Construction of cytomegalovirus promoter-driven gene expression system in Laodelphax striatellus

The small brown planthopper (SBPH, Laodelphax striatellus) is a significant rice pest, responsible for transmitting rice stripe virus (RSV) in a persistent and propagative manner. RSV is one of the most detrimental rice viruses, causing rice stripe disease, which results in considerable loss of rice grain yield. While RNA interference and gene knockout techniques have enabled gene downregulation in SBPH, no system currently exists for the overexpression of endogenous or exogenous genes. Consequently, the development of a protein expression system for SBPH is imperative to serve as a technical foundation for pest control and gene function investigations. This study aimed to construct an expression vector using the promoter of the constitutive-expressed tubulin gene of SBPH, and promoter of human cytomegalovirus (CMV). Fluorescence experiments demonstrated that both tubulin and CMV promoter could drive green fluorescent protein (GFP) expression in SBPH, and could also facilitate the expression of a nucleocapsid protein (NP) -GFP fusion protein containing viral NP with comparable efficiency. Through expression vector optimization, we have identified that the 3 tandem CMV promoters display a significantly higher promoter activity compared with both the 2 tandem CMV promoters and the single CMV promoter. In addition, the incorporation of Star polycation nanoparticles significantly enhanced the expression efficiency in SBPH. These results provide a promising technical platform for investigating gene functions in SBPH.

MicroRNA750-3p Targets Processing of Precursor 7 to Suppress Rice Black-Streaked Dwarf Virus Propagation in Vector Laodelphax striatellus

MicroRNAs (miRNAs) are non-coding RNAs, which, as members of the RNA interference pathway, play a pivotal role in antiviral infection. Almost 80% of plant viruses are transmitted by insect vectors; however, little is known about the interaction of the miRNAs of insect vectors with plant viruses. Here, we took rice black-streaked dwarf virus (RBSDV), a devastating virus to rice production in eastern Asia, and the small brown planthopper, (SBPH, Laodelphax striatellus) as a model to investigate the role of microRNA750-3p (miR750-3p) in regulating viral transmission. Our results showed that Ls-miR750-3p was downregulated in RBSDV-infected SBPH and predominately expressed in the midgut of SBPH. Injection with miR750-3p agomir significantly reduced viral accumulation, and the injection with the miR750-3p inhibitor, antagomir-750-3p, dramatically promoted the viral accumulation in SBPH, as detected using RT-qPCR and Western blotting. The processing of precursor 7 (POP7), a subunit of RNase P and RNase MRP, was screened, identified, and verified using a dual luciferase reporter assay as one target of miR750-3p. Knockdown of POP7 notably increased RBSDV viral propagation in SBPH and then increased the viral transmission rate by SBPH. Taken together, our data indicate that miR750-3p targets POP7 to suppress RBSDV infection in its insect vector. These results enriched the role of POP7 in modulating virus infection in host insects and shared new insight into the function of miRNAs in plant virus and insect vector interaction.

Small Brown Planthopper Nymph Infestation Regulates Plant Defenses by Affecting Secondary Metabolite Biosynthesis in Rice

The small brown planthopper (SBPH, Laodelphax striatellus) is one of the most destructive insect pests in rice (Oryza sativa), which is the world's major grain crop. The dynamic changes in the rice transcriptome and metabolome in response to planthopper female adult feeding and oviposition have been reported. However, the effects of nymph feeding remain unclear. In this study, we found that pre-infestation with SBPH nymphs increased the susceptibility of rice plants to SBPH infestation. We used a combination of broadly targeted metabolomic and transcriptomic studies to investigate the rice metabolites altered by SBPH feeding. We observed that SBPH feeding induced significant changes in 92 metabolites, including 56 defense-related secondary metabolites (34 flavonoids, 17 alkaloids, and 5 phenolic acids). Notably, there were more downregulated metabolites than upregulated metabolites. Additionally, nymph feeding significantly increased the accumulation of seven phenolamines and three phenolic acids but decreased the levels of most flavonoids. In SBPH-infested groups, 29 differentially accumulated flavonoids were downregulated, and this effect was more pronounced with infestation time. The findings of this study indicate that SBPH nymph feeding suppresses flavonoid biosynthesis in rice, resulting in increased susceptibility to SBPH infestation.

A rice plant expressing viral glycoprotein NSvc2-NS reduces the transmission of rice stripe virus by the small brown planthopper

BACKGROUND

Plant viruses transmitted by arthropod vectors threaten crop health worldwide. Rice stripe virus (RSV) is one of the most important rice viruses in East Asia and is transmitted by the small brown planthopper (SBPH). Previously, it was demonstrated that the viral glycoprotein NSvs2-N could mediate RSV infection of the vector midgut. Therefore, NSvc2-N protein could potentially be used to reduce RSV transmission by competitively blocking midgut receptors.

RESULTS

Here, we report that transgenic rice plants expressing viral glycoprotein can interfere with RSV acquisition and transmission by SBPH. The soluble fraction (30-268 amino acids, designated NSvs2-N_S ) of NSvs2-N was transformed into rice calli, which produced plants harboring the exogenous gene. When SBPH was fed on transgenic plants prior to RSV-infected rice (sequential feeding) and when insects were fed on RSV-infected transgenic plants (concomitant feeding), virus acquisition by the insect vector was inhibited, and subsequent viral titers were reduced. Immunofluorescence labeling also indicated that viral infection of the insect midgut was inhibited after SBPH was fed on transgenic plants. The system by which RSV infected insect cells in vitro was used to corroborate the role of NSvc2-N_S in reducing viral infection. After the cells were incubated with transgenic rice sap, the virus infection rate of the cells decreased significantly, and viral accumulation in the cells was lower than that in the control group.

CONCLUSION

These results demonstrated the negative effect of NSvs2-N_S transgenic plants on RSV transmission by insect vectors, which provides a novel and effective way to control plant viral diseases. © 2022 Society of Chemical Industry.

Chromosome-level Assembly, Dosage Compensation and Sex-biased Gene Expression in the Small Brown Planthopper, Laodelphax striatellus

In insects, sex chromosome differentiation often results in unequal gene dosages between sexes. Dosage compensation mechanisms evolve to balance gene expression, but the degree and mechanism of regulation often vary by insect species. In hemipteran species, the small brown planthopper (SBPH), Laodelphax striatellus, is an injurious crop pest, with a sex chromosome type XX in females and XO in males. This species offers the opportunity to study dosage compensation and sex-biased gene expression. In this study, we generated a chromosome-level genome of SBPH using Oxford Nanopore Technologies and high-throughput chromatin conformation capture (Hi-C) technology. We also sequenced RNA-seq data from 16 tissue samples to annotate the genome and analyze gene dosage compensation. We finally obtained a 510.2 megabases (Mb) genome with 99.12% of the scaffolds anchored on 15 chromosomes (14 autosomes and 1 X chromosome) and annotated 16,160 protein-coding genes based on full-length cDNA sequencing data. Furthermore, we found complete dosage compensation in all L. striatellus somatic tissues, but lack of dosage compensation in gonad tissue testis. We also found that female-biased genes were significantly enriched on the X chromosome in all tissues, whereas male-biased genes in gonad tissues were enriched on autosomes. This study not only provides a high-quality genome assembly but also lays a foundation for a better understanding of the sexual regulatory network in hemipteran insects.

Rice Stripe Virus Modulates the Feeding Preference of Small Brown Planthopper from the Stems to Leaves of Rice Plants to Promote Virus Infection

Research on plant-virus-vector interactions has revealed that viruses can enhance their spread to new host plants by attracting nonviruliferous vectors to infected plants or driving viruliferous vectors to noninfected plants. However, whether viruses can also modulate the feeding preference of viruliferous vectors for different plant parts remains largely unknown. Here, by using rice stripe virus (RSV) and its vector, the small brown planthopper (SBPH), as a model, the effect of the virus on the feeding preference of its vector was studied by calculating the number of nonviruliferous and viruliferous SBPHs settling on different parts of rice plants. The results showed that the RSV-free SBPHs significantly preferred feeding on the stems of rice plants, whereas RSV-carrying SBPHs fed more on rice leaves. Moreover, the rice plants inoculated with RSV on the leaves showed more severe symptoms, with enhanced disease incidence and virus accumulation compared with rice plants inoculated at the top and bottom of stems, suggesting that the leaves are more susceptible to RSV than the stems of rice plants. These results demonstrate that RSV modulates the feeding preference of its transmitting vector SBPH from the stems to leaves of rice plants to promote virus infection. Interestingly, we also found that the leaves were more susceptible than the stems to rice black-streaked dwarf virus. This study proves that the feeding preference of insect vectors can be modulated by plant viruses to facilitate virus transmission.

Structural insights into the interaction between gabazine (SR-95531) and Laodelphax striatellus GABA receptors

γ-Aminobutyric acid receptors (GABARs) mediate fast inhibitory neurotransmission and are targets for insecticides. GABARs are composed of five subunits, the composition of which dictates the pharmacological characteristics of GABARs. Both competitive and noncompetitive GABAR antagonists can be used as insecticides. Gabazine is a potent competitive antagonist of mammalian α1β2γ2 GABARs; however, it is less potent against insect GABARs. To explore how gabazine interacts with GABARs, we examined whether the sensitivity of the small brown planthopper (Laodelphax striatellus) RDL GABAR (LsRDLR) to gabazine is increased when its amino acid residues are substituted with α1β2γ2 GABAR residues. In the results, two of the generated mutants showed enhanced gabazine sensitivity. Docking simulations of gabazine using LsRDLR homology models and an α1β2γ2 GABAR cryo-EM structure revealed that the accommodation of gabazine into the "aromatic box" in the orthosteric site lowered the binding energy. This information may help in designing GABAR-targeting insecticides with novel modes of action.

Decoyinine Induced Resistance in Rice against Small Brown Planthopper Laodelphax striatellus

Induced resistance against SBPH via microbial pesticides is considered as an eco-friendly and promising management approach. In this study, the induced resistance against SBPH in rice seedling by a new potential microbial pesticide, decoyinine (DCY), a secondary metabolite produced by Streptomyces hygroscopicus, was evaluated to investigate the effects of DCY on SBPH's biological and population parameters along with defense-related physiological and biochemical indices in rice against SBPH feeding. We found that DCY has potential to improve rice resistance and significantly reduced the fecundity of SBPH. Laboratory results revealed that DCY treated rice significantly changed SBPH's fecundity and population life table parameters. The concentrations of hydrogen peroxide (H2O2), soluble sugars and malondialdehyde (MDA) were significantly lower in DCY treated rice plants against SBPH infestation at 24, 48 and 96 hours post infestation (hpi), respectively. The concentrations of antioxidant enzymes, catalase (CAT) was significantly higher at 72 hpi, while super oxidase dismutase (SOD) and peroxidase (POD) concentrations were recorded higher at 96 hpi. The concentrations of synthases enzymes, phenyl alanine ammonia-lyase (PAL) was higher at 48 hpi, whereas polyphenol oxidase (PPO) concentration was maximum at 72 hpi against SBPH infestation. The results imply that DCY has unique properties to enhance rice resistance against SBPH by stimulating plant defensive responses. Microbial pesticides may be developed as an alternative to chemical pest control.

Membrane association of importin α facilitates viral entry into salivary gland cells of vector insects

The importin α family belongs to the conserved nuclear transport pathway in eukaryotes. However, the biological functions of importin α in the plasma membrane are still elusive. Here, we report that importin α, as a plasma membrane-associated protein, is exploited by the rice stripe virus (RSV) to enter vector insect cells, especially salivary gland cells. When the expression of three importin α genes was simultaneously knocked down, few virions entered the salivary glands of the small brown planthopper, Laodelphax striatellus Through hemocoel inoculation of virions, only importin α2 was found to efficiently regulate viral entry into insect salivary-gland cells. Importin α2 bound the nucleocapsid protein of RSV with a relatively high affinity through its importin β-binding (IBB) domain, with a dissociation constant K _D of 9.1 μM. Furthermore, importin α2 and its IBB domain showed a distinct distribution in the plasma membrane through binding to heparin in heparan sulfate proteoglycan. When the expression of importin α2 was knocked down in viruliferous planthoppers or in nonviruliferous planthoppers before they acquired virions, the viral transmission efficiency of the vector insects in terms of the viral amount and disease incidence in rice was dramatically decreased. These findings not only reveal the specific function of the importin α family in the plasma membrane utilized by viruses, but also provide a promising target gene in vector insects for manipulation to efficiently control outbreaks of rice stripe disease.

MicroRNA-315-5p promotes rice black-streaked dwarf virus infection by targeting a melatonin receptor in the small brown planthopper

BACKGROUND

MicroRNAs (miRNAs), a class of small non-coding endogenous RNAs, play key roles in various biological processes. Most plant viruses are transmitted by insect vectors. However, little is known about the function of miRNAs on plant virus-insect host interaction.

RESULTS

We investigated the role of miR-315-5p in regulation of plant viral infection in insects using a rice black-streaked dwarf virus (RBSDV) and small brown planthopper (SBPH) interaction system. Our results showed that miR-315-5p had the highest expression level in 2nd-instar nymph, and was highly expressed in the salivary gland and midgut in SBPH. miR-315-5p was in response to and regulated RBSDV infection in SBPH. Injection of miR-315-5p mimic, agomir-315, significantly increased the RBSDV accumulation, whereas injection of miR-315-5p inhibitor, antagomir-315, reduced virus accumulation in SBPH. Furthermore, a melatonin receptor was identified as a target gene of miR-315-5p by the dual luciferase reporter assay. Knockdown of the melatonin receptor significantly increased the expression of RBSDV coat protein gene S10 and replication related genes, S5-1, S6, and S9-1. Furthermore, treatment with melatonin receptor antagonist luzindole and activator agomelatine significantly increased and reduced RBSDV accumulation in SBPH, respectively. Compared to the control, miR-315-5p did not affect the efficiency of RBSDV acquisition in SBPH. However, the efficiency of RBSDV transmission was significantly reduced after injecting antagomir-315.

CONCLUSION

Taken together, our data reveal that miR-315-5p is beneficial for RBSDV infection in its insect vector by directly targeting a melatonin receptor. These findings provide a new insight to the function of miRNAs in virus-insect vector interaction. © 2021 Society of Chemical Industry.

A salivary calcium-binding protein from Laodelphax striatellus acts as an effector that suppresses defense in rice

BACKGROUND

Calcium (Ca²⁺ )-binding proteins in the saliva of herbivorous insects function as effectors to attenuate host plant defenses and thus improve insect feeding performance. Silencing these genes via transgenic plant-mediated RNAi is thus a promising pest control strategy. However, their sequences and functions in the small brown planthopper Laodelphax striatellus (SBPH) remain to be investigated.

RESULTS

We identified a putative EF-hand Ca²⁺ -binding protein (LsECP1) in SBPH watery saliva. LsECP1 was expressed extremely high in the salivary glands but at a low level during the egg stage. Transient LsECP1 expression in rice cells indicated its cytoplasm and nucleus localization. The bacterially expressed recombinant LsECP1 protein exhibited Ca²⁺ -binding activity. Rice plants fed by SBPH nymphs with knocked down LsECP1 exhibited higher levels of cytosolic Ca²⁺ , jasmonic acid (JA), jasmonoyl-isoleucine (JA-Ile) and hydrogen peroxide (H₂ O₂ ). Consistently, application of heterogeneously expressed LsECP1 protein suppressed wound-induced JA, JA-Ile and H₂ O₂ accumulation in rice. Thus, LsECP1 knockdown by dsRNA injection resulted in reduced feeding, fecundity and survival rates of SBPH reared on rice plants. Transgenic rice plants constitutively expressing LsECP1 dsRNA were produced, and plant-mediated LsECP1 knockdown enhanced rice resistance to SBPH.

CONCLUSION

SBPH LsECP1 acts as an effector to impair host rice defense responses and promotes SBPH performance. This discovery provides a potential gene target for plant-mediated RNAi-based pest management. © 2021 Society of Chemical Industry.

Host-Plant Induced Shifts in Microbial Community Structure in Small Brown Planthopper, Laodelphax striatellus (Homoptera: Delphacidae)

Microbiome associated with insects play vital roles in host ecology and physiology. The small brown planthopper (SBPH), Laodelphax striatellus, is a polyphagous insect pest that caused enormous damage to a wide range of cereal crops. Previous studies have assessed the effects of environmental factors, such as antibiotics, insecticide, and geographical habitat on the bacterial composition of SBPH. However, the influence of host plants on the microbial community in SBPH still unclear. Here, we characterized and compared the microbial community in three SBPH populations feeding on rice, barley, and wheat, respectively, using high-throughput amplicon sequencing. Our observations revealed that the microbiome harbored by SBPH was abundant and diverse. Ten phyla comprising 141 genera of bacteria were annotated, and four fungal phyla consisting of 47 genera were assigned. The bacteria belonging to the phylum Proteobacteria were the most prevalent and the fungi with the highest abundance were from the order Hypocreales. Comparative analysis showed that host plants could significantly induce structural changes of SBPH microbiome. Significant differences in abundance were observed in two main bacterial orders (Rickettsiales and Rhodospirillales) and three fungal classes (Sordariomycetes, an unclassified class in Ascomycota and Eurotiomycetes) among three host-adapted SBPH populations. Our results could broaden our understanding of interactions among SBPH, its microbial associates and host plants, and also represented the basis of future SBPH biological management.

Planthopper-Secreted Salivary Disulfide Isomerase Activates Immune Responses in Plants

The small brown planthopper (Laodelphax striatellus; SBPH) is a piercing-sucking insect that secretes salivary proteins into its plant host during feeding. However, the mechanisms by which these salivary proteins regulate plant defense responses remain poorly understood. Here, we identified the disulfide isomerase (LsPDI1) in the SBPH salivary proteome. LsPDI1 was highly expressed in the SBPH salivary glands and secreted into rice plants during feeding. Transient in planta LsPDI1 expression in the absence of signal peptide induced reactive oxygen species (ROS) burst, cell death, callose deposition, and jasmonic acid (JA) signaling pathway. Deletion mutant analysis revealed that either the a-b-b' or the b-b'-a' domains in LsPDI1 are required to induce cell death in plants. LsPDI1 and its orthologs were highly conserved among various planthopper species and strongly induced ROS burst and cell death in plants. Transient in Nicotiana benthamiana LsPDI1 expression impaired the performance of Spodoptera frugiperda and Myzus persicae on host plants. Hence, LsPDI1 is an important salivary elicitor that enhances plant resistance to insects by inducing the calcium, ROS, and JA signaling pathways. The findings of this study provide novel insights into the molecular mechanisms underlying plant-insect interactions.

Activation of Toll Immune Pathway in an Insect Vector Induced by a Plant Virus

The Toll pathway plays an important role in defense against infection of various pathogenic microorganisms, including viruses. However, current understanding of Toll pathway was mainly restricted in mammal and some model insects such as Drosophila and mosquitoes. Whether plant viruses can also activate the Toll signaling pathway in vector insects is still unknown. In this study, using rice stripe virus (RSV) and its insect vector (small brown planthopper, Laodelphax striatellus) as a model, we found that the Toll pathway was activated upon RSV infection. In comparison of viruliferous and non-viruliferous planthoppers, we found that four Toll pathway core genes (Toll, Tube, MyD88, and Dorsal) were upregulated in viruliferous planthoppers. When the planthoppers infected with RSV, the expressions of Toll and MyD88 were rapidly upregulated at the early stage (1 and 3 days post-infection), whereas Dorsal was upregulated at the late stage (9 days post-infection). Furthermore, induction of Toll pathway was initiated by interaction between a Toll receptor and RSV nucleocapsid protein (NP). Knockdown of Toll increased the proliferation of RSV in vector insect, and the dsToll-treated insects exhibited higher mortality than that of dsGFP-treated ones. Our results provide the first evidence that the Toll signaling pathway of an insect vector is potentially activated through the direct interaction between Toll receptor and a protein encoded by a plant virus, indicating that Toll immune pathway is an important strategy against plant virus infection in an insect vector.

Mitochondrial DNA diversity and population structure of Laodelphax striatellus across a broad geographic area in China

The small brown planthopper (SBPH), Laodelphax striatellus Fallén (Hemiptera: Delphacidae), is a crucial devastating rice pest in East Asia. To effectively control this pest, we investigate the genetic diversity, genetic differentiation and genetic structure of 49 populations in China based on a 596 bp fragment of the mitochondrial DNA cytochrome c oxidase subunit I (mtDNA COI) gene. Overall, 83 haplotypes were detected in 1253 mtDNA COI sequences. High levels of genetic variability (Hd = 0.756 ± 0.009, π = 0.00416 ± 0.00011) and genetic differentiation (F _ST = 0.262, p < .001) were observed. Bayesian inference phylogenetic and median-joining haplotype network analyses indicated no obvious geographical distribution pattern among haplotypes. Hierarchical AMOVA and SAMOVA revealed no genetically distinct groups and lack of obvious phylogeographic structure. Isolation by distance (IBD) analysis results demonstrated no correlation between genetic differentiation and geographic distance. Finally, the demographic history of SBPH examined by neutrality tests and mismatch distribution analyses illustrated a sudden population expansion at the large spatial scale in China.

Rice virus release from the planthopper salivary gland is independent of plant tissue recognition by the stylet

BACKGROUND

The transmission of plant viruses by arthropod vectors is closely related to feeding behavior. For persistently transmitted viruses, virus release means that virus moves through the salivary gland microvillus barriers of insects into plant via the stylet. However, whether virus release is dependent on plant tissue and component recognition by the stylet is unclear.

RESULTS

In this study, the small brown planthopper (SBPH) and two rice viruses transmitted by it were used as a model to explore this question. After the viruliferous insects penetrated a stretched membrane without plant tissue structure and ingested liquid food (rice sap, nutrient solution or water), both viruses were detected in the liquid food after only a 6 min inoculation access period, suggesting that the viruses were released from SBPH salivary gland independent of plant tissue and component recognition by the stylet. In subsequent electrical penetration graph (EPG) analysis, N4a-like and N4b-like waveforms, similar to N4a (phloem salivation before ingestion) and N4b (sieve element ingestion), were observed during SBPH penetrating the membrane, exhibiting normal feeding activity of planthopper on membrane, which further demonstrated that virus release from salivary gland was along with feeding activity, without the stylet sensing plant tissue. EPG analysis and identification of salivary proteins indicated more active feeding behavior and efficient salivation in viruliferous planthoppers.

CONCLUSION

These results suggest that the rice virus is released from insect salivary gland independent of plant tissue and component recognition by the stylet, and the simple virus release mode facilitates virus transmission by vectors. © 2020 Society of Chemical Industry.

A Plant Virus Ensures Viral Stability in the Hemolymph of Vector Insects through Suppressing Prophenoloxidase Activation

Large ratios of vector-borne plant viruses circulate in the hemolymph of their vector insects before entering the salivary glands to be transmitted to plants. The stability of virions in the hemolymph is vital in this process. Activation of the proteolytic prophenoloxidase (PPO) to produce active phenoloxidase (PO) is one of the major innate immune pathways in insect hemolymph. How a plant virus copes with the PPO immune reaction in its vector insect remains unclear. Here, we report that the PPO affects the stability of rice stripe virus (RSV), a notorious rice virus, in the hemolymph of a vector insect, the small brown planthopper. RSV suppresses PPO activation using viral nonstructural protein. Once the level of PO activity is elevated, RSV is melanized and eliminated from the hemolymph. Our work gives valuable clues for developing novel strategies for controlling the transmission of vector-borne plant viruses.

Most plant viruses require vector insects for transmission. Viral stability in the hemolymph of vector insects is a prerequisite for successful transmission of persistent plant viruses. However, knowledge of whether the proteolytic activation of prophenoloxidase (PPO) affects the stability of persistent plant viruses remains elusive. Here, we explored the interplay between rice stripe virus (RSV) and the PPO cascade of the vector small brown planthopper. Phenoloxidase (PO) activity was suppressed by RSV by approximately 60%. When the PPO cascade was activated, we found distinct melanization around RSV particles and serious damage to viral stability in the hemolymph. Viral suppression of PO activity was derived from obstruction of proteolytic cleavage of PPOs by binding of the viral nonstructural protein NS3. These results indicate that RSV attenuates the PPO response to ensure viral stability in the hemolymph of vector insects. Our research provides enlightening cues for controlling the transmission of vector-borne viruses.

Triazophos-induced vertical transmission of rice stripe virus is associated with host vitellogenin in the small brown planthopper Laodelphax striatellus

BACKGROUND

Rice stripe virus (RSV) is one of the most serious rice diseases in East Asia. The small brown planthopper (SBPH), Laodelphax striatellus, is an economically important rice pest. SBPH transmits RSV horizontally and vertically, resulting in serious rice economic losses. Exposure to sublethal doses of insecticides can induce many physiological and cellular changes in insects. However, the mechanism underlying triazophos-induced RSV vertical transmission remains unclear. Here, we investigated the role of vitellogenin (Vg) in triazophos-induced RSV vertical transmission.

RESULTS

RSV vertical transmission rates were significantly induced by sublethal exposure to triazophos. The transcript abundance of viral RNA3 segment (RNA3) and capsid protein (CP) in the ovaries of SBPH were also significantly increased. Triazophos induced the expression level of Vg in L. striatellus (LsVg) and increased the contents of ovarian protein and fat body protein. Knockdown of Vg significantly reduced the expression levels of LsVg and Vg receptor (LsVgR), and decreased RSV accumulations in the ovaries. Double-stranded Vg (dsVg)-mediated down-regulation could be rescued by exposure to triazophos. Vertical transmission rate of the dsVg-injected group was significantly decreased compared with the dsGFP-injected group, and triazophos significantly rescued the RSV vertical transmission rate of the dsVg-injected group.

CONCLUSION

Our results indicate that triazophos-induced RSV vertical transmission is associated with Vg. This work will help us to further elucidate sublethal doses of insecticides-mediated effects and develop new strategies for pest control. © 2019 Society of Chemical Industry.

Genome-Wide Single Nucleotide Polymorphisms are Robust in Resolving Fine-Scale Population Genetic Structure of the Small Brown Planthopper, Laodelphax striatellus (Fallén) (Hemiptera: Delphacidae)

Deciphering genetic structure and inferring migration routes of insects with high migratory ability have been challenging, due to weak genetic differentiation and limited resolution offered by traditional genotyping methods. Here, we tested the ability of double digest restriction-site associated DNA sequencing (ddRADseq)-based single nucleotide polymorphisms (SNPs) in revealing the population structure relative to 13 microsatellite markers by using four small brown planthopper populations as subjects. Using ddRADseq, we identified 230,000 RAD loci and 5,535 SNP sites, which were present in at least 80% of individuals across the four populations with a minimum sequencing depth of 10. Our results show that this large SNP panel is more powerful than traditional microsatellite markers in revealing fine-scale population structure among the small brown planthopper populations. In contrast to the mixed population structure suggested by microsatellites, discriminant analysis of principal components (DAPC) of the SNP dataset clearly separated the individuals into four geographic populations. Our results also suggest the DAPC analysis is more powerful than the principal component analysis (PCA) in resolving population genetic structure of high migratory taxa, probably due to the advantages of DAPC in using more genetic variation and the discriminant analysis function. Together, these results point to ddRADseq being a promising approach for population genetic and migration studies of small brown planthopper.

Evaluation of Rice stripe virus transmission efficiency by quantification of viral load in the saliva of insect vector

BACKGROUND

Persistent plant viruses transfer from insect gut to the hemolymph, and finally to the salivary glands before inoculation into the plant hosts with saliva during insect feeding. Virus accumulation in saliva is an important indicator for the transmission ability of an insect vector. In order to evaluate the transmission ability of the small brown planthopper to rice stripe virus (RSV), we successfully measured accumulation of RSV in the saliva of planthoppers via the absolute real-time quantitative polymerase chain reaction method by quantifying the copy numbers of viral genes.

RESULTS

After feeding on an artificial diet for 24 h, the copy numbers of viral genes of capsid protein (CP) and disease-specific protein (SP) can be detected in the saliva collected from as few as ten viruliferous planthoppers and ten non-viruliferous planthoppers after infected with RSV for 7 days. When the expression of planthopper G protein pathway suppressor 2 or c-Jun N-terminal kinase was knocked down, the copy numbers of CP and SP in the saliva varied accordingly.

CONCLUSION

Our study provided an accurate and convenient detection system to evaluate the transmission efficiency of RSV by small brown planthoppers, and this method may also be suitable for other persistent plant viruses. © 2019 Society of Chemical Industry.

Influences of two coexisting endosymbionts, CI-inducing Wolbachia and male-killing Spiroplasma, on the performance of their host Laodelphax striatellus (Hemiptera: Delphacidae)

The small brown planthopper Laodelphax striatellus (Hemiptera: Delphacidae) is reported to have the endosymbiont Wolbachia, which shows a strong cytoplasmic incompatibility (CI) between infected males and uninfected females. In the 2000s, female-biased L. striatellus populations were found in Taiwan, and this sex ratio distortion was the result of male-killing induced by the infection of another endosymbiont, Spiroplasma. Spiroplasma infection is considered to negatively affect both L. striatellus and Wolbachia because the male-killing halves the offspring of L. striatellus and hinders the spread of Wolbachia infection via CI. Spiroplasma could have traits that increase the fitness of infected L. striatellus and/or coexisting organisms because the coinfection rates of Wolbachia and Spiroplasma were rather high in some areas. In this study, we investigated the influences of the infection of these two endosymbionts on the development, reproduction, and insecticide resistance of L. striatellus in the laboratory. Our results show that the single-infection state of Spiroplasma had a negative influence on the fertility of L. striatellus, while the double-infection state had no significant influence. At late nymphal and adult stages, the abundance of Spiroplasma was lower in the double-infection state than in the single-infection state. In the double-infection state, the reduction of Spiroplasma density may be caused by competition between the two endosymbionts, and the negative influence of Spiroplasma on the fertility of host may be relieved. The resistance of L. striatellus to four insecticides was compared among different infection states of endosymbionts, but Spiroplasma infection did not contribute to increase insecticide resistance. Because positive influences of Spiroplasma infection were not found in terms of the development, reproduction, and insecticide resistance of L. striatellus, other factors improving the fitness of Spiroplasma-infected L. striatellus may be related to the high frequency of double infection in some L. striatellus populations.

The vitellogenin receptor has an essential role in vertical transmission of rice stripe virus during oogenesis in the small brown plant hopper

BACKGROUND

The small brown plant hopper (SBPH), Laodelphax striatellus Fallén, is one of the most destructive pests on rice. This pest transmits rice stripe virus (RSV) both horizontally and vertically, leading to major yield and economic losses in rice production. However, the way that RSV particles enter oocytes of SBPH remains largely unknown. Thus, identification of key factors involved in the interaction between SBPH and RSV in the ovary is crucial.

RESULTS

Transcriptome of non-viruliferous (NV) or high viruliferous (HV) SBPH ovaries at 24 and 48 h of emergence was sequenced. Differentially expressed genes analysis showed that vitellogenin receptor was significantly highly expressed in the ovary of the HV SBPH strains compared to NV strains. Quantitative real-time polymer chain reaction showed that the vitellogenin receptor in L. striatellus (LsVgR) was highly expressed in the ovaries of female adults and maintained a high level of expression at the early stage of ovary development. By using RNA interference, the expression of LsVgR in the ovaries of the HV strain was significantly decreased by 98.1%. RSV titer was reduced by 60.9% as quantified by viral RNA3 intergenic region and the transcripts of nucleocapsid protein gene (CP) reduced by 46.3%. The numbers of offspring hatched were significantly reduced in dsRNA-treated groups. The transcripts of CP were not affected by silencing LsVgR, whereas the abundance of RNA-dependent RNA polymerase increased by 15-fold in the member of surviving progenies.

CONCLUSION

Our results suggest that vitellogenin receptor participates in regulating RSV replication during oogenesis. © 2018 Society of Chemical Industry.

Olfactory co-receptor Orco stimulated by Rice stripe virus is essential for host seeking behavior in small brown planthopper

BACKGROUND

Laodelphax striatellus, the small brown planthopper (SBPH), is an economically important pest, besides sucking damage, which transmits rice viruses to cause severe damages to rice. In the process of virus transmission, the host orientation behavior of insect is mainly driven by olfaction. In this context, the molecular basis of olfaction in SBPH is of particular interest.

RESULTS

Here, we identified the gene that encodes olfactory receptor co-receptor (Orco) and analyzed its expression profiles in Rice stripe virus (RSV)-infected and RSV-free SBPH. It was found that LstrOrco shared high identity with other Orcos from different order insects. LstrOrco was mainly expressed in the head of SBPH, and its expression was significantly stimulated by RSV-infection. The behavioral bioassay revealed that viruliferous SBPH might have a stronger olfactory and seeking ability for rice than RSV-free insect. After silencing of LstrOrco expression, the olfaction and seeking behavior of nymphs for rice seedlings was significantly inhibited, mainly in the increase of the 'no response' percent and the prolongation of the response time.

CONCLUSION

These results suggested that Orco played an important role in olfactory signaling and seeking behavior of SBPH, which provided a basic for future development of olfactory-based agriculture pest management strategies. © 2018 Society of Chemical Industry.

Comparative Proteomic Analysis of Susceptible and Resistant Rice Plants during Early Infestation by Small Brown Planthopper

The small brown planthopper (Laodelphax striatellus Fallén, Homoptera, Delphacidae-SBPH) is one of the major destructive pests of rice (Oryza sativa L.). Understanding on how rice responds to SBPH infestation will contribute to developing strategies for SBPH control. However, the response of rice plant to SBPH is poorly understood. In this study, two contrasting rice genotypes, Pf9279-4 (SBPH-resistant) and 02428 (SBPH-susceptible), were used for comparative analysis of protein profiles in the leaf sheath of rice plants in responses to SBPH infestation. One hundred and thirty-two protein spots that were differentially expressed between the resistant and susceptible rice lines were identified with significant intensity differences (≥2-fold, P < 0.05) at 0, 6, and 12 h after SBPH infestation. Protein expression profile analysis in the leaf sheath of SBPH-resistant and SBPH-susceptible rice lines after SBPH infestation showed that proteins induced by SBPH feeding were involved mainly in stress response, photosynthesis, protein metabolic process, carbohydrate metabolic process, energy metabolism, cell wall-related proteins, amino acid metabolism and transcriptional regulation. Gene expression analysis of 24 differentially expressed proteins (DEPs) showed that more than 50% DEPs were positively correlated with their mRNA levels. Analysis of some physiological indexes mainly involved in the removal of oxygen reactive species showed that the levels of superoxide dismutase (SOD) and glutathione (GSH) were considerably higher in Pf9279-4 than 02428 during SBPH infestation. The catalase (CAT) activity and hydroxyl radical inhibition were lower in Pf9279-4 than 02428. Analysis of enzyme activities indicates that Pf9279-4 rice plants defend against SBPH through the activation of the pathway of the salicylic acid (SA)-dependent systemic acquired resistance. In conclusion, this study provides some insights into the molecular networks involved on cellular and physiological responses to SBPH infestation.

The c-Jun N-terminal kinase pathway of a vector insect is activated by virus capsid protein and promotes viral replication

No evidence has shown whether insect-borne viruses manipulate the c-Jun N-terminal kinase (JNK) signaling pathway of vector insects. Using a system comprising the plant virus Rice stripe virus (RSV) and its vector insect, the small brown planthopper, we have studied the response of the vector insect’s JNK pathway to plant virus infection. We found that RSV increased the level of Tumor Necrosis Factor-α and decreased the level of G protein Pathway Suppressor 2 (GPS2) in the insect vector. The virus capsid protein competitively bound GPS2 to release it from inhibiting the JNK activation machinery. We confirmed that JNK activation promoted RSV replication in the vector, whereas JNK inhibition caused a significant reduction in virus production and thus delayed the disease incidence of plants. These findings suggest that inhibition of insect vector JNK may be a useful strategy for controling the transmission of plant viruses.

DOI:http://dx.doi.org/10.7554/eLife.26591.001

There are over a thousand different viruses that infect plants. Many plant viruses are transmitted by insects that feed on the plants, much as mosquitoes spread diseases between people when feeding on blood. Often the plant virus can replicate inside the cells of the insect. However, unlike in the plant hosts, the viruses do not seem to cause disease in the insects that carry them.

Rice stripe disease is a major viral disease of rice that can reduce the crop’s yield by more than 50% in some areas. An insect called the small brown planthopper spreads the rice stripe virus between plants. Like other animals, insects have an immune system that protects them against viral infections. This means that the rice stripe virus must manipulate the planthopper’s immune system in order to replicate inside the insect’s cells. It was not clear how the virus did this, but answering this question could provide important clues to help scientists develop new ways to protect crops against plant viruses.

Wang, Zhao, Li et al. now show that rice stripe virus manipulates its insect host to produce more of a protein called TNF-α and less of a protein called GPS2. Moreover, a protein that makes up part of the virus also binds to GPS2. This stops GPS2 from inhibiting a conserved signaling pathway that involves an enzyme known as JNK. When the JNK signaling pathway becomes active, replication of the rice stripe virus inside the insect is accelerated. Further experiments showed that inhibiting JNK made it harder for the virus to replicate, which meant that it took longer for the disease to develop in rice plants.

These findings uncover a host of proteins that could be manipulated in insects to benefit rice agriculture. Such alterations could possibly be achieved through breeding or otherwise genetically modifying the insects to make them less able to carry viruses and then releasing them into wild populations. Alternatively, if further studies can identify chemicals that cause insect cells to alter the levels of the proteins, such chemicals could be administered to farmland to reduce the spread of viruses.

DOI:http://dx.doi.org/10.7554/eLife.26591.002

Bacterial microbiota in small brown planthopper populations with different rice viruses

The small brown planthopper (SBPH) is an important virus vector, transmitting Rice stripe virus (RSV), and Rice black-streaked dwarf virus (RBSDV). Insect symbionts play an essential role in the insect fitness, however, it is still unclear about their contributions to viral transmission by SBPH. Here, we investigated endosymbiont communities in non-viruliferous, RSV-infected, and RBSDV-infected SBPH populations using Illumina 16S rRNA gene MiSeq sequencing. In total, 281,803 effective sequences of the 16S rRNA gene were generated from different samples. Sequence analysis revealed the percentages of these bacterial groups in different SBPH populations on several taxonomic levels ranging from phyla to genera. The extremely consistent bacterial diversity and abundance indicated that RSV or RBSDV infection did not affect the composition and abundance of symbionts in SBPH. It was notable that Wolbachia was dominant in all populations. The symbiosis between Wolbachia and SBPH might be potentially studied and utilized to control pest SBPH in the future.

Expression pattern and pharmacological characterisation of two novel alternative splice variants of the glutamate-gated chloride channel in the small brown planthopper Laodelphax striatellus

BACKGROUND

Glutamate-gated chloride channels (GluCl) mediate fast inhibitory neurotransmission in invertebrate nervous systems. Although only one GluCl gene was presented in insects, it showed diverse alternative splicing that was speculated could affect channel function and pharmacology.

RESULTS

In this study, we isolated GluCl cDNAs from adults of the small brown planthopper (SBPH) Laodelphax striatellus and showed that six L. striatellus GluCl variants (LsGluCl-AS, LsGluCl-BS, LsGluCl-CS, LsGluCl-AL, LsGluCl-BL, LsGluCl-CL) were present in the SBPH. The expression patterns of six variants differed among developmental stages (egg, first- to fifth-instar nymphs, male and female adults) and among the body parts (head, thorax, abdomen, leg) of the female adult SBPH. All the transcripts were abundant in the head of the adult. When expressed in African clawed frog, Xenopus laevis, oocytes, the two functional variants (LsGluCl-AS, LsGluCl-AL) had similar EC₅₀ and IC₅₀ values for L-glutamate and channel blockers picrotoxinin and fipronil.

CONCLUSION

This study represents a comprehensive molecular, expression and pharmacological characterisation of GluCl in the SBPH. These findings should be useful in providing more opportunities to discover novel insect control chemicals. © 2016 Society of Chemical Industry.

Transcriptome Analysis Reveals the Response of Iron Homeostasis to Early Feeding by Small Brown Planthopper in Rice

It has been documented that planthopper attacks change iron (Fe) content of rice plants. To investigate whether planthopper attacks change rice Fe homeostasis at the molecular level, the response of rice Fe homeostasis to early feeding by small brown planthopper (SBPH) was examined by transcriptome profiling. Results showed that the concentration of Fe and nicotianamine decreased in resistant rice genotype and increased in susceptible rice genotype after attack by SBPH. Transcriptome profiling showed that 13 and 21 Fe homeostasis-related genes encoded enzymes that were involved in phytosiderophore biosynthesis and that Fe transporters and regulators displayed altered expression in resistant and susceptible rice genotypes, respectively, after attack by SBPH. This revealing response of Fe homeostasis to planthopper attack in rice at the molecular level provided new insight into rice plants' response to planthopper attack and uncovered a novel physiological response of rice to planthopper attack, which extended our knowledge of the early interaction between rice and SBPH.

Seasonal Changes in the Percentage of Rice Stripe Virus Viruliferous Laodelphax striatellus (Hemiptera: Delphacidae) in Paddy Fields in Japan

Rice stripe disease, which is caused by Rice stripe virus (RSV), is one of the most serious viral diseases of rice. RSV is transmitted in a persistent manner by Laodelphax striatellus (Fallén). The incidence of the disease can be estimated from the density of viruliferous vectors. Understanding seasonal changes of the percentage of viruliferous L. striatellus can facilitate forecasting and controlling the disease. In paddies, the percentage of viruliferous insects fluctuated in phase with the rate of detection of RSV-infected rice; it gradually increased from July to August, plateaued or temporarily declined in September, and increased sharply on ratoons in October. These findings indicate that horizontal transmission of RSV from diseased plants to vector insects occurred frequently, and the insects acquired RSV from the ratoons. However, the percentages of viruliferous insects overwintering in poaceous weeds, the main hosts for L. striatellus in winter, were lower than those in ratoons. Few L. striatellus that acquired RSV from ratoons seemed to move to overwintering sites and transmit the virus to the next generation. However, there was a tendency for the percentages of viruliferous overwintering insects to be higher on paddy ridges than in river levees. Insects could probably move from ratoons to poaceous weeds when the weeds were near a paddy. Although increasing percentage of viruliferous insects on ratoons seem to have relatively little impact on RSV dynamics in the next crop season, appropriate weed management around paddies is still needed to reduce the incidence of rice stripe disease.

Study of the long-distance migration of small brown planthoppers Laodelphax striatellus in China using next-generation sequencing

BACKGROUND

The small brown planthopper (Laodelphax striatellus) is a widespread insect pest of rice in East Asia. Previous studies have shown the long-distance migrations undertaken by L. striatellus, but have not provided molecular evidence to support this.

RESULTS

Long-distance immigration has occurred in the north-east coastal rice-growing region of China. Using the specific-locus amplified fragment sequencing technique, sequence data for 2.7 Gb of an abruptly increased population and 13 L. striatellus local populations from a range of regions in China that have serious rice stripe disease were obtained. A total of 2572 single nucleotide polymorphisms (SNPs) and 37 indels were detected, and the genotypes of many polymorphism sites were heterozygous in every sample, which indicated that there were rich genetic differences among the populations, and that the migration of insect pests accelerated the gene flow and increased the heterozygosity of L. striatellus populations. The genetic distance and the polymorphism markers among different populations showed that the abruptly increased population in Liaoning Province is close to several populations from Jiangsu Province and Shandong Province.

CONCLUSION

The vector that caused rice stripe disease in the north-east of China was an immigrant population; however, the population may be formed from several groups from different areas, such as Jiangsu and Shandong provinces.

Biochemical and molecular characterisation and cross-resistance in field and laboratory chlorpyrifos-resistant strains of Laodelphax striatellus (Hemiptera: Delphacidae) from eastern China

BACKGROUND

Laboratory selection is often employed in resistance mechanism studies because field-derived populations commonly do not have high enough resistance for such studies. In the present study, a field-collected Laodelphax striatellus population from eastern China was laboratory selected for chlorpyrifos resistance and susceptibility, and the developed strains, along with a field population, were studied for cross-resistance and resistance mechanisms at biochemical and molecular levels.

RESULTS

A 158.58-fold chlorpyrifos-resistant strain (JH-chl) and a chlorpyrifos-susceptible strain (JHS) were established after laboratory selection of 25 generations. Cross-resistance to deltamethrin, diazinon, methomyl, carbosulfan, acephate and imidacloprid were detected in JH-chl and a field-collected strain (JHF). Synergism and enzyme activity data suggested potential involvement of P450s and esterases in JH-chl as well as AChE alteration. Furthermore, CYP6AY3v2, CYP306A2v2, CYP353D1v2 and LSCE36 genes were significantly overexpressed in JH-chl (6.87-12.14-fold). Feeding of dsRNAs reduced the expression of the four target genes (35.6-56.8%) and caused significant adult mortality (75.21-88.45%), implying resistance reduction. However, mechanism(s) conferring chlorpyrifos resistance in JHF were unclear.

CONCLUSION

In contrast to previous reports, multiple overexpressed detoxification genes were potentially associated with chlorpyrifos resistance, as confirmed by RNAi feeding tests. Chlorpyrifos resistance exhibits cross-resistance with insecticides in the same and different classes.

Transcriptome Analysis of the Small Brown Planthopper, Laodelphax striatellus Carrying Rice stripe virus

Rice stripe virus (RSV), the type member of the genus Tenuivirus, transmits by the feeding behavior of small brown planthopper (SBPH), Laodelphax striatellus. To investigate the interactions between the virus and vector insect, total RNA was extracted from RSV-viruliferous SBPH (RVLS) and non-viruliferous SBPH (NVLS) adults to construct expressed sequence tag databases for comparative transcriptome analysis. Over 30 million bases were sequenced by 454 pyrosequencing to construct 1,538 and 953 of isotigs from the mRNA of RVLS and NVLS, respectively. The gene ontology (GO) analysis demonstrated that both libraries have similar GO structures, however, the gene expression pattern analysis revealed that 17.8% and 16.8% of isotigs were up- and down-regulated significantly in the RVLS, respectively. These RSV-dependently regulated genes possibly have important roles in the physiology of SBPH, transmission of RSV, and RSV and SBPH interaction.