Chitosan/double-stranded RNA nanoparticle-mediated RNA interference to silence chitin synthase genes through larval feeding in the African malaria mosquito (Anopheles gambiae)

Polymer-Coated Hydroxyapatite Nanocarrier for Double-Stranded RNA Delivery

Conventional synthetic insecticides have limited success due to insect resistance and negative effects on off-target biota and the environment. Although RNA interference (RNAi) is a tool that is becoming more widely utilized in pest control products, naked dsRNA has limited success in most taxa. Nanocarriers have shown promising results in enhancing the efficacy of this tool. In this study, we used a layer-by-layer electrostatic assembly where we synthesized poly(acrylic acid) (PAA)-coated hydroxyapatite (HA) nanoparticles (PAA-HA NPs) as inorganic nanocarriers, which were then coated with a layer of a cationic poly(amino acid), 10 kDa poly-l-arginine (PLR₁₀), to allow for binding of a layer of negatively charged dsRNA. Binding of PLR₁₀-PAA-HA NPs to dsRNA was found to increase as the mass ratio of NPs to dsRNA increased. In vitro studies with transgenic SF9 cells (from Spodoptera frugiperda) expressing the firefly luciferase gene showed a significant gene silencing (35% decrease) at a 5:1 NP-to-dsRNA ratio, while naked dsRNA was ineffective at gene silencing. There was a significant concentration-response relationship in knockdown; however, cytotoxicity was observed at higher concentrations. Confocal microscopy studies showed that dsRNA from PLR₁₀-PAA-HA NPs was not localized within endosomes, while naked dsRNA appeared to be entrapped within the endosomes. Overall, polymer-functionalized HA nanocarriers enabled dsRNA to elicit gene knockdown in cells, whereas naked dsRNA was not effective in causing gene knockdown.

Assessing the Risks of Topically Applied dsRNA-Based Products to Non-target Arthropods

RNA interference (RNAi) is a powerful technology that offers new opportunities for pest control through silencing of genes that are essential for the survival of arthropod pests. The approach relies on sequence-specificity of applied double-stranded (ds) RNA that can be designed to have a very narrow spectrum of both the target gene product (RNA) as well as the target organism, and thus allowing highly targeted pest control. Successful RNAi has been reported from a number of arthropod species belonging to various orders. Pest control may be achieved by applying dsRNA as foliar sprays. One of the main concerns related to the use of dsRNA is adverse environmental effects particularly on valued non-target species. Arthropods form an important part of the biodiversity in agricultural landscapes and contribute important ecosystem services. Consequently, environmental risk assessment (ERA) for potential impacts that plant protection products may have on valued non-target arthropods is legally required prior to their placement on the market. We describe how problem formulation can be used to set the context and to develop plausible pathways on how the application of dsRNA-based products could harm valued non-target arthropod species, such as those contributing to biological pest control. The current knowledge regarding the exposure to and the hazard posed by dsRNA in spray products for non-target arthropods is reviewed and suggestions are provided on how to select the most suitable test species and to conduct laboratory-based toxicity studies that provide robust, reliable and interpretable results to support the ERA.

Profiling of RNAi sensitivity after foliar dsRNA exposure in different European populations of Colorado potato beetle reveals a robust response with minor variability

In recent years, substantial effort was spent on the exploration and implementation of RNAi technology using double-stranded RNA (dsRNA) for pest management purposes. However, only few studies investigated the geographical variation in RNAi sensitivity present in field-collected populations of the targeted insect pest. In this baseline study, 2nd instar larvae of 14 different European populations of Colorado potato beetle (CPB), Leptinotarsa decemlineata, collected from nine different countries were exposed to a foliarly applied diagnostic dose of dsactin (dsact) to test for possible variations in RNAi response. Only minor variability in RNAi sensitivity was observed between populations. However, the time necessary to trigger a dsRNA-mediated phenotypic response varied significantly among populations, indicated by significant differences in mortality figures obtained five days after treatment. An inbred German laboratory reference strain D01 and a Spanish field strain E02 showed almost 100% mortality after foliar exposure to 30 ng dsactin (equal to 0.96 g/ha), whereas another Spanish strain E01 was least responsive and showed only 30% mortality. Calculated LD₅₀-values for foliarly applied dsact against strains D01 (most sensitive) and E01 (least sensitive) were 9.22 and 68.7 ng/leaf disc, respectively. The variability was not based on target gene sequence divergence or knock-down efficiency. Variability in expression of the core RNAi machinery genes dicer (dcr2a) and argonaute (ago2a) was observed but did not correlate with sensitivity. Interestingly, RT-qPCR data collected for all strains revealed a strong correlation between the expression level of dcr2a and ago2a (r 0.93) as well as ago2a and stauC (r 0.94), a recently described dsRNA binding protein in Coleopterans. Overall, this study demonstrates that sensitivity of CPB to sprayable RNAi slightly varies between strains but also shows that foliar RNAi as a control method works against all tested CPB populations collected across a broad geographic range in Europe. Thus, underpinning the potential of RNAi-based CPB control as a promising component in integrated pest management (IPM) and resistance management programs.

RNA Interference Is Enhanced by Knockdown of double-stranded RNases in the Yellow Fever Mosquito Aedes Aegypti

RNA interference (RNAi) techniques are being developed for a range of pest insect control technologies, including the sterile insect technique (SIT) and double-stranded RNA (dsRNA)-based insecticides. In SIT applications, where >99% of the released males should be sterile to meet industry standards, the efficiency of RNAi will need to be improved for many insect species if this technology is to be adopted. Endogenous dsRNases can impede dsRNA delivery in some insects, and, here, we investigated whether dsRNases in the midgut could limit RNAi efficacy in the mosquito Aedes aegypti. Ten putative dsRNases were identified in the Ae. aegypti genome, with two highly expressed in the midguts of larvae. Using an ex vivo assay, we observed that dsRNA was rapidly degraded within the mosquito larva’s gut. Double-stranded RNA targeting these two dsRNases, when fed to the larvae, effectively reduced gut dsRNase activity. When these dsRNase-specific dsRNAs were co-delivered with dsRNA targeting a cyan fluorescent protein (CFP) reporter gene, greater knockdown of CFP fluorescence was observed. These results suggest that inhibiting dsRNase activity could enable the implementation of RNAi-based mosquito control methods.

Insect Cuticular Chitin Contributes to Form and Function

Chitin contributes to the rigidity of the insect cuticle and serves as an attachment matrix for other cuticular proteins. Deficiency of chitin results in abnormal embryos, cuticular structural defects and growth arrest. When chitin is not turned over during molting, the developing insect is trapped inside the old cuticle. Partial deacetylation of cuticular chitin is also required for proper laminar organization of the cuticle and vertical pore canals, molting, and locomotion. Thus, chitin and its modifications strongly influence the structure of the exoskeleton as well as the physiological functions of the insect. Internal tendons and specialized epithelial cells called "tendon cells" that arise from the outer layer of epidermal cells provide attachment sites at both ends of adult limb muscles. Membrane processes emanating from both tendon and muscle cells interdigitate extensively to strengthen the attachment of muscles to the extracellular matrix (ECM). Protein ligands that bind to membrane-bound integrin complexes further enhance the adhesion between muscles and tendons. Tendon cells contain F-actin fiber arrays that contribute to their rigidity. In the cytoplasm of muscle cells, proteins such as talin and other proteins provide attachment sites for cytoskeletal actin, thereby increasing integrin binding and activation to mechanically couple the ECM with actin in muscle cells. Mutations in integrins and their ligands, as well as depletion of chitin deacetylases, result in defective locomotion and muscle detachment from the ECM. Thus, chitin in the cuticle and chitin deacetylases strongly influence the shape and functions of the exoskeleton as well as locomotion of insects.

Comparison of efficacy of RNAi mediated by various nanoparticles in the rice striped stem borer (Chilo suppressalis)

RNA interference (RNAi) has proven to be a very promising prospect for insect pest control. However, low RNAi efficacy limits further development of this biotechnology for use on lepidopteran insects, including the rice striped stem borer (SSB) (Chilo suppressalis), one of the major destructive rice pests. In this work, the application of various nanoparticles (NPs) by which double-stranded RNA (dsRNA) could be encapsulated was evaluated as an alternative delivery strategy to potentially increase the bioactivity of dsRNA. Three NPs, chitosan, carbon quantum dot (CQD), and lipofectamine2000, complexed with dsRNA (to target the glyceraldehyde-3-phosphate dehydrogenase gene (G3PDH)) were tested to examine their use in controlling SSB. Relative mRNA expressions were quantified using qPCR to evaluate knockdown efficiency of NP-dsRNA treated larvae, and the correlated dsRNA-mediated SSB larval mortality was tested. Thereafter, the content dynamics of hemolymph dsRNA after ingesting different NP-dsRNA were monitored in vivo; the hemolymph dsRNA content was in ratios of 5.67, 9.43, and 1 with chitosan, CQD, and lipofectamine2000 induced samples, respectively. The results demonstrated that all three tested NPs led to efficient feeding delivery by improving both dsRNA stability and cellular uptake equally. Furthermore, there was a strong correlation (r= 0.9854) between the hemolymph dsRNA contents and the average RNAi depletions in the non-gut tissues of SSB. Overall, our results strongly suggest that due to its strong endosomal escaping ability, CQD was the most efficient carrier for inducing systemic RNAi, and thereby causing effective gene silencing and mortality in SSB.

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.

Improving RNAi efficiency for pest control in crop species

The application of RNAi promotes the development of novel approaches toward plant protection in a sustainable way. Genetically modified crops expressing dsRNA have been developed as commercial products with great potential in insect pest management. Alternatively, some nontransformative approaches, including foliar spray, irrigation and trunk injection, are favorable in actual utilization. In this review, we summarize the recent progress and successful cases of RNAi-based pest management strategy, explore essential implications and possibilities to improve RNAi efficiency by delivery of dsRNA through transformative and nontransformative approaches, and highlight the remaining challenges and important issues related to the application of this technology.

Enhancing RNAi Efficiency to Decipher the Functional Response of Potential Genes in Bemisia tabaci AsiaII-1 (Gennadius) Through dsRNA Feeding Assays

Whitefly Bemisia tabaci is a global invasive pest that causes substantial losses to agricultural crops worldwide either by direct feeding or vectoring numerous plant viruses. Management with insecticides remains a big challenge due to its rapid resistance development potential as well as the impact of these chemicals on non-target organisms. Thus, in search of alternate and novel pest management strategies RNA interference (RNAi) has come up as potential future tool in this direction. The present study targets nine potential genes (Aquaporin (AQP), Calcitonin (CAL), CyclophilinB (CYCP), Knottin-1 (k-1), Heat shock proteins (Hsp20, Hsp40 and Hsp70), SWItch/Sucrose Non-fermentable (SNF7) and inhibitor of apoptosis (IAP) of whitefly that have been implicated to play a role in various vital physiological functions and virus transmission. The RNAi mediated knockdown efficiency of these genes has been improved through the conjugation of respective target gene dsRNA with CQD (carbon quantum dots) nanoparticles or simultaneous knockdown of dsRNA specific gut nucleases. The studies revealed that feeding of dsRNA (40 μg/ml sucrose diet) of the target gene(s) either conjugated with CQD or along with dsRNA against dsRNase (dsdsRNase) (40 μg/ml sucrose diet) enhanced the RNAi efficiency by 24-89% compared to whiteflies fed with naked dsRNA of the same target gene. The studies provide insights about the functional role of various genes in whitefly, which can possibly be exploited for the management of this pest in the future.

Alteration of insulin signaling to control insect pest by using transformed bacteria expressing dsRNA

BACKGROUND

Insulin/insulin-like growth factor signaling (IIS) is known to mediate larval growth and adult reproduction in the legume pod borer, Maruca vitrata (Lepidoptera: Crambidae). Four IIS components (InR, FOXO, Akt, and TOR) play crucial roles in the IIS pathway.

RESULTS

RNA interference (RNAi) against any of these four IIS component genes was effective in suppressing each target mRNA level by either hemocoelic injection or oral administration using gene-specific double-stranded RNAs (dsRNAs). These RNAi treatments interfered with larval growth, leading to small pupae or significant larval mortality. For massive production of dsRNA, transformed bacteria expressing dsRNAs of these four IIS components were prepared with L4440 expression vector and HT115 strain of Escherichia coli. The transformed bacteria killed the larvae in a dose-dependent manner by feeding administration. An ultra-sonication pretreatment was performed to impair bacterial membrane and increase dsRNA release from the bacteria in insect intestine. This pretreatment increased the insecticidal activity of these recombinant bacteria. To further increase dsRNA toxicity, its mixture with Bacillus thuringiensis (Bt) was prepared and showed significant increase of Bt insecticidal activity in the laboratory. The bacterial mixture also showed a high control efficacy (83.3%) in an adzuki bean (Vigna angularis) field infested by M. vitrata. Furthermore, such a dsRNA effect was specific for M. vitrata, but not for non-target insects.

CONCLUSION

The bacteria expressing dsRNA specific to IIS components can be used to develop dsRNA insecticide. © 2019 Society of Chemical Industry.

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.

Gene silencing by RNAi via oral delivery of dsRNA by bacteria in the South American tomato pinworm, Tuta absoluta

BACKGROUND

RNA interference (RNAi) has been evaluated in several insect pests as a novel strategy to be included in integrated pest management. Lepidopterans are recognized to be recalcitrant to gene silencing by RNAi. As such, double-stranded RNA (dsRNA) delivery needs to be adjusted to assure its stability until it reaches the target gene transcript for silencing. Gene silencing by RNAi offers the potential to be used in the control of Tuta absoluta (Meyrick), one of the main insect pests of tomato (Solanum lycopersicum) worldwide. Here, we tested the delivery of dsRNA expressed in Escherichia coli HT115(DE3) and supplied to larvae in an artificial diet by screening target genes for silencing. We tested six target genes: juvenile hormone inducible protein (JHP); juvenile hormone epoxide hydrolase protein (JHEH); ecdysteroid 25-hydroxylase (PHM); chitin synthase A (CHI); carboxylesterase (COE); and arginine kinase (AK).

RESULTS

Based on larval mortality, the duration of the larval stage in days, pupal weight, and the accumulation of the target gene transcript, we demonstrated the efficacy of bacterial dsRNA delivery for the functional effects on larval development. Providing dsRNA targeted to JHP, CHI, COE and AK by bacteria led to a significant decrease in transcript accumulation and an increase in larval mortality.

CONCLUSION

Bacteria expressing dsRNA targeting essential T. absoluta genes supplied in artificial diet are efficient to screen RNAi target-genes. The oral delivery of dsRNA by bacteria is a novel potential alternative for the control of T. absoluta based on RNAi. © 2019 Society of Chemical Industry.

Prospects, challenges and current status of RNAi through insect feeding

RNA interference is a phenomenon in which the introduction of double-stranded RNA (dsRNA) into cells triggers the degradation of the complementary messenger RNA in a sequence-specific manner. Suppressing expression of vital genes could lead to insect death, therefore this technology has been considered as a potential strategy for insect pest control. There are three main routes of dsRNA administration into insects: (i) injections to the hemolymph, (ii) topical, and (iii) feeding. In this review, we focus on dsRNA administration through feeding. We summarize novel strategies that have been developed to improve the efficacy of this method, such as the use of nano-based formulations, engineered microorganisms, and transgenic plants. We also expose the hurdles that have to be overcome in order to use this technique as a reliable pest management method. © 2019 Society of Chemical Industry.

Progress and prospects of arthropod chitin pathways and structures as targets for pest management

Chitin is a structural component of the arthropod cuticular exoskeleton and the peritrophic matrix of the gut, which play crucial roles in growth and development. In the past few decades, our understanding of the composition, biosynthesis, assembly, degradation, and regulation of chitinous structures has increased. Many chemicals have been developed that target chitin biosynthesis (benzoyphenyl ureas, etoxazole), chitin degradation (allosamidin, psammaplin), and chitin regulation (benzoyl hydrazines), thus resulting in molting deformities and lethality. In addition, proteins that disrupt chitin structures, such as lectins, proteases, and chitinases have been utilized to halt feeding and induce mortality. Chitin-degrading enzymes, such as chitinases are also useful for improving the efficacy of bio-insecticides. Transgenic plants, baculoviruses, fungi, and bacteria have been engineered to express chitinases from a variety of organisms for control of arthropod pests. In addition, RNA interference targeting genes involved in chitin pathways and structures are now being investigated for the development of environmentally friendly pest management strategies. This review describes the chemicals and proteins used to target chitin structures and enzymes for arthropod pest management, as well as pest management strategies based upon these compounds, such as plant-incorporated-protectants and recombinant entomopathogens. Recent advances in RNA interference-based pest management, and how this technology can be used to target chitin pathways and structures are also discussed.

Responses of two ladybird beetle species (Coleoptera: Coccinellidae) to dietary RNAi

BACKGROUND

One concern with the adoption of RNAi-based genetically engineered (GE) crops is the potential harm to valued non-target organisms. Species of Coccinellidae (Coleoptera) are important natural enemies and might be exposed to the insecticidal dsRNA produced by the plant. To assess their susceptibility to dietary RNAi, we fed Adalia bipunctata and Coccinella septempunctata with a dsRNA designed to target the vATPase A of the western corn rootworm, Diabrotica virgifera virgifera (Dvv dsRNA). Specific dsRNAs designed to target the vATPase A of the two ladybird beetle species served as positive controls.

RESULTS

Our results revealed that both species were sensitive to dietary RNAi when ingesting their own dsRNAs, with C. septempunctata being more sensitive than A. bipunctata. Dvv dsRNA also adversely affected the two ladybird beetles as indicated by a significantly (but marginally) prolonged developmental time for A. bipunctata and a significantly reduced survival rate for C. septempunctata. These results, however, were obtained at Dvv dsRNA concentrations that were orders of magnitude higher than expected to occur in the field. Gene expression analyses confirmed the bioactivity of the dsRNA treatments and the results from the feeding bioassays. These results are consistent with the bioinformatics analyses, which revealed a higher number of 21-nucleotide-long matches, a requirement for effective RNAi, of the Dvv dsRNA with the vATPase A of C. septempunctata (34 matches) than with that of A. bipunctata (six matches).

CONCLUSION

Feeding bioassays revealed that two ladybird species are responsive to dietary RNAi. The two species, however, differed in their sensitivity. © 2019 Society of Chemical Industry.

Safe nanotechnologies for increasing the effectiveness of environmentally friendly natural agrochemicals

Natural compounds and living organisms continue to play a limited role in crop protection, and few of them have reached the market, despite their attractiveness and the efforts made in research. Very often these products have negative characteristics compared to synthetic compounds, e.g., higher costs of production, lower effectiveness, lack of persistence, and inability to reach and penetrate the target plant. Conversely, nanotechnologies are having an enormous impact on all human activities, including agriculture, even if the production of some nanomaterials is not environmentally friendly or could have adverse effects on agriculture and the environment. Thus, certain nanomaterials could facilitate the development of formulated natural pesticides, making them more effective and more environmentally friendly. Nanoformulations can improve efficacy, reduce effective doses, and increase shelf-life and persistence. Such controlled-release products can improve delivery to the target pest. This review considers certain available nanomaterials and nanotechnologies for use in agriculture, discussing their properties and the feasibility of their use in sustainable crop protection, in particular, in improving the effectiveness of natural bio-based agrochemicals. © 2019 Society of Chemical Industry.

Climatic Conditions: Conventional and Nanotechnology-Based Methods for the Control of Mosquito Vectors Causing Human Health Issues

Climate variability is highly impacting on mosquito-borne diseases causing malaria and dengue fever across the globe. Seasonal variability change in temperature and rainfall patterns are impacting on human health. Mosquitoes cause diseases like dengue fever, yellow fever, malaria, Chikungunya, West Nile and Japanese encephalitis. According to estimations by health organizations, annually one million human deaths are caused by vector-borne diseases, and dengue fever has increased about 30-fold over the past 50 years. Similarly, over 200 million cases of malaria are being reported annually. Mosquito-borne diseases are sensitive to temperature, humidity and seasonal variability. Both conventional (environmental, chemical, mechanical, biological etc.) and nanotechnology-based (Liposomes, nano-suspensions and polymer-based nanoparticles) approaches are used for the eradication of Malaria and dengue fever. Now green approaches are used to eradicate mosquitoes to save human health without harming the environment. In this review, the impact of climatic conditions on mosquito-borne diseases along with conventional and nanotechnology-based approaches used for controlling malaria and dengue fever have been discussed. Important recommendations have been made for people to stay healthy.

Gene silencing through RNAi and antisense Vivo-Morpholino increases the efficacy of pyrethroids on larvae of Anopheles stephensi

Background

Insecticides are still at the core of insect pest and vector control programmes. Several lines of evidence indicate that ABC transporters are involved in detoxification processes against insecticides, including permethrin and other pyrethroids. In particular, the ABCG4 gene, a member of the G subfamily, has consistently been shown to be up-regulated in response to insecticide treatments in the mosquito malaria vector Anopheles stephensi (both adults and larvae).

Methods

To verify the actual involvement of this transmembrane protein in the detoxification process of permethrin, bioassays on larvae of An. stephensi, combining the insecticide with a siRNA, specifically designed for the inhibition of ABCG4 gene expression were performed. Administration to larvae of the same siRNA, labeled with a fluorescent molecule, was effected to investigate the systemic distribution of the inhibitory RNA into the larval bodies. Based on siRNA results, similar experiments using antisense Vivo-Morpholinos (Vivo-MOs) were effected. These molecules, compared to siRNA, are expected to guarantee a higher stability in environmental conditions and in the insect gut, and present thus a higher potential for future in-field applications.

Results

Bioassays using two different concentrations of siRNA, associated with permethrin, led to an increase of larval mortality, compared with results with permethrin alone. These outcomes confirm that ABCG4 transporter plays a role in the detoxification process against the selected insecticide. Moreover, after fluorescent labelling, it was shown the systemic dissemination of siRNA in different body districts of An. stephensi larvae, which suggest a potential systemic effect of the molecule. At the same time, results of Vivo-MO experiments were congruent with those obtained using siRNA, thus confirming the potential of ABCG4 inhibition as a strategy to increase permethrin susceptibility in mosquitoes. For the first time, Vivo-MOs were administered in water to larvae, with evidence for a biological effect.

Conclusions

Targeting ABCG4 gene for silencing through both techniques resulted in an increased pyrethroid efficacy. These results open the way toward the possibility to exploit ABCG4 inhibition in the context of integrated programmes for the control An. stephensi mosquitoes and malaria transmission.

Prolonged mosquitocidal activity of Siparuna guianensis essential oil encapsulated in chitosan nanoparticles

Background

The use of synthetic insecticides is one of the most common strategies for controlling disease vectors such as mosquitos. However, their overuse can result in serious risks to human health, to the environment, as well as to the selection of insecticidal resistant insect strains. The development of efficient and eco-friendly insect control is urgent, and essential oils have been presented as potential alternatives to synthetic insecticides. Moreover, nanoencapsulation techniques can enhance their efficiency by protecting from degradation and providing a controlled release rate.

Results

We assessed the potential of chitosan nanoparticles in encapsulating Siparuna guianensis essential oil, and maintaining its efficiency and prolonging its activity for the control of Aedes aegypti larvae. The encapsulation was characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA), with an encapsulation efficiency ranging from 84.8% to 88.0%. Toxicity studies have demonstrated efficacy against mosquito larvae over 50% for 19 days with 100% mortality during the first week. This persistent action is presumably due to the enhanced contact and slow and maintained release conferred by chitosan nanoparticles. Furthermore, the exposure of aquatic non-target organisms (e.g. embryos and small adult fishes) revealed adequate selectivity of these nanoparticles.

Conclusions

The encapsulation of S. guianensis essential oil in chitosan nanoparticles showed promising potential as a larvicide control alternative and should be considered within strategies for fighting Ae. aegypti.

Numerous outbreaks of infectious diseases such as dengue, zika, and chikungunya in tropical countries have occurred where the mosquito Aedes aegypti is the transmitting vector. In Brazil, these infections are responsible for deaths and severe sequelae. Thus, many efforts have been made by governmental and research groups to control these outbreaks. However, complete success in this control has so far remained unachieved. Parallel to the need to develop new technologies that contribute to the control of insects that transmit diseases, there is a growing societal awareness regarding the risks associated with the use of synthetic insecticides, which has led to a search for natural alternatives such as essential oils from plants. Thus, our group conducted experiments to evaluate the application of nanotechnology in obtaining an efficient prolonged release system to combat Ae. aegypti larvae using the essential oil of a plant native to the Cerrado and Amazonian forests. These results demonstrate that using a simple and easily scalable encapsulation technique; it is possible to keep the low toxicity against non-target organism and prolong the activity of an essential oil in water and maintain larval mortality at a significant level for more than a week with a single application.

Novel RNAi delivery systems in the control of medical and veterinary pests

RNA interference (RNAi) is a transformative technology with great potential to control, study or even protect insects and acarines through the knockdown of target gene expression. RNAi offers unprecedented levels of control, but fundamental to its successful deployment is the need to deliver 'trigger' RNA in an appropriate fashion giving due consideration to potential barriers of RNAi efficiency, safety, and the intended purpose of the knockdown. This short review focusses on recent innovations in RNAi delivery that are designed for, or could be adapted for use with, insect and acarine pests of medical or veterinary importance.

Silencing of the Chitin Synthase Gene Is Lethal to the Asian Citrus Psyllid, Diaphorina citri

Chitin synthase is a critical enzyme that catalyzes N-acetylglucosamine to form chitin, which plays an important role in the growth and development of insects. In this study, we identified a chitin synthase gene (CHS) with a complete open reading frame (ORF) of 3180 bp from the genome database of Diaphorina citri, encoding a protein of 1059 amino acid residues with the appropriate signature motifs (EDR and QRRRW). Reverse transcription-quantitative PCR (RT-qPCR) analysis suggested that D. citri CHS (DcCHS) was expressed throughout all developmental stages and all tissues. DcCHS had the highest expression level in the integument and fifth-instar nymph stage. Furthermore, the effects of diflubenzuron (DFB) on D. citri mortality and DcCHS expression level were investigated using fifth-instar nymph through leaf dip bioassay, and the results revealed that the nymph exposed to DFB had the highest mortality compared with control group (Triton-100). Silencing of DcCHS by RNA interference resulted in malformed phenotypes and increased mortality with decreased molting rate. In addition, transmission electron microscopy (TEM) and fluorescence in situ hybridization (FISH) also revealed corresponding ultrastructural defects. Our results suggest that DcCHS might play an important role in the development of D. citri and can be used as a potential target for psyllid control.

Natural Polysaccharides for siRNA Delivery: Nanocarriers Based on Chitosan, Hyaluronic Acid, and Their Derivatives

Natural polysaccharides are frequently used in the design of drug delivery systems due to their biocompatibility, biodegradability, and low toxicity. Moreover, they are diverse in structure, size, and charge, and their chemical functional groups can be easily modified to match the needs of the final application and mode of administration. This review focuses on polysaccharidic nanocarriers based on chitosan and hyaluronic acid for small interfering RNA (siRNA) delivery, which are highly positively and negatively charged, respectively. The key properties, strengths, and drawbacks of each polysaccharide are discussed. In addition, their use as efficient nanodelivery systems for gene silencing applications is put into context using the most recent examples from the literature. The latest advances in this field illustrate effectively how chitosan and hyaluronic acid can be modified or associated with other molecules in order to overcome their limitations to produce optimized siRNA delivery systems with promising in vitro and in vivo results.

Applications of Nanotechnology in Plant Growth and Crop Protection: A Review

In the era of climate change, global agricultural systems are facing numerous, unprecedented challenges. In order to achieve food security, advanced nano-engineering is a handy tool for boosting crop production and assuring sustainability. Nanotechnology helps to improve agricultural production by increasing the efficiency of inputs and minimizing relevant losses. Nanomaterials offer a wider specific surface area to fertilizers and pesticides. In addition, nanomaterials as unique carriers of agrochemicals facilitate the site-targeted controlled delivery of nutrients with increased crop protection. Due to their direct and intended applications in the precise management and control of inputs (fertilizers, pesticides, herbicides), nanotools, such as nanobiosensors, support the development of high-tech agricultural farms. The integration of biology and nanotechnology into nonosensors has greatly increased their potential to sense and identify the environmental conditions or impairments. In this review, we summarize recent attempts at innovative uses of nanotechnologies in agriculture that may help to meet the rising demand for food and environmental sustainability.

Applications of Nanotechnology in Plant Growth and Crop Protection: A Review

In the era of climate change, global agricultural systems are facing numerous, unprecedented challenges. In order to achieve food security, advanced nano-engineering is a handy tool for boosting crop production and assuring sustainability. Nanotechnology helps to improve agricultural production by increasing the efficiency of inputs and minimizing relevant losses. Nanomaterials offer a wider specific surface area to fertilizers and pesticides. In addition, nanomaterials as unique carriers of agrochemicals facilitate the site-targeted controlled delivery of nutrients with increased crop protection. Due to their direct and intended applications in the precise management and control of inputs (fertilizers, pesticides, herbicides), nanotools, such as nanobiosensors, support the development of high-tech agricultural farms. The integration of biology and nanotechnology into nonosensors has greatly increased their potential to sense and identify the environmental conditions or impairments. In this review, we summarize recent attempts at innovative uses of nanotechnologies in agriculture that may help to meet the rising demand for food and environmental sustainability.

Development of CS-TPP-dsRNA nanoparticles to enhance RNAi efficiency in the yellow fever mosquito, Aedes aegypti

Mosquito-borne diseases are a major threat to human health and are responsible for millions of deaths globally each year. Vector control is one of the most important approaches used in reducing the incidence of these diseases. However, increasing mosquito resistance to chemical insecticides presents challenges to this approach. Therefore, new strategies are necessary to develop the next generation vector control methods. Because of the target specificity of dsRNA, RNAi-based control measures are an attractive alternative to current insecticides used to control disease vectors. In this study, Chitosan (CS) was cross-linked to sodium tripolyphosphate (TPP) to produce nano-sized polyelectrolyte complexes with dsRNA. CS-TPP-dsRNA nanoparticles were prepared by ionic gelation method. The encapsulation efficiency, protection of dsRNA from nucleases, cellular uptake, in vivo biodistribution, larval mortality and gene knockdown efficiency of CS-TPP-dsRNA nanoparticles were determined. The results showed that at a 5:1 weight ratio of CS-TPP to dsRNA, nanoparticles of less than 200 nm mean diameter and a positive surface charge were formed. Confocal microscopy revealed the distribution of the fed CS-TPP-dsRNA nanoparticles in midgut, fat body and epidermis of yellow fever mosquito, Aedes aegypti larvae. Bioassays showed significant mortality of larvae fed on CS-TPP-dsRNA nanoparticles. These assays also showed knockdown of a target gene in CS-TPP-dsRNA nanoparticle fed larvae. These data suggest that CS-TPP nanoparticles may be used for delivery of dsRNA to mosquito larvae.

Downregulation of female doublesex expression by oral-mediated RNA interference reduces number and fitness of Anopheles gambiae adult females

Background

Mosquito-borne diseases affect millions worldwide, with malaria alone killing over 400 thousand people per year and affecting hundreds of millions. To date, the best strategy to prevent the disease remains insecticide-based mosquito control. However, insecticide resistance as well as economic and social factors reduce the effectiveness of the current methodologies. Alternative control technologies are in development, including genetic control such as the sterile insect technique (SIT). The SIT is a pivotal tool in integrated agricultural pest management and could be used to improve malaria vector control. To apply the SIT and most other newer technologies against disease transmitting mosquitoes, it is essential that releases are composed of males with minimal female contamination. The removal of females is an essential requirement because released females can themselves contribute towards nuisance biting and disease transmission. Thus, females need to be eliminated from the cohorts prior to release. Manual separation of Anopheles gambiae pupae or adult mosquitoes based on morphology is time consuming, is not feasible on a large scale and has limited the implementation of the SIT technique. The doublesex (dsx) gene is one of the effector switches of sex determination in the process of sex differentiation in insects. Both males and females have specific splicing variants that are expressed across the different life stages. Using RNA interference (RNAi) to reduce expression of the female specific (dsxF) variant of this gene has proven to have detrimental effects to the females in other mosquito species, such as Aedes aegypti. We tested oral RNAi on dsx (AgdsxF) in An. gambiae.

Methods

We studied the expression pattern of the dsx gene in the An. gambiae G3 strain. We knocked down AgdsxF expression in larvae through oral delivery of double stranded RNA (dsRNA) produced by bacteria and observed its effects in adults.

Results

Our results show that feeding of AgdsxF dsRNA can effectively reduce (> 66%) the mRNA of female dsx transcript and that there is a concomitant reduction in the number of female larvae that achieve adulthood. Control groups produced 52% (± 3.9% SE) of adult males and 48% (± 4.0% SE) females, while AgdsxF dsRNA treated groups had 72.1% (± 4.0% SE) males vs 27.8% females (± 3.3% SE). In addition, the female adults produce fewer progeny, 37.1% (± 8.2% SE) less than the controls. The knockdown was sex-specific and had no impact on total numbers of viable male adults, in the male dsx transcripts or male fitness parameters such as longevity or body size.

Conclusions

These findings indicate that RNAi could be used to improve novel mosquito control strategies that require efficient sex separation and male-only release of An. gambiae by targeting sex determination genes such as AgdsxF. The advantages of using RNAi in a controlled setting for mosquito rearing are numerous, as the dose and time of exposure are controlled, and the possibility of off-target effects and the waste of female production would be significantly reduced.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3437-4) contains supplementary material, which is available to authorized users.

Uptake and Bioactivity of Chitosan/Double-Stranded RNA Polyplex Nanoparticles in Caenorhabditis elegans

In this study, we investigated chitosan/dsRNA polyplex nanoparticles as RNAi agents in the nematode Caenorhabditis elegans. By measurement of an easily observed phenotype and uptake of fluorescently labeled dsRNA, we demonstrate that chitosan/dsRNA polyplex nanoparticles are considerably more effective at gene knockdown on a whole body concentration basis than naked dsRNA. Further, we show that chitosan/dsRNA polyplex nanoparticles introduce dsRNA into cells via a different mechanism than the canonical sid-1 and sid-2 pathway. Clathrin-mediated endocytosis is likely the main uptake mechanism. Finally, although largely reported as nontoxic, we have found that chitosan, as either polyplex nanoparticles or alone, is capable of downregulating the expression of myosin. Myosin is a critical component of growth and development in eukaryotes, and we have observed reductions in both growth rate and reproduction in chitosan exposed C. elegans. Given the increased potency, noncanonical uptake, and off-target effects that we identified, these findings highlight the need for a rigorous safety assessment of nano-RNAi products prior to deployment. Specifically, the potential adverse effects of the nanocarrier and its components need to be considered.

Biosynthesis, modifications and degradation of chitin in the formation and turnover of peritrophic matrix in insects

The peritrophic matrix (PM) is an extracellular, semi-permeable biocomposite that lines the midgut of most insects. The PM serves as the first defense in the midgut to resist microorganisms such as viruses, bacteria and other pathogens, and to protect epithelial cells from mechanical damage. The PM also separates the midgut lumen into different compartments, which play important roles in nutrient ingestion and digestion. The PM is a highly dynamic structure that consists mainly of chitin fibers cross-linked by proteins, glycoproteins, and proteoglycans. The PM is continuously biosynthesized, assembled, and degraded in response to feeding and development. Chitin chains are synthesized by several enzymes and organized in several hierarchical levels, in which various PM-associated proteins appear to be essential for maintaining the structural integrity and physiological function of the PM. This review summarizes research advances on molecular components of the PM and their functions, as well as related proteins and enzymes that contribute to PM formation and modification. Crucial gaps in our current understanding of the PM are also addressed.

RNAi-based bioinsecticide for Aedes mosquito control

Zika virus infection and dengue and chikungunya fevers are emerging viral diseases that have become public health threats. Their aetiologic agents are transmitted by the bite of genus Aedes mosquitoes. Without effective therapies or vaccines, vector control is the main strategy for preventing the spread of these diseases. Increased insecticide resistance calls for biorational actions focused on control of the target vector population. The chitin required for larval survival structures is a good target for biorational control. Chitin synthases A and B (CHS) are enzymes in the chitin synthesis pathway. Double-stranded RNA (dsRNA)-mediated gene silencing (RNAi) achieves specific knockdown of target proteins. Our goal in this work, a new proposed RNAi-based bioinsecticide, was developed as a potential strategy for mosquito population control. DsRNA molecules that target five different regions in the CHSA and B transcript sequences were produced in vitro and in vivo through expression in E. coli HT115 and tested by direct addition to larval breeding water. Mature and immature larvae treated with dsRNA targeting CHS catalytic sites showed significantly decreased viability associated with a reduction in CHS transcript levels. The few larval and adult survivors displayed an altered morphology and chitin content. In association with diflubenzuron, this bioinsecticide exhibited insecticidal adjuvant properties.

Liposome encapsulation and EDTA formulation of dsRNA targeting essential genes increase oral RNAi-caused mortality in the Neotropical stink bug Euschistus heros

BACKGROUND

The Neotropical stink bug Euschistus heros is a major pest in soybean fields. Development of highly species-specific pesticides based on RNA interference (RNAi) could provide a new sustainable and environmentally friendly control strategy.

RESULTS

Here, the potential of RNAi as a pest control tool against E. heros was assessed. First, target gene selection using a microinjection approach was performed. Seven of the 15 candidate genes tested exhibited > 95% mortality after hemolymph injection of 27.5 ng dsRNA. Subsequently, dsRNA was administered orally using different formulations: naked dsRNA, liposome-encapsulated-dsRNA and dsRNA formulated with EDTA. Liposome-encapsulated dsRNA targeting vATPase A and muscle actin led to significant mortality after 14 days (45% and 42%, respectively), whereas EDTA-formulated dsRNA did so for only one of the target genes. Ex vivo analysis of the dsRNA stability in collected saliva indicated a strong dsRNA-degrading capacity by E. heros saliva, which could explain the need for dsRNA formulations.

CONCLUSION

The results demonstrate that continuous ingestion of dsRNA with EDTA or liposome-encapsulated dsRNA can prevent dsRNA from being degraded enzymatically and suggest great potential for using these formulations in dsRNA delivery to use RNAi as a functional genomics tool or for pest management of stink bugs. © 2018 Society of Chemical Industry.

Molecular cloning, expression, and functional analysis of the chitin synthase 1 gene and its two alternative splicing variants in the white-backed planthopper, Sogatella furcifera (Hemiptera: Delphacidae)

Chitin synthase is responsible for chitin synthesis in the cuticles and cuticular linings of other tissues in insects. We cloned two alternative splicing variants of the chitin synthase 1 gene (SfCHS1) from the white-backed planthopper, Sogatella furcifera. The full-length cDNA of the two variants (SfCHS1a and SfCHS1b) consists of 6408 bp, contains a 4719-bp open reading frame encoding 1572 amino acids, and has 5′ and 3′ non-coding regions of 283 and 1406 bp, respectively. The two splicing variants occur at the same position in the cDNA sequence between base pairs 4115 and 4291, and consist of 177 nucleotides that encode 59 amino acids but show 74.6% identity at the amino acid level. Analysis in different developmental stages showed that expression of SfCHS1 and SfCHS1a were highest just after molting, whereas SfCHS1b reached its highest expression level 2 days after molting. Further, SfCHS1 and SfCHS1a were mainly expressed in the integument, whereas SfCHS1b was predominately expressed in the gut and fat body. RNAi-based gene silencing inhibited transcript levels of the corresponding mRNAs in S. furcifera nymphs injected with double-stranded RNA of SfCHS1, SfCHS1a, and SfCHS1b, resulted in malformed phenotypes, and killed most of the treated nymphs. Our results indicate that SfCHS1 may be a potential target gene for RNAi-based S. furcifera control.

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.

Preparation and Use of a Yeast shRNA Delivery System for Gene Silencing in Mosquito Larvae

The mosquito genome projects facilitated research in new facets of mosquito biology, including functional genetic studies in the dengue and Zika virus vector Aedes aegypti and the primary African malaria vector Anopheles gambiae. RNA interference (RNAi) has facilitated gene silencing experiments in both of these disease vector mosquito species and could one day be applied as a new method of vector control. Here, we describe a procedure for the genetic engineering of Saccharomyces cerevisiae (baker's yeast) that express short hairpin RNA (shRNA) corresponding to mosquito target genes of interest. Following cultivation, which facilitates inexpensive propagation of shRNA, the yeast is inactivated and prepared in a ready-to-use dry tablet formulation that is fed to mosquito larvae. Ingestion of the yeast tablets results in effective larval target gene silencing. This technically straightforward and affordable technique may be applicable to a wide variety of mosquito species and potentially to other arthropods that feed on yeast.

Aedes aegypti Galectin Competes with Cry11Aa for Binding to ALP1 To Modulate Cry Toxicity

The key step for the toxicity of Bacillus thuringiensis subsp. israelensis (Bti) is the interaction between toxins and putative receptors; thus, many studies focus on identification of new toxin receptors and engineering of toxins with higher affinity/specificity for receptors. In the larvae of Aedes aegypti, galectin-14 was one of the genes upregulated by Bti treatment. RNAi knockdown expression of galectin-14 and feeding recombinant galectin-14-thioredoxin fusion protein significantly affected survival of Ae. aegypti larvae treated with Bti toxins. Recombinant galectin-14 protein bound to brush border membrane vesicles (BBMVs) of Ae. aegypti larvae, ALP1 and APN2, and galectin-14 and Cry11Aa bound to BBMVs with a similarly high affinity. Competitive binding results showed that galectin-14 competed with Cry11Aa for binding to BBMVs and ALP1 to prevent effective binding of toxin to receptors. These novel findings demonstrated that midgut proteins other than receptors play an important role in modulating the toxicity of Cry toxins.

How do oral insecticidal compounds cross the insect midgut epithelium?

The use of oral insecticidal molecules (small molecules, peptides, dsRNA) via spray or plant mediated applications represents an efficient way to manage damaging insect species. With the exception of Bt toxins that target the midgut epithelium itself, most of these compounds have targets that lie within the hemocoel (body) of the insect. Because of this, one of the greatest factors in determining the effectiveness of an oral insecticidal compound is its ability to traverse the gut epithelium and enter the hemolymph. However, for many types of insecticidal compounds, neither the pathway taken across the gut nor the specific genes which influence uptake are fully characterized. Here, we review how different types of insecticidal compounds enter or cross the midgut epithelium through passive (diffusion) or active (transporter based, endocytosis) routes. A deeper understanding of how insecticidal molecules cross the gut will help to best utilize current insecticides and also provide for more rational design of future ones.

Nanotechnology for Plant Disease Management

Each year, 20%–40% of crops are lost due to plant pests and pathogens. Existing plant disease management relies predominantly on toxic pesticides that are potentially harmful to humans and the environment. Nanotechnology can offer advantages to pesticides, like reducing toxicity, improving the shelf-life, and increasing the solubility of poorly water-soluble pesticides, all of which could have positive environmental impacts. This review explores the two directions in which nanoparticles can be utilized for plant disease management: either as nanoparticles alone, acting as protectants; or as nanocarriers for insecticides, fungicides, herbicides, and RNA-interference molecules. Despite the several potential advantages associated with the use of nanoparticles, not many nanoparticle-based products have been commercialized for agricultural application. The scarcity of commercial applications could be explained by several factors, such as an insufficient number of field trials and underutilization of pest–crop host systems. In other industries, nanotechnology has progressed rapidly, and the only way to keep up with this advancement for agricultural applications is by understanding the fundamental questions of the research and addressing the scientific gaps to provide a rational and facilitate the development of commercial nanoproducts.

Comparative transcriptome analysis and RNA interference reveal CYP6A8 and SNPs related to pyrethroid resistance in Aedes albopictus

Wide and improper application of pyrethroid insecticides for mosquito control has resulted in widespread resistance in Aedes albopictus mosquitoes, an important dengue vector. Therefore, understanding the molecular regulation of insecticide resistance is urgently needed to provide a basis for developing novel resistance diagnostic methods and vector control approaches. We investigated the transcriptional profiles of deltamethrin-resistant and -susceptible Ae. albopictus by performing paired-end sequencing for RNA expression analysis. The analysis used 24 independent libraries constructed from 12 wild-caught resistant and 12 susceptible Ae. albopictus female adults. A total of 674,503,592 and 612,512,034 reads were obtained, mapped to the Ae. albopictus genome and assembled into 20,091 Ae. albopictus transcripts. A total of 1,130 significantly differentially expressed genes included 874 up-regulated genes and 256 down-regulated genes in the deltamethrin-resistant individuals. These differentially expressed genes code for cytochrome P450s, cuticle proteins, glutathione S-transferase, serine proteases, heat shock proteins, esterase, and others. We selected three highly differentially expressed candidate genes, CYP6A8 and two genes of unknown function (CCG013931 and CCG000656), to test the association between these 3 genes and deltamethrin resistance using RNAi through microinjection in adult mosquitoes and oral feeding in larval mosquitoes. We found that expression knockdown of these three genes caused significant changes in resistance. Further, we detected 1,162 single nucleotide polymorphisms (SNPs) with a frequency difference of more than 50%. Among them, 5 SNPs in 4 cytochrome P450 gene families were found to be significantly associated with resistance in a genotype-phenotype association study using independent field-collected mosquitoes of known resistance phenotypes. Altogether, a combination of novel individually based transcriptome profiling, RNAi, and genetic association study identified both differentially expressed genes and SNPs associated with pyrethroid resistance in Ae. albopictus mosquitoes, and laid a useful foundation for further studies on insecticide resistance mechanisms.

The mysteries of insect RNAi: A focus on dsRNA uptake and transport

RNA interference (RNAi) is becoming a practical tool to control insect pests. Many mysteries of how double-stranded RNA (dsRNA) is transported into, within, and between cells to generate an efficient RNAi response in insects are still to be unraveled. This review provides an overview of the evidence that supports a key role of endocytosis in the uptake of dsRNA on both cellular and tissue levels. Additionally, other components of cellular membrane transport and their impact on the efficiency of RNAi in insects are explored. It is now evident that the membrane transport and potentially dsRNA release from the endosome may comprise some of the limiting factors in insects that are recalcitrant to dsRNA. This review concludes with the apparent connection between gene products that are necessary for cellular trafficking of dsRNA and highly lethal RNAi targets.

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.

Development and characterization of the first dsRNA-resistant insect population from western corn rootworm, Diabrotica virgifera virgifera LeConte

The use of dsRNA to control insect pests via the RNA interference (RNAi) pathway is being explored by researchers globally. However, with every new class of insect control compounds, the evolution of insect resistance needs to be considered, and understanding resistance mechanisms is essential in designing durable technologies and effective resistance management strategies. To gain insight into insect resistance to dsRNA, a field screen with subsequent laboratory selection was used to establish a population of DvSnf7 dsRNA-resistant western corn rootworm, Diabrotica virgifera virgifera, a major maize insect pest. WCR resistant to ingested DvSnf7 dsRNA had impaired luminal uptake and resistance was not DvSnf7 dsRNA-specific, as indicated by cross resistance to all other dsRNAs tested. No resistance to the Bacillus thuringiensis Cry3Bb1 protein was observed. DvSnf7 dsRNA resistance was inherited recessively, located on a single locus, and autosomal. Together these findings will provide insights for dsRNA deployment for insect pest control.

Double-stranded RNA Oral Delivery Methods to Induce RNA Interference in Phloem and Plant-sap-feeding Hemipteran Insects

Phloem and plant sap feeding insects invade the integrity of crops and fruits to retrieve nutrients, in the process damaging food crops. Hemipteran insects account for a number of economically substantial pests of plants that cause damage to crops by feeding on phloem sap. The brown marmorated stink bug (BMSB), Halyomorpha halys (Heteroptera: Pentatomidae) and the Asian citrus psyllid (ACP), Diaphorina citri Kuwayama (Hemiptera: Liviidae) are hemipteran insect pests introduced in North America, where they are an invasive agricultural pest of high-value specialty, row, and staple crops and citrus fruits, as well as a nuisance pest when they aggregate indoors. Insecticide resistance in many species has led to the development of alternate methods of pest management strategies. Double-stranded RNA (dsRNA)-mediated RNA interference (RNAi) is a gene silencing mechanism for functional genomic studies that has potential applications as a tool for the management of insect pests. Exogenously synthesized dsRNA or small interfering RNA (siRNA) can trigger highly efficient gene silencing through the degradation of endogenous RNA, which is homologous to that presented. Effective and environmental use of RNAi as molecular biopesticides for biocontrol of hemipteran insects requires the in vivo delivery of dsRNAs through feeding. Here we demonstrate methods for delivery of dsRNA to insects: loading of dsRNA into green beans by immersion, and absorbing of gene-specific dsRNA with oral delivery through ingestion. We have also outlined non-transgenic plant delivery approaches using foliar sprays, root drench, trunk injections as well as clay granules, all of which may be essential for sustained release of dsRNA. Efficient delivery by orally ingested dsRNA was confirmed as an effective dosage to induce a significant decrease in expression of targeted genes, such as juvenile hormone acid O-methyltransferase (JHAMT) and vitellogenin (Vg). These innovative methods represent strategies for delivery of dsRNA to use in crop protection and overcome environmental challenges for pest management.

Increased RNAi Efficacy in Spodoptera exigua via the Formulation of dsRNA With Guanylated Polymers

Lepidoptera comprise some of the most devastating herbivorous pest insects worldwide. One of the most promising novel pest control strategies is exploiting the RNA interference (RNAi) mechanism to target essential genes for knockdown and incite toxic effects in the target species without harming other organisms in the ecosystem. However, many insects are refractory to oral RNAi, often due to rapid degradation of ingested dsRNA in their digestive system. This is the case for many lepidopteran insects, including the beet armyworm Spodoptera exigua, which is characterized by a very alkaline gut environment (pH > 9.0) and a strong intestinal nucleolytic activity. In this research, guanidine-containing polymers were developed to protect dsRNA against nucleolytic degradation, specifically in high pH environments. First, their ability to protect dsRNA against nucleolytic degradation in gut juice of the beet armyworm S. exigua was investigated ex vivo. Polymers with high guanidine content provided a strong protection against nucleolytic degradation at pH 11, protecting the dsRNA for up to 30 h. Next, cellular uptake of the dsRNA and the polyplexes in lepidopteran CF203 midgut cells was investigated by confocal microscopy, showing that the polymer also enhanced cellular uptake of the dsRNA. Finally, in vivo feeding RNAi bioassays demonstrated that using these guanidine-containing polymer nanoparticles led to an increased RNAi efficiency in S. exigua. Targeting the essential gene chitin synthase B, we observed that the mortality increased to 53% in the polymer-protected dsRNA treatment compared to only 16% with the naked dsRNA and found that polymer-protected dsRNA completely halted the development of the caterpillars. These results show that using guanylated polymers as a formulation strategy can prevent degradation of dsRNA in the alkaline and strongly nucleolytic gut of lepidopteran insects. Furthermore, the polymer also enhances cellular uptake in lepidopteran midgut cells. This new delivery strategy could be of great use in further fundamental research in lepidopterans, using RNAi as a research tool, and could lead to future applications for RNAi-based pest control of lepidopteran insects.

Delivery of lethal dsRNAs in insect diets by branched amphiphilic peptide capsules

Development of new and specific insect pest management methods is critical for overcoming pesticide resistance and collateral off-target killings. Gene silencing by feeding dsRNA to insects shows promise in this area. Here we described the use of a peptide nano-material, branched amphiphilic peptide capsules (BAPCs), that facilitates cellular uptake of dsRNA by insects through feeding. The insect diets included dsRNA with and without complexation with BAPCs. The selected insect species come from two different orders with different feeding mechanisms: Tribolium castaneum and Acyrthosiphon pisum. The gene transcripts tested (BiP and Armet) are part of the unfolded protein response (UPR) and suppressing their translation resulted in lethality. For Acyrthosiphon pisum, ingestion of BiP-dsRNA associated with BAPCs led to the premature death of the aphids (t_1/2=4-5days) compared to ingestion of the same amounts of free BiP-dsRNA (t_1/2=11-12days). Tribolium castaneum was effectively killed using a combination of BiP-dsRNA and Armet-dsRNA complexed with BAPCs; most dying as larvae or during eclosion (~75%). Feeding dsRNA alone resulted in fewer deaths (~30%). The results show that complexation of dsRNA with BAPCs enhanced the oral delivery of dsRNA over dsRNA alone.

Liposome mediated double-stranded RNA delivery to silence ribosomal protein P0 in the tick Rhipicephalus haemaphysaloides

Control of ticks has been achieved primarily by the application of acaricides, which has drawbacks such as environmental contamination leading to the selection of pesticide-resistant ticks. The potential of dsRNA to suppress genes critical for tick survival due to its sequence specificity suggests that dsRNAs could be developed as tailor-made pesticides. In this study, the dsRNA of P0 gene from the tick, Rhipicephalus haemaphysaloides, was evaluated as a potential anti-tick agent. Effects of using different dsRNA delivery methods were tested by quantitative RT-PCR and tick bioassays to determine survival, feeding and reproduction. The results showed that P0 dsRNAs could be effectively delivered into ticks and silenced by incubating with liposomes. Incubation time was found to be the most important factor in dsRNA delivery and gene silencing compared with liposome types and dsRNA concentration. The effects of P0 dsRNA treatment on ticks were found to be significant on blood feeding, molting or reproduction. These data show that anti-tick agents based on dsRNAs could have potential use in tick control.

Systemic RNAi in western corn rootworm, Diabrotica virgifera virgifera, does not involve transitive pathways

Western corn rootworm (WCR, Diabrotica virgifera virgifera LeConte) is highly sensitive to orally delivered double-stranded RNA (dsRNA). RNAi in WCR is systemic and spreads throughout the insect body. This raises the question whether transitive RNAi is a mechanism that functions in WCR to amplify the RNAi response via production of secondary siRNA. Secondary siRNA production is achieved through RNA-dependent RNA polymerase (RdRP) activity in other eukaryotic organisms, but RdRP has not been identified in WCR and any other insects. This study visualized the spread of the RNAi-mediated knockdown of Dv v-ATPase C mRNA throughout the WCR gut and other tissues using high-sensitivity branched DNA in situ hybridization. Furthermore, we did not detect either secondary siRNA production or transitive RNAi in WCR through siRNA sequence profile analysis. Nucleotide mismatched sequences introduced into either the sense or antisense strand of v-ATPase C dsRNAs were maintained in siRNAs derived from WCR fed with the mismatched dsRNAs in a strand specific manner. The distribution of all siRNAs was restricted to within the original target sequence regions, which may indicate the lack of new dsRNA synthesis leading to production of secondary siRNA. Thus, the systemic spread of RNAi in WCR may be derived from the original dsRNA molecules taken up from the gut lumen. These results indicate that the initial dsRNA dose is important for a lethal systemic RNAi response in WCR and have implications in developing effective dsRNA traits to control WCR and in resistance management to prolong the durability of RNAi trait technology.

Insect RNAi: Integrating a New Tool in the Crop Protection Toolkit

Protecting crops against insect pests is a major focus area in crop protection. Over the past two decades, biotechnological interventions, especially Bt proteins, have been successfully implemented across the world and have had major impacts on reducing chemical pesticide applications. As insects continue to adapt to insecticides, both chemical and protein-based, new methods, molecules, and modes of action are necessary to provide sustainable solutions. RNA interference (RNAi) has emerged as a significant tool to knock down or alter gene expression profiles in a species-specific manner. In the past decade, there has been intense research on RNAi applications in crop protection. This chapter looks at the current state of knowledge in the field and outlines the methodology, delivery methods, and precautions required in designing targets. Assessing the targeting of specific gene expression is also an important part of a successful RNAi strategy. The current literature on the use of RNAi in major orders of insect pests is reviewed, along with a perspective on the regulatory aspects of the approach. Risk assessment of RNAi would focus on molecular characterization, food/feed risk assessment, and environmental risk assessment. As more RNAi-based products come through regulatory systems, either via direct application or plant expression based, the impact of this approach on crop protection will become clearer.

Cytochrome P450 genes from the aquatic midge Chironomus tentans: Atrazine-induced up-regulation of CtCYP6EX3 enhanced the toxicity of chlorpyrifos

The open reading frames of 19 cytochrome P450 monooxygenase (CYP) genes were sequenced from Chironomus tentans, a commonly used freshwater invertebrate model. Phylogenetic analysis of the 19 CYPs along with a previously reported CYP (CtCYP4G33) revealed that they belong to three different clans, including 3 in CYP4, 15 in CYP3, and 2 in mitochondria clan. When third-instar larvae were exposed to atrazine at 5000 μg/L, the transcription of CtCYP6EX3, CtCYP6EV3, CtCYP9AT1 and CtCYPEX1 was significantly up-regulated. To examine whether CtCYP6EX3 played a role in oxidative activation of chlorpyrifos to chlorpyrifos-oxon, we evaluated larval susceptibility to chlorpyrifos after CtCYP6EX3 transcript was suppressed by RNAi. The larvae fed chitosan/dsCtCYP6EX3 nanoparticles showed a significantly decreased CtCYP6EX3 transcript (53.1%) as compared with the control larvae fed chitosan/dsGFP nanoparticles. When the CtCYP6EX3-silenced larvae were exposed to chlorpyrifos at 6 μg/L or its binary mixture with atrazine (chlorpyrifos at 3 μg/L and atrazine at 1000 μg/L), the larvae became less susceptible to the pesticides as their mortalities decreased by 24.1% and 20.5%, respectively. These results along with our previous findings suggested that the increased toxicity of chlorpyrifos was likely due to an enhanced oxidative process from chlorpyrifos to chlorpyrifos-oxon by CtCYP6EX3 as RNAi of CtCYP6EX3 led to decreased susceptibility of C. tentans larvae to chlorpyrifos alone and the binary mixture of atrazine and chlorpyrifos. However, further study would be necessary to validate our results by functional assays using heterologously expressed CtCYP6EX3 enzyme.

Accumulation of dsRNA in endosomes contributes to inefficient RNA interference in the fall armyworm, Spodoptera frugiperda

RNA interference (RNAi) efficiency varies among insects studied. The barriers for successful RNAi include the presence of double-stranded ribonucleases (dsRNase) in the lumen and hemolymph that could potentially digest double-stranded RNA (dsRNA) and the variability in the transport of dsRNA into and within the cells. We recently showed that the dsRNAs are transported into lepidopteran cells, but they are not processed into small interference RNAs (siRNAs) because they are trapped in acidic bodies. In the current study, we focused on the identification of acidic bodies in which dsRNAs accumulate in Sf9 cells. Time-lapse imaging studies showed that dsRNAs enter Sf9 cells and accumulate in acidic bodies within 20 min after their addition to the medium. CypHer-5E-labeled dsRNA also accumulated in the midgut and fat body dissected from Spodoptera frugiperda larvae with similar patterns observed in Sf9 cells. Pharmacological inhibitor assays showed that the dsRNAs use clathrin mediated endocytosis pathway for transport into the cells. We investigated the potential dsRNA accumulation sites employing LysoTracker and double labeling experiments using the constructs to express a fusion of green fluorescence protein with early or late endosomal marker proteins and CypHer-5E-labeled dsRNA. Interestingly, CypHer-5E-labeled dsRNA accumulated predominantly in early and late endosomes. These data suggest that entrapment of internalized dsRNA in endosomes is one of the major factors contributing to inefficient RNAi response in lepidopteran insects.

Implementing the sterile insect technique with RNA interference - a review

We review RNA interference (RNAi) of insect pests and its potential for implementing sterile insect technique (SIT)-related control. The molecular mechanisms that support RNAi in pest species are reviewed in detail, drawing on literature from a range of species including Drosophila melanogaster Meigen and Homo sapiens L. The underlying genes that enable RNAi are generally conserved across taxa, although variance exists in both their form and function. RNAi represents a plausible, non-GM system for targeting populations of insects for control purposes, if RNAi effector molecules can be delivered environmentally (eRNAi). We consider studies of eRNAi from across several insect orders and review to what extent taxonomy, genetics, and differing methods of double-stranded (ds) RNA synthesis and delivery can influence the efficiency of gene knockdown. Several factors, including the secondary structure of the target mRNA and the specific nucleotide sequence of dsRNA effector molecules, can affect the potency of eRNAi. However, taxonomic relationships between insects cannot be used to reliably forecast the efficiency of an eRNAi response. The mechanisms by which insects acquire dsRNA from their environment require further research, but the evidence to date suggests that endocytosis and transport channels both play key roles. Delivery of RNA molecules packaged in intermediary carriers such as bacteria or nanoparticles may facilitate their entry into and through the gut, and enable the evasion of host defence systems, such as toxic pH, that would otherwise attenuate the potential for RNAi.