Use of recombinant tobacco mosaic virus to achieve RNA interference in plants against the citrus mealybug, Planococcus citri (Hemiptera: Pseudococcidae)

Fluorescent Nanoparticle-RNAi-Mediated Silencing of Sterol Carrier Protein-2 Gene Expression Suppresses the Growth, Development, and Reproduction of Helicoverpa armigera

Helicoverpa armigera is a polyphagous destructive lepidopteran pest with strong Bacillus thuringiensis (Bt) resistance. Cholesterol, a vital component for insect growth, can only be obtained from food, and its transfer and metabolism are regulated by sterol carrier protein-2 (SCP-2). This study examined whether H. armigera SCP-2 (HaSCP-2) gene expression, involved in cholesterol absorption, can be silenced by nanocarrier fluorescent nanoparticle-RNA interference (FNP-RNAi) by larval feeding and whether the silencing affected H. armigera development. Fluorescence microscopy showed that nanoparticle-siRNA was distributed in Ha cells and the larval midgut. FNP-HaSCP-2 siRNA suppressed HaSCP-2 expression by 52.5% in H.armigera Ha cells. FNP can effectively help deliver siRNA into cells, protect siRNA, and is not affected by serum. FNP-siRNA in vivo biological assays showed that HaSCP-2 transcript levels were inhibited by 70.19%, 68.16%, and 67.66% in 3rd, 4th, and 5th instar larvae, leading to a decrease in the cholesterol level in the larval and prepupal fatbodies. The pupation rate and adult emergence were reduced to 26.0% and 56.52%, respectively. This study demonstrated that FNP could deliver siRNA to cells and improve siRNA knockdown efficiency. HaSCP-2 knockdown by FNP-siRNA in vivo hindered H. armigera growth and development. FNP could enhance RNAi efficiency to achieve pest control by SCP-2-targeted FNP-RNAi.

The role of polyplexes in developing a green sustainable approach in agriculture

Rise in global population has increased the food demands and thus the competition among farmers to produce more and more. In the race to obtain higher productivity, farmers have resorted to injudicious farming practices that include the reckless use of nitrogenous fertilizers and intensive cropping on farmlands. Such practices have paved the path for large scale infestations of crops and plants by pests thus affecting the plant productivity and crop vigour. There are several traditional techniques to control pest infestations in plants such as the use of chemical or bio-pesticides, and integrated pest management practices which face several drawbacks. Delivery of gene/nucleic acid in plants through genetic engineering approaches is a more sustainable and effective method of protection against pests. The technology of RNA interference (RNAi) provides a sustainable solution to counter pest control problems faced by other traditional techniques. The RNAi technique involves delivery of dsDNA/dsRNA or other forms of nucleic acids into target organisms thereby bringing about gene silencing. However, RNAi is also limited to its use because of their susceptibility to degradation wherein the use of cationic polymers can provide a tangible solution. Cationic polymers form stable complexes with the nucleic acids known as "polyplexes", which may be attributed to their high positive charge densities thus protecting the exogenous nucleic acids from extracellular degradation. The current paper focuses on the utility of nucleic acids as a sustainable tool for pest control in crops and the use of cationic polymers for the efficient delivery of nucleic acids in pests thus protecting the plant from infestations.

Plastid Transformation of Micro-Tom Tomato with a Hemipteran Double-Stranded RNA Results in RNA Interference in Multiple Insect Species

Plant-mediated RNA interference (RNAi) holds great promise for insect pest control, as plants can be transformed to produce double-stranded RNA (dsRNA) to selectively down-regulate insect genes essential for survival. For optimum potency, dsRNA can be produced in plant plastids, enabling the accumulation of unprocessed dsRNAs. However, the relative effectiveness of this strategy in inducing an RNAi response in insects using different feeding mechanisms is understudied. To investigate this, we first tested an in vitro-synthesized 189 bp dsRNA matching a highly conserved region of the v-ATPaseA gene from cotton mealybug (Phenacoccus solenopsis) on three insect species from two different orders that use leaf-chewing, lacerate-and-flush, or sap-sucking mechanisms to feed, and showed that the dsRNA significantly down-regulated the target gene. We then developed transplastomic Micro-tom tomato plants to produce the dsRNA in plant plastids and showed that the dsRNA is produced in leaf, flower, green fruit, red fruit, and roots, with the highest dsRNA levels found in the leaf. The plastid-produced dsRNA induced a significant gene down-regulation in insects using leaf-chewing and lacerate-and-flush feeding mechanisms, while sap-sucking insects were unaffected. Our results suggest that plastid-produced dsRNA can be used to control leaf-chewing and lacerate-and-flush feeding insects, but may not be useful for sap-sucking insects.

V-ATPase subunit a is required for survival and midgut development of Locusta migratoria

The vacuolar-type H⁺ -ATPase (V-ATPase) is an ATP-dependent proton pump, which regulates various cellular processes. To date, most functional studies on V-ATPases of insects have focused on subunits of the V1 complex, and there is little information on the VO genes. In this study, two cDNA sequences of LmV-ATPase a were identified in Locusta migratoria. RT-qPCR analysis revealed that LmV-ATPase a1 and LmV-ATPase a2 are differentially expressed in various tissues and developmental stages. Injection of dsRNA for the common region of LmV-ATPase a1 and LmV-ATPase a2 into third-instar nymphs resulted in a significant suppression of LmV-ATPase a. The injected nymphs ceased feeding, lost body weight and finally died at a mortality of 98.6%. Furthermore, aberrations of midgut epithelial cells, the accumulation of electron-lucent vesicles in the cytoplasm, and a partially damaged brush border were observed in dsLmV-ATPase a-injected nymphs using transmission electron microscopy. Especially, the mRNA level of wingles, and notch genes were dramatically down-regulated in the dsLmV-ATPase a-injected nymphs. Taken together, our results suggest that LmV-ATPase a is required for survival and midgut development of L. migratoria. Hence, this gene could be a good target for RNAi-based control against locusts.

Next Generation dsRNA-Based Insect Control: Success So Far and Challenges

RNA interference (RNAi) is a method of gene silencing where dsRNA is digested into small interfering RNA (siRNA) in the presence of enzymes. These siRNAs then target homologous mRNA sequences aided by the RNA-induced silencing complex (RISC). The mechanism of dsRNA uptake has been well studied and established across many living organisms including insects. In insects, RNAi is a novel and potential tool to develop future pest management means targeting various classes of insects including dipterans, coleopterans, hemipterans, lepidopterans, hymenopterans and isopterans. However, the extent of RNAi in individual class varies due to underlying mechanisms. The present review focuses on three major insect classes viz hemipterans, lepidopterans and coleopterans and the rationale behind this lies in the fact that studies pertaining to RNAi has been extensively performed in these groups. Additionally, these classes harbour major agriculturally important pest species which require due attention. Interestingly, all the three classes exhibit varying levels of RNAi efficiencies with the coleopterans exhibiting maximum response, while hemipterans are relatively inefficient. Lepidopterans on the other hand, show minimum response to RNAi. This has been attributed to many facts and few important being endosomal escape, high activity dsRNA-specific nucleases, and highly alkaline gut environment which renders the dsRNA unstable. Various methods have been established to ensure safe delivery of dsRNA into the biological system of the insect. The most common method for dsRNA administration is supplementing the diet of insects via spraying onto leaves and other commonly eaten parts of the plant. This method is environment-friendly and superior to the hazardous effects of pesticides. Another method involves submergence of root systems in dsRNA solutions and subsequent uptake by the phloem. Additionally, more recent techniques are nanoparticle- and Agrobacterium-mediated delivery systems. However, due to the novelty of these biotechnological methods and recalcitrant nature of certain crops, further optimization is required. This review emphasizes on RNAi developments in agriculturally important insect species and the major hurdles for efficient RNAi in these groups. The review also discusses in detail the development of new techniques to enhance RNAi efficiency using liposomes and nanoparticles, transplastomics, microbial-mediated delivery and chemical methods.

Strategies for enhancing the efficiency of RNA interference in insects

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

The Interplay Between Viruses and RNAi Pathways in Insects

As an overarching immune mechanism, RNA interference (RNAi) displays pathogen specificity and memory via different pathways. The small interfering RNA (siRNA) pathway is the primary antiviral defense mechanism against RNA viruses of insects and plays a lesser role in defense against DNA viruses. Reflecting the pivotal role of the siRNA pathway in virus selection, different virus families have independently evolved unique strategies to counter this host response, including protein-mediated, decoy RNA-based, and microRNA-based strategies. In this review, we outline the interplay between insect viruses and the different pathways of the RNAi antiviral response; describe practical application of these interactions for improved expression systems and for pest and disease management; and highlight research avenues for advancement of the field.

Evaluation of RNA Interference for Control of the Grape Mealybug Pseudococcus maritimus (Hemiptera: Pseudococcidae)

Simple Summary

RNA interference (RNAi) is a defense mechanism that protects insects from viruses by targeting and degrading RNA. This feature has been exploited to reduce the expression of endogenous RNA for determining functions of various genes and for killing insect pests by targeting genes that are vital for insect survival. When dsRNA matching perfectly to the target RNA is administered, the RNAi machinery dices the dsRNA into ~21 bp fragments (known as siRNAs) and one strand of siRNA is employed by the RNAi machinery to target and degrade the target RNA. In this study we used a cocktail of dsRNAs targeting grape mealybug’s aquaporin and sucrase genes to kill the insect. Aquaporins and sucrases are important genes enabling these insects to maintain water relations indispensable for survival and digest complex sugars in the diet of plant sap-feeding insects, including mealybugs. In our experiments, administration of dsRNA caused a reduction in expression of the target genes and an increase in insect mortality. These results provide support for the application of RNAi to control the grape mealybug.

Abstract

The grape mealybug Pseudococcus maritimus (Ehrhorn, 1900) (Hemiptera: Pseudococcidae) is a significant pest of grapevines (Vitis spp.) and a vector of disease-causing grape viruses, linked to its feeding on phloem sap. The management of this pest is constrained by the lack of naturally occurring resistance traits in Vitis. Here, we obtained proof of concept that RNA interference (RNAi) using double-stranded RNA (dsRNA) molecules against essential genes for phloem sap feeding can depress insect survival. The genes of interest code for an aquaporin (AQP) and a sucrase (SUC) that are required for osmoregulation in related phloem sap-feeding hemipteran insects (aphids and whiteflies). In parallel, we investigated the grape mealybug genes coding non-specific nucleases (NUC), which reduce RNAi efficacy by degrading administered dsRNA. Homologs of AQP and SUC with experimentally validated function in aphids, together with NUC, were identified in the published transcriptome of the citrus mealybug Planococcus citri by phylogenetic analysis, and sequences of the candidate genes were obtained for Ps. maritimus by PCR with degenerate primers. Using this first sequence information for Ps. maritimus, dsRNA was prepared and administered to the insects via an artificial diet. The treatment comprising dsRNA against AQP, SUC and NUC significantly increased insect mortality over three days, relative to dsRNA-free controls. The dsRNA constructs for AQP and NUC were predicted, from sequence analysis to have some activity against other mealybugs, but none of the three dsRNA constructs have predicted activity against aphids. This study provides the basis to develop in planta RNAi strategies against Ps. maritimus and other mealybug pests of grapevines.

Double-stranded RNA targeting vATPase B reveals a potential target for pest management of Henosepilachna vigintioctopunctata

The development of genetic based techniques, specifically RNA interference (RNAi), has emerged as a powerful tool in novel pest management strategies for pestiferous coleoptera. The 28-spotted ladybird beetle, Henosepilachna vigintioctopunctata, is a dynamic foliar pest of solenaceous plants, primarily potato plants, and has quickly become one of the most important pests attacking many crops in Asian countries. In this study, we demonstrate the efficacy of dietary RNAi targeting vATPase B, which led to significant gene silencing. Downstream effects of vATPase B silencing appeared to be both time- and partial dose-dependent. Our results indicate that silencing of vATPase B caused a significant decrease in survival rate, as well as reduced the food stuffs consumption and inhibited the overall development of H. vigintioctopunctata. Furthermore, results demonstrate expression of insect melanism related genes, TH and DDC, was significantly up regulated under the dsvATPase B (RNAi molecule designed against vATPase B) treatment. The impact of oral dsvATPase B delivery on the survival of 1st, 3rd instars, and adults was investigated through bacterially expressed dsRNA. The effectiveness of RNAi-based gene silencing in H. vigintioctopunctata provides a powerful reverse genetic tool for the functional annotation of its genes. This study demonstrates that vATPase B may represent a candidate gene for RNAi-based control of H. vigintioctopunctata.

Omics applications: towards a sustainable protection of tomato

Transcriptome data and gene expression analysis have a huge potential in the study of multiple relationships involving plants, pathogens, and pests, including the interactions with beneficial microorganisms such as endophytes or other functional groups. Next-generation sequencing (NGS) and other recent long-read-based sequencing approaches (i.e., nanopore and others) provide unprecedented tools allowing the fast identification of plant information processing systems, in situ and in real time, fundamental for crop management and pest regulation. Other -omics approaches such as metagenomics and metatranscriptomics allow high-resolution insights on the rhizosphere ecology. They may highlight key factors affecting belowground biodiversity or processes, modulating the expression of stress-responsive pathways. The application of miRNAs and other small RNAs is a relatively new field of application, with enormous potential for the selective activation of defense pathways. However, limitations concerning the stability of the RNA molecules and their effective delivery must be overcome.

The Roles of DNA Methyltransferases 1 (DNMT1) in Regulating Sexual Dimorphism in the Cotton Mealybug, Phenacoccus solenopsis

The cotton mealybug, Phenacoccus solenopsis, is an invasive pest that can cause massive damage to many host plants of agricultural importance. P. solenopsis is highly polyphagous, and shows extreme sexual dimorphism between males and females. The functions of DNA methyltransferase (DNMT) enzymes in the cotton mealybug have not been well studied. Here, we carried out an investigation of DNMTs in cotton mealybug to study their roles in sexual dimorphism. We found that the cotton mealybug has two copies of PsDnmt1, but Dnmt3 is absent. We then amplified the full-length cDNAs of PsDnmt1A (2,225 bp) and PsDnmt1B (2,862 bp) using rapid amplification cDNA ends (RACE). Quantitative reverse transcriptase PCR shows that both PsDnmt1A and PsDnmt1B are highly expressed in adult males, while the expression of PsDnmt1B is 30-fold higher in gravid females than in virgin females. We knocked down PsDnmt1A and PsDnmt1B with small interfering RNAs (siRNAs), and both genes were successfully down-regulated after 24 h or 72 h in adult females and pupa (t-test, p < 0.05). Down-regulating the expression of these two DNMT genes led to offspring lethality and abnormal body color in adult females. Furthermore, the silencing of PsDnmt1B induced abnormal wing development in emerged adult males. Our results provide evidence that PsDnmt1 plays a crucial role in regulating sexual dimorphism in the cotton mealybug.

The Use of Engineered Plant Viruses in a Trans-Kingdom Silencing Strategy Against Their Insect Vectors

Plants, plant viruses, and their vectors are co-evolving actors that co-exist and interact in nature. Insects are the most important vectors of plant viruses, serving as both carriers and hosts for the virus. This trans-kingdom interaction can be harnessed for the production of recombinant plant viruses designed to target insect genes via the RNAi machinery. The selection of the adequate viruses is important since they must infect and preferentially replicate in both the host plant and the insect vector. The routes of transmission that determine the extent of the infection inside the insect vary among different plant viruses. In the context of the proposed strategy, plant viruses that are capable of transversing the insect gut-hemocoel barrier and replicating in insect tissues are attractive candidates. Thus, the transmission of such viruses in a persistent and propagative manner is considered as a prerequisite for this strategy to be feasible, a characteristic that is found in viruses from the families Bunyaviridae, Reoviridae, and Rhabdoviridae. In addition, several RNA viruses are known that replicate in both plant and insect tissues via a yet unclarified transmission route. In this review, advances in knowledge of trans-kingdom transmission of plant viruses and future perspectives for their engineering as silencing vectors are thoroughly discussed.

Systemic RNAi of V-ATPase subunit B causes molting defect and developmental abnormalities in Periplaneta fuliginosa

The vacuolar (H⁺ )-ATPases (V-ATPases) are ATP-driven proton pumps with multiple functions in many organisms. In this study, we performed structural and functional analysis of vha55 gene that encodes V-ATPase subunit B in the smokybrown cockroach Periplaneta fuliginosa (Blattodea). We observed a high homology score of the deduced amino acid sequences between 10 species in seven orders. RNAi of the vha55 gene in P. fuliginosa caused nymphal/nymphal molting defects with incomplete shedding of old cuticles, growth inhibition, as well as bent and wrinkled cuticles of thoraxes and abdominal segments. Since growth inhibition caused by vha55 RNAi did not interfere in the commencement of cockroach molting, molting timing and body growth might be controlled by independent mechanism. Our study suggested V-ATPases might be a good candidate molecule for evolutionary and developmental studies of insect molting.

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

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

Evaluation of potential RNA-interference-target genes to control cotton mealybug, Phenacoccus solenopsis (Hemiptera: Pseudococcuidae)

RNA interference (RNAi) of vital insect genes is a potential tool for targeted pest control. However, selection of the right target genes is a challenge because the RNAi efficacy is known to vary among insect species. Cotton mealybug, Phenacoccus solenopsis, is a phloem-feeding economically important crop pest. We evaluated the RNAi of 2 vital genes, Bursicon (PsBur) and V-ATPase (PsV-ATPase) as potential targets in P. solenopsis for its control. PCR fragments of PsBur and PsV-ATPase were amplified using cDNA synthesized from the total RNA. The PCR amplicons were cloned into Potato virus X (PVX) to develop recombinant PVX for the inoculation of Nicotiana tabacum plants for bioassays with healthy P. solenopsis. Reverse-transcription-polymerase chain reaction (RT-PCR) was used to validate the expression of transgenes in the recombinant-PVX-inoculated plants (treated), and suppression of the target genes in the mealybugs exposed to them. The RT-PCR confirmed the expression of transgenes in the treated plants. Mealybug individuals on treated plants either died or showed physical deformities. Further, the population of mealybug was significantly reduced by feeding on N. tabacum expressing RNAi triggers against PsBur and PsV-ATPase. The results conclude that RNAi is activated in P. solenopsis by feeding on N. tabacum expressing RNAi triggering elements of PsBur and PsV-ATPase genes through recombinant PVX vector. Further, V-ATPase and Bursicon genes are potential targets for RNAi-mediated control of P. solenopsis.

Integration of Cell-Penetrating Peptides with Rod-like Bionanoparticles: Virus-Inspired Gene-Silencing Technology

Inspired by the high gene transfer efficiency of viral vectors and to avoid side effects, we present here a 1D rod-like gene-silencing vector based on a plant virus. By decorating the transacting activator of transduction (TAT) peptide on the exterior surface, the TAT-modified tobacco mosaic virus (TMV) achieves a tunable isoelectric point (from ∼3.5 to ∼9.6) depending on the TAT dose. In addition to enhanced cell internalization, this plant virus-based vector (TMV-TAT) acquired endo/lysosomal escape capacity without inducing lysosomal damage, resulting in both high efficiency and low cytotoxicity. By loading silencer green fluorescent protein (GFP) siRNA onto the TMV-TAT vector (siRNA@TMV-TAT) and interfering with GFP-expressing mouse epidermal stem cells (ESCs/GFP) in vitro, the proportion of GFP-positive cells could be knocked down to levels even lower than 15% at a concentration of ∼100% cell viability. Moreover, by interfering with GFP-expressing highly metastatic hepatocellular carcinoma (MHCC97-H/GFP) tumors in vivo, treatment with siRNA@TMV-TAT complexes for 10 days achieved a GFP-negative rate as high as 80.8%. This work combines the high efficiency of viral vectors and the safety of nonviral vectors and may provide a promising strategy for gene-silencing technology.

RNA Interference Mechanisms and Applications in Plant Pathology

The origin of RNA interference (RNAi), the cell sentinel system widely shared among eukaryotes that recognizes RNAs and specifically degrades or prevents their translation in cells, is suggested to predate the last eukaryote common ancestor ( 138 ). Of particular relevance to plant pathology is that in plants, but also in some fungi, insects, and lower eukaryotes, RNAi is a primary and effective antiviral defense, and recent studies have revealed that small RNAs (sRNAs) involved in RNAi play important roles in other plant diseases, including those caused by cellular plant pathogens. Because of this, and because RNAi can be manipulated to interfere with the expression of endogenous genes in an intra- or interspecific manner, RNAi has been used as a tool in studies of gene function but also for plant protection. Here, we review the discovery of RNAi, canonical mechanisms, experimental and translational applications, and new RNA-based technologies of importance to plant pathology.

Efficient Protein Expression and Virus-Induced Gene Silencing in Plants Using a Crinivirus-Derived Vector

Plant virus-based vectors are valuable tools for recombinant gene expression and functional genomics for both basic and applied research. In this study, Lettuce infectious yellows virus (LIYV) of the genus Crinivirus was engineered into a virus vector that is applicable for efficient protein expression and virus-induced gene silencing (VIGS) in plants. We examined gene replacement and “add a gene” strategies to develop LIYV-derived vectors for transient expression of the green fluorescent protein (GFP) reporter in Nicotiana benthamiana plants. The latter yielded higher GFP expression and was further examined by testing the effects of heterologous controller elements (CEs). A series of five vector constructs with progressively extended LIYV CP sgRNA CEs were tested, the longest CE gave the highest GFP expression but lower virus accumulation. The whitefly transmissibility of the optimized vector construct to other host plants, and the capability to accommodate and express a larger gene, a 1.8 kb β-glucuronidase (GUS) gene, were confirmed. Furthermore, the LIYV vector was also validated VIGS by silencing the endogenous gene, phytoene desaturase (PDS) in N. benthamiana plants, and the transgene GFP in N. benthamiana line 16c plants. Therefore, LIYV-derived vectors could provide a technical reference for developing vectors of other economically important criniviruses.

Characterization and RNAi-mediated knockdown of Chitin Synthase A in the potato tuber moth, Phthorimaea operculella

Chitin is a major component of insect exoskeleton, tracheal system and gut where it is synthesized by chitin synthase (CHS) enzymes. In this paper, we report the isolation and RNAi of chitin synthase A (PhoCHSA) from the potato tuber moth Phthorimaea operculella. The full-length cDNA of PhoCHSA is 5,627 bp with 4,689 bp open reading frame coding for 1,563 amino acids. Structural analysis of conceptual amino acid translation showed three distinct regions found in all known insect CHS proteins; N-terminus region having 9 transmembrane helices, middle catalytic region containing several conserved domains identified in insect CHS enzymes, and C-terminus region containing seven transmembrane spans. Phylogenetic analysis showed that PhoCHSA protein clustered with CHSA enzymes identified from insects from different insect orders. RNAi targeting three different regions of the gene showed different efficacy against potato tuber moth larvae and dsRNA targeting the 5′ region has the highest efficacy. Results were verified by qRT-PCR which showed that dsRNA targeting the 5′ region caused the highest reduction in PhoCHSA mRNA level. Our results show the importance of selecting the RNAi target region and that chitin synthase A can be a suitable RNAi target for the potato tuber moth control.

Viral Delivery of dsRNA for Control of Insect Agricultural Pests and Vectors of Human Disease: Prospects and Challenges

RNAi is applied as a new and safe method for pest control in agriculture but efficiency and specificity of delivery of dsRNA trigger remains a critical issue. Various agents have been proposed to augment dsRNA delivery, such as engineered micro-organisms and synthetic nanoparticles, but the use of viruses has received relatively little attention. Here we present a critical view of the potential of the use of recombinant viruses for efficient and specific delivery of dsRNA. First of all, it requires the availability of plasmid-based reverse genetics systems for virus production, of which an overview is presented. For RNA viruses, their application seems to be straightforward since dsRNA is produced as an intermediate molecule during viral replication, but DNA viruses also have potential through the production of RNA hairpins after transcription. However, application of recombinant virus for dsRNA delivery may not be straightforward in many cases, since viruses can encode RNAi suppressors, and virus-induced silencing effects can be determined by the properties of the encoded RNAi suppressor. An alternative is virus-like particles that retain the efficiency and specificity determinants of natural virions but have encapsidated non-replicating RNA. Finally, the use of viruses raises important safety issues which need to be addressed before application can proceed.

Oral delivery of double-stranded RNAs induces mortality in nymphs and adults of the Asian citrus psyllid, Diaphorina citri

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is one of the most important citrus pests. ACP is the vector of the phloem-limited bacteria Candidatus Liberibacter americanus and Candidatus Liberibacter asiaticus, the causal agents of the devastating citrus disease huanglongbing (HLB). The management of HLB is based on the use of healthy young plants, eradication of infected plants and chemical control of the vector. RNA interference (RNAi) has proven to be a promising tool to control pests and explore gene functions. Recently, studies have reported that target mRNA knockdown in many insects can be induced through feeding with double-stranded RNA (dsRNA). In the current study, we targeted the cathepsin D, chitin synthase and inhibitor of apoptosis genes of adult and nymph ACP by feeding artificial diets mixed with dsRNAs and Murraya paniculata leaves placed in dsRNAs solutions, respectively. Adult ACP mortality was positively correlated with the amount of dsRNA used. Both nymphs and adult ACP fed dsRNAs exhibited significantly increased mortality over time compared with that of the controls. Moreover, qRT-PCR analysis confirmed the dsRNA-mediated RNAi effects on target mRNAs. These results showed that RNAi can be a powerful tool for gene function studies in ACP and perhaps for HLB control.

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.

Comparative Analysis of RNAi-Based Methods to Down-Regulate Expression of Two Genes Expressed at Different Levels in Myzus persicae

With the increasing availability of aphid genomic data, it is necessary to develop robust functional validation methods to evaluate the role of specific aphid genes. This work represents the first study in which five different techniques, all based on RNA interference and on oral acquisition of double-stranded RNA (dsRNA), were developed to silence two genes, ALY and Eph, potentially involved in polerovirus transmission by aphids. Efficient silencing of only Eph transcripts, which are less abundant than those of ALY, could be achieved by feeding aphids on transgenic Arabidopsis thaliana expressing an RNA hairpin targeting Eph, on Nicotiana benthamiana infected with a Tobacco rattle virus (TRV)-Eph recombinant virus, or on in vitro-synthesized Eph-targeting dsRNA. These experiments showed that the silencing efficiency may differ greatly between genes and that aphid gut cells seem to be preferentially affected by the silencing mechanism after oral acquisition of dsRNA. In addition, the use of plants infected with recombinant TRV proved to be a promising technique to silence aphid genes as it does not require plant transformation. This work highlights the need to pursue development of innovative strategies to reproducibly achieve reduction of expression of aphid genes.

RNAi Efficiency, Systemic Properties, and Novel Delivery Methods for Pest Insect Control: What We Know So Far

In recent years, the research on the potential of using RNA interference (RNAi) to suppress crop pests has made an outstanding growth. However, given the variability of RNAi efficiency that is observed in many insects, the development of novel approaches toward insect pest management using RNAi requires first to unravel factors behind the efficiency of dsRNA-mediated gene silencing. In this review, we explore essential implications and possibilities to increase RNAi efficiency by delivery of dsRNA through non-transformative methods. We discuss factors influencing the RNAi mechanism in insects and systemic properties of dsRNA. Finally, novel strategies to deliver dsRNA are discussed, including delivery by symbionts, plant viruses, trunk injections, root soaking, and transplastomic plants.

Inoculation of Nicotiana tabacum with recombinant potato virus X induces RNA interference in the solenopsis mealybug, Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae)

OBJECTIVES

The chitin synthase 1 (CHS1) gene in Phenacoccus solenopsis (PsCHS1) was evaluated as a potential target of RNA interference (RNAi) by using Potato virus X (PVX) as a vector (recombinant PVX) for expressing RNAi triggering elements in Nicotiana tabacum L.

RESULTS

RT-PCR analysis confirmed the expression of PsCHS1 in N. tabacum inoculated with recombinant-PVX-PsCHS1 (treated). RT- and multiplex-PCR further showed a reduction in mRNA levels of the target gene in mealybugs feeding on treated plants. Mortality in parent adults and emerging nymphs (21 and 29%) exposed to the treated plants was significantly higher (P < 0.05) than those exposed to uninoculated (-ve control) or inoculated with non-recombinant PVX (PVX-control). The number of surviving adults and the combined number of adults and nymphs (47 and 60%) was significantly (P < 0.05) lower on the treated plants than the -ve (76%) or PVX (74%) control. The visual observations verified the physical deformities in mealybugs exposed to the treated plants.

CONCLUSION

chitin synthase 1 is a potential RNAi target in P. solenopsis and the recombinant PVX can be used as a tool to evaluate candidate RNAi triggering elements in plants.

Disruption of insect transmission of plant viruses

Plant-infecting viruses are transmitted by a diverse array of organisms including insects, mites, nematodes, fungi, and plasmodiophorids. Virus interactions with these vectors are diverse, but there are some commonalities. Generally the infection cycle begins with the vector encountering the virus in the plant and the virus is acquired by the vector. The virus must then persist in or on the vector long enough for the virus to be transported to a new host and delivered into the plant cell. Plant viruses rely on their vectors for breaching the plant cell wall to be delivered directly into the cytosol. In most cases, viral capsid or membrane glycoproteins are the specific viral proteins that are required for transmission and determinants of vector specificity. Specific molecules in vectors also interact with the virus and while there are few-identified to no-identified receptors, candidate recognition molecules are being further explored in these systems. Due to the specificity of virus transmission by vectors, there are defined steps that represent good targets for interdiction strategies to disrupt the disease cycle. This review focuses on new technologies that aim to disrupt the virus-vector interaction and focuses on a few of the well-characterized virus-vector interactions in the field. In closing, we discuss the importance of integration of these technologies with current methods for plant virus disease control.

Emerging strategies for RNA interference (RNAi) applications in insects

RNA interference (RNAi) in insects is a gene regulatory process that also plays a vital role in the maintenance and in the regulation of host defenses against invading viruses. Small RNAs determine the specificity of the RNAi through precise recognition of their targets. These small RNAs in insects comprise small interfering RNAs (siRNAs), micro RNAs (miRNAs) and Piwi interacting RNAs (piRNAs) of various lengths. In this review, we have explored different forms of the RNAi inducers that are presently in use, and their applications for an effective and efficient fundamental and practical RNAi research with insects. Further, we reviewed trends in next generation sequencing (NGS) technologies and their importance for insect RNAi, including the identification of novel insect targets as well as insect viruses. Here we also describe a rapidly emerging trend of using plant viruses to deliver the RNAi inducer molecules into insects for an efficient RNAi response.

Cloning, characterization and effect of TmPGRP-LE gene silencing on survival of Tenebrio molitor against Listeria monocytogenes infection

Peptidoglycan recognition proteins (PGRPs) are a family of innate immune molecules that recognize bacterial peptidoglycan. PGRP-LE, a member of the PGRP family, selectively binds to diaminopimelic acid (DAP)-type peptidoglycan to activate both the immune deficiency (Imd) and proPhenoloxidase (proPO) pathways in insects. A PGRP-LE-dependent induction of autophagy to control Listeria monocytogenes has also been reported. We identified and partially characterized a novel PGRP-LE homologue, from Tenebrio molitor and analyzed its functional role in the survival of the insect against infection by a DAP-type PGN containing intracellular pathogen, L. monocytogenes. The cDNA is comprised of an open reading frame (ORF) of 990 bp and encodes a polypeptide of 329 residues. TmPGRP-LE contains one PGRP domain, but lacks critical residues for amidase activity. Quantitative RT-PCR analysis showed a broad constitutive expression of the transcript at various stages of development spanning from larva to adult. RNAi mediated knockdown of the transcripts, followed by a challenge with L. monocytogenes, showed a significant reduction in survival rate of the larvae, suggesting a putative role of TmPGRP-LE in sensing and control of L. monocytogenes infection in T. molitor. These results implicate PGRP-LE as a defense protein necessary for survival of T. molitor against infection by L. monocytogenes.