Tissue-dependence and sensitivity of the systemic RNA interference response in the desert locust, Schistocerca gregaria

Assessing nanoparticle-enabled dsRNA delivery for oral RNAi in two orthopteran pests: Schistocerca gregaria and Melanoplus sanguinipes

Locusts and pest grasshoppers (Orthoptera: Acrididae) cause significant economic losses to agricultural crops and rangeland forage and can even cause humanitarian crises during periodic plagues. Current management methods for these insects rely heavily on broad-spectrum chemical insecticides and growth regulators, which can affect non-target organisms and may eventually develop resistance in the targeted species. Therefore, we assessed the potential of RNA interference (RNAi)-based alternative strategies that could supplement the current management methods. In insects, RNAi efficiency is known to vary with the method of double-stranded RNA (dsRNA) delivery. In this study, we tested two different delivery methods (injection and oral feeding) in the desert locust (Schistocerca gregaria) and the migratory grasshopper (Melanoplus sanguinipes) and showed that both species are sensitive to the injection but not to the oral feeding of dsRNA, likely due to high nuclease activity or poor uptake in the midgut. To address these limitations, we explored the utility of using nanoparticles that are often used for drug delivery in humans as a carrier (poly lactic-co-glycolic acid [PLGA] and poly(L-arginine)-polyethylene glycol [PLA-PEG]) for orally delivering dsRNA to the insect pests. Although the PLGA nanoparticles successfully permeated the digestive system into the hemolymph and the PLA-PEG-dsRNA complexes remained stable in the midgut juice and were detected in the fat body, neither dsRNA-encapsulating nanoparticle elicited gene knockdown upon oral feeding. These results suggest that nanoparticle-based oral delivery improves dsRNA stability and midgut permeation. However, additional barriers must be overcome to achieve efficient oral RNAi in these orthopteran pest species.

Transgenic expression of mAChR-C dsRNA in maize confers efficient locust control

Plant-mediated RNA interference (RNAi), in which double-stranded RNAs (dsRNAs) targeting insect genes are expressed in plants for insect ingestion, has shown great potential for the control of herbivorous insect pests. Locusts, which are among the most destructive agricultural insect pests, appear to be resistant to orally delivered naked dsRNA. Moreover, the feasibility of using plant-mediated RNAi to suppress target gene expression in locusts remains unclear. Using the migratory locust Locusta migratoria, we report that the C-type muscarinic acetylcholine receptor (mAChR-C), a G protein-coupled receptor (GPCR) belonging to the bioamine receptor subfamily, plays a pivotal role in chitin metabolism by regulating genes responsible for chitin synthesis and degradation. Knockdown of locust mAChR-C via injection of dsRNA caused defective nymph molting and metamorphosis, accompanied by malformation, arrested development, and impaired motility. Notably, locusts fed transgenic maize expressing locust mAChR-C dsRNAs exhibited defective phenotypes similar to those subjected to mAChR-C dsRNA injection. In contrast, ingestion of transgenic maize expressing locust mAChR-C dsRNA had no significant effects on non-target insects, including the fall armyworm Spodoptera frugiperda, the cotton bollworm Helicoverpa armigera, the Asian corn borer Ostrinia furnacalis, and the oriental armyworm Mythimna separata. Our results suggest that transgenic expression of locust mAChR-C dsRNA is an effective RNAi approach for locust control and offers a promising eco-friendly strategy for locust management.

Nutritional and hormonal regulation of mitochondrial biogenesis drives fat body remodeling for reproductive competence

INTRODUCTION

Insect fat body serves as a central hub for energy mobilization and protein synthesis. During larval metamorphosis, fat body undergoes programmed cell death and tissue disassembly. Following adult eclosion, fat body reconstructs with cell proliferation and becomes competent for large-scale vitellogenin (Vg) synthesis required for the maturation of dozens of eggs.

OBJECTIVES

This study aims to uncover the molecular mechanisms underlying the remodeling of fat body in acquisition of competence for massive Vg production.

METHODS

RNA-seq and metabolomics were used for identification of differentially expressed genes and metabolites. RNAi was applied for gene knockdown. Transmission electron microscope, MitoTracker staining, mitochondrial DNA quantification, ATP and citrate synthase assays were employed for examining mitochondrial biogenesis. Dual-luciferase reporter assay and EMSA were performed for transcriptional regulation. qRT-PCR and western blot were performed for measuring Vg synthesis.

RESULTS

Transcriptomic and metabolomic analyses revealed significant upregulation of genes and metabolites involved in mitochondrial biogenesis in the fat body of adult locusts. PGC-1α was highly expressed in adult fat body. Knockdown of PGC-1α reduced mitochondrial biogenesis, fat body cell number, Vg synthesis and ovarian development. CREBB bound to PGC-1α promoter and activated its transcription. CREBB depletion impaired mitochondrial biogenesis and fat body remodeling. Moreover, loss of TORC1 function suppressed CREBB function and PGC-1α expression, subsequently disrupting mitochondrial biogenesis and fat body remodeling. Juvenile hormone (JH) deprivation also decreased CREBB function and PGC-1α expression, which was reversible with JH treatment. Our results suggest that TORC1 and JH coordinate CREBB-upregulated PGC-1α expression, which promotes mitochondrial biogenesis and fat body remodeling for Vg synthesis and egg production.

CONCLUSION

The findings provide new insights into the molecular mechanisms of post-metamorphic fat body development, and highlight the role of JH/TORC1/CREBB/PGC-1α/mitochondrial biogenesis axis in insect reproduction. The data also offer potential targets for insect pest control.

Nano-delivery platform with strong protection and efficient delivery: preparation of self-assembled RNA pesticide with dual RNAi targets against Apolygus lucorum

BACKGROUND

RNA pesticide is regarded as the "third revolution in the history of pesticides". However, the double-stranded RNA (dsRNA) is easily degraded in the environment, and its delivery efficiency is not sufficient for pest management. This study aimed to construct a star polycation (SPc)-based delivery platform with strong protection and efficient delivery to develop a self-assembled RNA pesticide with dual RNA interference (RNAi) targets.

RESULTS

The nanocarrier SPc was applied to assemble with dsRNA via electrostatic interaction, hydrogen bond and Van der Waals force, and the self-complexation with SPc formed nanoscale dsRNA/SPc complex. The SPc could protect the dsRNA from the degradation by midgut fluid or RNase A, thus significantly increasing the stability of dsRNA under various environmental conditions. Meanwhile, the SPc was able to improve the translocation of dsRNA across insect cuticle, and increase its plant uptake. Then, dsECR-A and dsTre-1 fragments were individually screened, and the dsECR-A and dsTre-1 fragments with good control effects were co-expressed in pET28-BL21 (DE3) RNase III - system to prepare the dsECR-A + Tre-1/SPc complex. Both topical application and spraying of dsECR-A + Tre-1/SPc complex could effectively control a piercing-sucking agricultural pest Apolygus lucorum. The SPc-loaded dsECR-A + Tre-1 could up-regulate endocytosis-related genes and down-regulate cuticle biosynthesis-related genes, which primarily inhibited insect growth and development.

CONCLUSIONS

Our study comprehensively demonstrated the advantages of SPc-based dsRNA delivery platform, and developed a self-assembled RNA pesticide with dual RNAi targets, which provided a reference for the design of novel RNA pesticides.

RNAi of the elastomeric protein resilin reduces jump velocity and resilience to damage in locusts

Resilin, an elastomeric protein with remarkable physical properties that outperforms synthetic rubbers, is a near-ubiquitous feature of the power amplification mechanisms used by jumping insects. Catapult-like mechanisms, which incorporate elastic energy stores formed from a composite of stiff cuticle and resilin, are frequently used by insects to translate slow muscle contractions into rapid-release recoil movements. The precise role of resilin in these jumping mechanisms remains unclear, however. We used RNAi to reduce resilin deposition in the principal energy-storing springs of the desert locust (Schistocerca gregaria) before measuring jumping performance. Knockdown reduced the amount of resilin-associated fluorescence in the semilunar processes (SLPs) by 44% and reduced the cross-sectional area of the tendons of the hind leg extensor-tibiae muscle by 31%. This affected jumping in three ways: First, take-off velocity was reduced by 15% in knockdown animals, which could be explained by a change in the extrinsic stiffness of the extensor-tibiae tendon caused by the decrease in its cross-sectional area. Second, knockdown resulted in permanent breakages in the hind legs of 29% of knockdown locusts as tested by electrical stimulation of the extensor muscle, but none in controls. Third, knockdown locusts exhibited a greater decline in distance jumped when made to jump in rapid succession than did controls. We conclude that stiff cuticle acts as the principal elastic energy store for insect jumping, while resilin protects these more brittle structures against breakage from repeated use.

Nanocarrier-Based Eco-Friendly RNA Pesticides for Sustainable Management of Plant Pathogens and Pests

The production of healthy agricultural products has increased the demand for innovative and sustainable plant protection technologies. RNA interference (RNAi), described as post-transcriptional gene silencing, offers great opportunities for developing RNA pesticides for sustainable disease and pest control. Compared with traditional synthesized pesticides, RNA pesticides possess many advantages, such as strong targeting, good environmental compatibility, and an easy development process. In this review, we systematically introduce the development of RNAi technology, highlight the advantages of RNA pesticides, and illustrate the challenges faced in developing high-efficiency RNA pesticides and the benefits of nanocarriers. Furthermore, we introduce the process and mechanism of nanocarrier-mediated RNAi technology, summarize the applications of RNA pesticides in controlling plant pathogens and pests, and finally outline the current challenges and future prospects. The current review provides theoretical guidance for the in-depth research and diversified development of RNA pesticides, which can promote the development and practice of nanocarrier-mediated RNAi.

Challenges of Robust RNAi-Mediated Gene Silencing in Aedes Mosquitoes

In this study, we report the complexities and challenges associated with achieving robust RNA interference (RNAi)-mediated gene knockdown in the mosquitoes Aedes aegypti and Aedes albopictus, a pivotal approach for genetic analysis and vector control. Despite RNAi's potential for species-specific gene targeting, our independent efforts to establish oral delivery of RNAi for identifying genes critical for mosquito development and fitness encountered significant challenges, failing to reproduce previously reported potent RNAi effects. We independently evaluated a range of RNAi-inducing molecules (siRNAs, shRNAs, and dsRNAs) and administration methods (oral delivery, immersion, and microinjection) in three different laboratories. We also tested various mosquito strains and utilized microorganisms for RNA delivery. Our results reveal a pronounced inconsistency in RNAi efficacy, characterized by minimal effects on larval survival and gene expression levels in most instances despite strong published effects for the tested targets. One or multiple factors, including RNase activity in the gut, the cellular internalization and processing of RNA molecules, and the systemic dissemination of the RNAi signal, could be involved in this variability, all of which are barely understood in mosquitoes. The challenges identified in this study highlight the necessity for additional research into the underlying mechanisms of mosquito RNAi to develop more robust RNAi-based methodologies. Our findings emphasize the intricacies of RNAi application in mosquitoes, which present a substantial barrier to its utilization in genetic control strategies.

RNA Interference in Insects: From a Natural Mechanism of Gene Expression Regulation to a Biotechnological Crop Protection Promise

Insect pests rank among the major limiting factors in agricultural production worldwide. In addition to direct effect on crops, some phytophagous insects are efficient vectors for plant disease transmission. Large amounts of conventional insecticides are required to secure food production worldwide, with a high impact on the economy and environment, particularly when beneficial insects are also affected by chemicals that frequently lack the desired specificity. RNA interference (RNAi) is a natural mechanism gene expression regulation and protection against exogenous and endogenous genetic elements present in most eukaryotes, including insects. Molecules of double-stranded RNA (dsRNA) or highly structured RNA are the substrates of cellular enzymes to produce several types of small RNAs (sRNAs), which play a crucial role in targeting sequences for transcriptional or post-transcriptional gene silencing. The relatively simple rules that underlie RNAi regulation, mainly based in Watson-Crick complementarity, have facilitated biotechnological applications based on these cellular mechanisms. This includes the promise of using engineered dsRNA molecules, either endogenously produced in crop plants or exogenously synthesized and applied onto crops, as a new generation of highly specific, sustainable, and environmentally friendly insecticides. Fueled on this expectation, this article reviews current knowledge about the RNAi pathways in insects, and some other applied questions such as production and delivery of recombinant RNA, which are critical to establish RNAi as a reliable technology for insect control in crop plants.

RNAi turns 25:contributions and challenges in insect science

Since its discovery in 1998, RNA interference (RNAi), a Nobel prize-winning technology, made significant contributions to advances in biology because of its ability to mediate the knockdown of specific target genes. RNAi applications in medicine and agriculture have been explored with mixed success. The past 25 years of research on RNAi resulted in advances in our understanding of the mechanisms of its action, target specificity, and differential efficiency among animals and plants. RNAi played a major role in advances in insect biology. Did RNAi technology fully meet insect pest and disease vector management expectations? This review will discuss recent advances in the mechanisms of RNAi and its contributions to insect science. The remaining challenges, including delivery to the target site, differential efficiency, potential resistance development and possible solutions for the widespread use of this technology in insect management.

An oral dsRNA delivery system based on chitosan induces G protein-coupled receptor kinase 2 gene silencing for Apolygus lucorum control

RNA interference (RNAi) is recognized as a new and environmentally friendly pest control strategy due to its high specificity. However, the RNAi efficiency is relatively low in many sucking insect pests, such as Apolygus lucorum. Therefore, there is an urgent need to develop new and effective ways of dsRNA delivery. Bacterially expressed or T7 synthesized dsRNA targeting a G Protein-Coupled Receptor Kinase 2 gene was mixed with chitosan in a 1:2 ratio by mass. The size of the chitosan/dsRNA nanoparticles was 69 ± 12 nm, and the TEM and AFM images showed typical spherical or ellipsoidal structures. The chitosan nanoparticles protected the dsRNA from nuclease activity, and pH and temperature-dependent degradation, and the fluorescently-tagged nanoparticles were found to be stable on the surface of green bean plants (48 h) (Phaseolus vulgaris) and were absorbed by midgut epithelial cells and transported to hemolymph. Once fed to the A. lucorum nymph, chitosan/dsRNA could effectively inhibit the expression of the G protein-coupled receptor kinase 2 gene (70%), and led to significantly increase mortality (50%), reduced weight (26.54%) and a prolonged developmental period (8.04%). The feeding-based and chitosan-mediated dsRNA delivery method could be a new strategy for A. lucorum management, providing an effective tool for gene silencing of piercing-sucking insects.

The Study of Cell-Penetrating Peptides to Deliver dsRNA and siRNA by Feeding in the Desert Locust, Schistocerca gregaria

RNA(i) interference is a gene silencing mechanism triggered by double-stranded (ds)RNA, which promises to contribute to species-specific insect pest control strategies. The first step toward the application of RNAi as an insecticide is to enable efficient gene silencing upon dsRNA oral delivery. The desert locust, Schistocerca gregaria is a devastating agricultural pest. While this species is responsive to dsRNA delivered by intra-hemocoelic injection, it is refractory to orally delivered dsRNA. In this study, we evaluated the capacity of five cell-penetrating peptides (CPPs) to bind long dsRNA and protect it from the locust midgut environment. We then selected the CPP EB1 for further in vivo studies. EB1:dsRNA complexes failed to induce RNAi by feeding. Interestingly, we observed that intra-hemocoelic injection of small-interfering (si)RNAs does not result in a silencing response, but that this response can be obtained by injecting EB1:siRNA complexes. EB1 also protected siRNAs from midgut degradation activity. However, EB1:siRNA complexes failed as well in triggering RNAi when fed. Our findings highlight the complexity of the dsRNA/siRNA-triggered RNAi in this species and emphasize the multifactorial nature of the RNAi response in insects. Our study also stresses the importance of in vivo studies when it comes to dsRNA/siRNA delivery systems.

Both LmDicer-1 and two LmDicer-2s participate in siRNA-mediated RNAi pathway and contribute to high gene silencing efficiency in Locusta migratoria

Dicers belong to a class of large RNase III multidomain ribonucleases and are central components of the RNA interference (RNAi) pathways. In insects, Dicer-2 has been known to cleave long double-stranded RNA (dsRNA) in small interfering RNA (siRNA)-mediated-RNAi pathway. However, Dicer-1 is responsible for cleaving precursor microRNAs (pre28 miRNAs) in miRNA-mediated RNAi pathway. In this study, we identified one LmDicer-1 and two LmDicer-2 (LmDicer-2a and LmDicer-2b) genes in Locusta migratoria. The RNAi of RNAi assay showed that knockdown of each of the Dicer genes reduced RNAi efficiency against a target gene (Lmβ-Tubulin), suggesting that all these genes participated in the siRNA-mediated RNAi pathway. Sequence analyses of the siRNAs generated from dsLmβ-Tubulin after silencing each LmDicer gene showed no significant difference in the pattern of siRNAs mapped to dsLmβ-Tubulin. This result indicated that all the three LmDicers are capable of generating siRNAs from the dsRNA. We then generated recombinant proteins consisting of different domains using Escherichia coli expression system and incubated each recombinant protein with dsLmβ-Tubulin. We found that the recombinant Dicer proteins successfully cleaved dsLmβ-Tubulin. However, LmDicer-2a-R lacking dsRBD domain lost activity, suggesting that dsRBD domain is critical for Dicer function. Furthermore, overexpression of these proteins in Drosophila S2 cells improved RNAi efficiency. Our siRNA affinity chromatography and LC-MS/MS analysis identified LmDicer-2a, LmDicer-2b, LmR2D2, LmAgo2a, LmAgo1, LmStaufen and LmTARBP2 as constituents of RNA-induced silencing complex. Taken together, these data show that both LmDicer-1 and two LmDicer-2s all participate in siRNA-mediated RNAi pathway and likely contribute to high RNAi efficiency in L. migratoria.

RNAi-based pest control: Production, application and the fate of dsRNA

The limitations of conventional pesticides have raised the demand for innovative and sustainable solutions for plant protection. RNA Interference (RNAi) triggered by dsRNA has evolved as a promising strategy to control insects in a species-specific manner. In this context, we review the methods for mass production of dsRNA, the approaches of exogenous application of dsRNA in the field, and the fate of dsRNA after application. Additionally, we describe the opportunities and challenges of using nanoparticles as dsRNA carriers to control insects. Furthermore, we provide future directions to improve pest management efficiency by utilizing the synergistic effects of multiple target genes. Meanwhile, the establishment of a standardized framework for assessment and regulatory consensus is critical to the commercialization of RNA pesticides.

Development of RNAi Methods for the Mormon Cricket, Anabrus simplex (Orthoptera: Tettigoniidae)

Mormon crickets are a major rangeland pest in the western United States and are currently managed by targeted applications of non-specific chemical insecticides, which can potentially have negative effects on the environment. In this study, we took the first steps toward developing RNAi methods for Mormon crickets as a potential alternative to traditional broad-spectrum insecticides. To design an effective RNAi-based insecticide, we first generated a de novo transcriptome for the Mormon cricket and developed dsRNAs that could silence the expression of seven housekeeping genes. We then characterized the RNAi efficiencies and time-course of knockdown using these dsRNAs, and assessed their ability to induce mortality. We have demonstrated that it is possible to elicit RNAi responses in the Mormon cricket by injection, but knockdown efficiencies and the time course of RNAi response varied according to target genes and tissue types. We also show that one of the reasons for the poor knockdown efficiencies could be the presence of dsRNA-degrading enzymes in the hemolymph. RNAi silencing is possible in Mormon cricket, but more work needs to be done before it can be effectively used as a population management method.

RNA meets toxicology: efficacy indicators from the experimental design of RNAi studies for insect pest management

RNA interference (RNAi) selectively targets genes and silences their expression in vivo, causing developmental defects, mortality and altered behavior. Consequently, RNAi has emerged as a promising research area for insect pest management. However, it is not yet a viable alternative over conventional pesticides despite several theoretical advantages in safety and specificity. As a first step toward a more standardized approach, a machine learning algorithm was used to identify factors that predict trial efficacy. Current research on RNAi for pest management is highly variable and relatively unstandardized. The applied random forest model was able to reliably predict mortality ranges based on bioassay parameters with 72.6% accuracy. Response time and target gene were the most important variables in the model, followed by applied dose, double-stranded RNA (dsRNA) construct size and target species, further supported by generalized linear mixed effect modeling. Our results identified informative trends, supporting the idea that basic principles of toxicology apply to RNAi bioassays and provide initial guidelines standardizing future research similar to studies of traditional insecticides. We advocate for training that integrates genetic, organismal, and toxicological approaches to accelerate the development of RNAi as an effective tool for pest management. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Odor-Induced Vomiting Is Combinatorially Triggered by Palp Olfactory Receptor Neurons That Project to the Lobus Glomerulatus in Locust Brain

Although vomiting is commonly recognized as a protective reaction in response to toxic stimuli, the elaborate sensory processes and necessary molecular components are not fully clear, which is due to a lack of appropriate experimental animal models. Vomiting reflex to volatile chemicals renders locust one candidate for vomiting model. Here, we identified a panel of chemical cues that evoked evident vomiting in locust nymphs and demonstrated the selected combinatorial coding strategy that palps but not antennae olfactory receptor neurons (ORNs) employed. Specifically, knocking down individual palp odorant receptors (ORs) such as OR17, OR21, and OR22 attenuated the vomiting intensity evoked by E-2-hexenal and hexanal, while suppression of OR12 and OR22 augmented vomiting to E-2-hexenal and 2-hexanone, respectively. Furthermore, dual-RNAi treatment against OR17 or OR21 together with OR22 resulted in a much lower response intensity than that of individual OR suppression. Furthermore, OR12 was revealed in palp sensilla basiconica (pb) subtype 3 to tune the neuronal decaying activity to E-2-hexenal. Finally, anterograde labeling indicated that palp ORNs primarily projected into the lobus glomerulatus (LG), and the projection neurons (PNs) in the LG further projected into the accessary calyx (ACA). Together, the establishment of an olfaction-inducible vomiting model in locusts deepens the understanding of olfactory coding logics and provides an opportunity to clarify the neural basis underlying animal vomiting.

Increased RNAi Efficiency by dsEGFP-Induced Up-Regulation of Two Core RNAi Pathway Genes (OfDicer2 and OfAgo2) in the Asian Corn Borer (Ostrinia furnacalis)

Simple Summary

RNA interference (RNAi) has shown great potentials as a novel technology for insect pest management. However, numerous studies have shown that the efficiency of RNAi varies substantially among different insect species. For example, as a major insect pest of corn, the Asian corn borer (Ostrinia furnacalis) showed very low RNAi efficiency. Therefore, it is necessary to develop new strategies for enhancing RNAi efficiency in insects with low RNAi efficiency. In this study, six core RNAi pathway genes were identified and characterized from O. furnacalis transcriptome database. After dsEGFP was injected into O. furnacalis, the expression of the core RNAi pathway genes (OfDicer2 and OfAgo2) was significantly up-regulated in response to the exposure of dsEGFP. As a result, the RNAi efficiency against the target genes in certain tissues of O. furnacalis was significantly improved. These results suggest that RNAi efficiency can be improved by inducing the expression of key RNAi pathway genes in O. furnacalis.

Abstract

RNA interference (RNAi) is a sequence-specific gene silencing mechanism that holds great promise for effective management of agricultural pests. Previous studies have shown that the efficacy of RNAi varies among different insect species, which limits its wide spread application in the field of crop protection. In this study, we identified and characterized six core RNAi pathway genes including OfDicer1, OfDicer2, OfR2D2, OfAgo1, OfAgo2, and OfAgo3 from the transcriptomic database of the Asian corn borer (Ostrinia furnacalis). Domain analysis showed that the six deduced proteins contained the necessary functional domains. Insect developmental stage- and tissue-specific expression analysis showed that five genes were expressed in all the stages and tissues examined except OfAgo3, which showed low expression in larvae, and high expression in pupae and adults and in the midgut. RT-qPCR was performed to examine the response of these six genes to exogenous double-stranded RNA (dsRNA). Interestingly, the transcript levels of OfDicer2 and OfAgo2 were significantly enhanced after the injection of dsEGFP at different time points and tissues investigated. Consequently, the RNAi efficiency in targeting the insect endogenous genes can be greatly enhanced in the hemolymph or midgut. Taken together, our investigations suggest that RNAi efficiency can be enhanced by pre-injection of dsRNA to induce the RNAi core machinery genes, which could be a useful strategy to improving RNAi efficiency for studying gene functions under laboratory conditions.

Glucosamine-6-phosphate N-acetyltransferase gene silencing by parental RNA interference in rice leaf folder, Cnaphalocrocis medinalis (Lepidoptera: Pyralidae)

Parental RNAi (pRNAi) is a response of RNA interference in which treated insect pests progenies showed a gene silencing phenotypes. pRNAi of CmGNA gene has been studied in Cnaphalocrocis medinalis via injection. Our results showed significant reduction in ovulation per female that was 26% and 35.26% in G1 and G2 generations, respectively. Significant reduction of hatched eggs per female were observed 23.53% and 45.26% as compared to control in G1–G2 generations, respectively. We also observed the significant variation in the sex ratio between female (40% and 53%) in G1–G2 generations, and in male (65%) in G1 generation as compared to control. Our results also demonstrated the significant larval mortality (63% and 55%) and pupal mortality (55% and 41%), and significant reduction of mRNA expression level in G1 and G2 generations. Our findings have confirmed that effectiveness of pRNAi induced silencing on the CmGNA target gene in G1–G2 generations of C. medinalis. These results suggested the potential role of pRNAi in insect pest resistance management strategies.

Double-Strand RNA (dsRNA) Delivery Methods in Insects: Diaphorina citri

RNAi is a gene-silencing mechanism conserved in the vast majority of eukaryotes. It is widely used to study gene function in animals due to the ease of eliciting gene knockdown. Beyond research applications, RNAi technology based on exogenous dsRNA is a promising candidate for next generation insect pest control. An advantage of using RNAi is that design of dsRNA essentially requires only the sequence of the target gene. The greatest challenge, however, is dsRNA delivery for large-scale insect control. Delivery methods that have widely been used are oral, injection, or via soaking. Unfortunately, each insect presents its own challenges owing to the differences in the presence of dsRNA degrading enzymes, cellular uptake efficiency, expression of core RNAi machinery, the nature of the target gene, the concentration and persistence of the dsRNA, as well as the particular way of feeding of each insect, which together cause variations in the efficiency of RNAi. In this chapter, a protocol for the synthetic production of dsRNA is described along with three methods for delivery that have been successful in one of the more problematic insects, Diaphorina citri.

Guiding RNAi Design Through Characterization of Endogenous Small RNA Pathways

RNA interference (RNAi) is a common eukaryotic gene regulation process driven by small RNA effectors. Mechanisms that govern regulatory small noncoding RNA behavior have been extensively described in only a handful of organisms, which suggests that the most effective RNAi approach in many organisms, such as insect pests, remains to be determined. Taking advantage of advances in high-throughput sequencing, characterization of small RNA molecules can be achieved through bioinformatic approaches without the need for genetic experiments. This chapter describes pipelines for characterizing three main classes of small RNAs (microRNAs, small-interfering RNAs, and piwi-associated RNAs) using computationally determined small RNA biogenesis signatures. Obtaining information regarding the abundance of different small RNA classes through these pipelines will lead to a better-informed RNAi strategy, thereby identifying the most efficacious approach for RNAi.

Innate and adaptive resistance to RNAi: a major challenge and hurdle to the development of double stranded RNA-based pesticides

RNA interference (RNAi) is a post-transcriptional gene silencing process where short interfering RNAs degrade targeted mRNA. Exploration of gene function through reverse genetics is the major achievement of RNAi discovery. Besides, RNAi can be used as a potential strategy for the control of insect pests. This has led to the idea of developing RNAi-based pesticides. Differential RNAi efficiency in the different insect orders is the biggest biological obstacle in developing RNAi-based pesticides. dsRNA stability, the sensitivity of core RNAi machinery, uptake of dsRNA and amplification and spreading of the RNAi signal are the key factors responsible for RNAi efficiency in insects. This review discusses the physiological and adaptive factors responsible for reduced RNAi in insects that pose a major challenge in developing dsRNA- based pesticides.

Transcript level is a key factor affecting RNAi efficiency

Efficiency is the basis for the application of RNA interference (RNAi) technology. Actually, RNAi efficiency varies greatly among insect species, tissues and genes. Previous efforts have revealed the mechanisms for variation among insect species and tissues. Here, we investigated the reason for variable efficiency among the target genes in the same insect. First, we tested the genes sampled randomly from Tribolium castaneum, Locusta migratoria and Drosophila S2 cells for both their expression levels and sensitivity to RNAi. The results indicated that the genes with higher expression levels were more sensitive to RNAi. Statistical analysis showed that the correlation coefficients between transcript levels and knockdown efficiencies were 0.8036 (n = 90), 0.7255 (n = 18) and 0.9505 (n = 13), respectively in T. castaneum, L. migratoria and Drosophila S2 cells. Subsequently, ten genes with varied expression level in different tissues (midgut and carcass without midgut) of T. castaneum were tested. The results indicated that the higher knockdown efficiency was always obtained in the tissue where the target gene expressed higher. In addition, three genes were tested in different developmental stages, larvae and pupae of T. castaneum. The results found that when the expression level increased after insect pupation, these genes became more sensitive to RNAi. Thus, all the proofs support unanimously that transcript level is a key factor affecting RNAi sensitivity. This finding allows for a better understanding of the RNAi efficiency variation and lead to effective or efficient use of RNAi technology.

Strategies for enhancing the efficiency of RNA interference in insects

Low RNA interference (RNAi) efficiency in many insect pests has significantly prevented its widespread application for insect pest management. This article provides a comprehensive review of recent research in developing various strategies for enhancing RNAi efficiency. Our review focuses on the strategies in target gene selection and double-stranded RNA (dsRNA) delivery technologies. For target gene selection, genome-wide or large-scale screening strategies have been used to identify most susceptible target genes for RNAi. Other strategies include the design of dsRNA constructs and manipulate the structure of dsRNA to maximize the RNA efficiency for a target gene. For dsRNA delivery strategies, much recent research has focused on the applications of complexed or encapsulated dsRNA using various reagents, polymers, or peptides to enhance dsRNA stability and cellular uptake. Other dsRNA delivery strategies include genetic engineering of microbes (e.g. fungi, bacteria, and viruses) and plants to produce insect-specific dsRNA. The ingestion of the dsRNA-producing organisms or tissues will have lethal or detrimental effects on the target insect pests. This article also identifies obstacles to further developing RNAi for insect pest management and suggests future avenues of research that will maximize the potential for using RNAi for insect pest management. © 2021 Society of Chemical Industry.

Exogenous administration of dsRNA for the demonstration of RNAi in Maruca vitrata (lepidoptera: crambidae)

The polyphagous spotted pod borer, Maruca vitrata is an important agricultural pest that causes extensive damage on various food crops. Though the pest is managed by synthetic chemicals, exploration of biotechnological approaches for its control is important. RNAi-based gene silencing is one such tool that has been extensively used for functional genomics and is highly variable in insects. In view of this, we have attempted to demonstrate RNAi in M. vitrata through exogenous double-stranded RNA (dsRNA) administration targeting seven genes associated with midgut, chemosensory, cell signalling and development. Two modes of exogenous dsRNA delivery by either haemolymph injection and/or ingestion into third and late third instar larval stages respectively exhibited efficient silencing of specific transcripts. Furthermore, dsRNA injection into the haemolymph showed significant reduction of target gene expression compared to negative controls establishing this mode of delivery to be more efficient. Interestingly, haemolymph injection required lesser dsRNA and led to higher reduction of transcript level vis-à-vis ingestion as demonstrated in dsRNA Serine Protease 33 (ds-SP33)-fed larvae. Over-expression of key RNAi component DICER and detection of siRNA authenticated the presence of RNAi in M. vitrata. Additionally, we have identified inhibitor molecules like morpholine, piperidine, carboxamide and piperidine-carboxamide through in silico analysis for blocking the function of SP33 to demonstrate the utility of functional genomics. Thus, the present study establishes the usefulness of injection and ingestion approaches for exogenous dsRNA delivery into M. vitrata larvae for effective RNAi.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02741-8.

Characterization, expression patterns, and transcriptional responses of three core RNA interference pathway genes from Ostrinia nubilalis

RNA interference (RNAi) is commonly used in the laboratory to analyze gene function, and RNAi-based pest management strategies are now being employed. Unfortunately, RNAi is hindered by inefficient and highly-variable results when different insects are targeted, especially lepidopterans, such as the European corn borer (ECB), Ostrinia nubilalis (Lepidoptera: Crambidae). Previous efforts to achieve RNAi-mediated gene suppression in ECB revealed low RNAi efficiency with both double-stranded RNA (dsRNA) injection and ingestion. One mechanism that can affect RNAi efficiency in insects is the expression and function of core RNAi pathway genes, such as those encoding Argonaut 2 (Ago2), Dicer 2 (Dcr2), and a dsRNA binding protein (R2D2). To determine if deficiencies in these core RNAi pathway genes contribute to low RNAi efficiency in ECB, full-length complementary DNAs encoding OnAgo2, OnDcr2, and OnR2D2 were cloned, sequenced, and characterized. A comparison of domain architecture suggested that all three predicted proteins contained the necessary domains to function. However, a comparison of evolutionary distances revealed potentially important variations in the first RNase III domain of OnDcr2, the double-stranded RNA binding domains of OnR2D2, and both the PAZ and PIWI domains of OnAgo2, which may indicate functional differences in enzymatic activity between species. Expression analysis indicated that transcripts for all three genes were expressed in all developmental stages and tissues investigated. Interestingly, the introduction of non-target dsRNA into ECB second-instar larvae via microinjection did not affect OnAgo2, OnDcr2, or OnR2D2 expression. In contrast, ingestion of the same dsRNAs resulted in upregulation of OnDcr2 but downregulation of OnR2D2. The unexpected transcriptional responses of the core machinery and the divergence in amino-acid sequence between specific domains in each core RNAi protein may possibly contribute to low RNAi efficiency in ECB. Understanding the contributions of different RNAi pathway components is critical to adapting this technology for use in controlling lepidopteran pests that exhibit low RNAi efficiency.

Assessment of the function of gonad-specific PmAgo4 in viral replication and spermatogenesis in Penaeus monodon

Argonaute family is phylogenetically subdivided into Ago and Piwi subfamilies that operate either transcriptional or post-transcriptional regulation in association with particular types of small RNAs. Among the four members of Ago subfamily (PmAgo1-4) found in black tiger shrimp Penaeus monodon, PmAgo4 exhibits gonad-restricted expression and takes part in transposon repression as the Piwi subfamily. While PmAgo1-3 participate in RNA interference (RNAi)-based mechanism, the role of PmAgo4 in RNAi is still mysterious, and was therefore investigated in this study. The results showed that knockdown of PmAgo4 in shrimp testis did not have a significant effect on the potency of PmRab7 silencing by dsPmRab7. In addition, replication of YHV as well as YHV-induced cumulative mortality in PmAgo4-knockdown shrimp are comparable to the control shrimp, suggesting the irrelevant association of PmAgo4 with RNAi-mediated gene silencing and antiviral immunity. Since PmAgo4 did not function in common with the Ago subfamily, its potential function in gametogenesis of male shrimp was further investigated. The reduction of PmAgo4 transcript levels in male shrimp revealed significant defect in testicular maturity as measured by Testicular Index (TI). Moreover, the numbers of mature sperm in spermatophore of PmAgo4-knockdown shrimp were significantly decreased comparing with the control shrimp. Our studies thus suggest a distinctive role of PmAgo4 that is not consistent with a dsRNA-mediate gene regulation and virus replication, but has a key function in controlling spermatogenesis in P. monodon.

Multiple Argonaute family genes contribute to the siRNA-mediated RNAi pathway in Locusta migratoria

Argonautes (Ago) are important core proteins in RNA interference (RNAi) pathways of eukaryotic cells. Generally, it is thought that Ago1, Ago2 and Ago3 are involved in the miRNA (microRNA), siRNA (small interfering RNA) and piRNA (Piwi-interacting RNA)-mediated RNAi pathways, respectively. As a main component of the RNA-induced silencing complex (RISC), Ago2 plays an indispensable role in using siRNA to recognize and cut target messenger RNAs resulting in suppression of transcript levels, but the contributions of Ago1 and Ago3 to the siRNA-mediated RNAi pathway remain to be explored in many insect species. In this study, we investigated the contributions of four Ago genes (named LmAgo1, LmAgo2a and LmAgo2b and LmAgo3) to RNAi efficiency in Locusta migratoria by using both in vivo and in vitro experiments. Our results showed that suppression of each of the Ago genes significantly impaired RNAi efficiency when targeting Lmβ-tubulin transcripts, resulting in recovery of 48, 43.3, 61.4 or 26% of Lmβ-tubulin transcripts following RNAi-mediated suppression of LmAgo1, LmAgo2a, LmAgo2b, and LmAgo3, respectively. Furthermore, overexpression of LmAgo1, LmAgo2a, LmAgo2b, or LmAgo3 in a PAc5.1-V5/HisB vector and co-transfection with psicheck2 fluorescence vector in S2 cells reduced luciferase fluorescence by 38.3, 58.9, 53.3 or 55.6%, respectively. Taken together, our results showed that LmAgo1, LmAgo2a, LmAgo2b, and LmAgo3 each make significant contributions to RNAi efficiency in L. migratoria and suggest that the involvement of all four enzymes could be one of the major factors supporting robust RNAi responses observed in this species.

Precocious Downregulation of Krüppel-Homolog 1 in the Migratory Locust, Locusta migratoria, Gives Rise to An Adultoid Phenotype with Accelerated Ovarian Development but Disturbed Mating and Oviposition

Krüppel-homolog 1 (Kr-h1) is a zinc finger transcription factor maintaining the status quo in immature insect stages and promoting reproduction in adult insects through the transduction of the Juvenile Hormone (JH) signal. Knockdown studies have shown that precocious silencing of Kr-h1 in the immature stages results in the premature development of adult features. However, the molecular characteristics and reproductive potential of these premature adult insect stages are still poorly understood. Here we report on an adult-like or 'adultoid' phenotype of the migratory locust, Locusta migratoria, obtained after a premature metamorphosis induced by the silencing of LmKr-h1 in the penultimate instar. The freshly molted adultoid shows precocious development of adult features, corresponding with increased transcript levels of the adult specifier gene LmE93. Furthermore, accelerated ovarian maturation and vitellogenesis were observed in female adultoids, coinciding with elevated expression of LmCYP15A1 in corpora allata (CA) and LmKr-h1 and vitellogenin genes (LmVg) in fat body, whereas LmE93 and Methoprene-tolerant (LmMet) transcript levels decreased in fat body. In adultoid ovaries, expression of the Halloween genes, Spook (LmSpo) and Phantom (LmPhm), was elevated as well. In addition, the processes of mating and oviposition were severely disturbed in these females. L. migratoria is a well-known, swarm-forming pest insect that can destroy crops and harvests in some of the world's poorest countries. As such, a better understanding of factors that are capable of significantly reducing the reproductive potential of this pest may be of crucial importance for the development of novel locust control strategies.

Candidate genetic determinants of intraspecific variation in pea aphid susceptibility to RNA interference

RNA interference (RNAi) plays a key role in insect defense against viruses and transposable elements, and it is being applied as an experimental tool and for insect pest control. However, RNAi efficiency is highly variable for some insects, notably the pea aphid Acyrthosiphon pisum. In this study, we used natural variation in RNAi susceptibility of pea aphids to identify genes that influence RNAi efficiency. Susceptibility to orally-delivered dsRNA against the gut aquaporin gene AQP1 (ds-AQP1) varied widely across a panel of 83 pea aphid genotypes, from zero to total mortality. Genome-wide association between aphid performance on ds-AQP1 supplemented diet and aphid genetic variants yielded 103 significantly associated single nucleotide polymorphisms (SNPs), including variants in 55 genes, at the 10^-4 probability cut-off. When ds-AQP1 was co-administered with dsRNA against six candidate genes, aphid mortality was reduced for three (50%) genes: the orthologs of the Drosophila genes trachealess (CG42865), headcase (CG15532) and a gene coding a peritrophin-A domain (CG8192), indicating that these genes function to promote RNAi efficiency against AQP1 in the pea aphid. Aphid susceptibility (quantified as mortality) to ds-AQP1 was correlated with RNAi against a further gene, snakeskin with essential gut function unrelated to AQP1, for some but not all aphid genotypes tested, suggesting that the determinants of RNAi efficiency may be partly gene-specific. This study demonstrates high levels of natural variation in susceptibility to RNAi and demonstrates the value of harnessing this variation to identify genes influencing RNAi efficiency.

Double-Stranded RNA Technology to Control Insect Pests: Current Status and Challenges

Exploiting the RNA interference (RNAi) gene mechanism to silence essential genes in pest insects, leading to toxic effects, has surfaced as a promising new control strategy in the past decade. While the first commercial RNAi-based products are currently coming to market, the application against a wide range of insect species is still hindered by a number of challenges. In this review, we discuss the current status of these RNAi-based products and the different delivery strategies by which insects can be targeted by the RNAi-triggering double-stranded RNA (dsRNA) molecules. Furthermore, this review also addresses a number of physiological and cellular barriers, which can lead to decreased RNAi efficacy in insects. Finally, novel non-transgenic delivery technologies, such as polymer or liposomic nanoparticles, peptide-based delivery vehicles and viral-like particles, are also discussed, as these could overcome these barriers and lead to effective RNAi-based pest control.

Mechanisms, Applications, and Challenges of Insect RNA Interference

The RNA interference (RNAi) triggered by short/small interfering RNA (siRNA) was discovered in nematodes and found to function in most living organisms. RNAi has been widely used as a research tool to study gene functions and has shown great potential for the development of novel pest management strategies. RNAi is highly efficient and systemic in coleopterans but highly variable or inefficient in many other insects. Differences in double-stranded RNA (dsRNA) degradation, cellular uptake, inter- and intracellular transports, processing of dsRNA to siRNA, and RNA-induced silencing complex formation influence RNAi efficiency. The basic dsRNA delivery methods include microinjection, feeding, and soaking. To improve dsRNA delivery, various new technologies, including cationic liposome-assisted, nanoparticle-enabled, symbiont-mediated, and plant-mediated deliveries, have been developed. Major challenges to widespread use of RNAi in insect pest management include variable RNAi efficiency among insects, lack of reliable dsRNA delivery methods, off-target and nontarget effects, and potential development of resistance in insect populations.

Effects of FABP knockdown on flight performance of the desert locust, Schistocerca gregaria

During migratory flight, desert locusts rely on fatty acids as their predominant source of energy. Lipids mobilized in the fat body are transported to the flight muscles and enter the muscle cells as free fatty acids. It has been postulated that muscle fatty acid binding protein (FABP) is needed for the efficient translocation of fatty acids through the aqueous cytosol towards mitochondrial β-oxidation. To assess whether FABP is required for this process, dsRNA was injected into freshly emerged adult males to knock down the expression of FABP. Three weeks after injection, FABP and its mRNA were undetectable in flight muscle, indicating efficient silencing of FABP expression. At rest, control and treated animals exhibited no morphological or behavioral differences. In tethered flight experiments, both control and treated insects were able to fly continually in the initial, carbohydrate-fueled phase of flight, and in both groups, lipids were mobilized and released into the hemolymph. Flight periods exceeding 30 min, however, when fatty acids become the main energy source, were rarely possible for FABP-depleted animals, while control insects continued to fly for more than 2 h. These results demonstrate that FABP is an essential element of skeletal muscle energy metabolism in vivo.

Juvenile Hormone receptor Met is essential for ovarian maturation in the Desert Locust, Schistocerca gregaria

Juvenile hormones (JH) are key endocrine regulators produced by the corpora allata (CA) of insects. Together with ecdysteroids, as well as nutritional cues, JH coordinates different aspects of insect postembryonic development and reproduction. The function of the recently characterized JH receptor, Methoprene-tolerant (Met), appears to be conserved in different processes regulated by JH. However, its functional interactions with other hormonal signalling pathways seem highly dependent on the feeding habits and on the developmental and reproductive strategies employed by the insect species investigated. Here we report on the effects of RNA interference (RNAi) mediated SgMet knockdown during the first gonadotrophic cycle in female desert locusts (Schistocerca gregaria). This voracious, phytophagous pest species can form migrating swarms that devastate field crops and harvests in several of the world’s poorest countries. A better knowledge of the JH signalling pathway may contribute to the development of novel, more target-specific insecticides to combat this very harmful swarming pest. Using RNAi, we show that the JH receptor Met is essential for ovarian maturation, vitellogenesis and associated ecdysteroid biosynthesis in adult female S. gregaria. Interestingly, knockdown of SgMet also resulted in a significant decrease of insulin-related peptide (SgIRP) and increase of neuroparsin (SgNP) 3 and 4 transcript levels in the fat body, illustrating the existence of an intricate regulatory interplay between different hormonal factors. In addition, SgMet knockdown in females resulted in delayed display of copulation behaviour with virgin males, when compared with dsGFP injected control animals. Moreover, we observed an incapacity of adult dsSgMet injected female locusts to oviposit during the time of the experimental setup. As such, SgMet is an essential gene playing crucial roles in the endocrine communication necessary for successful reproduction of the desert locust.

RNA Interference in Insects: Protecting Beneficials and Controlling Pests

Insects constitute the largest and most diverse group of animals on Earth with an equally diverse virome. The main antiviral immune system of these animals is the post-transcriptional gene-silencing mechanism known as RNA(i) interference. Furthermore, this process can be artificially triggered via delivery of gene-specific double-stranded RNA molecules, leading to specific endogenous gene silencing. This is called RNAi technology and has important applications in several fields. In this paper, we review RNAi mechanisms in insects as well as the potential of RNAi technology to contribute to species-specific insecticidal strategies. Regarding this aspect, we cover the range of strategies considered and investigated so far, as well as their limitations and the most promising approaches to overcome them. Additionally, we discuss patterns of viral infection, specifically persistent and acute insect viral infections. In the latter case, we focus on infections affecting economically relevant species. Within this scope, we review the use of insect-specific viruses as bio-insecticides. Last, we discuss RNAi-based strategies to protect beneficial insects from harmful viral infections and their potential practical application. As a whole, this manuscript stresses the impact of insect viruses and RNAi technology in human life, highlighting clear lines of investigation within an exciting and promising field of research.

Molecular mechanisms influencing efficiency of RNA interference in insects

RNA interference (RNAi) is an endogenous, sequence-specific gene-silencing mechanism elicited by small RNA molecules. RNAi is a powerful reverse genetic tool, and is currently being utilized for managing insects and viruses. Widespread implementation of RNAi-based pest management strategies is currently hindered by inefficient and highly variable results when different insect species, strains, developmental stages, tissues, and genes are targeted. Mechanistic studies have shown that double-stranded ribonucleases (dsRNases), endosomal entrapment, deficient function of the core machinery, and inadequate immune stimulation contribute to limited RNAi efficiency. However, a comprehensive understanding of the molecular mechanisms limiting RNAi efficiency remains elusive. Recent advances in dsRNA stability in physiological tissues, dsRNA internalization into cells, the composition and function of the core RNAi machinery, as well as small-interfering RNA/double-stranded RNA amplification and spreading mechanisms are reviewed to establish a global understanding of the obstacles impeding wider understanding of RNAi mechanisms in insects. © 2018 Society of Chemical Industry.

Management of Pest Insects and Plant Diseases by Non-Transformative RNAi

Since the discovery of RNA interference (RNAi), scientists have made significant progress towards the development of this unique technology for crop protection. The RNAi mechanism works at the mRNA level by exploiting a sequence-dependent mode of action with high target specificity due to the design of complementary dsRNA molecules, allowing growers to target pests more precisely compared to conventional agrochemicals. The delivery of RNAi through transgenic plants is now a reality with some products currently in the market. Conversely, it is also expected that more RNA-based products reach the market as non-transformative alternatives. For instance, topically applied dsRNA/siRNA (SIGS - Spray Induced Gene Silencing) has attracted attention due to its feasibility and low cost compared to transgenic plants. Once on the leaf surface, dsRNAs can move directly to target pest cells (e.g., insects or pathogens) or can be taken up indirectly by plant cells to then be transferred into the pest cells. Water-soluble formulations containing pesticidal dsRNA provide alternatives, especially in some cases where plant transformation is not possible or takes years and cost millions to be developed (e.g., perennial crops). The ever-growing understanding of the RNAi mechanism and its limitations has allowed scientists to develop non-transgenic approaches such as trunk injection, soaking, and irrigation. While the technology has been considered promising for pest management, some issues such as RNAi efficiency, dsRNA degradation, environmental risk assessments, and resistance evolution still need to be addressed. Here, our main goal is to review some possible strategies for non-transgenic delivery systems, addressing important issues related to the use of this technology.

RNA interference in the cat flea, Ctenocephalides felis: Approaches for sustained gene knockdown and evidence of involvement of Dicer-2 and Argonaute2

Effective RNA interference (RNAi) methods have been developed in many pest species, enabling exploration of gene function. Until now RNAi had not been attempted in the cat flea, Ctenocephalides felis, although the development of RNAi approaches would open up potential avenues for control of this important pest. This study aimed to establish if an RNAi response occurs in adult C. felis upon exposure to double-stranded RNA (dsRNA), which administration methods for dsRNA delivery could bring about effective gene knockdown and to investigate dynamics of any RNAi response. Knockdown of 80% of GSTσ was achieved by intrahaemoceolic microinjection of dsGSTσ but this invasive technique was associated with relatively high mortality rates. Immersing C. felis in dsGSTσ or dsDicer-2 overnight resulted in 65% knockdown of GSTσ or 60% of Dicer-2, respectively, and the degree of knockdown was not improved by increasing the dsRNA concentration in the bathing solution. Unexpectedly, the greatest degree of knockdown was achieved with the continuous administration of dsRNA in whole blood via a membrane feeding system, resulting in 96% knockdown of GSTσ within 2 days and sustained up to, at least, 7 days. Thus, unlike in many other species, the gut nucleases do not impair the RNAi response to ingested dsRNA in C. felis. A modest, but significant, upregulation of Dicer-2 and Argonaute2 was detectable 3 h after exposure to exogenous dsRNA, implicating the short-interfering RNA pathway. To our knowledge this study represents the first demonstration of experimentally induced RNAi in the cat flea as well as giving insight into how the gene knockdown response progresses.

RNAi for Resistance Against Biotic Stresses in Crop Plants

RNA interference (RNAi)-based gene silencing has become one of the most successful strategies in not only identifying gene function but also in improving agronomical traits of crops by silencing genes of different pathogens/pests and also plant genes for improvement of desired trait. The conserved nature of RNAi pathway across different organisms increases its applicability in various basic and applied fields. Here we attempt to summarize the knowledge generated on the fundamental mechanisms of RNAi over the years, with emphasis on insects and plant-parasitic nematodes (PPNs). This chapter also reviews the rich history of RNAi research, gene regulation by small RNAs across different organisms, and application potential of RNAi for generating transgenic plants resistant to major pests_. But, there are some limitations too which restrict wider applications of this technology to its full potential. Further refinement of this technology in terms of resolving these shortcomings constitutes one of the thrust areas in present RNAi research. Nevertheless, its application especially in breeding agricultural crops resistant against biotic stresses will certainly offer the possible solutions for some of the breeding objectives which are otherwise unattainable.

RNA interference technology in crop protection against arthropod pests, pathogens and nematodes

Scientists have made significant progress in understanding and unraveling several aspects of double-stranded RNA (dsRNA)-mediated gene silencing during the last two decades. Now that the RNA interference (RNAi) mechanism is well understood, it is time to consider how to apply the acquired knowledge to agriculture and crop protection. Some RNAi-based products are already available for farmers and more are expected to reach the market soon. Tailor-made dsRNA as an active ingredient for biopesticide formulations is considered a raw material that can be used for diverse purposes, from pest control and bee protection against viruses to pesticide resistance management. The RNAi mechanism works at the messenger RNA (mRNA) level, exploiting a sequence-dependent mode of action, which makes it unique in potency and selectivity compared with conventional agrochemicals. Furthermore, the use of RNAi in crop protection can be achieved by employing plant-incorporated protectants through plant transformation, but also by non-transformative strategies such as the use of formulations of sprayable RNAs as direct control agents, resistance factor repressors or developmental disruptors. In this review, RNAi is presented in an agricultural context (discussing products that have been launched on the market or will soon be available), and we go beyond the classical presentation of successful examples of RNAi in pest-insect control and comprehensively explore its potential for the control of plant pathogens, nematodes and mites, and to fight against diseases and parasites in beneficial insects. Moreover, we also discuss its use as a repressor for the management of pesticide-resistant weeds and insects. Finally, this review reports on the advances in non-transformative dsRNA delivery and the production costs of dsRNA, and discusses environmental considerations. © 2017 Society of Chemical Industry.

Biochemical Comparison of dsRNA Degrading Nucleases in Four Different Insects

Double stranded RNAs (dsRNA) degrading nuclease is responsible for the rapid degradation of dsRNA molecules, and thus accounts for variations in RNA interference (RNAi) efficacy among insect species. Here, the biochemical properties and tissue-specific activities of dsRNA degrading nucleases in four insects (Spodoptera litura, Locusta migratoria, Periplaneta americana, and Zophobas atratus) from different orders were characterized using a modified assay method. The results revealed that all insect dsRNA degrading nucleases tested showed high activity in alkaline environments at optimal Mg2+ concentrations and elevated temperatures. We also found that enzymes from different insects varied in terms of their optimal reaction conditions and kinetic parameters. Whole body enzyme activity differed dramatically between insect species, although enzymes with higher substrate affinities (lower Km) were usually balanced by a smaller Vmax to maintain a proper level of degradative capacity. Furthermore, enzyme activities varied significantly between the four tested tissues (whole body, gut, hemolymph, and carcass) of the insect species. All the insects tested showed several hundred-fold higher dsRNA degrading activity in their gut than in other tissues. Reaction environment analysis demonstrated that physiological conditions in the prepared gut fluid and serum of different insects were not necessarily optimal for dsRNA degrading nuclease activity. Our data describe the biochemical characteristics and tissue distributions of dsRNA degrading activities in various insects, not only explaining why oral delivery of dsRNA often produces lower RNAi effects than injection of dsRNA, but also suggesting that dsRNA-degrading activities are regulated by physiological conditions. These results allow for a better understanding of the properties of dsRNA degrading nucleases, and will aid in the development of successful RNAi strategies in insects.

Insights into RNAi-based antiviral immunity in Lepidoptera: acute and persistent infections in Bombyx mori and Trichoplusia ni cell lines

The control of viral infections in insects is a current issue of major concern and RNA interference (RNAi) is considered the main antiviral immune response in this group of animals. Here we demonstrate that overexpression of key RNAi factors can help to protect insect cells against viral infections. In particular, we show that overexpression of Dicer2 and Argonaute2 in lepidopteran cells leads to improved defense against the acute infection of the Cricket Paralysis Virus (CrPV). We also demonstrate an important role of RNAi in the control of persistent viral infections, as the one caused by the Macula-like Latent Virus (MLV). Specifically, a direct interaction between Argonaute2 and virus-specific small RNAs is shown. Yet, while knocking down Dicer2 and Argonaute2 resulted in higher transcript levels of the persistently infecting MLV in the lepidopteran cells under investigation, overexpression of these proteins could not further reduce these levels. Taken together, our data provide deep insight into the RNAi-based interactions between insects and their viruses. In addition, our results suggest the potential use of an RNAi gain-of-function approach as an alternative strategy to obtain reduced viral-induced mortality in Lepidoptera, an insect order that encompasses multiple species of relevant economic value.

Comparative analysis of double-stranded RNA degradation and processing in insects

RNA interference (RNAi) based methods are being developed for pest management. A few products for control of coleopteran pests are expected to be commercialized soon. However, variability in RNAi efficiency among insects is preventing the widespread use of this technology. In this study, we conducted research to identify reasons for variability in RNAi efficiency among thirty-seven (37) insects belonging to five orders. Studies on double-stranded RNA (dsRNA) degradation by dsRNases and processing of labeled dsRNA to siRNA showed that both dsRNA degradation and processing are variable among insects belonging to different orders as well as among different insect species within the same order. We identified homologs of key RNAi genes in the genomes of some of these insects and studied their domain architecture. These data suggest that dsRNA digestion by dsRNases and its processing to siRNAs in the cells are among the major factors contributing to differential RNAi efficiency reported among insects.

The evolution of animal Argonautes: evidence for the absence of antiviral AGO Argonautes in vertebrates

In addition to mediating regulation of endogenous gene expression, RNA interference (RNAi) in plants and invertebrates plays a crucial role in defense against viruses via virus-specific siRNAs. Different studies have demonstrated that the functional diversity of RNAi in animals is linked to the diversification of the Argonaute superfamily, central components of RISCs (RNA induced silencing complexes). The animal Argonaute superfamily is traditionally grouped into AGO and PIWI Argonautes. Yet, by performing phylogenetic analyses and determining the selective evolutionary pressure in the metazoan Argonaute superfamily, we provide evidence for the existence of three conserved Argonaute lineages between basal metazoans and protostomes, namely siRNA-class AGO, miRNA-class AGO and PIWI Argonautes. In addition, it shown that the siRNA-class AGO lineage is characterized by high rates of molecular evolution, suggesting a role in the arms race with viruses, while the miRNA-class AGOs display strong sequence conservation. Interestingly, we also demonstrate that vertebrates lack siRNA-class AGO proteins and that vertebrate AGOs display low rates of molecular evolution. In this way, we provide supportive evidence for the loss of the antiviral siRNA-class AGO group in vertebrates and discuss the consequence hereof on antiviral immunity and the use of RNAi as a loss of function tool in these animals.

Oral delivery of dsRNA lipoplexes to German cockroach protects dsRNA from degradation and induces RNAi response

BACKGROUND

In the past years, the concept of RNAi application for insect pest control has been proposed, considering the disruption of vital genes. However, the efficiency of RNAi is variable between different insect groups, especially by oral delivery of dsRNA. The purpose of this study is to assess the possibilities of RNAi as a tool for pest control using oral delivery of the dsRNAs encapsulated by liposome in the German cockroach Blattella germanica, which is highly sensitive to RNAi by injection of dsRNAs.

RESULTS

Injecting dsRNA into the abdomen of B. germanica caused dramatic depletion of essential α-tubulin gene and mortality. In contrast, oral delivery of the naked dsRNA resulted in lower RNAi efficiency, accounting for rapid degradation of the dsRNA in the midgut of B. germanica. Notably, we have further demonstrated that continuous ingestion of dsRNA lipoplexes in which dsRNA was encapsulated with a cationic liposome carrier was sufficient to slow down the degradation of dsRNA in the midgut and to increase the mortality of the German cockroach by significantly inhibiting α-tubulin expression in the midgut.

CONCLUSION

We provide empirical evidence that the formulation of dsRNA lipoplexes could be a plausible approach for insect pest control based on RNAi. © 2016 Society of Chemical Industry.

Knockdown of nuclease activity in the gut enhances RNAi efficiency in the Colorado potato beetle, Leptinotarsa decemlineata, but not in the desert locust, Schistocerca gregaria

The responsiveness towards orally delivered dsRNA and the potency of a subsequent environmental RNA interference (RNAi) response strongly differs between different insect species. While some species are very sensitive to dsRNA delivery through the diet, others are not. The underlying reasons for this may vary, but degradation of dsRNA by nucleases in the gut lumen is believed to play a crucial role. The Colorado potato beetle, Leptinotarsa decemlineata, is a voracious defoliator of potato crops worldwide, and is currently under investigation for novel control methods based on dsRNA treatments. Here we describe the identification and characterization of two nuclease genes exclusively expressed in the gut of this pest species. Removal of nuclease activity in adults increased the sensitivity towards dsRNA and resulted in improved protection of potato plants. A similar strategy in the desert locust, Schistocerca gregaria, for which we show a far more potent nuclease activity in the gut juice, did however not lead to an improvement of the RNAi response. Possible reasons for this are discussed. Taken together, the present data confirm a negative effect of nucleases in the gut on the environmental RNAi response, and further suggest that interfering with this activity is a strategy worth pursuing for improving RNAi efficacy in insect pest control applications.

Geographic variation in RNAi sensitivity in the migratory locust

The RNA interference (RNAi) technology has been widely used in basic and applied research. It is known that RNAi works in some species but not in others, although the cause for this difference remains unclear. Here, we present inter- and intra-populational variations in RNAi sensitivity in the migratory locust Locusta migratoria, and provide information on the genetic background of such variations. In the four strains analyzed, originating from different Japanese localities, most individuals from two of the strains were sensitive to injections of double-stranded RNA (dsRNA) against the corazonin (CRZ) and ecdysone receptor genes, whereas those from the other two strains were resistant. Selection for individuals sensitive to dsCRZ produced a dramatic increase in the RNAi sensitivity in the following generations, although phenotypes also varied in the selected line, suggesting that several genes might control RNAi sensitivity. Reciprocal crosses between a sensitive and a resistant strain suggested that the resistant phenotype is dominant. The expression levels of nine RNAi-associated genes known for other organisms were not correlated with the variation in RNAi sensitivity observed in L. migratoria. Variations in RNAi sensitivity as the ones observed in this study should be considered when using RNAi in basic and applied research as well as in pest management.

Parameters for Successful Parental RNAi as An Insect Pest Management Tool in Western Corn Rootworm, Diabrotica virgifera virgifera

Parental RNAi (pRNAi) is an RNA interference response where the gene knockdown phenotype is observed in the progeny of the treated organism. pRNAi has been demonstrated in female western corn rootworms (WCR) via diet applications and has been described as a potential approach for rootworm pest management. However, it is not clear if plant-expressed pRNAi can provide effective control of next generation WCR larvae in the field. In this study, we evaluated parameters required to generate a successful pRNAi response in WCR for the genes brahma and hunchback. The parameters tested included a concentration response, duration of the dsRNA exposure, timing of the dsRNA exposure with respect to the mating status in WCR females, and the effects of pRNAi on males. Results indicate that all of the above parameters affect the strength of pRNAi phenotype in females. Results are interpreted in terms of how this technology will perform in the field and the potential role for pRNAi in pest and resistance management strategies. More broadly, the described approaches enable examination of the dynamics of RNAi response in insects beyond pRNAi and crop pests.

Israeli Acute Paralysis Virus Infection Leads to an Enhanced RNA Interference Response and Not Its Suppression in the Bumblebee Bombus terrestris

RNA interference (RNAi) is the primary antiviral defense system in insects and its importance for pollinator health is indisputable. In this work, we examined the effect of Israeli acute paralysis virus (IAPV) infection on the RNAi process in the bumblebee, Bombus terrestris, and whether the presence of possible functional viral suppressors could alter the potency of the host's immune response. For this, a two-fold approach was used. Through a functional RNAi assay, we observed an enhancement of the RNAi system after IAPV infection instead of its suppression, despite only minimal upregulation of the genes involved in RNAi. Besides, the presence of the proposed suppressor 1A and the predicted OrfX protein in IAPV could not be confirmed using high definition mass spectrometry. In parallel, when bumblebees were infected with cricket paralysis virus (CrPV), known to encode a suppressor of RNAi, no increase in RNAi efficiency was seen. For both viruses, pre-infection with the one virus lead to a decreased replication of the other virus, indicating a major effect of competition. These results are compelling in the context of Dicistroviridae in multi-virus/multi-host networks as the effect of a viral infection on the RNAi machinery may influence subsequent virus infections.