Host plant odours and their recognition by the odourant-binding proteins of Diaphorina citri Kuwayama (Hemiptera: Psyllidae)

AmelOBP4: an antenna-specific odor-binding protein gene required for olfactory behavior in the honey bee (Apis mellifera)

BACKGROUND

Odorant binding proteins (OBPs) initiate the process of odorant perception. Numerous investigations have demonstrated that OBPs bind a broad variety of chemicals and are more likely to carry pheromones or odor molecules with high binding affinities. However, few studies have investigated its effects on insect behavior. Previously, we found that AmelOBP4 has a significantly higher expression in the heads of foragers than that of nurses regardless of their ages, revealing its importance in foraging behaviour of the honey bee. RNA interference (RNAi) is the induction of sequence specific gene silencing by double-stranded RNA (dsRNA), it is a powerful tool that makes gene inactivation possible in organisms that were not amenable to genetic analysis before.

RESULTS

In this study, we found that AmelOBP4 had high expression levels in the antennae of both nurses and foragers, and could be successfully inhibited by feeding double stranded RNA of AmelOBP4 (dsAmelOBP4). Foragers with inhibited AmelOBP4 showed significantly lower sugar responsiveness than control bees, and also significantly reduced EAG response to plant volatiles of nonanal, linalool and 1-Octen-3ol. On the other hand, nurses with inhibited AmelOBP4 showed significantly reduced EAG response to brood pheromone of ethyl oleate, methyl linoleate, methyl palmitate and β-ocimene. Finally, the Y-tube choice assay showed nurses only exhibited a significantly reduced preference to ethyl oleate, but foragers exhibited significantly reduced preference to all these three plant volatiles.

CONCLUSIONS

The findings of our study suggested that AmelOBP4 plays an important role in the odorant binding process, especially in modulating olfactory behaviour in workers. Our results provide a foundation for exploring the olfactory mechanism of Apis mellifera.

Comparative transcriptomics of a generalist aphid, Myzus persicae and a specialist aphid, Lipaphis erysimi reveals molecular signatures associated with diversity of their feeding behaviour and other attributes

Introduction

Aphids are phloem sap-sucking insects and are a serious destructive pest of several crop plants. Aphids are categorized as "generalists" or "specialists" depending on their host range. Myzus persicae (Sulz.) is a generalist aphid with a broad host range while Lipaphis erysimi (Kalt.), a specialist aphid, has a narrow host range. Aphid infestation involves several sequential stages including host recognition and selection, overcoming primary plant defence barriers, feeding on phloem sap and detoxification of host defence responses. Information on the molecular basis of variations between generalist and specialist aphids with reference to the above processes is limited.

Methods

In the current study, we generated transcriptome data of M. persicae and L. erysimi from adult and nymph stages and analysed the differential expression of genes between adults of the generalist and specialist aphid and similarly, between nymphs of the two aphid species. We categorized these differentially expressed genes into nine different categories namely, chemosensation-related, plant cell wall degrading enzymes, detoxification-related, digestive enzymes, peptidases, carbohydrate-, lipid-, amino acid-metabolism and reproduction. We also identified putative effector molecules in both M. persicae and L. erysimi from the transcriptome data.

Results and discussion

Gene expression analysis identified 7688 and 8194 differentially expressed unigenes at adult and nymph stages, respectively of M. persicae and L. erysimi. M. persicae showed significantly higher levels of expression in a greater number of unigenes (5112 in adults and 5880 in nymphs) in contrast to the specialist, L. erysimi (2576 in adults and 2314 in nymphs) in both developmental stages. In addition, M. persicae displayed a greater number (350 in adults and 331 in nymphs) of upregulated unigenes involved in important processes such as host recognition, plant cell wall degradation, detoxification, digestion and metabolism, which correlate with its dynamic and polyphagous nature in contrast to the specialist (337 in adults and 251 in nymphs). We also observed a greater number of putative effectors in M. persicae (948 in adults and 283 in nymphs) than L. erysimi (797 in adults and 245 in nymphs). Based on our analysis, we conclude that the generalist aphid, M. persicae has a more diversified and stronger arsenal of genes that influence its polyphagous feeding behaviour and effective response to plant defence mechanisms against insect-herbivory. Our study provides a compendium of such candidate genes that would be most useful in studies on aphid biology, evolution and control.

Citrus psyllid management by collective involvement of plant resistance, natural enemies and entomopathogenic fungi

Crops face constant threats from insect pests, which can lead to sudden disasters and global famine. One of the most dangerous pests is the Asian citrus psyllid (ACP), which poses a significant threat to citrus plantations worldwide. Effective and adaptive management strategies to combat ACP are always in demand. Plant resistance (PR) is a key element in pest management, playing crucial roles such as deterring pests through antifeedant and repellant properties, while also attracting natural enemies of these pests. One effective and innovative approach is the use of entomopathogenic fungi (EPF) to reduce pest populations. Additionally, other natural enemies play an important role in controlling certain insect pests. Given the significance of PR, EPF, and natural arthropod enemies (NAE), this review highlights the benefits of these strategies against ACP, drawing on successful examples from recent research. Furthermore, we discuss how EPF can be effectively utilized in citrus orchards, proposing strategies to ensure its efficient use and safeguard food security in the future.

Three odorant-binding proteins of small hive beetles, Aethina tumida, participate in the response of bee colony volatiles

Aethina tumida (small hive beetle, SHB) is a rapidly spreading invasive parasite of bee colonies. The olfactory system plays a key role in insect behavior, and odorant-binding proteins (OBPs) are involved in the first step of the olfactory signal transduction pathway and the detection of host volatiles. However, the olfactory mechanism of OBPs in SHB-localized bee colonies is unclear. In this study, electroantennogram (EAG) and behavioral bioassay showed that only three compounds (2-heptanone, ocimene, and ethyl palmitate) from bee colonies triggered high electrophysiological and behavioral responses. Three antenna-specific OBP genes (OBP6, OBP11, and OBP19) were identified, and they were significantly expressed on adult days 6-7. Furthermore, by combining RNA interference (RNAi) with EAG, olfactometer bioassay, competitive fluorescence binding assays, and molecular docking, we found that these three OBP genes were involved in the recognition of 2-heptanone and ethyl palmitate, and AtumOBP6 is also involved in the recognition of ocimene. These data indicate that AtumOBP6, AtumOBP11, and AtumOBP19 play an important role in the olfactory response to bee colony volatiles. Our results provide new insights into the functions of the OBP families in A. tumida and help to explore more potential target genes for environmentally friendly pest control strategies.

Association of Non-host Crop Plants with Mandarin in Host Location and Survival of Diaphorina citri Kuwayama (Hemiptera: Psyllidae)

Research efforts have been made to develop novel tactics, such as those targeting behavioral control, for management of the Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Psyllidae), vector of the causal agent of citrus Huanglongbing. Here, we investigated whether association of "Ponkan" mandarin (Citrus reticulata) with volatiles from non-host crops: avocado, passion fruit or coffee, alters host location by the Asian citrus psyllid; and whether they can be temporary hosts for the Asian citrus psyllid. In wind tunnel assays, we found that the association of mandarin seedling with avocado plant volatiles reduced in 30% the number of psyllids sitting on host plants compared to the mandarin alone. In contrast, passion fruit plant volatiles facilitated host location by psyllids, which found mandarin seedlings faster than when exposed to mandarin alone. The association with coffee volatiles did not alter the attractiveness of mandarin to the Asian citrus psyllid. Survival and half-lethal time (LT50) of D. citri fed on non-host plants were longer than those insects with water only, but shorter than those fed on mandarin. Among the non-host plants, D. citri performed better in coffee, followed by avocado and passion fruit plants. Our results indicate that the association of mandarin with avocado plant can be beneficial for Asian citrus psyllid management.

Exploring the functional profiles of odorant binding proteins crucial for sensing key odorants in the new leaves of coconut palms in Rhynchophorus ferrugineus

The red palm weevil (RPW), Rhynchophorus ferrugineus (Curculionidae: Coleoptera) is a highly destructive global pest of coconut trees, with a preference for laying its eggs on new leaves. Females can identify where to lay eggs by using their sense of smell to detect specific odorants found in new leaves. In this study, we focused on the two odorants commonly found in new leaves by GC-MS: trans, trans-2,4-nonadienal and trans-2-nonenal. Our behavioral assays demonstrated a significant attraction of females to both of these odorants, with their electrophysiological responses being dose-dependent. Furthermore, we examined the expression patterns induced by these odorants in eleven RferOBP genes. Among them, RferOBP3 and RferOBP1768 exhibited the most significant and simultaneous upregulation. To further understand the role of these two genes, we conducted experiments with females injected with OBP-dsRNA. This resulted in a significant decrease in the expression of RferOBP3 and RferOBP1768, as well as impaired the perception of the two odorants. A fluorescence competitive binding assay also showed that both RferOBPs strongly bound to the odorants. Additionally, sequence analysis revealed that these two RferOBPs belong to the Minus-C family and possess four conserved cysteines. Molecular docking simulations showed strong interactions between these two RferOBPs and the odorant molecules. Overall, our findings highlight the crucial role of RferOBP3 and RferOBP1768 in the olfactory perception of the key odorants in coconut palm new leaves. This knowledge significantly improves our understanding of how RPW females locate sites for oviposition and lays the foundation for future research on the development of environmentally friendly pest attractants.

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.

Volatile Signals From Guava Plants Prime Defense Signaling and Increase Jasmonate-Dependent Herbivore Resistance in Neighboring Citrus Plants

Intercropping can reduce agricultural pest incidence and represents an important sustainable alternative to conventional pest control methods. Citrus intercropped with guava (Psidium guajava L.) has a lower incidence of Asian citrus psyllid (ACP, Diaphorina citri Kuwayama) and huanglongbing disease (HLB), but the mechanisms are still unknown. In this study, we tested whether volatile organic compounds (VOCs) emitted by guava plants play a role in plant-plant communications and trigger defense responses in sweet orange (Citrus sinensis L. Osbeck) in the laboratory. The results showed that the behavioral preference and developmental performance of ACP on citrus plants that were exposed to guava VOCs were suppressed. The expression of defense-related pathways involved in early signaling, jasmonate (JA) biosynthesis, protease inhibitor (PI), terpenoid, phenylpropanoid, and flavonoid biosynthesis was induced in guava VOC-exposed citrus plants. Headspace analysis revealed that guava plants constitutively emit high levels of (E)-β-caryophyllene and (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), which can induce the accumulation of JA and promote stronger defense responses of citrus to ACP feeding. In addition, exposure to guava VOCs also increased the indirect defense of citrus by attracting the parasitic wasp Tamarixia radiata. Together, our findings indicate that citrus plants can eavesdrop on the VOC cues emitted by neighboring intact guava plants to boost their JA-dependent anti-herbivore activities. The knowledge gained from this study will provide mechanisms underlying citrus-guava intercropping for the ecological management of insect pests.

Metabolomic analysis elucidates how shade conditions ameliorate the deleterious effects of greening (Huanglongbing) disease in citrus

Under tropical and subtropical environments, citrus leaves are exposed to excess sunlight, inducing photoinhibition. Huanglongbing (HLB, citrus greening), a devastating phloem-limited disease putatively caused by Candidatus Liberibacter asiaticus, exacerbates this challenge with additional photosynthetic loss and excessive starch accumulation. A combined metabolomics and physiological approach was used to elucidate whether shade alleviates the deleterious effects of HLB in field-grown citrus trees, and to understand the underlying metabolic mechanisms related to shade-induced morpho-physiological changes in citrus. Using metabolite profiling and multinomial logistic regression, we identified pivotal metabolites altered in response to shade. A core metabolic network associated with shade conditions was identified through pathway enrichment analysis and metabolite mapping. We measured physio-biochemical responses and growth and yield characteristics. With these, the relationships between metabolic network and the variables measured above were investigated. We found that moderate-shade alleviates sink limitation by preventing excessive starch accumulation and increasing foliar sucrose levels. Increased growth and fruit yield in shaded compared with non-shaded trees were associated with increased photosystem II efficiency and leaf carbon fixation pathway metabolites. Our study also shows that, in HLB-affected trees under shade, the signaling of plant hormones (auxins and cytokinins) and nitrogen supply were downregulated with reducing new shoot production likely due to diminished needs of cell damage repair and tissue regeneration under shade. Overall, our findings provide the first glimpse of the complex dynamics between cellular metabolites and leaf physiological functions in citrus HLB pathosystem under shade, and reveal the mechanistic basis of how shade ameliorates HLB disease.

An odorant-binding protein of Asian citrus psyllid, Diaphorina citri, participates in the response of host plant volatiles

BACKGROUND

Odorant-binding proteins (OBPs) in insects contribute to the sensitivity of the olfactory system and connect external odorants to olfactory receptor neurons. Determination of the chemosensory functions in Diaphorina citri, a vector of the citrus Huanglongbing pathogen, may help in developing a potential target for pest management.

RESULTS

Diaphorina citri showed dose-dependent electroantennogram recording (EAG) responses to 12 host plant volatiles. A two-choice behavioral trap experiment showed that four compounds (methyl salicylate, linalool, citral and R-(+)-limonene) that elicited high EAG responses also had significant attraction to adults. The expression profiles induced by these compounds were detected in nine OBP genes, DcitOBP1-9. DcitOBP3, DcitOBP6 and DcitOBP7 commonly showed significant upregulation or downregulation compared with the control. Microscale thermophoresis (MST) showed that the recombinant protein DcitOBP7 had high in vitro binding affinities (K_d  < 10 μm) to methyl salicylate, linalool and R-(+)-limonene, and moderate binding affinity to citral with a K_d value of 15.95 μm. Furthermore, RNA interference (RNAi)-suppressed messenger RNA (mRNA) expression of DcitOBP7 resulted in a significant reduction in EAG activity and in adult D. citri behavioral responses to tested volatiles and the preferred host, Murraya paniculata. The hydrophilic residue Arg107 of DcitOBP7 may have a key role in binding odorants via formation of hydrogen bonds.

CONCLUSION

These results show that DcitOBP7 plays an important role in the olfactory response. This finding may provide new insight into the functions of OBP families in D. citri and aid in the development of safe strategies for managing D. citri populations. © 2021 Society of Chemical Industry.

Discovery of behaviorally active semiochemicals in Aenasius bambawalei using a reverse chemical ecology approach

BACKGROUND

The invasive mealybug, Phenacoccus solenopsis, has caused serious damage to cotton crops throughout the world. Aenasius bambawalei is a dominant endoparasitoid of P. solenopsis. Exploration of behaviorally active semiochemicals may promote the efficacy of parasitoids used in biological control. Reverse chemical ecology, based on the physiological function of odorant-binding proteins (OBPs), provides an effective approach to screen behaviorally active compounds to target insect pests. Determination of the binding mechanisms and specificity towards different odorants in A. bambawalei may facilitate the development of more-efficient biological control strategies.

RESULTS

We characterized the expression profile and analyzed the binding affinity of OBP28 in A. bambawalei. AbamOBP28 showed high expression in the wings and antennae of both male and female A. bambawalei. A fluorescence competitive binding assay indicated that AbamOBP28 displayed strong binding affinity to most candidate ligands. Circular dichroism spectra demonstrated that 1-octen-3-one, myrcene, dodecane, 2,4,4-trimethyl-2-pentene, nonanal, and limonene elicited conformational changes in AbamOBP28. Electrophysiological and behavioral bioassays revealed that diethyl sebacate, 2,4,4-trimethyl-2-pentene, and 1-octen-3-one evoked significant electroantennography responses and functioned as attractants in A. bambawalei at specific concentrations. Furthermore, three-dimensional structure modeling and molecular docking showed that hydrogen bonds were formed by Glu1 and Ser75 of AbamOBP28 with diethyl sebacate, respectively.

CONCLUSION

These results demonstrate that AbamOBP28 is involved in the chemoreception of A. bambawalei. The identified protein provides a potential target for efficient enemy utilization and pest control, and the overall results may help develop protocols for more effective screening of behaviorally active semiochemicals. © 2021 Society of Chemical Industry.

Characterization of the binding ability of the odorant binding protein BminOBP9 of Bactrocera minax to citrus volatiles

BACKGROUND

Bactrocera minax, one of the most important citrus pests, oviposits exclusively on citrus fruit. In the insect olfactory system, odorant-binding proteins (OBPs) facilitate the initial recognition role of host odor molecules. The aim of this study was to characterize the functional OBPs of B. minax and identify specific volatile organic compounds in the Citrus genus as OBP targets.

RESULTS

BminOBP9 (BminGOBP99a), a closely related homolog of BdorGOBP99a, which reduces the egg-laying behavior of Bactrocera dorsalis through silencing technology, was cloned, expressed, and purified. The binding ability of BminOBP9 to 11 citrus volatiles was then examined using fluorescence competition binding assays (FCBA). The results demonstrated that BminOBP9 could bind to all tested citrus volatiles, as could BdorGOBP99a, ZcucGOBP99a, and ZtauGOBP99a. Interestingly, the binding ability of BminOBP9 was the strongest among the four, suggesting that BminOBP9 may have a function in the specific recognition of citrus volatiles. Furthermore, we aligned the above four proteins and found nine distinctive amino acid sites in BminOBP9. To identify the unique binding sites of BminOBP9, we produced the nine mutants using site-directed mutagenesis. Further FCBA showed that the binding ability of the nine mutants to citrus volatiles significantly reduced, and six of them (substitutes S24P, L36F, E53K, N68D, D112A, and S118R) had the weakest binding ability.

CONCLUSION

The results demonstrated that BminOBP9 was the specific protein involved in the perception of citrus host volatiles by B. minax. Moreover, BminOBP9 could prove efficient in screening the candidate odors for pest management. © 2020 Society of Chemical Industry.

Characterization of Volatile Organic Compounds of Healthy and Huanglongbing-Infected Navel Orange and Pomelo Leaves by HS-GC-IMS

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is the only natural vector of bacteria responsible for Huanglongbing (HLB), a worldwide destructive disease of citrus. ACP reproduces and develops only on the young leaves of its rutaceous host plants. Olfactory stimuli emitted by young leaves may play an important role in ACP control and HLB detection. In this study, volatile organic compounds (VOCs) from healthy and HLB-infected young leaves of navel orange and pomelo were analyzed by headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS). A total of 36 compounds (including dimers or polymers) were identified and quantified from orange and 10 from pomelo leaves. Some compounds showed significant differences in signal intensity between healthy and HLB-infected leaves and may constitute possible indicators for HLB infection. Principal component analysis (PCA) clearly discriminated healthy and HLB-infected leaves in both orange and pomelo. HS-GC-IMS was an effective method to identify VOCs from leaves. This study may help develop new methods for detection of HLB or find new attractants or repellents of ACP for prevention of HLB.