A look inside odorant-binding proteins in insect chemoreception

Compensatory effects of other olfactory genes after CRISPR/cas9 editing of BmOR56 in silkworm, Bombyx mori

Bombyx mori is an oligophagous economic insect. Cis-Jasmone is one of the main substances in mulberry leaf that attract silkworm for feeding and BmOR56 is its receptor. Potential interaction ways between BmOR56 and cis-Jasmone were explored, which included some crucial amino acids such as Gln172, Val173, Ser176, Lys182, His322, and Arg345. BmOR56 was edited using CRISPR/cas9 for Qiufeng, and a homozygous knockout strain QiufengM was obtained. Compared with Qiufeng, the feeding ability of QiufengM on mulberry leaf did not change significantly, but on artificial diet decreased significantly. QiufengM also showed a dependence on the concentration of mulberry leaf powder. The result indicated that other olfactory genes had a compensatory effect on the attractance of mulberry leaf after the loss of BmOR56. Transcriptome analysis of antennae showed that many genes differentially expressed between Qiufeng and QiufengM, which involved in olfactory system, glucose metabolism, protein metabolism, amino acid metabolism, and insect hormone biosynthesis. Particularly, BmIR21, BmOR53 and BmOR27 were significantly up-regulated, which may have a compensatory effect on BmOR56 loss. In addition, detoxification mechanism was activated and may cause the passivation of feeling external signals in silkworm.

Precision detection of select human lung cancer biomarkers and cell lines using honeybee olfactory neural circuitry as a novel gas sensor

Human breath contains biomarkers (odorants) that can be targeted for early disease detection. It is well known that honeybees have a keen sense of smell and can detect a wide variety of odors at low concentrations. Here, we employ honeybee olfactory neuronal circuitry to classify human lung cancer volatile biomarkers at different concentrations and their mixtures at concentration ranges relevant to biomarkers in human breath from parts-per-billion to parts-per-trillion. We also validated this brain-based sensing technology by detecting human non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) cell lines using the 'smell' of the cell cultures. Different lung cancer biomarkers evoked distinct spiking response dynamics in the honeybee antennal lobe neurons indicating that those neurons encoded biomarker-specific information. By investigating lung cancer biomarker-evoked population neuronal responses from the honeybee antennal lobe, we classified individual human lung cancer biomarkers successfully (88% success rate). When we mixed six lung cancer biomarkers at different concentrations to create 'synthetic lung cancer' vs. 'synthetic healthy' human breath, honeybee population neuronal responses were able to classify those complex breath mixtures reliably with exceedingly high accuracy (93-100% success rate with a leave-one-trial-out classification method). Finally, we employed this sensor to detect human NSCLC and SCLC cell lines and we demonstrated that honeybee brain olfactory neurons could distinguish between lung cancer vs. healthy cell lines and could differentiate between different NSCLC and SCLC cell lines successfully (82% classification success rate). These results indicate that the honeybee olfactory system can be used as a sensitive biological gas sensor to detect human lung cancer.

Identification and sex expression profiles of candidate chemosensory genes from Atherigona orientalis via the antennae and leg transcriptome analysis

Atherigona orientalis Schiner (1868) is an acknowledged agricultural pest owing to its feeding habits and breeding locations. This insect is a tropical and subtropical pest in fruits and vegetables, in which >50 varieties of fruits and vegetables in 26 families, such as Capsicum annuum, Lycopersicon esculentum, and Cucumis melo have been attacked. Moreover, A. orientalis may also develop in rotten crops and feces or insect carcasses, which are also considered one kind of sanitary pest and medical insect. At present, the invasion ranges of A. orientalis are still increasing and more preventive and management measures are to be processed. To gain a better understanding of the molecular mechanisms involved in olfactory reception in A. orientalis, the transcriptome of male and female antennae and legs was systematically analyzed. In total, 131 chemosensory-related genes, including 63 odorant receptors (ORs), 20 gustatory receptors (GRs), 18 ionotropic receptors (IRs), 27 odorant binding proteins (OBPs), 1 chemosensory protein (CSP), and 2 sensory neuron membrane proteins (SNMPs), were identified. The analysis focused on obtaining expression information of candidate olfactory genes at the transcriptomic level by examining the differentially expressed genes (DEGs) in all samples. Totally, 41 DEGs were identified between male antennae (MA) and female antennae (FA), including 32 ORs, 5 OBPs, 1 IR, 2 GRs and 1 SNMP. In MA versus male legs (ML), 78 DEGs were identified (45 ORs, 18 OBPs, 6 GRs, 6 IRs, 1 CSP and 2 SNMPs). In FA and female legs (FL), 96 DEGs were identified (51 ORs, 21 OBPs, 9 GRs, 12 IRs, 1 CSP and 2 SNMPs). For ML and FL, 3 DEGs were identified, including 2 ORs and 1 SNMP. Our results supplement valuable insights for future research on the chemoreception mechanisms in A. orientalis.

Plasticity of the olfactory behaviors in Bactrocera dorsalis under various physiological states and environmental conditions

Insects rely heavily on their olfactory system for various behaviors, including foraging, mating, and oviposition. Numerous studies have demonstrated that insects can adjust their olfactory behaviors in response to different physiological states and environmental conditions. This flexibility allows them to perceive and process odorants according to different conditions. The Oriental fruit fly, Bactrocera dorsalis, is a highly destructive and invasive pest causing significant economic losses to fruit and vegetable crops worldwide. The olfactory behavior of B. dorsalis exhibits strong plasticity, resulting in its successful invasion. To enhance our understanding of B. dorsalis' olfactory behavior and explore potential strategies for behavior control, we have reviewed recent literature on its olfactory plasticity and potential molecular mechanisms.

Role of odorant binding protein C12 in the response of Tribolium castaneum to chemical agents

Tribolium castaneum is a worldwide pest of stored grain that mainly damages flour, and not only causes serious loss of flour quality but also leads to deterioration of flour quality. Chemical detection plays a key role in insect behavior, and the role of odorant-binding proteins (OBPs) in insect chemical detection has been widely studied. OBPs can interact with small molecule compounds and thereby modulate variation in insecticide susceptibility in insects. In this study, a total of 65 small molecule compounds are selected to investigate the bound effect with TcOBP C12. The molecular docking results showed that β-caryophyllene, (-)-catechin, butylated hydroxytoluene, diphenyl phthalate and quercetin were the top five compounds, with docking binding energies of -6.11, -5.25, -5.09, -5.05, and - 5.03 Kcal/mol, respectively. Molecular dynamics analysis indicated that odorant binding protein C12 (TcOBP C12) exhibited high binding affinity to all five tested chemical ligands, evidenced by fluorescence quenching assay in vitro. In addition, the contact toxicity assay results suggested that these chemical agents caused a dose-dependent increase in mortality rate for T. castaneum adults. The TcOBP C12 gene was upregulated >2 times after a 24-h exposure, indicating that OBP C12 may play an important role for T. castaneum in response to these chemical agents. In conclusion, our results provide a theoretical basis for future insecticide experiments and pest management.

Identification of odorant-binding proteins and functional analysis of antenna-specific BhorOBP28 in Batocera horsfieldi (Hope)

BACKGROUND

The important wood-boring pest Batocera horsfieldi has evolved a sensitive olfactory system to locate host plants. Odorant-binding proteins (OBPs) are thought to play key roles in olfactory recognition. Therefore, exploring the physiological function of OBPs could facilitate a better understanding of insect chemical communications.

RESULTS

In this research, 36 BhorOBPs genes were identified via transcriptome sequencing of adults' antennae from B. horsfieldi, and most BhorOBPs were predominantly expressed in chemosensory body parts. Through fluorescence competitive binding and fluorescence quenching assays, the antenna-specific BhorOBP28 was investigated and displayed strong binding affinities forming stable complexes with five volatiles, including (+)-α-Pinene, (+)-Limonene, β-Pinene, (-)-Limonene, and (+)-Longifolene, which could also elicit conformation changes when they were interacting with BhorOBP28. Batocera horsfieldi females exhibited a preference for (-)-Limonene, and a repellent response to (+)-Longifolene. Feeding dsOBP19 produced by a bacteria-expressed system with a newly constructed vector could lead to the knockdown of BhorOBP28, and could further impair B. horsfieldi attraction to (-)-Limonene and repellent activity of (+)-Longifolene. The analysis of site-directed mutagenesis revealed that Leu7, Leu72, and Phe121 play a vital role in selectively binding properties of BhorOBP28.

CONCLUSION

By modeling the molecular mechanism of olfactory recognition, these results demonstrate that BhorOBP28 is involved in the chemoreception of B. horsfieldi. The bacterial-expressed dsRNA delivery system gains new insights into potential population management strategies. Through the olfactory process concluded that discovering novel behavioral regulation and environmentally friendly control options for B. horsfieldi in the future. © 2024 Society of Chemical Industry.

Binding Properties of Odorant Binding Protein 37 in Plagiodera versicolora to Host Volatile, o-Cymene

The chemosensory system plays an important role in the host plants location. Plagiodera versicolora (Coleoptera: Chrysomelidae) is a worldwide leaf-eating forest pest that feeds exclusively on salicaceous trees. There is no function study of odorant binding proteins (OBPs) in P. versicolora. In the current study, we found that PverOBP37 has a high expression in male and female antennae, heads, and legs by quantitative real-time PCR. The binding properties of PverOBP37 to 18 host plant volatiles were determined by fluorescence competition binding assays. The results showed that PverOBP37 could bind to the host plant volatile, o-cymene. Furthermore, four candidate key amino acid residues (F8, Y50, F103, and R107) of PverOBP37 to o-cymene were identified by molecular docking. The functional assay to confirm Y50, F103, and R107 mutations were key amino acid residues of PverOBP37 involved in the binding to o-cymene. Knockdown of PverOBP37 and Y-tube behavioral bioassays of mated females led to a significantly reduced attraction to o-cymene. This study not only revealed the molecular mechanism of PverOBP37 but also suggested that PverOBP37 is essential to detect host plant volatiles as cues to search for egg-laying sites in P. versicolora.

Binding characteristics of pheromone-binding protein 1 in Glyphodes pyloalis to organophosphorus insecticides: Insights from computational and experimental approaches

Glyphodes pyloalis (Lepidoptera: Pyralidae) is one of the major pests in mulberry production in China, which has developed resistance to various insecticides. Chemoreception is one of the most crucial physiological tactics in insects, playing a pivotal role in recognizing chemical stimuli in the environment, including noxious stimuli such as insecticides. Herein, we obtained recombinant pheromone-binding protein 1 (GpylPBP1) that exhibited antennae-biased expression in G. pyloalis. Ligand-binding assays indicated that GpylPBP1 had the binding affinities to two organophosphorus insecticides, with a higher binding affinity to chlorpyrifos than to phoxim. Computational simulations showed that a mass of nonpolar amino acid residues formed the binding pocket of GpylPBP1 and contributed to the hydrophobic interactions in the bindings of GpylPBP1 to both insecticides. Furthermore, the binding affinities of three GpylPBP1 mutants (F12A, I52A, and F118A) to both insecticides were all significantly reduced compared to those of the GpylPBP1-wild type, suggesting that Phe12, Ile52, and Phe118 residues were crucial binding sites and played crucial roles in the bindings of GpylPBP1 to both insecticides. Our findings can be instrumental in elucidating the effects of insecticides on olfactory recognition in moths and facilitating the development of novel pest management strategies using PBPs as targets based on insect olfaction.

Rational design, synthesis and binding mechanisms of novel benzyl geranate derivatives as potential eco-friendly aphid repellents

BACKGROUND

The push-pull strategy is considered as a promising eco-friendly method for pest management. Plant volatile organic compounds (PVOCs) act as semiochemicals constitute the key factor in implementing this strategy. Benzyl alcohol and geraniol, as functional PVOCs, were reported to regulate insect behavior, showing the potential application in pest control. Using geraniol as lead, a geraniol derivative 5i with fine repellent activity was discovered in our previous work. In order to explore novel, eco-friendly aphid control agents, a series of benzyl geranate derivatives was designed and synthesized using 5i as the lead and benzyl alcohol as the active fragment.

RESULTS

Benzyl alcohol was firstly evaluated to have repellent activity to Acyrthosiphon pisum. Based on this repellent fragment, a series of novel benzyl geranate derivatives was rationally designed and synthesized using a scaffold-hopping strategy. Among them, compound T9, with a binding affinity (Kd  = 0.43 μm) and a substantial repellency of 64.7% against A. pisum, is the most promising compound. Molecule docking showed that hydrophobic and hydrogen-bonding interactions substantially influenced the binding affinity of compounds with ApisOBP9. Additionally, T9 exhibited low-toxicity to honeybees and ladybugs.

CONCLUSION

Using a simple scaffold-hopping strategy combined with active fragment benzyl alcohol, a new derivative T9, with high aphid-repellency and low-toxicity to nontarget organisms, can be considered as a novel potential eco-friendly aphid control agent for sustainable agriculture. © 2023 Society of Chemical Industry.

Odorant-binding protein 19 in Monochamus alternatus involved in the recognition of a volatile strongly emitted from ovipositing host pines

Monochamus alternatus is the primary carrier of pine wood nematodes, which pose a serious threat to Pinus spp. in many countries. Newly emerging M. alternatus adults feed on heathy host pines, while matured adults transfer to stressed host pines for mating and oviposition. Several odorant-binding proteins (OBPs) of M. alternatus have been proved to aid in the complex process of host location. To clarify the corresponding relations between OBPs and pine volatiles, more OBPs need to be studied. In this research, MaltOBP19 showed a specific expression in the antennae and mouthparts of M. alternatus, and it was marked in 4 types of antenna sensilla by immunolocalization. Fluorescence binding assays demonstrated the high binding affinity of MaltOBP19 with camphene and myrcene in vitro. In Y-tube olfactory experiments, M. alternatus adults were attracted by camphene and RNAi of OBP19 via microinjection significantly decreased their attraction index. Myrcene induced phobotaxis, but RNAi had no significant effect on this behavior. Further, we found that ingesting dsOBP19 produced by a bacteria-expressed system with a newly constructed vector could lead to the knockdown of MaltOBP19. These results suggest that MaltOBP19 may play a role in the process of host conversion via the recognition of camphene, which has been identified to be strongly released in stressed host pines. In addition, it is proved that knockdown of OBP can be achieved by oral administration of bacteria-expressed double-stranded RNA in M. alternatus adults, providing a new perspective in the control of M. alternatus.

The Sequence Characteristics and Binding Properties of the Odorant-Binding Protein SvelOBP1 from Sympiezomias velatus (Coleoptera: Curculionidae) to Jujube Volatiles

Sympiezomias velatus (Chevrolat) (Coleoptera: Curculionidae) has caused serious damage on jujube trees (Ziziphus jujuba Mill) in northern China. Semiochemicals emerging from the host are essential in the process of insects identifying and localizing the host. The highly expressed odorant-binding protein 1 of S. velatus (SvelOBP1) was assumed to play a possible role in the recognition of host volatiles. In this study, SvelOBP1 was cloned based on the antennal transcriptome of S. velatus. The recombinant SvelOBP1 protein was expressed in Escherichia coli and purified by Ni-NTA resin. The predicted protein SvelOBP1 belonged to a classic OBP subfamily. The expression patterns revealed that SvelOBP1 was mainly expressed in the antennae of both males and females, whereas the expression of SvelOBP1 in other body parts could be neglected. The fluorescence binding assay indicated that SvelOBP1 displayed very strong binding affinities to dibutyl benzene-1,2-dicarboxylate and (Z)-hex-3-en-1-ol (Ki = 6.66 ± 0.03 and 7.98 ± 0.06 μM). The molecular docking results showed that residues Trp114, Phe115 and Asp110 may be involved in binding to both dibutyl benzene-1,2-dicarboxylate and (Z)-hex-3-en-1-ol and may have a great impact on odorant recognition of S. velatus. Our results provide evidence that SvelOBP1 might participate in the olfactory molecular perception of S. velatus and would promote the development of pest attractants for S. velatus control.

The Plant Volatile-Sensing Mechanism of Insects and Its Utilization

Plants and insects are engaged in a tight relationship, with phytophagous insects often utilizing volatile organic substances released by host plants to find food and egg-laying sites. Using plant volatiles as attractants for integrated pest management is vital due to its high efficacy and low environmental toxicity. Using naturally occurring plant volatiles combined with insect olfactory mechanisms to select volatile molecules for screening has proved an effective method for developing plant volatile-based attractant technologies. However, the widespread adoption of this technique is still limited by the lack of a complete understanding of molecular insect olfactory pathways. This paper first describes the nature of plant volatiles and the mechanisms of plant volatile perception by insects. Then, the attraction mechanism of plant volatiles to insects is introduced with the example of Cnaphalocrocis medinalis. Next, the progress of the development and utilization of plant volatiles to manage pests is presented. Finally, the functions played by the olfactory system of insects in recognizing plant volatiles and the application prospects of utilizing volatiles for green pest control are discussed. Understanding the sensing mechanism of insects to plant volatiles and its utilization will be critical for pest management in agriculture.

An Antennae-Enriched Odorant-Binding Protein EonuOBP43 Mediate the Behavioral Response of the Tea Green Leafhopper, Empoasca onukii Matsuda to the Host and Nonhost Volatiles

Olfaction is crucial for Empoasca onukii Matsuda to recognize odors from the host and nonhost plants, and it has been proposed that odorant binding proteins are directly required for odorant discrimination and represent potential targets of interest for pest control. Here, we cloned EonuOBP43 and expressed the recombinant EonuOBP43 protein. Furthermore, competitive fluorescence binding assays with 19 ligands indicated that terpenoids and alkanes showed a relatively higher than for other classes of chemicals. Additionally, ligand docking and site-directed mutagenesis results revealed that seven hydrophobic residues, including Val-86, Met-89, Phe-90, Ile-104, Ile-105, Leu-130, and Val-134, played a key role in the binding of EonuOBP43 to plant volatiles. In olfactometer tests, E. onukii were significantly attracted to α-farnesene and repelled to β-caryophyllene, and dsOBP43 treated adult lost response to α-farnesene and β-caryophyllene. In summary, our results demonstrated that EonuOBP43 may function as a carrier in the process of sensing plant compounds of E. onukii.

The antennal transcriptome analysis and characterizations of odorant-binding proteins in Megachile saussurei (Hymenoptera, Megachilidae)

BACKGROUND

Odorant-binding proteins (OBPs) are essential in insect's daily behaviors mediated by olfactory perception. Megachile saussurei Radoszkowski (Hymenoptera, Megachilidae) is a principal insect pollinating alfalfa (Medicago sativa) in Northwestern China. The olfactory function have been less conducted, which provides a lot of possibilities for our research.

RESULTS

Our results showed that 20 OBPs were identified in total. Multiple sequence alignment analysis indicated MsauOBPs were highly conserved with a 6-cysteine motif pattern and all belonged to the classic subfamily, coding 113-196 amino acids and sharing 41.32%-99.12% amino acid identity with known OBPs of other bees. Phylogenetic analysis indicated there were certain homologies existed among MsauOBPs and most sequences were clustered with that of Osmia cornuta (Hymenoptera, Megachilidae). Expression analysis showed the identified OBPs were mostly enriched in antennae instead of other four body parts, especially the MsauOBP2, MsauOBP3, MsauOBP4, MsauOBP8, MsauOBP11 and MsauOBP17, in which the MsauOBP2, MsauOBP4 and MsauOBP8 presented obvious tissue-biased expression pattern. Molecular docking results indicated MsauOBP4 might be the most significant protein in recognizing alfalfa flower volatile 3-Octanone, while MsauOBP13 might be the most crucial protein identifying (Z)-3-hexenyl acetate. It was also found the lysine was a momentous hydrophilic amino acid in docking simulations.

CONCLUSION

In this study, we identified and analyzed 20 OBPs of M. saussurei. The certain homology existed among these OBPs, while some degree of divergence could also be noticed, indicating the complex functions that different MsauOBPs performed. Besides, the M. saussurei and Osmia cornuta were very likely to share similar physiological functions as most of their OBPs were clustered together. MsauOBP4 might be the key protein in recognizing 3-Octanone, while MsauOBP13 might be the key protein in binding (Z)-3-hexenyl acetate. These two proteins might contribute to the alfalfa-locating during the pollination process. The relevant results may help determine the highly specific and effective attractants for M. saussurei in alfalfa pollination and reveal the molecular mechanism of odor-evoked pollinating behavior between these two species.

Expression and purification of Drosophila OBP44a with the aids of LC-MS and NMR

The production of highly purified native soluble proteins in large quantities is crucial for studying protein structure and function. Odorant binding proteins (OBPs) are small, soluble, extracellular proteins with multiple disulfide bonds, whose functions include, but are not limited to, binding hydrophobic molecules and delivering them to their corresponding receptors expressed on insect olfactory receptor neurons. Expression of proteins with multiple disulfide bonds like OBPs usually results in insolubility and low yield, which has been a significant barrier to understanding their biological roles and physiological functions. In the E. coli system, expression of OBPs often results in insoluble inclusion bodies or a limited amount of periplasmic soluble proteins. Although expression of OBPs in eukaryotic systems such as Sf9 insect cells or yeast Pichia pastoris can increase the solubility of the protein, the process remains insufficient. Additionally, monitoring the purity and native apo state of the protein is critical for establishing the correct conformation of the protein. In this study, we employed an E. coli host with an altered intracellular environment to produce cytosolic soluble OBP44a protein, which yielded over 100 mg/L. We monitored the integrity of disulfide bonds throughout the purification process using LC-MS and used NMR to ensure the final product adopted a single conformation. Our study presents an efficient method for obtaining large quantities of soluble proteins in a single conformation, which enables extensive in vitro studies of secreted proteins like OBPs.

Host plant recognition by two odorant-binding proteins in Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

BACKGROUND

Rhynchophorus ferrugineus, the red palm weevil (RPW), is a key pest that attacks many economically important palm species and that has evolved a sensitive and specific olfactory system to seek palm hosts. Odorant-binding proteins (OBPs) not only play crucial roles in its olfactory perception process but are also important molecular targets for the development of new approaches for pest management.

RESULTS

Analysis of the tissue expression profiles of RferOBP8 and RferOBP11 revealed that these two Rhynchophorus ferrugineus odorant binding proteins (RferOBPs) exhibited high expression in the antennae and showed sexual dimorphism. We analyzed the volatiles of seven host plants by gas chromatography-mass spectrometry and screened 13 potential ligands by molecular docking. The binding affinity of two recombinant OBPs to aggregation pheromones and 13 palm odorants was tested by fluorescence competitive binding assays. The results revealed that eight tested palm volatiles and ferrugineol have high binding affinities with RferOBP8 or RferOBP11. Behavioral trials showed that these eight odor compounds could elicit an attraction response in adult RPW. RNA interference analysis indicated that the reduction in the expression levels of the two RferOBPs led to a decrease in behavioral responses to these volatiles.

CONCLUSION

These results suggest that RferOBP8 and RferOBP11 are involved in mediating the responses of RPW to palm volatiles and to aggregation pheromones and may play important roles in RPW host-seeking. This study also provides a theoretical foundation for the promising application of novel molecular targets in the development of new behavioral interference strategies for RPW management in the future. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Biomimetic affinity sensor for the ultrasensitive detection of neonicotinoids

Multiple pesticides are often used in combination to protect crops from pests. This makes rapid on-site detection of pesticide contamination challenging. Herein, we describe a method for simultaneous detection of diverse neonicotinoid pesticides using a sensor that combines neonicotinoid-specific odorant-binding protein 2 (OBP2), which was cloned from an insect chemical sensing protein and modified gold nanoparticles with local surface plasmon resonance (LSPR)-based digital nanoplasmonometry (DiNM). When neonicotinoid pesticides bind to OBP2 on gold nanoparticles, the induced LSPR shift peak wavelength is too small to be measured using conventional LSPR immunoassays. DiNM records and compares the scattered image intensity in two adjacent wavelength bands, A and B, centered on the LSPR peak. It considers both the peak shift and the relative intensity change in these two bands, resulting in a significant LSPR signal enhancement. Then the spectral-image contrast was computed as the signal response. Using this approach, we obtained excellent limits of detection (LODs) of 1.4, 1.5, and 4.5 ppb for the neonicotinoids imidacloprid, acetamiprid, and dinotefuran, respectively. Blind tests demonstrated high positive and negative rates for teas, approximately 85 and 100%, respectively. Recombinant OBP2 produced in E. coli offers several advantages over antibodies, including high yield, time savings, and cost effectiveness. Moreover, this method is highly selective and sensitive to neonicotinoids, making it practical for field use.

Roles of insect odorant binding proteins in communication and xenobiotic adaptation

Odorant binding proteins (OBPs) are small water-soluble proteins mainly associated with olfaction, facilitating the transport of odorant molecules to their relevant receptors in the sensillum lymph. While traditionally considered essential for olfaction, recent research has revealed that OBPs are engaged in a diverse range of physiological functions in modulating chemical communication and defense. Over the past 10 years, emerging evidence suggests that OBPs play vital roles in purifying the perireceptor space from unwanted xenobiotics including plant volatiles and pesticides, potentially facilitating xenobiotic adaptation, such as host location, adaptation, and pesticide resistance. This multifunctionality can be attributed, in part, to their structural variability and effectiveness in transporting, sequestering, and concealing numerous hydrophobic molecules. Here, we firstly overviewed the classification and structural properties of OBPs in diverse insect orders. Subsequently, we discussed the myriad of functional roles of insect OBPs in communication and their adaptation to xenobiotics. By synthesizing the current knowledge in this field, our review paper contributes to a comprehensive understanding of the significance of insect OBPs in chemical ecology, xenobiotic adaptation, paving the way for future research in this fascinating area of study.

Screening of behaviorally active compounds based on the interaction between two chemosensory proteins and mung bean volatiles in Callosobruchus chinensis

Chemosensory proteins (CSPs) are involved in the earliest steps of the olfactory process by binding and transporting odorants and play a crucial role in the insect's search for food and egg-laying sites. In the present study, the tissue expression profiles showed that both CchiCSP3 and CchiCSP5 of Callosobruchus chinensis were highly expressed in the adult antennae. Subsequently, the recombinant CchiCSP3 and CchiCSP5 proteins were analysed using fluorescence competitive binding assays, and both showed binding affinities for the three mung bean volatiles. Molecular docking and site-directed mutagenesis revealed four key amino acid residues in CchiCSP3 (L47, W80, Y81, and L84) and CchiCSP5 (Y28, K46, L49, and I72). Electroantennogram (EAG) and dual-choice biobehavioral assays showed that the antennae of adult C. chinensis were electrophysiologically active in response to stimulation with all three behaviorally active compounds and that octyl 4-methoxycinnamate and β-ionone had a significant luring effect on adult C. chinensis, whereas vanillin had a significant avoidance effect. Our study screened three effective behaviorally active compounds based on the involvement of two CchiCSPs in the recognition of mung bean volatiles, providing an opportunity to develop an alternative control strategy using behavioral disruptors to limit the impact of pests.

The ecology of nutrient sensation and perception in insects

Insects are equipped with neurological, physiological, and behavioral tools to locate potential food sources and assess their nutritional quality based on volatile and chemotactile cues. We summarize current knowledge on insect taste perception and the different modalities of reception and perception. We suggest that the neurophysiological mechanisms of reception and perception are closely linked to the species-specific ecology of different insects. Understanding these links consequently requires a multidisciplinary approach. We also highlight existing knowledge gaps, especially in terms of the exact ligands of receptors, and provide evidence for a perceptional hierarchy suggesting that insects have adapted their reception and perception to preferentially perceive nutrient stimuli that are important for their fitness.

Expression and functional characterization of odorant-binding protein 2 in the predatory mite Neoseiulus barkeri

Olfaction plays a crucial role for arthropods in foraging, mating, and oviposition. The odorant-binding protein (OBP) gene is considered one of the most important olfactory genes. However, little is known about its functions in predatory mites. Here, we used Neoseiulus barkeri, an important commercialized natural pest control, to explore the chemosensory characteristics of OBP. In this study, N. barkeri was attracted by methyl salicylate (MeSA) and showed higher crawling speeds under MeSA treatment. Then, we identified and cloned an OBP gene named Nbarobp2 and analyzed its expression profiles in the predatory mite. Nbarobp2 was 663 bp, was highly expressed in larval and nymphal stages, and was significantly upregulated in N. barkeri under MeSA treatment. Nbarobp2 encoded 202 amino acid residues with a molecular weight of 23 kDa (after removing the signal peptide). Sequence comparisons revealed that the OBPs in Arachnida shared 6 conserved cysteine sites, but were distinguishable from the OBPs of Insecta on the phylogenetic tree. RNA interference, Western blotting, and binding affinity assays further proved that Nbarobp2 was involved in volatile perception in predatory mites. This study shed light on the functional characteristics of OBPs in predatory mites, providing a new insight for better biological control.

Identification of odorant receptors of Tribolium confusum in response to limonene repellent activity

Tribolium confusum is an important storage pest showing significant resistance to various chemical pesticides, development of botanical pesticides is an effective strategy to resolve above problem and decrease utilization of chemical pesticides. Present study attempts to explore the molecular mechanism about the repellent activity of limonene. When treatment concentration of limonene was 200.00 μg/cm2, the repellent level remained at grade V after 24 hours. Our study showed that limonene could be distinguished by T. confusum antenna with a maximal electroantennography test value of 0.90 mV. Simultaneously, 88 upregulated and 98 downregulated genes were sequenced in limonene-repellent T. confusum, and RT-qPCR analysis showed that four down-regulated and one up-regulated OR genes play an important role in the response to limonene. The repellent rate was decreased by 22.13% mediated with a knockdown of dsTconOR93, while the EAG value of the female and male adults was reduced to 0.26 mV (49.06%) and 0.20 mV (54.05%), respectively. In conclusion, limonene had a strong repellent activity against T. confusum and TconOR93 gene was determined to be a major effector in perception of limonene. This study provides a basis for the development of limonene as a novel botanical pesticide for the control to storage pests, which will reduce the utilization of chemical pesticides and postpone the development of resistance.

Antennal transcriptome analysis of odorant-binding proteins and characterization of GOBP2 in the variegated cutworm Peridroma saucia

Odorant-binding proteins (OBPs) are expressed at extremely high concentrations in the chemo-sensilla lymph of insects and have long been thought to be crucial for delivering the semiochemicals to the odorant receptors. They are represented by multiple classes: general odorant-binding proteins (GOBP1 and GOBP2) and pheromone-binding proteins. In the current study, we identified a total of 35 OBPs in the antennal transcriptome of Peridroma saucia, a worldwide pest that causes serious damage to various crops. A gene expression value (TPM, transcripts per million) analysis revealed that seven OBPs (PsauPBP1/2/3, PsauGOBP1/2, PsauOBP6, and PsauOBP8) were highly abundant in the antennae. Next, we focused on the expression and functional characterization of PsauGOBP2. Real-time quantitative-PCR analysis demonstrated that PsauGOBP2 was predominantly expressed in the antennae of both sexes. Fluorescence binding assays showed that the recombinant PsauGOBP2 strongly binds to the female sex pheromone components Z11-16: Ac (Ki = 4.2 μM) and Z9-14: Ac (Ki = 4.9 μM) and binds moderately (6 µM ≤ Ki ≤ 13 µM) to the host plant volatiles phenylethyl acetate, β-myrcene, and dodecanol. Further 3D structural modeling and molecular docking revealed that several crucial amino acid residues are involved in ligand binding. The results not only increase our understanding of the olfactory system of P. saucia but also provide insights into the function of PsauGOBP2 that has implications for developing sustainable approaches for P. saucia management.

Influence of pH on indole-dependent heterodimeric interactions between Anopheles gambiae odorant-binding proteins OBP1 and OBP4

Insect Odorant Binding Proteins (OBPs) constitute important components of their olfactory apparatus, as they are essential for odor recognition. OBPs undergo conformational changes upon pH change, altering their interactions with odorants. Moreover, they can form heterodimers with novel binding characteristics. Anopheles gambiae OBP1 and OBP4 were found capable of forming heterodimers possibly involved in the specific perception of the attractant indole. In order to understand how these OBPs interact in the presence of indole and to investigate the likelihood of a pH-dependent heterodimerization mechanism, the crystal structures of OBP4 at pH 4.6 and 8.5 were determined. Structural comparison to each other and with the OBP4-indole complex (3Q8I, pH 6.85) revealed a flexible N-terminus and conformational changes in the α4-loop-α5 region at acidic pH. Fluorescence competition assays showed a weak binding of indole to OBP4 that become further impaired at acidic pH. Additional Molecular Dynamic and Differential Scanning Calorimetry studies displayed that the influence of pH on OBP4 stability is significant compared to the modest effect of indole. Furthermore, OBP1-OBP4 heterodimeric models were generated at pH 4.5, 6.5, and 8.5, and compared concerning their interface energy and cross-correlated motions in the absence and presence of indole. The results indicate that the increase in pH may induce the stabilization of OBP4 by increasing its helicity, thereby enabling indole binding at neutral pH that further stabilizes the protein and possibly promotes the creation of a binding site for OBP1. A decrease in interface stability and loss of correlated motions upon transition to acidic pH may provoke the heterodimeric dissociation allowing indole release. Finally, we propose a potential OBP1-OBP4 heterodimer formation/disruption mechanism induced by pH change and indole binding.

Predicted structure of odorant-binding protein 12 from Monochamus alternatus (Hope) suggests a mechanism of flexible odorant-binding

Odorant-binding proteins (OBPs) are thought to bind and deliver hydrophobic odorants from the environment to receptors on insect sensory neurons, and have been used to screen behaviorally active compounds of insects. In order to screen behaviorally active compounds for Monochamus alternatus by OBPs, we cloned full length of Obp12 coding sequence from M. alternatus and proved secretion property of MaltOBP12, then tested binding affinities of recombinant MaltOBP12 to 12 pine volatiles in vitro. We confirmed MaltOBP12 has binding affinities to 9 pine volatiles. The structure of MaltOBP12 and protein-ligand interactions were further analyzed by homology modeling, molecular docking, site-directed mutagenesis, and ligand-binding assays. These results demonstrated that the binding pocket of MaltOBP12 consists of several large aromatic and hydrophobic residues, and four aromatic residues (Tyr50, Phe109, Tyr112, Phe122) are essential for odorant-binding; ligands adopt extensive hydrophobic interactions with an overlapping subset of residues in the binding pocket. Finally, based on non-directional hydrophobic interactions, MaltOBP12 binds odorants flexibly. These findings will not only help us understand how OBPs flexibly bind odorants but also promote to screen of behaviourally active compounds by computer methods to prevent M. alternatus in the future.

Transcriptomic Responses Underlying the High Virulence of Black Queen Cell Virus and Sacbrood Virus following a Change in Their Mode of Transmission in Honey Bees (Apis mellifera)

BACKGROUND

Over the last two decades, honey bees (Apis mellifera) have suffered high rates of colony losses that have been attributed to a variety of factors, chief among which are viral pathogens, such as deformed wing virus (DWV), whose virulence has increased because of vector-based transmission by the invasive, ectoparasitic varroa mite (Varroa destructor). A shift in the experimental mode of transmission of the black queen cell virus (BQCV) and sacbrood virus (SBV) from fecal/food-oral (direct horizontal) to vector-mediated (indirect horizontal) transmission also results in high virulence and viral titers in pupal and adult honey bees. Agricultural pesticides represent another factor that acts independently or in interaction with pathogens, and they are also thought to cause colony loss. Understanding the molecular mechanisms underlying the higher virulence following a vector-based mode of transmission provides deeper insight into honey bee colony losses, as does determining whether or not host-pathogen interactions are modulated by exposure to pesticides.

METHODS

Through an experimental design with controlled laboratory, we investigated the effects of the modes of transmission of BQCV and SBV (feeding vs. vector-mediated via injection) alone or in combination with chronic exposure to sublethal and field-realistic concentrations of flupyradifurone (FPF), a novel agricultural insecticide, on honey bee survival and transcription responses by using high-throughput RNA sequencing (RNA-seq) analysis.

RESULTS

Co-exposure to viruses via feeding (VF) or injection (VI) and FPF insecticide had no statistically significant interactive effect on their survival compared to, respectively, VF or VI treatments alone. Transcriptomic analysis revealed a distinct difference in the gene expression profiles of bees inoculated with viruses via injection (VI) and exposed to FPF insecticide (VI+FPF). The number of differentially expressed genes (DEGs) at log2 (fold-change) > 2.0 in VI bees (136 genes) or/and VI+FPF insecticide (282 genes) was very high compared to that of VF bees (8 genes) or the VF+FPF insecticide treatment (15 genes). Of these DEGs, the expression in VI and VI+FPF bees of some immune-related genes, such as those for antimicrobial peptides, Ago2, and Dicer, was induced. In short, several genes encoding odorant binding proteins, chemosensory proteins, odor receptors, honey bee venom peptides, and vitellogenin were downregulated in VI and VI+FPF bees.

CONCLUSIONS

Given the importance of these suppressed genes in honey bees' innate immunity, eicosanoid biosynthesis, and olfactory associative function, their inhibition because of the change in the mode of infection with BQCV and SBV to vector-mediated transmission (injection into haemocoel) could explain the high virulence observed in these viruses when they were experimentally injected into hosts. These changes may help explain why other viruses, such as DWV, represent such a threat to colony survival when transmitted by varroa mites.

Candidate odorant-binding protein and chemosensory protein genes in the turnip aphid Lipaphis erysimi

The turnip aphid, Lipaphis erysimi Kaltenbach, inflicts heavy damage on cruciferous crops worldwide. In these insects, olfactory perception is crucial for mating, host location, and oviposition. Both odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are responsible for the delivery of host odorants and pheromones during initial molecular interactions. In this study, antennal and body transcriptomes of L. erysimi were generated through the deep sequencing of RNA libraries. A dataset of 11 LeryOBP and four LeryCSP transcripts was identified among assembled unigenes and subjected to sequence analysis. Phylogenetic analysis found a one-to-one orthologous relationship between LeryOBP/LeryCSP and its corresponding homologs from other aphid species. Further quantitative real-time PCR analyses across developmental stages and tissues showed that five LeryOBP genes (i.e., LeryGOBP, LeryOBP6, LeryOBP7, LeryOBP9, and LeryOBP13) and LeryCSP10 were specifically or significantly elevated in the antennae compared with other tissues. Moreover, two transcripts (i.e., LeryGOBP and LeryOBP6) exhibited remarkably higher expression levels in alate aphids, implying their potentially functional role in the perception of new host plant locations. These results present the identification and expression of OBP/CSP genes in L. erysimi, providing valuable insights into their putative role in olfactory signal transduction.

Transcriptome analysis of anorexic and preferred silkworms (Bombyx mori) on artificial diet

The silkworm, Bombyx mori, is an important oligophagous economic insect and feeding habits of different silkworm varieties to artificial diet are different. Research on the mechanisms of feeding habits on artificial diet, and breeding of silkworm varieties adapted on artificial diet, which is a necessary condition for industrial silkworm rearing, is currently lacking. For an artificial diet, Xin was anorexic, whereas Haoyue A showed a strong appetite. When the two varieties were crossed, the F₁ generation showed a poor appetite for the artificial diet and had a setae dispersion rate of <50 %. However, the F₂ generation, self-bred progeny of F₁, had a good appetite for the artificial diet, with a setae dispersion rate of 70 %. Herein, transcriptome analysis was conducted on the F₂ generation, comparing individuals with anorexic and preferred feeding habits, and 2188 differential genes were identified, with 1524 genes up-regulated and 934 genes down-regulated. Several genes were identified to contribute to feeding habits, such as genes involved olfactory system, energy supply, and cell proliferation and differentiation. GO enrichment revealed a large number of DEGs related to behavior, growth, signaling, developmental process, response to stimulation, and other pathways. Furthermore, proteins closely related to feeding were expressed differently. Some DEGs were selected for qRT-PCR, and results indicated the reliability of the DEG results. The DEGs between individuals with anorexic and preferred feeding habits were screened by RNA-Seq technology, which provides a reliable reference to study molecule mechanisms of feeding habits on artificial diet.

Involvement of Holotrichia parallela odorant-binding protein 3 in the localization of oviposition sites

Organic fertilizers-derived volatiles attract Holotrichia parallela during oviposition. However, the mechanisms underlying the perception of oviposition cues in H. parallela remain unclear. Here, H. parallela odorant-binding protein 3 (HparOBP3) was identified as a key OBP. Bioinformatics analysis showed that HparOBP3 clustered together with Holotrichia oblita OBP8. HparOBP3 was mainly expressed in the antennae of both sexes. Recombinant HparOBP3 exhibited distinct binding affinities towards 22 compounds released by organic fertilizers. After 48 h of RNA interference (RNAi), the expression of HparOBP3 in male and female antennae was decreased by 90.77 % and 82.30 %, respectively. In addition, silencing of HparOBP3 significantly reduced the electrophysiological responses and tropism of males to cis-3-hexen-1-ol, 1-hexanol, and (Z)-β-ocimene as well as females to cis-3-hexen-1-ol, 1-hexanol, benzaldehyde, and (Z)-β-ocimene. Molecular docking indicated that hydrophobic residues Leu-83, Leu-87, Phe-108, and Ile-120 of HparOBP3 were important amino acids for interacting with ligands. Mutation of the key residue, Leu-83, significantly diminished the binding ability of HparOBP3. Furthermore, acrylic plastic arena bioassays showed that the attraction and oviposition indexes of organic fertilizers to H. parallela were reduced by 55.78 % and 60.11 %, respectively, after silencing HparOBP3. These results suggest that HparOBP3 is essential in mediating the oviposition behavior of H. parallela.

Expression, affinity, and binding mode analysis of antennal-binding protein X in the variegated cutworm Peridroma saucia (Hübner)

The variegated cutworm Peridroma saucia (Hübner) is a worldwide pest that causes serious damage to many crops. Odorant-binding proteins (OBPs) are small soluble proteins involved in the first step of odorant reception. In moths, antennal-binding protein Xs (ABPXs) represent a main subfamily of classic OBPs. However, their functions remain unclear. Here, we cloned the ABPX gene from the antennae of P. saucia. RT-qPCR and western-blot analyses showed that PsauABPX is antenna-predominant and male-biased. Further temporal expression investigation indicated that the expression of PsauABPX started 1 day before eclosion and reached the highest 3 days after eclosion. Next, fluorescence binding assays revealed that recombinant PsauABPX had high binding affinities with P. saucia female sex pheromone components Z11-16: Ac and Z9-14: Ac. Then, molecular docking, molecular dynamics simulation, and site-directed mutagenesis were employed to identify key amino acid residues involved in the binding of PsauABPX to Z11-16: Ac and Z9-14: Ac. The results demonstrated that Val-32, Gln-107 and Tyr-114 are essential for the binding to both sex pheromones. This study not only give us insight into the function and binding mechanism of ABPXs in moths, but could also be used to explore novel strategies to control P. saucia.

Current understandings of olfactory molecular events in the Asian corn borer, Ostrinia furnacalis (Lepidoptera: Crambidae)

The Asian corn borer Ostrinia furnacalis (Lepidoptera: Crambidae) is a serious corn pest with widespread distribution in East Asia. Its olfactory mechanism is a focus of scientific study, aiming to find good ways to control this pest. Molecular events are considered to be important in olfactory mechanism. Current understandings of olfactory molecular events in O. furnacalis, mainly involving sex pheromones and olfactory proteins, were summarized to provide a reference for further studies. O. furnacalis sex pheromone contains two components E-12-tetradecenyl acetate and Z-12-tetradecenyl acetate, which may be recognized and bound by the pheromone binding proteins OfurPBP3 and OfurPBP2, and then transported to the odorant receptors (ORs) OfurOR4 and OfurOR6 to activate them. The ORs OfurOR8, OfurOR7 and OfurOR5b mainly respond to the sex pheromone components of other Ostrinia species, E-11-tetradecenyl acetate, Z-11-tetradecenyl acetate and Z-9-tetradecenyl acetate. The OR OfurOR27 responds strongly to plant odorants nonanal, octanal and 1-octanol. Much work remains to be done to fully understand odorants with olfactory activity to O. furnacalis and the functions of its olfactory proteins. These studies will help to reveal olfactory mechanism in O. furnacalis, with the aim of regulating its behaviors to control this pest.

A plant virus enhances odorant-binding protein 5 (OBP5) in the vector whitefly for more actively olfactory orientation to the host plant

BACKGROUND

The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is a notorious agricultural pest and the effective vector of many plant viruses worldwide. Cucurbit chlorotic yellows virus (CCYV), exclusively transmitted by B. tabaci in a semipersistent manner, is a serious causal agent in cucurbit crops in many countries. Plant viruses can manipulate the behaviors of insect vectors to promote the spread of themselves, but underlying mechanisms are remaining unclear.

RESULTS

In this study, our observations indicated that B. tabaci, when carrying CCYV, oriented more actively to the host plant cucumber. Transcriptome analysis and quantitative polymerase chain reaction with reverse transcription analysis showed that the odorant-binding protein 5 (OBP5) was upregulated with viral acquisition. Sequence and phylogenetic analysis showed that BtabOBP5 was highly homologous with nine OBPs from other hemipteran insects. In addition, OBP5-silenced whiteflies significantly altered their orientation behavior towards cucumber plants and towards some typical volatile organic compounds released from cucumbers.

CONCLUSION

This study described a novel mechanism by which the olfactory system of vector insects could be regulated by a semipersistent plant virus, thereby affecting insect olfactory behavior and relationship with host plants. These results provided a basis for developing potential olfaction-based pest management strategies in the future. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Odorant-Binding Proteins and Chemosensory Proteins in Spodoptera frugiperda: From Genome-Wide Identification and Developmental Stage-Related Expression Analysis to the Perception of Host Plant Odors, Sex Pheromones, and Insecticides

Spodoptera frugiperda is a worldwide generalist pest with remarkable adaptations to environments and stresses, including developmental stage-related behavioral and physiological adaptations, such as diverse feeding preferences, mate seeking, and pesticide resistance. Insects’ odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are essential for the chemical recognition during behavioral responses or other physiological processes. The genome-wide identification and the gene expression patterns of all these identified OBPs and CSPs across developmental stage-related S. frugiperda have not been reported. Here, we screened for genome-wide SfruOBPs and SfruCSPs, and analyzed the gene expression patterns of SfruOBPs and SfruCSPs repertoires across all developmental stages and sexes. We found 33 OBPs and 22 CSPs in the S. frugiperda genome. The majority of the SfruOBP genes were most highly expressed in the adult male or female stages, while more SfruCSP genes were highly expressed in the larval or egg stages, indicating their function complementation. The gene expression patterns of SfruOBPs and SfruCSPs revealed strong correlations with their respective phylogenic trees, indicating a correlation between function and evolution. In addition, we analyzed the chemical-competitive binding of a widely expressed protein, SfruOBP31, to host plant odorants, sex pheromones, and insecticides. Further ligands binding assay revealed a broad functional related binding spectrum of SfruOBP31 to host plant odorants, sex pheromones, and insecticides, suggesting its potential function in food, mate seeking, and pesticide resistance. These results provide guidance for future research on the development of behavioral regulators of S. frugiperda or other environmentally friendly pest-control strategies.

Antennal Transcriptome Analysis of Olfactory Genes and Characterization of Odorant Binding Proteins in Odontothrips loti (Thysanoptera: Thripidae)

To identify odors in complex environments accurately, insects have evolved multiple olfactory proteins. In our study, various olfactory proteins of Odontothrips loti Haliday, an oligophagous pest that primarily affects Medicago sativa (alfalfa), were explored. Specifically, 47 putative olfactory candidate genes were identified in the antennae transcriptome of O. loti, including seven odorant-binding proteins (OBPs), nine chemosensory proteins (CSPs), seven sensory neuron membrane proteins (SNMPs), eight odorant receptors (ORs), and sixteen ionotropic receptors (IRs). PCR analysis further confirmed that 43 out of 47 genes existed in O. loti adults, and O.lotOBP1, O.lotOBP4, and O.lotOBP6 were specifically expressed in the antennae with a male-biased expression pattern. In addition, both the fluorescence competitive binding assay and molecular docking showed that p-Menth-8-en-2-one, a component of the volatiles of the host, had strong binding ability to the O.lotOBP6 protein. Behavioral experiments showed that this component has a significant attraction to both female and male adults, indicating that O.lotOBP6 plays a role in host location. Furthermore, molecular docking reveals potential active sites in O.lotOBP6 that interact with most of the tested volatiles. Our results provide insights into the mechanism of O. loti odor-evoked behavior and the development of a highly specific and sustainable approach for thrip management.

Identification and sex-specific expression of chemosensory genes in the antennal transcriptomes of Pachyrhinus yasumatsui (Coleoptera: Curculionidae)

Pachyrhinus yasumatsui Kono et Morimoto is a major pest of Chinese jujube, which is widespread in northern China and causes severe economic losses in the jujube industry. Chemosensory genes play crucial roles in insect behaviors. Currently, little is known about chemosensory genes in P. yasumatsui. In the present study, antennal transcriptomes of female and male adult P. yasumatsui were annotated. In total, 113 genes involved in chemosensory functions were identified, including 41 odorant receptors, 28 odorant-binding proteins, 16 ionotropic receptors, 15 chemosensory proteins, 9 gustatory receptors, and 4 sensory neuron membrane proteins. Subsequently, the phylogenetic analyses of these olfactory-related proteins in P. yasumatsui were conducted using multiple sequence alignment. Furthermore, sex-specific expression levels of 113 genes were analyzed based on fragments per kilobase of transcript per million mapped reads (FPKM). Then, the quantitative real-time PCR (RT-qPCR) was used to quantify gene expression profiles of 28 P. yasumatsui OBPs (PyasOBPs) and 15 CSPs (PyasCSPs). The results revealed that 20 PyasOBPs and 13 PyasCSPs exhibited significantly higher expression in the antennae than in the bodies, suggesting that they might have functions in olfaction. Moreover, some OBPs and CSPs (PyasOBP6, PyasOBP7, PyasOBP16, PyasOBP21, and PyasCSP4) exhibited female-biased expression, indicating that they might take part in several female-specific behaviors. This study will promote the understanding of olfactory mechanism in P. yasumatsui, and our findings lay the groundwork for developing environmentally friendly pest management measures.

Identification and Functional Characterization of General Odorant Binding Proteins in Orthaga achatina

The olfactory system in insects are crucial for recognition of host plants and oviposition sites. General odorant binding proteins (GOBPs) are thought to be involved in detecting odorants released by host plants. Orthaga achatina (Lepidoptera: Pyralidae) is one of the most serious pests of camphor trees, Cinnamomum camphora (L.) Presl, an important urban tree species in southern China. In this study, we study the GOBPs of O. achatina. Firstly, two full-length GOBP genes (OachGOBP1 and OachGOBP2) were successfully cloned according to transcriptome sequencing results, and real-time quantitative PCR measurements showed that both GOBP genes were specifically expressed in the antennae of both sexes, proposing their important roles in olfaction. Then, both GOBP genes were heterologous expressed in Escherichia coli and fluorescence competitive binding assays were conducted. The results showed that OachGOBP1 could bind Farnesol (K_i = 9.49 μM) and Z11-16: OH (K_i = 1.57 μM). OachGOBP2 has a high binding affinity with two camphor plant volatiles (Farnesol, K_i = 7.33 μM; α-Phellandrene, K_i = 8.71 μM) and two sex pheromone components (Z11-16: OAc, K_i = 2.84 μM; Z11-16: OH, K_i = 3.30 μM). These results indicate that OachGOBP1 and OachGOBP2 differ in terms of odorants and other ligands. Furthermore, key amino acid residues that bind to plant volatiles were identified in GOBPs using 3-D structure modeling and ligand molecular docking, predicting the interactions between the GOBPs and the host plant volatiles.

A Highly Expressed Antennae Odorant-Binding Protein Involved in Recognition of Herbivore-Induced Plant Volatiles in Dastarcus helophoroides

Natural enemies such as parasitoids and parasites depend on sensitive olfactory to search for their specific hosts. Herbivore-induced plant volatiles (HIPVs) are vital components in providing host information for many natural enemies of herbivores. However, the olfactory-related proteins involved in the recognition of HIPVs are rarely reported. In this study, we established an exhaustive tissue and developmental expression profile of odorant-binding proteins (OBPs) from Dastarcus helophoroides, an essential natural enemy in the forestry ecosystem. Twenty DhelOBPs displayed various expression patterns in different organs and adult physiological states, suggesting a potential involvement in olfactory perception. In silico AlphaFold2-based modeling and molecular docking showed similar binding energies between six DhelOBPs (DhelOBP4, 5, 6, 14, 18, and 20) and HIPVs from Pinus massoniana. While in vitro fluorescence competitive binding assays showed only recombinant DhelOBP4, the most highly expressed in the antennae of emerging adults could bind to HIPVs with high binding affinities. RNAi-mediated behavioral assays indicated that DhelOBP4 was an essential functional protein for D. helophoroides adults recognizing two behaviorally attractive substances: p-cymene and γ-terpinene. Further binding conformation analyses revealed that Phe 54, Val 56, and Phe 71 might be the key binding sites for DhelOBP4 interacting with HIPVs. In conclusion, our results provide an essential molecular basis for the olfactory perception of D. helophoroides and reliable evidence for recognizing the HIPVs of natural enemies from insect OBPs' perspective.

Artificial Olfactory Biohybrid System: An Evolving Sense of Smell

The olfactory system can detect and recognize tens of thousands of volatile organic compounds (VOCs) at low concentrations in complex environments. Bioelectronic nose (B-EN), which mimics olfactory systems, is becoming an emerging sensing technology for identifying VOCs with sensitivity and specificity. B-ENs integrate electronic sensors with bioreceptors and pattern recognition technologies to enable medical diagnosis, public security, environmental monitoring, and food safety. However, there is currently no commercially available B-EN on the market. Apart from the high selectivity and sensitivity necessary for volatile organic compound analysis, commercial B-ENs must overcome issues impacting sensor operation and other problems associated with odor localization. The emergence of nanotechnology has provided a novel research concept for addressing these problems. In this work, the structure and operational mechanisms of biomimetic olfactory systems are discussed, with an emphasis on the development and immobilization of materials. Various biosensor applications and current developments are reviewed. Challenges and opportunities for fulfilling the potential of artificial olfactory biohybrid systems in fundamental and practical research are investigated in greater depth.

Functional disparity of four pheromone-binding proteins from the plum fruit moth Grapholita funebrana Treitscheke in detection of sex pheromone components

Grapholita funebrana, also known as the plum fruit moth, is an oligophagous pest species that causes enormous economic losses of the fruits of Rosaceae. An eco-friendly method for the control of G. funebrana besides chemical control has not yet been developed. The sex pheromone communication system plays an important role in moth courtship and mating, in which pheromone-binding proteins (PBPs) are critical. In this research, we identified four PBPs, namely, GfunPBP1.1, GfunPBP1.2, GfunPBP2, and GfunPBP3, from the antennae of G. funebrana. The results of real-time quantitative PCR (RT-qPCR) showed that all four GfunPBPs were overwhelmingly expressed in the antennae and that GfunPBP1.2 and GfunPBP2 showed male-biased expression patterns, whereas GfunPBP1.1 and GfunPBP3 were equally expressed between sexes. The results of ligand-binding assays illustrated that although all four recombinant GfunPBPs (rGfunPBPs) had binding activity with the tested sex pheromone compounds, their preferred ligands were significantly different. rGfunPBP2 had the strongest binding affinity to Z8-12:Ac and Z8-12:OH; rGfunPBP1.1 preferred to bind Z8-14:Ac, Z10-14:Ac, and 12:OH more than to the other three GfunPBPs; and rGfunPBP1.2 exhibited stronger binding affinity to E8-12:Ac than to the other rGfunPBPs. Molecular docking results demonstrated that hydrophobic forces, especially van der Waals forces and hydrogen bonds, were the most important forces that maintained GfunPBP-pheromone ligand complexes. This study will improve our understanding of the sex pheromone recognition mechanisms of G. funebrana and promote the development of novel strategies for controlling G. funebrana.

Identification and expression profile analysis of chemosensory genes in pine needle gall midge, Thecodiplosis japonensis (Diptera: Cecidomyiidae)

Insects have highly specialized and sensitive olfactory systems involving several chemosensory genes to locate their mates and hosts or escape from predators. Pine needle gall midge, Thecodiplosis japonensis (Diptera: Cecidomyiidae), has invaded China since 2016 and caused serious damage. Till now, there is no environmentally friendly measure to control this gall midge. Screening molecules with high affinity to target odorant-binding protein to develop highly efficient attractants is a potential pest management method. However, the chemosensory genes in T. japonensis are still unclear. We identified 67 chemosensory-related genes in the transcriptomes of antennae, including 26 OBPs, 2 CSPs, 17 ORs, 3 SNMPs, 6 GRs, and 13 IRs, using high throughput sequencing. Phylogenetic analysis of these six chemosensory gene families among Dipteran was performed to classify and predict the functions. The expression profiles of OBPs, CSPs and ORs were validated by quantitative real-time PCR. 16 of the 26 OBPs were biased expressed in antennae. TjapORco and TjapOR5 were highly expressed in the antenna of unmated male and female adults. The functions of related OBPs and ORs genes were also discussed. These results provide a basis for the functional research on chemosensory genes at the molecular level.

Perception and kairomonal response of the coccinellid predator (Harmonia axyridis) to the fall armyworm (Spodoptera frugiperda) sex pheromone

Pheromone cues released from hosts or prey are of crucial importance to natural enemies for prey and habitat location. The use of herbivorous insect sex pheromones has long been considered as a potential pest control alternative that is non-toxic and harmless to beneficials. We hypothesized that Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), a major predatory coccinellid beetle of the devastating migratory pest Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae), could perceive and use the sex pheromone of S. frugiperda to locate its habitat. Here we tested the electrophysiological and behavioral responses of H. axyridis to the two components Z7-12:Ac and Z9-14:Ac of S. frugiperda sex pheromone by using electroantennography (EAG) and Y-tube bioassay. The 3D modeling of H. axyridis odorant-binding proteins (HaxyOBPs) and molecular docking were also performed. The results showed that both female and male H. axyridis exhibited significantly higher electrophysiological and behavioral responses to Z9-14:Ac at the concentrations of 0.001, 0.01, and 0.1 μg/μL, while no significant electrophysiological and behavioral responses of H. axyridis were observed to Z7-12:Ac. The blend of Z7-12:Ac and Z9-14:Ac at the ratio of 1:100 had a significant attraction to both male and female H. axyridis at the concentrations of 0.01 and 0.1 μg/μL based on electrophysiological and behavioral assays, but no significant behavioral responses were observed at the ratios of 1:9. According to the 3D modeling of HaxyOBPs and molecular docking, HaxyOBP12 has a good affinity with Z9-14:Ac. Z9-14:Ac is bound to the HaxyOBP12 by hydrogen bonding and hydrophobic interactions. However, there were no credible docking results between HaxyOBPs and Z7-12:Ac. Our findings revealed that H. axyridis can perceive Z9-14:Ac and could use it as a chemical cue to locate prey habitat. We speculated that Z7-12:Ac, which showed some antagonistic effect toward the response of H. axyridis to Z9-14:Ac, could improve the adaptability of S. frugiperda in the presence of predators. This study provides new insights into the application of pheromones to manipulate natural enemy behavior for pest control.

Odorant-binding Protein 10 From Bradysia odoriphaga (Diptera: Sciaridae) Binds Volatile Host Plant Compounds

Bradysia odoriphaga (Diptera: Sciaridae) is a major insect pest of seven plant families including 30 commercial crops in Asia. The long-term use of chemical pesticides leads to problems such as insect resistance, environmental issues, and food contamination. Against this background, a novel pest control method should be developed. In insects, odorant-binding proteins (OBPs) transport odor molecules, including pheromones and plant volatiles, to olfactory receptors. Here, we expressed and characterized the recombinant B. odoriphaga OBP BodoOBP10, observing that it could bind the sulfur-containing compounds diallyl disulfide and methyl allyl disulfide with Ki values of 8.01 μM and 7.00 μM, respectively. Homology modeling showed that the BodoOBP10 3D structure was similar to that of a typical OBP. Both diallyl disulfide and methyl allyl disulfide bound to the same site on BodoOBP10, mediated by interactions with six hydrophobic residues Met70, Ile75, Thr89, Met90, Leu93, and Leu94, and one aromatic residue, Phe143. Furthermore, silencing BodoOBP10 expression via RNAi significantly reduced the electroantennogram (EAG) response to diallyl disulfide and methyl allyl disulfide. These findings suggest that BodoOBP10 should be involved in the recognition and localization of host plants.

Plant volatile ligands for male-biased MmedOBP14 stimulate orientation behavior of the parasitoid wasp Microplitis mediator

As an important class of chemosensory-associated proteins, odorant binding proteins (OBPs) play a key role in the perception of olfactory signals for insects. Parasitoid wasp Microplitis mediator relies on its sensitive olfactory system to locate host larvae of Noctuidae and Geometridae. In the present study, MmedOBP14, a male-biased OBP in M. mediator, was functionally investigated. In fluorescence competitive binding assays, the recombinant MmedOBP14 showed strong binding abilities to five plant volatiles: β-ionone, 3,4-dimethylacetophenone, 4-ethylacetophenone, acetophenone and ocimene. Homology modeling and molecular docking results indicated that the binding sites of all five ligands were similar and concentrated in the binding pocket of MmedOBP14. Except acetophenone, the remaining four ligands at 1, 10 and 100 μg/μL caused strong antennal electrophysiological responses in adults M. mediator, and males showed more obvious EAG responses to most ligands than females. In behavioral trials, males were attracted by low concentrations of MmedOBP14 ligands, whereas high doses of β-ionone and acetophenone had a repellent effect on males. Moreover, 1 μg/μL of 3,4-dimethylacetophenone showed the strongest attractiveness to female wasps. These findings suggest that MmedOBP14 may play a more important role in the perception of plant volatiles for male wasps to locate habitat, supplement nutrition and search partners.

Proteomic Characterization of Drosophila melanogaster Proboscis

Drosophila melanogaster flies use their proboscis to taste and distinguish edible compounds from toxic compounds. With their proboscis, flies can detect sex pheromones at a close distance or by contact. Most of the known proteins associated with probosci's detection belong to gustatory receptor families. To extend our knowledge of the proboscis-taste proteins involved in chemo-detection, we used a proteomic approach to identify soluble proteins from Drosophila females and males. This investigation, performed with hundreds of dissected proboscises, was initiated by the chromatographic separation of tryptic peptides, followed by tandem mass spectrometry, allowing for femtomole detection sensitivity. We found 586 proteins, including enzymes, that are involved in intermediary metabolism and proteins dedicated to various functions, such as nucleic acid metabolism, ion transport, immunity, digestion, and organ development. Among 60 proteins potentially involved in chemosensory detection, we identified two odorant-binding proteins (OBPs), i.e., OBP56d (which showed much higher expression in females than in males) and OBP19d. Because OBP56d was also reported to be more highly expressed in the antennae of females, this protein can be involved in the detection of both volatile and contact male pheromone(s). Our proteomic study paves the way to better understand the complex role of Drosophila proboscis in the chemical detection of food and pheromonal compounds.

Multi-omics analysis the differences of VOCs terpenoid synthesis pathway in maintaining obligate mutualism between Ficus hirta Vahl and its pollinators

INRODUCTION

Volatile organic compounds (VOCs) emitted by the receptive syconia of Ficus species is a key trait to attract their obligate pollinating fig wasps. Ficus hirta Vahl is a dioecious shrub, which is pollinated by a highly specialized symbiotic pollinator in southern China. Terpenoids are the main components of VOCs in F. hirta and play ecological roles in pollinator attraction, allelopathy, and plant defense. However, it remains unclear that what molecular mechanism difference in terpenoid synthesis pathways between pre-receptive stage (A-phase) and receptive stage (B-phase) of F. hirta syconia.

METHODS

Transcriptome, proteome and Gas Chromatography-Mass Spectrometer (GC-MS) were applied here to analyze these difference.

RESULTS AND DISCUSSION

Compared to A-phase syconia, the genes (ACAT2, HMGR3, GGPS2, HDR, GPS2, TPS2, TPS4, TPS10-4, TPS14) related to the terpenoid synthesis pathway had higher expression level in receptive syconia (B-phase) according to transcriptome sequencing. Seven differentially expressed transcription factors were screened, namely bHLH7, MYB1R1, PRE6, AIL1, RF2b, ANT, VRN1. Specifically, bHLH7 was only specifically expressed in B-phase. 235 differentially expressed proteins (DEPs) were mainly located in the cytoplasm and chloroplasts. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the DEPs were mainly enriched in the metabolic process. A total of 9 terpenoid synthesis proteins were identified in the proteome. Among them, 4 proteins in methylerythritol phosphate (MEP) pathway were all down-regulated. Results suggested the synthesis of terpenoids precursors in B-phase bracts were mainly accomplished through the mevalonic acid (MVA) pathway in cytoplasm. Correlation analysis between the transcriptome and proteome, we detected a total of 1082 transcripts/proteins, three of which are related to stress. From the VOCs analysis, the average percent of monoterpenoids emitted by A-phase and B-phase syconia were 8.29% and 37.08%, while those of sesquiterpenes were 88.43% and 55.02% respectively. Monoterpenes (camphene, myrcene, camphor, menthol) were only detected in VOCs of B-phase syconia. To attract pollinators, B-phase syconia of F. hirta need more monoterpenoids and less sesquiterpenes. We speculate that transcription factor bHLH7 may regulate the terpenoid synthesis pathway between A- and B-phase syconia. Our research provided the first global analysis of mechanism differences of terpenoid synthesis pathways between A and B phases in F. hirta syconia.

Comparison and Functional Analysis of Odorant-Binding Proteins and Chemosensory Proteins in Two Closely Related Thrips Species, Frankliniella occidentalis and Frankliniella intonsa (Thysanoptera: Thripidae) Based on Antennal Transcriptome Analysis

Two closely related thrips species, Frankliniella occidentalis and Frankliniella intonsa, are important pests on agricultural and horticultural crops. They have several similarities, including occurrence patterns, host range, and aggregation pheromone compounds. However, there are very few reports about the chemosensory genes and olfactory mechanisms in these two species. To expand our knowledge of the thrips chemosensory system, we conducted antennal transcriptome analysis of two thrips species, and identified seven odorant-binding proteins (OBPs) and eight chemosensory proteins (CSPs) in F. occidentalis, as well as six OBPs and six CSPs in F. intonsa. OBPs and CSPs showed high sequence identity between the two thrips species. The RT-qPCR results showed that the orthologous genes FoccOBP1/3/4/5/6, FintOBP1/3/4/6, FoccCSP1/2/3, and FintCSP1/2 were highly expressed in male adults. Molecular docking results suggested that orthologous pairs FoccOBP4/FintOBP4, FoccOBP6/FintOBP6, and FoccCSP2/FintCSP2 might be involved in transporting the major aggregation pheromone compound neryl (S)-2-methylbutanoate, while orthologous pairs FoccOBP6/FintOBP6, FoccCSP2/FintCSP2, and FoccCSP3/FintCSP3 might be involved in transporting the minor aggregation pheromone compound (R)-lavandulyl acetate. These results will provide a fundamental basis for understanding the molecular mechanisms of pheromone reception in the two thrips species.

Identification and Expression Analysis of Odorant-Binding and Chemosensory Protein Genes in Virus Vector Nephotettix cincticeps

The insect odorant binding proteins (OBPs) and chemosensory proteins (CSPs) are involved in the perception and discrimination of insects to host odor cues. Nephotettix cincticeps, one of the destructive pests of rice plants, not only directly damages hosts by sucking, but also indirectly transmits plant viruses in the field. Previous study found that two rice volatiles ((E)-β-caryophyllene and 2-heptanol) induced by rice dwarf virus (RDV) mediated the olfactory behavior of N. cincticeps, which may promote virus dispersal. However, the OBPs and CSPs in N. cincticeps are still unknown. In this study, to identify the OBP and CSP genes in N. cincticeps, transcriptomic analyses were performed. In total, 46,623 unigenes were obtained. Twenty putative OBP and 13 CSP genes were discovered and identified. Phylogenetic analyses revealed that five putative OBPs belonged to the plus-C OBP family, and the other classic OBPs and CSPs were distributed among other orthologous groups. A total of 12 OBP and 10 CSP genes were detected, and nine OBP and three CSP genes were highly expressed in N. cincticeps antennae compared with other tissues. This study, for the first time, provides a valuable resource to well understand the molecular mechanism of N. cincticeps in the perception and discrimination of the two volatiles induced by RDV infection.

Recent Insights into Insect Olfactory Receptors and Odorant-Binding Proteins

Human and insect olfaction share many general features, but insects differ from mammalian systems in important ways. Mammalian olfactory neurons share the same overlying fluid layer in the nose, and neuronal tuning entirely depends upon receptor specificity. In insects, the olfactory neurons are anatomically segregated into sensilla, and small clusters of olfactory neurons dendrites share extracellular fluid that can be independently regulated in different sensilla. Small extracellular proteins called odorant-binding proteins are differentially secreted into this sensillum lymph fluid where they have been shown to confer sensitivity to specific odorants, and they can also affect the kinetics of the olfactory neuron responses. Insect olfactory receptors are not G-protein-coupled receptors, such as vertebrate olfactory receptors, but are ligand-gated ion channels opened by direct interactions with odorant molecules. Recently, several examples of insect olfactory neurons expressing multiple receptors have been identified, indicating that the mechanisms for neuronal tuning may be broader in insects than mammals. Finally, recent advances in genome editing are finding applications in many species, including agricultural pests and human disease vectors.

Gut bacteria induce oviposition preference through ovipositor recognition in fruit fly

Gut bacteria play important roles in insect life cycle, and various routes can be used by insects to effectively transmit their gut bacteria. However, it is unclear if the gut bacteria can spread by actively attracting their insect hosts, and the recognition mechanisms of host insects are poorly understood. Here, we explore chemical interactions between Bactrocera dorsalis and its gut bacterium Citrobacter sp. (CF-BD). We found that CF-BD could affect the development of host ovaries and could be vertically transmitted via host oviposition. CF-BD could attract B. dorsalis to lay eggs by producing 3-hexenyl acetate (3-HA) in fruits that were hosts of B. dorsalis. Furthermore, we found that B. dorsalis could directly recognize CF-BD in fruits with their ovipositors in which olfactory genes were expressed to bind 3-HA. This work reports an important mechanism concerning the active spread of gut bacteria in their host insects.

Discovery of Novel Potential Aphid Repellents: Geranic Acid Esters Containing Substituted Aromatic Rings

Aphids are one of the most damaging agricultural pests. For the sake of novel eco-friendly compounds with good activity for aphid control, a series of novel geranic acid esters containing substituted aromatic rings were designed by inverting ester groups of lead compounds. All compounds were characterized by HRMS, ¹H-NMR, and ¹³C-NMR. In order to identify the effect of inversion ester groups on activity, a bioassay was conducted. The results showed that the repellent activity against Acyrthosiphon pisum (A. pisum) and the binding affinity with the odorant-binding protein 9 from A. pisum (ApisOBP9) of the compounds were increased after inversion of the ester groups. Particularly, 5f showed the best repellent activity (repellency proportion: 55.6%) and binding affinity (1/Ki: 0.49 µM). Meanwhile, the structure–activity relationships revealed that the introduction of meta-substitution of the benzene ring and halogen atoms, such as Cl and Br, facilitated the biological activity. The further molecular docking results demonstrated that hydrogen bonding interactions and hydrophobic interactions were vital for the binding affinity with ApisOBP9. Additionally, all compounds were predicted to be eco-friendly and their volatile physicochemical properties have been enhanced compared to the leads. The present results provide valuable clues for the further rational design of aphids’ behavioral control agents.