The green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae), salivary protein NcSP75 is a key effector for successful phloem ingestion

Identification and expression patterns of somatic piRNAs and PIWI genes in Riptortus pedestris (Hemiptera: Alydidae)

RNA interference (RNAi)-based gene silencing is a feasible and sustainable technology for the management of hemipteran pests by double-stranded RNA involvement, including small-interfering RNA, microRNA, and Piwi-interacting RNA (piRNA) pathways, that may help to decrease the usage of chemical insecticides. However, only a few data are available on the somatic piRNAs and their biogenesis genes in Riptortus pedestris, which serves as a significant pest of soybean (Glycine max). In this study, two family members of the PIWI gene were identified and characterized in R. pedestris, containing Argonaute3 (RpAgo3) and Aubergine (RpAub) genes with conserved protein domains, and their clusters were validated by phylogenetic analysis. In addition, they were widely expressed in all developmental stages of the whole body of R. pedestris and had lower expression levels in R. pedestris guts under different rearing conditions based on previous transcriptome sequencing. Furthermore, abundant clean reads were filtered to a total number of 45,998 piRNAs with uridine bias at the first nucleotide (nt) position and 26-32 nt in length by mapping onto the reference genome of R. pedestris according to our previous whole-transcriptome sequencing. Finally, our data revealed that gut bacterial changes were significantly positively or negatively associated with differentially expressed piRNAs among the five comparison groups with Pearson correlation analysis. In conclusion, these findings paved new avenues for the application of RNAi-based biopesticides for broad-spectrum hemipteran pest control.

Transcriptomic response of citrus psyllid salivary glands to the infection of citrus Huanglongbing pathogen

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), is the key vector insect transmitting the Candidatus Liberibacter asiaticus (CLas) bacterium that causes the devastating citrus greening disease (Huanglongbing, HLB) worldwide. The D. citri salivary glands (SG) exhibit an important barrier against the transmission of HLB pathogen. However, knowledge on the molecular mechanism of SG defence against CLas infection is still limited. In the present study, we compared the SG transcriptomic response of CLas-free and CLas-infected D. citri using an illumine paired-end RNA sequencing. In total of 861 differentially expressed genes (DEGs) in the SG upon CLas infection, including 202 upregulated DEGs and 659 downregulated DEGs were identified. Functional annotation analysis showed that most of the DEGs were associated with cellular processes, metabolic processes, and the immune response. Gene ontology and Kyoto Encyclopaedia of Genes and Genomes enrichment analyses revealed that these DEGs were enriched in pathways involving carbohydrate metabolism, amino acid metabolism, the immune system, the digestive system, the lysosome, and endocytosis. A total of 16 DEGs were randomly selected to further validate the accuracy of RNA-Seq dataset by reverse-transcription quantitative polymerase chain reaction. This study provides substantial transcriptomic information regarding the SG of D. citri in response to CLas infection, which may shed light on the molecular interaction between D. citri and CLas, and provides new ideas for the prevention and control of citrus psyllid.

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.

Advances of herbivore-secreted elicitors and effectors in plant-insect interactions

Diverse molecular processes regulate the interactions between insect herbivores and their host plants. When plants are exposed to insects, elicitors induce plant defenses, and complex physiological and biochemical processes are triggered, such as the activation of the jasmonic acid (JA) and salicylic acid (SA) pathways, Ca2+ flux, reactive oxygen species (ROS) burst, mitogen-activated protein kinase (MAPK) activation, and other responses. For better adaptation, insects secrete a large number of effectors to interfere with plant defenses on multiple levels. In plants, resistance (R) proteins have evolved to recognize effectors and trigger stronger defense responses. However, only a few effectors recognized by R proteins have been identified until now. Multi-omics approaches for high-throughput elicitor/effector identification and functional characterization have been developed. In this review, we mainly highlight the recent advances in the identification of the elicitors and effectors secreted by insects and their target proteins in plants and discuss their underlying molecular mechanisms, which will provide new inspiration for controlling these insect pests.

A leafhopper saliva protein mediates horizontal transmission of viral pathogens from insect vectors into rice phloem

Numerous insects transmit viruses together with saliva to plant phloem, but the roles of saliva components remain elusive. Here, we report that calcium-binding protein (CBP), a universal insect saliva protein, is modified to benefit horizontal transmission of a devastating rice reovirus into plant phloem. CBP effectively competes with virus-induced filaments to target and traverse actin-based apical plasmalemma into saliva-stored cavities in salivary glands of leafhopper vector. Thus, the inhibition of CBP expression by viral infection facilitates filament-mediated viral secretion into salivary cavities and then into plant phloem. Furthermore, virus-mediated reduction of CBP secretion causes an increase of cytosolic Ca²⁺ levels in rice, triggering substantial callose deposition and H₂O₂ production. Thus, viruliferous vectors encounter stronger feeding barriers, probe more frequently, and secrete more saliva into plants, ultimately enhancing viral transmission. We thus conclude that the inhibition of CBP secretion facilitates viral secretion and increases host defense response to benefit viral transmission.

CBP, a calcium binding protein found in insect saliva, allows for the transmission of the devastating rice gall dwarf virus into plant phloem. This interaction with CBP is compounded by stronger feeding barriers, more frequent probing behavior, and increased saliva secretion into plants by insect vectors, all increasing the likelihood of viral transmission.

Arms and ammunitions: effectors at the interface of rice and it's pathogens and pests

The plant immune system has evolved to resist attack by pathogens and pests. However, successful phytopathogens deliver effector proteins into plant cells where they hijack the host cellular machinery to suppress the plant immune responses and promote infection. This manipulation of the host cellular pathways is done by the pathogen using various enzymatic activities, protein- DNA or protein- protein interactions. Rice is one the major economically important crops and its yield is affected by several pathogens and pests. In this review, we summarize the various effectors at the plant- pathogen/ pest interface for the major pathogens and pests of rice, specifically, on the mode of action and target genes of the effector proteins. We then compare this across the major rice pathogens and pests in a bid to understand probable conserved pathways which are under attack from pathogens and pests in rice. This analysis highlights conserved patterns of effector action, as well as unique host pathways targeted by the pathogens and pests.

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.

Proteomics of the Honeydew from the Brown Planthopper and Green Rice Leafhopper Reveal They Are Rich in Proteins from Insects, Rice Plant and Bacteria

Honeydew is a watery fluid excreted by plant sap-feeding insects. It is a waste product for the insect hosts. However, it plays important roles for other organisms, such as serving as a nutritional source for beneficial insects and bacteria, as well as elicitors and effectors modulating plant responses. In this study, shotgun LC-MS/MS analyses were used to identify the proteins in the honeydew from two important rice hemipteran pests, the brown planthopper (Nilaparvata lugens, BPH) and green rice leafhopper (Nephotettix cincticeps, GRH). A total of 277 and 210 proteins annotated to insect proteins were identified in the BPH and GRH honeydews, respectively. These included saliva proteins that may have similar functions as the saliva proteins, such as calcium-binding proteins and apolipophorin, involved in rice plant defenses. Additionally, a total of 52 and 32 Oryza proteins were identified in the BPH and GRH honeydews, respectively, some of which are involved in the plant immune system, such as Pathogen-Related Protein 10, ascorbate peroxidase, thioredoxin and glutaredoxin. Coincidently, 570 and 494 bacteria proteins were identified from the BPH and GRH honeydews, respectively, which included several well-known proteins involved in the plant immune system: elongation factor Tu, flagellin, GroEL and cold-shock proteins. The results of our study indicate that the insect honeydew is a complex fluid cocktail that contains abundant proteins from insects, plants and microbes, which may be involved in the multitrophic interactions of plants-insects-microbes.

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.

Research advances in plant-insect molecular interaction

Acute and precise signal perception and transduction are essential for plant defense against insects. Insect elicitors-that is, the biologically active molecules from insects' oral secretion (which contains regurgitant and saliva), frass, ovipositional fluids, and the endosymbionts-are recognized by plants and subsequently induce a local or systematic defense response. On the other hand, insects secrete various types of effectors to interfere with plant defense at multiple levels for better adaptation. Jasmonate is a main regulator involved in plant defense against insects and integrates with multiple pathways to make up the intricate defense network. Jasmonate signaling is strictly regulated in plants to avoid the hypersensitive defense response and seems to be vulnerable to assault by insect effectors at the same time. Here, we summarize recently identified elicitors, effectors, and their target proteins in plants and discuss their underlying molecular mechanisms.

The salivary gland proteome of root-galling grape phylloxera (Daktulosphaira vitifoliae Fitch) feeding on Vitis spp

The successful parasitisation of a plant by a phytophagous insect is dependent on the delivery of effector molecules into the host. Sedentary gall forming insects, such as grape phylloxera (Daktulosphaira vitifoliae Fitch, Phylloxeridae), secrete multiple effectors into host plant tissues that alter or modulate the cellular and molecular environment to the benefit of the insect. The identification and characterisation of effector proteins will provide insight into the host-phylloxera interaction specifically the gall-induction processes and potential mechanisms of plant resistance. Using proteomic mass spectrometry and in-silico secretory prediction, 420 putative effectors were determined from the salivary glands or the root-feeding D. vitifoliae larvae reared on Teleki 5C (V. berlandieri x V. riparia). Among them, 170 conserved effectors were shared between D. vitifoliae and fourteen phytophagous insect species. Quantitative RT-PCR analysis of five conserved effector candidates (protein disulfide-isomerase, peroxidoredoxin, peroxidase and a carboxypeptidase) revealed that their gene expression decreased, when larvae were starved for 24 h, supporting their assignment as effector molecules. The D. vitifoliae effectors identified here represent a functionally diverse group, comprising both conserved and unique proteins that provide new insight into the D. vitifoliae-Vitis spp. interaction and the potential mechanisms by which D. vitifoliae establishes the feeding site, suppresses plant defences and modulates nutrient uptake.

Characterization of multicopper oxidase genes in the green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae), with focus on salivary gland-specific genes

Multicopper oxidase (MCO) enzymes are present ubiquitously and act on diverse substrates. Recently, the presence of multiple MCO genes has been described in many insects. Based on sialotranscriptome data, we identified and comprehensively characterized six MCO genes: NcLac1S, 1G, and 2-5 in the green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae). NcLac1S and NcLac1G belong to the MCO1 ortholog of other insects. NcLac2 forms a clade with MCO2s involved in the sclerotization and pigmentation of the cuticle. NcLac3 and NcLac4 form a clade with NlMCO3 -5 of the hemipteran Nilaparvata luges. NcLac5 forms a clade with MCORPs (MCO-related proteins) that lack amino acid residues normally highly conserved in copper-coordinated MCOs. NcLac1S and NcLac3 were specifically expressed in the salivary glands; whereas NcLac5 was primarily expressed in the salivary glands. Only NcLac3 protein is considered to have laccase activity in the salivary glands and salivary sheaths ejected by the insect. NcLac1G expression was relatively high in the testis. NcLac2 and NcLac4 were specifically expressed in the integument and in Malpighian tubules, respectively. Knockdown by RNA interference (RNAi) of either NcLac2 and NcLac5 in nymphs caused high mortality. All NcLac2-knockdown nymphs showed depigmentation and soft cuticle, and eventually died, as did other MCO2-knockdown insects. DsNcLac5-injected nymphs (third, fourth, and fifth-instar) showed high mortality, but injection into adults had no effect on survival or number of eggs deposited, suggesting that NcLac5 is not essential for survival after molting (eclosion). NcLac5 could be a promising target gene for control of N. cincticeps.

Induction of RNAi Core Machinery's Gene Expression by Exogenous dsRNA and the Effects of Pre-exposure to dsRNA on the Gene Silencing Efficiency in the Pea Aphid (Acyrthosiphon pisum)

The pea aphid, Acyrthosiphon pisum, is an important agricultural pest and biological model organism, and RNA interference (RNAi) is an important tool for functional genomics and for insect pest management. However, the efficiency of RNAi in pea aphids is variable, limiting its application in aphids. In this study, we present optimized conditions for inducing and increasing the gene silencing efficiency of RNAi in pea aphids. The optimal gene silencing of the target Aphunchback gene was achieved by injecting 600 ng double-stranded (ds) RNA, and the highest mRNA depletion rate (74%) was detected at 36 h after injection. Moreover, the same gene silencing conditions were used to achieve transcript silencing for nine different genes in the pea aphid, although the silencing efficiencies for the different genes varied. Furthermore, the pre-exposure of aphids to dsRNA (600 ng dsGFP) led to significant hunchback silencing following a secondary exposure to 60 ng of dshunchback, a dose which did not lead to gene silencing when independently injected. The information presented here can be exploited to develop more efficient RNAi bioassays for pea aphids, both as gene functional study tools and an insect pest control strategy.