RNAi for Insect Control: Current Perspective and Future Challenges

Hypericum japonicum: a Double-Headed Sword to Combat Vector Control and Cancer

Mosquito control with naturally derived herbal insecticides has gained much momentum, with the increased insecticide resistance of vectors and the multiple infectious diseases spread by them. Yet, recent studies also suggest that mosquitoes could probably transmit some cancerous cells or cancer-causing viruses from one individual to another between their blood meals. The current research thus focused on the screening and characterization of novel plants with both mosquitocidal and anticancerous properties. Accordingly, different solvent extracts of Hypericum japonicum, a key plant in Chinese medicine, were screened for its larvicidal efficacy using the fourth instar larvae of Aedes aegypti (major vector of Dengue and chikungunya). Methanolic extracts of the plant showed effective larvicidal property with LC₅₀ 7.37 ppm and LC₉₀11.59 ppm values. The anticancerous property of the plant extract was also evaluated by in vitro cytotoxicity assay against Daltons Lymphoma Ascites (DLA) cells. The results indicated that H. japonicum plant extracts at very low concentrations of LC₅₀0.95 ppm and LC₉₀1.85 ppm were potent cytotoxic agents. To the best of our knowledge, this is the first and the foremost report of Hypericum japonicum as a potent mosquitocidal and anticancerous agent. Identification and characterization of such plant-derived bioactive plants thus could serve as a double-headed sword against the spread of infectious diseases and cancer.

RNase If -treated quantitative PCR for dsRNA quantitation of RNAi trait in genetically modified crops

Background

RNA interference (RNAi) technology has been widely used to knockdown target genes via post-transcriptional silencing. In plants, RNAi is used as an effective tool with diverse applications being developed such as resistance against insects, fungi, viruses, and metabolism manipulation. To develop genetically modified (GM) RNAi traits for insect control, a transgene is created and composed of an inversely-repeated sequence of the target gene with a spacer region inserted between the repeats. The transgene design is subject to form a self-complementary hairpin RNA (hpRNA) and the active molecules are > 60 bp doubled-stranded RNA (dsRNA) derived from the hpRNA. However, in some cases, an undesirable intermediate such as single-stranded RNA (ssRNA) may be formed, which is not an active molecule. The aforementioned characteristics of RNAi traits lead to increase the challenges for RNAi-derived dsRNA quantitation.

Results

To quantify the dsRNA and distinguish it from the ssRNA in transgenic maize, an analytical tool is required to be able to effectively quantify dsRNA which contains a strong secondary structure. Herein, we develop a modified qRT-PCR method (abbreviated as RNase I_f -qPCR) coupled with a ssRNA preferred endonuclease (i.e., RNase I_f). This method enables the precise measurement of the active molecules (i.e., dsRNA) derived from RNAi traits of GM crops and separately quantifies the dsRNA from ssRNA. Notably, we also demonstrate that the RNase I_f -qPCR is comparable to a hybridization-based method (Quantigene Plex 2.0).

Conclusions

To our best knowledge, this is the first report of a method combining RNase I_f with modified qRT-PCR protocol. The method represents a reliable analytical tool to quantify dsRNA for GM RNAi crops. It provides a cost-effective and feasible analytical tool for general molecular laboratory without using additional equipment for other methods. The RNase I_f -qPCR method demonstrates high sensitivity (to 0.001 pg/ μL of dsRNA), precision and accuracy. In this report, we demonstrated the deployment of this method to characterize the RNAi events carrying v-ATPase C in maize during trait development process. The method can be utilized in any application which requires the dsRNA quantification such as double-stranded RNA virus or sprayable dsRNA as herbicide.

Electronic supplementary material

The online version of this article (10.1186/s12896-018-0413-6) contains supplementary material, which is available to authorized users.

Nucleases as a barrier to gene silencing in the cotton boll weevil, Anthonomus grandis

RNA interference (RNAi) approaches have been applied as a biotechnological tool for controlling plant insect pests via selective gene down regulation. However, the inefficiency of RNAi mechanism in insects is associated with several barriers, including dsRNA delivery and uptake by the cell, dsRNA interaction with the cellular membrane receptor and dsRNA exposure to insect gut nucleases during feeding. The cotton boll weevil (Anthonomus grandis) is a coleopteran in which RNAi-mediated gene silencing does not function efficiently through dsRNA feeding, and the factors involved in the mechanism remain unknown. Herein, we identified three nucleases in the cotton boll weevil transcriptome denoted AgraNuc1, AgraNuc2, and AgraNuc3, and the influences of these nucleases on the gene silencing of A. grandis chitin synthase II (AgraChSII) were evaluated through oral dsRNA feeding trials. A phylogenetic analysis showed that all three nucleases share high similarity with the DNA/RNA non-specific endonuclease family of other insects. These nucleases were found to be mainly expressed in the posterior midgut region of the insect. Two days after nuclease RNAi-mediated gene silencing, dsRNA degradation by the gut juice was substantially reduced. Notably, after nucleases gene silencing, the orally delivered dsRNA against the AgraChSII gene resulted in improved gene silencing efficiency when compared to the control (non-silenced nucleases). The data presented here demonstrates that A. grandis midgut nucleases are effectively one of the main barriers to dsRNA delivery and emphasize the need to develop novel RNAi delivery strategies focusing on protecting the dsRNA from gut nucleases and enhancing its oral delivery and uptake to crop insect pests.

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.

Control of Western Corn Rootworm (Diabrotica virgifera virgifera) Reproduction through Plant-Mediated RNA Interference

RNA interference (RNAi) in transgenic maize has recently emerged as an alternative mode of action for western corn rootworm (Diabrotica virgifera virgifera) control which can be combined with protein-based rootworm control options for improved root protection and resistance management. Currently, transgenic RNAi-based control has focused on suppression of genes that when silenced lead to larval mortality. We investigated control of western corn rootworm reproduction through RNAi by targeting two reproductive genes, dvvgr and dvbol, with the goal of reducing insect fecundity as a new tool for pest management. The results demonstrated that exposure of adult beetles, as well as larvae to dvvgr or dvbol dsRNA in artificial diet, caused reduction of fecundity. Furthermore, western corn rootworm beetles that emerged from larval feeding on transgenic maize roots expressing dvbol dsRNA also showed significant fecundity reduction. This is the first report of reduction of insect reproductive fitness through plant-mediated RNAi, demonstrating the feasibility of reproductive RNAi as a management tool for western corn rootworm.

RNAi-mediated gene silencing in Rhynchophorus ferrugineus (Oliver) (Coleoptera: Curculionidae)

RNA interference (RNAi) is a powerful strategy for gene function analysis, and it is also widely studied in view of a promising use in pest control. The red palm weevil, Rhynchophorus ferrugineus (Oliver) (Coleoptera: Curculionidae), is one of the most devastating pests of palm in the world. Conventional pest management practices are not adequate to control this insect, thus the development of efficient approaches with minimal environmental impact are needed. In this work, the potential of RNAi in R. ferrugineus has been investigated through the silencing of three different genes (α-amylase, V-ATPase, Ecdysone receptor). For each gene we tested two different doses (1,500 and 5,500 ng) and two delivery techniques (injection and ingestion), evaluating both gene knockdown and mortality on insects. Results show that RNAi mediated gene silencing in R. ferrugineus varies from gene to gene, and that the response is dose-dependent, with stronger effects when dsRNA was administered by injection. In parallel, the same study was carried out with the model organism Tribolium castaneum Herbst (Coleoptera: Tenebrionidae), with results showing a different pattern of response, although the two insects belong to the same order.

Molecular characterisation of two α-esterase genes involving chlorpyrifos detoxification in the diamondback moth, Plutella xylostella

BACKGROUND

Carboxylesterases (CarEs) are involved in metabolic detoxification of dietary and environmental xenobiotics in insects. However, owing to the complexity of the protein family, the involvement of CarEs in insecticide metabolism in Plutella xylostella has not been fully elucidated. This study aimed to characterise two CarE genes and assess their potential roles in response to chlorpyrifos in P. xylostella.

RESULTS

Synergistic tests showed that triphenyl phosphate decreased the resistance of the third-instar larvae to chlorpyrifos. The treatment of the third-instar larvae with chlorpyrifos at the LC₃₀ dose led to a significant increase in CarE activity. Two CarE cDNAs (Pxae18 and Pxae28) were subsequently sequenced and characterised. Both genes were expressed predominantly in the larval midgut. Most importantly, two CarE genes showed significantly higher expression in the chlorpyrifos-resistant strain than in the susceptible strain. RNAi knockdown of Pxae18 and Pxae28 significantly increased the mortality to chlorpyrifos from 40% in the control to 73.8 and 63.3% respectively.

CONCLUSION

RNAi knockdown of Pxae18 and Pxae28 significantly inhibited detoxification ability and increased the mortality in P. xylostella. The results indicate that these two CarE genes play important roles in the detoxification of chlorpyrifos in P. xylostella. © 2016 Society of Chemical Industry.

Influence of various stressors on the expression of core genes of the small interfering RNA pathway in the oriental fruit fly, Bactrocera dorsalis

RNA interference (RNAi)-based technology has emerged as a potential tool for controlling insect pests, however, previous studies found that the efficiency of RNAi in Bactrocera dorsalis was variable. In nature, insects often meet various challenges, such as pathogen infections, extreme temperatures, lack of nutrition and heavy metals. To better understand the association of the stressors with efficiency of RNAi, in the current study we tested the expression of three core genes, dicer2 (Bddcr2), r2d2 (Bdr2d2) and argonaute2 (Bdago2), of the small interfering RNA (siRNA) pathway of B. dorsalis upon various stressors. Our results showed that all three genes were upregulated by the infection of invertebrate iridescent virus 6, which suggested a function of the siRNA pathway against viral infection. The loading of FeCl₃ could also increase the expression of Bddcr2. The treatments of Escherichia coli, extremely high (40°C) and low (0°C) temperatures, as well as starvation, could negatively influence the expression of Bddcr2 and/or Bdago2. In total, our results showed that various stressors could influence the expression of core components of B. dorsalis siRNA pathway. This highlights further speculation on the RNAi efficiency upon these stressors. Considering the complexity and variation of RNAi efficiency in different conditions, these results provide initial aspects in possible environmental stressors to influence the activity of the siRNA pathway, but the real impact of RNAi efficiency posed by these stressors requires further studies.

RNA interference of Dicer-1 and Argonaute-1 increasing the sensitivity of Aphis gossypii Glover (Hemiptera: Aphididae) to plant allelochemical

Plant allelochemicals are a group of important defensive agents of plants, which have been documented to be deleterious to insect herbivores. In the present study, we found that the expression level of Dicer-1 was significantly increased when Aphis gossypii adults fed on plant allelochemicals (tannic acid and gossypol) incorporated artificial diets. Consider that miRNAs play great regulatory roles in various biological processes, this suggested that miRNAs may be related to the regulation of enzymes involved in metabolism of plant allelochemicals in A. gossypii. To further reveal the roles of miRNAs in the response of A. gossypii against plant allelochemicals, both Dicer-1 and Argonaute-1, an important component of the RNA-induced silencing complex (RISC) in miRNA pathway, were silenced using systemic RNA interference (RNAi). The results indicated that silence of Dicer-1 reduced the expression of miRNAs, and resulted in a high mortality of A. gossypii when fed on both tannic acid and gossypol. The silencing of Argonaute-1 resulted in the mortality of A. gossypii by the treatment of tannic acid significantly increased compared with control, however, the sensitivity of A. gossypii to gossypol was not significantly changed. It suggested that miRNAs play potential regulatory roles in the response of A. gossypii to plant allelochemicals. These results should be useful to understand the molecular mechanisms of the cotton aphid adaption to plant allelochemicals.

Investigating Engineered Ribonucleoprotein Particles to Improve Oral RNAi Delivery in Crop Insect Pests

Genetically modified (GM) crops producing double-stranded RNAs (dsRNAs) are being investigated largely as an RNA interference (RNAi)-based resistance strategy against crop insect pests. However, limitations of this strategy include the sensitivity of dsRNA to insect gut nucleases and its poor insect cell membrane penetration. Working with the insect pest cotton boll weevil (Anthonomus grandis), we showed that the chimeric protein PTD-DRBD (peptide transduction domain—dsRNA binding domain) combined with dsRNA forms a ribonucleoprotein particle (RNP) that improves the effectiveness of the RNAi mechanism in the insect. The RNP slows down nuclease activity, probably by masking the dsRNA. Furthermore, PTD-mediated internalization in insect gut cells is achieved within minutes after plasma membrane contact, limiting the exposure time of the RNPs to gut nucleases. Therefore, the RNP provides an approximately 2-fold increase in the efficiency of insect gene silencing upon oral delivery when compared to naked dsRNA. Taken together, these data demonstrate the role of engineered RNPs in improving dsRNA stability and cellular entry, representing a path toward the design of enhanced RNAi strategies in GM plants against crop insect pests.

The immunomodulatory protein RH36 is relating to blood-feeding success and oviposition in hard ticks

An immunomodulatory protein designated RH36 was identified in the tick Rhipicephalus haemaphysaloides. The cDNA sequence of RH36 has 844bp and encodes a deduced protein with a predicted molecular weight of 24kDa. Bioinformatics analysis indicated that RH36 presented a degree of similarity of 34.36% with the immunomodulatory protein p36 from the tick Dermacentor andersoni. The recombinant RH36 (rRH36) expressed in Sf9 insect cells suppressed the T-lymphocyte mitogen-driven in vitro proliferation of splenocytes and the expression of several cytokines such as IL-2, IL-12, and TNF-α. Furthermore, the proliferation of splenocytes isolated from rRH36-inoculated mice was significantly lower than that in control mice, suggesting that rRH36 could directly suppress immune responses in vivo. In addition, microarray analysis of splenocytes indicated that the expression of several immunomodulatory genes was downregulated by rRH36. The silencing of the RH36 gene by RNAi led to a 37.5% decrease in the tick attachment rate 24h after placement into the rabbit ears, whereas vaccination with RH36 caused a 53.06% decrease in the tick engorgement rate. Unexpectedly, RNAi induced a significant decrease in the oviposition rate, ovary weight at day 12 after engorgement, and egg-hatching rate. The effects of RH36 on blood feeding and oviposition were further confirmed by vaccination tests using the recombinant protein. These results indicate that RH36 is a novel member of immunosuppressant proteins and affects tick blood feeding and oviposition.

RNA interference of three up-regulated transcripts associated with insecticide resistance in an imidacloprid resistant population of Leptinotarsa decemlineata

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is a major agricultural pest of potatoes in the Central Sands production region of Wisconsin. Previous studies have shown that populations of L. decemlineata have become resistant to many classes of insecticides, including the neonicotinoid insecticide, imidacloprid. Furthermore, L. decemlineata has multiple mechanisms of resistance to deal with a pesticide insult, including enhanced metabolic detoxification by cytochrome p450s and glutathione S-transferases. With recent advances in the transcriptomic analysis of imidacloprid susceptible and resistant L. decemlineata populations, it is possible to investigate the role of candidate genes involved in imidacloprid resistance. A recently annotated transcriptome analysis of L. decemlineata was obtained from select populations of L. decemlineata collected in the Central Sands potato production region, which revealed a subset of mRNA transcripts constitutively up-regulated in resistant populations. We hypothesize that a portion of the up-regulated transcripts encoding for genes within the resistant populations also encode for pesticide resistance and can be suppressed to re-establish a susceptible phenotype. In this study, a discrete set of three up-regulated targets were selected for RNA interference experiments using a resistant L. decemlineata population. Following the successful suppression of transcripts encoding for a cytochrome p450, a cuticular protein, and a glutathione synthetase protein in a select L. decemlineata population, we observed reductions in measured resistance to imidacloprid that strongly suggest these genes control essential steps in imidacloprid metabolism in these field populations.

RNA interference as a gene silencing tool to control Tuta absoluta in tomato (Solanum lycopersicum)

RNA interference (RNAi), a gene-silencing mechanism that involves providing double-stranded RNA molecules that match a specific target gene sequence, is now widely used in functional genetic studies. The potential application of RNAi-mediated control of agricultural insect pests has rapidly become evident. The production of transgenic plants expressing dsRNA molecules that target essential insect genes could provide a means of specific gene silencing in larvae that feed on these plants, resulting in larval phenotypes that range from loss of appetite to death. In this report, we show that the tomato leafminer ( Tuta absoluta ), a major threat to commercial tomato production, can be targeted by RNAi. We selected two target genes (Vacuolar ATPase-A and Arginine kinase) based on the RNAi response reported for these genes in other pest species. In view of the lack of an artificial diet for T. absoluta, we used two approaches to deliver dsRNA into tomato leaflets. The first approach was based on the uptake of dsRNA by leaflets and the second was based on "in planta-induced transient gene silencing" (PITGS), a well-established method for silencing plant genes, used here for the first time to deliver in planta-transcribed dsRNA to target insect genes. Tuta absoluta larvae that fed on leaves containing dsRNA of the target genes showed an ∼60% reduction in target gene transcript accumulation, an increase in larval mortality and less leaf damage. We then generated transgenic 'Micro-Tom' tomato plants that expressed hairpin sequences for both genes and observed a reduction in foliar damage by T. absoluta in these plants. Our results demonstrate the feasibility of RNAi as an alternative method for controlling this critical tomato pest.

RNAi-mediated plant protection against aphids

Aphids (Aphididae) are major agricultural pests that cause significant yield losses of crop plants each year by inflicting damage both through the direct effects of feeding and by vectoring harmful plant viruses. Expression of double-stranded RNA (dsRNA) directed against suitable insect target genes in transgenic plants has been shown to give protection against pests through plant-mediated RNA interference (RNAi). Thus, as a potential alternative and effective strategy for insect pest management in agricultural practice, plant-mediated RNAi for aphid control has received close attention in recent years. In this review, the mechanism of RNAi in insects and the so far explored effective RNAi target genes in aphids, their potential applications in the development of transgenic plants for aphid control and the major challenges in this regard are reviewed, and the future prospects of using plant-mediated RNAi for aphid control are discussed. This review is intended to be a helpful insight into the generation of aphid-resistant plants through plant-mediated RNAi strategy. © 2016 Society of Chemical Industry.

CRISPR/Cas9-mediated targeted gene mutagenesis in Spodoptera litura

Custom-designed nuclease technologies such as the clustered regularly interspaced short palindromic repeat (CRISPR)-associated (Cas) system provide attractive genome editing tools for insect functional genetics. The targeted gene mutagenesis mediated by the CRISPR/Cas9 system has been achieved in several insect orders including Diptera, Lepidoptera and Coleoptera. However, little success has been reported in agricultural pests due to the lack of genomic information and embryonic microinjection techniques in these insect species. Here we report that the CRISPR/Cas9 system induced efficient gene mutagenesis in an important Lepidopteran pest Spodoptera litura. We targeted the S. litura Abdominal-A (Slabd-A) gene which is an important embryonic development gene and plays a significant role in determining the identities of the abdominal segments of insects. Direct injection of Cas9 messenger RNA and Slabd-A-specific single guide RNA (sgRNA) into S. litura embryos successfully induced the typical abd-A deficient phenotype, which shows anomalous segmentation and ectopic pigmentation during the larval stage. A polymerase chain reaction-based analysis revealed that the Cas9/sgRNA complex effectively induced a targeted mutagenesis in S. litura. These results demonstrate that the CRISPR/Cas9 system is a powerful tool for genome manipulation in Lepidopteran pests such as S. litura.

Reduced stability and intracellular transport of dsRNA contribute to poor RNAi response in lepidopteran insects

RNA interference (RNAi) has become a widely used reverse genetic tool to study gene function in eukaryotic organisms and is being developed as a technology for insect pest management. The efficiency of RNAi varies among organisms. Insects from different orders also display differential efficiency of RNAi, ranging from highly efficient (coleopterans) to very low efficient (lepidopterans). We investigated the reasons for varying RNAi efficiency between lepidopteran and coleopteran cell lines and also between the Colorado potato beetle, Leptinotarsa decemlineata and tobacco budworm, Heliothis virescens. The dsRNA either injected or fed was degraded faster in H. virescens than in L. decemlineata. Both lepidopteran and coleopteran cell lines and tissues efficiently took up the dsRNA. Interestingly, the dsRNA administered to coleopteran cell lines and tissues was taken up and processed to siRNA whereas the dsRNA was taken up by lepidopteran cell lines and tissues but no siRNA was detected in the total RNA isolated from these cell lines and tissues. The data included in this paper showed that the degradation and intracellular transport of dsRNA are the major factors responsible for reduced RNAi efficiency in lepidopteran insects.

The next generation of insecticides: dsRNA is stable as a foliar-applied insecticide

BACKGROUND

RNAi is a powerful tool used to study gene function. It also has been hypothesized to be a promising new method for control of insect pests on crops, although the perceived instability of dsRNA in the environment has constrained thinking about the options for this new type of pest control.

RESULTS

We confirmed that foliar application of Colorado potato beetle dsRNA actin is highly effective for control, demonstrated that treatment with actin-dsRNA protects potato plants for at least 28 days under greenhouse conditions and found that the dsRNA is not readily removed by water once dried on the leaves.

CONCLUSION

These new results suggest that foliar application of dsRNA could be a valuable control strategy for some pests. Technological aspects of spraying dsRNA that need to be considered in the future are discussed.

No impact of DvSnf7 RNA on honey bee (Apis mellifera L.) adults and larvae in dietary feeding tests

The honey bee (Apis mellifera L.) is the most important managed pollinator species worldwide and plays a critical role in the pollination of a diverse range of economically important crops. This species is important to agriculture and historically has been used as a surrogate species for pollinators to evaluate the potential adverse effects for conventional, biological, and microbial pesticides, as well as for genetically engineered plants that produce pesticidal products. As part of the ecological risk assessment of MON 87411 maize, which expresses a double-stranded RNA targeting the Snf7 ortholog (DvSnf7) in western corn rootworm (Diabrotica virgifera virgifera), dietary feeding studies with honey bee larvae and adults were conducted. Based on the mode of action of the DvSnf7 RNA in western corn rootworm, the present studies were designed to be of sufficient duration to evaluate the potential for adverse effects on larval survival and development through emergence and adult survival to a significant portion of the adult stage. Testing was conducted at concentrations of DvSnf7 RNA that greatly exceeded environmentally relevant exposure levels based on expression levels in maize pollen. No adverse effects were observed in either larval or adult honey bees at these high exposure levels, providing a large margin of safety between environmental exposure levels and no-observed-adverse-effect levels.

Diet-delivered RNAi in Helicoverpa armigera--Progresses and challenges

Helicoverpa armigera (the cotton bollworm) is a significant agricultural pest endemic to Afro-Eurasia and Oceania. Gene suppression via RNA interference (RNAi) presents a potential avenue for management of the pest, which is highly resistant to traditional insecticide sprays. This article reviews current understanding on the fate of ingested double-stranded RNA in H. armigera. Existing in vivo studies on diet-delivered RNAi and their effects are summarized and followed by a discussion on the factors and hurdles affecting the efficacy of diet-delivered RNAi in H. armigera.

Validating the importance of two acetylcholinesterases in insecticide sensitivities by RNAi in Pardosa pseudoannulata, an important predatory enemy against several insect pests

The pond wolf spider (Pardosa pseudoannulata) is an important predatory enemy against several insect pests and showed relative different sensitivities to organophosphate and carbamate insecticides compared to insect pests. In our previous studies, two acetylcholinesterases were identified in P. pseudoannulata and played important roles in insecticide sensitivities. In order to understand the contributions of the two acetylcholinesterases to insecticide sensitivities, we firstly employed the RNAi technology in the spider. For a suitable microinjection RNAi method, the injection site, injection volume and interference time were optimized, which then demonstrated that the injection RNAi method was applicable in this spider. With the new RNAi method, it was revealed that both Pp-AChE1 and Pp-AChE2, encoded by genes Ppace1 and Ppace2, were the targets of organophosphate insecticides, but Pp-AChE1 would be more important. In contrast, the carbamate acted selectively on Pp-AChE1. The results showed that Pp-AChE1 was the major catalytic enzyme in P. pseudoannulata and the major target of organophosphate and carbamate insecticides. In a word, an RNAi method was established in the pond wolf spider, which further validated the importance of two acetylcholinesterases in insecticide sensitivities in this spider.

Chitosan, Carbon Quantum Dot, and Silica Nanoparticle Mediated dsRNA Delivery for Gene Silencing in Aedes aegypti: A Comparative Analysis

In spite of devastating impact of mosquito borne pathogens on humans, widespread resistance to chemical insecticides and environmental concerns from residual toxicity limit mosquito control strategies. We tested three nanoparticles, chitosan, carbon quantum dot (CQD), and silica complexed with dsRNA, to target two mosquito genes (SNF7 and SRC) for controlling Aedes aegypti larvae. Relative mRNA levels were quantified using qRT-PCR to evaluate knockdown efficiency in nanoparticle-dsRNA treated larvae. The knockdown efficiency of target genes correlated with dsRNA mediated larval mortality. Among the three nanoparticles tested, CQD was the most efficient carrier for dsRNA retention, delivery, and thereby causing gene silencing and mortality in Ae. aegypti.

The novel ABC transporter ABCH1 is a potential target for RNAi-based insect pest control and resistance management

Insect pests cause serious crop damage and develop high-level resistance to chemical insecticides and Bacillus thuringiensis (Bt) insecticidal Cry toxins. A new promising approach for controlling them and overcoming this resistance is RNA interference (RNAi). The RNAi-based insect control strategy depends on the selection of suitable target genes. In this study, we cloned and characterized a novel ABC transporter gene PxABCH1 in diamondback moth, Plutella xylostella (L.). Phylogenetic analysis showed that PxABCH1 is closely related to ABCA and ABCG subfamily members. Spatial-temporal expression detection revealed that PxABCH1 was expressed in all tissues and developmental stages, and highest expressed in head and male adult. Midgut sequence variation and expression analyses of PxABCH1 in all the susceptible and Bt-resistant P. xylostella strains and the functional analysis by sublethal RNAi demonstrated that Cry1Ac resistance was independent of this gene. Silencing of PxABCH1 by a relatively high dose of dsRNA dramatically reduced its expression and resulted in larval and pupal lethal phenotypes in both susceptible and Cry1Ac-resistant P. xylostella strains. To our knowledge, this study provides the first insight into ABCH1 in lepidopterans and reveals it as an excellent target for RNAi-based insect pest control and resistance management.

De novo transcriptome assembly and analysis to identify potential gene targets for RNAi-mediated control of the tomato leafminer (Tuta absoluta)

BACKGROUND

Providing double-stranded RNA (dsRNA) to insects has been proven to silence target genes, and this approach has emerged as a potential method to control agricultural pests by engineering plants to express insect dsRNAs. A critical step of this technology is the screening of effective target genes essential for insect development and/or survival. The tomato leafminer (Tuta absoluta Meyrick) is a major Solanum lycopersicum (tomato) pest that causes significant yield losses and has recently invaded Europe, from where it is spreading at an alarming rate. To explore RNA interference (RNAi) against T. absoluta, sequence information on potential target genes is necessary, but only a few sequences are available in public databases.

RESULTS

We sequenced six libraries from RNA samples from eggs, adults, and larvae at four stages, obtaining an overall total of around 245 million reads. The assembled T. absoluta transcriptome contained 93,477 contigs with an average size of 1,574 bp, 59.8 % of which presented positive Blast hits, with 19,995 (21.4 %) annotated by gene ontology. From the transcriptome, most of the core genes of the RNAi mechanism of Lepidoptera were identified indicating the potential suitability of T. absoluta for gene silencing. No contigs displayed significant similarity with a RNA-dependent RNA Polymerase. Genes from the juvenile hormone and ecdysteroid biosynthetic pathways were identified, representing potential target genes for systemic silencing. Comparisons of transcript profiles among stages revealed 1,577 genes differentially expressed at earlier larval stages, from which potential gene targets were identified. Five of these genes were evaluated using in vitro transcribed dsRNA absorbed by tomato leaflets, which were fed to 1(st) instar T. absoluta larvae, resulting in significant reduction of larval body weight while exhibiting significant knockdown for three of the genes.

CONCLUSIONS

The transcriptome we generated represents a valuable genomic resource for screening potential gene targets that affect the development or survival of T. absoluta larvae. Five novel genes that showed greater expression at the 1(st) larval stage were demonstrated to be effective potential RNAi targets by reducing larval weight and can be considered good candidates for use in RNAi-mediated crop protection.

Small RNAs--the secret agents in the plant-pathogen interactions

Eukaryotic regulatory small RNAs (sRNAs) that induce RNA interference (RNAi) are involved in a plethora of biological processes, including host immunity and pathogen virulence. In plants, diverse classes of sRNAs contribute to the regulation of host innate immunity. These immune-regulatory sRNAs operate through distinct RNAi pathways that trigger transcriptional or post-transcriptional gene silencing. Similarly, many pathogen-derived sRNAs also regulate pathogen virulence. Remarkably, the influence of regulatory sRNAs is not limited to the individual organism in which they are generated. It can sometimes extend to interacting species from even different kingdoms. There they trigger gene silencing in the interacting organism, a phenomenon called cross-kingdom RNAi. This is exhibited in advanced pathogens and parasites that produce sRNAs to suppress host immunity. Conversely, in host-induced gene silencing (HIGS), diverse plants are engineered to trigger RNAi against pathogens and pests to confer host resistance. Cross-kingdom RNAi opens up a vastly unexplored area of research on mobile sRNAs in the battlefield between hosts and pathogens.

Movement of regulatory RNA between animal cells

Recent studies suggest that RNA can move from one cell to another and regulate genes through specific base-pairing. Mechanisms that modify or select RNA for secretion from a cell are unclear. Secreted RNA can be stable enough to be detected in the extracellular environment and can enter the cytosol of distant cells to regulate genes. Mechanisms that import RNA into the cytosol of an animal cell can enable uptake of RNA from many sources including other organisms. This role of RNA is akin to that of steroid hormones, which cross cell membranes to regulate genes. The potential diagnostic use of RNA in human extracellular fluids has ignited interest in understanding mechanisms that enable the movement of RNA between animal cells. Genetic model systems will be essential to gain more confidence in proposed mechanisms of RNA transport and to connect an extracellular RNA with a specific biological function. Studies in the worm C. elegans and in other animals have begun to reveal parts of this novel mechanism of cell-to-cell communication. Here, I summarize the current state of this nascent field, highlight the many unknowns, and suggest future directions.

Molecular interactions and immune responses between Maize fine streak virus and the leafhopper vector Graminella nigrifrons through differential expression and RNA interference

Graminella nigrifrons is the only known vector for Maize fine streak virus (MFSV). In this study, we used real-time quantitative PCR to compare the expression profiles of transcripts that putatively function in the insect immune response: four peptidoglycan recognition proteins (PGRP-SB1, -SD, -LC and LB), Toll, spaetzle, defensin, Dicer-2 (Dcr-2), Argonaut-2 (Ago-2) and Arsenic resistance protein 2 (Ars-2). Except for PGRP-LB and defensin, transcripts involved in humoral pathways were significantly suppressed in G. nigrifrons fed on MFSV-infected maize. The abundance of three RNA interference (RNAi) pathway transcripts (Dcr-2, Ago-2, Ars-2) was significantly lower in nontransmitting relative to transmitting G. nigrifrons. Injection with double-stranded RNA (dsRNA) encoding segments of the PGRP-LC and Dcr-2 transcripts effectively reduced transcript levels by 90 and 75% over 14 and 22 days, respectively. MFSV acquisition and transmission were not significantly affected by injection of either dsRNA. Knock-down of PGRP-LC resulted in significant mortality (greater than 90%) at 27 days postinjection, and resulted in more abnormal moults relative to those injected with Dcr-2 or control dsRNA. The use of RNAi to silence G. nigrifrons transcripts will facilitate the study of gene function and pathogen transmission, and may provide approaches for developing novel targets of RNAi-based pest control.

Conversations between kingdoms: small RNAs

Humans, animals, and plants are constantly under attack from pathogens and pests, resulting in severe consequences on global human health and crop production. Small RNA (sRNA)-mediated RNA interference (RNAi) is a conserved regulatory mechanism that is involved in almost all eukaryotic cellular processes, including host immunity and pathogen virulence. Recent evidence supports the significant contribution of sRNAs and RNAi to the communication between hosts and some eukaryotic pathogens, pests, parasites, or symbiotic microorganisms. Mobile silencing signals—most likely sRNAs—are capable of translocating from the host to its interacting organism, and vice versa. In this review, we will provide an overview of sRNA communications between different kingdoms, with a primary focus on the advances in plant-pathogen interaction systems.

Transcriptome analysis and systemic RNAi response in the African sweetpotato weevil (Cylas puncticollis, Coleoptera, Brentidae)

The African sweetpotato weevil (SPW) Cylas puncticollis Boheman is one of the most important constraints of sweetpotato production in Sub-Saharan Africa and yet is largely an uncharacterized insect pest. Here, we report on the transcriptome analysis of SPW generated using an Illumina platform. More than 213 million sequencing reads were obtained and assembled into 89,599 contigs. This assembly was followed by a gene ontology annotation. Subsequently, a transcriptome search showed that the necessary RNAi components relevant to the three major RNAi pathways, were found to be expressed in SPW. To address the functionality of the RNAi mechanism in this species, dsRNA was injected into second instar larvae targeting laccase2, a gene which encodes an enzyme involved in the sclerotization of insect exoskeleton. The body of treated insects showed inhibition of sclerotization, leading eventually to death. Quantitative Real Time PCR (qPCR) confirmed this phenotype to be the result of gene silencing. Together, our results provide valuable sequence data on this important insect pest and demonstrate that a functional RNAi pathway with a strong and systemic effect is present in SPW and can further be explored as a new strategy for controlling this important pest.

Biology, ecology, and control of elaterid beetles in agricultural land

Wireworms, the larvae of click beetles (Coleoptera: Elateridae), have had a centuries-long role as major soil insect pests worldwide. With insecticidal control options dwindling, research on click beetle biology and ecology is of increasing importance in the development of new control tactics. Methodological improvements have deepened our understanding of how larvae and adults spatially and temporarily utilize agricultural habitats and interact with their environment. This progress, however, rests with a few pest species, and efforts to obtain comparable knowledge on other economically important elaterids are crucial. There are still considerable gaps in our understanding of female and larval ecology; movement of elaterids within landscapes; and the impact of natural enemies, cultivation practices, and environmental change on elaterid population dynamics. This knowledge will allow generation of multifaceted control strategies, including cultural, physical, and chemical measures, tailored toward species complexes and crops across a range of appropriate spatial scales.

Enhancement of larval RNAi efficiency by over-expressing Argonaute2 in Bombyx mori

RNA interference has been described as a powerful genetic tool for gene functional analysis and a promising approach for pest management. However, RNAi efficiency varies significantly among insect species due to distinct RNAi machineries. Lepidopteran insects include a large number of pests as well as model insects, such as the silkworm, Bombyx mori. However, only limited success of in vivo RNAi has been reported in lepidoptera, particularly during the larval stages when the worms feed the most and do the most harm to the host plant. Enhancing the efficiency of larval RNAi in lepidoptera is urgently needed to develop RNAi-based pest management strategies. In the present study, we investigate the function of the conserved RNAi core factor, Argonaute2 (Ago2), in mediating B. mori RNAi efficiency. We demonstrate that introducing BmAgo2 dsRNA inhibits the RNAi response in both BmN cells and embryos. Furthermore, we establish several transgenic silkworm lines to assess the roles of BmAgo2 in larval RNAi. Over-expressing BmAgo2 significantly facilitated both dsRNA-mediated larval RNAi when targeting DsRed using dsRNA injection and shRNA-mediated larval RNAi when targeting BmBlos2 using transgenic shRNA expression. Our results show that BmAgo2 is involved in RNAi in B. mori and provides a promising approach for improving larval RNAi efficiency in B. mori and in lepidopteran insects in general.

Core RNAi machinery and gene knockdown in the emerald ash borer (Agrilus planipennis)

The RNA interference (RNAi) technology has been widely used in insect functional genomics research and provides an alternative approach for insect pest management. To understand whether the emerald ash borer (Agrilus planipennis), an invasive and destructive coleopteran insect pest of ash tree (Fraxinus spp.), possesses a strong RNAi machinery that is capable of degrading target mRNA as a response to exogenous double-stranded RNA (dsRNA) induction, we identified three RNAi pathway core component genes, Dicer-2, Argonaute-2 and R2D2, from the A. planipennis genome sequence. Characterization of these core components revealed that they contain conserved domains essential for the proteins to function in the RNAi pathway. Phylogenetic analyses showed that they are closely related to homologs derived from other coleopteran species. We also delivered the dsRNA fragment of AplaScrB-2, a β-fructofuranosidase-encoding gene horizontally acquired by A. planipennis as we reported previously, into A. planipennis adults through microinjection. Quantitative real-time PCR analysis on the dsRNA-treated beetles demonstrated a significantly decreased gene expression level of AplaScrB-2 appearing on day 2 and lasting until at least day 6. This study is the first record of RNAi applied in A. planipennis.

Systemic RNA interference in locusts: reverse genetics and possibilities for locust pest control

RNA interference (RNAi) is a biological process triggered by double stranded (ds)RNA that results in sequence-dependent mRNA degradation. Because of its high specificity, this post-transcriptional gene silencing mechanism is a widely used tool for reverse genetics in several insect species. In particular, locusts possess a very robust and sensitive RNAi response that has already been exploited to investigate a diverse range of important physiological processes. These orthopteran insects constitute important model organisms in several areas of entomology, but they can also become voracious swarming pests that threaten the agricultural production in large parts of the world. In comparison to the widely applied chemical insecticides, the RNAi-technology could contribute to the development of a novel generation of insecticides, with high species-specificity. In this article, we discuss the potential of the RNAi-technology in loss of function studies in locusts, as well as to control locust populations.

Biological mechanisms determining the success of RNA interference in insects

Insects constitute the largest group of animals on this planet, having a huge impact on our environment, as well as on our quality of life. RNA interference (RNAi) is a posttranscriptional gene silencing mechanism triggered by double-stranded (ds)RNA fragments. This process not only forms the basis of a widely used reverse genetics research method in many different eukaryotes but also holds great promise to contribute to the species-specific control of agricultural pests and to combat viral infections in beneficial and disease vectoring insects. However, in many economically important insect species, such as flies, mosquitoes, and caterpillars, systemic delivery of naked dsRNA does not trigger effective gene silencing. Although many components of the RNAi pathway have initially been deciphered in the fruit fly, Drosophila melanogaster, it will be of major importance to investigate this process in a wider variety of species, including dsRNA-sensitive insects such as locusts and beetles, to elucidate the factors responsible for the remarkable variability in RNAi efficiency, as observed in different insects. In this chapter, we review the current knowledge on the RNAi pathway, as well as the most recent insights into the mechanisms that might determine successful RNAi in insects.