CLONING, EXPRESSION, AND FUNCTIONAL ANALYSIS OF THREE ODORANT-BINDING PROTEINS OF THE ORIENTAL FRUIT MOTH, Grapholita molesta (BUSCK) (LEPIDOPTERA: TORTRICIDAE)

Functional analysis of Ophraella communa Lesage OcomOBP11 in recognition of Ambrosia artemisiifolia L. volatiles

Odorant binding proteins (OBPs) play a critical role in insect survival by facilitating the detection of environmental odors. This study identified and characterized OcomOBP11 in the leaf beetle Ophraella communa Lesage. Sequence and phylogenetic analyses demonstrated that OcomOBP11 belongs to the Minus-C OBP family. Tissue-specific expression analysis revealed that OcomOBP11 showed the highest relative expression in the testis. Functional assays demonstrated that OcomOBP11 has strong binding affinity to 24 of the 26 volatiles from Ambrosia artemisiifolia L., a primary host plant. Molecular docking showed that OcomOBP11 formed hydrophobic interactions with β-caryophyllene, α-pinene, camphene, octanal, and nonanal and hydrogen bonds with octanal and nonanal. RNAi-mediated knockdown of OcomOBP11 significantly reduced the electrophysiological (EAG) response and behavioral preference of O. communa for these ligands. These findings suggest that OcomOBP11 plays an important role in host plant localization, thus providing guidance for future biological invasion control strategies.

An odorant receptor mediates the avoidance of Plutella xylostella against parasitoid

BACKGROUND

Ecosystems are brimming with myriad compounds, including some at very low concentrations that are indispensable for insect survival and reproduction. Screening strategies for identifying active compounds are typically based on bioassay-guided approaches.

RESULTS

Here, we selected two candidate odorant receptors from a major pest of cruciferous plants-the diamondback moth Plutella xylostella-as targets to screen for active semiochemicals. One of these ORs, PxylOR16, exhibited a specific, sensitive response to heptanal, with both larvae and adult P. xylostella displaying heptanal avoidance behavior. Gene knockout studies based on CRISPR/Cas9 experimentally confirmed that PxylOR16 mediates this avoidance. Intriguingly, rather than being involved in P. xylostella-host plant interaction, we discovered that P. xylostella recognizes heptanal from the cuticular volatiles of the parasitoid wasp Cotesia vestalis, possibly to avoid parasitization.

CONCLUSIONS

Our study thus showcases how the deorphanization of odorant receptors can drive discoveries about their complex functions in mediating insect survival. We also demonstrate that the use of odorant receptors as a screening platform could be efficient in identifying new behavioral regulators for application in pest management.

The Odorant-Binding Protein 1 Mediates the Foraging Behavior of Grapholita molesta Larvae

Since eggs are laid directly on fruit skin, it is typically believed that food odor has little impact on the foraging of Grapholita molesta larvae. It is crucial to note that larvae that hatch on twigs and leaves could need some sort of identification system when foraging. Here, 22 GmolOBP genes were identified from the G. molesta larval transcriptome via the comparison of conserved domain and homology in the protein level. GmolOBP1 had strong affinities for important pear-fruit volatiles, which caused larvae strong behavioral responses. However, after GmolOBP1 silencing, the larvae lost their attraction to methyl salicylate, α-farnesene, butyl acetate, ethyl butanoate, and ethyl hexanoate, and the effects of larvae seeking various pears were significantly reduced. Consequently, GmolOBP1 was required for the reception of pear volatiles and was involved in mediating how G. molesta larvae foraged. Our research revealed the GmolOBP1 foraging signal recognition mechanism as well as potential molecular targets for field pest management.

Involvement of three chemosensory proteins in perception of host plant volatiles in the tea green leafhopper, Empoasca onukii

Chemosensory proteins (CSPs) can bind and transport odorant molecules, which are believed to be involved in insect chemoreception. Here, we investigated three CSPs in perception of volatiles in Empoasca onukii. Expression profiles showed that although EonuCSP4, EonuCSP 6-1 and EonuCSP6-2 were ubiquitously expressed in heads, legs, thoraxes and abdomen, they were all highly expressed in the antennae of E. onukii. Further, fluorescence competitive binding assays revealed that EonuCSP4 and 6-1 had binding affinities for three plant volatiles, suggesting their possible involvement in the chemosensory process. Among them, EonuCSP6-1 showed relatively high binding affinities for benzaldehyde. Behavioral assays revealed that the adults of E. onukii showed a significant preference for two compounds including benzaldehyde. The predicted three-dimensional (3D) structures of these 3 CSP have the typical six α-helices, which form the hydrophobic ligand-binding pocket. We therefore suggest that Eoun6-1 might be involved in the chemoreception of the host-related volatiles for E. onukii. Our data may provide a chance of finding a suitable antagonist of alternative control strategies which block the perception of chemosensory signals in pest, preventing the food- orientation behaviors.

Ligand-binding specificities of four odorant-binding proteins in Conogethes punctiferalis

Odorant-binding proteins (OBPs) play essential roles in lepidopteran insects' perception of host volatiles by binding and transporting hydrophobic ligands. The yellow peach moth (YPM), Conogethes punctiferalis (Guenée), is a serious agricultural pest, with broad host range and cryptic feeding habits. However, few studies about YPM perceiving pheromones and host plant odorants have been reported. In this study, four OBP genes (CpunOBP8, CpunOBP9, CpunABP, and CpunGOBP2) were cloned from the antennae of YPM. The recombinant proteins were expressed and purified by prokaryotic expression system, with their binding affinities to 26 ligands being tested. Four CpunOBPs all had six conserved cysteine residues, which were typical structural characteristics of classical OBPs. The fluorescence competitive binding assay indicated that CpunOBP8 and CpunABP could not only exhibit high binding affinities to female sex pheromones, but also to host plant odorants. For example, CpunOBP8 bound strongly with cis-10-hexadecenal, hexadecanal, and so forth, whereas CpunABP bound with cis-10-hexadecenal, camphene, and 3-carene. Comparatively, CpunOBP9 and CpunGOBP2 could only bind with host plant odorants, with CpunOBP9 binding strongly to 3-methyl-1-butanol, hexyl acetate, and so forth, while CpunGOBP2 displaying the widest binding spectra and correlating with 3-carene, pentyl acetate, and so forth. The results indicated that on the one hand, each of the four CpunOBPs had its specific binding spectra when binding and transporting olfactory ligands; on the other hand, the same ligand might be bound to more than one CpunOBPs, which would provide information for the potential application of semiochemicals in controlling YPM.

Functional analysis of the odorant receptor coreceptor in odor detection in Grapholita molesta (lepidoptera: Tortricidae)

The olfactory system must detect and discriminate various semiochemicals in the environment. In response to such diversity, insects have evolved a family of odorant-gated ion channels composed of a common receptor (coreceptor, Orco) and a ligand-binding tuning odorant receptor (OR) that confers odour specificity. This study aims to examine the expression pattern of Orco gene of Grapholita molesta (GmolOrco) and to elucidate the role of GmolOrco in detecting G. molesta sex pheromone and green leaf volatiles by using gene silencing via RNA interference (RNAi) coupled antennal electrophysiological (EAG). Multiple sequence alignment showed that GmolOrco shared high sequence similarities with the Orco ortholog of lepidopterans. The results of real-time quantitative PCR detection demonstrated that GmolOrco was predominantly expressed in adult antennae and had the highest expression quantity in adult period among the different developmental stages. Compared with the noninjected controls, GmolOrco expression in GmolOrcodouble-stranded RNA (dsRNA)-injected males was reduced to 39.92% and that in females was reduced to 40.43%. EAG assays showed that the responses of GmolOrco-dsRNA injected males to sex pheromones (Z)-8-dodecenyl acetate (Z8-12:OAc) and (Z)-8-dodecenyl alcohol (Z8-12:OH) were significantly reduced, and the GmolOrco-dsRNA-injected female to green leaf volatile (Z)-3-hexenyl acetate also significantly declined. We inferred that Orco-mediated olfaction was different in male and female G. molesta adults and was mainly involved in recognizing the sex pheromones released by female moths.

Molecular and Binding Characteristics of OBP5 of Bradysia odoriphaga (Diptera: Sciaridae)

Odorant-binding proteins (OBPs) capture and transport semiochemicals to olfactory receptors (OR) and function in the first step in insect olfaction. In the present study, we cloned a full-length cDNA sequence of BodoOBP5 from the insect pest Bradysia odoriphaga (Diptera: Sciaridae). Real-time PCR (qRT-PCR) analysis revealed that BodoOBP5 was expressed at higher levels in female adults than in other developmental stages. In the different tissues, BodoOBP5 was highly expressed in the female antennae, whereas low levels were expressed in the head and the male antennae, expression was negligible in other tissues. The recombinant protein of BodoOBP5 was successfully expressed with a bacterial system. Competitive binding assays with nine host plant volatiles and a putative sex pheromone revealed that purified BodoOBP5 strongly bound to two sulfur compounds (methyl allyl disulfide and diallyl disulfide); the corresponding dissolution constants (Ki) were 10.38 and 9.23 μM, respectively. Molecular docking indicated that Leu99, Leu103, Ala143, Tyr107, Phe142, and Trp144 in the hydrophobic cavity of BodoOBP5 are the key residues mediating the interaction of BodoOBP5 with methyl allyl disulfide and diallyl disulfide. RNAi-based Y-tube olfactometer assay indicated that there is no significant difference in methyl allyl disulfide and diallyl disulfide. The results of this study increase our understanding of the binding of BodoOBP5 with plant volatiles, facilitating the development of novel ways to control B. odoriphaga.

Identification of chemosensory genes by antennal transcriptome analysis and expression profiles of odorant-binding proteins in parasitoid wasp Aulacocentrum confusum

The endoparasitoid wasp, Aulacocentrum confusum (Hymenoptera: Braconidae), is a preponderant natural enemy of the larvae of Glyphodes pyloalis Walker (Lepidoptera: Pyralidae), which is a destructive pest of mulberry trees. We first constructed the antennal transcriptome database of A. confusum. In total, we obtained 48,262,304 clean reads from the dataset and assembled 24,324 unigenes. A total of 12,690 (52.17%) unigenes indicated significant similarity (E-value < 10^-5) compared to known protein sequences of other species from the NCBI non-redundant protein database. Gene ontology (GO) and cluster of orthologous groups (COG) analyses were used to determine the functional categories of these genes. A total of 84 putative chemosensory genes were identified from the antennal transcriptome of A. confusum, including 11 putative odorant-binding protein (OBP) genes, six chemosensory protein (CSP) genes, 44 olfactory receptor (OR) genes (including one olfactory co-receptor, Orco), 19 ionotropic receptor (IR) genes, and four sensory neuron membrane protein (SNMP) genes. Results of qPCR assays indicated that among of 11 AconOBPs, nine AconOBP genes were significantly expressed in the antennae of A. confusum adults. AconOBP8 was significantly expressed in the abdomen and AconOBP10 was highly expressed in the thorax. These findings can build a basis for further study on the processes of chemosensory perception in A. confusum at the molecular level.

Functional Characterization of Chemosensory Protein AmalCSP5 From Apple Buprestid Beetle, Agrilus mali (Coleoptera: Buprestidae)

In the sensitive and complex chemo-sensation system of insects, chemosensory proteins (CSPs) can facilitate the transfer of chemical information and play important roles for variable behaviors of insects. We cloned the chemosensory protein AmalCSP5 from antennae of the apple buprestid beetle (Agrilus mali Matsumura), a serious invasive pest of wild apple trees. Expression profiling showed that AmalCSP5 was expressed in various tissues, suggesting its significance in multiple physiological activities and behaviors of A. mali. AmalCSP5 was preferentially expressed in female antennae and male abdomens. AmalCSP5 was able to bind a variety of test volatiles, especially alcohols and esters. AmalCSP5 exhibited good binding affinity for all five test secondary compounds (i.e., procyanidin, phlorizin, kaemferol, chlorogenic acid, and rutin), suggesting its preferential binding abilities to nonvolatile host plant secondary metabolites and critical roles in gustatory perception of nonvolatiles. Tyr27 and Ser69 of AmalCSP5 could form hydrogen bonds with hexyl benzoate and hexyl hexanoate, respectively. Procyanidin, the best ligand among all test compounds, could form hydrogen bonds with three amino acid residues (i.e., Arg7, Leu8, and Lys41) of AmalCSP5. Thus, high ligand binding affinity for AmalCSP5 seemed to be dependent mainly on the formation of hydrogen bonds. The putative key amino acid residues of AmalCSP5 can be used as molecular targets for designing and screening new attractants and repellents for A. mali. Our results provide insights into binding interactions of AmalCSP5 with volatile and nonvolatile ligands, and a firm basis for developing eco-friendly management strategies of A. mali.

Functional Analysis of Odorant-Binding Proteins 12 and 17 from Wheat Blossom Midge Sitodiplosis mosellana Géhin (Diptera: Cecidomyiidae)

Simple Summary

Sitodiplosis mosellana is one of the most destructive pests of wheat. Adults rely highly on wheat spike volatiles to search and locate oviposition sites. Insect odorant-binding proteins (OBPs) are important in binding and transporting host plant volatiles to the olfactory receptors. Therefore, OBP-based behavioral interference is believed to be a novel and effective pest management strategy. The objectives of this study were to clone two S. mosellana female antenna-enriched OBP genes (SmosOBP12 and SmosOBP17), determine the functions of the encoded SmosOBP proteins in binding wheat volatiles, and investigate behavioral responses of female S. mosellana to odorant molecules. Results indicated that SmosOBP12 had a broader ligand-binding spectrum than SmosOBP17 to wheat volatiles. Female S. mosellana showed intensive response to 3-hexanol, 1-octen-3-ol, D-panthenol, 3-carene, (Z)-3-hexenylacetate, hexyl acetate, methyl salicylate, heptyl acetate, ethyl heptanoate, α-farnesene, and ocimene. Notably, all these compounds except α-farnesene exhibited strong affinity to SmosOBP12. In conclusion, SmosOBP12 may play more crucial roles than SmosOBP17 in perception and transportation of biologically active host volatiles. This information has enhanced our molecular understanding of the S. mosellana olfaction, which could also serve as an important reference for developing attractants or repellents to control this pest.

Abstract

The wheat blossom midge Sitodiplosis mosellana, one of the most disastrous wheat pests, depends highly on olfactory cues to track suitable plants. To better understand the olfactory recognition mechanisms involved in host selection, in the present study we cloned two S. mosellana adult antenna-specific odorant binding protein (OBP) genes, SmosOBP12 and SmosOBP17, and evaluated bacterially expressed recombinant proteins for their selectivity and sensitivity for host wheat volatiles using the fluorescence-based ligand binding assay. The results showed that both SmosOBPs effectively bound alcohol, ester, ketone, and terpenoid compounds. Particularly, SmosOBP12 had significantly higher affinities (K_i < 10.5 μM) than SmosOBP17 (K_i2 > 0.1 μM) to 3-hexanol, 1-octen-3-ol, D-panthenol, 3-carene, (Z)-3-hexenylacetate, hexyl acetate, methyl salicylate, heptyl acetate, and ethyl heptanoate. Consistently, S. mosellana females were attracted to all these chemicals in a behavioral assay using Y-tube olfactometer. SmosOBP12 also bound aldehyde, but neither bound alkanes. Notably, SmosOBP12 exhibited strong affinity to ocimene (K_i = 8.2 μM) that repelled S. mosellana. SmosOBP17, however, was insensitive to this compound. Taken together, our results indicate that SmosOBP12 may play a greater role than SmosOBP17 in perceiving these biologically active plant volatiles.

The general odorant receptor GmolOR9 from Grapholita molesta (Lepidoptera: Tortricidae) is mainly tuned to eight host-plant volatiles

Grapholita molesta is one of the most destructive fruit pests distributed worldwide. Odorant receptors (ORs) located on the dendritic membrane of chemosensory neurons are deemed to be key molecules for sensing exogenous chemical signals. In this study, GmolOR9, a general OR from G. molesta, was functionally characterized. Quantitative real-time polymerase chain reaction revealed that GmolOR9 was more highly expressed in adults than in other stages, including eggs, larvae, and pupae. GmolOR9 expression was highly significantly more in the antennae of females than in those of males, and the highest level occurred in the antennae of 3-day-old female adults. GmolOR9 was broadly tuned to eight of 47 odorant components tested, including (Z)-3-hexenyl acetate, butyl propionate, ethyl hexanoate, ethyl heptanoate, 1-hexanol, (Z)-3-hexenol, 2-ethyl-1-hexanol, and linalool, by in vitro heterologous expression. Furthermore, electroantennogram responses indicated that the effects of dsOR9-injected females to (Z)-3-hexenyl acetate dramatically decreased. These results suggested that GmolOR9 might be involved in detecting host-plant volatiles. Moreover, (Z)-3-hexenyl acetate might serve as a potential attractant for the biological control of G. molesta.

Molecular and functional characterization of three odorant-binding proteins from the wheat blossom midge, Sitodiplosis mosellana

Sitodiplosis mosellana, a periodic but devastating wheat pest, relies on wheat spike volatiles as a cue in selecting hosts for oviposition. Insect odorant-binding proteins (OBPs) are thought to play essential roles in filtering, binding and transporting hydrophobic odorant molecules to specific receptors. To date, the molecular mechanisms underlying S. mosellana olfaction are poorly understood. Here, three S. mosellana antenna-specific OBP genes, SmosOBP11, 16 and 21, were cloned and bacterially expressed. Binding properties of the recombinant proteins to 28 volatiles emitted from wheat spikes were investigated using fluorescence competitive binding assays. Sequence analysis suggested that these SmosOBPs belong to the Classic OBP subfamily. Ligand-binding analysis showed that all three SmosOBPs preferentially bound alcohol, ester and ketone compounds, and SmosOBP11 and 16 also selectively bound terpenoid compounds. In particular, the three SmosOBPs had high binding affinities (K_i < 20 μmol/L) to 3-hexanol and cis-3-hexenylacetate that elicited strong electroantennogram (EAG) response from female antennae. In addition, SmosOBP11 displayed significantly higher binding (K_i < 8 μmol/L) than SmosOBP16 and 21 to 1-octen-3-ol, D-panthenol, α-pinene and heptyl acetate which elicited significant EAG response, suggesting that SmosOBP11 plays a major role in recognition and transportation of these volatiles. These findings have provided important insight into the molecular mechanism by which S. mosellana specifically recognizes plant volatiles for host selection, and have facilitated identification of effective volatile attractants that are potentially useful for pest monitoring and trapping.

Functional Analysis of the Chemosensory Protein GmolCSP8 From the Oriental Fruit Moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae)

Chemosensory proteins (CSPs) belong to a family of small water-soluble proteins that can selectively bind and transport odorant molecules for olfactory communication in insects. To date, their definite physiological functions in olfaction remain controversial when compared with odorant binding proteins (OBPs). To investigate the functions of CSPs in the oriental fruit moth Grapholita molesta, we determined the tissue expression patterns and binding properties of the CSP, GmolCSP8. The key binding sites of GmolCSP8 with a representative ligand were evaluated using molecular flexible docking, site-directed mutagenesis and ligand-binding experiments. Multiple sequence alignment and phylogenetic analysis showed that GmolCSP8 possesses a typical conserved four cysteines motif and shares high sequence identity with some CSP members of other Lepidopteran insects. GmolCSP8 was predominantly expressed in the wings and antennae of both male and female adults and may be involve in contact chemoreception. Recombinant GmolCSP8 (rGmolCSP8) exhibited specific-binding affinities to small aliphatic alcohols (C4–12) and had the strongest binding affinity to 1-hexanol. The three-dimensional structure of GmolCSP8 was constructed using the structure of sgCSP4 as a template. Site-directed mutagenesis and ligand-binding experiments confirmed that Thr27 is the key binding site in GmolCSP8 for 1-hexanol binding, because this residue can form hydrogen bond with the oxygen atom of the hydroxyl group in 1-hexanol, and Leu30 may play an important role in binding to 1-hexanol. We found that pH significantly affected the binding affinities of rGmolCSP8 to ligand, revealing that ligand-binding and -release by this protein is related to a pH-dependent conformational transition. Based on these results, we infer that GmolCSP8 may participate in the recognition and transportation of 1-hexanol and other small aliphatic alcohols.

Structural Insights into Two Representative Conformations of the Complex Formed by Grapholita molesta (Busck) Pheromone Binding Protein 2 and Z-8-Dodecenyl Acetate

Grapholita molesta is a notorious fruit borer globally, causing severe damage to fruit production. To control the pest, one commonly used mean is pheromone-mediated management. As an important sex pheromone, Z-8-dodecenyl acetate (Z8-12: Ac), is often coformulated with other active ingredients to regulate the behavior of G. molesta. To uncover its interactions with G. molesta pheromone binding protein 2 (GmolPBP2) is used to help develop insect attractants. During 200 ns molecular dynamics simulations, two representative conformations of the GmolPBP2-Z8-12: Ac complex are selected. Conformation II at the time of 14-106 ns is dominantly maintained by the hydrophobic interactions and hydrogen bond. In Conformation I, which lasts from 106 to 200 ns, the hydrophobic interactions are enhanced while the hydrogen bond is quite weakened, due to the formation of a more sophisticated hydrophobic binding pocket and the enlargement of hydrogen bond distance. Taking the two conformations as a whole, the affinity between GmolPBP2 and Z8-12: Ac is crucially determined by three hot-spots including Phe11, Trp36, and Ile51. These results would provide a basis for the discovery, optimization, and design of leading compounds potentially active to attract G. molesta.

Molecular Identification, Expression, and Functional Analysis of a General Odorant-Binding Protein 1 of Asian Citrus Psyllid

For insects, odorant-binding proteins (OBPs) play an essential role in binding and transporting semiochemicals through the sensillum lymph to olfactory receptor neurons within the antennal sensilla. In the present study, the full-length cDNA encoding a general odorant-binding protein 1 (DcitOBP1, accession number KY475564) was cloned from the antennae of Diaphorina citri using RACE-PCR, and qRT-PCR analysis revealed that the DcitOBP1 gene was expressed mainly in the antennae of D. citri. In molecular docking assay, the results showed that DcitOBP1 protein has better binding affinities to the 12 selected host-plant volatile compounds. Then, the recombinant DcitOBP1 protein was expressed in Escherichia coli. After removed His-Tag, the binding properties of purified DcitOBP1 protein to the selected host-plant volatile compounds were investigated in a fluorescence ligand-binding assay, similar, but more obviously binding properties of DcitOBP1 protein result were obtained, the dissociation constant (KD) value of DcitOBP1/1-NPN complex was 6.440 ± 0.521, and the DcitOBP1 protein showed high binding affinities (IC50 < 100 μM) to six of the selected ligands, namely methyl salicylate, α-phellandrene, (1R)-(+)-α-pinene, 3-carene, β-caryophyllene, and α-caryophyllene. Additionally, the behavior bioassays were also showed that D. citri had significant behavioral responses toward to α-caryophyllene, β-caryophyllene, (1R)-(+)-α-pinene, and α-phellandrene. Our investigation infer that the DcitOBP1 protein might play a crucial role in host-plant volatile odorants' perception in D. citri, and these results also have been supplied previous insight evidence into the physiological functions of the DcitOBP1 protein of D. citri.

Molecular and Functional Characterization of Odorant Binding Protein 7 From the Oriental Fruit Moth Grapholita molesta (Busck) (Lepidoptera: Tortricidae)

Odorant-binding proteins (OBPs) are widely and abundantly distributed in the insect sensillar lymph and are essential for insect olfactory processes. The OBPs can capture and transfer odor molecules across the sensillum lymph to odorant receptors and trigger the signal transduction pathway. In this study, a putative OBP gene, GmolOBP7, was cloned using specific-primers, based on the annotated unigene which forms the antennal transcriptome of Grapholita molesta. Real-time PCR (qRT-PCR) analysis revealed that GmolOBP7 was highly expressed in the wings of males and the antennae of both male and female adult moths, while low levels were expressed in other tissues. The recombinant GmolOBP7 (rGmolOBP7) was successfully expressed and purified via Ni-ion affinity chromatography. The results of binding assays revealed that rGmolOBP7 exhibited a high binding affinity to the minor sex pheromone 1-dodecanol containing K_i of 7.48 μM and had high binding capacities to the host-plant volatiles, such as pear ester, lauraldehyde and α-ocimene. RNA-interference experiments were performed to further assess the function of GmolOBP7. qRT-PCR showed that the levels of mRNA transcripts significantly declined in 1 and 2 day old male and female moths, treated with GmolOBP7 dsRNA, compared with non-injection controls. The EAG responses of dsRNA-injected males and females to pear ester, as well as the EAG responses of dsRNA-injected males to 1-dodecanol, were significantly reduced compared to the GFP-dsRNA-injected and non-injected controls. We therefore infer that GmolOBP7 has a dual function in the perception and recognition of the host-plant volatiles and sex pheromones.

Degradation of sex pheromone and plant volatile components by an antennal glutathione S-transferase in the oriental fruit moth,Grapholita molesta Busck (Lepidoptera: Tortricidae)

Insect antennae have a primary function of perceiving and discerning odorant molecules including sex pheromones and host plant volatiles. The assumption that genes highly expressed in the antennae may have an olfactory-related role associated with signal transduction. Here, one delta subfamily glutathione S-transferase (GST) gene (GmolGSTD1) was obtained from an antennal transcriptome of Grapholita molesta. Quantitative real-time polymerase chain reaction results revealed that GmolGSTD1 was mainly expressed in antennae and the expression levels were significantly higher in female antennae than in male antennae. The recombinant GmolGSTD1 (rGmolGSTD1) showed glutathione-conjugating activity toward 1-chloro-2,4-dinitrobenzene (CDNB) as substrates. The pH range for optimal rGmolGSTD1 enzyme activity was 6.0-6.5, and rGmolGSTD1 enzyme activity had maximal peaks at 35-40°C. Spectrophotometric analysis indicated that insecticides had weak inhibitory effects on the activity of rGmolGSTD1 with the inhibitory rates of 28.82% for chlorpyrifos, 22.27% for lambda-cyhalothrin, 18.07% for bifenthrin, 20.42% for acetamiprid, 17.57% for thiamethoxam, 25.67% for metaflumizone, 27.43% for abamectin, and 7.24% for chlorbenzuron. rGmolGSTD1 exhibited high degradation activity to the sex pheromone component (Z)-8-dodecenyl alcohol and the host plant volatile butyl hexanoate with the degradation efficiency of 75.01% and 48.54%, respectively. We speculate that GmolGSTD1 works in inactivating odorant molecules and maintaining sensitivity to olfactory communication of G. molesta.

Distinct binding affinities of odorant-binding proteins from the natural predator Chrysoperla sinica suggest different strategies to hunt prey

Chrysoperla sinica is an important natural predator of many notorious agricultural pests. Understanding its olfactory mechanism can help enhance the effectiveness of C. sinica in biological control. In the present study, we investigated the tissue expression patterns of 12 odorant-binding protein (OBP) genes from C. sinica (CsinOBPs). The results of quantitative real-time polymerase chain reaction (qPCR) showed that CsinOBP1, CsinOBP2, CsinOBP3, CsinOBP4, CsinOBP6, CsinOBP7, CsinOBP9, CsinOBP10, and CsinOBP12 were predominantly expressed in the antennae of both sexes, indicating their roles in olfaction. Additionally, the qPCR analysis revealed that the 12 CsinOBP genes had distinct expression patterns, while the motif-pattern investigation suggested that the OBPs had different ligands. The ligand-binding assay showed that CsinOBP1 and CsinOBP10 had broader binding spectra than did the other OBPs. Thus, CsinOBP1 was able to bind not only plant volatiles (such as farnesol, cis-3-hexenyl hexanoate, geranylacetone, β-ionone, 2-tridecanone, and trans-nerolidol) but also the aphid alarm pheromone (E)-β-farnesene. On the other hand, CsinOBP2 and CsinOBP6 exhibited relatively narrow binding spectra, only binding ethyl benzoate. The study also identified several compounds that can potentially be used to develop slow-release agents attracting C. sinica and to improve search strategies for insect pest control.

Odorant Receptors and Odorant-Binding Proteins as Insect Pest Control Targets: A Comparative Analysis

Recently, two alternative targets in insect periphery nerve system have been explored for environmentally-friendly approaches in insect pest management, namely odorant-binding proteins (OBPs) and odorant receptors (ORs). Located in insect antennae, OBPs are thought to be involved in the transport of odorants to ORs for the specific signal transduction of behaviorally active odorants. There is rich information on OBP binding affinity and molecular docking to bioactive compounds as well as ample 3D crystal structures due to feasible production of recombinant proteins. Although these provide excellent opportunities for them to be considered as pest control targets and a tool to design pest control agents, the debates on their binding specificity represent an obstacle. On the other hand, ORs have recently been functionally characterized with increasing evidence for their specificity, sensitivity and functional roles in pest behaviors. However, a major barrier to use ORs for semiochemical discovery is the lack of 3D crystal structures. Thus, OBPs and ORs have not been analyzed comparatively together so far for their feasibility as pest control targets. Here, we summarize the state of OBPs and ORs research in terms of its application in insect pest management. We discuss the suitability of both proteins as pest control targets and their selection toward the discovery of new potent semiochemicals. We argue that both proteins represent promising targets for pest control and can be used to identify new super-ligands likely present in nature and with reduced risk of resistance development than insect pesticides currently used in agriculture. We discuss that with the massive identification of OBPs through RNA-seq and improved binding affinity measurements, these proteins could be reconsidered as suitable targets for semiochemical discovery.

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).

Expression Profiles and Functional Characterization of Two Odorant-Binding Proteins From the Apple Buprestid Beetle Agrilus mali (Coleoptera: Buprestidae)

The apple buprestid beetle, Agrilus mali Matsumura (Coleoptera: Buprestidae), can respond to various volatiles, but the underlying mechanism of odorant perception for this insect is poorly understood. Here, we cloned A. mali's odorant-binding proteins 3 (AmalOBP3) and 8 (AmalOBP8) and characterized their expression patterns and binding profiles. Sequence and phylogenetic analyses showed that AmalOBP3 and AmalOBP8 were distributed in the classic and minus-C OBP subfamily, respectively. AmalOBP3 was specifically and abundantly expressed in antennae of both sexes. AmalOBP8 displayed high transcript levels in antennae of both sexes, abdomens of males, and wings of both sexes. Both AmalOBPs exhibited much higher expression in male antennae than in female antennae, suggesting that they could be important in perception of male-specific olfactory cues (e.g., some sex pheromones). Out of the 40 odorant ligands tested, AmalOBP3 and AmalOBP8 bound to 15 and 21 different odorants, respectively, indicating a distinct and selective binding profile for them. Both AmalOBPs seemed to have very strong binding affinity to aliphatic alcohols and aldehydes with 12 to 15 carbon atoms. Alcohols, esters, and terpenoids were more likely to be good ligands for both AmalOBPs than aldehydes and alkanes. Together with its broad expression in different tissues, strong binding with higher numbers of putative ligands for AmalOPB8 means that this protein can have more extensive functional roles in chemosensation of A. mali. Our results provide insights into the molecular basis of chemosensation in A. mali, as well as a basis for developing detection, monitoring, and management tools for this serious pest.

De novo characterization of microRNAs in oriental fruit moth Grapholita molesta and selection of reference genes for normalization of microRNA expression

MicroRNAs (miRNAs) are a group of endogenous non-coding small RNAs that have critical regulatory functions in almost all known biological processes at the post-transcriptional level in a variety of organisms. The oriental fruit moth Grapholita molesta is one of the most serious pests in orchards worldwide and threatens the production of Rosacea fruits. In this study, a de novo small RNA library constructed from mixed stages of G. molesta was sequenced through Illumina sequencing platform and a total of 536 mature miRNAs consisting of 291 conserved and 245 novel miRNAs were identified. Most of the conserved and novel miRNAs were detected with moderate abundance. The miRNAs in the same cluster normally showed correlated expressional profiles. A comparative analysis of the 79 conserved miRNA families within 31 arthropod species indicated that these miRNA families were more conserved among insects and within orders of closer phylogenetic relationships. The KEGG pathway analysis and network prediction of target genes indicated that the complex composed of miRNAs, clock genes and developmental regulation genes may play vital roles to regulate the developmental circadian rhythm of G. molesta. Furthermore, based on the sRNA library of G. molesta, suitable reference genes were selected and validated for study of miRNA transcriptional profile in G. molesta under two biotic and six abiotic experimental conditions. This study systematically documented the miRNA profile in G. molesta, which could lay a foundation for further understanding of the regulatory roles of miRNAs in the development and metabolism in this pest and might also suggest clues to the development of genetic-based techniques for agricultural pest control.

DIFFERENTIAL PROTEOME ANALYSIS OF THE MALE AND FEMALE ANTENNAE FROM Holotrichia parallela

To understand the olfactory mechanisms of Holotrichia parallela antennae in detecting volatile compounds in the environment, protein profiles of H. parallela antennae were analyzed using two-dimensional electrophoresis followed by mass spectrometry and bioinformatics analyses. Approximately 1,100 protein spots in silver staining gel were detected. Quantitative image analysis revealed that in total 47 protein spots showed significant changes in different genders of adult antennae. Thirty-five differentially expressed proteins were identified by Matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF/TOF) tandem mass spectrometer, among which 65.7% are involved in carbohydrate and energy metabolism, antioxidant system, transport, and amino acid/nucleotide metabolism. Some proteins identified here have not been reported previously in insect antennae. Identified male-biased proteins included odorant-binding protein 4, pheromone-binding protein-related protein 2, odorant-binding protein 14, prophenoloxidase-I, acyl-CoA dehydrogenase, aldo-keto reductase-like, carbamoyl phosphate synthetase, etc. whereas some proteins are female biased, such as antennae-rich cytochrome P450, aldehyde dehydrogenase, and putative glutamine synthetase. Alterations in the levels of some proteins were further confirmed by real time polymerase chain reaction (RT-PCR). The proteomic resources displayed here are valuable for the discovery of proteins from H. parallela antennae.

Binding Properties of General Odorant Binding Proteins from the Oriental Fruit Moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae)

Background

The oriental fruit moth Grapholita molesta is a host-switching pest species. The adults highly depend on olfactory cues in locating optimal host plants and oviposition sites. Odorant binding proteins (OBPs) are thought to be responsible for recognizing and transporting hydrophobic odorants across the aqueous sensillum lymph to stimulate the odorant receptors (ORs) within the antennal sensilla and activate the olfactory signal transduction pathway. Exploring the physiological function of these OBPs could facilitate understanding insect chemical communications.

Methodology/Principal Finding

Two antennae-specific general OBPs (GOBPs) of G. molesta were expressed and purified in vitro. The binding affinities of G. molesta GOBP1 and 2 (GmolGOBP1 and 2) for sex pheromone components and host plant volatiles were measured by fluorescence ligand-binding assays. The distribution of GmolGOBP1 and 2 in the antennal sensillum were defined by whole mount fluorescence immunohistochemistry (WM-FIHC) experiments. The binding sites of GmolGOBP2 were predicted using homology modeling, molecular docking and site-directed mutagenesis. Both GmolGOBP1 and 2 are housing in sensilla basiconica and with no differences in male and female antennae. Recombinant GmolGOBP1 (rGmolGOBP1) exhibited broad binding properties towards host plant volatiles and sex pheromone components; rGmolGOBP2 could not effectively bind host plant volatiles but showed specific binding affinity with a minor sex pheromone component dodecanol. We chose GmolGOBP2 and dodecanol for further homology modeling, molecular docking, and site-directed mutagenesis. Binding affinities of mutants demonstrated that Thr9 was the key binding site and confirmed dodecanol bonding to protein involves a hydrogen bond. Combined with the pH effect on binding affinities of rGmolGOBP2, ligand binding and release of GmolGOBP2 were related to a pH-dependent conformational transition.

Conclusion

Two rGmolGOBPs exhibit different binding characteristics for tested ligands. rGmolGOBP1 has dual functions in recognition of host plant volatiles and sex pheromone components, while rGmolGOBP2 is mainly involved in minor sex pheromone component dodecanol perception. This study also provides empirical evidence for the predicted functions of key amino acids in recombinant protein ligand-binding characteristics.