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

Exploring the role of CYP6AB328 in spinetoram resistance and growth and development of Phthorimaea absoluta

Phthorimaea absoluta is a major agricultural pest, affecting tomatoes and other solanaceous crops. Insect cytochrome P450 is a key enzyme that metabolizes xenobiotics (insecticides and plant toxins) and regulates endogenous compounds, but the functions of specific CYP genes in P. absoluta remain unclear. This study analyzed the expression pattern of 97 CYP genes in two regional populations of P. absoluta from Xinjiang, China. CYP6AB328 was identified as the most significantly overexpressed in the strain from Yining city (YN) compared to the strain from Alaer city (Ala), its expression level exhibited a positively correlated with the accumulating resistance of spinetoram. Following the cloning and sequence analysis of the target gene, it was named CYP6AB328. Additionally, a leaflet delivery system demonstrated the relatively stable presence of dsCYP6AB328 in the leaves from 12 to 24 h. The expression level of CYP6AB328 was significantly reduced by 68.9 % in 2nd instar larvae treated with 7.5 μg/200 μL dsCYP6AB328 at 48 h. Knockdown CYP6AB328 significantly increased susceptibility to spinetoram in the SPI-S strain (belongs to YN strain) and markedly decreased the spinetoram resistance ratio in the resistant strain (SPI-R: 250.57-fold). Notably, silencing CYP6AB328 inhibited nearly all 1st instar larvae fully mining the leaves, resulting in mortality up to 95.3 %, while in 2nd instar larvae, it prolonged leaf-mining time, reduced leaf damage, extended the development time of 2nd to 4th instar, caused 18 % larval abnormality and achieved an 84.4 % mortality on the 6th day of treatment. In summary, our findings indicate that CYP6AB328 plays an important role in promoting development of spinetoram resistance and growth and development of P. absoluta.

Spray-induced gene silencing for crop protection: recent advances and emerging trends

The RNA-based spray-induced gene silencing (SIGS) technology represents an ecologically sustainable approach to crop protection and pathogen management. Following the recent approval of Ledprona as the first sprayable double-stranded RNA (dsRNA) biopesticide by the EPA at the end of 2023, SIGS has emerged as a focal point in both academic and industrial sectors. This review analyzes recent advances and emerging trends in SIGS. The application of SIGS for crop protection, including the control of insects, fungal pathogens, and viruses, is briefly summarized. Distinguishing this review from others, we delve into practical aspects of the technology, such as the selection and screening of target genes, large-scale production methods, and delivery systems, highlighting major advancements in these areas and also addressing the remaining questions and issues, particularly concerning safety concerns and controlling harmful weeds. Finally, this review emphasizes the emerging trends in SIGS technology, particularly its integration with nanotechnology and other methodologies. Collectively, the rapid progress in SIGS studies is poised to accelerate the maturation and application of this technology.

Plant defense metabolites influence the interaction between an insect herbivore and an entomovirus

The tri-trophic interaction of plants, insect herbivores, and entomoviruses is an important topic in ecology and pest control. The susceptibility of insect herbivores to entomoviruses (e.g., nucleopolyhedroviruses) is influenced by host plants; however, the role of plant secondary metabolites in determining such susceptibility is poorly understood. Metabolomic analyses of Brassica oleracea, Glycine max, and Ipomoea aquatica plants, which differ in how they affect the susceptibility of Spodoptera exigua to nucleopolyhedroviruses among 14 plants, suggested that the plant secondary metabolites genistein, kaempferol, quercitrin, and coumarin play a role in influencing nucleopolyhedroviruses susceptibility. Subsequently, transcriptomic analysis of caterpillars, treated with nucleopolyhedroviruses alone or with one of these four phenolics, identified four genes (CYP340K4, CXE18, GSTe, and GSTe1) that were significantly downregulated by the phenolics. Functional characterization of these genes suggested that their downregulation significantly increased larval sensitivity to nucleopolyhedroviruses and altered aspects of the immune response. Our findings provide new insight into the role of plant defense metabolites in influencing the interactions between insect herbivores and entomopathogens and identify plant secondary metabolites as potential synergists of viral agents for the control of agricultural pests.

Ex vivo delivery of dsRNA targeting ryanodine receptors for control of Tuta absoluta

BACKGROUND

RNA interference (RNAi) is an endogenous eukaryote viral defence mechanism representing a unique form of post-transcriptional gene silencing. Owing to its high specificity, this technology is being developed for use in dsRNA-based biopesticides for control of pest insects. Whilst many lepidopteran species are recalcitrant to RNAi, Tuta absoluta, a polyphagous insect responsible for extensive crop damage, is sensitive. Ryanodine receptors (RyRs) are intracellular calcium channels regulating calcium ion (Ca2+) release. The chemical pesticide class of diamides functions agonistically against lepidopteran RyR, resulting in uncontrolled Ca2+ release, feeding cessation and death. Resistance to diamides has emerged in T. absoluta, derived from RyR point mutations.

RESULTS

RNAi was used to target RyR transcripts of T. absoluta. Data presented here demonstrate the systemic use of exogenous T. absoluta RyR-specific (TaRy) dsRNA in tomato plants (Solanum lycopersicum) to significantly downregulate expression of the target gene, resulting in significant insect mortality and reduced leaf damage. Using a leaflet delivery system, daily dosing of 3 μg TaRy dsRNA for 72 h resulted in 50% downregulation of the target gene and 50% reduction in tomato leaf damage. Corrected larval mortality and adult emergence were reduced by 38% and 33%, respectively. TaRy dsRNA demonstrated stability in tomato leaves ≤72 h after dosing.

CONCLUSIONS

This work identifies TaRy as a promising target for RNAi control of this widespread crop pest. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Optimizing the production of dsRNA biocontrols in microbial systems using multiple transcriptional terminators

Crop pests and pathogens annually cause over $220 billion in global crop damage, with insects consuming 5%-20% of major grain crops. Current crop pest and disease control strategies rely on insecticidal and fungicidal sprays, plant genetic resistance, transgenes, and agricultural practices. Double-stranded RNA (dsRNA) is emerging as a novel sustainable method of plant protection as an alternative to traditional chemical pesticides. Successful commercialization of dsRNA-based biocontrols requires the economical production of large quantities of dsRNA combined with suitable delivery methods to ensure RNAi efficacy against the target pest. In this study, we have optimized the design of plasmid DNA constructs to produce dsRNA biocontrols in Escherichia coli, by employing a wide range of alternative synthetic transcriptional terminators before measurement of dsRNA yield. We demonstrate that a 7.8-fold increase of dsRNA was achieved using triple synthetic transcriptional terminators within a dual T7 dsRNA production system compared to the absence of transcriptional terminators. Moreover, our data demonstrate that batch fermentation production dsRNA using multiple transcriptional terminators is scalable and generates significantly higher yields of dsRNA generated in the absence of transcriptional terminators at both small-scale batch culture and large-scale fermentation. In addition, we show that application of these dsRNA biocontrols expressed in E. coli cells results in increased insect mortality. Finally, novel mass spectrometry analysis was performed to determine the precise sites of transcriptional termination at the different transcriptional terminators providing important further mechanistic insight.

Chitin Deacetylase 1 Gene as an Optimal RNAi-Based Target for Controlling the Tomato Leaf Miner Tuta absoluta

Chitin is a critical component of both the exoskeleton and internal structures of insects, which can protect insects from mechanical damage, dehydration and pathogen infection, and plays a significant role in the molting process. Chitin deacetylases (CDAs), key enzymes involved in chitin metabolism, are widely distributed among arthropods and microorganisms. In this study, we identified a CDA gene, TaCDA1, in the invasive insect species Tuta absoluta (Meyrick). Sequence analysis demonstrated a high degree of similarity to CDAs in other insects, revealing the presence of three conserved domains. Quantitative analysis showed that the TaCDA1 gene exhibited peak expression during the pupal stage, particularly within the epidermis. The suppression of TaCDA1 expression through RNA interference in T. absoluta pupae significantly impacted the expression of genes associated with chitin metabolism, increasing mortality and developmental abnormalities during the pupa-adult transition and reducing the pupal weight. Furthermore, soaking gene-specific dsRNA resulted in elevated mortality rates during the larva-pupa transition, causing the inability to form new cuticles or undergo ecdysis, as confirmed by subsequent histological observations. The oral administration of dsTaCDA1 + sucrose solution did not significantly impact NtCDA1 expression or the mortality rate compared to the dsGFP + sucrose solution control in the non-target insect Nesidiocoris tenuis. This study demonstrated that TaCDA1 is a potential and safe target for pest control of T. absoluta.

RNA interference: a promising biotechnological approach to combat plant pathogens, mechanism and future prospects

Plant pathogens and other biological pests represent significant obstacles to crop Protection worldwide. Even though there are many effective conventional methods for controlling plant diseases, new methods that are also effective, environmentally safe, and cost-effective are required. While plant breeding has traditionally been used to manipulate the plant genome to develop resistant cultivars for controlling plant diseases, the emergence of genetic engineering has introduced a completely new approach to render plants resistant to bacteria, nematodes, fungi, and viruses. The RNA interference (RNAi) approach has recently emerged as a potentially useful tool for mitigating the inherent risks associated with the development of conventional transgenics. These risks include the use of specific transgenes, gene control sequences, or marker genes. Utilizing RNAi to silence certain genes is a promising solution to this dilemma as disease-resistant transgenic plants can be generated within a legislative structure. Recent investigations have shown that using target double stranded RNAs via an effective vector system can produce significant silencing effects. Both dsRNA-containing crop sprays and transgenic plants carrying RNAi vectors have proven effective in controlling plant diseases that threaten commercially significant crop species. This article discusses the methods and applications of the most recent RNAi technology for reducing plant diseases to ensure sustainable agricultural yields.

RNAi targeting Nav and CPR via leaf delivery reduces adult emergence and increases the susceptibility to λ-cyholthin in Tuta absoluta (Meyrick)

The tomato leafminer, Tuta absoluta (Meyrick), one of the most economically destructive pests of tomato, causes severe yields losses of tomato production globally. Rapid evolution of insecticide resistance requires the development of alternative control strategy for this pest. RNA interference (RNAi) represents a promising, innovative control strategy against key agricultural insect pests, which has recently been licensed for Colorado Potato Beetle control. Here two essential genes, voltage-gated sodium channel (Nav) and NADPH-cytochrome P450 reductase (CPR) were evaluated as targets for RNAi using an ex vivo tomato leaf delivery system. Developmental stage-dependent expression profiles showed TaNav was most abundant in adult stages, whereas TaCPR was highly expressed in larval and adult stages. T. absoluta larvae feeding on tomato leaflets treated with dsRNA targeting TaNav and TaCPR showed significant knockdown of gene expression, leading to reduction in adult emergence. Additionally, tomato leaves treated with dsRNA targeting these two genes were significantly less damaged by larval feeding and mining. Furthermore, bioassay with LC30 doses of λ-cyholthin showed that silencing TaNav and TaCPR increased T. absoluta mortality about 32.2 and 17.4%, respectively, thus indicating that RNAi targeting TaNav and TaCPR could increase the susceptibility to λ-cyholthin in T. absoluta. This study demonstrates the potential of using RNAi targeting key genes, like TaNav and TaCPR, as an alternative technology for the control of this most destructive tomato pests in the future.

Screening and functional validation of the core detoxification genes conferring broad-spectrum response to insecticides in Spodoptera frugiperda

BACKGROUND

Fall armyworm, Spodoptera frugiperda, a formidable agricultural pest, has developed resistance to various synthetic insecticides. However, how S. frugiperda utilizes its limited energy and resources to deal with various insecticides remains largely unexplored.

RESULTS

We utilized transcriptome sequencing to decipher the broad-spectrum adaptation mechanism of S. frugiperda to eight insecticides with distinct modes-of-action. Analysis of the Venn diagram revealed that 1014 upregulated genes and 778 downregulated genes were present in S. frugiperda treated with at least five different insecticides, compared to the control group. Exposure to various insecticides led to the significant upregulation of eight cytochrome P450 monooxygenases (P450s), four UDP glucosyltransferases (UGTs), two glutathione-S-transferases (GSTs) and two ATP-binding cassette transporters (ABCs). Among them, the sfCYP340AD3 and sfCYP4G74 genes were demonstrated to respond to stress from six different insecticides in S. frugiperda, as evidenced by RNA interference and toxicity bioassays. Furthermore, homology modeling and molecular docking analyses showed that sfCYP340AD3 and sfCYP4G74 possess strong binding affinities to a variety of insecticides.

CONCLUSION

Collectively, these findings showed that S. frugiperda utilizes a battery of core detoxification genes to cope with the exposure of synthetic insecticides. This study also sheds light on the identification of efficient insecticidal targets gene and the development of resistance management strategies in S. frugiperda, thereby facilitating the sustainable control of this serious pest. © 2024 Society of Chemical Industry.

RNA interference in insects: the link between antiviral defense and pest control

RNA interference (RNAi) is a form of gene silencing triggered by double-stranded RNA (dsRNA) that operates in all eukaryotic cells. RNAi has been widely investigated in insects to determine the underlying molecular mechanism, to investigate its role in systemic antiviral defense, and to develop strategies for pest control. When insect cells are infected by viruses, viral dsRNA signatures trigger a local RNAi response to block viral replication and generate virus-derived DNA that confers systemic immunity. RNAi-based insect pest control involves the application of exogenous dsRNA targeting genes essential for insect development or survival, but the efficacy of this approach has limited potency in many pests through a combination of rapid dsRNA degradation, inefficient dsRNA uptake/processing, and ineffective RNAi machinery. This could be addressed by dsRNA screening and evaluation, focusing on dsRNA design and off-target management, as well as dsRNA production and delivery. This review summarizes recent progress to determine the role of RNAi in antiviral defense and as a pest control strategy in insects, addressing gaps between our fundamental understanding of the RNAi mechanism and the exploitation of RNAi-based pest control strategies.

Knockdown of the glutamate-gated chloride channel gene decreases emamectin benzoate susceptibility in the fall armyworm, Spodoptera frugiperda

Emamectin benzoate (EB), a derivative of avermectin, is the primary insecticide used to control the fall armyworm (FAW) in China. However, the specific molecular targets of EB against FAW remain unclear. In this study, we cloned the glutamate-gated chloride channel (GluCl) gene, which is known to be a primary molecular target for avermectin. We first investigated the transcript levels of SfGluCl in FAW and found that the expression level of SfGluCl in the head and nerve cord was significantly higher than that in other tissues. Furthermore, we found that the expression level of SfGluCl was significantly higher in eggs than that in other developmental stages, including larvae, pupae, and adults. Additionally, we identified three variable splice forms of SfGluCl in exons 3 and 9 and found that their splice frequencies remained unaffected by treatment with the LC50 of EB. RNAi mediated knockdown of SfGluCl showed a significant reduction of 42% and 65% after 48 and 72 h of dsRNA feeding, respectively. Importantly, knockdown of SfGluCl sifgnificantly reduced LC50 and LC90 EB treatment induced mortality of FAW larvae by 15% and 44%, respectively, compared to the control group feeding by dsEGFP. In contrast, there were no significant changes in the mortality of FAW larvae treated with the control insecticides chlorantraniliprole and spinetoram. Finally, molecular docking simulations revealed that EB bound to the large amino-terminal extracellular domain of SfGluCl by forming five hydrogen bonds, two alkyl hydrophobic interactions and one salt bridge. These findings strongly suggest that GluCl may serve as one of the molecular targets of EB in FAW, shedding light on the mode of action of this important insecticide.

RNAi turns 25:contributions and challenges in insect science

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

PxRdl2 dsRNA increased the insecticidal activities of GABAR-targeting compounds against Plutella xylostella

The utilization of RNA interference (RNAi) for pest management has garnered global interest. The bioassay results suggested the knockout of the PxRdl2 gene significantly increased the insecticidal activities of the γ-aminobutyric acid receptor (GABAR)-targeting compounds (fipronil, two pyrazoloquinazolines, and two isoxazolines), thereby presenting a viable target gene for RNAi-mediated pest control. Consequently, we suggest enhancing the insecticidal activities of GABAR-targeting compounds by knockdown the transcript level of PxRdl2. Furthermore, PxRdl2 dsRNA was expressed in HT115 Escherichia coli to reduce costs and protect dsRNA against degradation. In comparison to in vitro synthesized dsRNA, the recombinant bacteria (ds-B) exhibited superior interference efficiency and greater stability when exposed to UV irradiation. Collectively, our results provide a strategy for insecticide spray that combines synergistically with insecticidal activities by suppressing PxRdl2 using ds-B and may be beneficial for reducing the usage of insecticide and slowing pest resistance.

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

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

An Optimized/Scale Up-Ready Protocol for Extraction of Bacterially Produced dsRNA at Good Yield and Low Costs

Double-stranded RNA (dsRNA) can trigger RNA interference (RNAi) and lead to directed silencing of specific genes. This natural defense mechanism and RNA-based products have been explored for their potential as a sustainable and ecofriendly alternative for pest control of species of agricultural importance and disease vectors. Yet, further research, development of new products and possible applications require a cost-efficient production of dsRNA. In vivo transcription of dsRNA in bacterial cells has been widely used as a versatile and inducible system for production of dsRNA combined with a purification step required to extract the dsRNA. Here, we optimized an acidic phenol-based protocol for extraction of bacterially produced dsRNA at low cost and good yield. In this protocol, bacterial cells are efficiently lysed, with no viable bacterial cells present in the downstream steps of the purification. Furthermore, we performed a comparative dsRNA quality and yield assessment of our optimized protocol and other protocols available in the literature and confirmed the cost-efficiency of our optimized protocol by comparing the cost of extraction and yields of each extraction method.

Current Scenario of Exogenously Induced RNAi for Lepidopteran Agricultural Pest Control: From dsRNA Design to Topical Application

Invasive insects cost the global economy around USD 70 billion per year. Moreover, increasing agricultural insect pests raise concerns about global food security constraining and infestation rising after climate changes. Current agricultural pest management largely relies on plant breeding-with or without transgenes-and chemical pesticides. Both approaches face serious technological obsolescence in the field due to plant resistance breakdown or development of insecticide resistance. The need for new modes of action (MoA) for managing crop health is growing each year, driven by market demands to reduce economic losses and by consumer demand for phytosanitary measures. The disabling of pest genes through sequence-specific expression silencing is a promising tool in the development of environmentally-friendly and safe biopesticides. The specificity conferred by long dsRNA-base solutions helps minimize effects on off-target genes in the insect pest genome and the target gene in non-target organisms (NTOs). In this review, we summarize the status of gene silencing by RNA interference (RNAi) for agricultural control. More specifically, we focus on the engineering, development and application of gene silencing to control Lepidoptera through non-transforming dsRNA technologies. Despite some delivery and stability drawbacks of topical applications, we reviewed works showing convincing proof-of-concept results that point to innovative solutions. Considerations about the regulation of the ongoing research on dsRNA-based pesticides to produce commercialized products for exogenous application are discussed. Academic and industry initiatives have revealed a worthy effort to control Lepidoptera pests with this new mode of action, which provides more sustainable and reliable technologies for field management. New data on the genomics of this taxon may contribute to a future customized target gene portfolio. As a case study, we illustrate how dsRNA and associated methodologies could be applied to control an important lepidopteran coffee pest.

Stabilized Double-Stranded RNA Strategy Improves Cotton Resistance to CBW (Anthonomus grandis)

Cotton is the most important crop for fiber production worldwide. However, the cotton boll weevil (CBW) is an insect pest that causes significant economic losses in infested areas. Current control methods are costly, inefficient, and environmentally hazardous. Herein, we generated transgenic cotton lines expressing double-stranded RNA (dsRNA) molecules to trigger RNA interference-mediated gene silencing in CBW. Thus, we targeted three essential genes coding for chitin synthase 2, vitellogenin, and ecdysis-triggering hormone receptor. The stability of expressed dsRNAs was improved by designing a structured RNA based on a viroid genome architecture. We transformed cotton embryos by inserting a promoter-driven expression cassette that overexpressed the dsRNA into flower buds. The transgenic cotton plants were characterized, and positive PCR transformed events were detected with an average heritability of 80%. Expression of dsRNAs was confirmed in floral buds by RT-qPCR, and the T1 cotton plant generation was challenged with fertilized CBW females. After 30 days, data showed high mortality (around 70%) in oviposited yolks. In adult insects fed on transgenic lines, chitin synthase II and vitellogenin showed reduced expression in larvae and adults, respectively. Developmental delays and abnormalities were also observed in these individuals. Our data remark on the potential of transgenic cotton based on a viroid-structured dsRNA to control CBW.

Strategies for the production of dsRNA biocontrols as alternatives to chemical pesticides

Current crop pest control strategies rely on insecticidal and fungicidal sprays, plant genetic resistance, transgenes and agricultural practices. However, many insects, plant viruses, and fungi have no current means of control or have developed resistance against traditional pesticides. dsRNA is emerging as a novel sustainable method of plant protection as an alternative to traditional chemical pesticides. The successful commercialisation of dsRNA based biocontrols for effective pest management strategies requires the economical production of large quantities of dsRNA combined with suitable delivery methods to ensure RNAi efficacy against the target pest. A number of methods exist for the production and delivery of dsRNA based biocontrols and here we review alternative methods currently employed and emerging new approaches for their production. Additionally, we highlight potential challenges that will need to be addressed prior to widespread adoption of dsRNA biocontrols as novel sustainable alternatives to traditional chemical pesticides.

Control of a sap-sucking insect pest by plastid-mediated RNA interference

Expression of double-stranded RNAs in plastids offers great potential for the efficient control of chewing insects. However, many insect pests do not consume plant tissue but rather feed on the host plant by sucking sap from the vascular system. Whether or not plastid-mediated RNA interference (RNAi) can be employed to control sap-sucking insects is unknown. Here, we show that five species of sap-sucking hemipteran insects acquire plastid RNA upon feeding on plants. We generated both nuclear transgenic and transplastomic tobacco plants expressing double-stranded RNAs targeting the MpDhc64C gene, a newly identified efficient target gene of RNAi whose silencing causes lethality to the green peach aphid Myzus persicae. In a whole-plant bioassay, transplastomic plants exhibited significant resistance to aphids, as evidenced by reduced insect survival, impaired fecundity, and decreased weight of survivors. The protective effect was comparable with that conferred by the best-performing nuclear transgenic plants. We found that the proportion of aphids on mature leaves of transplastomic plants was significantly lower compared with that of nuclear transgenic plants. When aphids were allowed to infest only the mature leaves, transplastomic plants grew significantly faster and were overall better protected from the pest compared with nuclear transgenic plants. When monitored by electrical-penetration-graph analyses and aphid avoidance response experiments, the insects displayed remarkable alterations in feeding behavior, which was different in nuclear transgenic and transplastomic plants, likely reflecting specific avoidance strategies to toxic RNA molecules. Taken together, our study demonstrates that plastid-mediated RNAi provides an efficient strategy for controlling at least some sap-sucking insect pests, even though there is most likely no or only very little chloroplast RNA in the sap.

First Report on CRISPR/Cas9-Based Genome Editing in the Destructive Invasive Pest Tuta Absoluta (Meyrick) (Lepidoptera: Gelechiidae)

The tomato leaf miner Tuta absoluta (Meyrick) is one of the world’s most destructive pests of tomato, and because of its severe economic impacts, as well as the development of pesticide resistance, the species has been intensively studied, especially in regard to the identification of targets for T. absoluta control. However, functional genomic studies of T. absoluta have been constrained by a lack of effective genetic tools. Therefore, the aim of the present study was to develop a CRISPR/Cas9 zygote microinjection protocol for generating heritable mutations in T. absoluta, using the ommochrome synthesis gene cinnabar as an easily evaluated target gene. The injection of fertilised eggs with Cas9 protein and four sgRNAs, which targeted cinnabar exon 3, resulted in a mutagenesis rate of 31.9% for eggs reaching adulthood, and cinnabar mutagenesis resulted in either red or mosaic eye colour phenotypes. As such, this study is the first to report a complete and detailed CRISPR/Cas9 workflow for the efficient genome editing of the globally important invasive pest T. absoluta. The application of this robust genome-editing tool to T. absoluta will greatly facilitate the discovery of suitable RNAi control targets and the subsequent development of novel control strategies.

Maf regulates the overexpression of CYP307A1, which is involved in the fitness advantage of bistrifluron-resistant Spodoptera litura (Fab.) (Noctuidae: Lepidoptera)

Spodoptera litura is a widely distributed multifeeding pest, that has developed high resistance to many chemical insecticides. In the present study, a bistrifluron-resistant (Bis-SEL) strain showing 113.8-fold resistance ratio relative to a bistrifluron-susceptible (Bis-UNSEL) strain was obtained and showed a fitness advantage (resurgence). First, we found that the observed resurgence might have resulted from Maf transcription factor overexpression in the Bis-SEL strain, which would influence the synthesis of ecdysone and chitin. Additionally, a co-expression relationship between Maf and CYP307A1 was verified by weighted correlation network analysis (WGCNA) and qRT-PCR, and the expression of CYP307A1, a key gene in ecdysone synthesis, was significantly downregulated by Maf interference. The assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) and a yeast one-hybrid showed that Maf could bind to the cncc:maf-s element in the CYP307A1 promoter region. The synthesis of ecdysone, which stimulated chitin synthesis, was also decreased significantly following Maf and CYP307A1 interference. Therefore, the upregulation of Maf expression leaded to the upregulation CYP307A1 expression, which led to an increase in the synthesis of ecdysone, resulting in resurgence accompanied by resistance to bistrifluron.

RNA Interference Suppression of v-ATPase B and Juvenile Hormone Binding Protein Genes Through Topically Applied dsRNA on Tomato Leaves: Developing Biopesticides to Control the South American Pinworm, Tuta absoluta (Lepidoptera: Gelechiidae)

The South American pinworm Tuta absoluta (Meyrick) (Family: Gelechiidae) is one of the most devastating lepidopteran pests in the developing countries of South America, Africa, and Asia. This pest is classified as the most serious threat for tomato production worldwide. In the present study, we analyzed RNAi-mediated control through exogenously applied dsRNA delivery on tomato. The dsRNA treatments were made to target the juvenile hormone binding protein and the v-ATPase B. Both mRNA targets were cloned, validated by sequencing, and used to produce each dsRNA. After treatments the relative transcript expression was analyzed using qRTPCR to assess to efficacy of RNAi. A leaf-dip assay was used to provide late 2nd instar larvae three feeding access periods: 24, 48, and 72 h, to evaluate the effect of gene silencing of each target. Larvae were fed tomato leaves coated with five different RNAi concentrations (10, 20, 30, 40, and 50 micrograms/centimeter-squared), that suppressed two genes (juvenile hormone protein, JHBP, and vacuolar-type adenosine triphosphatase enzyme, v-ATPase). Treatments with dsRNA showed a significant increase in mortality at 24, 48, and 72 h after ingestion (P < 0.01, α = 0.05), along with reduced leaf damage, and increased feeding deterrence. The results suggest that these two RNAi products may provide a suitable treatment for control of this and other lepidopteran pests.

RNA interference in Pardosa pseudoannulata, an important predatory enemy against several insect pests, through ingestion of dsRNA-expressing Escherichia coli

RNA interference is an important technology for gene functional research in many organisms. The pond wolf spider (Pardosa pseudoannulata) is an important natural enemy of rice field pests. To facilitate large-scale gene functional research in this spider species and others, we developed an RNA interference (RNAi) method via ingestion of bacteria expressing dsRNA. The dsRNA targeting a cytochrome P450 monooxygenase (cyp41g2) was expressed in Escherichia coli HT115 (DE3). And then the bacterial suspension was fed to 14-20 days old spiderlings. The mRNA abundance of the target gene was significantly reduced after 3-day's ingestion of bacteria expressing dsRNA, and between day 5 and 7, RNAi efficiency remained stable. Thus, we selected 5 days as the optimum interference time. Furthermore, the bacteria resuspension containing 20 ng/μl dsRNA was selected as the optimum concentration. To evaluate the applicability of this method, three other genes with different tissue expression pattern were also selected as targets. And the mRNA abundance of all the four target genes was significantly reduced with RNAi efficiency between 66.0% and up to 86.9%. The results demonstrated that the oral delivery of bacteria expressing dsRNA would be an effective RNAi method for the gene functional study in P. pseudoannulata.

Lab-to-Field Transition of RNA Spray Applications - How Far Are We?

The drastic loss of biodiversity has alarmed the public and raised sociopolitical demand for chemical pesticide-free plant production, which is now treated by governments worldwide as a top priority. Given this global challenge, RNAi-based technologies are rapidly evolving as a promising substitute to conventional chemical pesticides. Primarily, genetically modified (GM) crops expressing double-stranded (ds)RNA-mediating gene silencing of foreign transcripts have been developed. However, since the cultivation of GM RNAi crops is viewed negatively in numerous countries, GM-free exogenous RNA spray applications attract tremendous scientific and political interest. The sudden rise in demand for pesticide alternatives has boosted research on sprayable RNA biopesticides, generating significant technological developments and advancing the potential for field applications in the near future. Here we review the latest advances that could pave the way for a quick lab-to-field transition for RNA sprays, which, as safe, selective, broadly applicable, and cost-effective biopesticides, represent an innovation in sustainable crop production. Given these latest advances, we further discuss technological limitations, knowledge gaps in the research, safety concerns and regulatory requirements that need to be considered and addressed before RNA sprays can become a reliable and realistic agricultural approach.

Advances in the Development of Microbial Double-Stranded RNA Production Systems for Application of RNA Interference in Agricultural Pest Control

RNA interference (RNAi) is a valuable and revolutionary technology that has been widely applied in medicine and agriculture. The application of RNAi in various industries requires large amounts of low-cost double-stranded RNA (dsRNA). Chemical synthesis can only produce short dsRNAs; long dsRNAs need to be synthesized biologically. Several microbial chassis cells, such as Escherichia coli, Saccharomyces cerevisiae, and Bacillus species, have been used for dsRNA synthesis. However, the titer, rate of production, and yield of dsRNA obtained by these microorganism-based strategies is still low. In this review, we summarize advances in microbial dsRNA production, and analyze the merits and faults of different microbial dsRNA production systems. This review provides a guide for dsRNA production system selection. Future development of efficient microbial dsRNA production systems is also discussed.

RNA Interference-Based Forest Protection Products (FPPs) Against Wood-Boring Coleopterans: Hope or Hype?

Forest insects are emerging in large extension in response to ongoing climatic changes, penetrating geographic barriers, utilizing novel hosts, and influencing many hectares of conifer forests worldwide. Current management strategies have been unable to keep pace with forest insect population outbreaks, and therefore novel and aggressive management strategies are urgently required to manage forest insects. RNA interference (RNAi), a Noble Prize-winning discovery, is an emerging approach that can be used for forest protection. The RNAi pathway is triggered by dsRNA molecules, which, in turn, silences genes and disrupts protein function, ultimately causing the death of the targeted insect. RNAi is very effective against pest insects; however, its proficiency varies significantly among insect species, tissues, and genes. The coleopteran forest insects are susceptible to RNAi and can be the initial target, but we lack practical means of delivery, particularly in systems with long-lived, endophagous insects such as the Emerald ash borer, Asian longhorn beetles, and bark beetles. The widespread use of RNAi in forest pest management has major challenges, including its efficiency, target gene selection, dsRNA design, lack of reliable dsRNA delivery methods, non-target and off-target effects, and potential resistance development in wood-boring pest populations. This review focuses on recent innovations in RNAi delivery that can be deployed against forest pests, such as cationic liposome-assisted (lipids), nanoparticle-enabled (polymers or peptides), symbiont-mediated (fungi, bacteria, and viruses), and plant-mediated deliveries (trunk injection, root absorption). Our findings guide future risk analysis of dsRNA-based forest protection products (FPPs) and risk assessment frameworks incorporating sequence complementarity-based analysis for off-target predictions. This review also points out barriers to further developing RNAi for forest pest management and suggests future directions of research that will build the future use of RNAi against wood-boring coleopterans.

Methods for Delivery of dsRNAs for Agricultural Pest Control: The Case of Lepidopteran Pests

RNA interference (RNAi) is a natural mechanism of gene regulation, highly conserved in eukaryotes. Since the elucidation of the gene silencing mechanism, RNAi became an important tool used in insect reverse genetics. The demonstration of effective target-gene silencing by ingestion of double-stranded RNA (dsRNA) produced by transgenic plants indicated the RNAi potential to be used in insect pest management, particularly in agriculture. However, the efficiency of gene silencing by RNAi in insects may vary according to the target taxa, and lepidopteran species have been shown to be quite recalcitrant to RNAi. Developing transgenic plants is a time-consuming and labor-intensive process, so alternative oral delivery systems are required to develop and optimize RNAi settings, such as selecting an efficient target gene, and dsRNA design, length, and stability, among other features. We have developed delivery systems to evaluate dsRNAs to silence genes from two important lepidopteran crop pests of tomato (Solanum lycopersicum) and sugarcane (Saccharum × officinarum): Tuta absoluta (Meyrick), the South American Tomato Pinworm, and Diatraea saccharalis (Fabricius), the Sugarcane Borer, respectively. The protocol described here can be used in similar species and includes (a) direct oral delivery by droplets containing dsRNA; (b) oral delivery by tomato leaflets that absorbed dsRNA solution; (c) delivery by Escherichia coli expressing dsRNA; and (d) delivery by transgenic plants expressing dsRNA.

The fitness advantages of bistrifluron resistance related to chitin synthase A in Spodoptera litura (Fab.) (Noctuidae: Lepidoptera)

BACKGROUND

Spodoptera litura is one of the major agricultural pests in China, and it has developed serious resistance to many traditional chemical insecticides. In the present study, the bistrifluron-resistant (Bis-SEL) strain accompanied by a higher oviposition, 113.8-fold RR compared to the bistrifluron-susceptible (Bis-UNSEL) strain, was obtained by bidirectional screening. A comparison of their gonad coefficiency and genes related to oviposition or resistance was used to elucidate the resurgence mechanism.

RESULTS

The ovarian index, oviposition, and potential egg production in the Bis-SEL strain of female adults were significantly higher than those in the Bis-UNSEL strain, and the length of ovariole in the Bis-SEL strain was also significantly elongated. The protein contents of vitellogenin (Vg) and vitellogenin receptor (VgR) in the Bis-UNSEL strain were lower than those in the Bis-SEL strain, consistent with their gene expressions levels, and there was a significantly positive linear correlation between Vg and VgR protein contents, further confirming that resistant strains have high reproductive fitness. Moreover, the chitin synthase A in the Bis-SEL strain was clearly up-regulated, and a mutation (H866Y) near the QRRRW in the catalytic domain caused a rise in the hydrogen bond between UDP-GlcNAc and chitin synthase, and its chitin content was higher than that in the Bis-UNSEL strain. Nevertheless, the sensitivity of the Bis-SEL strain to bistrifluron was significantly recovered when it was knocked down though RNA interference.

CONCLUSION

The fitness advantages of bistrifluron resistance may be related to the up-regulation and mution of chitin synthase A. © 2021 Society of Chemical Industry.

Engineering insects from the endosymbiont out

Insects are an incredibly diverse group of animals with species that benefit and harm natural ecosystems, agriculture, and human health. Many insects have consequential associations with microbes: bacterial symbionts may be embedded in different insect tissues and cell types, inherited across insect generations, and required for insect survival and reproduction. Genetically engineering insect symbionts is key to understanding and harnessing these associations. We summarize different types of insect-bacteria relationships and review methods used to genetically modify endosymbiont and gut symbiont species. Finally, we discuss recent studies that use this approach to study symbioses, manipulate insect-microbe interactions, and influence insect biology. Further progress in insect symbiont engineering promises to solve societal challenges, ranging from controlling pests to protecting pollinator health.

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.

Programming gene expression in multicellular organisms for physiology modulation through engineered bacteria

A central goal of synthetic biology is to predictably and efficiently reprogram living systems to perform computations and carry out specific biological tasks. Although there have been many advances in the bio-computational design of living systems, these advances have mainly been applied to microorganisms or cell lines; programming animal physiology remains challenging for synthetic biology because of the system complexity. Here, we present a bacteria-animal symbiont system in which engineered bacteria recognize external signals and modulate animal gene expression, twitching phenotype, and fat metabolism through RNA interference toward gfp, sbp-1, and unc-22 gene in C. elegans. By using genetic circuits in bacteria to control these RNA expressions, we are able to program the physiology of the model animal Caenorhabditis elegans with logic gates. We anticipate that engineered bacteria can be used more extensively to program animal physiology for agricultural, therapeutic, and basic science applications.

dsRNA-Mediated Pest Management of Tuta absoluta Is Compatible with Its Biological Control Agent Nesidiocoris tenuis

Simple Summary

The zoophytophagous mirid bug Nesidiocoris tenuis is an efficient predator of the tomato leafminer, Tuta absoluta. RNA interference (RNAi) targeting the alphaCOP (αCOP) (Coatomer subunit alpha protein) gene of N. tenuis (Nt-αCOP) was proven to be functional in N. tenuis, causing downregulation of gene expression, mortality and sub-lethal effects. In contrast, when N. tenuis were fed with dsRNA (dsTa-αCOP) targeting the ortholog αCOP gene of T. absoluta, no lethal nor sub-lethal effects were observed. These results indicate the compatibility of this biocontrol agent along with RNAi-mediated management in order to suppress T. absoluta efficiently in tomato crop.

Abstract

RNAi-mediated insect pest management has recently shown promising results against the most serious pest of tomato, the tomato leafminer, Tuta absoluta. This study aimed to investigate whether dsRNA (dsTa-αCOP) designed to target the T. absoluta-αCOP gene could cause adverse effects to its biocontrol agent, the mirid predator, Nesidiocoris tenuis. Oral exposure of N. tenuis to dsRNA (dsNt-αCOP) designed to target N. tenuis-αCOP resulted in a 61%, 67% and 55% reduction in its transcript level in comparison to the sucrose, dsGFP and dsTa-αCOP treatments, respectively. In addition, significantly higher mortality of 57% was recorded in dsNt-αCOP-treated N. tenuis when compared to the sucrose (7%), dsGFP (10%) and dsTa-αCOP (10%) treatments. Moreover, the predation rate of ~33–39 Ephestia kuehniella eggs per N. tenuis adult dramatically reduced to almost half in the surviving dsNt-αCOP-treated N. tenuis. This worst-case exposure scenario confirmed for the first time that the RNAi machinery is functional in this species and that the risk of exposure through the oral route is possible. In contrast, dsTa-αCOP did not cause any sub-lethal effects to N. tenuis upon oral exposure. Oral exposure of T. absoluta to dsTa-αCOP resulted in 50% mortality. In the context of a biosafety risk assessment of RNAi-mediated insect management, investigating the effects on non-target organisms is essential in order to include this method as part of an integrated pest management strategy. Based on our laboratory assays, RNAi-mediated control is compatible with the biological control of T. absoluta by its natural enemy N. tenuis, adding the RNAi approach in the armoire of integrated pest management of T. absoluta.