(E)-β-farnesene synthase genes affect aphid (Myzus persicae) infestation in tobacco (Nicotiana tabacum)

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.

Genetic Diversity Analysis Reveals Potential of the Green Peach Aphid (Myzus persicae) Resistance in Ethiopian Mustard

Brassica carinata (BBCC, 2n = 34) is commonly known as Ethiopian mustard, Abyssinian mustard, or carinata. Its excellent agronomic traits, including resistance to biotic and abiotic stresses, make it a potential genetic donor for interspecific hybridization. Myzus persicae (green peach aphid, GPA) is one of the most harmful pests of Brassica crops, significantly effecting the yield and quality. However, few aphid-resistant Brassica crop germplasms have been utilized in breeding practices, while the underlying biochemical basis of aphid resistance still remains poorly understood. In this study, we examined the genetic diversity of 75 B. carinata accessions and some plant characteristics that potentially contribute to GPA resistance. Initially, the morphological characterization showed abundant diversity in the phenotypic traits, with the dendrogram indicating that the genetic variation of the 75 accessions ranged from 0.66 to 0.98. A population structure analysis revealed that these accessions could be grouped into two main subpopulations and one admixed group, with the majority of accessions (86.67%) clustering in one subpopulation. Subsequently, there were three GPA-resistant B. carinata accessions, BC13, BC47, and BC51. The electrical penetration graph (EPG) assay detected resistance factors in the leaf mesophyll tissue and xylem. The result demonstrated that the Ethiopian mustard accessions were susceptible when the phloem probing time, the first probe time, and the G-wave time were 20.51-32.51 min, 26.36-55.54 s, and 36.18-47.84 min, respectively. In contrast, resistance of the Ethiopian mustard accessions was observed with the phloem probing time, the first probe time, and G-wave time of 41.18-70.78 min, 181.07-365.85 s, and 18.03-26.37 min, respectively. In addition, the epidermal characters, leaf anatomical structure, glucosinolate composition, defense-related enzyme activities, and callose deposition were compared between the resistant and susceptible accessions. GPA-resistant accessions had denser longitudinal leaf structure, higher wax content on the leaf surface, higher indole glucosinolate level, increased polyphenol oxidase (PPO) activity, and faster callose deposition than the susceptible accessions. This study validates that inherent physical and chemical barriers are evidently crucial factors in the resistance against GPA infestation. This study not only provide new insights into the biochemical basis of GPA resistance but also highlights the GPA-resistant B. carinata germplasm resources for the future accurate genetic improvement of Brassica crops.

Plant Volatiles Mediate Host Selection of Sitodiplosis mosellana (Diptera: Cecidomyiidae) among Wheat Varieties

Sitodiplosis mosellana is a major wheat pest that oviposits on spikes, and resistant wheat varieties have been released. However, wheat spike volatiles mediating S. mosellana host selection or resistance are largely unknown. In this study, we found that the highly susceptible wheat varieties Xinong 822, Xinong 88, and Xiaoyan 22 were preferred for S. mosellana oviposition, and their spike volatiles were more attractive to females compared to the resistant varieties Kenong 1006, Shanmai 139, and Jinmai 47. Importantly, we found five odor components evoking obvious concentration-dependent electroantennogram (EAG) and behavioral response. Notably, 3-hexanol, cis-3-hexenylacetate, and hexyl acetate strongly attracted females, whereas ocimene, a dominant component of three resistant varieties, and α-farnesene, absent in Xinong 88, repelled females. Significant attraction was also observed in a synthetic blend mimicking Xinong 822 volatiles. These results suggest that these wheat volatiles are involved in host selection of S. mosellana and provide a basis for development of semichemical-based pest management.

Recent Advances in the Biosynthesis of Farnesene Using Metabolic Engineering

Farnesene, as an important sesquiterpene isoprenoid polymer of acetyl-CoA, is a renewable feedstock for diesel fuel, polymers, and cosmetics. It has been widely applied in agriculture, medicine, energy, and other fields. In recent years, farnesene biosynthesis is considered a green and economical approach because of its mild reaction conditions, low environmental pollution, and sustainability. Metabolic engineering has been widely applied to construct cell factories for farnesene biosynthesis. In this paper, the research progress, common problems, and strategies of farnesene biosynthesis are reviewed. They are mainly described from the perspectives of the current status of farnesene biosynthesis in different host cells, optimization of the metabolic pathway for farnesene biosynthesis, and key enzymes for farnesene biosynthesis. Furthermore, the challenges and prospects for future farnesene biosynthesis are discussed.

Taxonomic Insights and Its Type Cyclization Correlation of Volatile Sesquiterpenes in Vitex Species and Potential Source Insecticidal Compounds: A Review

Sesquiterpenes (SS) are secondary metabolites formed by the bonding of 3 isoprene (C5) units. They play an important role in the defense and signaling of plants to adapt to the environment, face stress, and communicate with the outside world, and their evolutionary history is closely related to their physiological functions. This review considers their presence and extensively summarizes the 156 sesquiterpenes identified in Vitextaxa, emphasizing those with higher concentrations and frequency among species and correlating with the insecticidal activities and defensive responses reported in the literature. In addition, we classify the SS based on their chemical structures and addresses cyclization in biosynthetic origin. Most relevant sesquiterpenes of the Vitex genus are derived from the germacredienyl cation mainly via bicyclogermacrene and germacrene C, giving rise to aromadrendanes, a skeleton with the highest number of representative compounds in this genus, and 6,9-guaiadiene, respectively, indicating the production of 1.10-cyclizing sesquiterpene synthases. These enzymes can play an important role in the chemosystematics of the genus from their corresponding routes and cyclizations, constituting a new approach to chemotaxonomy. In conclusion, this review is a compilation of detailed information on the profile of sesquiterpene in the Vitex genus and, thus, points to new unexplored horizons for future research.

Spatiotemporal expression profiling of the farnesyl diphosphate synthase genes in aphids and analysis of their associations with the biosynthesis of alarm pheromone

The alarm behavior plays a key role in the ecology of aphids, but the site and molecular mechanism for the biosynthesis of aphid alarm pheromone are largely unknown. Farnesyl diphosphate synthase (FPPS) catalyzes the synthesis of FPP, providing the precursor for the alarm pheromone (E)-β-farnesene (EβF), and we speculate that FPPS is closely associated with the biosynthetic pathway of EβF. We firstly analyzed the spatiotemporal expression of FPPS genes by using quantitative reverse transcription-polymerase chain reaction, showing that they were expressed uninterruptedly from the embryonic stage to adult stage, with an obvious increasing trend from embryo to 4th-instar in the green peach aphid Myzus persicae, but FPPS1 had an overall significantly higher expression level than FPPS2; both FPPS1 and FPPS2 exhibited the highest expression in the cornicle area. This expression pattern was verified in Acyrthosiphon pisum, suggesting that FPPS1 may play a more important role in aphids and the cornicle area is most likely the site for EβF biosynthesis. We thus conducted a quantitative measurement of EβF in M. persicae by gas chromatography-mass spectrometry. The data obtained were used to perform an association analysis with the expression data, revealing that the content of EβF per aphid was significantly correlated with the mean weight per aphid (r = 0.8534, P = 0.0307) and the expression level of FPPS1 (r = 0.9134, P = 0.0109), but not with that of FPPS2 (r = 0.4113, P = 0.4179); the concentration of EβF per milligram of aphid was not correlated with the mean weight per aphid or the expression level of FPPS genes. These data suggest that FPPS1 may play a key role in the biosynthesis of aphid alarm pheromone.

Plasmid engineering of aphid alarm pheromone in tobacco seedlings affects the preference of aphids

Plants producing sufficient amount of aphid alarm pheromone by expressing (E)-β-Farnesene (EβF) synthase gene may contribute to plant protection by reducing aphid populations. However, terpene biosynthesis varies among plant species and developmental stages. In the present study, volatile headspace analysis of tobacco seedlings with MaβFS1 (an EβF synthase from the Asian peppermint Mentha asiatica) failed to generate EβF. We further targeted MaβFS1 to the tobacco plastid, using a chloroplast targeting sequence, either with or without the AtFPS1 gene for the biosynthesis of the precursor farnesyl diphosphate. When both MaβFS1 and AtFPS1 genes were targeted to the chloroplast, low levels of EβF were detected in stably transformed tobacco seedlings; resulting in specific repellence of the green peach aphid, Myzus persicae. These data indicate that redirecting the EβF biosynthetic pathway from its natural cytosolic location to the chloroplast is a valid strategy. This redirecting strategy may be very useful for other crop plants that do not naturally produce EβF or other repellent volatiles.

(E)-β-farnesene synthase gene affects aphid behavior in transgenic Medicago sativa

BACKGROUND

Pea aphid (Acyrthosiphon pisum Harris) is one of the major pests in alfalfa crops, causing significant yield losses. (E)-β-farnesene (EβF), an alarm pheromone released by pea aphid, is generic to many species of aphids, and is used to minimize potential danger from predators and parasitoids by avoiding the source of the pheromone.

RESULTS

In this study, EβF synthase gene was constructed into a plant expression vector, and overexpressed in alfalfa (Medicago sativa L.), with expression among transgenic lines confirmed by qRT-PCR. Subcellular localization analysis showed that EβF synthase gene was expressed in the plasma membrane and nucleus of the leaf. GC/MS of extraction from transgenic alfalfa indicated emission of EβF ranging from 5.92 to 13.09 ng day^-1 g^-1 fresh tissue. Behavior assays in Y-olfactometers demonstrated that transgenic alfalfa expressing AaEβF gene could repel pea aphids, with aphids taking a significantly longer time to select a transgenic line compared with the control line (P < 0.01).

CONCLUSION

We have demonstrated a potentially valuable strategy of using EβF synthase genes for aphid control in alfalfa. © 2018 Society of Chemical Industry.

Modification of chrysanthemum odour and taste with chrysanthemol synthase induces strong dual resistance against cotton aphids

Aphids are pests of chrysanthemum that employ plant volatiles to select host plants and ingest cell contents to probe host quality before engaging in prolonged feeding and reproduction. Changes in volatile and nonvolatile metabolite profiles can disrupt aphid-plant interactions and provide new methods of pest control. Chrysanthemol synthase (CHS) from Tanacetum cinerariifolium represents the first committed step in the biosynthesis of pyrethrin ester insecticides, but no biological role for the chrysanthemol product alone has yet been documented. In this study, the TcCHS gene was over-expressed in Chrysanthemum morifolium and resulted in both the emission of volatile chrysanthemol (ca. 47 pmol/h/gFW) and accumulation of a chrysanthemol glycoside derivative, identified by NMR as chrysanthemyl-6-O-malonyl-β-D-glucopyranoside (ca. 1.1 mM), with no detrimental phenotypic effects. Dual-choice assays separately assaying these compounds in pure form and as part of the headspace and extract demonstrated independent bioactivity of both components against the cotton aphid (Aphis gossypii). Performance assays showed that the TcCHS plants significantly reduced aphid reproduction, consistent with disturbance of aphid probing activities on these plants as revealed by electropenetrogram (EPG) studies. In open-field trials, aphid population development was very strongly impaired demonstrating the robustness and high impact of the trait. The results suggest that expression of the TcCHS gene induces a dual defence system, with both repellence by chrysanthemol odour and deterrence by its nonvolatile glycoside, introducing a promising new option for engineering aphid control into plants.

Host plants and obligate endosymbionts are not the sources for biosynthesis of the aphid alarm pheromone

(E)-β-farnesene (EβF) is the major component of the alarm pheromone of many aphid species, but where EβF is synthesized in aphids is only partly understood. There are at least three most possible sources for the alarm pheromone: host plants, aphid obligate endosymbiont and aphids themselves. Here we eliminated the possibility of host plants and the obligate endosymbiont Buchnera aphidicola as the sources for EβF released by aphids. We excluded the possible effects of host plants on EβF biosynthesis by rearing aphids on non-plant diets. Both the diet-reared aphids, including the cotton aphid Aphis gossypii and the green peach aphid Myzus persicae, could still release EβF based on solid-phase micro-extraction combined with gas chromatography-mass spectrometer analysis. Meanwhile, we treated host aphids with antibiotics to fully eliminate Buchnera bacteria. Though the treatment seriously affected the development and fecundity of host aphids, the treated aphids could still release EβF, and there was no significant difference in the EβF concentration as per the aphid weight under different rearing conditions. Taken together, our experimental results suggest that host plants and obligate endosymbionts are not the sources for EβF released by aphids, indicating that it is most probably the aphid itself synthesizes the alarm pheromone.

Use of slow-release plant infochemicals to control aphids: a first investigation in a Belgian wheat field

Using infochemicals to develop a push–pull strategy in pest control is a potential way to promote sustainable crop production. Infochemicals from plant essential oils were mixed with paraffin oil for slow release in field experiments on wheat to control the population density of cereal aphids and to enhance their natural enemies. (Z)-3-Hexenol (Z3H) attracted Metopolophum dirhodum and Sitobion avenae, the predominant species on wheat in Belgium, and may be a useful infochemical for aphid control by attracting aphids away from field plots. Release of (E)-β-farnesene (EBF) or a garlic extract (GE) led to a significant decrease in the abundance of wheat aphids. The main natural enemies of cereal aphids found were lacewings (47.8%), hoverflies (39.4%), and ladybirds (12.8%). Ladybird abundance varied little before the end of the wheat-growing season. Our results suggest that these chemicals can form the basis of a “push–pull” strategy for aphid biological control, with GE and EBF acting as a pest- and beneficial-pulling stimulus and Z3H for aphid pulling.

Expressing an (E)-β-farnesene synthase in the chloroplast of tobacco affects the preference of green peach aphid and its parasitoid

(E)-β-Farnesene (EβF) synthase catalyses the production of EβF, which for many aphids is the main or only component of the alarm pheromone causing the repellence of aphids and also functions as a kairomone for aphids' natural enemies. Many plants possess EβF synthase genes and can release EβF to repel aphids. In order to effectively recruit the plant-derived EβF synthase genes for aphid control, by using chloroplast transit peptide (CTP) of the small subunit of Rubisco (rbcS) from wheat (Triticum aestivum L.), we targeted AaβFS1, an EβF synthase gene from sweet wormwood (Artemisia annua L.), to the chloroplast of tobacco to generate CTP + AaβFS1 transgenic lines. The CTP + AaβFS1 transgenic tobacco plants could emit EβF at a level up to 19.25 ng/day per g fresh tissues, 4-12 fold higher than the AaβFS1 transgenic lines without chloroplast targeting. Furthermore, aphid/parasitoid behavioral bioassays demonstrated that the CTP + AaβFS1 transgenic tobacco showed enhanced repellence to green peach aphid (Myzus persicae) and attracted response of its parasitoid Diaeretiella rapae, thus affecting aphid infestation at two trophic levels. These data suggest that the chloroplast is an ideal subcellular compartment for metabolic engineering of plant-derived EβF synthase genes to generate a novel type of transgenic plant emitting an alarm pheromone for aphid control.

The first crop plant genetically engineered to release an insect pheromone for defence

Insect pheromones offer potential for managing pests of crop plants. Volatility and instability are problems for deployment in agriculture but could be solved by expressing genes for the biosynthesis of pheromones in the crop plants. This has now been achieved by genetically engineering a hexaploid variety of wheat to release (E)-β-farnesene (Eβf), the alarm pheromone for many pest aphids, using a synthetic gene based on a sequence from peppermint with a plastid targeting amino acid sequence, with or without a gene for biosynthesis of the precursor farnesyl diphosphate. Pure Eβf was produced in stably transformed wheat lines with no other detectable phenotype but requiring targeting of the gene produced to the plastid. In laboratory behavioural assays, three species of cereal aphids were repelled and foraging was increased for a parasitic natural enemy. Although these studies show considerable potential for aphid control, field trials employing the single and double constructs showed no reduction in aphids or increase in parasitism. Insect numbers were low and climatic conditions erratic suggesting the need for further trials or a closer imitation, in the plant, of alarm pheromone release.

Engineering plants for aphid resistance: current status and future perspectives

The current status of development of transgenic plants for improved aphid resistance, and the pros and cons of different strategies are reviewed and future perspectives are proposed. Aphids are major agricultural pests that cause significant yield losses of crop plants each year. Excessive dependence on insecticides for aphid control is undesirable because of the development of insecticide resistance, the potential negative effects on non-target organisms and environmental pollution. Transgenic plants engineered for resistance to aphids via a non-toxic mode of action could be an efficient alternative strategy. In this review, the distribution of major aphid species and their damages on crop plants, the so far isolated aphid-resistance genes and their applications in developments of transgenic plants for improved aphid resistance, and the pros and cons of these strategies are reviewed and future perspectives are proposed. Although the transgenic plants developed through expressing aphid-resistant genes, manipulating plant secondary metabolism and plant-mediated RNAi strategy have been demonstrated to confer improved aphid resistance to some degree. So far, no aphid-resistant transgenic crop plants have ever been commercialized. This commentary is intended to be a helpful insight into the generation and future commercialization of aphid-resistant transgenic crops in a global context.

Probing the mechanism of 1,4-conjugate elimination reactions catalyzed by terpene synthases

The reaction mechanisms of (E)-β-farnesene synthase (EBFS) and isoprene synthase (ISPS), enzymes that catalyze a formal regiospecific 1,4-conjugate elimination of hydrogen diphosphate from (E,E)-farnesyl and dimethylallyl diphosphate (FDP and DMADP) to generate the semiochemicals (E)-β-farnesene and isoprene, respectively, were probed with substrate analogs and kinetic measurements. The results support stepwise reaction mechanisms through analogous enzyme-bound allylic cationic intermediates. For EBFS, we demonstrate that the elimination reaction can proceed via the enzyme-bound intermediate trans-nerolidyl diphosphate, while for ISPS the intermediacy of 2-methylbut-3-enyl 2-diphosphate can be inferred from the product outcome when deuterated DMADPs are used as substrates. Possible implications derived from the mechanistic details of the EBFS-catalyzed reaction for the evolution of sesquiterpene synthases are discussed.