Salivary proteins of plant-feeding hemipteroids - implication in phytophagy

Proteomic Analysis of Salivary Secretions from the Tea Green Leafhopper, Empoasca flavescens Fabrecius

Saliva plays a crucial role in shaping the compatibility of piercing-sucking insects with their host plants. Understanding the complex composition of leafhopper saliva is important for developing effective and eco-friendly control strategies for the tea green leafhopper, Empoasca flavescens Fabrecius, a major piercing-sucking pest in Chinese tea plantations. This study explored the saliva proteins of tea green leafhopper adults using a custom collection device, consisting of two layers of Parafilm stretched over a sucrose diet. A total of 152 proteins were identified using liquid chromatography-tandem mass spectrometry (LC-MS/MS) following the filter-aided sample preparation (FASP). These proteins were categorized into six groups based on their functions, including enzymes, transport proteins, regulatory proteins, cell structure proteins, other proteins, and unknown proteins. Bioinformatics analyses predicted 16 secreted proteins, which were successfully cloned and transcriptionally analyzed across various tissues and developmental stages. Genes encoding putative salivary secretory proteins, including Efmucin1, EfOBP1, EfOBP2, EfOBP3, Efmucin2, low-density lipoprotein receptor-related protein (EfLRP), EFVg1, and EFVg2, exhibited high expressions in salivary gland (SG) tissues and feeding-associated expressions at different developmental stages. These findings shed light on the potential elicitors or effectors mediating the leafhopper feeding and defense responses in tea plants, providing insights into the coevolution of tea plants and leafhoppers. The study's conclusions open avenues for the development of innovative leafhopper control technologies that reduce the reliance on pesticides in the tea industry.

Roles of herbivorous insects salivary proteins

The intricate relationship between herbivorous insects and plants has evolved over millions of years, central to this dynamic interaction are salivary proteins (SPs), which mediate key processes ranging from nutrient acquisition to plant defense manipulation. SPs, sourced from salivary glands, intestinal regurgitation or acquired through horizontal gene transfer, exhibit remarkable functional versatility, influencing insect development, behavior, and adhesion mechanisms. Moreover, SPs play pivotal roles in modulating plant defenses, to induce or inhibit plant defenses as elicitors or effectors. In this review, we delve into the multifaceted roles of SPs in herbivorous insects, highlighting their diverse impacts on insect physiology and plant responses. Through a comprehensive exploration of SP functions, this review aims to deepen our understanding of plant-insect interactions and foster advancements in both fundamental research and practical applications in plant-insect interactions.

Discovery of Three Bemisia tabaci Effectors and Their Effect on Gene Expression in Planta

Bemisia tabaci (whitefly) is a polyphagous agroeconomic pest species complex. Two members of this species complex, Mediterranean (MED) and Middle-East-Asia Minor 1 (MEAM1), have a worldwide distribution and have been shown to manipulate plant defenses through effectors. In this study, we used three different strategies to identify three MEAM1 proteins that can act as effectors. Effector B1 was identified using a bioinformatics-driven effector-mining strategy, whereas effectors S1 and P1 were identified in the saliva of whiteflies collected from artificial diet and in phloem exudate of tomato on which nymphs were feeding, respectively. These three effectors were B. tabaci specific and able to increase whitefly fecundity when transiently expressed in tobacco plants (Nicotiana tabacum). Moreover, they reduced growth of Pseudomonas syringae pv. tabaci in Nicotiana benthamiana. All three effectors changed gene expression in planta, and B1 and S1 also changed phytohormone levels. Gene ontology and KEGG pathway enrichment analysis pinpointed plant-pathogen interaction and photosynthesis as the main enriched pathways for all three effectors. Our data thus show the discovery and validation of three new B. tabaci MEAM1 effectors that increase whitefly fecundity and modulate plant immunity. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Identification of salivary proteins of the cowpea aphid Aphis craccivora by transcriptome and LC-MS/MS analyses

Aphid salivary proteins mediate the interaction between aphids and their host plants. Moreover, these proteins facilitate digestion, detoxification of secondary metabolites, as well as activation and suppression of plant defenses. The cowpea aphid, Aphis craccivora, is an important sucking pest of leguminous crops worldwide. Although aphid saliva plays an important role in aphid plant interactions, knowledge of the cowpea aphid salivary proteins is limited. In this study, we performed transcriptomic and LC-MS/MS analyses to identify the proteins present in the salivary glands and saliva of A. craccivora. A total of 1,08,275 assembled transcripts were identified in the salivary glands of aphids. Of all these assembled transcripts, 53,714 (49.11%) and 53,577 (49.48%) transcripts showed high similarity to known proteins in the Nr and UniProt databases, respectively. A total of 2159 proteins were predicted as secretory proteins from the salivary gland transcriptome dataset, which contain digestive enzymes, detoxification enzymes, previously known effectors and elicitors, and potential proteins whose functions have yet to be determined. The proteomic analysis of aphid saliva resulted in the identification of 171 proteins. Tissue-specific expression of selected genes using RT-PCR showed that three genes were expressed only in the salivary glands. Overall, our results provide a comprehensive repertoire of cowpea aphid salivary proteins from the salivary gland and saliva, which will be a good resource for future effector functional studies and might also be useful for sustainable aphid management.

Comparative transcriptomic analysis of salivary glands between the zoophytophagous Cyrtorhinus lividipennis and the phytozoophagous Apolygus lucorum

BACKGROUND

Saliva plays a crucial role in shaping the feeding behavior of insects, involving processes such as food digestion and the regulation of interactions between insects and their hosts. Cyrtorhinus lividipennis serves as a predominant natural enemy of rice pests, while Apolygus lucorum, exhibiting phytozoophagous feeding behavior, is a destructive agricultural pest. In this study, a comparative transcriptome analysis, incorporating the published genomes of C.lividipennis and A.lucorum, was conducted to reveal the role of salivary secretion in host adaptation.

RESULTS

In contrast to A.lucorum, C.lividipennis is a zoophytophagous insect. A de novo genome analysis of C.lividipennis yielded 19,706 unigenes, including 16,217 annotated ones. On the other hand, A.lucorum had altogether 20,111 annotated genes, as obtained from the published official gene set (20,353 unigenes). Functional analysis of the top 1,000 salivary gland (SG)-abundant genes in both insects revealed that the SG was a dynamically active tissue engaged in protein synthesis and secretion. Predictions of other tissues and signal peptides were compared. As a result, 94 and 157 salivary proteins were identified in C.lividipennis and A.lucorum, respectively, and were categorized into 68 and 81 orthogroups. Among them, 26 orthogroups were shared, potentially playing common roles in digestion and detoxification, including several venom serine proteases. Furthermore, 42 and 55 orthogroups were exclusive in C.lividipennis and A.lucorum, respectively, which were exemplified by a hyaluronidase in C.lividipennis that was associated with predation, while polygalacturonases in A.lucorum were involved in mesophyll-feeding patterns.

CONCLUSIONS

Findings in this study provide a comprehensive insight into saliva secretions in C.lividipennis and A.lucorum via a transcriptome approach, reflecting the intricate connections between saliva secretions and feeding behaviors. It is found that conserved salivary secretions are involved in shaping the overlapping feeding patterns, while a plethora of unique salivary secretions may drive the evolution of specific feeding behaviors crucial for their survival. These results enhance our understanding of the feeding mechanisms in different insects from the perspective of saliva and contribute to future environmentally friendly pest control by utilizing predatory insects.

Orientus ishidae (Hemiptera: Cicadellidae): Biology, Direct Damage and Preliminary Studies on Apple Proliferation Infection in Apple Orchard

The mosaic leafhopper, Orientus ishidae (Matsumura), is an Asian species widespread in Europe that can cause leaf damage in wild trees and transmit disease phytoplasmas to grapevines. Following an O. ishidae outbreak reported in 2019 in an apple orchard in northern Italy, the biology and damage caused by this species to apples were investigated during 2020 and 2021. Our studies included observations on the O. ishidae life cycle, leaf symptoms associated to its trophic activity, and its capability to acquire "Candidatus Phytoplasma mali," a causal agent of Apple Proliferation (AP). The results indicate that O. ishidae can complete the life cycle on apple trees. Nymphs emerged between May and June, and adults were present from early July to late October, with the peak of flight between July and early August. Semi-field observations allowed for an accurate description of leaf symptoms that appeared as a distinct yellowing after a one-day exposure. In field experiments, 23% of the leaves were found damaged. In addition, 16-18% of the collected leafhoppers were found carrying AP phytoplasma. We conclude that O. ishidae has the potential to be a new apple tree pest. However, further studies are required to better understand the economic impact of the infestations.

Sex-biased proteomic response to tomato spotted wilt virus infection of the salivary glands of Frankliniella occidentalis, the western flower thrips

Successful transmission of tomato spotted wilt virus (TSWV) by Frankliniella occidentalis requires robust infection of the salivary glands (SGs) and virus delivery to plants during salivation. Feeding behavior and transmission efficiency are sexually-dimorphic traits of this thrips vector species. Proteins secreted from male and female SG tissues, and the effect of TSWV infection on the thrips SG proteome are unknown. To begin to discern thrips factors that facilitate virus infection of SGs and transmission by F. occidentalis, we used gel- and label-free quantitative and qualitative proteomics to address two hypotheses: (i) TSWV infection modifies the composition and/or abundance of SG-expressed proteins in adults; and (ii) TSWV has a differential effect on the male and female SG proteome and secreted saliva. Our study revealed a sex-biased SG proteome for F. occidentalis, and TSWV infection modulated the SG proteome in a sex-dependent manner as evident by the number, differential abundance, identities and generalized roles of the proteins. Male SGs exhibited a larger proteomic response to the virus than female SGs. Intracellular processes modulated by TSWV in males indicated perturbation of SG cytoskeletal networks and cell-cell interactions, i.e., basement membrane (BM) and extracellular matrix (ECM) proteins, and subcellular processes consistent with a metabolic slow-down under infection. Several differentially-abundant proteins in infected male SGs play critical roles in viral life cycles of other host-virus pathosystems. In females, TSWV modulated processes consistent with tissue integrity and active translational and transcriptional regulation. A core set of proteins known for their roles in plant cell-wall degradation and protein metabolism were identified in saliva of both sexes, regardless of virus infection status. Saliva proteins secreted by TSWV-infected adults indicated energy generation, consumption and protein turnover, with an enrichment of cytoskeletal/BM/ECM proteins and tricarboxylic acid cycle proteins in male and female saliva, respectively. The nonstructural TSWV protein NSs - a multifunctional viral effector protein reported to target plant defenses against TSWV and thrips - was identified in female saliva. This study represents the first description of the SG proteome and secretome of a thysanopteran and provides many candidate proteins to further unravel the complex interplay between the virus, insect vector, and plant host.

Haemaphysalis longicornis calreticulin is not an effective molecular tool for tick bite diagnosis and disruption of tick infestations

BACKGROUND

Tick calreticulin (CRT) is a calcium-binding protein secreted into the host during blood feeding. It has been used as a biomarker of tick exposure and has potential as an anti-tick vaccine, but there is no information about these uses for Haemaphysalis longicornis CRT (HlCRT). We synthesized recombinant H. longicornis CRT (rHlCRT) and evaluated its potential for tick bite diagnosis and for disrupting tick infestations.

METHODS

The responses of mice and rabbits exposed to H. longicornis ticks were measured with ELISA to determine the antibody level against rHlCRT. To evaluate the effects of rHlCRT-induced anti-tick immunity, engorgement weight, tick engorgement index (TEI), feeding duration, ecdysis rate, and egg weight per engorged tick were compared between ticks fed on immunized and normal mice.

RESULTS

Mean anti-tick CRT antibody levels in sera collected from mice at 1 and 15 days after primary tick exposure were not significantly different from the mean antibody levels in negative control mice that were not bitten by ticks (all P values > 0.05). No significant anti-HlCRT IgG responses developed in mice after second exposure to tick bites compared with the level of anti-HlCRT antibody response in negative control mice (all P values > 0.25). For rabbits, no significant differences in the antibody levels were observed in animals before challenge infestation and after tick exposures, and in animals after two tick exposures (all P values > 0.10). There were no significant differences in the body weight of ticks fed on immunized and normal mice (all P values > 0.15). No significant differences in TEI were observed between ticks fed on immunized mice and normal control mice (all P values > 0.50). There were no significant differences in feeding duration for female ticks, and feeding duration and ecdysis rate for nymphs in the experimental and control groups (all P values > 0.10 for feeding duration and P value = 0.19 for ecdysis rate). We did not observe a significant difference in egg weight per tick in the rHlCRT-immunized and the control groups (P = 0.88).

CONCLUSIONS

HlCRT in H. longicornis tick saliva proteins appears to be nonimmunogenic to mammalian hosts like mice and rabbits. Vaccination with rHlCRT did not generate effective immunity against parthenogenetic and bisexual H. longicornis nymphs or female ticks. These results indicate that HlCRT is not a suitable molecular candidate for H. longicornis tick bite diagnosis and not effective for the disruption of tick infestations.

The larval saliva of an endoparasitic wasp, Pteromalus puparum, suppresses host immunity

In the lengthy co-evolution between insects and their animal or plant hosts, insects have evolved a wide range of salivary strategies to help evade host defenses. Although there is a very large literature on saliva of herbivorous and hematophagous insects, little attention has been focused on the saliva of parasitoid wasps. Some parasitoid species are natural enemies that effectively regulate insect population sizes in nature that they are applied for biological control of agricultural pests. Here, we demonstrate the influence of the endoparasitoid, Pteromalus puparum, larval saliva on the cellular and humoral immunity of its host. Larval saliva increases mortality of hemocytes, and inhibits hemocyte spreading, a specific cellular immune action. We report that high saliva concentrations inhibit host cellular encapsulation of foreign invaders. The larval saliva also inhibits melanization in host hemolymph. The saliva inhibits the growth of some bacterial species, Bacillus subtilis, Staphylococcus aureus and Pseudomonas aeruginosa in vitro. This may promote larvae fitness by protecting them from infections. Insight into such functions of parasitic wasp saliva provides a new insight into host-parasitoid relationships and possibly leads to new agricultural pest management technologies.

Analysis of the Destructive Effect of the Halyomorpha halys Saliva on Tomato by Computer Tomographical Imaging and Antioxidant Capacity Measurement

Simple Summary

Halyomorpha halys is a devastating agricultural pest, and digestion starts with an extra-oral phase as the injury is inflicted by injected saliva enzymes into the plant tissues. We carried out a noninvasive imaging assay assisted by computer tomography (CT) of through damaged tomatoes caused by H. halys. It was intended to support the imaging results by further laboratory analytical approaches, such as the antioxidant capacity, which have been used as a stress indicator. Our results confirmed that the noninvasive approach may provide new data for the cognition of the degree of damage induced by this important pest. The important finding is the proof of escalation of the lesions as a function of bug number and the exposure time. Volume and structure deviation in tomatoes were justified by the antioxidant activity. In summary, our method can be suitable for the qualitative inspection of tomato items.

Abstract

Qualitative and quantitative parameters of tomatoes are impaired by Halyomorpha halys Stål (Hemiptera: Pentatomidae), which cause severe economic losses worldwide. Our aims were to assess H. halys-induced tissue damage in tomatoes via computer tomography and to confirm the results of imaging obtained by analytical methods. Our examination confirmed the intensification of the change in the inner structure of damaged tomatoes as a function of time. The tendency of this destruction triggered by bug saliva grew exponentially from the exocarp layer to the inner placenta. The destruction of the plant tissue was aggravated by an increase in the number of bugs, as it was unequivocally evinced by the shell thickness assays. The results of the assessment of the antioxidant capacity of tomato mesocarp showed a distinct decrease in the antioxidant capacity of the samples obtained from H. halys-infested tomatoes. This indicates that the ferric-reducing antioxidant power value was related to the degradation processes of the mesocarp tissue in tomato fruit caused by the watery saliva released by H. halys. The presented experimental method can be suitable for the qualitative control of the vegetable items intended for trade, which can help for the isolation of tomatoes damaged by bugs immediately after harvest.

Whole-body transcriptome mining for candidate effectors from Diuraphis noxia

Background

Proteins within aphid saliva play a crucial role as the molecular interface between aphids and their host plants. These salivary effectors modulate plant responses to favour aphid feeding and facilitate infestation. The identification of effectors from economically important pest species is central in understanding the molecular events during the aphid-plant interaction. The Russian wheat aphid (Diuraphis noxia, Kurdjumov) is one such pest that causes devastating losses to wheat and barley yields worldwide. Despite the severe threat to food security posed by D. noxia, the non-model nature of this pest and its host has hindered progress towards understanding this interaction. In this study, in the absence of a salivary gland transcriptome, whole-body transcriptomics data was mined to generate a candidate effector catalogue for D. noxia.

Results

Mining the transcriptome identified 725 transcripts encoding putatively secreted proteins amongst which were transcripts specific to D. noxia. Six of the seven examined D. noxia putative effectors, termed DnE’s (Diuraphis noxia effectors) exhibited salivary gland-specific expression. A comparative analysis between whole-body D. noxia transcriptome data versus the head and body transcriptomes from three other aphid species allowed us to define a catalogue of transcripts putatively upregulated in D. noxia head tissue. Five of these were selected for RT-qPCR confirmation, and were found to corroborate the differential expression predictions, with a further three confirmed to be highly expressed in D. noxia salivary gland tissue.

Conclusions

Determining a putative effector catalogue for D. noxia from whole-transcriptome data, particularly the identification of salivary-specific sequences potentially unique to D. noxia, provide the basis for future functional characterisation studies to gain further insight into this aphid-plant interaction. Furthermore, due to a lack of publicly available aphid salivary gland transcriptome data, the capacity to use comparative transcriptomics to compile a list of putative effector candidates from whole-body transcriptomics data will further the study of effectors in various aphid species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08712-4.

Diversification of the phytophagous lineages of true bugs (Insecta: Hemiptera: Heteroptera) shortly after that of the flowering plants

More than 95% of phytophagous true bug (Hemiptera: Heteroptera) species belong to four superfamilies: Miroidea (Cimicomorpha), Pentatomoidea, Coreoidea, and Lygaeoidea (all Pentatomomorpha). These iconic groups of highly diverse, overwhelmingly phytophagous insects include several economically prominent agricultural and silvicultural pest species, though their evolutionary history has not yet been well resolved. In particular, superfamily- and family-level phylogenetic relationships of these four lineages have remained controversial, and the divergence times of some crucial nodes for phytophagous true bugs have hitherto been little known, which hampers a better understanding of the evolutionary processes and patterns of phytophagous insects. In the present study, we used 150 species and concatenated nuclear and mitochondrial protein-coding genes and rRNA genes to infer the phylogenetic relationships within the Terheteroptera (Cimicomorpha + Pentatomomorpha) and estimated their divergence times. Our results support the monophyly of Cimicomorpha, Pentatomomorpha, Miroidea, Pentatomoidea, Pyrrhocoroidea, Coreoidea, and Lygaeoidea. The phylogenetic relationships across phytophagous lineages are largely congruent at deep nodes across the analyses based on different datasets and tree-reconstructing methods with just a few exceptions. Estimated divergence times and ancestral state reconstructions for feeding habit indicate that phytophagous true bugs explosively radiated in the Early Cretaceous-shortly after the angiosperm radiation-with the subsequent diversification of the most speciose clades (Mirinae, Pentatomidae, Coreinae, and Rhyparochromidae) in the Late Cretaceous.

Heat Shock 70 kDa Protein Cognate 3 of Brown Planthopper Is Required for Survival and Suppresses Immune Response in Plants

The brown planthopper (Nilaparvata lugens) is a monophagous pest of rice (Oryza sativa), which threatens food security around the world. Insect Heat shock proteins 70 kDa (Hsp70s) play a key role in insect growth and development, however, if they also modulate the plant physiological processes is still unclear. In this study, we identified the Heat shock 70 kDa protein cognate 3 (NlHSC70-3) of BPH from compared protein profiles of Nipponbare tissues after BPH infestation via LC/MS. NlHSC70-3 has a predicted signal peptide and displays high transcription levels in the salivary glands, which further supported that it is secreted into plants by BPH during the feeding process. Using RNA interference (RNAi), we showed that NlHSC70-3 is indispensable for the survival of BPH on rice. Most importantly, NlHSC70-3 mediates the plant immune responses including cell death, flg22-induced ROS burst and defense-related gene expression in N. benthamiana. These results demonstrate that NlHSC70-3 may function as an effector manipulating plant physiological processes to facilitate pest survival on rice, which provides a new potential target for future pest control.

Effects of aromatic compounds on movement activity of Pyrrhocoris apterus in the conditions of a laboratory experiment

In the conditions of an urban environment, insects, including Pyrrhocoris apterus (Linnaeus, 1758) (Hemiptera, Pyrrhocoridae), are subject to dozens of volatiles: solvents, food flavorings, cosmetic products. For firebugs, some attract them, some scare them off, and to some they are indifferent. In this article, in the conditions of laboratory experiment, we evaluated the influence of chemical substances on rates of movement of firebugs. Under the influence of the substances we studied, there occurred no significant increase or decrease in the activity of firebugs. The most significant effect on rates of movement of firebugs was exerted by solvent, white spirit and solvent 646 p – firebugs travelled 12 cm distance for 24.2%, 49.6% and 58.7% of the time the control group did. Movement activity of firebugs was insignificantly decreased by aromatic extracts Royаl milk extract, Butcher’s broom extract and chrysalis oil: firebugs travelled 12 cm distance for 132.7%, 135.2% and 146.8% of time the control group. Aromatic substances likely interact with receptors on antennae of firebugs, and therefore the insects need additional time for orienting reactions, deciding whether those substances signal presence of potential food objects in the accessibility zone.

High population levels lead Glycaspis brimblecombei (Hemiptera: Aphalaridae) to unrecorded feeding and oviposition behaviors on Eucalyptus urograndis plants

The red gum lerp psyllid, Glycaspis brimblecombei Moore, 1964 (Hemiptera: Aphalaridae), an insect pest originating in Australia and which feeds only on Eucalyptus L'Hér. (Myrtales: Myrtaceae) plants, has spread to several countries. The populations of this insect commonly reach high populations on Eucalyptus plants since its entry into Brazil, and also indicated an unrecorded behavioral. The objectives of this study were to describe a peculiar adaptation in the feeding habit of G. brimblecombei and to register the new habit. The oviposition and feeding by G. brimblecombei, commonly, on the leaves of Eucalyptus, started to occur, also, on lignified twigs. This suggests a not yet recorded adaptation of this insect to reduce insect × plant intraspecific competition.

Screening Candidate Effectors of the Bean Bug Riptortus pedestris by Proteomic and Transcriptomic Analyses

The damage of Riptortus pedestris is exceptional by leading soybean plants to keep green in late autumn. Identification of the salivary proteins is essential to understand how the pest-plant interaction occurs. Here, we have tried to identify them by a combination of proteomic and transcriptomic analyses. The transcriptomes of salivary glands from R. pedestris males, females and nymphs showed about 28,000 unigenes, in which about 40% had open reading frames (ORFs). Therefore, the predicted proteins in the transcriptomes with secretion signals were obtained. Many of the top 1,000 expressed transcripts were involved in protein biosynthesis and transport, suggesting that the salivary glands produce a rich repertoire of proteins. In addition, saliva of R. pedestris males, females and nymphs was collected and proteins inside were identified. In total, 155, 20, and 11 proteins were, respectively, found in their saliva. We have tested the tissue-specific expression of 68 genes that are likely to be effectors, either because they are homologs of reported effectors of other sap-feeding arthropods, or because they are within the top 1,000 expressed genes or found in the salivary proteomes. Their potential functions in regulating plant defenses were discussed. The datasets reported here represent the first step in identifying effectors of R. pedestris.

Identification of salivary proteins in the whitefly Bemisia tabaci by transcriptomic and LC-MS/MS analyses

The whitefly Bemisia tabaci is a notorious agricultural pest of many crops worldwide. Although it is thought that B. tabaci secretes saliva into the host plant to counter plant defenses, knowledge on the whitefly salivary proteome is limited. Here, we characterized the gene/protein repertoires of B. tabaci salivary glands and secreted saliva by transcriptomic and liquid chromatography tandem mass spectroscopy analyses. A total of 698 salivary gland-enriched unigenes and 171 salivary proteins were identified. Comparative analysis between the B. tabaci salivary proteins and those of different arthropod species revealed numerous similarities in proteins associated with binding, hydrolysis, and oxidation-reduction, which demonstrates a degree of conservation across herbivorous saliva. There were 74 proteins only identified in B. tabaci saliva, of which 34 were B. tabaci-specific. In addition, 13 salivary proteins, of which 11 were B. tabaci-specific, were differentially regulated when B. tabaci fed on different hosts. Our results provide a good resource for future functional studies of whitefly salivary effectors, and might be useful in pest management.

Up-Regulated Salivary Proteins of Brown Marmorated Stink Bug Halyomorpha halys on Plant Growth-Promoting Rhizobacteria-Treated Plants

Plant Growth-Promoting Rhizobacteria (PGPR) induce systemic resistance (SR) in plants, decreasing the development of phytopathogens. The FZB42 strain of Bacillus velezensis is known to induce an SR against pathogens in various plant species. Previous studies suggested that it could also influence the interactions between plants and associated pests. However, insects have developed several strategies to counteract plant defenses, including salivary proteins that allow the insect escaping detection, manipulating defensive pathways to its advantage, deactivating early signaling processes, or detoxifying secondary metabolites. Because Brown Marmorated Stink Bug (BMSB) Halyomorpha halys is highly invasive and polyphagous, we hypothesized that it could detect the PGPR-induced systemic defenses in the plant, and efficiently adapt its salivary compounds to counteract them. Therefore, we inoculated a beneficial rhizobacterium on Vicia faba roots and soil, previous to plant infestation with BMSB. Salivary gland proteome of BMSB was analyzed by LC-MS/MS and a label-free quantitative proteomic method. Among the differentially expressed proteins, most were up-regulated in salivary glands of insects exposed to PGPR-treated plants for 24 h. We could confirm that BMSB was confronted with a stress during feeding on PGPR-treated plants. The to-be-confirmed defensive state of the plant would have been rapidly detected by the invasive H. halys pest, which consequently modified its salivary proteins. Among the up-regulated proteins, many could be associated with a role in plant defense counteraction, and more especially in allelochemicals detoxification or sequestration.

Aphid salivary protein Mp1 facilitates infestation by binding phloem protein 2-A1 in Arabidopsis

We have previously demonstrated that Arabidopsis (Arabidopsis thaliana) phloem protein PP2-A1 is an integral component of resistance to the green peach aphid (Myzus persicae). Here, we report that M. persicae overcomes the resistance of PP2-A1 by using the salivary protein Mp1 as an energetic effector and an interactor of AtPP2-A1. Using the RNA interference technique, we demonstrated that Mp1 plays an essential role in the phloem-feeding activity of M. persicae. When the Mp1 gene was silenced, aphids incurred serious impairments not only in phloem-feeding activity, but also in survival and fertility. In essence, phloem-feeding activity was attributed to the molecular interaction between Mp1 and AtPP2-A1. The Mp1 and AtPP2-A1 interactions were localized to plant cell membranes by co-immunoprecipitation and bimolecular fluorescence complementation experiments. Furthermore, the interaction was found to be required for aphid feeding on Arabidopsis phloem. Overall, our results suggest that Mp1 is an important effector of M. persicae and interacts with AtPP2-A1 to facilitate infestation in the plant tissue by this insect.

Tripartite parasitic and symbiotic interactions as a possible mechanism of horizontal gene transfer

Herbivory is a highly sophisticated feeding behavior that requires abilities of plant defense suppression, phytochemical detoxification, and plant macromolecule digestion. For plant-sucking insects, salivary glands (SGs) play important roles in herbivory by secreting and injecting proteins into plant tissues to facilitate feeding. Little is known on how insects evolved secretory SG proteins for such specialized functions. Here, we investigated the composition and evolution of secretory SG proteins in the brown marmorated stink bug (Halyomorpha halys) and identified a group of secretory SG phospholipase C (PLC) genes with highest sequence similarity to the bacterial homologs. Further analyses demonstrated that they were most closely related to PLCs of Xenorhabdus, a genus of Gammaproteobacteria living in symbiosis with insect-parasitizing nematodes. These suggested that H. halys might acquire these PLCs from Xenorhabdus through the mechanism of horizontal gene transfer (HGT), likely mediated by a nematode during its parasitizing an insect host. We also showed that the original HGT event was followed by gene duplication and expansion, leading to functional diversification of the bacterial-origin PLC genes in H. halys. Thus, this study suggested that an herbivore might enhance adaptation through gaining genes from an endosymbiont of its parasite in the tripartite parasitic and symbiotic interactions.

Delivery of Rice Gall Dwarf Virus Into Plant Phloem by Its Leafhopper Vectors Activates Callose Deposition to Enhance Viral Transmission

Rice gall dwarf virus (RGDV) and its leafhopper vector Recilia dorsalis are plant phloem-inhabiting pests. Currently, how the delivery of plant viruses into plant phloem via piercing-sucking insects modulates callose deposition to promote viral transmission remains poorly understood. Here, we initially demonstrated that nonviruliferous R. dorsalis preferred feeding on RGDV-infected rice plants than viruliferous counterpart. Electrical penetration graph assay showed that viruliferous R. dorsalis encountered stronger physical barriers than nonviruliferous insects during feeding, finally prolonging salivary secretion and ingestion probing. Viruliferous R. dorsalis feeding induced more defense-associated callose deposition on sieve plates of rice phloem. Furthermore, RGDV infection significantly increased the cytosolic Ca²⁺ level in rice plants, triggering substantial callose deposition. Such a virus-mediated insect feeding behavior change potentially impedes insects from continuously ingesting phloem sap and promotes the secretion of more infectious virions from the salivary glands into rice phloem. This is the first study demonstrating that the delivery of a phloem-limited virus by piercing-sucking insects into the plant phloem activates the defense-associated callose deposition to enhance viral transmission.

Transcriptomic and proteomic analysis of putative digestive proteases in the salivary gland and gut of Empoasca (Matsumurasca) onukii Matsuda

Background

Infestation by tea green leafhoppers (Empoasca (Matsumurasca) onukii) can cause a series of biochemical changes in tea leaves. As a typical cell-rupture feeder, E. onukii secretes proteases while using its stylet to probe the tender shoots of tea plants (Camellia sinensis). This study identified and analyzed proteases expressed specifically in the salivary gland (SG) and gut of E. onukii through enzymatic activity assays complemented with an integrated analysis of transcriptomic and proteomic data.

Results

In total, 129 contigs representing seven types of putative proteases were identified. Transcript abundance of digestive proteases and enzymatic activity assays showed that cathepsin B-like protease, cathepsin L-like protease, and serine proteases (trypsin- and chymotrypsin-like protease) were highly abundant in the gut but moderately abundant in the SG. The abundance pattern of digestive proteases in the SG and gut of E. onukii differed from that of other hemipterans, including Nilaparvata lugens, Laodelphax striatellus, Acyrthosiphum pisum, Halyomorpha halys and Nephotettix cincticeps. Phylogenetic analysis showed that aminopeptidase N-like proteins and serine proteases abundant in the SG or gut of hemipterans formed two distinct clusters.

Conclusions

Altogether, this study provides insightful information on the digestive system of E. onukii. Compared to five other hemipteran species, we observed different patterns of proteases abundant in the SG and gut of E. onukii. These results will be beneficial in understanding the interaction between tea plants and E. onukii.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07578-2.

Diel activity patterns and arrestment behaviour in host associations of green mirids (Creontiades dilutus)

Many plant bugs (Miridae) are generalist herbivores that feed on multiple host species. The reasons these bugs move across hosts and the behavioural mechanisms responsible for their retention at specific hosts remain elusive. Green mirids (Creontiades dilutus) are endemic to Australia. These insects are important pests of cotton (Gossypium hirsutum) and even in low numbers can cause substantial damage to crops. These bugs are also present in relatively much higher numbers on pigeon pea (Cajanus cajan) planted alongside cotton fields, and evidence shows they move across these crops in both directions. Observations of these highly mobile insects in the field are challenging, but indirect evidence suggests that they may be nocturnal. This study evaluated: (1) the diel (24 h) period in which C. dilutus adults were most active, (2) whether they respond to plant volatiles immediately prior to landing on host substrates, and (3) if their presence on a host is in response to attraction or arrestment cues. The results suggest that C. dilutus bugs are typically most active early in the evenings, after remaining motionless during the day (unless disturbed). Their movement (at night) was arrested by hosts prior to touching plant tissues. There was no evidence to suggest that these bugs are attracted by volatiles beyond 2 cm. These outcomes demonstrate that insect behaviours need to be investigated within their typical activity periods, and that arrestment cues possibly play a central role in the host finding process of generalist C. dilutus and probably, therefore, other mirid species.

Proteomic and transcriptomic analyses of saliva and salivary glands from the Asian citrus psyllid, Diaphorina citri

Salivary secretions play critical roles in interactions among insects, insect-vectored pathogens, and host plants. The Asian citrus psyllid Diaphorina citri is a sap-sucking Hemipteran that serves as a vector for Candidatus Liberibacter asiaticus, the causal agent of citrus greening disease ("Huanglongbing" or HLB). D. citri continuously injects saliva into host plants using specialized stylets so as to feed and transmit the HLB pathogen. Knowledge on the composition and function of salivary proteins of this pest is very limited. In this study, proteomic and transcriptomic approaches were adopted to characterize the protein composition of the saliva and salivary glands in D. citri. A total of 246 and 483 proteins were identified in saliva and dissected salivary glands, respectively, via LC-MS/MS analyses. Comparative analyses of the identified proteins were performed between D. citri and other reported Hemipteran insect species. Transcription levels of the genes coding for the identified proteins were determined via RNA-sequencing among different tissues including salivary glands and other digestive tissues. Identification of putative effectors that are expressed exclusively or abundantly in salivary glands provides the foundation for future functional studies towards the understanding of their roles in interactions among D. citri, HLB pathogen, and their citrus host. BIOLOGICAL SIGNIFICANCE: This is a systematic analysis on proteins in saliva and dissected salivary glands. A high percentage of novel proteins have been identified due to the large amounts of samples collected. This report gives a more comprehensive repertoire of potential effector proteins that may be possibly involved in modulating host defense, altering nutrient metabolism, and facilitating Ca. L. asiaticus transmission.

Simplified and rapid staining of leafhopper salivary sheaths in plant tissues for electrical penetration graph waveform correlations

Herbivorous insects in the order Hemiptera use piercing-sucking mouthparts to utilize plant sap. Among them salivary sheath feeders penetrate into the plant by their flexible stylets to reach vascular elements. Manoeuvering stylets in plant tissues is aided by the creation of salivary sheaths, which solidify from proteinous gelling saliva and remain as lasting artefacts in the plant tissues. Studying their structure reveals hidden details of the feeding behaviour and the transmission of pathogens in case of vector insects. One important aspect of studying salivary sheaths is that it can be used to confirm the biological function of electropenetrography (EPG) waveform patterns. Previously, complex and vaguely documented histological methods have been used to observe salivary sheath structure. Building on existing methodologies, we report a simplified histological procedure where each step was optimized to offer a rapid process that does not require special equipment, can be applied to many samples, has good success rate and a low cost of errors in terms of time and materials. We describe the procedure, using a Psammotettix alienus - barley model system, in three steps. (i) Clarification of entire plant parts and pre-staining salivary sheaths with aqueous fuchsin. This step allows to identify salivary sheath starting points on the surface. (ii) Knowing salivary sheath location, using hand sectioning, produce a single c. 60 μm section that contains the entire salivary sheath. (iii) Counterstain the section with methylene green and, after further clarification, study under light microscope in a glycerol - ethanol embedding solution, without fixed mounting.

Apolygus lucorum genome provides insights into omnivorousness and mesophyll feeding

Apolygus lucorum (Miridae) is an omnivorous pest that occurs worldwide and is notorious for the serious damage it causes to various crops and substantial economic losses. Although some studies have examined the biological characteristics of the mirid bug, no reference genome is available in Miridae, limiting in-depth studies of this pest. Here, we present a chromosome-scale reference genome of A. lucorum, the first sequenced Miridae species. The assembled genome size was 1.02 Gb with a contig N50 of 785 kb. With Hi-C scaffolding, 1,016 Mb contig sequences were clustered, ordered and assembled into 17 large scaffolds with scaffold N50 length 68 Mb, each corresponding to a natural chromosome. Numerous transposable elements occur in this genome and contribute to the large genome size. Expansions of genes associated with omnivorousness and mesophyll feeding such as those related to digestion, chemosensory perception, and detoxification were observed in A. lucorum, suggesting that gene expansion contributed to its strong environmental adaptability and severe harm to crops. We clarified that a salivary enzyme polygalacturonase is unique in mirid bugs and has significantly expanded in A. lucorum, which may contribute to leaf damage from this pest. The reference genome of A. lucorum not only facilitates biological studies of Hemiptera as well as an understanding of the damage mechanism of mesophyll feeding, but also provides a basis on which to develop efficient control technologies for mirid bugs.

Extraoral digestion: outsourcing the role of the hemipteran midgut

Extraoral digestion allows for breakdown of dietary components before they reach the midgut for final enzymatic degradation and absorption. In the Hemiptera, this is achieved by the secretion of enzyme-rich fluids from the salivary gland, with the combination of protein and mRNA from these tissues termed the sialome. Separate channels within the hemipteran stylets allow for secretion of saliva and ingestion of predigested material in a non-reflux mechanism. Both feeding mode and diet type influence the composition of the hemipteran sialome, as illustrated by 1) differences in protease abundance between hematophagous and predatory heteropteran sialomes, 2) diet specific aminopeptidase-N genes among aphid biotypes, and 3) adaptation-induced sialome variation in related cicada populations. Despite challenges associated with incomplete genome annotation, -omics analysis of the sialomes of diverse hemipteran species will enhance understanding of both sialome function and the evolution of extraoral digestion within the order.

A Transcriptomic and Proteomic Analysis of the Diaphorina citri Salivary Glands Reveals Genes Responding to Candidatus Liberibacter asiaticus

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is the principal vector of the Candidatus Liberibacter asiaticus (CLas) bacterium that causes Huanglongbing (HLB) disease. The D. citri salivary glands (SG) is an important barrier to the transmission of CLas. Despite its importance, the transcriptome and proteome of SG defense against CLas are unstudied in D. citri. In the present study, we generated a comparative transcriptome dataset of the SG in infected and uninfected D. citri using an Illumina RNA-Seq technology. We obtained 407 differentially expressed genes (DEGs), including 159 upregulated DEGs and 248 downregulated DEGs. Functional categories showed that many DEGs were associated with the ribosome, the insecticide resistance, the immune response and the digestion in comparison with CLas-infected SG and CLas-free SG. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases confirmed that metabolism and immunity were important functions in the SG. Among the DEGs, 68 genes (35 upregulated and 33 downregulated) encoding putative-secreted proteins were obtained with a signal peptide, suggesting that these genes may play important roles in CLas infection. A total of 673 SG proteins were identified in uninfected D. citri by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) analysis, and 30 DEGs (15 upregulated and 15 downregulated) were found using the local tBLASTP programs. Among the 30 DEGs, many DEGs mainly involved in the metabolism and cellular processes pathways. This study provides basic transcriptome and proteome information for the SG in D. citri, and helps illuminate the molecular interactions between CLas and D. citri.

Proteomic Mapping of Multifunctional Complexes Within Triatomine Saliva

Triatomines are hematophagous insects that transmit Trypanosoma cruzi, the etiological agent of Chagas disease. This neglected tropical disease represents a global health issue as it is spreading worldwide. The saliva of Triatominae contains miscellaneous proteins crucial for blood feeding acquisition, counteracting host's hemostasis while performing vasodilatory, anti-platelet and anti-coagulant activities, besides modulating inflammation and immune responses. Since a set of biological processes are mediated by protein complexes, here, the sialocomplexomes (salivary protein complexes) of five species of Triatominae were studied to explore the protein-protein interaction networks. Salivary multiprotein complexes from Triatoma infestans, Triatoma dimidiata, Dipetalogaster maxima, Rhodnius prolixus, and Rhodnius neglectus were investigated by Blue-Native- polyacrylamide gel electrophoresis coupled with liquid chromatography tandem mass spectrometry. More than 70 protein groups, uncovering the landscape of the Triatominae salivary interactome, were revealed. Triabin, actin, thioredoxin peroxidase and an uncharacterized protein were identified in sialocomplexes of the five species, while hexamerin, heat shock protein and histone were identified in sialocomplexes of four species. Salivary proteins related to triatomine immunity as well as those required during blood feeding process such as apyrases, antigen 5, procalins, and nitrophorins compose different complexes. Furthermore, unique proteins for each triatomine species were revealed. This study represents the first Triatominae sialocomplexome reference to date and shows that the approach used is a reliable tool for the analysis of Triatominae salivary proteins assembled into complexes.

Gene silencing of Diaphorina citri candidate effectors promotes changes in feeding behaviors

Insect effectors are mainly secreted by salivary glands, modulate plant physiology and favor the establishment and transmission of pathogens. Feeding is the principal vehicle of transmission of Candidatus Liberibacter asiaticus (Ca. Las) by the Asian citrus psyllid (ACP), Diaphorina citri. This study aimed to predict putative ACP effectors that may act on the Huanglongbing (HLB) pathosystem. Bioinformatics analysis led to the identification of 131 candidate effectors. Gene expression investigations were performed to select genes that were overexpressed in the ACP head and modulated by Ca. Las. To evaluate the actions of candidate effectors on D. citri feeding, six effectors were selected for gene silencing bioassays. Double-stranded RNAs (dsRNAs) of the target genes were delivered to D. citri adults via artificial diets for five days. RNAi silencing caused a reduction in the ACP lifespan and decreased the salivary sheath size and honeydew production. Moreover, after dsRNA delivery of the target genes using artificial diet, the feeding behaviors of the insects were evaluated on young leaves from citrus seedlings. These analyses proved that knockdown of D. citri effectors also interfered with ACP feeding abilities in planta, causing a decrease in honeydew production and reducing ACP survival. Electrical penetration graph (EPG) analysis confirmed the actions of the effectors on D. citri feeding behaviors. These results indicate that gene silencing of D. citri effectors may cause changes in D. citri feeding behaviors and could potentially be used for ACP control.

Manipulation of Jasmonate Signaling by Plant Viruses and Their Insect Vectors

Plant viruses pose serious threats to stable crop yield. The majority of them are transmitted by insects, which cause secondary damage to the plant host from the herbivore-vector's infestation. What is worse, a successful plant virus evolves multiple strategies to manipulate host defenses to promote the population of the insect vector and thereby furthers the disease pandemic. Jasmonate (JA) and its derivatives (JAs) are lipid-based phytohormones with similar structures to animal prostaglandins, conferring plant defenses against various biotic and abiotic challenges, especially pathogens and herbivores. For survival, plant viruses and herbivores have evolved strategies to convergently target JA signaling. Here, we review the roles of JA signaling in the tripartite interactions among plant, virus, and insect vectors, with a focus on the molecular and biochemical mechanisms that drive vector-borne plant viral diseases. This knowledge is essential for the further design and development of effective strategies to protect viral damages, thereby increasing crop yield and food security.

Salivary Protein 1 of Brown Planthopper Is Required for Survival and Induces Immunity Response in Plants

The brown planthopper (BPH), Nilaparvata lugens Stål, is one of the major pests of rice. It uses its stylet to penetrate rice phloem, feeding on rice sap and causing direct damage to rice or even plant death. During the feeding process, BPHs secrete saliva into plant tissues, which plays crucial roles in the plant-insect interactions. However, little is known about how the salivary proteins secreted by BPH affect feeding ability and how they induce plant immune responses. Here, we identified an N. lugens Salivary Protein 1 (NlSP1) by screening salivary proteome and characterized its functions in BPH and plants. NlSP1 induces cell death, H₂O₂ accumulation, the expression of defense-related genes, and callose deposition in planta. The active region of NlSP1 that induces plant cell death is located in its N-terminal region. Inhibition of NlSP1 expression in BPHs reduced their feeding ability and had a lethal effect on them. Most importantly, we demonstrated that NlSP1 was able to be secreted into rice plant during feeding process and form a complex with certain interacting partner of rice. These results provide a detailed characterization of a salivary protein from BPHs and offers new insights into our understanding of rice-BPH interaction.

Assessing the Role of Differential Herbivore Performance Among Plant Species in Associational Effects Involving the Invasive Stink Bug Bagrada hilaris (Hemiptera: Pentatomidae)

Co-occurring plant species can influence the extent of damage to each other by altering the activity or abundance of a shared herbivore. One mechanism by which neighboring host plants exacerbate damage to a focal host is if the neighbor amplifies herbivore populations. We studied the performance of a shared herbivore on a native and an invasive plant, to estimate how strongly the presence of the invasive plant increases local herbivore abundance-in a system in which highly asymmetric spillover herbivory may occur. Specifically, we conducted a series of greenhouse experiments that measured reproduction, development, and survival of the invasive stink bug Bagrada hilaris Burmeister on an invasive annual forb, Sahara mustard (Brassica tournefortii), or a native perennial shrub, four-winged saltbush (Atriplex canescens). All measured aspects of stink bug performance revealed consistently greater performance on Br. tournefortii. Indeed, A. canescens appears to be insufficient for Ba. hilaris to complete its development. Nonetheless, preliminary damage assessments found that both plant species were used as feeding hosts, putative feeding lesions were a more reliable indicator of herbivory than was the degree of yellowing, and higher Ba. hilaris abundance was generally associated with greater sublethal damage to A. canescens. Thus, A. canescens appears to be susceptible to Ba. hilaris herbivory, though more research is needed to assess fitness impacts of this novel herbivore. Our results indicate that differential herbivore performance among host plants may be an important contributor to observed patterns of abundance of a shared herbivore and spillover herbivory between plants.

A salivary effector enables whitefly to feed on host plants by eliciting salicylic acid-signaling pathway

Phloem-feeding insects feed on plant phloem using their stylets. While ingesting phloem sap, these insects secrete saliva to circumvent plant defenses. Previous studies have shown that, to facilitate their feeding, many phloem-feeding insects can elicit the salicylic acid- (SA-) signaling pathway and thus suppress effective jasmonic acid defenses. However, the molecular basis for the regulation of the plant's defense by phloem-feeding insects remains largely unknown. Here, we show that Bt56, a whitefly-secreted low molecular weight salivary protein, is highly expressed in the whitefly primary salivary gland and is delivered into host plants during feeding. Overexpression of the Bt56 gene in planta promotes susceptibility of tobacco to the whitefly and elicits the SA-signaling pathway. In contrast, silencing the whitefly Bt56 gene significantly decreases whitefly performance on host plants and interrupts whitefly phloem feeding with whiteflies losing the ability to activate the SA pathway. Protein-protein interaction assays show that the Bt56 protein directly interacts with a tobacco KNOTTED 1-like homeobox transcription factor that decreases whitefly performance and suppresses whitefly-induced SA accumulation. The Bt56 orthologous genes are highly conserved but differentially expressed in different species of whiteflies. In conclusion, Bt56 is a key salivary effector that promotes whitefly performance by eliciting salicylic acid-signaling pathway.

Transcriptomic and Expression Analysis of the Salivary Glands in Brown Planthoppers, Nilaparvata lugens (Hemiptera: Delphacidae)

The brown planthopper (BPH), Nilaparvata lugens Stål (Hemiptera: Delphacidae), is a serious rice pest because of its destructive feeding. The salivary glands of the BPH play a key role in successful feeding. In this study, we explored the salivary gland transcriptome (sialotranscriptome) of adult BPHs using Illumina sequencing and a total of 55,913 transcripts and 45,421 unigenes were obtained. We identified one reference gene RPL9 (Ribosomal protein L9) and 19 salivary protein genes from the BPH sialotranscripome, which were categorized as those involved in sugar metabolism, extra-oral digestion of cell wall components, detoxification, and suppression of plant defenses. Tissue expression profiles of 19 salivary protein genes analysis revealed that the expression level of alpha-glucosidase family 31 had no difference in five tissues, suggesting that it may have functions in the whole-body parts. Glucose dehydrogenase (flavine adenine dinucleotide, quinone)-like was expressed highly in the salivary gland, which might play putative role in insect feeding. Glucose dehydrogenase (acceptor) was expressed the highest level in head without salivary gland. Other salivary protein genes were highly expressed in gut or malpighian tubule, suggesting that they may play roles in digestive and detoxification mechanism. Moreover, we detected RPL9 is one of the genes that is most consistently present for acquisition of gene expression in different tissues. Thus, RPL9 can be a new reference for expression studies of BPH. The obtained BPH sialotranscripome provides a list of genes that have potential roles in feeding and interaction between BPHs and rice plants.

Proteomic analysis of watery saliva secreted by white-backed planthopper, Sogatella furcifera

The white-backed planthopper, Sogatella furcifera, is a phloem sap feeder that secretes watery and gelling saliva during feeding. In this study, we identified the major proteins in watery saliva of S. furcifera by shotgun LC-MS/MS analysis combined with transcriptomic analysis. A total of 161 proteins were identified, which were divided into 8 function categories, including enzymes, transporter, calcium ion binding protein, salivary sheath protein, cytoskeleton protein, DNA-, RNA-, and protein-binding or regulating proteins, other non-enzyme proteins and unknown proteins. Gene expression pattern of 11 secretory proteins were analyzed by real time quantitative-PCR. We detected the mucin-like protein, which had a unique expression level in salivary gland, most likely to be a candidate effector involved in regulation of plant defense. This study identified the watery saliva component of S. furcifera and it provided a list of proteins which may play a role in interaction between S. furcifera and rice.

Proteomic analysis of labial saliva of the generalist cabbage looper (Trichoplusia ni) and its role in interactions with host plants

Insect saliva is one of the first secretions to come in contact with plants during feeding. The composition and role of caterpillar saliva has not been as thoroughly studied as that of sucking insects. This study focuses on characterizing the proteome of the cabbage looper (Trichoplusia ni) saliva using iTRAQ labeling and LC-MS/MS. We also measured how the saliva proteome changed when larvae were reared on different diets - cabbage, tomato, and an artificial pinto bean diet. We identified 254 proteins in the saliva out of which 63 were differentially expressed. A large percentage (56%) of the proteins identified function in protein metabolism, followed by proteins involved in vesicle transport (6%) and oxidoreductase activity (5%), among other categories. Several proteins identified are antioxidants or reactive oxygen species (ROS) scavengers. Among these ROS scavengers, we identified a catalase and further analyzed its gene expression and enzymatic activity. We also applied commercial, purified catalase on tomato and measured the activity of defensive proteins - trypsin proteinase inhibitor, polyphenol oxidase and peroxidase. Catalase gene expression was significantly higher in the salivary glands of larvae fed on tomato. Also, catalase suppressed the induction of tomato trypsin proteinase inhibitor levels, but not the induction of polyphenol oxidase or peroxidase. These results add to our understanding of proteomic plasticity in saliva and its role in herbivore offense against plant defenses.

Evaluation of mature soybean pods as a food source for two pod-sucking bugs, Riptortus pedestris (Hemiptera: Alydidae) and Halyomorpha halys (Hemiptera: Pentatomidae)

Riptortus pedestris (Fabricius) and Halyomorpha halys (Stål) cause injury to soybeans by piercing and sucking pods and seeds. Growers believe that new damage decreases near to harvest despite the occurrence of these bugs at that time. As this question has never been assessed, we evaluated two diets: a) mature soybean pods (dried shell + dried soybean seeds) and b) dried soybean seeds for the two bugs by assessing their biological, behavioral, and morphological attributes on each diet in laboratory. While nymphs of both species were able to develop and adults able to reproduce on the tested diets, bugs fed on pods had longer development times and 2.2 to 5.0 times higher mortality rates than bugs fed on seeds. Furthermore, adult longevity of R. pedestris and H. halys fed on pods was 8.4 and 7.5 days shorter, respectively, than that of bugs fed on seeds. However, pod feeding had no effect on adult fecundity or egg viability. In a behavioral choice test, adult R. pedestris preferred seeds over pods and probed seeds longer than pods. On average, adult H. halys also preferred seeds over pods, although 15.6% of H. halys showed the reverse, preferring pods over seeds. The proboscis length and estimated depth of stylet penetration into the host tissue of both nymphs and adults of both species was much greater than the thickness of the pod shell, suggesting that mouthpart structure does not explain the negative effects of pods vs. seeds. In conclusion, mature soybean pods were found to be a suitable food source for both R. pedestris and H. halys despite some negative effects, and thus careful attention should be paid to the population levels of these two bugs approaching harvest to reduce economic damage in soybean.

PAMPs, PRRs, effectors and R-genes associated with citrus-pathogen interactions

BACKGROUND

Recent application of molecular-based technologies has considerably advanced our understanding of complex processes in plant-pathogen interactions and their key components such as PAMPs, PRRs, effectors and R-genes. To develop novel control strategies for disease prevention in citrus, it is essential to expand and consolidate our knowledge of the molecular interaction of citrus plants with their pathogens.

SCOPE

This review provides an overview of our understanding of citrus plant immunity, focusing on the molecular mechanisms involved in the interactions with viruses, bacteria, fungi, oomycetes and vectors related to the following diseases: tristeza, psorosis, citrus variegated chlorosis, citrus canker, huanglongbing, brown spot, post-bloom, anthracnose, gummosis and citrus root rot.

Genomics of Lepidoptera saliva reveals function in herbivory

Lepidoptera herbivores deposit copious amounts of saliva when feeding. Their saliva is produced by the paired mandibular and labial glands and evidence indicates that it may play an important role in allowing an herbivore to establish on its host plant. Genomic studies of Lepidoptera saliva are beginning to reveal the role of saliva in herbivory. Molecules involved in digestion, detoxification, immunity, defense against plant secondary chemicals, chemoreception and so on have been identified using high throughput genomic tools. These genomic tools have also revealed changes that occur in Lepidoptera saliva when caterpillars feed on different host plants. However, there are other factors either biotic or abiotic (e.g., larval stage, larval health, temperature, water stress, etc.) that might also affect its composition. Though further functional and ecological studies are still necessary to fully understand the role of Lepidoptera saliva on herbivory, here we review current trends.

A salivary EF-hand calcium-binding protein of the brown planthopper Nilaparvata lugens functions as an effector for defense responses in rice

The brown planthopper (BPH), Nilaparvata lugens (Stål) (Hemiptera: Delphacidae), a major pest of rice in Asia, is able to successfully puncture sieve tubes in rice with its piercing stylet and then to ingest phloem sap. How BPH manages to continuously feed on rice remains unclear. Here, we cloned the gene NlSEF1, which is highly expressed in the salivary glands of BPH. The NlSEF1 protein has EF-hand Ca²⁺-binding activity and can be secreted into rice plants when BPH feed. Infestation of rice by BPH nymphs whose NlSEF1 was knocked down elicited higher levels of Ca²⁺ and H₂O₂ but not jasmonic acid, jasmonoyl-isoleucine (JA-Ile) and SA in rice than did infestation by control nymphs; Consistently, wounding plus the recombination protein NlSEF1 suppressed the production of H₂O₂ in rice. Bioassays revealed that NlSEF1-knockdown BPH nymphs had a higher mortality rate and lower feeding capacity on rice than control nymphs. These results indicate that the salivary protein in BPH, NlSEF1, functions as an effector and plays important roles in interactions between BPH and rice by mediating the plant’s defense responses.

The Salivary Protein Repertoire of the Polyphagous Spider Mite Tetranychus urticae: A Quest for Effectors

The two-spotted spider mite Tetranychus urticae is an extremely polyphagous crop pest. Alongside an unparalleled detoxification potential for plant secondary metabolites, it has recently been shown that spider mites can attenuate or even suppress plant defenses. Salivary constituents, notably effectors, have been proposed to play an important role in manipulating plant defenses and might determine the outcome of plant-mite interactions. Here, the proteomic composition of saliva from T. urticae lines adapted to various host plants-bean, maize, soy, and tomato-was analyzed using a custom-developed feeding assay coupled with nano-LC tandem mass spectrometry. About 90 putative T. urticae salivary proteins were identified. Many are of unknown function, and in numerous cases belonging to multimembered gene families. RNAseq expression analysis revealed that many genes coding for these salivary proteins were highly expressed in the proterosoma, the mite body region that includes the salivary glands. A subset of genes encoding putative salivary proteins was selected for whole-mount in situ hybridization, and were found to be expressed in the anterior and dorsal podocephalic glands. Strikingly, host plant dependent expression was evident for putative salivary proteins, and was further studied in detail by micro-array based genome-wide expression profiling. This meta-analysis revealed for the first time the salivary protein repertoire of a phytophagous chelicerate. The availability of this salivary proteome will assist in unraveling the molecular interface between phytophagous mites and their host plants, and may ultimately facilitate the development of mite-resistant crops. Furthermore, the technique used in this study is a time- and resource-efficient method to examine the salivary protein composition of other small arthropods for which saliva or salivary glands cannot be isolated easily.

Transcriptomic and Expression Analysis of the Salivary Glands in White-Backed Planthoppers, Sogatella furcifera

The white-backed planthopper (WBPH), Sogatella furcifera (Horváth), is one of the serious rice pests because of its destructive feeding. The salivary glands of the WBPH play an important role in the feeding behaviour. Currently, however, very little is known about the salivary glands at the molecular level. We sequenced the salivary gland transcriptome (sialotranscripome) of adult WBPHs using the Illumina sequencing. A total of 65,595 transcripts and 51,842 unigenes were obtained from salivary glands. According to annotations against the Nr database, many of the unigenes identified were associated with the most studied enzymes in hemipteran saliva. In the present study, we identified 32 salivary protein genes from the WBPH sialotranscripome, which were categorized as those involved in sugar metabolism, detoxification, suppression of plant defense responses, immunity-related responses, general digestion, and other phytophagy processes. Tissue expression profiles analysis revealed that four of 32 salivary protein genes (multicopper oxidase 4, multicopper oxidase 6, carboxylesterase and uridine phosphorylase 1 isform X2) were primarily expressed in the salivary gland, suggesting that they played putative role in insect-rice interactions. 13 of 32 salivary protein genes were primarily expressed in gut, which might play putative role in digestive and detoxify mechanism. Development expression profiles analysis revealed that the expression level of 26 of 32 salivary protein genes had no significant difference, suggesting that they may play roles in every developmental stages of salivary gland of WBPH. The other six genes have a high expression level in the salivary gland of adult. 31 of 32 genes (except putative acetylcholinesterase 1) have no significant difference in male and female adult, suggesting that their expression level have no difference between sexes. This report analysis of the sialotranscripome for the WBPH, and the transcriptome provides a foundational list of the genes involved in feeding. Our data will be useful to investigate the mechanisms of interaction between the WBPH and the host plant.

Identification of the secreted watery saliva proteins of the rice brown planthopper, Nilaparvata lugens (Stål) by transcriptome and Shotgun LC-MS/MS approach

The rice brown planthopper, Nilaparvata lugens (Stål), a major rice insect pest in Asia, is a vascular bundle-feeder that ejects gelling and watery saliva during the feeding process. Although major proteins in the salivary glands of N. lugens have been identified using 2D PAGE, very little is known about the secreted saliva of this insect. In this study, we identified the major proteins in the secreted watery saliva of N. lugens, via collecting from a sucrose diet that adult planthoppers had fed upon through a membrane of stretched parafilm, and using shotgun LC-MS/MS analysis with reference to transcriptome database of salivary glands of N. lugens. A total of 107 proteins were identified in the watery saliva of N. lugens, over 80% of which showed significant similarity to known proteins. When annotated by the Blast2GO suite, 29 proteins had catalytic activity and 24 proteins were binding proteins. The saliva enzymes included oxidoreductases, hydrolases, phosphatases, peptidases (proteases), kinases, transferases, and lyases. Binding proteins in N. lugens watery saliva included ATP-binding, lipophorin, calcium-binding, actin-binding and DNA-, RNA-, and chromatin-binding proteins. Other non-enzymatic proteins, such as ubiquitins, heat shock proteins, ribosomal proteins, and immunoglobulin proteins were also found in N. lugens watery saliva. This is the first study to identify, characterize and list the proteins in watery saliva of N. lugens, which might be involved in planthopper-rice interactions.

Comparative transcriptomics and proteomics of three different aphid species identifies core and diverse effector sets

BACKGROUND

Aphids are phloem-feeding insects that cause significant economic losses to agriculture worldwide. While feeding and probing these insects deliver molecules, called effectors, inside their host to enable infestation. The identification and characterization of these effectors from different species that vary in their host range is an important step in understanding the infestation success of aphids and aphid host range variation. This study employs a multi-disciplinary approach based on transcriptome sequencing and proteomics to identify and compare effector candidates from the broad host range aphid Myzus persicae (green peach aphid) (genotypes O, J and F), and narrow host range aphids Myzus cerasi (black cherry aphid) and Rhopalosiphum padi (bird-cherry oat aphid).

RESULTS

Using a combination of aphid transcriptome sequencing on libraries derived from head versus body tissues as well as saliva proteomics we were able to predict candidate effectors repertoires from the different aphid species and genotypes. Among the identified conserved or core effector sets, we identified a significant number of previously identified aphid candidate effectors indicating these proteins may be involved in general infestation strategies. Moreover, we identified aphid candidate effector sequences that were specific to one species, which are interesting candidates for further validation and characterization with regards to species-specific functions during infestation. We assessed our candidate effector repertoires for evidence of positive selection, and identified 49 candidates with DN/DS ratios >1. We noted higher rates of DN/DS ratios in predicted aphid effectors than non-effectors. Whether this reflects positive selection due to co-evolution with host plants, or increased neofunctionalization upon gene duplication remains to be investigated.

CONCLUSION

Our work provides a comprehensive overview of the candidate effector repertoires from three different aphid species with varying host ranges. Comparative analyses revealed candidate effectors that are most likely are involved in general aspects of infestation, whereas others, that are highly divergent, may be involved in specific processes important for certain aphid species. Insights into the overlap and differences in aphid effector repertoires are important in understanding how different species successfully infest different ranges of plant species.

Performances of survival, feeding behavior, and gene expression in aphids reveal their different fitness to host alteration

Insect populations feeding on different plant species are under selection pressure to adapt to these differences. A study integrating elements of the ecology, behavior, and gene expression of aphids on different host plants has not yet been well-explored. The present study explores the relationship between host fitness and survival, feeding behavior, and salivary gland gene expression of a pea (Pisum sativum) host race of Acyrthosiphon pisum feeding on a common host Vicia faba and on three genetically-related hosts (Vicia villosa, Medicago truncatula, and Medicago sativa). Life table data indicated that aphids on non-favored hosts exhibited small size, low reproduction rate, slow population increase and individual development, and long lifespan. Electrical penetration graph results showed that the aphids spent significantly less time in passive ingestion of phloem sap on all non-preferred host plants before acclimation. After a period of acclimation on M. truncatula and V. villosa, pea host race individuals showed improved feeding behavior. No individuals of the pea host race completed its life history on M. sativa. Interestingly, the number of host-specific differentially-expressed salivary gland genes was negatively correlated with the fitness of aphids on this host plant. This study provided important cues in host plant specialization in aphids.