RNA Interference towards the Potato Psyllid, Bactericera cockerelli, Is Induced in Plants Infected with Recombinant Tobacco mosaic virus (TMV)

UV-LASER adjuvant-surfactant-facilitated delivery of mobile dsRNA to tomato plant vasculature and evidence of biological activity by gene knockdown in the potato psyllid

BACKGROUND

Double-stranded RNA (dsRNA) biopesticides are of interest for the abatement of insect vectors of pathogenic bacteria such as 'Candidatus Liberibacter', which infects both its psyllid and plant hosts. Silencing of genes essential for psyllids, or for Liberibacter, is anticipated to lead to mortality or impeded bacterial multiplication. Foliar delivery is preferred for biopesticide application; however, the cuticle impedes dsRNA penetration into the vasculature. Here, conditions were established for wounding tomato leaves using ultraviolet light amplification by stimulated emissions of radiation (UV-LASER) to promote dsRNA penetration into leaves and vasculature.

RESULTS

UV-LASER treatment with application of select adjuvants/surfactants resulted in vascular delivery of 100-, 300- and 600-bp dsRNAs that, in general, were correlated with size. The 100-bp dsRNA required no pretreatment, whereas 300- and 600-bp dsRNAs entered the vasculature after UV-LASER treatment only and UV-LASER adjuvant/surfactant treatment, respectively. Of six adjuvant/surfactants evaluated, plant-derived oil combined with an anionic organosilicon compound performed most optimally. Localization of dsRNAs in the tomato vasculature was documented using fluorometry and fluorescence confocal microscopy. The biological activity of in planta-delivered dsRNA (200-250 bp) was determined by feeding third-instar psyllids on tomato leaves post UV-LASER adjuvant/surfactant treatment, with or without psyllid cdc42- and gelsolin dsRNAs. Gene knockdown was quantified by quantitative, real-time polymerase chain reaction with reverse transcription (RT-qPCR) amplification. At 10 days post the ingestion-access period, knockdown of cdc42 and gelsolin expression was 61% and 56%, respectively, indicating that the dsRNAs delivered to the tomato vasculature were mobile and biologically active.

CONCLUSION

Results indicated that UV-LASER adjuvant/surfactant treatments facilitated the delivery of mobile, biologically active dsRNA molecules to the plant vasculature. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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

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

Advances in RNA Interference for Plant Functional Genomics: Unveiling Traits, Mechanisms, and Future Directions

RNA interference (RNAi) is a conserved molecular mechanism that plays a critical role in post-transcriptional gene silencing across diverse organisms. This review delves into the role of RNAi in plant functional genomics and its applications in crop improvement, highlighting its mechanistic insights and practical implications. The review begins with the foundational discovery of RNAi's mechanism, tracing its origins from petunias to its widespread presence in various organisms. Various classes of regulatory non-coding small RNAs, including siRNAs, miRNAs, and phasiRNAs, have been uncovered, expanding the scope of RNAi-mediated gene regulation beyond conventional understanding. These RNA classes participate in intricate post-transcriptional and epigenetic processes that influence gene expression. In the context of crop enhancement, RNAi has emerged as a powerful tool for understanding gene functions. It has proven effective in deciphering gene roles related to stress resistance, metabolic pathways, and more. Additionally, RNAi-based approaches hold promise for integrated pest management and sustainable agriculture, contributing to global efforts in food security. This review discusses RNAi's diverse applications, such as modifying plant architecture, extending shelf life, and enhancing nutritional content in crops. The challenges and future prospects of RNAi technology, including delivery methods and biosafety concerns, are also explored. The global landscape of RNAi research is highlighted, with significant contributions from regions such as China, Europe, and North America. In conclusion, RNAi remains a versatile and pivotal tool in modern plant research, offering novel avenues for understanding gene functions and improving crop traits. Its integration with other biotechnological approaches such as gene editing holds the potential to shape the future of agriculture and sustainable food production.

Gene silencing of cathepsins B and L using CTV-based, plant-mediated RNAi interferes with ovarial development in Asian citrus psyllid (ACP), Diaphorina citri

Diaphorina citri Kuwayama (Hemiptera: Liviidae) is a vector of the bacteria Candidatus Liberibacter americanus (CLam) and Candidatus Liberibacter asiaticus (CLas), which are phloem-restricted and associated with the most important and destructive worldwide citrus disease, Huanglongbing (HLB). Currently, no cure for HLB has been described. Therefore, measures have focused on reducing D. citri populations. In these insects, cathepsin B (DCcathB) and L (DCcathL) enzymes play an important role in digestion, and are involved in embryogenesis, immune defense, and ecdysis. In this study, we used a CTV-based vector to deliver dsRNA (CTV-dsRNA) into Citrus macrophylla plants targeting DCcathB and DCcathL genes in D. citri that fed on the phloem of these CTV-RNAi infected plants. Subsequently, we evaluated expression of DCcathB and DCcathL genes as well as the Vitellogenin (Vg) gene by RT-qPCR in D. citri fed on CTV-dsRNA occurring in plant phloem. It was found that a defective phenotype in D. citri females as a result of knockdown of DCcathB and DCcathL genes mediated by CTV dsRNA. These results showed that Psyllids fed on plants treated with the CTV-dsRNA exhibited downregulation of the Vg gene, one of the most important genes associated with embryogenic and female development, which was associated with dsRNA-mediated silencing of the two cathepsin genes. Based on our findings, a CTV-based strategy for delivering RNAi via plants that targets DCcathB and DCcathL genes may represent a suitable avenue for development of dsRNA-based tools to manage D. citri that limits the spread of HLB.

FAR knockout significantly inhibits Chilo suppressalis survival and transgene expression of double-stranded FAR in rice exhibits strong pest resistance

Chilo suppressalis is one of the most prevalent and damaging rice pests, causing significant economic losses each year. Chemical control is currently the primary method of controlling C. suppressalis. However, the indiscriminate use of chemical insecticides increases pest resistance, pollutes the environment and poses a significant health threat to humans and livestock, highlighting the need to find safer, more pest-specific and more effective alternatives to pest control. Plant-mediated RNA interference (RNAi) is a promising agricultural pest control method that is highly pest-specific and has less of an impact on the environment. Using multi-sgRNAs/Cas9 technology to delete Fatty acyl-CoA reductase (FAR) of C. suppressalis in the G0 generation, we show that downregulating FAR transcription may significantly increase the mortality rate and darken the epidermis of C. suppressalis compared with the control. Subsequently, we developed dsFAR transgenic rice lines using Agrobacterium-mediated genetic transformation and then screened three strains expressing dsFAR at high levels using transcriptional level analysis. Using transgenic rice stems, a laboratory feeding bioassay indicated that at least one line (L#10) displayed a particularly high level of insect resistance, with an insect mortality rate of more than 80%. In the field trials, dsFAR transgenic rice displayed high levels of resistance to C. suppressalis damage. Collectively, these results suggest the potential of a new environment-friendly, species-specific strategy for rice pest management.

Characteristics of environmental RNAi in potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Psylloidea: Triozidae)

RNA interference (RNAi) has potential to become a major tool for integrated management of insect pests of agricultural crops based on sequence-specificity and low doses of rapidly biodegradable dsRNA. Deploying ‘environmental RNAi’ for control of insect vectors of plant pathogens is of increasing interest for combatting emerging plant diseases. Hemipteran insect vectors, including psyllids, are vascular feeders, making their development difficult to control specifically by targeting with pesticidal chemistries. Psyllids transmit “Candidatus Liberibacter solanacearum” the causal organism of potato zebra chip and tomato vein greening diseases, transmitted, respectively, by the potato or tomato psyllid (PoP). Until now, the optimal effective concentration(s) of double-stranded RNA (dsRNA) required for significant gene knockdown and RNAi persistence in PoP have not been determined. The objective of this study was to optimize RNAi in young PoP adults and 3rd instars for screening by oral delivery of dsRNAs. The minimal effective dsRNA concentrations required for robust knockdown and persistence were evaluated by delivering seven concentrations spanning 0.1 ng/μL to 500 ng/μL over post ingestion-access periods (IAP) ranging from 48 h to 12 days. The PoP gene candidates evaluated as targets were vacuolar ATPase subunit A, clathrin heavy chain, and non-fermenting protein 7, which were evaluated for knockdown by qPCR amplification. The minimum and/or the second most effective dsRNA concentration resulting in effective levels of gene knockdown was 100 ng/μL for all three targets. Higher concentrations did not yield further knockdown, indicating potential RISC saturation at the higher doses. Gene silencing post-IAP of 100 ng/μL dsRNA persisted for 3–5 days in adults and nymphs, with the PoP 3rd instar, followed by teneral and mature adults, respectively, exhibiting the most robust RNAi-response.

Going Viral: Virus-Based Biological Control Agents for Plant Protection

The most economically important biotic stresses in crop production are caused by fungi, oomycetes, insects, viruses, and bacteria. Often chemical control is still the most commonly used method to manage them. However, the development of resistance in the different pathogens/pests, the putative damage on the natural ecosystem, the toxic residues in the field, and, thus, the contamination of the environment have stimulated the search for saferalternatives such as the use of biological control agents (BCAs). Among BCAs, viruses, a major driver for controlling host populations and evolution, are somewhat underused, mostly because of regulatory hurdles that make the cost of registration of such host-specific BCAs not affordable in comparison with the limited potential market. Here, we provide a comprehensive overview of the state of the art of virus-based BCAs against fungi, bacteria, viruses, and insects, with a specific focus on new approaches that rely on not only the direct biocidal virus component but also the complex ecological interactions between viruses and their hosts that do not necessarily result in direct damage to the host.

Future of Bacterial Disease Management in Crop Production

Bacterial diseases are a constant threat to crop production globally. Current management strategies rely on an array of tactics, including improved cultural practices; application of bactericides, plant activators, and biocontrol agents; and use of resistant varieties when available. However, effective management remains a challenge, as the longevity of deployed tactics is threatened by constantly changing bacterial populations. Increased scrutiny of the impact of pesticides on human and environmental health underscores the need for alternative solutions that are durable, sustainable, accessible to farmers, and environmentally friendly. In this review, we discuss the strengths and shortcomings of existing practices and dissect recent advances that may shape the future of bacterial disease management. We conclude that disease resistance through genome modification may be the most effective arsenal against bacterial diseases. Nonetheless, more research is necessary for developing novel bacterial disease management tactics to meet the food demand of a growing global population.

The Plant V-ATPase

V-ATPase is the dominant proton pump in plant cells. It contributes to cytosolic pH homeostasis and energizes transport processes across endomembranes of the secretory pathway. Its localization in the trans Golgi network/early endosomes is essential for vesicle transport, for instance for the delivery of cell wall components. Furthermore, it is crucial for response to abiotic and biotic stresses. The V-ATPase's rather complex structure and multiple subunit isoforms enable high structural flexibility with respect to requirements for different organs, developmental stages, and organelles. This complexity further demands a sophisticated assembly machinery and transport routes in cells, a process that is still not fully understood. Regulation of V-ATPase is a target of phosphorylation and redox-modifications but also involves interactions with regulatory proteins like 14-3-3 proteins and the lipid environment. Regulation by reversible assembly, as reported for yeast and the mammalian enzyme, has not be proven in plants but seems to be absent in autotrophic cells. Addressing the regulation of V-ATPase is a promising approach to adjust its activity for improved stress resistance or higher crop yield.

Knockdown of ecdysteroid synthesis genes results in impaired molting and high mortality in Bactericera cockerelli (Hemiptera: Triozidae)

BACKGROUND

RNA-mediated interference (RNAi) has become a promising biopesticide technology with which to direct sequence-specific gene knockdown of key targets in the potato psyllid (PoP) Bactericera cockerelli, resulting in significant mortality. In this study, three strategically selected target genes, ATF4, C7 and D24, essential for the biosynthesis and regulation of ecdysteroids, were evaluated for knockdown and mortality using oral delivery of individual, paired and all three double-stranded RNAs (dsRNAs), in five replicated experiments. Knockdown was determined as the fold-change in gene expression using a quantitative polymerase chain reaction.

RESULTS

Knockdown of the D24 target, at 39%-45%, resulted in 51% PoP mortality by 10 days post-ingestion (dpi) of dsRNA. Knockdown of C7, at 38%-61%, resulted in 53% mortality by 10 dpi, whereas dsD24 ingestion resulted in 65% mortality by 10 dpi when dsD24 and dsC7 were co-delivered. Three phenotypes, INCOMEC, PREMEC and SWOLLEN, were observed at a frequency of 4%-12%, and are consistent with incomplete ecdysis in immature and/or adult PoP. Adult PoP exhibiting INCOMEC survived for several days but were unable to mate or fly, whereas SWOLLEN and PREMEC were lethal to the immature instars. Knockdown of ATF4 did not result in the mortality or malformations in immature and adult PoP.

CONCLUSIONS

Compared with knockdown of individual D24 and C7 targets, significantly greater RNAi penetrance was achieved following delivery of combined dsRNAs. The highest knockdown that resulted in incomplete ecdysis and/or mortality was obtained for targets with predicted involvement in the same or interacting pathway(s). Knockdown of ATF4 was apparently "rescued" by uncharacterized compensatory gene(s) or effects. © 2022 Society of Chemical Industry.

Induction of resistance to Myzus persicae-nicotianae in Cucumber mosaic virus infected tobacco plants using silencing of CMV-2b gene

Aphids such as tobacco aphid Myzus persicae-nicotianae, are among the most important plant viral vectors and plant viruses encode genes to interact with their vectors. Cucumber mosaic virus (CMV) encodes 2b protein as a suppressor of plant immune and it plays a vital role in CMV accumulation and susceptibility to aphid vectors. In this study, the resistance of tobacco plants (Nicotiana tabacum) to M. p. nicotianae was evaluated by silencing of 2b in CMV-infected plants. However, the pFGC-C.h silencing gene construct was transiently expressed using Agrobacterium tumefacience, LBA 4404 in tobacco leaves, and four days later, the plants were mechanically inoculated by CMV (Kurdistan isolate), and then, 15 days post-inoculation 1 nonviruliferous aphid was placed on each leaf for evaluation of resistance to M. p. nicotianae. To evaluate the tobacco plants resistance and susceptibility to M. p. nicotianae, the number of aphids existent per tobacco leaf, life table and, demographic parameters were recorded and used as a comparison indicator. The obtained results were analyzed using the age-stage, two-sex life table. The highest number of aphids was recorded on the control CMV-infected plants, while the lowest number on CMV infected leaves expressing CMV-2b silencing construct (pFGC-C.h). The obtained data revealed the lowest rate for all of intrinsic rate of natural increase (rm) (0.246/day), the rate of reproduction (r0) (17.04 females/generation), and finite rate of increase (λ) (1.279/day), on the pFGC-C.h treatment. The maximum generation time (T) (11.834 days) was observed on (V) treatment. However, the collected data revealed induction of resistance to tobacco aphids by silencing of CMV-2b in CMV infected plants.

Development of a Ligation-Independent Cloning-Based Dual Vector System for RNA Interference in Plants

RNA interference (RNAi) is an evolutionarily conserved post-transcriptional gene silencing mechanism that responds to double-stranded RNA (dsRNA) by sequence-specific downregulation of target genes. The dsRNA-mediated RNAi technology has become one of the most widely used and powerful tools for functional genomic studies in diverse organisms. However, its application has been limited due to the technical difficulty of making RNAi constructs caused by the inverted repeat structure that is required for the formation of hairpin RNA. Here, we present a ligation-independent cloning-based dual vector-mediated RNAi system for silencing specific genes in plants. This approach is simple, efficient, and cost-effective and can be readily adapted to other binary vectors for functional analysis of target genes and the development of sustainable disease and pest control strategies in a broad range of plant species.

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.

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.

Effectiveness of orally-delivered double-stranded RNA on gene silencing in the stinkbug Plautia stali

Development of a reliable method for RNA interference (RNAi) by orally-delivered double-stranded RNA (dsRNA) is potentially promising for crop protection. Considering that RNAi efficiency considerably varies among different insect species, it is important to seek for the practical conditions under which dsRNA-mediated RNAi effectively works against each pest insect. Here we investigated RNAi efficiency in the brown-winged green stinkbug Plautia stali, which is notorious for infesting various fruits and crop plants. Microinjection of dsRNA into P. stali revealed high RNAi efficiency-injection of only 30 ng dsRNA into last-instar nymphs was sufficient to knockdown target genes as manifested by their phenotypes, and injection of 300 ng dsRNA suppressed the gene expression levels by 80% to 99.9%. Knockdown experiments by dsRNA injection showed that multicopper oxidase 2 (MCO2), vacuolar ATPase (vATPase), inhibitor of apoptosis (IAP), and vacuolar-sorting protein Snf7 are essential for survival of P. stali, as has been demonstrated in other insects. By contrast, P. stali exhibited very low RNAi efficiency when dsRNA was orally administered. When 1000 ng/μL of dsRNA solution was orally provided to first-instar nymphs, no obvious phenotypes were observed. Consistent with this, RT-qPCR showed that the gene expression levels were not affected. A higher concentration of dsRNA (5000 ng/μL) induced mortality in some cohorts, and the gene expression levels were reduced to nearly 50%. Simultaneous oral administration of dsRNA against potential RNAi blocker genes did not improve the RNAi efficiency of the target genes. In conclusion, P. stali shows high sensitivity to RNAi with injected dsRNA but, unlike the allied pest stinkbugs Halyomorpha halys and Nezara viridula, very low sensitivity to RNAi with orally-delivered dsRNA, which highlights the varied sensitivity to RNAi across different species and limits the applicability of the molecular tool for controlling this specific insect pest.

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.

Cytochrome P450s CYP380C6 and CYP380C9 in green peach aphid facilitate its adaptation to indole glucosinolate-mediated plant defense

BACKGROUND

Overexpressing CIRCADIAN CLOCK ASSOCIATED1 in Arabidopsis thaliana (CCA1-ox) increases indole glucosinolate production and resistance to green peach aphid (Myzus persicae). Little is known of how aphids respond to this group of plant defense compounds or of the underlying molecular mechanism.

RESULTS

Aphids reared on CCA1-ox for over 40 generations (namely the CCA population) became less susceptible to CCA1-ox than aphids maintained on the wild-type Col-0 (namely the COL population). This elevated tolerance was transgenerational as it remained for at least eight generations after the CCA population was transferred to Col-0. Intriguingly, transcriptome analysis indicated that all differential cytochrome P450 monooxygenase genes (MpCYPs), primarily MpCYP4s, MpCYP380s and MpCYP6s, were more highly expressed in the CCA population. Application of a P450 inhibitor to the CCA population resulted in decreased aphid reproduction on CCA1-ox, which was not observed if aphids were reared on Col-0. When indole glucosinolate biosynthesis in CCA1-ox was blocked using virus-induced gene silencing, the effect of the P450 inhibitor on the CCA population was attenuated, affirming the essential role played by MpCYPs in counteracting the defense mechanism in CCA1-ox that is low or absent in Col-0. Furthermore, we used host-induced gene silencing to identify MpCYP380C6 and MpCYP380C9 that specifically facilitated the CCA population to cope with CCA1-mediated plant defense. Expression profiles revealed their possible contribution to the transgenerational tolerance observed in aphids.

CONCLUSION

MpCYP380C6 and MpCYP380C9 in aphids play a crucial role in mitigating indole glucosinolate-mediated plant defense, and this effect is transgenerational.

RNA interference approaches for plant disease control

Plant diseases caused by a variety of pathogens can have severe effects on crop plants and even plants in natural ecosystems. Despite many effective conventional approaches to control plant diseases, new, efficacious, environmentally sound and cost-effective approaches are needed, particularly with our increasing human population and the effects on crop production and plant health caused by climate change. RNA interference (RNAi) is a gene regulation and antiviral response mechanism in eukaryotes; transgenic and non transgenic plant-based RNAi approaches have shown great effectiveness and potential to target specific plant pathogens and help control plant diseases, especially when no alternatives are available. Here we discuss ways in which RNAi has been used against different plant pathogens, and some new potential applications for plant disease control.

Silencing arginine kinase/integrin β1 subunit by transgenic plant expressing dsRNA inhibits the development and survival of Plutella xylostella

BACKGROUND

Plutella xylostella is a devastating agricultural insect pest of cruciferous plants, including crops. Plant-mediated RNA interference (RNAi) is currently being developed for plant protection. In this study, we investigated the response of P. xylostella exposed to transgenic Arabidopsis thaliana plants that expressed double-stranded RNA (dsRNA) targeting P. xylostella genes of arginine kinase (PxAK) and integrin β₁ subunit (Pxβ).

RESULTS

Transgenic plants producing dsRNAs of the 384-bp fragment of PxAK (dsAK plants), the 497-bp fragment of Pxβ (dsβ plants), and the 881 bp of the combination of both genes (dsAK-β plants) were generated and verified. Insect bioassay with these transgenic plants showed that the development of P. xylostella was affected, causing longer developmental time, and lower pupal weight and pupation rate. P. xylostella mortality rates were 25.0% when exposed to dsAK plants, 22.5% with dsβ plants, and 30.0% with dsAK-β plants, which were all higher than 7.5% for the wild-type plant. PxAK and Pxβ in P. xylostella were suppressed by 26.6-79.7% at the transcription level by the transgenic plants.

CONCLUSION

These results suggest that plant-mediated RNAi targeting single gene or both PxAK and Pxβ may have the potential to control P. xylostella. © 2019 Society of Chemical Industry.

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.

The functional difference of eight chitinase genes between male and female of the cotton mealybug, Phenacoccus solenopsis

The cotton mealybug Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae) is a polyphagous insect that attacks tens of plant and causes substantial economic loss. Insect chitinases are required to remove the old cuticle to allow for continued growth and development. Though insect chitinases have been well studied in tens of insects, their functions in mealybug are still not addressed. Here, we sequenced the transcriptomes of adult males and females, from which eight chitinase genes were identified. We then used the method of rapid amplification of cDNA ends to amplify their full length. Phylogenetic analysis indicated that these genes clustered into five subgroups. Among which, group II PsCht2 had the longest transcript and was highly expressed at second instar nymph. PsCht10, PsCht3-3 and PsIDGF were highly expressed in the adult females, whereas PsCht4 and PsCht4-1 were significantly expressed at the male pupa and adult male. Next, we knocked down all eight chitinase genes by feeding the double-stranded RNA. Knockdown of PsCht4 or PsCht4-1 led to the failure of moult and, silencing PsCht5 resulted in pupation defect, while silencing PsCht10 led to small body size, suggesting these genes have essential roles in development and can be used as a potential target for pest control.

miR403a and SA Are Involved in NbAGO2 Mediated Antiviral Defenses Against TMV Infection in Nicotiana benthamiana

RNAi (RNA interference) is an important defense response against virus infection in plants. The core machinery of the RNAi pathway in plants include DCL (Dicer Like), AGO (Argonaute) and RdRp (RNA dependent RNA polymerase). Although involvement of these RNAi components in virus infection responses was demonstrated in Arabidopsis thaliana, their contribution to antiviral immunity in Nicotiana benthamiana, a model plant for plant-pathogen interaction studies, is not well understood. In this study, we investigated the role of N. benthamiana NbAGO2 gene against TMV (Tomato mosaic virus) infection. Silencing of NbAGO2 by transient expression of an hpRNA construct recovered GFP (Green fluorescent protein) expression in GFP-silenced plant, demonstrating that NbAGO2 participated in RNAi process in N. benthamiana. Expression of NbAGO2 was transcriptionally induced by both MeSA (Methylsalicylate acid) treatment and TMV infection. Down-regulation of NbAGO2 gene by amiR-NbAGO2 transient expression compromised plant resistance against TMV infection. Inhibition of endogenous miR403a, a predicted regulatory microRNA of NbAGO2, reduced TMV infection. Our study provides evidence for the antiviral role of NbAGO2 against a Tobamovirus family virus TMV in N. benthamiana, and SA (Salicylic acid) mediates this by induction of NbAGO2 expression upon TMV infection. Our data also highlighted that miR403a was involved in TMV defense by regulation of target NbAGO2 gene in N. Benthamiana.

Plant-Mediated Silencing of the Whitefly Bemisia tabaci Cyclophilin B and Heat Shock Protein 70 Impairs Insect Development and Virus Transmission

The whitefly B. tabaci is a global pest and transmits extremely important plant viruses especially begomoviruses, that cause substantial crop losses. B. tabaci is one of the top invasive species worldwide and have developed resistance to all major pesticide classes. One of the promising alternative ways for controlling this pest is studying its genetic makeup for identifying specific target proteins which are critical for its development and ability to transmit viruses. Tomato yellow leaf curl virus (TYLCV) is the most economically important and well-studied begomovirus transmitted by B. tabaci, in a persistent-circulative manner. Recently, we reported that B. tabaci Cyclophilin B (CypB) and heat shock protein 70 proteins (hsp70) interact and co-localize with TYLCV in the whitefly midgut, on the virus transmission pathway, and that both proteins have a significant role in virus transmission. Here, we extended the previous work and used the Tobacco rattle virus (TRV) plant-mediated RNA silencing system for knocking down both genes and testing the effect of their silencing on whitefly viability and virus transmission. Portions of these two genes were cloned into TRV constructs and tomato plants were infected and used for whitefly feeding and transmission experiments. Following whitefly feeding on TRV-plants, the expression levels of cypB and hsp70 in adult B. tabaci significantly decreased over 72 h feeding period. The knockdown in the expression of both genes was further shown in the first generation of silenced whiteflies, where phenotypic abnormalities in the adult, wing, nymph and bacteriosomes development and structure were observed. Additionally, high mortality rates that reached more than 80% among nymphs and adults were obtained. Finally, silenced whitefly adults with both genes showed decreased ability to transmit TYLCV under lab conditions. Our results suggest that plant-mediated silencing of both cypB and hsp70 have profound effects on whitefly development and its ability to transmit TYLCV.

Molecular mechanisms influencing efficiency of RNA interference in insects

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

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

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

Double-Stranded RNAs High-Efficiently Protect Transgenic Potato from Leptinotarsa decemlineata by Disrupting Juvenile Hormone Biosynthesis

RNA interference (RNAi) has been developed for plant pest control. In this study, hairpin-type double-stranded RNA (dsRNA) targeting the juvenile hormone (JH) acid methyltransferase ( JHAMT) gene ( dsJHAMT) was introduced in potato plants via Agrobacterium-mediated transformation. The results indicated that the transcriptional RNA of dsJHAMT accumulated in the transgenic plants. The transcripts and proteins of the L. decemlineata JHAMT gene were significantly reduced in larvae feeding on dsJHAMT transgenic foliage. The dsJHAMT had a significant negative effect on the growth and development of L. decemlineata, especially resulting in less oviposition. Importantly, in the field trials, transgenic plants are high-efficiently protected from insect damage mainly because surviving insects laid fewer or no eggs. Even full protection from beetle damage can be acquired by continuously lowering insect population size at large scale in the field over the years. Therefore, the transgenic plants expressing dsJHAMT successfully provided an additional option for plant pest control.

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.

Accumulation of 24 nucleotide transgene-derived siRNAs is associated with crinivirus immunity in transgenic plants

RNA silencing is a conserved antiviral defence mechanism that has been used to develop robust resistance against plant virus infections. Previous efforts have been made to develop RNA silencing-mediated resistance to criniviruses, yet none have given immunity. In this study, transgenic Nicotiana benthamiana plants harbouring a hairpin construct of the Lettuce infectious yellows virus (LIYV) RNA-dependent RNA polymerase (RdRp) sequence exhibited immunity to systemic LIYV infection. Deep sequencing analysis was performed to characterize virus-derived small interfering RNAs (vsiRNAs) generated on systemic LIYV infection in non-transgenic N. benthamiana plants as well as transgene-derived siRNAs (t-siRNAs) derived from the immune-transgenic plants before and after LIYV inoculation. Interestingly, a similar sequence distribution pattern was obtained with t-siRNAs and vsiRNAs mapped to the transgene region in both immune and susceptible plants, except for a significant increase in t-siRNAs of 24 nucleotides in length, which was consistent with small RNA northern blot results that showed the abundance of t-siRNAs of 21, 22 and 24 nucleotides in length. The accumulated 24-nucleotide sequences have not yet been reported in transgenic plants partially resistant to criniviruses, and thus may indicate their correlation with crinivirus immunity. To further test this hypothesis, we developed transgenic melon (Cucumis melo) plants immune to systemic infection of another crinivirus, Cucurbit yellow stunting disorder virus (CYSDV). As predicted, the accumulation of 24-nucleotide t-siRNAs was detected in transgenic melon plants by northern blot. Together with our findings and previous studies on crinivirus resistance, we propose that the accumulation of 24-nucleotide t-siRNAs is associated with crinivirus immunity in transgenic plants.

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.

A Systematic Study of RNAi Effects and dsRNA Stability in Tribolium castaneum and Acyrthosiphon pisum, Following Injection and Ingestion of Analogous dsRNAs

RNA interference (RNAi) effects in insects are highly variable and may be largely dependent upon the stability of introduced double-stranded RNAs to digestion by nucleases. Here, we report a systematic comparison of RNAi effects in susceptible red flour beetle (Tribolium castaneum) and recalcitrant pea aphid (Acyrthosiphon pisum) following delivery of dsRNAs of identical length targeting expression of V-type ATPase subunit E (VTE) and inhibitor of apoptosis (IAP) genes. Injection and ingestion of VTE and IAP dsRNAs resulted in up to 100% mortality of T. castaneum larvae and sustained suppression (>80%) of transcript levels. In A. pisum, injection of VTE but not IAP dsRNA resulted in up to 65% mortality and transient suppression (ca. 40%) of VTE transcript levels. Feeding aphids on VTE dsRNA reduced growth and fecundity although no evidence for gene suppression was obtained. Rapid degradation of dsRNAs by aphid salivary, haemolymph and gut nucleases contrasted with stability in T. castaneum larvae where it appears that exo-nuclease activity is responsible for relatively slow digestion of dsRNAs. This is the first study to directly compare RNAi effects and dsRNA stability in receptive and refractory insect species and provides further evidence that dsRNA susceptibility to nucleases is a key factor in determining RNAi efficiency.

A transgenic strategy for controlling plant bugs (Adelphocoris suturalis) through expression of double-stranded RNA homologous to fatty acyl-coenzyme A reductase in cotton

Plant bugs (Miridae species), which are sap-sucking insects, have emerged as major pests of cotton in China. Most Miridae species are not sensitive to commercial Bacillus thuringiensis (Bt) cotton, resulting in significant economic losses and an increased application of insecticide, which eventually may compromise the future of Bt cotton. We demonstrate that FATTY ACYL-COA REDUCTASE (AsFAR) plays an essential role in the reproduction of the bug Adelphocoris suturalis. Down-regulation of AsFAR expression by injection of double-stranded RNA suppresses ovarian development and female fertility, resulting in females producing few viable offspring. To determine the viability of an RNA interference approach to limit FAR expression and reproductive ability in A. suturalis, a dsRNA targeting the AsFAR gene (dsAsFAR) of A. suturalis was expressed in transgenic cotton plants. AsFAR transcription levels were significantly downregulated in A. suturalis feeding on the transgenic plants. In contained field trials, the transgenic cotton lines significantly suppressed the development of A. suturalis populations and were resistant to damage caused by plant bug infestation. These results suggest a new strategy for the management of plant bug pests of cotton.

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.

Knockdown of Mythimna separata chitinase genes via bacterial expression and oral delivery of RNAi effectors

Background

RNAi (RNA interference) is a technology for silencing of target genes via sequence-specific manner. RNAi technology has been used for development of anti-pathogenic crops. In 2007, development of transgenic plants resistant to insect herbivore using RNAi technology was first reported, leading to a burst of efforts aimed at exploitation of RNAi mechanism and control strategy against variety of insect species based on this technique. Mythimna separata belongs to noctuidae family of lepidoptera and is posing threat to crops of economic importance. Recently, outbreaks of M. separata severely threatens corn production in Northern China, calling for new control approaches.

Results

Chitinase genes were chosen as the target genes as they were expressed predominantly in the gut tissue and were reported to be ideal silencing targets in several insect species. Interfering sequences against the target genes were cloned into the L4440 vector to produce sequence specific dsRNAs (double-stranded RNAs). Recombinant L4440 vectors were transformed into Escherichia coli strain HT115 (DE3) which was defective in dsRNA degradation activity, so preserving the dsRNA from degradation by cellular machinery. The bacteria were mixed with artificial diet and were fed to M. separata. We showed that oral delivery of bacterially expressed dsRNA would lead to RNAi effects in the recipient insect. Quantitative real-time PCR results showed that expression level of target MseChi1 and MseChi2 genes in gut tissue of M. separata were down-regulated after oral delivery of engineered bacteria expressing the corresponding dsRNA. Sequence-specific siRNA (small interfering RNA) was detected in recipient insects, supporting the existence of siRNA-mediated silencing effects in M. separata. Furthermore, knockdown of MseChi1 and MseChi2 resulted in increased mortality and reduced body weight of the feeding larvae.

Conclusion

We reported a simple and low cost experimental procedure to silence M. separata endogenous gene expression. Our research provides both an experimental foundation for using RNAi technology to control M. separata and also a useful research tool for loss-of-function study of important developmental and regulatory genes in this insect species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-017-0328-7) contains supplementary material, which is available to authorized users.

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.

Silencing of Mythimna separata chitinase genes via oral delivery of in planta-expressed RNAi effectors from a recombinant plant virus

OBJECTIVES

To evaluate transient expression of RNA interference (RNAi) effectors in Nicotiana benthamiana plants by using recombinant virus vectors and also oral delivery of the effectors for silencing of Mythimna separata endogenous gene expression.

RESULTS

Mythimna separata is a serious pest of corn production in China. To evaluate RNAi approaches to target specific RNAs in M. separate, we cloned fragments of the M. separata chitinase sequences into a virus vector in order to produce RNAi effectors during virus infection and replication in plants. When the infected plants were fed to M. separata, expression levels of target MseChi1 and MseChi2 genes were down-regulated by 76 and 45 %, respectively, and sequence-specific siRNAs were detected in recipient insects. RNAi-based silencing of chitinase genes also led to body weight decreases by 43 %.

CONCLUSION

Our research demonstrates target mRNA knockdown and suggests a promising application for controlling of M. separata by in planta expression of RNAi effectors using a recombinant plant virus.

Morphological abnormalities and cell death in the Asian citrus psyllid (Diaphorina citri) midgut associated with Candidatus Liberibacter asiaticus

Candidatus Liberibacter asiaticus (CLas) is a phloem-limited, gram-negative, fastidious bacterium that is associated with the development of citrus greening disease, also known as Huanglongbing (HLB). CLas is transmitted by the Asian citrus psyllid (ACP) Diaphorina citri, in a circulative manner. Two major barriers to transmission within the insect are the midgut and the salivary glands. We performed a thorough microscopic analysis within the insect midgut following exposure to CLas-infected citrus trees. We observed changes in nuclear architecture, including pyknosis and karyorrhexis as well as changes to the actin cytoskeleton in CLas-exposed midgut cells. Further analyses showed that the changes are likely due to the activation of programmed cell death as assessed by Annexin V staining and DNA fragmentation assays. These results suggest that exposure to CLas-infected trees induces apoptotic responses in the psyllid midgut that should be further investigated. Understanding the adaptive significance of the apoptotic response has the potential to create new approaches for controlling HLB.

Molecular and biological characterization of highly infectious transcripts from full-length cDNA clones of broad bean wilt virus 1

Broad bean wilt virus 1 (BBWV-1), genus Fabavirus, has a genome composed of two single-stranded positive-sense RNAs of ∼5.8 (RNA1) and 3.4kb (RNA2). Full-length cDNA clones of both genomic RNAs (pBenR1 and pBenR2) from BBWV-1 isolate Ben were constructed under the control of the T7 promoter. In vitro derived capped transcripts were infectious in Nicotiana benthamiana, Chenopodium quinoa and Vicia faba plants. The biological activity of viral transcripts was not affected by extra bases at the 5'-terminus introduced during in vitro transcription. Virions derived from the infectious cDNA clones displayed similar viral infectivity and accumulation, as well as symptom induction as the wild-type BBWV-1 isolate.

Identification of Novel and Conserved microRNAs in Homalodisca vitripennis, the Glassy-Winged Sharpshooter by Expression Profiling

The glassy-winged sharpshooter (GWSS) Homalodisca vitripennis (Hemiptera: Cicadellidae), is a xylem-feeding leafhopper and an important vector of the bacterium Xylella fastidiosa; the causal agent of Pierce’s disease of grapevines. MicroRNAs are a class of small RNAs that play an important role in the functional development of various organisms including insects. In H. vitripennis, we identified microRNAs using high-throughput deep sequencing of adults followed by computational and manual annotation. A total of 14 novel microRNAs that are not found in the miRBase were identified from adult H. vitripennis. Conserved microRNAs were also found in our datasets. By comparison to our previously determined transcriptome sequence of H. vitripennis, we identified the potential targets of the microRNAs in the transcriptome. This microRNA profile information not only provides a more nuanced understanding of the biological and physiological mechanisms that govern gene expression in H. vitripennis, but may also lead to the identification of novel mechanisms for biorationally designed management strategies through the use of microRNAs.

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.

Transgenic Cotton Plants Expressing Double-stranded RNAs Target HMG-CoA Reductase (HMGR) Gene Inhibits the Growth, Development and Survival of Cotton Bollworms

RNA interference (RNAi) has been developed as a powerful technique in the research of functional genomics as well as plant pest control. In this report, double-stranded RNAs (dsRNA) targeting 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) gene, which catalyze a rate-limiting enzymatic reaction in the mevalonate pathway of juvenile hormone (JH) synthesis in cotton bollworm, was expressed in cotton plants via Agrobacterium tumefaciens-mediated transformation. PCR and Sothern analysis revealed the integration of HMGR gene into cotton genome. RT-PCR and qRT-PCR confirmed the high transcription level of dsHMGR in transgenic cotton lines. The HMGR expression both in transcription and translation level was significantly downregulated in cotton bollworms (helicoverpa armigera) larvae after feeding on the leaves of HMGR transgenic plants. The transcription level of HMGR gene in larvae reared on transgenic cotton leaves was as much as 80.68% lower than that of wild type. In addition, the relative expression level of vitellogenin (Vg, crucial source of nourishment for offspring embryo development) gene was also reduced by 76.86% when the insect larvae were fed with transgenic leaves. The result of insect bioassays showed that the transgenic plant harboring dsHMGR not only inhibited net weight gain but also delayed the growth of cotton bollworm larvae. Taken together, transgenic cotton plant expressing dsRNAs successfully downregulated HMGR gene and impaired the development and survival of target insect, which provided more option for plant pest control.

Indirect Effects of One Plant Pathogen on the Transmission of a Second Pathogen and the Behavior of its Potato Psyllid Vector

Plant pathogens can influence the behavior and performance of insect herbivores. Studies of these associations typically focus on tripartite interactions between a plant host, a plant pathogen, and its insect vector. An unrelated herbivore or pathogen might influence such interactions. This study used a model system consisting of Tobacco mosaic virus (TMV), the psyllid Bactericera cockerelli Sulc, and tomatoes to investigate multipartite interactions among a pathogen, a nonvector, and a plant host, and determine whether shifts in host physiology were behind potential interactions. Additionally, the ability of TMV to affect the success of another pathogen, 'Candidatus Liberibacter solanacearum,' which is transmitted by the psyllid, was studied. In choice trials, psyllids preferred nearly fourfold noninfected plants to TMV-infected plants. No-choice bioassays demonstrated that there was no difference in psyllid development between TMV-infected and control plants; oviposition was twice as high on control plants. Following inoculation by psyllids, 'Candidatus Liberibacter solanacearum' titers were lower in TMV-infected plants than control plants. TMV-infected plants had lower levels of amino acids and sugars but little differences in phenolics and terpenoids, relative to control plants. Possibly, these changes in sugars are associated with a reduction in psyllid attractiveness in TMV-infected tomatoes resulting in decreased infection of 'Candidatus Liberibacter solanacearum.'

RNA interference against gut osmoregulatory genes in phloem-feeding insects

In planta RNAi (i.e. plants engineered to synthesize active RNAi molecules) has great potential as a strategy to control insect crop pests. This study investigated the impact of RNAi against osmoregulatory genes expressed in the gut of two phloem-feeding species, the green peach aphid Myzus persicae and the potato/tomato psyllid Bactericera cockerelli. The target genes comprising candidate gut sucrase, aquaporin and sugar transporter genes were identified by mining insect genomic and transcriptomic datasets for genes orthologous to empirically-tested osmoregulatory genes of the pea aphid Acyrthosiphon pisum. Insects feeding on plants with RNAi against the target genes exhibited elevated hemolymph osmotic pressure (a predicted effect of perturbed osmotic function) and some reduction in performance, especially offspring production in M. persicae and mortality in B. cockerelli, associated with up to 50% reduction in mean expression of the target genes. The effects were particularly pronounced for insects treated with RNAi against multiple osmoregulatory genes, i.e. combinatorial RNAi, suggesting that the partial silencing of multiple genes with related roles can yield greater functional impairment than RNAi against a single gene. These results demonstrate the potential of RNAi against osmoregulatory genes, but further advances to improve the efficacy of RNAi in phloem-feeding insects are required to achieve effective pest control.

Engineered chloroplast dsRNA silences cytochrome p450 monooxygenase, V-ATPase and chitin synthase genes in the insect gut and disrupts Helicoverpa zea larval development and pupation

In the past two decades, chloroplast genetic engineering has been advanced to achieve high-level protein accumulation but not for down-regulation of targeted genes. Therefore, in this report, lepidopteran chitin synthase (Chi), cytochrome P450 monooxygenase (P450) and V-ATPase dsRNAs were expressed via the chloroplast genome to study RNA interference (RNAi) of target genes in intended hosts. PCR and Southern blot analysis confirmed homoplasmy and site-specific integration of transgene cassettes into the chloroplast genomes. Northern blots and real-time qRT-PCR confirmed abundant processed and unprocessed dsRNA transcripts (up to 3.45 million copies of P450 dsRNAs/μg total RNA); the abundance of cleaved dsRNA was greater than the endogenous psbA transcript. Feeding of leaves expressing P450, Chi and V-ATPase dsRNA decreased transcription of the targeted gene to almost undetectable levels in the insect midgut, likely after further processing of dsRNA in their gut. Consequently, the net weight of larvae, growth and pupation rates were significantly reduced by chloroplast-derived dsRNAs. Taken together, successful expression of dsRNAs via the chloroplast genome for the first time opens the door to study RNA interference/processing within plastids. Most importantly, dsRNA expressed in chloroplasts can be utilized for gene inactivation to confer desired agronomic traits or for various biomedical applications, including down-regulation of dysfunctional genes in cancer or autoimmune disorders, after oral delivery of dsRNA bioencapsulated within plant cells.

Transient plant transformation mediated by Agrobacterium tumefaciens: Principles, methods and applications

Agrobacterium tumefaciens is widely used as a versatile tool for development of stably transformed model plants and crops. However, the development of Agrobacterium based transient plant transformation methods attracted substantial attention in recent years. Transient transformation methods offer several applications advancing stable transformations such as rapid and scalable recombinant protein production and in planta functional genomics studies. Herein, we highlight Agrobacterium and plant genetics factors affecting transfer of T-DNA from Agrobacterium into the plant cell nucleus and subsequent transient transgene expression. We also review recent methods concerning Agrobacterium mediated transient transformation of model plants and crops and outline key physical, physiological and genetic factors leading to their successful establishment. Of interest are especially Agrobacterium based reverse genetics studies in economically important crops relying on use of RNA interference (RNAi) or virus-induced gene silencing (VIGS) technology. The applications of Agrobacterium based transient plant transformation technology in biotech industry are presented in thorough detail. These involve production of recombinant proteins (plantibodies, vaccines and therapeutics) and effectoromics-assisted breeding of late blight resistance in potato. In addition, we also discuss biotechnological potential of recombinant GFP technology and present own examples of successful Agrobacterium mediated transient plant transformations.

Silencing the HaAK gene by transgenic plant-mediated RNAi impairs larval growth of Helicoverpa armigera

Insect pests have caused noticeable economic losses in agriculture, and the heavy use of insecticide to control pests not only brings the threats of insecticide resistance but also causes the great pollution to foods and the environment. Transgenic plants producing double-stranded RNA (dsRNA) directed against insect genes have been is currently developed for protection against insect pests. In this study, we used this technology to silence the arginine kinase (AK) gene of Helicoverpa armigera (HaAK), encoding a phosphotransferase that plays a critical role in cellular energy metabolism in invertebrate. Transgenic Arabidopsis plants producing HaAK dsRNA were generated by Agrobacterium-mediated transformation. The maximal mortality rate of 55% was reached when H. armigera first-instar larvae were fed with transgenic plant leaves for 3 days, which was dramatically higher than the 18% mortality recorded in the control group. Moreover, the ingestion of transgenic plants significantly retarded larval growth, and the transcript levels of HaAK were also knocked down by up to 52%. The feeding bioassays further indicated that the inhibition efficiency was correlated with the integrity and concentration of the produced HaAK dsRNA in transgenic plants. These results strongly show that the resistance to H. armigera was improved in transgenic Arabidopsis plants, suggesting that the RNAi targeting of AK has the potential for the control of insect pests.

RNAi-mediated resistance to viruses in genetically engineered plants

RNA interference (RNAi) has emerged as a leading technology in designing genetically modified crops engineered to resist viral infection. The last decades have seen the development of a large number of crops whose inherent posttranscriptional gene silencing mechanism has been exploited to target essential viral genes through the production of dsRNA that triggers an endogenous RNA-induced silencing complex (RISC), leading to gene silencing in susceptible viruses conferring them with resistance even before the onset of infection. Selection and breeding events have allowed for establishing this highly important agronomic trait in diverse crops. With improved techniques and the availability of new data on genetic diversity among several viruses, significant progress is being made in engineering plants using RNAi with the release of a number of commercially available crops. Biosafety concerns with respect to consumption of RNAi crops, while relevant, have been addressed, given the fact that experimental evidence using miRNAs associated with the crops shows that they do not pose any health risk to humans and animals.

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.

The challenge of RNAi-mediated control of hemipterans

The post-transcriptional gene silencing mechanism RNA interference (RNAi) has potential as a crop protection strategy against important pest insects. Here we focus on Hemiptera pests, comprising some of the most devastating pest organisms as aphids, whiteflies, psyllids, bedbugs and kissing bugs. At first, a state-of-the-art overview is provided of the progress in RNAi in Hemiptera, as well as on the challenges when developing new RNAi-based pest control strategies against hemipteran pests, such as the delivery of dsRNA and degradation in the insect body. We also discuss the variability in RNAi efficiency as observed between species and experiments, and the factors potentially responsible for this phenomenon.