. Some of the genes reviewed here are newly studied members of gene families that hold potential for biotechnological applications related to commercial and pharmacological value. Some members of conifer gene families can also be exploited for their potential in phytoremediation applications.

Oleoresin defenses in conifers: chemical diversity, terpene synthases and limitations of oleoresin defense under climate change

Conifers have evolved complex oleoresin terpene defenses against herbivores and pathogens. In co-evolved bark beetles, conifer terpenes also serve chemo-ecological functions as pheromone precursors, chemical barcodes for host identification, or nutrients for insect-associated microbiomes. We highlight the genomic, molecular and biochemical underpinnings of the large chemical space of conifer oleoresin terpenes and volatiles. Conifer terpenes are predominantly the products of the conifer terpene synthase (TPS) gene family. Terpene diversity is increased by cytochromes P450 of the CYP720B class. Many conifer TPS are multiproduct enzymes. Multisubstrate CYP720B enzymes catalyse multistep oxidations. We summarise known terpenoid gene functions in various different conifer species with reference to the annotated terpenoid gene space in a spruce genome. Overall, biosynthesis of terpene diversity in conifers is achieved through a system of biochemical radiation and metabolic grids. Expression of TPS and CYP720B genes can be specific to individual cell types of constitutive or traumatic resin duct systems. Induced terpenoid transcriptomes in resin duct cells lead to dynamic changes of terpene composition and quantity to fend off herbivores and pathogens. While terpenoid defenses have contributed much to the evolutionary success of conifers, under new conditions of climate change, these defences may become inconsequential against range-expanding forest pests.

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.

A molecular and genomic reference system for conifer defence against insects

Insect pests are part of natural forest ecosystems contributing to forest rejuvenation but can also cause ecological disturbance and economic losses that are expected to increase with climate change. The white pine or spruce weevil (Pissodes strobi) is a pest of conifer forests in North America. Weevil-host interactions with various spruce (Picea) species have been explored as a genomic and molecular reference system for conifer defence against insects. Interactions occur in two major phases of the insect life cycle. In the exophase, adult weevils are free-moving and display behaviour of host selection for oviposition that is affected by host traits. In the endophase, insects live within the host where mobility and development from eggs to young adults are affected by a complex system of host defences. Genetic resistance exists in several spruce species and involves synergism of constitutive and induced chemical and physical defences that comprise the conifer defence syndrome. Here, we review conifer defences that disrupt the weevil life cycle and mechanisms by which trees resist weevil attack. We highlight molecular and genomic aspects and a possible role for the weevil microbiome. Knowledge of this conifer defence system is supporting forest health strategies and tree breeding for insect resistance.

Research advances in cytochrome P450-catalysed pharmaceutical terpenoid biosynthesis in plants

Terpenoids, the biggest class of plant secondary metabolites, have a wide range of significant physiological roles, while many of them are important natural drugs. Biosynthesis of pharmaceutical terpenoids in plants is a fairly complex process, most of which involves cytochrome P450 (CYP450) monooxygenases. CYP450 enzymes are versatile biocatalysts that play critical roles in terpenoid skeleton modification and structural diversity. Therefore, the discovery and identification of CYP450 genes is significant for elucidating the terpenoid biosynthetic pathway. This review summarizes the progress and cloning strategies relating to CYP450s in pharmaceutical terpenoid biosynthesis of the past decade.

Promiscuous terpene synthases from Prunella vulgaris highlight the importance of substrate and compartment switching in terpene synthase evolution

The mint family (Lamiaceae) is well documented as a rich source of terpene natural products. More than 200 diterpene skeletons have been reported from mints, but biosynthetic pathways are known for just a few of these. We crossreferenced chemotaxonomic data with publicly available transcriptomes to select common selfheal (Prunella vulgaris) and its highly unusual vulgarisin diterpenoids as a case study for exploring the origins of diterpene skeletal diversity in Lamiaceae. Four terpene synthases (TPS) from the TPS-a subfamily, including two localised to the plastid, were cloned and functionally characterised. Previous examples of TPS-a enzymes from Lamiaceae were cytosolic and reported to act on the 15-carbon farnesyl diphosphate. Plastidial TPS-a enzymes using the 20-carbon geranylgeranyl diphosphate are known from other plant families, having apparently arisen independently in each family. All four new enzymes were found to be active on multiple prenyl-diphosphate substrates with different chain lengths and stereochemistries. One of the new enzymes catalysed the cyclisation of geranylgeranyl diphosphate into 11-hydroxy vulgarisane, the likely biosynthetic precursor of the vulgarisins. We uncovered the pathway to a rare diterpene skeleton. Our results support an emerging paradigm of substrate and compartment switching as important aspects of TPS evolution and diversification.

Cytochrome P450 enzymes: A driving force of plant diterpene diversity

In plant terpene biosynthesis, oxidation of the hydrocarbon backbone produced by terpene synthases is typically carried out by cytochrome P450 oxygenases (CYPs). The modifications introduced by CYPs include hydroxylations, sequential oxidations at one position and ring rearrangements and closures. These reactions significantly expand the structural diversity of terpenoids, but also provide anchoring points for further decorations by various transferases. In recent years, there has been a significant increase in reports of CYPs involved in plant terpene pathways. Plant diterpenes represent an important class of metabolites that includes hormones and a number of industrially relevant compounds such as pharmaceutical, aroma or food ingredients. In this review, we provide a comprehensive survey on CYPs reported to be involved in plant diterpene biosynthesis to date. A phylogenetic analysis showed that only few CYP clans are represented in diterpene biosynthesis, namely CYP71, CYP85 and CYP72. Remarkably few CYP families and subfamilies within those clans are involved, indicating specific expansion of these clades in plant diterpene biosynthesis. Nonetheless, the evolutionary trajectory of CYPs of specialized diterpene biosynthesis is diverse. Some are recently derived from gibberellin biosynthesis, while others have a more ancient history with recent expansions in specific plant families. Among diterpenoids, labdane-related diterpenoids represent a dominant class. The availability of CYPs from diverse plant species able to catalyze oxidations in specific regions of the labdane-related backbones provides opportunities for combinatorial biosynthesis to produce novel diterpene compounds that can be screened for biological activities of interest.

Functional characterization of the cytochrome P450 monooxygenase CYP71AU87 indicates a role in marrubiin biosynthesis in the medicinal plant Marrubium vulgare

BACKGROUND

Horehound (Marrubium vulgare) is a medicinal plant whose signature bioactive compounds, marrubiin and related furanoid diterpenoid lactones, have potential applications for the treatment of cardiovascular diseases and type II diabetes. Lack of scalable plant cultivation and the complex metabolite profile of M. vulgare limit access to marrubiin via extraction from plant biomass. Knowledge of the marrubiin-biosynthetic enzymes can enable the development of metabolic engineering platforms for marrubiin production. We previously identified two diterpene synthases, MvCPS1 and MvELS, that act sequentially to form 9,13-epoxy-labd-14-ene. Conversion of 9,13-epoxy-labd-14-ene by cytochrome P450 monooxygenase (P450) enzymes can be hypothesized to facilitate key functional modification reactions in the formation of marrubiin and related compounds.

RESULTS

Mining a M. vulgare leaf transcriptome database identified 95 full-length P450 candidates. Cloning and functional analysis of select P450 candidates showing high transcript abundance revealed a member of the CYP71 family, CYP71AU87, that catalyzed the hydroxylation of 9,13-epoxy-labd-14-ene to yield two isomeric products, 9,13-epoxy labd-14-ene-18-ol and 9,13-epoxy labd-14-ene-19-ol, as verified by GC-MS and NMR analysis. Additional transient Nicotiana benthamiana co-expression assays of CYP71AU87 with different diterpene synthase pairs suggested that CYP71AU87 is specific to the sequential MvCPS1 and MvELS product 9,13-epoxy-labd-14-ene. Although the P450 products were not detectable in planta, high levels of CYP71AU87 gene expression in marrubiin-accumulating tissues supported a role in the formation of marrubiin and related diterpenoids in M. vulgare.

CONCLUSIONS

In a sequential reaction with the diterpene synthase pair MvCPS1 and MvELS, CYP71AU87 forms the isomeric products 9,13-epoxy labd-14-ene-18/19-ol as probable intermediates in marrubiin biosynthesis. Although its metabolic relevance in planta will necessitate further genetic studies, identification of the CYP71AU87 catalytic activity expands our knowledge of the functional landscape of plant P450 enzymes involved in specialized diterpenoid metabolism and can provide a resource for the formulation of marrubiin and related bioactive natural products.

Cytosolic lipid droplets as engineered organelles for production and accumulation of terpenoid biomaterials in leaves

Cytosolic lipid droplets are endoplasmic reticulum-derived organelles typically found in seeds as reservoirs for physiological energy and carbon to fuel germination. Here, we report synthetic biology approaches to co-produce high-value sesqui- or diterpenoids together with lipid droplets in plant leaves. The formation of cytosolic lipid droplets is enhanced in the transient Nicotiana benthamiana system through ectopic production of WRINKLED1, a key regulator of plastid fatty acid biosynthesis, and a microalgal lipid droplet surface protein. Engineering of the pathways providing the universal C5-building blocks for terpenoids and installation of terpenoid biosynthetic pathways through direction of the enzymes to native and non-native compartments boost the production of target terpenoids. We show that anchoring of distinct biosynthetic steps onto the surface of lipid droplets leads to efficient production of terpenoid scaffolds and functionalized terpenoids. The co-produced lipid droplets "trap" the terpenoids in the cells.

Functions of stone cells and oleoresin terpenes in the conifer defense syndrome

Conifers depend on complex defense systems against herbivores. Stone cells (SC) and oleoresin are physical and chemical defenses of Sitka spruce that have been separately studied in previous work. Weevil oviposit at the tip of the previous year's apical shoot (PYAS). We investigated interactions between weevil larvae and trees in controlled oviposition experiments with resistant (R) and susceptible (S) Sitka spruce. R trees have an abundance of SC in the PYAS cortex. SC are mostly absent in S trees. R trees and S trees also differ in the composition of oleoresin terpenes. Transcriptomes of R and S trees revealed differences in long-term weevil-induced responses. Performance of larvae was significantly reduced on R trees compared with S trees under experimental conditions that mimicked natural oviposition behavior at apical shoot tips and may be attributed to the effects of SC. In oviposition experiments designed for larvae to feed below the area of highest SC abundance, larvae showed an unusual feeding behavior and oleoresin appeared to function as the major defense. The results support a role for both SC and oleoresin terpenes and possible synergies between these traits in the defense syndrome of weevil-resistant Sitka spruce.

Identification, expression, and phylogenetic analyses of terpenoid biosynthesis-related genes in secondary xylem of loblolly pine (Pinus taeda L.) based on transcriptome analyses

Loblolly pine (Pinus taeda L.) is one of the most important species for oleoresin (a mixture of terpenoids) in South China. The high oleoresin content of loblolly pine is associated with resistance to bark beetles and other economic benefits. In this study, we conducted transcriptome analyses of loblolly pine secondary xylem to gain insight into the genes involved in terpenoid biosynthesis. A total of 372 unigenes were identified as being critical for oleoresin production, including genes for ATP-binding cassette (ABC) transporters, the cytochrome P450 (CYP) protein family, and terpenoid backbone biosynthesis enzymes. Six key genes involved in terpenoid biosynthetic pathways were selected for multiple sequence alignment, conserved motif prediction, and phylogenetic and expression profile analyses. The protein sequences of all six genes exhibited a higher degree of sequence conservation, and upstream genes were relatively more conserved than downstream genes in terpenoid biosynthetic pathways. The N-terminal regions of these sequences were less conserved than the C-terminal ends, as the N-terminals were quite diverse in both length and composition. The phylogenetic analyses revealed that most genes originated from gene duplication after species divergence, and partial genes exhibited incomplete lineage sorting. In addition, the expression profile analyses showed that all six genes exhibited high expression levels during the high-oleoresin-yielding phase.

A database-driven approach identifies additional diterpene synthase activities in the mint family (Lamiaceae)

Members of the mint family (Lamiaceae) accumulate a wide variety of industrially and medicinally relevant diterpenes. We recently sequenced leaf transcriptomes from 48 phylogenetically diverse Lamiaceae species. Here, we summarize the available chemotaxonomic and enzyme activity data for diterpene synthases (diTPSs) in the Lamiaceae and leverage the new transcriptomes to explore the diTPS sequence and functional space. Candidate genes were selected with an intent to evenly sample the sequence homology space and to focus on species in which diTPS transcripts were found, yet from which no diterpene structures have been previously reported. We functionally characterized nine class II diTPSs and 10 class I diTPSs from 11 distinct plant species and found five class II activities, including two novel activities, as well as a spectrum of class I activities. Among the class II diTPSs, we identified a neo-cleroda-4(18),13E-dienyl diphosphate synthase from Ajuga reptans, catalyzing the likely first step in the biosynthesis of a variety of insect-antifeedant compounds. Among the class I diTPSs was a palustradiene synthase from Origanum majorana, leading to the discovery of specialized diterpenes in that species. Our results provide insights into the diversification of diterpene biosynthesis in the mint family and establish a comprehensive foundation for continued investigation of diterpene biosynthesis in the Lamiaceae.

Assessment of flux through oleoresin biosynthesis in epithelial cells of loblolly pine resin ducts

The shoot system of pines contains abundant resin ducts, which harbor oleoresins that play important roles in constitutive and inducible defenses. In a pilot study, we assessed the chemical diversity of oleoresins obtained from mature tissues of loblolly pine trees (Pinus taeda L.). Building on these data sets, we designed experiments to assess oleoresin biosynthesis in needles of 2-year-old saplings. Comparative transcriptome analyses of single cell types indicated that genes involved in the biosynthesis of oleoresins are significantly enriched in isolated epithelial cells of resin ducts, compared with those expressed in mesophyll cells. Simulations using newly developed genome-scale models of epithelial and mesophyll cells, which incorporate our data on oleoresin yield and composition as well as gene expression patterns, predicted that heterotrophic metabolism in epithelial cells involves enhanced levels of oxidative phosphorylation and fermentation (providing redox and energy equivalents). Furthermore, flux was predicted to be more evenly distributed across the metabolic network of mesophyll cells, which, in contrast to epithelial cells, do not synthesize high levels of specialized metabolites. Our findings provide novel insights into the remarkable specialization of metabolism in epithelial cells.