RNAi-Induced Electrophysiological and Behavioral Changes Reveal two Pheromone Binding Proteins of Helicoverpa armigera Involved in the Perception of the Main Sex Pheromone Component Z11–16:Ald

An Odorant Receptor Expressed in Both Antennae and Ovipositors Regulates Benzothiazole-Induced Oviposition Behavior in Bactrocera dorsalis

The oriental fruit fly,Bactrocera dorsalis (Hendel), is a notorious pest of fruit crops, causing severe damage to fleshy fruits during oviposition and larval feeding. Gravid females locate suitable oviposition sites by detecting the host volatiles. Here, the oviposition preference of antenna-removed females and the electrophysiological response of ovipositors to benzothiazole indicated that both antennae and ovipositors are involved in perceiving benzothiazole. Subsequently, odorant receptors (ORs) expressed in both antennae and ovipositors were screened, and BdorOR43a-1 was further identified to respond to benzothiazole using voltage-clamp recording. Furthermore, BdorOR43a-1-/- mutants were obtained using the CRISPR/Cas9 system and their oviposition preference to benzothiazole was found to be significantly altered compared to WT females, suggesting that BdorOR43a-1 is one of the important ORs for benzothiazole perception. Our results not only demonstrate the important role of antennae and ovipositors in benzothiazole-induced oviposition but also elucidate on the OR responsible for benzothiazole perception in B. dorsalis.

Oleic acid triggers burial behavior in a termite population through an odorant binding protein

Social insects maintain hygienic conditions through their social immunity behaviors. Among these behaviors, burial behavior of termites is central for protecting healthy individuals from corpses. Many factors trigger burial behavior, and it is generally believed that chemicals released by corpses, such as oleic acid, are the most important cues for triggering burial behavior in termites. However, the contribution of the olfactory system to this behavior remains unclear. Here we report an odorant binding protein (OBP) that transports oleic acid and triggers burial behavior in Coptotermes formosanus Shiraki. We demonstrated that CforOBP7 is highly expressed in the antennae of workers. Fluorescent competition binding experiments exhibited that CforOBP7 has a strong affinity for oleic acid. Furthermore, the antennal response to oleic acid was significantly reduced, and oleic acid-triggered burial behavior was also inhibited in CforOBP7-silenced termites. We conclude that CforOBP7 governs the burial behavior of C. formosanus triggered by oleic acid.

Molecular mechanism of sex pheromone perception in male Mythimna loreyi revealed by in vitro system

BACKGROUND

Mythimna loreyi is an important agricultural pest with a sensitive sex pheromone communication system. To clarify the pheromone binding proteins (PBPs) and pheromone receptors (PRs) involved in sex pheromone perception is important for both understanding the molecular olfactory mechanism and developing a new pest control strategy in M. loreyi.

RESULTS

First, the electroantennogram (EAG) assay showed that male M. loreyi displayed the highest response to the major sex pheromone component Z9-14:Ac, and higher responses to two minor components, Z7-12:Ac and Z11-16:Ac. Second, the fluorescence competition binding assay showed that PBP1 bound all three pheromones and other tested compounds with high or moderate affinity, while PBP2 and PBP3 each bound only one pheromone component and few other compounds. Third, functional study using the Xenopus oocyte system demonstrated that, of the six candidate PRs, PR2 was weakly sensitive to the major pheromone Z9-14:Ac, but was strongly sensitive to pheromone analog Z9-14:OH; PR3 was strongly and specifically sensitive to a minor component Z7-12:Ac; PR4 and OR33 were both weakly sensitive to another minor component, Z11-16:Ac. Finally, phylogenetic relationship and ligand profiles of PRs were compared among six species from two closely related genera Mythimna and Spodoptera, suggesting functional shifts of M. loreyi PRs toward Spodoptera PRs.

CONCLUSION

Functional differentiations were revealed among three PBPs and six PRs in sex pheromone perception, laying an important basis for understanding the molecular mechanism of sex pheromone perception and for developing new control strategies in M. loreyi. © 2023 Society of Chemical Industry.

Chemosensory Protein 2 of Male Athetis lepigone Is Involved in the Perception of Sex Pheromones and Maize Volatiles

In moths, the interactions between chemosensory proteins (CSPs) and sex pheromones have yet to be comprehensively investigated. Here, we examined the function of AlepCSP2 in male Athetis lepigone based on protein expression, molecular docking, site-directed mutagenesis, fluorescence competitive binding analyses, and RNA interference (RNAi) experiments. We found that AlepCSP2 showed strong binding affinity for two sex pheromones and five maize volatiles and that binding was optimal under neutral conditions. Furthermore, we identified six amino acids as being key residues involved in the interaction between AlepCSP2 and multiple ligands. Further RNAi showed that siCSP2 males displayed consistently lower electroantennography responses to two sex pheromones and three maize volatiles at different dosages tested, and the mating rate also decreased significantly by 37.50%. These findings will contribute to characterizing the binding mechanisms of moth CSPs to sex pheromones and host volatiles and also identify unique targets for developing novel pest behavior disruptors.

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.

Molecular Characterization and Expression Patterns of Two Pheromone-Binding Proteins from the Diurnal Moth Phauda flammans (Walker) (Lepidoptera: Zygaenoidea: Phaudidae)

Sex pheromone-binding proteins (PBPs) play an important role in sex pheromone recognition in Lepidoptera. However, the mechanisms of chemical communication mediating the response to sex pheromones remain unclear in the diurnal moths of the superfamily Zygaenoidea. In this study, Phauda flammans (Walker) (Lepidoptera: Zygaenoidea: Phaudidae) was used as a model insect to explore the molecular mechanism of sex pheromone perception in the superfamily Zygaenoidea. Two novel pheromone-binding proteins (PflaPBP1 and PflaPBP2) from P. flammans were identified. The two pheromone-binding proteins were predominantly expressed in the antennae of P. flammans male and female moths, in which PflaPBP1 had stronger binding affinity to the female sex pheromones Z-9-hexadecenal and (Z, Z, Z)-9, 12, 15-octadecatrienal, PflaPBP2 had stronger binding affinity only for (Z, Z, Z)-9, 12, 15-octadecatrienal, and no apparent binding affinity to Z-9-hexadecenal. The molecular docking results indicated that Ile 170 and Leu 169 are predicted to be important in the binding of the sex pheromone to PflaPBP1 and PflaPBP2. We concluded that PflaPBP1 and PflaPBP2 may be responsible for the recognition of two sex pheromone components and may function differently in female and male P. flammans. These results provide a foundation for the development of pest control by exploring sex pheromone blocking agents and the application of sex pheromones and their analogs for insect pests in the superfamily Zygaenoidea.

Silencing of Cytochrome P450 Gene AgoCYP6CY19 Reduces the Tolerance to Host Plant in Cotton- and Cucumber-Specialized Aphids, Aphis gossypii

Cytochrome P450 monooxygenases play important roles in insect metabolism and detoxification of toxic plant substances. However, the function of CYP6 family genes in degrading plant toxicants in Aphis gossypii has yet to be elucidated. In this study, AgoCYP6CY19, an A. gossypii CYP gene that differentially expresses in cotton- and cucumber-specialized aphids, was characterized. Spatiotemporal expression profiling revealed that AgoCYP6CY19 expression was higher in second instar nymph and 7 day old adults than in other developmental stages. Although the expression of AgoCYP6CY19 was significantly higher in cotton-specialized aphids, AgoCYP6CY19 silencing significantly increased larval and adult mortality and reduced total fecundity in both cotton- and cucumber-specialized aphids. What is more, the expression of AgoCYP6CY19 was significantly induced after the cotton-specialized and cucumber-specialized aphids fed on epigallocatechin gallate (EGCG) and cucurbitacin B (CucB), respectively. These findings demonstrate that AgoCYP6CY19 plays a pivotal role in toxic plant substance detoxification and metabolism. Functional knowledge about plant toxicity tolerance genes in this major pest can provide new insights into insect detoxification of toxic plant substances and insecticides and offer new targets for agricultural pest control strategies.

AlepPBP2, but not AlepPBP3, may involve in the recognition of sex pheromones and maize volatiles in Athetis lepigone

Athetis lepigone Möschler (Lepidoptera, Noctuidae) is a common maize pest in Europe and Asia. However, there is no long-term effective management strategy is available yet to suppress its population. Adults rely heavily on olfactory cues to locate their optimal host plants and oviposition sites. Pheromone-binding proteins (PBPs) are believed to be responsible for recognizing and transporting different odorant molecules to interact with receptor membrane proteins. In this study, the ligand-binding specificities of two AlepPBPs (AlepPBP2 and AlepPBP3) for sex pheromone components and host plant (maize) volatiles were measured by fluorescence ligand-binding assay. The results demonstrated that AlepPBP2 had a high affinity with two pheromones [(Z)-7-dodecenyl acetate, Ki = 1.11 ± 0.1 μM, (Z)-9-tetradecenyl acetate, Ki = 1.32 ± 0.15 μM] and ten plant volatiles, including (-)-limonene, α-pinene, myrcene, linalool, benzaldehyde, nonanal, 2-hexanone, 3-hexanone, 2-heptanone and 6-methyl-5-hepten-2-one. In contrast, we found that none of these chemicals could bind to AlepPBP3. Our results clearly show no significant differences in the functional characterization of the binding properties between AlepPBP2 and AlepPBP3 to sex pheromones and host plant volatiles. Furthermore, molecular docking was employed for further detail on some crucial amino acid residues involved in the ligand-binding of AlepPBP2. These findings will provide valuable information about the potential protein binding sites necessary for protein-ligand interactions which appear as attractive targets for the development of novel technologies and management strategies for insect pests.

Expression Profiling and Functional Analysis of Candidate Odorant Receptors in Galeruca daurica

Simple Summary

Odorant receptors (ORs) play an important role in the olfactory system in insects. However, there is no functional research on the odorant receptors of Galeruca daurica. In this study, 21 OR genes were identified from the transcriptome database of G. daurica adults. Most GdauORs were mainly expressed in antennae, and the expression levels of GdauORs in adults were affected by age. When GdauOR4, GdauOR15, and GdauORco were silenced by RNAi, the electrophysiological responses to host plant volatiles were significantly decreased.

Abstract

Galeruca daurica (Joannis) is an oligophagous pest in the grasslands of Inner Mongolia, China, which feed mainly on Allium spp. Odorant receptors (ORs) play an important role in the olfactory system in insects, and function together with olfactory co-receptor (ORco). In this study, 21 OR genes were identified from the transcriptome database of G. daurica adults, and named GdauOR1-20 and GdauORco. The expression profiles were examined by RT-qPCR and RNA interference (RNAi) and electroantennogram (EAG) experiments were conducted to further identify the olfactory functions of GdauOR4, GdauOR11, GdauOR15, and GdauORco. It was found that 15 GdauORs (OR1, OR3-6, OR8, OR11-13, OR15, OR17-20, and ORco) were mainly expressed in antennae, and the expression levels of GdauORs in adults were affected by age. When GdauOR4, GdauOR15, and GdauORco were silenced by RNAi, the electrophysiological responses to host plant volatiles were significantly decreased in G. daurica. This study lays a necessary foundation for clarifying the mechanism on finding host plants in G. daurica.

Functional differentiation of three pheromone binding proteins in Orthaga achatina using mixed-type sex pheromones

Pheromone-binding proteins (PBPs) play important roles in perception of insect sex pheromones, functioning to recognize and transport pheromone components onto the olfactory receptors of the odorant sensing neurons. Orthaga achatina, a serious pest of camphor trees, uses a mixture of three Type I (Z11-16:OAc, Z11-16:OH and Z11-16:Ald) and one Type II (Z3,Z6,Z9,Z12,Z15-23:H) sex pheromone components in its sex communication, in which Z11-16:OAc is the major component and others are minor components. In this study, we for the first time demonstrated that the three PBPs differentiated in recognition among pheromone components in a moth using mixed-type sex pheromones. First, tissue expression study showed that all three PBPs of O. achatina were expressed only in antennae and highly male-biased, suggesting their involvement in perception of the sex pheromones. Second, the three PBPs were expressed in Escherichia coli and the binding affinities of PBPs to four sex pheromone components and some pheromone analogs were determined by the fluorescence competition binding assays. The results showed that OachPBP1 bound all four sex pheromone components with high binding affinity, while OachPBP2 had high or moderate binding affinity only to three Type I components, and OachPBP3 had high binding affinity only to three minor pheromone components. Furthermore, key amino acid residues that bind to sex pheromone components were identified in three PBPs by 3-D structure modeling and ligand molecular docking, predicting the interactions between PBPs and pheromone components. Our study provides a fundamental insight into the olfactory mechanism in moths that use mixed-type sex pheromones.

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.

Structural Insights into the Ligand-Binding and -Releasing Mechanism of Helicoverpa armigera Pheromone-Binding Protein PBP1

Cotton bollworm (Helicoverpa armigera) is a worldwide agricultural pest in which the transport of pheromones is indispensable and perceived by pheromone-binding proteins (PBPs). However, three-dimensional structure, pheromone binding, and releasing mechanisms of PBPs are not completely illustrated. Here, we solved three structures of the cotton bollworm HarmPBP1 at different pH values and its complex with ligand, Z-9-hexadecenal. Although apo-HarmPBP1 adopts a common PBP scaffold of six α-helices surrounding a predominantly hydrophobic central pocket, the conformation is greatly distinct from other apo-PBPs. The Z-9-hexadecenal is bound mainly by hydrophobic interaction. The pheromone can enter this cavity through an opening between the helices α5 and α6, as well as the loop between α3 and α4. Structural analysis suggests that ligand entry into the pocket is followed by a shift of Lys94 and Lys138, which may act as a lid at the opening of the pocket. Acidic pH will cause a subtle structural change of the lid, which in turn affects its ligand-binding ability, differently from other family proteins. Taken together, this study provides structural bases for the interactions between pheromones and PBPs, the pH-induced conformational switch, and the design of small inhibitors to control cotton bollworms by disrupting male–female chemosensory communication.

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.

Molecular characterization of sex pheromone binding proteins from Holotrichia oblita (Coleoptera: Scarabaeida)

Pheromone binding proteins (PBPs), a subfamily of the odorant binding proteins (OBPs), capture and transfer sex pheromones across the sensillum lymph to pheromone receptors and initiate insect courtship and mating. In this study, we functionally characterized ten OBPs from the black chafer, Holotrichia oblita (HoblOBPs), among which six HoblOBPs (HoblOBP2, 4, 5, 8, 9 and 24) were shown to recognize sex pheromones using electroantennography assays (EAG) and in vitro fluorescence competitive binding assays. Insect tropism to sex pheromones was significantly reduced after those genes were knocked down in vivo, e.g. HoblOBP24 RNAi reduced the tropism of H. oblita to methyl glycinate by 34%. Furthermore, molecular docking revealed key residues for the binding of the six HoblOBPs with sex pheromones. And hydrogen bonds and hydrophobic forces were shown to be the main forces in the binding of the six HoblOBPs and their sex pheromone ligands. Our study characterized six H. oblita PBPs and their binding abilities to sex pheromone ligands. The results will improve our understanding on the olfactory mechanisms that H. oblita utilizes to recognize sex pheromones, and will promote the development of novel strategies for controlling H. oblita and other insect pests.

Olfactory Sensilla and Olfactory Genes in the Parasitoid Wasp Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae)

Simple Summary

Parasitic wasps are the major natural enemies of many organisms, and therefore they are broadly used in the biological control of numerous agricultural and horticultural pests. For example, Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) is a tiny natural egg parasitoid of various agricultural pest insects, including Plutella xylostella, Helicoverpa armigera, Spodoptera frugiperda and Ectomyelois ceratoniae. However, how T. pretiosum seek and localise host insect eggs is still not clear. The olfactory system is critical in guiding insect behaviours, including mating, feeding and oviposition, which play pivotal roles in the interactions between parasitoid wasps and their hosts. This project aimed to investigate T. pretiosum major olfactory tissue (antennae) and the olfactory genes, including odorant binding proteins (OBPs) and odorant receptors (ORs). T. pretiosum adult antennae were examined under scanning electron microscopy, and four types of olfactory sensilla were observed. Using T. pretiosum genome, 22 OBPs and 105 ORs were identified, which were further compared with olfactory genes of other Hymenoptera insect species. The expression patterns of OBPs between T. pretiosum male and female adults were examined to identify female- or male-specific OBPs. This study enriches our knowledge of T. pretiosum olfactory system and will help better use it in the integrated pest management (IPM) for many insect pest species.

Abstract

Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) is a tiny natural egg parasitoid of several agricultural pest insects, which has been widely used in the biological control for Plutella xylostella, Helicoverpa armigera, Spodoptera frugiperda and Ectomyelois ceratoniae. However, limited studies have been conducted on T. pretiosum olfactory system, which is critical in regulating insect behaviours. In this study, T. pretiosum adult antennae were investigated under ascanning electron microscopy (SEM). Four types of olfactory sensilla were observed, including chaetica sensilla (CS), trichoid sensilla (TS), faleate sensilla (FS) and placoid sensilla (PS). Using T. pretiosum genome, 22 putative odorant binding proteins (OBPs) and 105 odorant receptors (ORs) were identified, which were further compared with olfactory genes of Apis mellifera, Nasonia vitripennis and Diachasma alloeum. The expression patterns of OBPs between T. pretiosum male and female adults were examined by quantitative real time PCR (qRT-PCR) approaches. Three female-specific OBPs (TpreOBP19, TpreOBP15 and TpreOBP3) were identified, which may play crucial roles in T. pretiosum host-seeking and oviposition behaviours. This study enriches our knowledge of T. pretiosum olfactory genes and improves our understanding of its olfactory system.

RNA Interference-Based Forest Protection Products (FPPs) Against Wood-Boring Coleopterans: Hope or Hype?

Forest insects are emerging in large extension in response to ongoing climatic changes, penetrating geographic barriers, utilizing novel hosts, and influencing many hectares of conifer forests worldwide. Current management strategies have been unable to keep pace with forest insect population outbreaks, and therefore novel and aggressive management strategies are urgently required to manage forest insects. RNA interference (RNAi), a Noble Prize-winning discovery, is an emerging approach that can be used for forest protection. The RNAi pathway is triggered by dsRNA molecules, which, in turn, silences genes and disrupts protein function, ultimately causing the death of the targeted insect. RNAi is very effective against pest insects; however, its proficiency varies significantly among insect species, tissues, and genes. The coleopteran forest insects are susceptible to RNAi and can be the initial target, but we lack practical means of delivery, particularly in systems with long-lived, endophagous insects such as the Emerald ash borer, Asian longhorn beetles, and bark beetles. The widespread use of RNAi in forest pest management has major challenges, including its efficiency, target gene selection, dsRNA design, lack of reliable dsRNA delivery methods, non-target and off-target effects, and potential resistance development in wood-boring pest populations. This review focuses on recent innovations in RNAi delivery that can be deployed against forest pests, such as cationic liposome-assisted (lipids), nanoparticle-enabled (polymers or peptides), symbiont-mediated (fungi, bacteria, and viruses), and plant-mediated deliveries (trunk injection, root absorption). Our findings guide future risk analysis of dsRNA-based forest protection products (FPPs) and risk assessment frameworks incorporating sequence complementarity-based analysis for off-target predictions. This review also points out barriers to further developing RNAi for forest pest management and suggests future directions of research that will build the future use of RNAi against wood-boring coleopterans.

Identification and characterization of odorant binding proteins in the diamondback moth, Plutella xylostella

Odorant binding proteins (OBPs) are a group of soluble proteins functioning as odorant carriers in insect antennae, mouth parts and other chemosensory organs. However, multiple insect OBPs have been detected in other tissues and various functions have been proposed. Therefore, a detailed expression profile including stages, tissues and sexes where OBPs are expressed will assist in building the links to their potential functions, enhancing the functional studies of insect OBPs. Here, we identified 39 putative OBP genes from its genome and transcriptome sequences of diamondback moth (DBM), Plutella xylostella. The expression patterns of identified PxylOBPs were further investigated from eggs, larvae, pupae, virgin adults, mated adults, larval midgut, larval heads, adult antennae, adult heads and adult tarsi. Moreover, P. xylostella larvae and adults with and without host plants for 5 h were utilized to study the interactions between OBP expression and host plants. The results showed that most PxylOBPs were highly expressed in male and female adult antennae. The expression levels of certain PxyOBPs could be regulated by mating activities and feeding host plants. This study advances our knowledge of P. xylostella OBPs, which may help develop new strategies for more environmentally sustainable management of P. xylostella.

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.

Identification of Olfactory Genes From the Greater Wax Moth by Antennal Transcriptome Analysis

The olfactory system is used by insects to find hosts, mates, and oviposition sites. Insects have different types of olfactory proteins, including odorant-binding proteins (OBPs), chemosensory proteins (CSPs), odorant receptors (ORs), ionotropic receptors (IRs), and sensory neuron membrane proteins (SNMPs) to perceive chemical cues from the environment. The greater wax moth, Galleria mellonella, is an important lepidopteran pest of apiculture. However, the molecular mechanism underlying odorant perception in this species is unclear. In this study, we performed transcriptome sequencing of G. mellonella antennae to identify genes involved in olfaction. A total of 42,544 unigenes were obtained by assembling the transcriptome. Functional classification of these unigenes was determined by searching against the Gene Ontology (GO), eukaryotic orthologous groups (KOG), and the Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. We identified a total of 102 olfactory-related genes: 21 OBPs, 18 CSPs, 43 ORs, 18 IRs, and 2 SNMPs. Results from BLASTX best hit and phylogenetic analyses showed that most of the genes had a close relationship with orthologs from other Lepidoptera species. A large number of OBPs and CSPs were tandemly arrayed in the genomic scaffolds and formed gene clusters. Reverse transcription-quantitative PCR results showed that GmelOBP19 and GmelOR47 are mainly expressed in male antennae. This work provides a transcriptome resource for olfactory genes in G. mellonella, and the findings pave the way for studying the function of these genes.

Characterizing the Role of Orco Gene in Detecting Aggregation Pheromone and Food Resources in Protaetia brevitarsis Leiws (Coleoptera: Scarabaeidae)

An accurate olfactory system for recognizing semiochemicals and environmental chemical signals plays crucial roles in survival and reproduction of insects. Among all olfaction-related proteins, olfactory receptors (ORs) contribute to the conversion of chemical stimuli to electric signals and thereby are vital in odorant recognition. Olfactory receptor co-receptor (Orco), one of the most conserved ORs, is extremely essential in recognizing odorants through forming a ligand-gated ion channel complex with conventional ligand-binding odorant receptors. We have previously identified aggregation pheromone in Protaetia brevitarsis (Coleoptera: Scarabaeidae), a native agricultural and horticultural pest in East-Asia. However, to our best knowledge, its olfaction recognition mechanisms are still veiled. To illustrate how P. brevitarsis recognize aggregation pheromone and host plants, in the present study we cloned and sequenced the full-length Orco gene from P. brevitarsis antennae (named PbreOrco) and found that PbreOrco is highly conserved and similar to Orcos from other Coleoptera insects. Our real-time quantitative PCR (qRT-PCR) results showed that PbreOrco is mainly expressed in antenna. We also demonstrated that silencing PbreOrco using RNA interference through injecting dsOrco fragment significantly inhibited PbreOrco expression in comparison with injecting control dsGFP and subsequently revealed using electroantennogram and behavioral bioassays that decreasing PbreOrco transcript abundance significantly impaired the responses of P. brevitarsis to intraspecific aggregation pheromone and prolonged the time of P. brevitarsis spending on food seeking. Overall, our results demonstrated that PbreOrco is crucial in mediating odorant perception in P. brevitarsis.

Coordinative mediation of the response to alarm pheromones by three odorant binding proteins in the green peach aphid Myzus persicae

Odorant binding proteins (OBPs) play an essential role for insect chemosensation in insect peripheral nervous systems of antennae. Each antennal sensilla contains more than one OBP at high concentrations but the interactions and cooperation between co-localized OBPs are rarely reported. In present study, we cloned, expressed and purified eight OBPs of the green peach aphid Myzus persicae. The effects of knocking down the expression of these OBP genes by RNAi on the electrophysiological and behavioural responses of M. persicae to the aphid alarm pheromone, (E)-β-farnesene (EβF) were investigated. The results showed that the aphids could still be repelled by EβF when the expression of each of three OBP genes was individually knocked down. However, the simultaneous knockdown of MperOBP3/7/9 expression significantly reduced the electrophysiological response and the repellent behaviours of M. persicae to EβF than the single OBP gene knockdown (P < 0.05). Rather than a normal saturation binding curve of individual OBP, the binding curve of MperOBP3/7/9 is bell-shaped with a higher affinity for the fluorescent probe N-phenyl-1-naphthylamine (1-NPN). The competitive binding assays confirmed that MperOBP3, MperOBP7, MperOBP9 and MperOBP3/7/9 mixture exhibited a stronger binding affinity for EβF, than for sex pheromones and plant volatiles with a dissociation constant of 2.5 μM, 1.1 μM, 3.9 μM and 1.0 μM, respectively. The competitive binding curve of MperOBP3/7/9 mixture to EβF is shallow without bottom plateau, suggesting a conformational change and a rapid dissociation after the displacement of all 1-NPN (in vivo after the saturation binding of all OBPs by EβF). The interaction between OBPs and formation of a heterogeneous unit may facilitate the delivery EβF to the OR at electrophysiological and behavioural levels during insect odorant signal transduction thus mediate M. persicae response to the alarm pheromone EβF.

Functional characteristic analysis of three odorant-binding proteins from the sweet potato weevil (Cylas formicarius) in the perception of sex pheromones and host plant volatiles

BACKGROUND

The sweet potato weevil, Cylas formicarius, is the most serious pest of sweet potato worldwide. The molecular mechanism of sex pheromone recognition in C. formicarius has not been reported. Odorant-binding proteins (OBPs) play a critical role in selectively binding and transporting pheromones or other odors to the surface of olfactory receptor neurons through the aqueous sensillar lymph, therefore the function of sweet potato OBPs is worth studying.

RESULTS

Herein, the CforOBP1-3 genes encoding three classical OBPs were cloned in C. formicarius by reverse transcription-polymerase chain reaction. Phylogenetic analysis showed that CforOBP1-3 were homologous genes, but the relationship between CforOBP2 and CforOBP3 was closest among the three genes. In addition, real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) assays demonstrated that the expression of CforOBP1 was higher in the antennae and legs of female and male insects, while CforOBP2 and CforOBP3 were mainly expressed in the antennae of male insects. The fluorescent competitive binding assay results indicated that CforOBP1-3 had strong binding affinities to sex pheromones and other tested ligands. Finally, the mRNA expression of CforOBP1-3 was successfully inhibited by RNA interference, and in vivo behavioral experiments showed that CforOBP1-3-deficient C. formicarius was partly anosmic and lost some of its ability to locate sex pheromones and host plant volatiles.

CONCLUSION

These results suggested that CforOBP1 was shown to be involved in the process of weevils feeding and finding sweet potato, and CforOBP2-3 were mainly involved in the mating behavior of adult male weevils.

Odorant-binding proteins involved in sex pheromone and host-plant recognition of the sugarcane borer Chilo infuscatellus (Lepidoptera: Crambidae)

BACKGROUND

Pheromone-binding proteins (PBPs) are responsible for transporting sex pheromones and general odorant-binding proteins (GOBPs) have been proposed to transport host-plant volatiles. A large number of OBPs have been identified from Lepidoptera species. However, olfactory molecular biology and physiology studies on PBP and GOBP in sugarcane pests are limited. Chilo infuscatellus is one of the most widely distributed pests in sugarcane-producing areas.

RESULTS

Three PBPs (CinfPBP1, CinfPBP2 and CinfPBP3) and two GOBPs (CinfGOBP1 and CinfGOBP2) were identified, and five olfactory gene transcripts were abundantly expressed in antennae of C. infuscatellus. Binding assays showed that CinfPBP1-3 exhibited strong binding affinity for the sex pheromone components Z11-16:OH and 16:OH of C. infuscatellus. Meanwhile, CinfGOBP1-2 had high binding affinity with host-plant volatiles from sugarcane (Saccharum officinarum). Field-trapping results suggested that four volatile components, octadecane, (Z)-3-hexen-1-ol, α-terpineol and hexadecane from host plants and sex pheromone mixed baits have synergistic roles in attracting C. infuscatellus adult moths.

CONCLUSION

Functional characterization of CinfPBPs and CinfGOBPs in C. infuscatellus could help us find new environmentally friendly alternatives to conventional pest control using pesticides in sugarcane fields. © 2020 Society of Chemical Industry.

Molecular mechanisms of olfactory detection in insects: beyond receptors

Insects thrive in diverse ecological niches in large part because of their highly sophisticated olfactory systems. Over the last two decades, a major focus in the study of insect olfaction has been on the role of olfactory receptors in mediating neuronal responses to environmental chemicals. In vivo, these receptors operate in specialized structures, called sensilla, which comprise neurons and non-neuronal support cells, extracellular lymph fluid and a precisely shaped cuticle. While sensilla are inherent to odour sensing in insects, we are only just beginning to understand their construction and function. Here, we review recent work that illuminates how odour-evoked neuronal activity is impacted by sensillar morphology, lymph fluid biochemistry, accessory signalling molecules in neurons and the physiological crosstalk between sensillar cells. These advances reveal multi-layered molecular and cellular mechanisms that determine the selectivity, sensitivity and dynamic modulation of odour-evoked responses in insects.

Identification of cytochrome P450, odorant-binding protein, and chemosensory protein genes involved in Type II sex pheromone biosynthesis and transportation in the tea pest, Scopula subpunctaria

Sex pheromone-based pest management technology has been widely used to monitor and control insect pests in the agricultural, forestry, and public health sectors. Scopula subpunctaria is a widespread tea pest in China with Type II sex pheromone components. However, limited information is available on the biosynthesis and transportation of Type II sex pheromone components. In this study, we constructed an S. subpunctaria sex pheromone gland (PG) transcriptome and obtained 85,246 transcripts. Cytochrome P450 monooxygenases (CYPs) thought to epoxidize dienes and trienes to epoxides in the PG and odorant-binding proteins (OBPs) and chemosensory genes (CSPs) thought to be responsible for the binding and transportation of sex pheromone components. In present study, a total of 79 CYPs, 29 OBPs and 17 CSPs were identified. We found that SsubCYP341A and SsubCYP341B_ortholog1 belonged to the CYP341 family and were more highly expressed in the PG than in the female body. Of these, SsubCYP341A was the seventh-most PG-enriched CYP in the PG transcriptome. Two CYP4 members, CYP340BD_ortholog2 and CYP4G, were the top two most PG-enriched CYPs. Tissue expression and phylogenetic tree analysis showed that SsubOBP25, 27, and 28 belonged to the moth pheromone-binding protein family; they were distinctly expressed in the antennae and were more abundant in male antennae than in female antennae. SsubCSP16 was distributed into the same clade as CSPs from other moths that showed high binding affinities to sex pheromone components. It indicated that all the above-mentioned genes could be involved in sex pheromone biosynthesis or transportation. Our study provides large-scale PG sequence information that can be used to identify potential targets for the biological control of S. subpunctaria by disrupting its sex pheromone biosynthesis and transportation pathways.

Identification and Expression Profiles of 14 Odorant-Binding Protein Genes From Pieris rapae (Lepidoptera: Pieridae)

The small white butterfly, Pieris rapae (L.), is an important insect pest of Brassica crops. This species utilize olfactory cues to find their hosts and mates. However, the molecular mechanism underlying the olfactory perception in this species remains unclear. Here, we identified 14 odorant-binding proteins (OBP) genes-essential for insect olfaction-in P. rapae by exploring a previously published transcriptome dataset. Proteins encoded by all of these genes contain N-terminal signal peptides and six positionally conserved cysteine residues, which are characteristic of insect OBPs. These OBPs displayed high amino acid identity with their respective orthologs in other lepidopterans, and several conserved motifs were identified within these OBPs. Phylogenetic analysis showed that these OBPs were well segregated from each other and clustered into different branches. PrapOBP1 and PrapOBP2 were clustered into the 'general odorant-binding protein' clade, and PrapOBP3 and PrapOBP4 fall into the 'pheromone-binding protein' clade. The 14 OBP genes were located on seven genomic scaffolds. Of these, PrapOBP1, 2, 3, and 4 were located on scaffold332, whereas PrapOBP5, 6, 7, 8, and 9 were located on scaffold116. Ten of the 14 genes had antenna-biased expression. Of these, PrapOBP1, 2, 4, and 13 were enriched in male antennae, whereas PrapOBP7 and PrapOBP10 were female-biased. Our findings suggest that these OBPs may be involved in olfactory communication. To the best of our knowledge, this is the first report on the identification and characterization of OBPs in P. rapae, and our findings provide a solid foundation for studying the functions of these genes.

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.

RNA-based biocontrol compounds: current status and perspectives to reach the market

Facing current climate challenges and drastically reduced chemical options for plant protection, the exploitation of RNA interference (RNAi) as an agricultural biotechnology tool has unveiled possible new solutions to the global problems of agricultural losses caused by pests and other biotic and abiotic stresses. While the use of RNAi as a tool in agriculture is still limited to a few transgenic crops, and only adopted in restricted parts of the world, scientists and industry are already seeking innovations in leveraging and exploiting the potential of RNAi in the form of RNA-based biocontrol compounds for external applications. Here, we highlight the expanding research and development pipeline, commercial landscape and regulatory environment surrounding the pursuit of RNA-based biocontrol compounds with improved environmental profiles. The commitments of well-established agrochemical companies to invest in research endeavours and the role of start-up companies are crucial for the successful development of practical applications for these compounds. Additionally, the availability of standardized guidelines to tackle regulatory ambiguities surrounding RNA-based biocontrol compounds will help to facilitate the entire commercialization process. Finally, communication to create awareness and public acceptance will be key to the deployment of these compounds. © 2019 Society of Chemical Industry.

Molecular Characterization and Key Binding Sites of Sex Pheromone-Binding Proteins from the Meadow Moth, Loxostege sticticalis

The meadow moth, Loxostege sticticalis, is a typical agricultural pest that uses sex pheromones to mediate mating behavior; however, the mechanism underlying the selectivity of its pheromone-binding proteins (PBPs) remains unknown. In this study, LstiPBP1 and LstiPBP3 were cloned, expressed, and purified, and the fluorescence binding assay showed that LstiPBP1 binds to the major sex pheromone component, E-11-tetradecenol (E11-14:OH), with high affinity; moreover, E11-14:OH could evoke a significant antennal electrophysiological response and attract L. sticticalis males. After LstiPBP1 was silenced, both the antennal response and attractiveness of E11-14:OH decreased significantly. Molecular docking predicted that a hydrogen bonding site, Leu37, played key role in the binding of LstiPBP1 to E11-14:OH. After Leu37 was mutated, the E11-14:OH-binding affinity decreased drastically. These results suggest that LstiPBP1 participates in E11-14:OH recognition and could be used as a target gene to disturb the mating behavior of L. sticticalis and develop new odorants for pest control.

Transcriptome and gene expression analysis of three developmental stages of the coffee berry borer, Hypothenemus hampei

Coffee production is a global industry valued at approximately 173 billion US dollars. One of the main challenges facing coffee production is the management of the coffee berry borer (CBB), Hypothenemus hampei, which is considered the primary arthropod pest of coffee worldwide. Current control strategies are inefficient for CBB management. Although biotechnological alternatives, including RNA interference (RNAi), have been proposed in recent years to control insect pests, characterizing the genetics of the target pest is essential for the successful application of these emerging technologies. In this study, we employed RNA-seq to obtain the transcriptome of three developmental stages of the CBB (larva, female and male) to increase our understanding of the CBB life cycle in relation to molecular features. The CBB transcriptome was sequenced using Illumina Hiseq and assembled de novo. Differential gene expression analysis was performed across the developmental stages. The final assembly produced 29,434 unigenes, of which 4,664 transcripts were differentially expressed. Genes linked to crucial physiological functions, such as digestion and detoxification, were determined to be tightly regulated between the reproductive and nonreproductive stages of CBB. The data obtained in this study help to elucidate the critical roles that several genes play as regulatory elements in CBB development.

The sensilla trichodea-biased EoblPBP1 binds sex pheromones and green leaf volatiles in Ectropis obliqua Prout, a geometrid moth pest that uses Type-II sex pheromones

Pheromone-binding proteins (PBPs) are considered to play critical roles in sex pheromone detection. Lepidopteran moths can be divided into two taxa, those that use Type-I sex pheromones, such as C10-C18 unsaturated aldehydes, alcohols and acetates, and those that use Type-II pheromones, which are C17-C23 polyunsaturated hydrocarbons and their epoxide derivatives. To date, nearly all the characterized PBPs have been reported in moths with Type-I sex pheromones, and the physiological functions of PBPs in moths that use Type-II sex pheromones remains unclear. In the present study we functionally examine EoblPBP1 in Ectropis obliqua Prout, an important geometrid moth pest that uses Type-II sex pheromones. The phylogenetic analysis of the sequence indicated that EoblPBP1 clustered together with ScerPBP1, a geometrid PBP for detecting Type-II sex pheromones. Scanning electron microscopy showed that E. obliqua moths of both sexes mainly had six types of antennal sensilla, including two types of sensilla trichodea, Str-I and Str-II, sensilla basiconica (Sba), sensilla styloconica (Sst), sensilla chaetica (Sch) and sensilla auricillica (Sau). Of these, Str-I was confirmed to be male moth-specific and had five different subtypes. Fluorescence in situ hybridization revealed that EoblPBP1 was primarily expressed at the base of Str-I. A comparative binding assay showed that recombinant EoblPBP1 bound three sex pheromone components of E. obliqua, demonstrating its involvement in the detection of Type-II sex pheromones. Besides, EoblPBP1 also highly bound unsaturated acetates pheromones and the green leaf volatiles. These results indicate that PBP1 is associated with detecting Type-II sex pheromones in E. obliqua but cannot differentiate Type-II sex pheromones from Type-I sex pheromones or green leaf volatiles. Our findings provide a foundation for further study on molecular basis of Type-II sex pheromone recognition in lepidopteran moths.

Key Amino Residues Determining Binding Activities of the Odorant Binding Protein AlucOBP22 to Two Host Plant Terpenoids of Apolygus lucorum

Odorant binding proteins (OBPs) are considered to be highly expressed at antennae sensillum lymph and play crucial roles in detection of insect host plant volatiles. The polyphagous mirid bug Apolygus lucorum is one of a series of insect pests on many important agricultural crops that heavily rely on sophisticated olfaction to locate host plants. Previously, putative OBP genes and their tissue-related expression patterns in this pest species have been clarified. In this study, we characterized the ligand spectrum and the molecular binding mechanism of the antennae-biased AlucOBP22 to host plant volatiles of A. lucorum. Frist, the recombinant AlucOBP22 protein was constructed and purified, and its binding affinities to selected host plant volatiles were assessed. Two terpenoids, β-ionone and β-caryophyllene, could highly bind to AlucOBP22. Next, three-dimensional model prediction indicated that AlucOBP22 employed six α-helices to form a typical pocket for ligand accommodation. Molecular docking analysis suggested that both β-ionone and β-caryophyllene were located at the AlucOBP22 pocket with some hydrophobic amino acid residues close to the two chemicals, suggesting that hydrophobic interactions might be crucial for ligand-specific binding. Finally, site-directed mutagenesis combined with fluorescence binding assays revealed that mutants of five hydrophobic residues Leu5, Ile40, Met41, Val44, and Met45 displayed significantly decreased or completely abolished binding affinities to the two ligands. Our findings showed the specific binding characteristic of AlucOBP22 and suggested that hydrophobic residues and their hydrophobic interactions were involved in AlucOBP22 binding to terpenoids, which provided new insights into the molecular interaction mechanisms of hemipteran insect OBPs to host plant odors.

Expression patterns and ligand binding characterization of Plus-C odorant-binding protein 14 from Adelphocoris lineolatus (Goeze)

Odorant-binding proteins (OBPs) can bind and transport hydrophobic odorants across the sensillum lymph to the olfactory receptors (ORs) and play crucial roles in insect chemosensory systems. Although the ligand spectra of classical OBPs have been extensively characterized, little is known about OBPs in the Plus-C subgroup. Here, we focus on AlinOBP14, a Plus-C OBP from the hemipteran mirid bug pest Adelphocoris lineolatus (Goeze). Quantitative real-time PCR experiments suggest that AlinOBP14 is ubiquitously expressed at different developmental stages but is highly expressed in the adult head, the non-chemosensory organ. Fluorescence-based competitive binding assays show that β-ionone, nerolidol, farnesol and insect juvenile hormone III (JHIII) strongly bind to AlinOBP14. No significant internal binding pocket is predicted by homology modeling. Instead, the long N-terminal and C-terminal regions and parts of several α-helixes form a cupped cavity to accommodate ligands. Molecular docking reveals that the four potential ligands have distinct binding orientations, implying different roles of the N-terminal extension in ligand recognition. This hypothesis is further confirmed via a ligand binding assay in which the recombinant N-terminal mutant AlinOBP14 displays comparable binding affinities for β-ionone and trans, trans-farnesol but decreased binding affinities for nerolidol and JHIII. Thus, our current study is the first to characterize the ligand binding spectra of a Plus-C OBP in hemipteran insect species and reveals that N-terminal extensions could be required for its recognition of putative ligands.

Expressional and functional comparisons of two general odorant binding proteins in Agrotis ipsilon

Insect general odorant binding proteins (GOBPs) have been long thought to bind and transport host plant volatiles to the olfactory receptors on the dendrite membrane of the olfactory neurons. Recent studies indicate that they can also bind female sex pheromones. In present study, two GOBP genes, AipsGOBP1 and AipsGOBP2 were cloned from the adult antennae of Agrotis ipsilon. Tissue expression profiles indicated that both of them are antennae-specific and more abundant in the female antennae than in the male antennae. Temporal expression profiles showed that both AipsGOBP1 and AipsGOBP2 began to express in antennae 3 days prior to adult emergence from pupae, and reached their highest expression level 3 and 4 days after adult emergence, respectively. Mating increased their expression in the female antennae but reduced their expression in the male antennae. In situ hybridization and immunolocalization demonstrated that both AipsGOBP1 and AipsGOBP2 are expressed and co-localized in sensilla basiconica and sensilla trichodea of both sexes. AipsGOBP2 exhibited a high binding affinity in vitro with the two major sex pheromone components Z7-12:Ac and Z9-14:Ac and the four plant volatiles cis-3-hexen-1-ol, oleic acid, dibutyl phthalate and β-caryophyllene with Ki values less than 5 μM. AipsGOBP1, on the other hand, showed medium binding affinities with the five A. ipsilon sex pheromones and six plant volatiles. AipsGOBP2 also showed a broader ligand-binding spectrum and a greater ligand-binding affinity than AipsGOBP1 with the tested aldehyde and alcohol sex pheromones of Lepidoptera species. Taken together, our results indicate that AipsGOBP2 may play greater roles than AipsGOBP1 does in binding sex pheromones and host plant volatiles.

Molecular and functional characterization of three odorant binding proteins from the oriental fruit moth Grapholita molesta (Busck) (Lepidoptera: Tortricide)

Odorant binding proteins (OBPs) act in recognizing odor molecules and their most well-studied functions are transporting odors across the sensillum lymph to olfactory receptor neurons within the insect antennal sensillum. The adults of Grapholita molesta highly depend on olfactory cues in locating host plants and selecting oviposition sites, in which OBPs play an important role in perceiving and recognizing host plant volatiles. Exploring the physiological function of OBPs could facilitate our understanding of their importance in insects' chemical communication. In this study, three OBP genes were cloned and named GmolOBP4, GmolOBP5, and GmolOBP10. Quantitative real-time PCR results indicated that GmolOBP4 and GmolOBP10 were predominantly expressed in adult antennae and GmolOBP5 was expressed in multiple tissues, including head, legs, and wings in addition to antennae. The binding affinities of the three recombinant GmolOBPs (rGmolOBPs) with four sex pheromone components and twenty-nine host plant volatiles were measured using 1-N-Phenyl-naphthylamine as a fluorescence probe. The three rGmolOBPs exhibited specific binding properties to potential ligands, GmolOBP4 and GmolOBP10 bound to minor sex pheromone components, such as (Z)-8-dodecenyl alcohol and dodecanol, respectively. rGmolOBP4 showed intermediate binding ability with hexanal, benzyl alcohol, and pear ester, rGmolOBP5 had a weak affinity for benzaldehyde, pear ester and, methyl jasmonate, and rGmolOBP10 showed strong binding capacity toward hexanol, decanol, and α-ocimene. We speculate that the GmolOBP4 and GmolOBP10 have dual functions in perception and recognition of host plant volatiles and sex pheromone components, while GmolOBP5 may serve other function(s).

Identification of Chemosensory Genes Based on the Transcriptomic Analysis of Six Different Chemosensory Organs in Spodoptera exigua

Insects have a complex chemosensory system that accurately perceives external chemicals and plays a pivotal role in many insect life activities. Thus, the study of the chemosensory mechanism has become an important research topic in entomology. Spodoptera exigua Hübner (Lepidoptera: Noctuidae) is a major agricultural polyphagous pest that causes significant agricultural economic losses worldwide. However, except for a few genes that have been discovered, its olfactory and gustatory mechanisms remain uncertain. In the present study, we acquired 144,479 unigenes of S. exigua by assembling 65.81 giga base reads from 6 chemosensory organs (female and male antennae, female and male proboscises, and female and male labial palps), and identified many differentially expressed genes in the gustatory and olfactory organs. Analysis of the transcriptome data obtained 159 putative chemosensory genes, including 24 odorant binding proteins (OBPs; 3 were new), 19 chemosensory proteins (4 were new), 64 odorant receptors (57 were new), 22 ionotropic receptors (16 were new), and 30 new gustatory receptors. Phylogenetic analyses of all genes and SexiGRs expression patterns using quantitative real-time polymerase chain reactions were investigated. Our results found that several of these genes had differential expression features in the olfactory organs compared to the gustatory organs that might play crucial roles in the chemosensory system of S. exigua, and could be utilized as targets for future functional studies to assist in the interpretation of the molecular mechanism of the system. They could also be used for developing novel behavioral disturbance agents to control the population of the moths in the future.

Various Bee Pheromones Binding Affinity, Exclusive Chemosensillar Localization, and Key Amino Acid Sites Reveal the Distinctive Characteristics of Odorant-Binding Protein 11 in the Eastern Honey Bee, Apis cerana

Odorant-binding proteins (OBPs) are the critical elements responsible for binding and transporting odors and pheromones in the sensitive olfactory system in insects. Honey bees are representative social insects that have complex odorants and pheromone communication systems relative to solitary insects. Here, we first cloned and characterized OBP11 (AcerOBP11), from the worker bees antennae of Eastern honey bee, Apis cerana. Based on sequence and phylogenetic analysis, most sequences homologous to AcerOBP11 belong to the typical OBPs family. The transcriptional expression profiles showed that AcerOBP11 was expressed throughout the developmental stages and highly specifically expressed in adult antennae. Using immunofluorescence localization, AcerOBP11 in worker bee's antennae was only localized in the sensilla basiconica (SB) near the fringe of each segment. Fluorescence ligand-binding assay showed that AcerOBP11 protein had strong binding affinity with the tested various bee pheromones components, including the main queen mandibular pheromones (QMPs), methyl p-hydroxybenzoate (HOB), and (E)-9-oxo-2-decanoic acid (9-ODA), alarm pheromone (n-hexanol), and worker pheromone components. AcerOBP11 also had strong binding affinity to plant volatiles, such as 4-Allylveratrole. Based on the docking and site-directed mutagenesis, two key amino acid residues (Ile97 and Ile140) were involved in the binding of AcerOBP11 to various bee pheromones. Taken together, we identified that AcerOBP11 was localized in a single type of antennal chemosensilla and had complex ligand-binding properties, which confer the dual-role with the primary characteristics of sensing various bee pheromones and secondary characteristics of sensing general odorants. This study not only prompts the theoretical basis of OBPs-mediated bee pheromones recognition of honey bee, but also extends the understanding of differences in pheromone communication between social and solitary insects.

Identification, Expression Patterns, and Functional Characterization of Chemosensory Proteins in Dendroctonus armandi (Coleoptera: Curculionidae: Scolytinae)

The Chinese white pine beetle, Dendroctonus armandi Tsai and Li (Coleoptera: Curculionidae: Scolytinae), is a serious pest of coniferous forests in China. Thus, there is considerable interest in developing eco-friendly pest-control methods, with the use of semiochemicals as a distinct possibility. Olfaction is extremely important for fitness of D. armandi because it is the primary mechanism through which the insect locates hosts and mates. Thus, here we characterized nine full-length genes encoding chemosensory proteins (CSPs) from D. armandi. The genes were ubiquitously and multiply expressed across different developmental stages and adult tissues, indicating various roles in developmental metamorphosis, olfaction, and gustation. Ligand-binding assays implied that DarmCSP2 may be the carrier of D. armandi pheromones and various plant host volatiles. These volatiles were identified through RNA interference of DarmCSP2 as: (+)-α-pinene, (+)-β-pinene, (−)-β-pinene, (+)-camphene, (+)-3-carene, and myrcene. The systematic chemosensory functional analysis of DarmCSP2 in this study clarified the molecular mechanisms underlying D. armandi olfaction and provided a theoretical foundation for eco-friendly pest control.

Silencing the Odorant Binding Protein RferOBP1768 Reduces the Strong Preference of Palm Weevil for the Major Aggregation Pheromone Compound Ferrugineol

In insects, perception of the environment-food, mates, and prey-is mainly guided by chemical signals. The dynamic process of signal perception involves transport to odorant receptors (ORs) by soluble secretory proteins, odorant binding proteins (OBPs), which form the first stage in the process of olfactory recognition and are analogous to lipocalin family proteins in vertebrates. Although OBPs involved in the transport of pheromones to ORs have been functionally identified in insects, there is to date no report for Coleoptera. Furthermore, there is a lack of information on olfactory perception and the molecular mechanism by which OBPs participate in the transport of aggregation pheromones. We focus on the red palm weevil (RPW) Rhynchophorus ferrugineus, the most devastating quarantine pest of palm trees worldwide. In this work, we constructed libraries of all OBPs and selected antenna-specific and highly expressed OBPs for silencing through RNA interference. Aggregation pheromone compounds, 4-methyl-5-nonanol (ferrugineol) and 4-methyl-5-nonanone (ferruginone), and a kairomone, ethyl acetate, were then sequentially presented to individual RPWs. The results showed that antenna-specific RferOBP1768 aids in the capture and transport of ferrugineol to ORs. Silencing of RferOBP1768, which is responsible for pheromone binding, significantly disrupted pheromone communication. Study of odorant perception in palm weevil is important because the availability of literature regarding the nature and role of olfactory signaling in this insect may reveal likely candidates representative of animal olfaction and, more generally, of molecular recognition. Knowledge of OBPs recognizing the specific pheromone ferrugineol will allow for designing biosensors for the detection of this key compound in weevil monitoring in date palm fields.

Chemosensory Gene Families in Ectropis grisescens and Candidates for Detection of Type-II Sex Pheromones

Tea grey geometrid (Ectropis grisescens), a devastating chewing pest in tea plantations throughout China, produces Type-II pheromone components. Little is known about the genes encoding proteins involved in the perception of Type-II sex pheromone components. To investigate the olfaction genes involved in E. grisescens sex pheromones and plant volatiles perception, we sequenced female and male antennae transcriptomes of E. grisescens. After assembly and annotation, we identified 153 candidate chemoreception genes in E. grisescens, including 40 odorant-binding proteins (OBPs), 30 chemosensory proteins (CSPs), 59 odorant receptors (ORs), and 24 ionotropic receptors (IRs). The results of phylogenetic, qPCR, and mRNA abundance analyses suggested that three candidate pheromone-binding proteins (EgriOBP2, 3, and 25), two candidate general odorant-binding proteins (EgriOBP1 and 29), six pheromone receptors (EgriOR24, 25, 28, 31, 37, and 44), and EgriCSP8 may be involved in the detection of Type-II sex pheromone components. Functional investigation by heterologous expression in Xenopus oocytes revealed that EgriOR31 was robustly tuned to the E. grisescens sex pheromone component (Z,Z,Z)-3,6,9-octadecatriene and weakly to the other sex pheromone component (Z,Z)-3,9-6,7-epoxyoctadecadiene. Our results represent a systematic functional analysis of the molecular mechanism of olfaction perception in E. grisescens with an emphasis on gene encoding proteins involved in perception of Type-II sex pheromones, and provide information that will be relevant to other Lepidoptera species.

Identification and Characterization of Odorant Binding Proteins in the Forelegs of Adelphocoris lineolatus (Goeze)

The chemosensory system is essential for insects to detect exogenous compounds, and odorant binding proteins (OBPs) play crucial roles in odorant binding and transduction. In the alfalfa plant bug Adelphocoris lineolatus, an important pest of multiple crops, our understanding of the physiological roles of antenna-biased OBPs has increased dramatically, whereas OBPs related to gustation have remained mostly unexplored. In this study, we employed RNA sequencing and RACE PCR methods to identify putative OBPs from the adult forelegs of both sexes. Eight candidate OBPs were identified, and three OBPs (AlinOBP15, 16, and 17) were novel. Full-length sequence alignment and phylogenetic analyses suggested that these three candidate OBPs had characteristics typical of the insect OBP family. AlinOBP16 and 17 displayed six highly conserved cysteines, placing them in the classic OBP subfamily, whereas AlinOBP15 resembled AlinOBP14 and clustered with the Plus-C clade. Quantitative real-time PCR (qRT-PCR) revealed distinct and significant tissue- and sex-biased expression patterns. AlinOBP15 was highly expressed in female heads, and AlinOBP16 and 17 were strongly expressed in female antennae. In particular, AlinOBP11, the most abundant OBP gene in our foreleg transcriptome dataset, was predominately expressed in adult legs. Furthermore, four types of sensilla hairs were observed on the forelegs of adult A. lineolatus, including sensilla trichodea, setae, and two types of sensilla chaetica (Sch1 and Sch2). Anti-AlinOBP11 antiserum strongly labeled the outer sensillum lymph of Sch2, implying that it has important gustatory functions in A. lineolatus. Our current findings provide evidence that OBPs can be functionally expressed in the tarsal gustatory sensilla of hemipteran mirid species, broadening our understanding of OBP chemosensory function in insects and facilitating the discovery of new functional targets for the regulation of insect host-searching behaviors.