Defense of pyrethrum flowers: repelling herbivores and recruiting carnivores by producing aphid alarm pheromone

Nocturnal burst emissions of germacrene D from the open disk florets of pyrethrum flowers induce moths to oviposit on a nonhost and improve pollination success

Recent studies show that nocturnal pollinators may be more important to ecosystem function and food production than is currently appreciated. Here, we describe an agricultural field study of pyrethrum (Tanacetum cinerariifolium) flower pollination. Pyrethrum is genetically self-incompatible and thus is reliant on pollinators for seed set. Our pollinator exclusion experiment showed that nocturnal insects, particularly moths, significantly contribute to seed set and quality. We discovered that the most abundant floral volatile, the sesquiterpene (-)-germacrene D (GD), is key in attracting the noctuid moths Peridroma saucia and Helicoverpa armigera. Germacrene D synthase (GDS) gene expression regulates the specific GD production and accumulation in flowers, which, in contrast to related species, lose the habit of closing at night. We did observe that female moths also oviposited on pyrethrum leaves and flower peduncles, but found that only a small fraction of those eggs hatched. Larvae were severely stunted in development, most likely due to the presence of pyrethrin defense compounds. This example of exploitative mutualism, which blocks the reproductive success of the moth pollinator and depends on nocturnal interactions, is placed into an ecological context to explain why it may have developed.

Understanding pyrethrin biosynthesis: toward and beyond natural pesticide overproduction

Pyrethrins are natural insecticides biosynthesised by Asteraceae plants, such as Tanacetum cinerariifolium and have a long history, dating back to ancient times. Pyrethrins are often used as low-persistence and safe insecticides to control household, horticultural, and agricultural insect pests. Despite its long history of use, pyrethrin biosynthesis remains a mystery, presenting a significant opportunity to improve yields and meet the growing demand for organic agriculture. To achieve this, both genetic modification and non-genetic methods, such as chemical activation and priming, are indispensable. Plants use pyrethrins as a defence against herbivores, but pyrethrin biosynthesis pathways are shared with plant hormones and signal molecules. Hence, the insight that pyrethrins may play broader roles than those traditionally expected is invaluable to advance the basic and applied sciences of pyrethrins.

Aphid alarm pheromone mimicry in transgenic Chrysanthemum morifolium: insights into the potential of (E)-β-farnesene for aphid resistance

(E)-β-Farnesene (EBF) serves as the primary component of the alarm pheromone used by most aphid pest species. Pyrethrum (Tanacetum cinerariifolium) exhibits tissue-specific regulation of EBF accumulation and release, effectively mimicking the aphid alarm signal, deterring aphid attacks while attracting aphid predators. However, cultivated chrysanthemum (Chrysanthemum morifolium), a popular and economically significant flower, is highly vulnerable to aphid infestations. In this study, we investigated the high expression of the pyrethrum EBF synthase (TcEbFS) gene promoter in the flower head and stem, particularly in the parenchyma cells. Subsequently, we introduced the TcEbFS gene, under the control of its native promoter, into cultivated chrysanthemum. This genetic modification led to increased EBF accumulation in the flower stem and young flower bud, which are the most susceptible tissues to aphid attacks. Analysis revealed that aphids feeding on transgenic chrysanthemum exhibited prolonged probing times and extended salivation durations during the phloem phase, indicating that EBF in the cortex cells hindered their host-location behavior. Interestingly, the heightened emission of EBF was only observed in transgenic chrysanthemum flowers after mechanical damage. Furthermore, we explored the potential of this transgenic chrysanthemum for aphid resistance by comparing the spatial distribution and storage of terpene volatiles in different organs and tissues of pyrethrum and chrysanthemum. This study provides valuable insights into future trials aiming for a more accurate replication of alarm pheromone release in plants. It highlights the complexities of utilizing EBF for aphid resistance in cultivated chrysanthemum and calls for further investigations to enhance our understanding of this defense mechanism.

Visual-, Olfactory-, and Nectar-Taste-Based Flower Aposematism

Florivory, i.e., flower herbivory, of various types is common and can strongly reduce plant fitness. Flowers suffer two very different types of herbivory: (1) the classic herbivory of consuming tissues and (2) nectar theft. Unlike the non-reversibility of consumed tissues, nectar theft, while potentially reducing a plant's fitness by lowering its attraction to pollinators, can, in various cases, be fixed quickly by the production of additional nectar. Therefore, various mechanisms to avoid or reduce florivory have evolved. Here, I focus on one of the flowers' defensive mechanisms, aposematism, i.e., warning signaling to avoid or at least reduce herbivory via the repelling of herbivores. While plant aposematism of various types was almost ignored until the year 2000, it is a common anti-herbivory defense mechanism in many plant taxa, operating visually, olfactorily, and, in the case of nectar, via a bitter taste. Flower aposematism has received only very little focused attention as such, and many of the relevant publications that actually demonstrated herbivore repellence and avoidance learning following flower signaling did not refer to repellence as aposematism. Here, I review what is known concerning visual-, olfactory-, and nectar-taste-based flower aposematism, including some relevant cases of mimicry, and suggest some lines for future research.

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.

Chemistry, biosynthesis and biology of floral volatiles: roles in pollination and other functions

Covering: 2010 to 2023Floral volatiles are a chemically diverse group of plant metabolites that serve multiple functions. Their composition is shaped by environmental, ecological and evolutionary factors. This review will summarize recent advances in floral scent research from chemical, molecular and ecological perspectives. It will focus on the major chemical classes of floral volatiles, on notable new structures, and on recent discoveries regarding the biosynthesis and the regulation of volatile emission. Special attention will be devoted to the various functions of floral volatiles, not only as attractants for different types of pollinators, but also as defenses of flowers against enemies. We will also summarize recent findings on how floral volatiles are affected by abiotic stressors, such as increased temperatures and drought, and by other organisms, such as herbivores and flower-dwelling microbes. Finally, this review will indicate current research gaps, such as the very limited knowledge of the isomeric pattern of chiral compounds and its importance in interspecific interactions.

Expression and functional analysis of an odorant binding protein PopeOBP16 from Phthorimaea operculella (Zeller)

Odorant binding proteins (OBPs) are essential proteins in the peripheral olfactory system, responsible for odorant recognition and transport to olfactory receptors. Phthorimaea operculella (potato tuber moth) is an important oligophagous pest on Solanaceae crops in many countries and regions. PopeOBP16 is one of the OBPs in potato tuber moth. This study examined the expression profiles of PopeOBP16. The results of qPCR indicated that PopeOBP16 was highly expressed in the antennae of adults, especially in males, suggesting that it may be involved in odor recognition in adults. The electroantennogram (EAG) was used to screen candidate compounds with the antennae of P. operculella. The relative affinities of PopeOBP16 to 27 host volatiles and two sex pheromone components with the highest relative EAG responses were examined with competitive fluorescence-based binding assays. PopeOBP16 had the strongest binding affinity with the plant volatiles: nerol, 2-phenylethanol, linalool, 1,8-cineole, benzaldehyde, β-pinene, d-limonene, terpinolene, α-terpinene, and the sex pheromone component trans-4, cis-7, cis-10-tridecatrien-1-ol acetate. The results provide a foundation for further research into the functioning of the olfactory system and the potential development of green chemistry for control of the potato tuber moth.

Analysis of apocarotenoid volatiles from lettuce (Lactuca sativa) induced by insect herbivores and characterization of carotenoid cleavage dioxygenase gene

Plant apocarotenoids have been shown to have a diverse biological role in herbivore-plant interactions. Despite their importance, little is known about herbivores' effect on apocarotenoid emissions in Lactuca sativa. In this study, we examined changes in apocarotenoid emissions in lettuce leaves after infestation by two insects, viz., Spodoptera littoralis larvae and Myzus persicae aphids. We found that β-ionone and β-cyclocitral showed higher concentrations than the other apocarotenoids, with a significant increase as per the intensity of infestation of both herbivore species. Furthermore, we performed functional characterization of Lactuca sativa carotenoid cleavage dioxygenase 1 (LsCCD1) genes. Three LsCCD1 genes were overexpressed in E. coli strains, and recombinant proteins were assayed for cleavage activity on an array of carotenoid substrates. The LsCCD1 protein cleaved β-carotene at the 9,10 (9',10') positions producing β-ionone. The transcript analysis of LsCCD1 genes revealed differential expression patterns under varying levels of herbivores' infestation, but the results were inconsistent with the pattern of β-ionone concentrations. Our results suggest that LsCCD1 is involved in the production of β-ionone, but other regulatory factors might be involved in its induction in response to herbivory. These results provide new insights into apocarotenoid production in response to insect herbivory in lettuce.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03511-4.

Molecular module of CmMYB15-like-Cm4CL2 regulating lignin biosynthesis of chrysanthemum (Chrysanthemum morifolium) in response to aphid (Macrosiphoniella sanborni) feeding

Lignin is a major component of plant cell walls and a conserved basic defense mechanism in higher plants deposited in response to aphid infection. However, the molecular mechanisms of lignin biosynthesis in response to aphid infection and the effect of lignin on aphid feeding behavior remain unclear. We report that 4-Coumarate:coenzyme A ligase 2 (Cm4CL2), a gene encoding a key enzyme in the lignin biosynthesis pathway, is induced by aphid feeding, resulting in lignin deposition and reduced aphid attack. Upstream regulator analysis showed that the expression of Cm4CL2 in response to aphid feeding was directly upregulated by CmMYB15-like, an SG2-type R2R3-MYB transcription factor. CmMYB15-like binds directly to the AC cis-element in the promoter region of Cm4CL2. Genetic validation demonstrated that CmMYB15-like was induced by aphid infection and contributed to lignin deposition and cell wall thickening, which consequently enhanced aphid resistance in a Cm4CL2-dependent manner. This study is the first to show that the CmMYB15-like-Cm4CL2 module regulates lignin biosynthesis in response to aphid feeding.

Microneedle-based interstitial fluid extraction for drug analysis: Advances, challenges, and prospects

Similar to blood, interstitial fluid (ISF) contains exogenous drugs and biomarkers and may therefore substitute blood in drug analysis. However, current ISF extraction techniques require bulky instruments and are both time-consuming and complicated, which has inspired the development of viable alternatives such as those relying on skin or tissue puncturing with microneedles. Currently, microneedles are widely employed for transdermal drug delivery and have been successfully used for ISF extraction by different mechanisms to facilitate subsequent analysis. The integration of microneedles with sensors enables in situ ISF analysis and specific compound monitoring, while the integration of monitoring and delivery functions in wearable devices allows real-time dose modification. Herein, we review the progress in drug analysis based on microneedle-assisted ISF extraction and discuss the related future opportunities and challenges.

Defensive functions and potential ecological conflicts of floral stickiness

Stickiness of vegetative tissues has evolved multiple times in different plant families but is rare and understudied in flowers. While stickiness in general is thought to function primarily as a defense against herbivores, it may compromise mutualistic interactions (such as those with pollinators) in reproductive tissues. Here, we test the hypothesis that stickiness on flower petals of the High-Andean plant, Bejaria resinosa (Ericaceae), functions as a defense against florivores. We address ecological consequences and discuss potential trade-offs associated with a repellant trait expressed in flowers that mediate mutualistic interactions. In surveys and manipulative experiments, we assess florivory and resulting fitness effects on plants with sticky and non-sticky flowers in different native populations of B. resinosa in Colombia. In addition, we analyze the volatile and non-volatile components in sticky and non-sticky flower morphs to understand the chemical information context within which stickiness is expressed. We demonstrate that fruit set is strongly affected by floral stickiness but also varies with population. While identifying floral stickiness as a major defensive function, our data also suggest that the context-dependency of chemical defense functionality likely arises from differential availability of primary pollinators and potential trade-offs between chemical defense with different modes of action.

Odorant-Binding Protein 3-Oriented Rational Design and Discovery of Novel Jasmonate Derivatives as Potential Aphid-Repellent Agents

Odorant-binding protein (OBP) is a potential target for developing insect behavior control agents due to its properties in transporting semiochemicals. In this study, 12 novel jasmonic acid (JA) derivatives were rationally designed and synthesized based on the binding features between Acyrthosiphon pisum OBP3 (ApisOBP3) and compound D1 [(E)-3,7-dimethylocta-2,6-dien-1-yl 2-(3-oxo-2-pentylcyclopentyl)acetate] with a binding affinity (K_d) of 26.79 μM. Most novel JA derivatives displayed better binding affinities than D1 (K_d = 1-26 μM). Among them, compound 6b [(E)-3,7-dimethylocta-2,6-dien-1-yl-2-((Z)-3-((acryloyloxy)imino)-2-pentylcyclopentyl)acetate] is the most promising compound with an excellent K_d of 1.33 μM and a significant repellent activity with repellent rates of 50-60% against A. pisum and Myzus persicae. Both hydrophobic and electrostatic interactions were found to contribute significantly to the binding of 6b to ApisOBP3. This study provides significant guidance for the rational design and efficient identification of novel aphid repellents based on aphid OBPs.

Dynamic regulation of volatile terpenoid production and emission from Chrysanthemum morifolium capitula

Flower-associated communities consist of both mutualistic and antagonistic organisms. We have limited knowledge on how flowers regulate volatiles to balance their defense against antagonists and the attraction of beneficial organisms necessary for reproductive success. Asteraceae is the largest family among flowering plants. Its representatives are characterized by unique inflorescence called capitulum, which has been reduced to a reproduction unit resembling a single flower. Here, we chose Chrysanthemum morifolium, a model species of Asteraceae, to investigate how the capitulum balances the accumulation and emission of floral terpenoid volatiles that are implicated in defense and pollinator attraction, respectively. Our results showed that the capitula of C. morifolium produce and emit complex mixtures of monoterpenoids and sesquiterpenoids. The highest concentrations of terpenoids were detected in the bud stage of the capitula. In contrast, the capitulum reached the highest emission level prior to full blooming. The disc florets were the dominant organs of terpenoid accumulation and emission in the full-openness stage. To understand the molecular basis of volatile terpenoid biosynthesis in C. morifolium, experiments were designed to study terpene synthase (TPS) genes, which are pivotal for terpene biosynthesis. Eight CmCJTPS genes were identified in the transcriptomes of C. morifolium, and the proteins encoded by five genes were found to be biochemically functional. CmCJTPS5 and CmCJTPS8 were the multi-product enzymes catalyzing the monoterpenoid and sesquiterpenoid formation, which closely matched the major terpenoids produced in the flower heads. The five functional terpene synthase genes exhibited similar temporal expression patterns but diverse spatial expression levels, suggesting tissue-specific functions. Altogether, our results illustrate the dynamic patterns of accumulation and emission of floral volatile terpenoids implicated in defense and attracting pollinators in C. morifolium, for which both the regulation of TPS gene expression and the regulation of release may play critical roles.

Caterpillar-Induced Rice Volatile (E)-β-Farnesene Impairs the Development and Survival of Chilo suppressalis Larvae by Disrupting Insect Hormone Balance

Significant research progress has recently been made on establishing the roles of tps46 in rice defense. (E)-β-farnesene (Eβf) is a major product of tps46 activity but its physiological functions and potential mechanisms against Chilo suppressalis have not yet been clarified. In the present study, C. suppressalis larvae were artificially fed a diet containing 0.8 g/kg Eβf and the physiological performance of the larvae was evaluated. In response to Eβf treatment, the average 2nd instar duration significantly increased from 4.78 d to 6.31 d while that of the 3rd instar significantly increased from 5.70 d to 8.00 d compared with the control. There were no significant differences between the control and Eβf-fed 4th and 5th instars in terms of their durations. The mortalities of the 2nd and 3rd Eβf-fed instars were 21.00-fold and 6.39-fold higher, respectively, than that of the control. A comparative transcriptome analysis revealed that multiple differentially expressed genes are involved in insect hormone biosynthesis. An insect hormone assay on the 3rd instars disclosed that Eβf disrupted the balance between the juvenile hormone and ecdysteroid levels. Eβf treatment increased the juvenile hormones titers but not those of the ecdysteroids. The qPCR results were consistent with those of the RNA-Seq. The foregoing findings suggested that Eβf impairs development and survival in C. suppressalis larvae by disrupting their hormone balance. Moreover, Eβf altered the pathways associated with carbohydrate and xenobiotic metabolism as well as those related to cofactors and vitamins in C. suppressalis larvae. The discoveries of this study may contribute to the development and implementation of an integrated control system for C. suppressalis infestations in rice.

Exploring the Relationship between Trichome and Terpene Chemistry in Chrysanthemum

Chrysanthemum is a popular ornamental plant with a long history of cultivation. Both the leaf and flowerhead of Chrysanthemum are known to produce diverse secondary metabolites, particularly terpenoids. Here we aimed to determine the relationship between terpene chemistry and the trichome traits in Chrysanthemum. In our examination of three cultivars of C. morifilium and three accessions of C. indicum, all plants contained T-shaped trichomes and biseriate peltate glandular trichomes. The biseriate peltate glandular trichome contained two basal cells, two stalk cells, six secondary cells and a subcuticular space, while the non-glandular T-shaped trichome was only composed of stalk cells and elongated cells. Histochemical staining analysis indicated that the biseriate peltate glandular trichome contained terpenoids and lipid oil droplets but not the T-shaped trichome. Next, experiments were performed to determine the relationship between the accumulation and emission of the volatile terpenoids and the density of trichomes on the leaves and flowerheads in all six Chrysanthemum cultivars\accessions. A significant correlation was identified between the monoterpenoid and sesquiterpenoid content and the density of glandular trichomes on the leaves, with the correlation coefficients being 0.88, 0.86 and 0.90, respectively. In contrast, there was no significant correlation between the volatile terpenoid content and the density of T-shaped trichomes on the leaves. In flowerheads, a significant correlation was identified between the emission rate of terpenoids and the number of glandular trichomes on the disc florets, with a correlation coefficient of 0.95. Interestingly, the correlation between the density of glandular trichomes and concentrations of terpenoids was insignificant. In summary, the relationship between trichomes and terpenoid chemistry in Chrysanthemum is clearly established. Such knowledge may be helpful for breeding aromatic Chrysanthemum cultivars by modulating the trichome trait.

Ribozyme-mediated CRISPR/Cas9 gene editing in pyrethrum (Tanacetum cinerariifolium) hairy roots using a RNA polymerase II-dependent promoter

BACKGROUND

Traditional CRISPR/Cas9 systems that rely on U6 or U3 snRNA promoters (RNA polymerase III-dependent promoters) can only achieve constitutive gene editing in plants, hampering the functional analysis of specifically expressed genes. Ribozyme-mediated CRISPR/Cas9 systems increase the types of promoters which can be used to transcribe sgRNA. Therefore, such systems allow specific gene editing; for example, transcription of the artificial gene Ribozyme-sgRNA-Ribozyme (RGR) is initiated by an RNA polymerase II-dependent promoter. Genetic transformation is indispensable for editing plant genes. In certain plant species, including pyrethrum, genetic transformation remains challenging to do, limiting the functional verification of novel CRISPR/Cas9 systems. Thus, this study's aim was to develop a simple Agrobacterium rhizogenes-mediated hairy root transformation system to analyze the function of a ribozyme-mediated CRISPR/Cas9 system in pyrethrum.

RESULTS

A hairy root transformation system for pyrethrum is described, with a mean transformation frequency of 7%. Transgenic hairy roots transformed with the pBI121 vector exhibited significantly increased beta-glucuronidase staining as a visual marker of transgene expression. Further, a ribozyme-based CRISPR/Cas9 vector was constructed to edit the TcEbFS gene, which catalyzes synthesis of the defense-related compound (E)-β-farnesene in pyrethrum. The vector was transferred into the hairy roots of pyrethrum and two stably transformed hairy root transgenic lines obtained. Editing of the TcEbFS gene in the hairy roots was evaluated by gene sequencing, demonstrating that both hairy root transgenic lines had DNA base loss at the editing target site. Gas chromatography-mass spectrometry showed that the (E)-β-farnesene content was significantly decreased in both hairy root transgenic lines compared with the empty vector control group. Altogether, these results show that RGR can be driven by the CaMV35S promoter to realize TcEbFS gene editing in pyrethrum hairy roots.

CONCLUSION

An A. rhizogenes-mediated hairy root transformation and ribozyme-mediated CRISPR/Cas9 gene editing system in pyrethrum was established, thereby facilitating gene editing in specific organs or at a particular developmental stage in future pyrethrum research.

The Ecological Significance of Aphid Cornicles and Their Secretions

Aphid cornicles are abdominal appendages that secrete an array of volatile and nonvolatile compounds with diverse ecological functions. The emission of alarm pheromones yields altruistic benefits for clone-mates in the aphid colony, which is essentially a superorganism with a collective fate. Secreted droplets also contain unsaturated triglycerides, fast-drying adhesives that can be lethal when smeared on natural enemies but more often impede their foraging efficiency. The longest cornicles have evolved in aphids that feed in exposed locations and are likely used to scent-mark colony intruders. Reduced cornicles are associated with reliance on alternative defenses, such as the secretion of protective waxes or myrmecophily. Root-feeding and gall-forming lifestyles provide protected feeding sites and are associated with an absence of cornicles. In some eusocial gall-formers, soldier morphs become repositories of cornicle secretion used to defend the gall, either as menopausal apterae that defend dispersing alatae or as sterile first instars that dispatch predators with their stylets and use cornicle secretions as a construction material for gall repair. Collectively, the evidence is consistent with an adaptive radiation of derived cornicle functions molded by the ecological lifestyle of the aphid lineage.

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.

Responses to predation risk cues and alarm pheromones affect plant virus transmission by an aphid vector

Herbivores assess predation risk in their environment by identifying visual, chemical, and tactile predator cues. Detection of predator cues can induce risk-avoidance behaviors in herbivores that affect feeding, dispersal, and host selection in ways that minimize mortality and reproductive costs. For herbivores that transmit plant pathogens, including many aphids, changes in herbivore behavior in response to predator cues may also affect pathogen spread. However, few studies have assessed how aphid behavioral responses to different types of predator cues affect pathogen transmission. Here, we conducted greenhouse experiments to assess whether responses of pea aphids (Acyrthosiphon pisum) to predation risk and alarm pheromone (E-β-Farnesene), an aphid alarm signal released in response to predation risk, affected transmission of Pea enation mosaic virus (PEMV). We exposed A. pisum individuals to risk cues, and quantified viral titer in aphids and pea (Pisum sativum) host plants across several time periods. We also assessed how A. pisum responses to risk cues affected aphid nutrition, reproduction, and host selection. We show that exposure to predator cues and alarm pheromone significantly reduced PEMV acquisition and inoculation. Although vectors avoided hosts with predator cues, predator cues did not alter vector reproduction or reduce nutrient acquisition. Overall, these results suggest that non-consumptive effects of predators may indirectly decrease the spread of plant pathogens by altering vector behavior in ways that reduce vector competence and pathogen transmission efficiency.

New slow release mixture of (E)-β-farnesene with methyl salicylate to enhance aphid biocontrol efficacy in wheat ecosystem

Semiochemical use is a promising way to reduce damage from pests by improving natural control in agro-ecosystems. The aphid alarm pheromone (E)-β-farnesene (EβF) and herbivore-induced methyl salicylate (MeSA) are two volatile cues to induce changes in aphid behavior with functional significance. Because of limitations related to the volatility and oxidization of EβF and MeSA under natural conditions, slow-release and antioxidant techniques should be developed and optimized before application. Here, a slow-release alginate bead of EβF mixed with MeSA was first designed and manufactured. We hypothesized that a mixture of these two semiochemicals could be effective in controlling Sitobion miscanthi in wheat crops. Both MeSA and EβF in alginate beads were released stably and continuously for at least 15 days in the laboratory, whereas EβF in paraffin oil and pure MeSA were released for only 2 and 7 days, respectively. In 2018 field experiments, EβF and MeSA alone or in association significantly decreased the abundance of alate and apterous aphids. An increased abundance of mummified aphids enhanced by higher parasitism rates was observed when using EβF and MeSA in association, with a significant reduction of apterous abundance, more so than EβF or MeSA alone. In 2019, plots treated with a mixture of EβF and MeSA showed significantly decreased abundance of alate and apterous aphids with higher parasitism rates compared with the control. The new slow-release alginate bead containing a mixture of EβF with MeSA could be the most efficient formulation to control S. miscanthi population by attracting parasitoids in the wheat agro-ecosystem. © 2021 Society of Chemical Industry. © 2021 Society of Chemical Industry.

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.

Methyl Salicylate and Sesquiterpene Emissions Are Indicative for Aphid Infestation on Scots Pine

Biotic stresses on forest trees are caused by various pest insects and plant pathogens. Attack by these parasites is known to induce the emissions of various biogenic volatile organic compounds (BVOCs), and the profile of these emissions often differs between infested and healthy plants. This difference in emission profile can be used for the non-destructive early-stage diagnosis of the stressor organism. We studied how phloem feeding by a large pine aphid (Cinara pinea Mordvilko) on the branch bark of Scots pine (Pinus sylvestris L.) affects BVOC emissions compared to those of healthy plants in two experiments. We found that in aphid-infested plants, methyl salicylate (MeSA) emissions significantly increased, and the emission rates were dependent on aphid density on the studied branch. Aphid infestation did not significantly affect total monoterpene emission, while the emissions of total sesquiterpenes were substantially higher in aphid-infested saplings than in uninfested plants. Sesquiterpene (E, E)-α-farnesene was emitted at increased rates in both experiments, and the aphid alarm pheromone sesquiterpene (E)-β-farnesene, only in the experiment with higher aphid pressure. We conclude that the rapid increase in MeSA emissions is the most reliable indicator of aphid infestation in pine trees together with (E, E)-α-farnesene.