Sensilla localization and sex pheromone recognition of odorant binding protein OBP4 in the mirid plant bug Adelphocoris lineolatus (Goeze)

Identification of chemosensory genes and antennal sensilla in Nassophasis sp. (Coleoptera: Rhynchophorinae)

Phytophagous insects rely on plant volatiles to select and locate hosts for feeding or reproduction and their olfactory system is essential for detecting plant volatiles. The stem-boring pest, Nassophasis sp. damages Dendrobium and causes economic losses. Currently, there are no effective methods for its control. However, understanding the morphological and molecular basis of its olfactory system may identify new pathways for their management and control. In this study, we observed the stemborer's antennal sensilla using scanning electron microscopy, and transcriptome sequencing was undertaken to annotate and analyze its chemosensory genes. Results showed that the antennal morphology is similar between males and females, with five types of antennal sensilla observed: sensilla chaetica (SC), sensilla trichodea (ST), sensilla brush (SB), sensilla basiconica (SBA) and sensilla gemmiformium (SG). Sexual dimorphism was not observed in sensilla type, but in the length of SBA and SG. A total of 70 olfactory-related genes were annotated, including 16 odorant binding proteins (OBP), 5 chemosensory proteins (CSPs), 26 olfactory receptors (ORs), 9 gustatory receptors (GRs), 10 ionotropic receptors (IRs), and 4 sensory neuron membrane proteins (SNMPs). Most genes were highly expressed and 14 of these genes were only expressed in the head, and 7 genes in the abdomen. This study provides a theoretical basis for the olfactory perception of Nassophasis sp. and a scientific basis for developing new pest control strategies.

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.

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.

Predation evaluation of the green lacewing, Chrysopa pallens on the pink tea mite pest, Acaphylla theae (Watt) (Acarina: Eriophyidae)

A better understanding of predator-prey interactions is crucial for the development of biological control strategies. The green lacewing, Chrysopa pallens, is a well-known generalist predator and reportedly functions as one of the most important biological control agents of insect pests. However, information regarding C. pallens' predation on tea plant pests, particularly notorious tea mites, remains largely unknown. In this study, we focused on the predator-prey relationship between C. pallens and an important tea mite pest, Acaphylla theae. We designed species-specific primers for the detection of A. theae DNA and established a PCR-based DNA gut content analysis assay. These results demonstrated that the primers were A. theae-specific and suitable for its molecular identification. The laboratory feeding experiment showed that the detectability success (DS50) of A. theae DNA remaining in C. pallens' guts was 2.9 h. We then performed a molecular detection of field predation, and achieved a 23.53% positive detection rate of A. theae DNA in the guts of field-collected C. pallens. This, for the first time, provides direct evidence that C. pallens can prey on A. theae in tea plantations. Finally, we tested the prey preference and estimated the predation ability of C. pallens on different developmental stages of A. theae. The results revealed that C. pallens had no significant preference for different developmental stages of A. theae. The functional responses of C. pallens' predation on different densities of A. theae at different developmental stages followed a Type II Holling model. The initial attack rate (a') ranged from 0.735 to 0.858 and the handling time (Th) was approximately 0.01. This study is the first to demonstrate the trophic interactions between C. pallens and A. theae and provides evidence for the development of biological control strategies against A. theae using C. pallens as a candidate predator.

Molecular and Functional Characterization of Pheromone Binding Protein 2 from Cyrtotrachelus buqueti (Coleoptera: Curculionidae)

Pheromone-binding proteins (PBPs) play important roles in binding and transporting sex pheromones. However, the PBP genes identified in coleopteran insects and their information sensing mechanism are largely unknown. Cyrtotrachelus buqueti (Coleoptera: Curculionidae) is a major insect pest of bamboo plantations. In this study, a novel PBP gene, CbuqPBP2, from C. buqueti was functionally characterized. CbuqPBP2 was more abundantly expressed in the antennae of both sexes than other body parts, and its expression level was significantly male-biased. Fluorescence competitive binding assays showed that CbuqPBP2 exhibited the strongest binding affinity to dibutyl phthalate (Ki = 6.32 μM), followed by styrene (Ki = 11.37 μM), among twelve C. buqueti volatiles. CbuqPBP2, on the other hand, showed high binding affinity to linalool (Ki = 10.55), the main volatile of host plant Neosinocalamus affinis. Furthermore, molecular docking also demonstrated the strong binding ability of CbuqPBP2 to dibutyl phthalate, styrene, and linalool, with binding energy values of -5.7, -6.6, and -6.0 kcal/mol, respectively, and hydrophobic interactions were the prevailing forces. The knockdown of CbuqPBP2 expression via RNA interference significantly reduced the electroantennography (EAG) responses of male adults to dibutyl phthalate and styrene. In conclusion, these results will be conducive to understanding the olfactory mechanisms of C. buqueti and promoting the development of novel strategies for controlling this insect pest.

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.

Foreleg Transcriptomic Analysis of the Chemosensory Gene Families in Plagiodera versicolora (Coleoptera: Chrysomelidae)

Plagiodera versicolora (Coleoptera: Chrysomelidae) is a worldwide leaf-eating forest pest in salicaceous trees. The forelegs play important roles in the chemoreception of insects. In this study, we conducted a transcriptome analysis of adult forelegs in P. versicolora and identified a total of 53 candidate chemosensory genes encoding 4 chemosensory proteins (CSPs), 19 odorant binding proteins (OBPs), 10 odorant receptors (ORs), 10 gustatory receptors (GRs), 6 ionotropic receptors (IRs), and 4 sensory neuron membrane proteins (SNMPs). Compared with the previous antennae transcriptome data, 1 CSP, 4 OBPs, 1 OR, 3 IRs, and 4 GRs were newly identified in the forelegs. Subsequently, the tissue expression profiles of 10 P. versicolora chemosensory genes were performed by real-time quantitative PCR. The results showed that PverOBP25, PverOBP27, and PverCSP6 were highly expressed in the antennae of both sexes. PverCSP11 and PverIR9 are predominately expressed in the forelegs than in the antennae. In addition, the expression levels of PverGR15 in female antennae and forelegs were significantly higher than those in the male antennae, implying that it may be involved in some female-specific behaviors such as oviposition site seeking. This work would greatly further the understanding of the chemoreception mechanism in P. versicolora.

Identification and Expression Profiles of Putative Soluble Chemoreception Proteins from Lasioderma serricorne (Coleoptera: Anobiidae) Antennal Transcriptome

The cigarette beetle, Lasioderma serricorne (Fabricius) (Coleoptera: Anobiidae), is a destructive stored product pest worldwide. Adult cigarette beetles are known to rely on host volatiles and pheromones to locate suitable habitats for oviposition and mating, respectively. However, little is known about the chemosensory mechanisms of these pests. Soluble chemoreception proteins are believed to initiate olfactory signal transduction in insects, which play important roles in host searching and mating behaviors. In this study, we sequenced the antennal transcriptome of L. serricorne and identified 14 odorant-binding proteins (OBPs), 5 chemosensory proteins (CSPs), and 2 Niemann-Pick C2 proteins (NPC2). Quantitative realtime PCR (qPCR) results revealed that several genes (LserOBP2, 3, 6, and 14) were predominantly expressed in females, which might be involved in specific functions in this gender. The five LserOBPs (LserOBP1, 4, 8, 10, and 12) that were highly expressed in the male antennae might encode proteins involved in specific functions in males. These findings will contribute to a better understanding of the olfactory system in this stored product pest and will assist in the development of efficient and environmentally friendly strategies for controlling L. serricorne.

The Adelphocoris lineolatus OBP4: Support for evolutionary and functional divergence of a mirid pheromone-binding protein from that found in lepidopteran moths

Pheromone-binding proteins (PBPs) have been extensively investigated in lepidopteran moths, but their evolution and function in hemipteran species remain unclear. Our previous study demonstrated that an odorant-binding protein, OBP4, of the mirid bug Adelphocoris lineolatus functions as a candidate hemipteran PBP but clustered with lepidopteran antennae-binding proteins (ABPs) rather than in the PBP/general odorant-binding protein (GOBP) clade. In this study, we hypothesized that origin and function of PBPs in hemipteran bugs may differ from those of lepidopteran moths. To test this hypothesis, we first constructed a phylogenetic tree using insect OBPs from sister hemipteran and holometabolous lineages, and the results indicated that neither OBP4 nor other types of candidate PBPs of mirid bugs clustered with the lepidopteran PBP/GOBP clade. Then, a fluorescence competitive binding assay was employed to determine binding affinities of recombinant OBP4 protein to host plant volatiles, with functional groups different from A. lineolatus sex pheromone components. The results revealed that OBP4 highly bound the female adult attractant 3-hexanone and 15 other mirid bug biologically active plant volatiles. Finally, we examined cellular expression profiles of OBP4 in putative antennal sensilla that are related to female A. lineolatus host plant location. The fluorescence in situ hybridization and immunocytochemical labeling assay showed that the OBP4 gene was highly expressed in the multiporous olfactory sensilla medium-long sensilla basiconica rather than in the short sensilla basiconica or uniporous sensilla chaetica. These results, together with those of our previous studies, indicate that OBP4 not only functions in recognition of bug-produced sex pheromones in males, but is probably involved in detection of host plant volatiles in both A. lineolatus sexes. Our findings support the hypothesis that the origin and function of PBPs in hemipteran bugs differ from those of well-known PBPs in lepidopteran moths, which provides a novel perspective on evolutionary mechanisms of sex pheromone communication across insect orders.

Ligand-binding properties of odorant-binding protein 6 in Athetis lepigone to sex pheromones and maize volatiles

BACKGROUND

Athetis lepigone, a noctuid moth feeding on more than 30 different crops worldwide, has evolved a sophisticated, sensitive, and specific chemosensory system to detect and discriminate exogenous chemicals. Odorant-binding proteins (OBPs) are the most important agent in insect chemosensory systems to be explored as an alternative target for environmentally friendly approaches to pest management.

RESULTS

To investigate the olfactory function of A. lepigone OBPs (AlepOBPs), AlepOBP6 was identified and expressed in Escherichia coli. The binding affinity of the recombinant OBP to 20 different ligands was then examined using a competitive binding approach. The results revealed that AlepOBP6 can bind to two sex pheromones and ten maize volatiles, and its conformation stability is pH dependent. We also carried out a structure-function study using different molecular approaches, including structure modeling, molecular docking, and a mutation functional assay to identify amino acid residues (M39, V68, W106, Q107, and Y114) involved in the binding of AlepOBP6 to both sex pheromones and maize volatiles in A. lepigone.

CONCLUSION

These results suggest that AlepOBP6 is likely involved in mediating the responses of A. lepigone to sex pheromones and maize volatiles, which may play a pivotal function in mating, feeding, and oviposition behaviors. This study not only provides new insight into the binding mechanism of OBPs to sex pheromones and host volatiles in moths, but also contributes to the discovery of novel target candidates for developing efficient behavior disruptors to control A. lepigone in the future. © 2021 Society of Chemical Industry.

Functional Characterization of a Candidate Sex Pheromone Receptor AlinOR33 Involved in the Chemoreception of Adelphocoris lineolatus

Sex pheromones are deemed to play a significant role in sexual communication of most insects. Although many sex pheromone components in mirid bugs have been identified, the roles of odorant receptors in sex pheromone perception in Adelphocoris spp. (Hemiptera: Miridae) remain unknown so far. Here, AlinOR33, a candidate sex pheromone receptor in Adelphocoris lineolatus was functionally characterized. Phylogenetic analysis showed that AlinOR33 clustered with the sex pheromone receptor AlucOR4 fromApolygus lucorum. Quantitative real-time PCR measurement revealed that the expression of AlinOR33 increased gradually from nymph to adult stage and reached its peak in the antennae of 3-day-old mated male bugs. The subsequent in situ hybridization demonstrated that AlinOR33 was mainly expressed in sensilla trichoid on the antennae of A. lineolatus. In the two-electrode voltage clamp recordings, AlinOR33/AlinOrco was specifically tuned to four sex pheromone components including butyl butyrate, hexyl hexanoate, trans-2-hexenyl butyrate and hexyl butyrate, and especially most sensitive to the major component trans-2-hexenyl butyrate. After dsAlinOR33 injection, the electroantennogram responses of males to four sex pheromone components were reduced significantly (∼50%). Compared to control bugs, dsAlinOR33-injected male bugs almost lost behavioral preference for trans-2-hexenyl butyrate. Furthermore, the wingbeat frequency of dsAlinOR33-injected male bugs notably declined. Therefore, we conclude that as a candidate sex pheromone receptor, AlinOR33 plays essential roles in the sexual behavior of A. lineolatus.

Functional Characterization of Olfactory Proteins Involved in Chemoreception of Galeruca daurica

Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) play a fundamental role in insect olfaction. Galeruca daurica (Joannis) is a new pest with outbreak status in the Inner Mongolia grasslands, northern China. In this study, six olfactory protein genes (GdauOBP1, GdauOBP6, GdauOBP10, GdauOBP15, GdauCSP4, and GdauCSP5) were cloned by RACE and expressed by constructing a prokaryotic expression system. Their binding affinities to 13 compounds from host volatiles (Allium mongolicum) were determined by fluorescence-binding assay. In order to further explore the olfactory functions of GdauOBP15 and GdauCSP5, RNA interference (RNAi) and electroantennogram (EAG) experiments were conducted. Ligand-binding assays showed that the binding properties of the six recombinant proteins to the tested volatiles were different. GdauOBP6, GdauOBP15, GdauCSP4, and GdauCSP5 could bind several tested ligands of host plants. It was suspected that GdauOBP6, GdauOBP15, GdauCSP4, and GdauCSP5 were related to the host location in G. daurica. We also found that there were different EAG responses between males and females when the GdauOBP15 and GdauCSP5 genes were silenced by RNAi. The EAG response of G. daurica females to 2-hexenal was significantly decreased in dsRNA-OBP15-injected treatment compared to the control, and the dsRNA-CSP5-treated females significantly reduced EAG response to eight tested host volatiles (1,3-dithiane, 2-hexenal, methyl benzoate, dimethyl trisulfide, myrcene, hexanal, 1,3,5-cycloheptatriene, and p-xylene). However, the EAG response had no significant difference in males. Both GdauOBP15 and GdauCSP5 may have different functions between males and females in G. daurica and may play more important roles in females searching for host plants.

An Overview of Antennal Esterases in Lepidoptera

Lepidoptera are used as a model for the study of insect olfactory proteins. Among them, odorant degrading enzymes (ODEs), that degrade odorant molecules to maintain the sensitivity of antennae, have received less attention. In particular, antennal esterases (AEs; responsible for ester degradation) are crucial for intraspecific communication in Lepidoptera. Currently, transcriptomic and genomic studies have provided AEs in several species. However, efforts in gene annotation, classification, and functional assignment are still lacking. Therefore, we propose to combine evidence at evolutionary, structural, and functional level to update ODEs as well as key information into an easier classification, particularly of AEs. Finally, the kinetic parameters for putative inhibition of ODEs are discussed in terms of its role in future integrated pest management (IPM) strategies.

Broadly Tuned Odorant Receptor AlinOR59 Involved in Chemoreception of Floral Scent in Adelphocoris lineolatus

Plant volatiles such as floral scent compounds play a crucial role in mediating insect host locating, mate search, and oviposition sites selection. The alfalfa plant bug, Adelphocoris lineolatus (Goeze), is a seriously polyphagous herbivore of alfalfa and cotton that has an obvious preference for flowering host plants. In this study, we focused on the role of an odorant receptor AlinOR59 in the perception of plant volatiles in A. lineolatus. In situ hybridization showed that AlinOR59 was coexpressed with the coreceptor AlinORco in the ORNs cell located in the long curved sensilla trichodea on antennae of both genders. The Xenopus oocytes expression coupled with two-electrode voltage clamp recordings demonstrated that AlinOR59 responded to 15 plant volatiles. In electroantennogram assays, all of the above 15 compounds could excite electrophysiological responses in the antennae of adult bugs. Furthermore, an important floral scent compound, methyl salicylate, was utilized to evaluate the behavioral responses of A. lineolatus. It was found that adult bugs of both genders were significantly attracted to methyl salicylate. Taken together, our findings suggest that AlinOR59 plays a crucial role in the perception of floral scents in A. lineolatus and could be used as a potential target to design novel olfactory regulators for the management of bugs.

Functional characterization of a binding protein for Type-II sex pheromones in the tea geometrid moth Ectropis obliqua Prout

The tea geometrid moth Ectropis obliqua Prout is one of the most serious moth pests in tea plants, and its sex pheromones have been identified as typical Type-II polyunsaturated hydrocarbons and epoxide derivatives. Therefore, the E. obliqua male olfactory system provides a good model to study the molecular basis of Type-II sex pheromone recognition as well as functional gene evolution towards structurally different types of moth sex pheromones. In this study, we identified the full-length sequence of a pheromone-binding protein, EoblPBP2 and revealed that it clustered together with the lepidopteran PBP2 subfamily, which binds Type I acetate pheromones. These findings suggest that the EoblPBP2 sequence and physiological function are conserved, although E. obliqua evolved Type II hydrocarbon and epoxide sex pheromones structurally different from Type I acetates. To examine this hypothesis, we studied the expression patterns and in vitro functions of EoblPBP2 in detail. Quantitative real-time PCR experiments showed that EoblPBP2 was predominantly expressed in male E. obliqua antennae. Fluorescence in situ hybridization further demonstrated that the EoblPBP2 gene was abundantly expressed in the pheromone-sensitive sensilla trichodea Str-I in male E. obliqua. The physiological function of recombinant EoblPBP2 was then examined using a competitive binding assay. The results showed that EoblPBP2 had high affinities for three E. obliqua Type II sex pheromone components and Type I acetate pheromones in comparison to some plant volatiles. These results indicate that PBP2 is involved in the detection of Type II pheromones in E. obliqua and it still retains high binding affinities to acetate pheromones and some green leaf ester volatiles.