Coexpression Analysis Identifies Two Oxidoreductases Involved in the Biosynthesis of the Monoterpene Acid Moiety of Natural Pyrethrin Insecticides in Tanacetum cinerariifolium

Exploring the co-operativity of secretory structures for defense and pollination in flowering plants

MAIN CONCLUSION

In flowers multiple secretory systems cooperate to deliver specialized metabolites to support specific roles in defence and pollination. The collective roles of cell types, enzymes, and transporters are discussed. The interplay between reproductive strategies and defense mechanisms in flowering plants has long been recognized, with trade-offs between investment in defense and reproduction predicted. Glandular trichomes and secretory cavities or ducts, which are epidermal and internal structures, play a pivotal role in the secretion, accumulation, and transport of specialized secondary metabolites, and contribute significantly to defense and pollination. Recent investigations have revealed an intricate connection between these two structures, whereby specialized volatile and non-volatile metabolites are exchanged, collectively shaping their respective ecological functions. However, a comprehensive understanding of this profound integration remains largely elusive. In this review, we explore the secretory systems and associated secondary metabolism primarily in Asteraceous species to propose potential shared mechanisms facilitating the directional translocation of these metabolites to diverse destinations. We summarize recent advances in our understanding of the cooperativity between epidermal and internal secretory structures in the biosynthesis, secretion, accumulation, and emission of terpenes, providing specific well-documented examples from pyrethrum (Tanacetum cinerariifolium). Pyrethrum is renowned for its natural pyrethrin insecticides, which accumulate in the flower head, and more recently, for emitting an aphid alarm pheromone. These examples highlight the diverse specializations of secondary metabolism in pyrethrum and raise intriguing questions regarding the regulation of production and translocation of these compounds within and between its various epidermal and internal secretory systems, spanning multiple tissues, to serve distinct ecological purposes. By discussing the cooperative nature of secretory structures in flowering plants, this review sheds light on the intricate mechanisms underlying the ecological roles of terpenes in defense and pollination.

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.

TcbHLH14 a Jasmonate Associated MYC2-like Transcription Factor Positively Regulates Pyrethrin Biosynthesis in Tanacetum cinerariifolium

Natural pyrethrins have high application value, and are widely used as a green pesticide in crop pest prevention and control. Pyrethrins are mainly extracted from the flower heads of Tanacetum cinerariifolium; however, the natural content is low. Therefore, it is essential to understand the regulatory mechanisms underlying the synthesis of pyrethrins through identification of key transcription factors. We identified a gene encoding a MYC2-like transcription factor named TcbHLH14 from T. cinerariifolium transcriptome, which is induced by methyl jasmonate. In the present study, we evaluated the regulatory effects and mechanisms of TcbHLH14 using expression analysis, a yeast one-hybrid assay, electrophoretic mobility shift assay, and overexpression/virus-induced gene silencing experiments. We found that TcbHLH14 can directly bind to the cis-elements of the pyrethrins synthesis genes TcAOC and TcGLIP to activate their expression. The transient overexpression of TcbHLH14 enhanced expression of the TcAOC and TcGLIP genes. Conversely, transient silencing of TcbHLH14 downregulated the expression of TcAOC and TcGLIP and reduced the content of pyrethrins. In summary, these results indicate that the potential application of TcbHLH14 in improving the germplasm resources and provide a new insight into the regulatory network of pyrethrins biosynthesis of T. cinerariifolium to further inform the development of engineering strategies for increasing pyrethrins contents.

TcbZIP60 positively regulates pyrethrins biosynthesis in Tanacetum cinerariifolium

Pyrethrins, synthesized in the perennial plant Tanacetum cinerariifolium, are a class of terpene mixtures with high insecticidal activity and low human toxicity, which are widely used in plant-derived pesticides. Numerous studies have identified multiple pyrethrins biosynthesis enzymes, which can be enhanced by exogenous hormones such as methyl jasmonate (MeJA). However, the mechanism by which hormone signaling regulates pyrethrins biosynthesis and the potential involvement of certain transcription factors (TFs) remain unclear. In this study, we found that the expression level of a TF in T. cinerariifolium was significantly increased after treatment with plant hormones (MeJA, abscisic acid). Subsequent analysis identified this TF as a member of the basic region/leucine zipper (bZIP) family and was thus named TcbZIP60. TcbZIP60 was localized in the nucleus, suggesting that it is involved in the transcription process. The expression profiles of TcbZIP60 were similar to those of pyrethrins synthesis genes in different flower organs and at different flowering stages. Furthermore, TcbZIP60 could directly bind to the E-box/G-box motifs in the promoters of the pyrethrins synthesis genes TcCHS and TcAOC to activate their expression. Transient overexpression of TcbZIP60 increased the expression levels of pyrethrins biosynthesis genes, leading to the significant accumulation of pyrethrins. Silencing of TcbZIP60 significantly downregulated pyrethrins accumulation and the expression of related genes. Overall, our results reveal a novel TF, TcbZIP60, that regulates both the terpenoid and jasmonic acid pathways of pyrethrins biosynthesis in T. cinerariifolium.

TcMYB8, a R3-MYB Transcription Factor, Positively Regulates Pyrethrin Biosynthesis in Tanacetum cinerariifolium

Pyrethrins are a mixture of terpenes, with insecticidal properties, that accumulate in the aboveground parts of the pyrethrum (Tanacetum cinerariifolium). Numerous studies have been published on the positive role of MYB transcription factors (TFs) in terpenoid biosynthesis; however, the role of MYB TFs in pyrethrin biosynthesis remains unknown. Here, we report the isolation and characterization of a T. cinerariifolium MYB gene encoding a R3-MYB protein, TcMYB8, containing a large number of hormone-responsive elements in its promoter. The expression of the TcMYB8 gene showed a downward trend during the development stage of flowers and leaves, and was induced by methyl jasmonate (MeJA), salicylic acid (SA), and abscisic acid (ABA). Transient overexpression of TcMYB8 enhanced the expression of key enzyme-encoding genes, TcCHS and TcGLIP, and increased the content of pyrethrins. By contrast, transient silencing of TcMYB8 decreased pyrethrin contents and downregulated TcCHS and TcGLIP expression. Further analysis indicated that TcMYB8 directly binds to cis-elements in proTcCHS and proTcGLIP to activate their expression, thus regulating pyrethrin biosynthesis. Together, these results highlight the potential application of TcMYB8 for improving the T. cinerariifolium germplasm, and provide insight into the pyrethrin biosynthesis regulation network.

Achievements and perspectives of synthetic biology in botanical insecticides

Botanical insecticides are the origin of all insecticidal compounds. They have been widely used to control pests in crops for a long time. Currently, the commercial production of botanical insecticides extracted from plants is limited because of insufficient raw material supply. Synthetic biology is a promising and effective approach for addressing the current problems of the production of botanical insecticides. It is an emerging biological research hotspot in the field of botanical insecticides. However, the biosynthetic pathways of many botanical insecticides are not completely elucidated. On the other hand, the cytotoxicity of botanical pesticides and low efficiency of these biosynthetic enzymes in new hosts make it still challenging for their heterologous production. In the present review, we summarized the recent developments in the heterologous production of botanical insecticides, analyzed the current challenges, and discussed the feasible production strategies, focusing on elucidating biosynthetic pathways, enzyme engineering, host engineering, and cytotoxicity engineering. Looking to the future, synthetic biology promises to further advance heterologous production of more botanical pesticides.

Synthesis of 4-methylvaleric acid, a precursor of pogostone, involves a 2-isobutylmalate synthase related to 2-isopropylmalate synthase of leucine biosynthesis

We show here that the side chain of pogostone, one of the major components of patchouli oil obtained from Pogostemon cablin and possessing a variety of pharmacological activities, is derived from 4-methylvaleric acid. We also show that 4-methylvaleric acid is produced through the one-carbon α-ketoacid elongation pathway with the involvement of the key enzyme 2-isobutylmalate synthase (IBMS), a newly identified enzyme related to isopropylmalate synthase (IPMS) of leucine (Leu) biosynthesis. Site-directed mutagenesis identified Met¹³² in the N-terminal catalytic region as affecting the substrate specificity of PcIBMS1. Even though PcIBMS1 possesses the C-terminal domain that in IPMS serves to mediate Leu inhibition, it is insensitive to Leu. The observation of the evolution of IBMS from IPMS, as well as previously reported examples of IPMS-related genes involved in making glucosinolates in Brassicaceae, acylsugars in Solanaceae, and flavour compounds in apple, indicate that IPMS genes represent an important pool for the independent evolution of genes for specialised metabolism.

Draft Genome of Tanacetum Coccineum: Genomic Comparison of Closely Related Tanacetum-Family Plants

The plant Tanacetum coccineum (painted daisy) is closely related to Tanacetum cinerariifolium (pyrethrum daisy). However, T. cinerariifolium produces large amounts of pyrethrins, a class of natural insecticides, whereas T. coccineum produces much smaller amounts of these compounds. Thus, comparative genomic analysis is expected to contribute a great deal to investigating the differences in biological defense systems, including pyrethrin biosynthesis. Here, we elucidated the 9.4 Gb draft genome of T. coccineum, consisting of 2,836,647 scaffolds and 103,680 genes. Comparative analyses of the draft genome of T. coccineum and that of T. cinerariifolium, generated in our previous study, revealed distinct features of T. coccineum genes. While the T. coccineum genome contains more numerous ribosome-inactivating protein (RIP)-encoding genes, the number of higher-toxicity type-II RIP-encoding genes is larger in T. cinerariifolium. Furthermore, the number of histidine kinases encoded by the T. coccineum genome is smaller than that of T. cinerariifolium, suggesting a biological correlation with pyrethrin biosynthesis. Moreover, the flanking regions of pyrethrin biosynthesis-related genes are also distinct between these two plants. These results provide clues to the elucidation of species-specific biodefense systems, including the regulatory mechanisms underlying pyrethrin production.

Overexpression of TcCHS Increases Pyrethrin Content When Using a Genotype-Independent Transformation System in Pyrethrum (Tanacetum cinerariifolium)

Pyrethrum (Tanacetum cinerariifolium) is one of the most important industrial crops for the extraction of pyrethrins, which are natural insecticidal compounds. Progress in pyrethrum molecular breeding with the objective of increasing pyrethrin content has been slow for lack of a suitable gene transfer system. Regeneration recalcitrance is a crucial barrier to establishing a genetic transformation system in pyrethrum. Therefore, in this study, an Agrobacterium-mediated transformation system in pyrethrum was developed using shoot apical meristems from germinated seedlings. Factors affecting transformation efficiency were optimized. Optimal conditions included explants at the "no true leaf" stage with a half apical meristem, an Agrobacterium tumefaciens cell density of OD₆₀₀ = 0.5, two days of cocultivation, and the incorporation of 1.5 mg L^-1 6-BA and 30 mg L^-1 kanamycin into the selection medium. Under the optimized conditions, two expression cassettes (proTcCHS-GUS and proRbcS-TcCHS) were successfully transformed into pyrethrum. Polymerase chain reaction (PCR), Southern blotting, reverse-transcription quantitative PCR (RT-qPCR), and histochemical staining confirmed the identity of proTcCHS-GUS transgenic plants. PCR and RT-qPCR analyses confirmed the identity of proRbcS-TcCHS transgenic plants. The transformation efficiency was 0.83% (5 transgenic lines/600 infected explants). The relative concentration of pyrethrins in proRbcS-TcCHS transformants (OX T0-1: 1.50% or OX T0-2: 1.24%) was higher than that in nontransformed plants (WT: 0.76%). Thus, the genetic transformation system overcame the low regeneration efficiency and integrated a foreign gene into the pyrethrum genome. The new system is a suitable and effective tool for creating high-yielding cultivars of pyrethrum.

Research Progress on the Synthetic Biology of Botanical Biopesticides

The production and large-scale application of traditional chemical pesticides will bring environmental pollution and food safety problems. With the advantages of high safety and environmental friendliness, botanical biopesticides are in line with the development trend of modern agriculture and have gradually become the mainstream of modern pesticide development. However, the traditional production of botanical biopesticides has long been faced with prominent problems, such as limited source and supply, complicated production processes, and excessive consumption of resources. In recent years, the rapid development of synthetic biology will break through these bottlenecks, and many botanical biopesticides are produced using synthetic biology, such as emodin, celangulin, etc. This paper reviews the latest progress and application prospect of synthetic biology in the development of botanical pesticides so as to provide new ideas for the analysis of synthetic pathways and heterologous and efficient production of botanical biopesticides and accelerate the research process of synthetic biology of natural products.

Single-cell RNA-sequencing of Nicotiana attenuata corolla cells reveals the biosynthetic pathway of a floral scent

High-throughput single-cell RNA sequencing (scRNA-Seq) identifies distinct cell populations based on cell-to-cell heterogeneity in gene expression. By examining the distribution of the density of gene expression profiles, we can observe the metabolic features of each cell population. Here, we employ the scRNA-Seq technique to reveal the entire biosynthetic pathway of a flower volatile. The corolla of the wild tobacco Nicotiana attenuata emits a bouquet of scents that are composed mainly of benzylacetone (BA). Protoplasts from the N. attenuata corolla limbs and throat cups were isolated at three different time points, and the transcript levels of > 16 000 genes were analyzed in 3756 single cells. We performed unsupervised clustering analysis to determine which cell clusters were involved in BA biosynthesis. The biosynthetic pathway of BA was uncovered by analyzing gene co-expression in scRNA-Seq datasets and by silencing candidate genes in the corolla. In conclusion, the high-resolution spatiotemporal atlas of gene expression provided by scRNA-Seq reveals the molecular features underlying cell-type-specific metabolism in a plant.

Engineering of tomato type VI glandular trichomes for trans-chrysanthemic acid biosynthesis, the acid moiety of natural pyrethrin insecticides

Glandular trichomes, known as metabolic cell factories, have been proposed as highly suitable for metabolically engineering the production of plant high-value specialized metabolites. Natural pyrethrins, found only in Dalmatian pyrethrum (Tanacetum cinerariifolium), are insecticides with low mammalian toxicity and short environmental persistence. Type I pyrethrins are esters of the monoterpenoid trans-chrysanthemic acid with one of the three rethrolone-type alcohols. To test if glandular trichomes can be made to synthesize trans-chrysanthemic acid, we reconstructed its biosynthetic pathway in tomato type VI glandular trichomes, which produce large amounts of terpenoids that share the precursor dimethylallyl diphosphate (DMAPP) with this acid. This was achieved by coexpressing the trans-chrysanthemic acid pathway related genes including TcCDS encoding chrysanthemyl diphosphate synthase and the fusion gene of TcADH2 encoding the alcohol dehydrogenase 2 linked with TcALDH1 encoding the aldehyde dehydrogenase 1 under the control of a newly identified type VI glandular trichome-specific metallocarboxypeptidase inhibitor promoter. Whole tomato leaves harboring type VI glandular trichomes expressing all three aformentioned genes had a concentration of total trans-chrysanthemic acid that was about 1.5-fold higher (by mole number) than the levels of β-phellandrene, the dominant monoterpene present in non-transgenic leaves, while the levels of β-phellandrene and the representative sesquiterpene β-caryophyllene in transgenic leaves were reduced by 96% and 81%, respectively. These results suggest that the tomato type VI glandular trichome is an alternative platform for the biosynthesis of trans-chrysanthemic acid by metabolic engineering.

Jellynolide A, pokepola esters, and sponalisolides from the aquaculture sponge Spongia officinalis L

Jellynolide A, an unreported bicyclic diterpenoid with an unprecedented penta-substituted carbon skeleton which implied an irregular biogenic pathway, together with four pairs of rare phosphate triesters, (±)-pokepola ester B-E, one undescribed related racemic furanoterpenoid, (±)-sponalisolide C, one undescribed furanoterpenoid, (-)-sponalisolide D, and two known (±)-sponalisolide B and dendrolasin carboxylic acid were isolated from the aquaculture Spongia officinalis L. Their structures were elucidated by comprehensive spectroscopic analysis, quantum chemical calculation of NMR parameters, and electronic circular dichroism (ECD). The plausible biosynthetic pathway of jellynolide A was proposed. (±)-Pokepola ester C exhibited significant inhibition against Wnt, HIF1 signaling pathways. (+)-Pokepola ester B and (-)-pokepola ester D showed moderate cytotoxicity activities.

TcMYC2 regulates Pyrethrin biosynthesis in Tanacetum cinerariifolium

Pyrethrins constitute a class of terpene derivatives with high insecticidal activity and are mainly synthesized in the capitula of the horticulturally important plant, Tanacetum cinerariifolium. Treatment of T. cinerariifolium with methyl jasmonate (MeJA) in the field induces pyrethrin biosynthesis, but the mechanism linking MeJA with pyrethrin biosynthesis remains unclear. In this study, we explored the transcription factors involved in regulating MeJA-induced pyrethrin biosynthesis. A single spray application of MeJA to T. cinerariifolium leaves rapidly upregulated the expression of most known pyrethrin biosynthesis genes and subsequently increased the total pyrethrin content in the leaf. A continuous 2-week MeJA treatment resulted in enhanced pyrethrin content and increased trichome density. TcMYC2, a key gene in jasmonate signaling, was screened at the transcriptome after MeJA treatment. TcMYC2 positively regulated expression of the pyrethrin biosynthesis genes TcCHS, TcAOC, and TcGLIP by directly binding to E-box/G-box motifs in the promoters. The stable overexpression of TcMYC2 in T. cinerariifolium hairy roots significantly increased the expression of TcAOC and TcGLIP. Further transient overexpression and viral-induced gene-silencing experiments demonstrated that TcMYC2 positively promoted pyrethrin biosynthesis. Collectively, the results reveal a novel molecular mechanism for MeJA-induced pyrethrin biosynthesis in T. cinerariifolium involving TcMYC2.

Characterization of a Cytosolic Acyl-Activating Enzyme Catalyzing the Formation of 4-Methylvaleryl-CoA for Pogostone Biosynthesis in Pogostemon Cablin

Pogostone, a compound with various pharmaceutical activities, is a major constituent of the essential oil preparation called Pogostemonis Herba, which is obtained from the plant Pogostemon cablin. The biosynthesis of pogostone has not been elucidated, but 4-methylvaleryl-CoA (4MVCoA) is a likely precursor. We analyzed the distribution of pogostone in P. cablin using gas chromatography-mass spectrometry (GC-MS) and found that pogostone accumulates at high levels in the main stems and leaves of young plants. A search for the acyl-activating enzyme (AAE) that catalyzes the formation of 4MVCoA from 4-methylvaleric acid was launched, using an RNAseq-based approach to identify 31 unigenes encoding putative AAEs including the PcAAE2, the transcript profile of which shows a strong positive correlation with the distribution pattern of pogostone. The protein encoded by PcAAE2 was biochemically characterized in vitro and shown to catalyze the formation of 4MVCoA from 4-methylvaleric acid. Phylogenetic analysis showed that PcAAE2 is closely related to other AAE proteins in P. cablin and other species that are localized to the peroxisomes. However, PcAAE2 lacks a peroxisome targeting sequence 1 (PTS1) and is localized in the cytosol.

Transcriptional Responses and GCMS Analysis for the Biosynthesis of Pyrethrins and Volatile Terpenes in Tanacetum coccineum

Natural pyrethrins have been widely used as natural pesticides due to their low mammalian toxicity and environmental friendliness. Previous studies have mainly focused on Tanacetumcinerariifolium, which contains high levels of pyrethrins and volatile terpenes that play significant roles in plant defense and pollination. However, there is little information on T. coccineum due to its lower pyrethrin content and low commercial value. In this study, we measured the transcriptome and metabolites of the leaves (L), flower buds (S1), and fully blossomed flowers (S4) of T. coccineum. The results show that the expression of pyrethrins and precursor terpene backbone genes was low in the leaves, and then rapidly increased in the S1 stage before decreasing again in the S4 stage. The results also show that pyrethrins primarily accumulated at the S4 stage. However, the content of volatile terpenes was consistently low. This perhaps suggests that, despite T. coccineum and T. cinerariifolium having similar gene expression patterns and accumulation of pyrethrins, T. coccineum attracts pollinators via its large and colorful flowers rather than via inefficient and metabolically expensive volatile terpenes, as in T. cinerariifolium. This is the first instance of de novo transcriptome sequencing reported for T. coccineum. The present results could provide insights into pyrethrin biosynthetic pathways and will be helpful for further understanding how plants balance the cost–benefit relationship between plant defense and pollination.

Chemical and biological studies of natural and synthetic products for the highly selective control of pest insect species

Tanacetum cinerariifolium was known to produce pyrethrins, but the mechanism of pyrethrin biosynthesis was largely unclear. The author showed that the non-mevalonate and oxylipin pathways underlie biosynthesis of the acid and alcohol moieties, respectively, and a GDSL lipase joins the products of these pathways. A blend of the green leaf volatiles and (E)-β-farnesene mediates the induction of wounding responses to neighboring intact conspecies by enhancing pyrethrin biosynthesis. Plants fight against herbivores underground as well as aboveground, and, in soy pulps, some fungi produce compounds selectively modulating ion channels in insect nervous system. The author proposed that indirect defense of plants occurs where microorganisms produce defense substances in the rhizosphere. Broad-spectrum pesticides, including neonicotinoids, may affect non-target organisms. The author discovered co-factors enabling functional expression of insect nicotinic acetylcholine receptors (nAChRs). This led to understanding the mechanism of insect nAChR-neonicotinoid interactions, thus paving new avenues for controlling pests and disease vectors.

Introgression of the sesquiterpene biosynthesis from Solanum habrochaites to cultivated tomato offers insights into trichome morphology and arthropod resistance

Cultivated tomatoes harboring the plastid-derived sesquiterpenes from S. habrochaites have altered type-VI trichome morphology and unveil additional genetic components necessary for piercing-sucking pest resistance. Arthropod resistance in the tomato wild relative Solanum habrochaites LA1777 is linked to specific sesquiterpene biosynthesis. The Sesquiterpene synthase 2 (SsT2) gene cluster on LA1777 chromosome 8 controls plastid-derived sesquiterpene synthesis. The main genes at SsT2 are Z-prenyltransferase (zFPS) and Santalene and Bergamotene Synthase (SBS), which produce α-santalene, β-bergamotene, and α-bergamotene in LA1777 round-shaped type-VI glandular trichomes. Cultivated tomatoes have mushroom-shaped type-VI trichomes with much smaller glands that contain low levels of monoterpenes and cytosolic-derived sesquiterpenes, not presenting the same pest resistance as in LA1777. We successfully transferred zFPS and SBS from LA1777 to cultivated tomato (cv. Micro-Tom, MT) by a backcrossing approach. The trichomes of the MT-Sst2 introgressed line produced high levels of the plastid-derived sesquiterpenes. The type-VI trichome internal storage-cavity size increased in MT-Sst2, probably as an effect of the increased amount of sesquiterpenes, although it was not enough to mimic the round-shaped LA1777 trichomes. The presence of high amounts of plastid-derived sesquiterpenes was also not sufficient to confer resistance to various tomato piercing-sucking pests, indicating that the effect of the sesquiterpenes found in the wild S. habrochaites can be insect specific. Our results provide for a better understanding of the morphology of S. habrochaites type-VI trichomes and paves the way to obtain insect-resistant tomatoes.

Silent constraints: the hidden challenges faced in plant metabolic engineering

Metabolic engineering is embraced as a method to sustainably enhance production of valuable phytochemicals with beneficial properties. However, successful production of these compounds in plants is not always predictable even when the pathways are fully known, frequently due to the lack of comprehensive understanding of plant metabolism as a whole, and interconnections between different primary, secondary, and hormone metabolic networks. Here, we highlight critical hidden constraints, including substrate availability, silent metabolism, and metabolic crosstalk, that impair engineering strategies. We explore how these constraints have historically been manifested in engineering attempts and propose how modern advancements will enable future strategies to overcome these impediments.

How Plants Synthesize Pyrethrins: Safe and Biodegradable Insecticides

Natural pyrethrin insecticides produced by Dalmatian pyrethrum (Tanacetum cinerariifolium) have low mammalian toxicity and short environmental persistence, providing an alternative to widely used synthetic agricultural insecticides that pose a threat to human health and the environment. A recent surge of interest in the use of pyrethrins as agricultural insecticides coincides with the discovery of several new genes in the pyrethrin biosynthetic pathway. Elucidation of this pathway facilitates efforts to breed improved pyrethrum varieties and to engineer plants with improved endogenous defenses or hosts for heterologous pyrethrin production. We describe the current state of knowledge related to global pyrethrum production, the pyrethrin biosynthetic pathway and its regulation, and recent efforts to engineer the pyrethrin pathway in diverse plant hosts.

More is better: the diversity of terpene metabolism in plants

All plants synthesize a diverse array of terpenoid metabolites. Some are common to all, but many are synthesized only in specific taxa and presumably evolved as adaptations to specific ecological conditions. While the basic terpenoid biosynthetic pathways are common in all plants, recent discoveries have revealed many variations in the way plants synthesized specific terpenes. A major theme is the much greater number of substrates that can be used by enzymes belonging to the terpene synthase (TPS) family. Other recent discoveries include non-TPS enzymes that catalyze the formation of terpenes, and novel transport mechanisms.

Novel routes towards bioplastics from plants: elucidation of the methylperillate biosynthesis pathway from Salvia dorisiana trichomes

Plants produce a large variety of highly functionalized terpenoids. Functional groups such as partially unsaturated rings and carboxyl groups provide handles to use these compounds as feedstock for biobased commodity chemicals. For instance, methylperillate, a monoterpenoid found in Salvia dorisiana, may be used for this purpose, as it carries both an unsaturated ring and a methylated carboxyl group. The biosynthetic pathway of methylperillate in plants is still unclear. In this work, we identified glandular trichomes from S. dorisiana as the location of biosynthesis and storage of methylperillate. mRNA from purified trichomes was used to identify four genes that can encode the pathway from geranyl diphosphate towards methylperillate. This pathway includes a (-)-limonene synthase (SdLS), a limonene 7-hydroxylase (SdL7H, CYP71A76), and a perillyl alcohol dehydrogenase (SdPOHDH). We also identified a terpene acid methyltransferase, perillic acid O-methyltransferase (SdPAOMT), with homology to salicylic acid OMTs. Transient expression in Nicotiana benthamiana of these four genes, in combination with a geranyl diphosphate synthase to boost precursor formation, resulted in production of methylperillate. This demonstrates the potential of these enzymes for metabolic engineering of a feedstock for biobased commodity chemicals.

Jasmonic acid is not a biosynthetic intermediate to produce the pyrethrolone moiety in pyrethrin II

Pyrethrum (Tanacetumcinerariifolium) produces insecticidal compounds known as pyrethrins. Pyrethrins are esters; the acid moiety is either trans-chrysanthemic acid or pyrethric acid and the alcohol moiety of pyrethrins is either pyrethrolone, cinerolone, or jasmolone. It was generally accepted that cis-jasmone was biosynthetic intermediate to produce the alcohol moieties of pyrethrin, and the biosynthetic origin of the cis-jasmone was postulated to be jasmonic acid. However, there was no direct evidence to prove this hypothesis. In order to uncover the origin of pyrethrolone moiety in pyrethrin II, feeding experiments were performed employing deuterium- and ¹³C-labeled compounds as substrates, and the expected labeled compounds were analyzed using UPLC MS/MS system. It was found that the pyrethrolone moiety in pyrethrin II was derived from 12-oxo-phytodienoic acid (OPDA), iso-OPDA and cis-jasmone but not from methyl jasmonate and 3-oxo-2-(2′-[Z]-pentenyl)-cyclopentane-1-hexanoic acid. The results supported that the biosynthesis of the pyrethrolone moiety in pyrethrin II partially used part of the jasmonic acid biosynthetic pathway, but not whole.

A Trichome-Specific, Plastid-Localized Tanacetum cinerariifolium Nudix Protein Hydrolyzes the Natural Pyrethrin Pesticide Biosynthetic Intermediate trans-Chrysanthemyl Diphosphate

Tanacetum cinerariifolium flowers synthesize six pyrethrins that function as effective insecticides. trans-Chrysanthemol is an early intermediate in the synthesis of the monoterpene moiety of pyrethrins. Previously, the pyrethrum enzyme chrysanthemyl diphosphate synthase (TcCDS) was shown to catalyze the formation of the prenyl diphosphate compound chrysanthemyl diphosphate (CPP) by condensing two molecules of dimethylallyl diphosphate (DMAPP). Later work also showed that with a low concentration of DMAPP, TcCDS can also remove the diphosphate group to give chrysanthemol. The removal of the phosphate groups from other prenyl diphosphates, such as DMAPP, isopentenyl diphosphate (IPP) and geranyl diphosphate (GPP), was previously shown to occur in two steps. In those cases, the first phosphate group is removed by a member of the Nudix hydrolase protein family, and the second by other unidentified phosphatases. These previously characterized Nudix proteins involved in the hydrolysis of prenyl diphosphates were shown to be cytosolic. Here we report that a plastidic Nudix protein from pyrethrum, designated TcNudix1, has high specificity for CPP and can hydrolyze it to chrysanthemol monophosphate (CMP). TcNudix1 is expressed specifically in the trichomes of the ovaries, where chrysanthemol is produced. TcNudix1 expression patterns and pathway reconstitution experiments presented here implicate the TcNudix1 protein in the biosynthesis of chrysanthemol.

Draft genome of Tanacetum cinerariifolium, the natural source of mosquito coil

Pyrethrum (Tanacetum cinerariifolium), which is a perennial Asteraceae plant with white daisy-like flowers, is the original source of mosquito coils and is known for the biosynthesis of the pyrethrin class of natural insecticides. However, the molecular basis of the production of pyrethrins by T. cinerariifolium has yet to be fully elucidated. Here, we present the 7.1-Gb draft genome of T. cinerariifolium, consisting of 2,016,451 scaffolds and 60,080 genes predicted with high confidence. Notably, analyses of transposable elements (TEs) indicated that TEs occupy 33.84% of the genome sequence. Furthermore, TEs of the sire and oryco clades were found to be enriched in the T. cinerariifolium-specific evolutionary lineage, occupying a total of 13% of the genome sequence, a proportion approximately 8-fold higher than that in other plants. InterProScan analysis demonstrated that biodefense-related toxic proteins (e.g., ribosome inactivating proteins), signal transduction-related proteins (e.g., histidine kinases), and metabolic enzymes (e.g., lipoxygenases, acyl-CoA dehydrogenases/oxygenases, and P450s) are also highly enriched in the T. cinerariifolium genome. Molecular phylogenetic analysis detected a variety of enzymes with genus-specific multiplication, including both common enzymes and others that appear to be specific to pyrethrin biosynthesis. Together, these data identify possible novel components of the pyrethrin biosynthesis pathway and provide new insights into the unique genomic features of T. cinerariifolium.

A Promiscuous CYP706A3 Reduces Terpene Volatile Emission from Arabidopsis Flowers, Affecting Florivores and the Floral Microbiome

Flowers are essential but vulnerable plant organs, exposed to pollinators and florivores; however, flower chemical defenses are rarely investigated. We show here that two clustered terpene synthase and cytochrome P450 encoding genes (TPS11 and CYP706A3) on chromosome 5 of Arabidopsis (Arabidopsis thaliana) are tightly coexpressed in floral tissues, upon anthesis and during floral bud development. TPS11 was previously reported to generate a blend of sesquiterpenes. By heterologous coexpression of TPS11 and CYP706A3 in yeast (Saccharomyces cerevisiae) and Nicotiana benthamiana, we demonstrate that CYP706A3 is active on TPS11 products and also further oxidizes its own primary oxidation products. Analysis of headspace and soluble metabolites in cyp706a3 and 35S:CYP706A3 mutants indicate that CYP706A3-mediated metabolism largely suppresses sesquiterpene and most monoterpene emissions from opening flowers, and generates terpene oxides that are retained in floral tissues. In flower buds, the combined expression of TPS11 and CYP706A3 also suppresses volatile emissions and generates soluble sesquiterpene oxides. Florivory assays with the Brassicaceae specialist Plutella xylostella demonstrate that insect larvae avoid feeding on buds expressing CYP706A3 and accumulating terpene oxides. Composition of the floral microbiome appears also to be modulated by CYP706A3 expression. TPS11 and CYP706A3 simultaneously evolved within Brassicaceae and form the most versatile functional gene cluster described in higher plants so far.plantcell;31/12/2947/FX1F1fx1.

Characterization of Composition and Antifungal Properties of Leaf Secondary Metabolites from Thirteen Cultivars of Chrysanthemum morifolium Ramat

Chrysanthemum morifolium Ramat is an ornamental plant of worldwide cultivation. Like many other species in the family Asteraceae, C. morifolium is a rich producer of secondary metabolites. There are two objectives in this study: (I) to determine and compare the diversity of apolar secondary metabolites among different cultivars of C. morifolium and (II) to compare their properties as antifungal agents. To attain these objectives, we selected 13 cultivars of C. morifolium that are commonly used for making chrysanthemum tea as experimental materials. Leaves at the same developmental stage were collected from respective mature plants and subjected to organic extraction. The extracts were analyzed using gas chromatography–mass spectrometry. A total of 37 apolar secondary metabolites including 26 terpenoids were detected from the 13 cultivars. These 13 cultivars can be largely divided into three chemotypes based on chemical principal components analysis. Next, the extracts from the 13 cultivars were examined in in vitro assays for their antifungal properties against three species of pathogenic fungi: Fusarium oxysporum, Magnaporthe oryzae, and Verticillium dahliae. Significant variability in antifungal activity of the leaf extracts among different cultivars was observed. The 13 cultivars can be divided into four groups based on their antifungal activities, which could be partly correlated to the contents of terpenoids. In short, this study reveals large variations in chemical composition, particularly of terpenoids, of leaf secondary metabolites among different cultivars of C. morifolium and their different abilities in functioning as antifungal agents.

Pyrethrin Biosynthesis: The Cytochrome P450 Oxidoreductase CYP82Q3 Converts Jasmolone To Pyrethrolone

The plant pyrethrum (Tanacetum cinerariifolium) synthesizes highly effective natural pesticides known as pyrethrins. Pyrethrins are esters consisting of an irregular monoterpenoid acid and an alcohol derived from jasmonic acid (JA). These alcohols, referred to as rethrolones, can be jasmolone, pyrethrolone, or cinerolone. We recently showed that jasmolone is synthesized from jasmone, a degradation product of JA, in a single hydroxylation step catalyzed by jasmone hydroxylase (TcJMH). TcJMH belongs to the CYP71 clade of the cytochrome P450 oxidoreductase family. Here, we used coexpression analysis, heterologous gene expression, and in vitro biochemical assays to identify the enzyme responsible for conversion of jasmolone to pyrethrolone. A further T cinerariifolium cytochrome P450 family member, CYP82Q3 (designated Pyrethrolone Synthase; TcPYS), appeared to catalyze the direct desaturation of the C1-C2 bond in the pentyl side chain of jasmolone to produce pyrethrolone. TcPYS is highly expressed in the trichomes of the ovaries in pyrethrum flowers, similar to TcJMH and other T cinerariifolium genes involved in JA biosynthesis. Thus, as previously shown for biosynthesis of the monoterpenoid acid moiety of pyrethrins, rethrolones are synthesized in the trichomes. However, the final assembly of pyrethrins occurs in the developing achenes. Our data provide further insight into pyrethrin biosynthesis, which could ultimately be harnessed to produce this natural pesticide in a heterologous system.

Pyrethric acid of natural pyrethrin insecticide: complete pathway elucidation and reconstitution in Nicotiana benthamiana

In the natural pesticides known as pyrethrins, which are esters produced in flowers of Tanacetum cinerariifolium (Asteraceae), the monoterpenoid acyl moiety is pyrethric acid or chrysanthemic acid. We show here that pyrethric acid is produced from chrysanthemol in six steps catalyzed by four enzymes, the first five steps occurring in the trichomes covering the ovaries and the last one occurring inside the ovary tissues. Three steps involve the successive oxidation of carbon 10 (C10) to a carboxylic group by TcCHH, a cytochrome P450 oxidoreductase. Two other steps involve the successive oxidation of the hydroxylated carbon 1 to give a carboxylic group by TcADH2 and TcALDH1, the same enzymes that catalyze these reactions in the formation of chrysanthemic acid. The ultimate result of the actions of these three enzymes is the formation of 10-carboxychrysanthemic acid in the trichomes. Finally, the carboxyl group at C10 is methylated by TcCCMT, a member of the SABATH methyltransferase family, to give pyrethric acid. This reaction occurs mostly in the ovaries. Expression in N. benthamiana plants of all four genes encoding aforementioned enzymes, together with TcCDS, a gene that encodes an enzyme that catalyzes the formation of chrysanthemol, led to the production of pyrethric acid.

Non-volatile natural products in plant glandular trichomes: chemistry, biological activities and biosynthesis

The investigation methods, chemistry, bioactivities, and biosynthesis of non-volatile natural products involving 489 compounds in plant glandular trichomes are reviewed.

The Repetitive DNA Composition in the Natural Pesticide Producer Tanacetum cinerariifolium: Interindividual Variation of Subtelomeric Tandem Repeats

Dalmatian pyrethrum (Tanacetum cinerariifolium (Trevir.) Sch. Bip.), a plant species endemic to the east Adriatic coast, is used worldwide for production of the organic insecticide, pyrethrin. Most studies concerning Dalmatian pyrethrum have focused on its morphological and biochemical traits relevant for breeding. However, little is known about the chromosomal evolution and genome organization of this species. Our study aims are to identify, classify, and characterize repetitive DNA in the T. cinerariifolium genome using clustering analysis of a low coverage genomic dataset. Repetitive DNA represents about 71.63% of the genome. T. cinerariifolium exhibits linked 5S and 35S rDNA configuration (L-type). FISH reveals amplification of interstitial telomeric repeats (ITRs) in T. cinerariifolium. Of the three newly identified satellite DNA families, TcSAT1 and TcSAT2 are located subterminally on most of T. cinerariifolium chromosomes, while TcSAT3 family is located intercalary within the longer arm of two chromosome pairs. FISH reveals high levels of polymorphism of the TcSAT1 and TcSAT2 sites by comparative screening of 28 individuals. TcSAT2 is more variable than TcSAT1 regarding the number and position of FISH signals. Altogether, our data highlights the dynamic nature of DNA sequences associated with subtelomeres in T. cinerariifolium and suggests that subtelomeres represent one of the most dynamic and rapidly evolving regions in eukaryotic genomes.

Jasmone Hydroxylase, a Key Enzyme in the Synthesis of the Alcohol Moiety of Pyrethrin Insecticides

Pyrethrins are synthesized by the plant pyrethrum (Tanacetum cinerariifolium), a chrysanthemum relative. These compounds possess efficient insecticidal properties and are not toxic to humans and most vertebrates. Pyrethrum flowers, and to a smaller extent leaves, synthesize six main types of pyrethrins, which are all esters of a monoterpenoid acid moiety and an alcohol moiety derived from jasmonic acid. Here, we identified and characterized the enzyme responsible for the conversion of jasmone, a derivative of jasmonic acid, to jasmolone. Feeding pyrethrum flowers with jasmone resulted in a 4-fold increase in the concentration of free jasmolone as well as smaller but significant proportional increases in free pyrethrolone and all three type I pyrethrins. We used floral transcriptomic data to identify cytochrome P450 genes whose expression patterns were most highly correlated with that of a key gene in pyrethrin biosynthesis, T. cinerariifolium chrysanthemyl diphosphate synthase The candidate genes were screened for jasmone hydroxylase activity through transient expression in Nicotiana benthamiana leaves fed with jasmone. The expression of only one of these candidate genes produced jasmolone; therefore, this gene was named T. cinerariifolium jasmolone hydroxylase (TcJMH) and given the CYP designation CYP71AT148. The protein encoded by TcJMH localized to the endoplasmic reticulum, and microsomal preparations from N. benthamiana leaves expressing TcJMH were capable of catalyzing the hydroxylation of jasmone to jasmolone in vitro, with a Km value of 53.9 µm TcJMH was expressed almost exclusively in trichomes of floral ovaries and was induced in leaves by jasmonate.

Production of trans-chrysanthemic acid, the monoterpene acid moiety of natural pyrethrin insecticides, in tomato fruit

The pyrethrum plant, Tanacetum cinerariifolium (Asteraceae) synthesizes a class of compounds called pyrethrins that have strong insecticidal properties but are safe to humans. Class I pyrethrins are esters of the monoterpenoid trans-chrysanthemic acid with one of three jasmonic-acid derived alcohols. We reconstructed the trans-chrysanthemic acid biosynthetic pathway in tomato fruits, which naturally produce high levels of the tetraterpene pigment lycopene, an isoprenoid which shares a common precursor, dimethylallyl diphosphate (DMAPP), with trans-chrysanthemic acid. trans-Chrysanthemic acid biosynthesis in tomato fruit was achieved by expressing the chrysanthemyl diphosphate synthase gene from T. cinerariifolium, encoding the enzyme that uses DMAPP to make trans-chrysanthemol, under the control of the fruit specific promoter PG, as well as an alcohol dehydrogenease (ADH) gene and aldehyde dehydrogenase (ALDH) gene from a wild tomato species, also under the control of the PG promoter. Tomato fruits expressing all three genes had a concentration of trans-chrysanthemic acid that was about 1.7-fold higher (by weight) than the levels of lycopene present in non-transgenic fruit, while the level of lycopene in the transgenic plants was reduced by 68%. Ninety seven percent of the diverted DMAPP was converted to trans-chrysanthemic acid, but 62% of this acid was further glycosylated. We conclude that the tomato fruit is an alternative platform for the biosynthesis of trans-chrysanthemic acid by metabolic engineering.