RNAi knockdown of a salivary transcript leading to lethality in the pea aphid, Acyrthosiphon pisum

Fusion dsRNA designs incorporating multiple target sequences can enhance the aphid control capacity of an RNAi-based strategy

BACKGROUND

RNA interference (RNAi) is the sequence-dependent suppression of gene expression by double-stranded RNA (dsRNA). This is a promising strategy for the control of insect pests because dsRNA can be rationally designed to maximize efficacy and biosafety, the latter by using sequences that are found in target pests but are safe for non-target insects. However, this has yet to be optimized in aphids, destructive sap-sucking pests that also transmit plant viruses. We used the green peach aphid (Myzus persicae) as a case study to optimize the efficiency of RNAi by applying a novel fusion dsRNA design.

RESULTS

Comparative transcriptomics revealed a number of genes that are induced in feeding aphids, and eight candidate genes were chosen as RNAi targets. To improve RNAi efficiency, our fusion dsRNA design approach combined optimal gene fragments (highly conserved in several aphid species but with less homology in beneficial insects such as the predator ladybeetle Propylea japonica) from three candidate genes. We compared this RNAi-based biological control approach with conventional chemical control using imidacloprid. We found that the fusion dsRNA strategy inhibited the aphid population to a significantly greater extent than single-target RNAi and did not affect ladybeetle fitness, allowing an additive effect between RNAi and natural predation, whereas imidacloprid was harmful to aphids and ladybeetles.

CONCLUSION

Our fusion dsRNA design approach enhances the ability of RNAi to control aphids without harming natural predators. © 2024 Society of Chemical Industry.

Coupling clearing and hybridization chain reaction approaches to investigate gene expression in organs inside intact insect heads

Detecting RNA molecules within their natural environment inside intact arthropods has long been challenging, particularly in small organisms covered by a tanned and pigmented cuticle. Here, we have developed a methodology that enables high-resolution analysis of the spatial distribution of transcripts of interest without having to dissect tiny organs or tissues, thereby preserving their integrity. We have combined an in situ amplification approach based on hybridization chain reaction, which enhances the signal-to-noise ratio, and a clearing approach that allows the visualization of inner organs beneath the cuticle. We have implemented this methodology for the first time in Hemiptera, mapping two salivary aphid (Acyrthosiphon pisum) transcripts, the effector c002 and the salivary sheath protein SHP. With a multiplex approach, we could simultaneously detect different mRNAs in mounted pea aphid head-thorax samples and show that they were distributed in distinct secretory cells of salivary glands. RESEARCH HIGHLIGHTS: Combining hybridisation chain reaction and clearing allows the detection of transcripts in intact aphids heads. The transcripts of the two salivary proteins c002 and SHP are compartmentalized in distinct secretory cells of the principal glands.

In vivo interference of pea aphid endosymbiont Buchnera groEL gene by synthetic peptide nucleic acids

The unculturable nature of intracellular obligate symbionts presents a significant challenge for elucidating gene functionality, necessitating the development of gene manipulation techniques. One of the best-studied obligate symbioses is that between aphids and the bacterial endosymbiont Buchnera aphidicola. Given the extensive genome reduction observed in Buchnera, the remaining genes are crucial for understanding the host-symbiont relationship, but a lack of tools for manipulating gene function in the endosymbiont has significantly impeded the exploration of the molecular mechanisms underlying this mutualism. In this study, we introduced a novel gene manipulation technique employing synthetic single-stranded peptide nucleic acids (PNAs). We targeted the critical Buchnera groEL using specially designed antisense PNAs conjugated to an arginine-rich cell-penetrating peptide (CPP). Within 24 h of PNA administration via microinjection, we observed a significant reduction in groEL expression and Buchnera cell count. Notably, the interference of groEL led to profound morphological malformations in Buchnera, indicative of impaired cellular integrity. The gene knockdown technique developed in this study, involving the microinjection of CPP-conjugated antisense PNAs, provides a potent approach for in vivo gene manipulation of unculturable intracellular symbionts, offering valuable insights into their biology and interactions with hosts.

Identification of salivary proteins of the cowpea aphid Aphis craccivora by transcriptome and LC-MS/MS analyses

Aphid salivary proteins mediate the interaction between aphids and their host plants. Moreover, these proteins facilitate digestion, detoxification of secondary metabolites, as well as activation and suppression of plant defenses. The cowpea aphid, Aphis craccivora, is an important sucking pest of leguminous crops worldwide. Although aphid saliva plays an important role in aphid plant interactions, knowledge of the cowpea aphid salivary proteins is limited. In this study, we performed transcriptomic and LC-MS/MS analyses to identify the proteins present in the salivary glands and saliva of A. craccivora. A total of 1,08,275 assembled transcripts were identified in the salivary glands of aphids. Of all these assembled transcripts, 53,714 (49.11%) and 53,577 (49.48%) transcripts showed high similarity to known proteins in the Nr and UniProt databases, respectively. A total of 2159 proteins were predicted as secretory proteins from the salivary gland transcriptome dataset, which contain digestive enzymes, detoxification enzymes, previously known effectors and elicitors, and potential proteins whose functions have yet to be determined. The proteomic analysis of aphid saliva resulted in the identification of 171 proteins. Tissue-specific expression of selected genes using RT-PCR showed that three genes were expressed only in the salivary glands. Overall, our results provide a comprehensive repertoire of cowpea aphid salivary proteins from the salivary gland and saliva, which will be a good resource for future effector functional studies and might also be useful for sustainable aphid management.

SmCSP4 from aphid saliva stimulates salicylic acid-mediated defence responses in wheat by interacting with transcription factor TaWKRY76

Aphid salivary proteins are critical in modulating plant defence responses. Grain aphid Sitobion miscanthi is an important wheat pest worldwide. However, the molecular basis for the regulation of the plant resistance to cereal aphids remains largely unknown. Here, we show that SmCSP4, a chemosensory protein from S. miscanthi saliva, is secreted into wheat plants during aphid feeding. Delivery of SmCSP4 into wheat leaves activates salicylic acid (SA)-mediated plant defence responses and subsequently reduces aphid performance by deterring aphid feeding behaviour. In contrast, silencing SmCSP4 gene via nanocarrier-mediated RNAi significantly decreases the ability of aphids to activate SA defence pathway. Protein-protein interaction assays showed that SmCSP4 directly interacts with wheat transcriptional factor TaWRKY76 in plant nucleus. Furthermore, TaWRKY76 directly binds to the promoter of SA degradation gene Downy Mildew Resistant 6 (DMR6) and regulates its gene expression as transcriptional activator. SmCSP4 secreted by aphids reduces the transcriptional activation activity of TaWRKY76 on DMR6 gene expression, which is proposed to result in increases of SA accumulation and enhanced plant immunity. This study demonstrated that SmCSP4 acts as salivary elicitor that is involved in activating SA signalling defence pathway of wheat by interacting with TaWRKY76, which provide novel insights into aphid-cereal crops interactions and the molecular mechanism on induced plant immunity.

Inefficient uptake of small interfering RNAs is responsible for their inability to trigger RNA interference in Colorado potato beetle cells

There has been limited success in the usage of exogenous small interference RNA (siRNA) or small hairpin RNA (shRNA) to trigger RNA interference (RNAi) in insects. Instead, long double-stranded RNAs (dsRNA) are used to induce knockdown of target genes in insects. Here, we compared the potency of si/sh RNAs and dsRNA in Colorado potato beetle (CPB) cells. CPB cells showed highly efficient RNAi response to dsRNA. However, si/sh RNAs were inefficient in triggering RNAi in CPB cells. Confocal microscopy observations of Cy3 labeled-si/sh RNA cellular uptake revealed reduced si/sh RNA uptake compared to dsRNA. si/sh RNAs were stable in the conditioned media of CPB cells. Although in a small amount, when internalized by CPB cells, the si/sh RNAs were processed by the Dicer enzyme. Lipid-mediated transfection and chimeric dsRNA approaches were used to improve the delivery of si/sh RNAs. Our results suggest that the uptake of si/sh RNAs is inefficient in CPB cells, resulting in ineffective RNAi response. However, with the help of effective delivery methods, si/sh RNA could be a useful option for developing target-specific RNAi-mediated biopesticides.

Double-stranded RNA mediated knockdown of sucrase gene induced mortality and reduced offspring production in Aphis gossypii

The importance of gut sucrase in maintaining osmotic equilibrium and utilizing phloem contents as a carbon source has been widely investigated and proven in sap-sucking insects. In the present study, silencing of Aphis gossypii sucrase1 (Agsuc1) was carried out by double-stranded RNA (dsRNA), which would be lethal to it due to disruption of osmotic balance. The dsRNA corresponding to Agsuc1 was synthesized by two methods, i.e., in vitro synthesis using T7/SP6 RNA polymerase and in vivo synthesis by bacterial expression, i.e., Escherichia coli strain HT115 transformed with the L4440 vector system. Oral delivery of double-stranded Agsuc1 synthesized in vitro (dsAgsuc1) and in vivo (HT115Agsuc1) induced around 50% mortality in nymphs of A. gossypii. Moreover, the number of offspring produced by the survived aphids decreased by 39-43%. Parthenogenetic reproduction of the aphids is the critical factor for their fast population build-up, leading to yield losses of economic significance. Thus, the present study demonstrated that the silencing of the Agsuc1 gene reduced the aphid population by killing it and inhibited the population buildup by reducing the number of offspring produced by the survived aphids, likely to result in a significant reduction in crop damage. The production of dsRNA by bacterial expression is a cost-effective method. It has the potential to be used as a biopesticide. The sucrase gene is an excellent putative target gene for RNAi against A. gossypii. It could be used to develop a transgenic plant that produces dsAgsuc1 to keep A. gossypii populations below the economic threshold level.

Transgenic East African Highland Banana Plants Are Protected against Radopholus similis through Host-Delivered RNAi

The burrowing nematode Radopholus similis is considered a major problem of intensive banana cultivation. It can cause extensive root damage resulting in the toppling disease of banana, which means that plants fall to the ground. Soaking R. similis in double-stranded (ds) RNA of the nematode genes Rps13, chitin synthase (Chs-2), Unc-87, Pat-10 or beta-1,4-endoglucanase (Eng1a) suppressed reproduction on carrot discs, from 2.8-fold (Chs-2) to 7-fold (Rps13). The East African Highland Banana cultivar Nakitembe was then transformed with constructs for expression of dsRNA against the same genes, and for each construct, 30 independent transformants were tested with nematode infection. Four months after transfer from in vitro culture to the greenhouse, the banana plants were transferred to a screenhouse and inoculated with 2000 nematodes per plant, and thirteen weeks later, they were analyzed for several parameters including plant growth, root necrosis and final nematode population. Plants with dsRNA constructs against the nematode genes were on average showing lower nematode multiplication and root damage than the nontransformed controls or the banana plants expressing dsRNA against the nonendogenous gene. In conclusion, RNAi seems to efficiently protect banana against damage caused by R. similis, opening perspectives to control this pest.

Target gene selection for RNAi-based biopesticides against the hawthorn spider mite, Amphitetranychus viennensis (Acari: Tetranychidae)

BACKGROUND

Recently, RNA interference (RNAi)-based biopesticide, a species-specific pest control alternative, has been deregulated and commercialized in the US and Canada. The hawthorn spider mite, Amphitetranychus viennensis Zacher, is a major pest for rosaceous plants, which has been controlled primarily by synthetic pesticides. To address the emerging resistance issues in A. viennensis, we initiated a project to develop RNAi-based biopesticides.

RESULTS

In this study, we (i) developed a dietary RNAi system for A. viennensis using leaf disc, (ii) assessed the suitability of multiple control genes to distinguish sequence-specific silencing from non-specific effects within this RNAi system, and (iii) screened for the target gene candidates. As a result, β-Glucuronidase (GUS), an enzyme derived from E. coli and a broadly used reporter for plants is the appropriate control for A. viennensis RNAi, while green fluorescent protein (GFP), is not suitable due to its significantly higher mortality than the other controls. For target gene screening, suppression was confirmed for all the candidates, including two housekeeping genes (Vacuolar-type H + -ATPase subunit A (V-ATPase A) and Glyceraldehyde 3-phosphate dehydrogenase, (GAPDH)), and three genes associated with development (ATP-dependent RNA Helicase DDX3Y (Belle), CREB-binding protein (CBP), and Farnesoic acid O-methyltransferase (FaMet)). Knocking down of V-ATPase A resulted in the highest mortality (~ 90%) and reduced fecundity (over 90%) than other candidates. As for the genes associated with development, suppression of Belle and CBP, led to approximately 65% mortality, as well as 86% and 40% reduction in fecundity, respectively. Silencing of FaMet, however, had negligible biological impacts on A. viennensis.

CONCLUSION

The combined efforts not only establish an effective dsRNA delivery method, but also provide potential target genes for RNAi-based biopesticides against A. viennensis, a devastating invasive pest for fruit trees and woody ornamental plants throughout Asia and Europe. © 2023 Society of Chemical Industry.

RNA Interference Past and Future Applications in Plants

Antisense RNA was observed to elicit plant disease resistance and post-translational gene silencing (PTGS). The universal mechanism of RNA interference (RNAi) was shown to be induced by double-stranded RNA (dsRNA), an intermediate produced during virus replication. Plant viruses with a single-stranded positive-sense RNA genome have been instrumental in the discovery and characterization of systemic RNA silencing and suppression. An increasing number of applications for RNA silencing have emerged involving the exogenous application of dsRNA through spray-induced gene silencing (SIGS) that provides specificity and environmentally friendly options for crop protection and improvement.

Horizontally transferred genes as RNA interference targets for aphid and whitefly control

RNA interference (RNAi)-based technologies are starting to be commercialized as a new approach for agricultural pest control. Horizontally transferred genes (HTGs), which have been transferred into insect genomes from viruses, bacteria, fungi or plants, are attractive targets for RNAi-mediated pest control. HTGs are often unique to a specific insect family or even genus, making it unlikely that RNAi constructs targeting such genes will have negative effects on ladybugs, lacewings and other beneficial predatory insect species. In this study, we sequenced the genome of a red, tobacco-adapted isolate of Myzus persicae (green peach aphid) and bioinformatically identified 30 HTGs. We then used plant-mediated virus-induced gene silencing (VIGS) to show that several HTGs of bacterial and plant origin are important for aphid growth and/or survival. Silencing the expression of fungal-origin HTGs did not affect aphid survivorship but decreased aphid reproduction. Importantly, although there was uptake of plant-expressed RNA by Coccinella septempunctata (seven-spotted ladybugs) via the aphids that they consumed, we did not observe negative effects on ladybugs from aphid-targeted VIGS constructs. To demonstrate that this approach is more broadly applicable, we also targeted five Bemisia tabaci (whitefly) HTGs using VIGS and demonstrated that knockdown of some of these genes affected whitefly survival. As functional HTGs have been identified in the genomes of numerous pest species, we propose that these HTGs should be explored further as efficient and safe targets for control of insect pests using plant-mediated RNA interference.

Molecular Mechanisms Underlying Host Plant Specificity in Aphids

Aphids are serious pests of agricultural and ornamental plants and important model systems for hemipteran-plant interactions. The long evolutionary history of aphids with their host plants has resulted in a variety of systems that provide insight into the different adaptation strategies of aphids to plants and vice versa. In the past, various plant-aphid interactions have been documented, but lack of functional tools has limited molecular studies on the mechanisms of plant-aphid interactions. Recent technological advances have begun to reveal plant-aphid interactions at the molecular level and to increase our knowledge of the mechanisms of aphid adaptation or specialization to different host plants. In this article, we compile and analyze available information on plant-aphid interactions, discuss the limitations of current knowledge, and argue for new research directions. We advocate for more work that takes advantage of natural systems and recently established molecular techniques to obtain a comprehensive view of plant-aphid interaction mechanisms.

Silencing an aphid-specific gene SmDSR33 for aphid control through plant-mediated RNAi in wheat

Grain aphid (Sitobion miscanthi) is one of the most dominant and devastating insect pests in wheat, which causes substantial losses to wheat production each year. Engineering transgenic plants expressing double strand RNA (dsRNA) targeting an insect-specific gene has been demonstrated to provide an alternative environmentally friendly strategy for aphid management through plant-mediated RNA interference (RNAi). Here we identified and characterized a novel potential RNAi target gene (SmDSR33) which was a gene encoding a putative salivary protein. We then generated stable transgenic wheat lines expressing dsRNA for targeted silencing of SmDSR33 in grain aphids through plant-mediated RNAi. After feeding on transgenic wheat plants expressing SmDSR33-dsRNA, the attenuated expression levels of SmDSR33 in aphids were observed when compared to aphids feeding on wild-type plants. The decreased SmDSR33 expression levels thus resulted in significantly reduced fecundity and survival, and decreased reproduction of aphids. We also observed altered aphid feeding behaviors such as longer duration of intercellular stylet pathway and shorter duration of passive ingestion in electroneurography assays. Furthermore, both the surviving aphids and their offspring exhibited decreased survival rates and fecundity, indicating that the silencing effect could be persistent and transgenerational in grain aphids. The results demonstrated that SmDSR33 can be selected as an effective RNAi target for wheat aphid control. Silencing of an essential salivary protein gene involved in ingestion through plant-mediated RNAi could be exploited as an effective strategy for aphid control in wheat.

Production of Sitobion avenae-resistant Triticum aestivum cvs using laccase as RNAi target and its systemic movement in wheat post dsRNA spray

Among the wheat biotic stresses, Sitobion avenae is one of the main factors devastating the wheat yield per hectare. The study's objective was to find out the laccase (lac) efficacy; as a potential RNAi target against grain aphids. The Sitobion avenae lac (Salac) was confirmed by Reverse Transcriptase-PCR. Gene was sequenced and accession number "ON703252" was allotted by GenBank. ERNAi tool was used to design 143 siRNA and one dsRNA target. 69% mortality and 61% reduction in lac expression were observed 8D-post lac DsRNA feeding. Phylogenetic analysis displayed the homology of grain aphid lac gene with peach potato, pea, and Russian wheat aphids. While Salac protein was found similar to the Russian grain, soybean, pea, and cedar bark aphid lac protein multi-copper oxidase. The dsRNAlac spray-induced silencing shows systematic translocation from leaf to root; with maximum lac expression found in the root, followed by stem and leaf 9-13D post-spray; comparison to control. RNAi-GG provides the Golden Gate cloning strategy with a single restriction ligation reaction used to achieve lac silencing. Agrobacterium tumefaciens mediated in planta and in-vitro transformation was used in the study. In vitro transformation, Galaxy 2012 yielded a maximum transformation efficiency (1.5%), followed by Anaj 2017 (0.8%), and Punjab (0.2%). In planta transformation provides better transformation efficiencies with a maximum in Galaxy 2012 (16%), and a minimum for Punjab (5%). Maximum transformation efficiency was achieved for all cultivars with 250 μM acetosyringone and 3h co-cultivation. Galaxy 2012 exhibited maximum transformation efficiency, and aphid mortality post-feeding transgenic wheat.

Addressing the challenges of symbiont-mediated RNAi in aphids

Because aphids are global agricultural pests and models for bacterial endosymbiosis, there is a need for reliable methods to study and control their gene function. However, current methods available for aphid gene knockout and knockdown of gene expression are often unreliable and time consuming. Techniques like CRISPR-Cas genome editing can take several months to achieve a single gene knockout because they rely on aphids going through a cycle of sexual reproduction, and aphids often lack strong, consistent levels of knockdown when fed or injected with molecules that induce an RNA interference (RNAi) response. In the hopes of addressing these challenges, we attempted to adapt a new method called symbiont-mediated RNAi (smRNAi) for use in aphids. smRNAi involves engineering a bacterial symbiont of the insect to continuously supply double-stranded RNA (dsRNA) inside the insect body. This approach has been successful in thrips, kissing bugs, and honeybees. We engineered the laboratory Escherichia coli strain HT115 and the native aphid symbiont Serratia symbiotica CWBI-2.3^T to produce dsRNA inside the gut of the pea aphid (Acyrthosiphon pisum) targeting salivary effector protein (C002) or ecdysone receptor genes. For C002 assays, we also tested co-knockdown with an aphid nuclease (Nuc1) to reduce RNA degradation. However, we found that smRNAi was not a reliable method for aphid gene knockdown under our conditions. We were unable to consistently achieve the expected phenotypic changes with either target. However, we did see indications that elements of the RNAi pathway were modestly upregulated, and expression of some targeted genes appeared to be somewhat reduced in some trials. We conclude with a discussion of the possible avenues through which smRNAi, and aphid RNAi in general, could be improved in the future.

Role of Acrostyle Cuticular Proteins in the Retention of an Aphid Salivary Effector

To avoid the activation of plant defenses and ensure sustained feeding, aphids are assumed to use their mouthparts to deliver effectors into plant cells. A recent study has shown that effectors detected near feeding sites are differentially distributed in plant tissues. However, the precise process of effector delivery into specific plant compartments is unknown. The acrostyle, a cuticular organ located at the tip of maxillary stylets that transiently binds plant viruses via its stylin proteins, may participate in this specific delivery process. Here, we demonstrate that Mp10, a saliva effector released into the plant cytoplasm during aphid probing, binds to the acrostyles of Acyrthosiphon pisum and Myzus persicae. The effector probably interacts with Stylin-03 as a lowered Mp10-binding to the acrostyle was observed upon RNAi-mediated reduction in Stylin-03 production. In addition, Stylin-03 and Stylin-01 RNAi aphids exhibited changes in their feeding behavior as evidenced by electrical penetration graph experiments showing longer aphid probing behaviors associated with watery saliva release into the cytoplasm of plant cells. Taken together, these data demonstrate that the acrostyle also has effector binding capacity and supports its role in the delivery of aphid effectors into plant cells.

Assessment of a zinc finger protein gene (MPZC3H10) as potential RNAi target for green peach aphid Myzus persicae control

BACKGROUND

RNA interference (RNAi) has potential application in pest control, and selection of the specific target gene is one of the key steps in RNAi. As an important effector, the zinc finger protein (ZFP) gene has high similarity among aphid species, and may have potential use in an RNAi-based pest control strategy. This study assessed the control efficiency of an RNAi target, MPZC3H10, a CCCH-type ZFP gene, against green peach aphid.

RESULTS

ZC3H10 amino acid sequence similarity is more than 97.71% among the five tested aphid species: Myzus persicae, Aphis citricidus, Acyrthosiphon pisum, Diuraphis noxia and Rhopalosiphum maidis. However, no homologous sequence was found in the transcriptome of their ladybeetle predator, Propylaea japonica. Spatial expression patterns revealed that MPZC3H10 showed high expression in the muscle and fat body of M. persicae. The RNAi bioassay revealed that silencing of MPZC3H10 resulted in high mortality (53.33%) in M. persicae. By contrast, there were no observed negative effects on the growth and development of P. japonica when fed on aphids treated with double-stranded RNA (dsRNA) or injected with a "high dose" of dsRNA.

CONCLUSION

Targeting MPZC3H10 showed promising efficiency for green peach aphid control via artificially designed dsRNA, and was safe for the predatory ladybeetle. © 2022 Society of Chemical Industry.

The salivary effector protein Sg2204 in the greenbug Schizaphis graminum suppresses wheat defence and is essential for enabling aphid feeding on host plants

Aphids secrete diverse repertoires of salivary effectors into host plant cells to promote infestation by modulating plant defence. The greenbug Schizaphis graminum is an important cereal aphid worldwide. However, the secreted effectors of S. graminum are still uncharacterized. Here, 76 salivary proteins were identified from the watery saliva of S. graminum using transcriptome and proteome analyses. Among them, a putative salivary effector Sg2204 was significantly up-regulated during aphid feeding stages, and transient overexpression of Sg2204 in Nicotiana benthamiana inhibited cell death induced by BAX or INF1. Delivering Sg2204 into wheat via the type III secretion system of Pseudomonas fluorescens EtAnH suppressed pattern-triggered immunity (PTI)-associated callose deposition. The transcript levels of jasmonic acid (JA)- and salicylic acid (SA)-associated defence genes of wheat were significantly down-regulated, and the contents of both JA and SA were also significantly decreased after delivery of Sg2204 into wheat leaves. Additionally, feeding on wheat expressing Sg2204 significantly increased the weight and fecundity of S. graminum and promoted aphid phloem feeding. Sg2204 was efficiently silenced via spray-based application of the nanocarrier-mediated transdermal dsRNA delivery system. Moreover, Sg2204-silenced aphids induced a stronger wheat defence response and resulted in negative impacts on aphid feeding behaviour, survival and fecundity. Silencing of Sg2204 homologues from four aphid species using nanocarrier-delivered dsRNA also significantly reduced aphid performance on host plants. Thus, our study characterized the salivary effector Sg2204 of S. graminum involved in promoting host susceptibility by suppressing wheat defence, which can also be regarded as a promising RNAi target for aphid control.

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.

Salivary Effector Sm9723 of Grain Aphid Sitobion miscanthi Suppresses Plant Defense and Is Essential for Aphid Survival on Wheat

Aphid salivary effectors play important roles in modulating plant defense responses. The grain aphid Sitobion miscanthi is one of the most economically important cereal aphids worldwide. However, little information is available on the identification and functional analysis of salivary effectors of S. miscanthi. In this study, a candidate salivary effector Sm9723 was identified, which was specifically expressed in aphid salivary glands and highly induced during the aphid feeding phase. Transient overexpression of Sm9723 in Nicotiana benthamiana suppressed BAX and INF1-induced cell death. Further, Sm9723 overexpression inhibited N. benthamiana defense responses by reducing pattern-triggered immunity associated callose deposition and expression levels of jasmonic and salicylic acid-associated defense genes. In addition, the salivary effector Sm9723 of S. miscanthi was effectively silenced through nanocarrier-mediated dsRNA delivery system. After silencing Sm9723, fecundity and survival of S. miscanthi decreased significantly, and the aphid feeding behavior was also negatively affected. These results suggest salivary effector Sm9723 is involved in suppressing plant immunity and is essential in enabling aphid virulence, which could be applied as potential target gene for RNAi-mediated pest control of S. miscanthi.

Positional effects of double-stranded RNAs targeting β-Actin gene affect RNA interference efficiency in Colorado potato beetle

Pesticide resistance in pests drives the development of RNA interference (RNAi)-based technology as a novel approach for pest control. To investigate the effects of the positional dependency of double-stranded RNAs (dsRNAs), we newly designed four different 200 bp dsRNAs targeting Colorado potato beetle (CPB) β-Actin gene, termed as dsACT200-1 to dsACT200-4, to compare their insecticidal activity to CPB larvae together with our previously used 200 bp and 700 bp dsRNAs (dsACT200 and dsACT700), respectively (He et al., 2020a). Each of dsRNAs harbors different numbers of expected siRNAs predicted by sequence-based prediction platform, dsACT200 and dsACT200-2 have a relatively higher number of siRNA than other 200 bps dsRNAs. When CPB larvae were fed with in vitro synthesized dsRNA-painted potato leaves, all the tested dsRNAs showed significant effects to protect against CPB larvae. Combined with the survival rate of CPB larvae, β-Actin gene expression level and the surviving CPB larvae weight, various positional dsRNAs from the same allele showed different plant protection activity against CPB larvae and partially correlated with the predicted siRNA numbers and distribution on the target sequence. This study suggests the specific allelic locus for rational dsRNA design triggering RNAi efficiency for target gene silencing is an essential factor in enhancing the insecticidal activity.

Destruxin A inhibits scavenger receptor B mediated melanization in Aphis citricola

BACKGROUND

Destruxin A (DA) is a mycotoxin secreted by entomogenous fungi, such as Metarhizium anisopliae, which has broad-spectrum insecticidal activity. Insect innate immunity provides resistance against the invasion of entomopathogenic fungi. Previous studies have shown that DA could inhibit the immune response, however, the suppressive mechanism of DA on prophenoloxidase system is still unknown.

RESULTS

Based on the transcriptome of Aphis citricola, we screened the scavenger receptor class B(AcSR-B)and identified that it significantly responds to DA. Spatio-temporal expression analysis showed that AcSR-B is highly expressed in adult stage and is mainly distributed in the abdominal region. We further revealed that both M. anisopliae and Escherichia coli could suppress the expression of AcSR-B at 24 h, and that the expressed recombinant protein rAcSR-B possessed agglutination activity to M. anisopliae and E. coli. DA could suppress the protein expression of AcSR-B. In addition, RNA interference of AcSR-B caused death of A. citricola in a dose-dependent manner, and RNA interference of AcSR-B increased mortality in A. citricola under the same lethal concentration of DA. The inhibiting effect of AcSR-B silencing was similar with the DA treatment upon phenol oxidase (PO) activity of A. citricola hemolymph. DA could not decrease PO activity further after AcSR-B silencing.

CONCLUSION

Destruxin A inhibits melanization by suppressing AcSR-B in A. citricola. Our findings are helpful in understanding the underlying molecular mechanism of the DA suppressing immune system, and uncover a potential molecular target for double-stranded RNA (dsRNA) insecticides.

Differential gene expression in aphids following virus acquisition from plants or from an artificial medium

BACKGROUND

Poleroviruses, such as turnip yellows virus (TuYV), are plant viruses strictly transmitted by aphids in a persistent and circulative manner. Acquisition of either virus particles or plant material altered by virus infection is expected to induce gene expression deregulation in aphids which may ultimately alter their behavior.

RESULTS

By conducting an RNA-Seq analysis on viruliferous aphids fed either on TuYV-infected plants or on an artificial medium containing purified virus particles, we identified several hundreds of genes deregulated in Myzus persicae, despite non-replication of the virus in the vector. Only a few genes linked to receptor activities and/or vesicular transport were common between the two modes of acquisition with, however, a low level of deregulation. Behavioral studies on aphids after virus acquisition showed that M. persicae locomotion behavior was affected by feeding on TuYV-infected plants, but not by feeding on the artificial medium containing the purified virus particles. Consistent with this, genes potentially involved in aphid behavior were deregulated in aphids fed on infected plants, but not on the artificial medium.

CONCLUSIONS

These data show that TuYV particles acquisition alone is associated with a moderate deregulation of a few genes, while higher gene deregulation is associated with aphid ingestion of phloem from TuYV-infected plants. Our data are also in favor of a major role of infected plant components on aphid behavior.

Glucosamine-6-phosphate N-acetyltransferase gene silencing by parental RNA interference in rice leaf folder, Cnaphalocrocis medinalis (Lepidoptera: Pyralidae)

Parental RNAi (pRNAi) is a response of RNA interference in which treated insect pests progenies showed a gene silencing phenotypes. pRNAi of CmGNA gene has been studied in Cnaphalocrocis medinalis via injection. Our results showed significant reduction in ovulation per female that was 26% and 35.26% in G1 and G2 generations, respectively. Significant reduction of hatched eggs per female were observed 23.53% and 45.26% as compared to control in G1–G2 generations, respectively. We also observed the significant variation in the sex ratio between female (40% and 53%) in G1–G2 generations, and in male (65%) in G1 generation as compared to control. Our results also demonstrated the significant larval mortality (63% and 55%) and pupal mortality (55% and 41%), and significant reduction of mRNA expression level in G1 and G2 generations. Our findings have confirmed that effectiveness of pRNAi induced silencing on the CmGNA target gene in G1–G2 generations of C. medinalis. These results suggested the potential role of pRNAi in insect pest resistance management strategies.

Host-mediated attenuation of gut sucrase in mustard aphid Lipaphis erysimi impaired its parthenogenetic reproduction on Indian mustard Brassica juncea

BACKGROUND

The nefarious hemipteran mustard aphid (Lipaphis erysimi) inflicts colossal yield losses in Brassica crops including Indian mustard (Brassica juncea). Lack of an accessible resistance source has been the primary impediment in breeding varietal resistance against aphids. In recent years, in planta RNAi-mediated resistance has been demonstrated in model plants as a potential tool for protection against insect pests. However, translational application in crop species is imperative for critical assessment of this technology in breeding effective resistance.

RESULTS

The essential role of sucrase 1 (SUC1) in mitigating osmotic pressure imposed by sucrose-rich phloem sap inside the insect gut is corroborated by its expression pattern in L. erysimi. Transgenic lines of Indian mustard were developed expressing SUC1 hairpin RNA for its host-mediated delivery into the infesting aphids. The expression of the dsRNA encoding cassette, and generation of siRNA molecules in transgenic B. juncea lines were verified by quantitative reverse transcription (RT)-PCR, stem-loop RT-PCR and Northern hybridization. Rearing of L. erysimi on the transgenic lines resulted in 22-40% reduction in aphid fecundity. The observed retardation in aphid reproduction was coherent with the detection of SUC1-specific siRNA molecules and attenuation of the SUC1 transcript level in L. erysimi fed on the transgenic lines.

CONCLUSION

Augmenting varietal resistance can substantially reduce usage of toxic agrochemicals in crop protection. This attempt was the first successful demonstration of host-mediated RNAi of an aphid gene in any Brassica crop. It paves the way for more rigorous attempt of engineering RNAi-based resistance against aphids in Brassica crops. © 2021 Society of Chemical Industry.

Double-Strand RNA (dsRNA) Delivery Methods in Insects: Diaphorina citri

RNAi is a gene-silencing mechanism conserved in the vast majority of eukaryotes. It is widely used to study gene function in animals due to the ease of eliciting gene knockdown. Beyond research applications, RNAi technology based on exogenous dsRNA is a promising candidate for next generation insect pest control. An advantage of using RNAi is that design of dsRNA essentially requires only the sequence of the target gene. The greatest challenge, however, is dsRNA delivery for large-scale insect control. Delivery methods that have widely been used are oral, injection, or via soaking. Unfortunately, each insect presents its own challenges owing to the differences in the presence of dsRNA degrading enzymes, cellular uptake efficiency, expression of core RNAi machinery, the nature of the target gene, the concentration and persistence of the dsRNA, as well as the particular way of feeding of each insect, which together cause variations in the efficiency of RNAi. In this chapter, a protocol for the synthetic production of dsRNA is described along with three methods for delivery that have been successful in one of the more problematic insects, Diaphorina citri.

OUP accepted manuscript

Homology of highly divergent genes often cannot be determined from sequence similarity alone. For example, we recently identified in the aphid Hormaphis cornu a family of rapidly evolving bicycle genes, which encode novel proteins implicated as plant gall effectors, and sequence similarity search methods yielded few putative bicycle homologs in other species. Coding sequence-independent features of genes, such as intron-exon boundaries, often evolve more slowly than coding sequences, however, and can provide complementary evidence for homology. We found that a linear logistic regression classifier using only structural features of bicycle genes identified many putative bicycle homologs in other species. Independent evidence from sequence features and intron locations supported homology assignments. To test the potential roles of bicycle genes in other aphids, we sequenced the genome of a second gall-forming aphid, Tetraneura nigriabdominalis and found that many bicycle genes are strongly expressed in the salivary glands of the gall forming foundress. In addition, bicycle genes are strongly overexpressed in the salivary glands of a non-gall forming aphid, Acyrthosiphon pisum, and in the non-gall forming generations of H. cornu. These observations suggest that Bicycle proteins may be used by multiple aphid species to manipulate plants in diverse ways. Incorporation of gene structural features into sequence search algorithms may aid identification of deeply divergent homologs, especially of rapidly evolving genes involved in host-parasite interactions.

Establishing RNAi for basic research and pest control and identification of the most efficient target genes for pest control: a brief guide

RNA interference (RNAi) has emerged as a powerful tool for knocking-down gene function in diverse taxa including arthropods for both basic biological research and application in pest control. The conservation of the RNAi mechanism in eukaryotes suggested that it should-in principle-be applicable to most arthropods. However, practical hurdles have been limiting the application in many taxa. For instance, species differ considerably with respect to efficiency of dsRNA uptake from the hemolymph or the gut. Here, we review some of the most frequently encountered technical obstacles when establishing RNAi and suggest a robust procedure for establishing this technique in insect species with special reference to pests. Finally, we present an approach to identify the most effective target genes for the potential control of agricultural and public health pests by RNAi.

RNAi Crop Protection Advances

RNAi technology is a versatile, effective, safe, and eco-friendly alternative for crop protection. There is plenty of evidence of its use through host-induced gene silencing (HIGS) and emerging evidence that spray-induced gene silencing (SIGS) techniques can work as well to control viruses, bacteria, fungi, insects, and nematodes. For SIGS, its most significant challenge is achieving stability and avoiding premature degradation of RNAi in the environment or during its absorption by the target organism. One alternative is encapsulation in liposomes, virus-like particles, polyplex nanoparticles, and bioclay, which can be obtained through the recombinant production of RNAi in vectors, transgenesis, and micro/nanoencapsulation. The materials must be safe, biodegradable, and stable in multiple chemical environments, favoring the controlled release of RNAi. Most of the current research on encapsulated RNAi focuses primarily on oral delivery to control insects by silencing essential genes. The regulation of RNAi technology focuses on risk assessment using different approaches; however, this technology has positive economic, environmental, and human health implications for its use in agriculture. The emergence of alternatives combining RNAi gene silencing with the induction of resistance in crops by elicitation and metabolic control is expected, as well as multiple silencing and biotechnological optimization of its large-scale production.

Biotechnological interventions for the sustainable management of a global pest, whitefly (Bemisia tabaci)

Whiteflies (Bemisia tabaci) are polyphagous invasive hemipteran insects that cause serious losses of important crops by directly feeding on phloem sap and transmitting pathogenic viruses. These insects have emerged as a major threat to global agriculture and food security. Chemically synthesized insecticides are currently the only option to control whiteflies, but the ability of whiteflies to evolve resistance against insecticides has made the management of these insects very difficult. Natural host-plant resistance against whiteflies identified in some crop plants has not been exploited to a great extent. Genetic engineering approaches, such as transgenics and RNA interference (RNAi), are potentially useful for the control of whiteflies. Transgenic plants harboring insecticidal toxins/lectins developed via nuclear or chloroplast transformation are a promising vehicle for whitefly control. Double-stranded RNAs (dsRNAs) of several insect genes, delivered either through microinjection into the insect body cavity or orally via an artificial diet and transiently or stably expressed in transgenic plants, have controlled whiteflies in model plants and in some crops at the laboratory level, but not at the field level. In this review, we highlight the merits and demerits of each delivery method along with strategies for sustained delivery of dsRNAs via fungal entomopathogen/endosymbiont or nontransgenic RNAi approaches, foliar sprays, root absorption or nanocarriers as well as the factors affecting efficient RNAi and their biosafety issues. Genome sequencing and transcriptome studies of whitefly species are facilitating the selection of appropriate genes for RNAi and gene-editing technology for the efficient and resilient management of whiteflies and their transmitted viruses.

Trehalose and glucose levels regulate feeding behavior of the phloem-feeding insect, the pea aphid Acyrthosiphon pisum Harris

Trehalose serves multifarious roles in growth and development of insects. In this study, we demonstrated that the high trehalose diet increased the glucose content, and high glucose diet increased the glucose content but decreased the trehalose content of Acyrthosiphon pisum. RNA interference (RNAi) of trehalose-6-phosphate synthase gene (ApTPS) decreased while RNAi of trehalase gene (ApTRE) increased the trehalose and glucose contents. In the electrical penetration graph experiment, RNAi of ApTPS increased the percentage of E2 waveform and decreased the percentage of F and G waveforms. The high trehalose and glucose diets increased the percentage of E2 waveform of A. pisum red biotype. The correlation between feeding behavior and sugar contents indicated that the percentage of E1 and E2 waveforms were increased but np, C, F and G waveforms were decreased in low trehalose and glucose contents. The percentage of np, E1 and E2 waveforms were reduced but C, F and G waveforms were elevated in high trehalose and glucose contents. The results suggest that the A. pisum with high trehalose and glucose contents spent less feeding time during non-probing phase and phloem feeding phase, but had an increased feeding time during probing phase, stylet work phase and xylem feeding phase.

Lipophorin receptor regulates the cuticular hydrocarbon accumulation and adult fecundity of the pea aphid Acyrthosiphon pisum

Cuticular hydrocarbons form a barrier that protects terrestrial insects from water loss via the epicuticle. Lipophorin loads and transports lipids, including hydrocarbons, from one tissue to another. In some insects, the lipophorin receptor (LpR), which binds to lipophorin and accepts its lipid cargo, is essential for female fecundity because it mediates the incorporation of lipophorin by developing oocytes. However, it is unclear whether LpR is involved in the accumulation of cuticular hydrocarbons and its precise role in aphid reproduction remains unknown. We herein present the results of our molecular characterization, phylogenetic analysis, and functional annotation of the pea aphid (Acyrthosiphon pisum) LpR gene (ApLpR). This gene was transcribed throughout the A. pisum life cycle, but especially during the embryonic stage and in the abdominal cuticle. Furthermore, we optimized the RHA interference (RNAi) parameters by determining the ideal dose and duration for gene silencing in the pea aphid. We observed that the RNAi-based ApLpR suppression significantly decreased the internal and cuticular hydrocarbon contents as well as adult fecundity. Additionally, a deficiency in cuticular hydrocarbons increased the susceptibility of aphids to desiccation stress, with decreased survival rates under simulated drought conditions. Moreover, ApLpR expression levels significantly increased in response to the desiccation treatment. These results confirm that ApLpR is involved in transporting hydrocarbons and protecting aphids from desiccation stress. Furthermore, this gene is vital for aphid reproduction. Therefore, the ApLpR gene of A. pisum may be a novel RNAi target relevant for insect pest management.

Improvement of host-induced gene silencing efficiency via polycistronic-tRNA-amiR expression for multiple target genes and characterization of RNAi mechanism in Mythimna separata

Host-induced gene silencing (HIGS) emerged as a new strategy for pest control. However, RNAi efficiency is reported to be low in Lepidoptera, which are composed of many important crop pests. To address this, we generated transgenic plants to develop HIGS effects in a maize pest, Mythimna separata (Lepidoptera, Noctuidae), by targeting chitinase encoding genes. More importantly, we developed an artificial microRNA (amiR) based PTA (polycistronic-tRNA-amiR) system for silencing multiple target genes. Compared with hpRNA (hairpin RNA), transgenic expression of a PTA cassette including an amiR for the gut-specific dsRNA nuclease gene MsREase, resulted in improved knockdown efficiency and caused more pronounced developmental abnormalities in recipient insects. When target gene siRNAs were analysed after HIGS and direct dsRNA/siRNA feeding, common features such as sense polarity and siRNA hotspot regions were observed, however, they differed in siRNA transitivity and major 20-24nt siRNA species. Core RNAi genes were identified in M. separata, and biochemical activities of MsAGO2, MsSID1 and MsDcr2 were confirmed by EMSA (electrophoretic mobility shift assay) and dsRNA cleavage assays, respectively. Taken together, we provide compelling evidence for the existence of the RNAi mechanism in M. separata by analysis of both siRNA signatures and RNAi machinery components, and the PTA system could potentially be useful for future RNAi control of lepidopteran pests.

Accelerated delivery of dsRNA in lepidopteran midgut cells by a Galanthus nivalis lectin (GNA)-dsRNA-binding domain fusion protein

Lepidopteran insects are highly refractory to oral RNA interference (RNAi). Degradation, impaired cellular uptake and intracellular transport of double-stranded RNA (dsRNA) are considered the major factors responsible for the reduced RNAi efficiency in these insects. In this study, the potential of lectins to improve dsRNA delivery and RNAi efficacy was evaluated. First, a fusion protein consisting of the Galanthus nivalis agglutinin (GNA) and a dsRNA binding domain was developed, further referred to as GNA:dsRBD (GNAF). Then, its ability to increase dsRNA uptake and transfection efficiency in lepidopteran midgut cells was evaluated, as well as its ability to protect and promote the RNAi response in the beet armyworm Spodoptera exigua. Confocal microscopy analysis showed that GNAF-complexed dsRNA was internalized faster in Choristoneura fumiferana midgut CF1 cells (1 min) compared to naked dsRNA (>1 h). The faster uptake was also correlated with an increased RNAi efficiency in these CF1 cells. In vivo feeding bioassays with GNAF-complexed dsRNA led to an increased mortality in S. exigua compared to the controls. By targeting the essential gene V-ATPase A, we observed that the mortality increased to 48% in the GNAF-dsRNA treatment compared to only 8.3% and 6.6% in the control treatments with the naked dsRNA and the GNAF, respectively.

Involvement of clathrin-dependent endocytosis in cellular dsRNA uptake in aphids

RNAi is an essential technology for studying gene function in eukaryotes, and is also considered to be a potential strategy for pest control. However, the mechanism behind the cellular uptake of dsRNA in aphids, a group of important agricultural sucking pests, remains unknown. Here, using the pea aphid Acyrthosiphon pisum as model for aphids, we identified two core genes of clathrin-dependent endocytosis (CDE), Apchc and Apvha16. We confirmed that expression of Apchc, Apvha16 and RNAi core component genes (ApAgo2, ApDcr2 and ApR2d2) were simultaneously induced at 12 h after feeding dsRNA. By using an RNAi-of-RNAi approach, we demonstrated that suppression of Apchc and Apvha16 transcripts by RNAi significantly impaired RNAi efficiency of selected reporter genes (RGs), including ApGNBP1, Apmts and Aphb, suggesting the involvement of CDE in cellular dsRNA uptake in aphids. Further confirmation was also provided using two inhibitors, chlorpromazine (CPZ) and bafilomycin A1 (BafA1). Administration of CPZ and of BafA1 both led to an impaired silencing efficiency of the RGs in the pea aphid. Finally, these RNAi-of-RNAi results were reconfirmed in the peach aphid Myzus persicae. Taking these findings together, we conclude that CDE is involved in cellular dsRNA uptake in aphids.

Exogenous administration of dsRNA for the demonstration of RNAi in Maruca vitrata (lepidoptera: crambidae)

The polyphagous spotted pod borer, Maruca vitrata is an important agricultural pest that causes extensive damage on various food crops. Though the pest is managed by synthetic chemicals, exploration of biotechnological approaches for its control is important. RNAi-based gene silencing is one such tool that has been extensively used for functional genomics and is highly variable in insects. In view of this, we have attempted to demonstrate RNAi in M. vitrata through exogenous double-stranded RNA (dsRNA) administration targeting seven genes associated with midgut, chemosensory, cell signalling and development. Two modes of exogenous dsRNA delivery by either haemolymph injection and/or ingestion into third and late third instar larval stages respectively exhibited efficient silencing of specific transcripts. Furthermore, dsRNA injection into the haemolymph showed significant reduction of target gene expression compared to negative controls establishing this mode of delivery to be more efficient. Interestingly, haemolymph injection required lesser dsRNA and led to higher reduction of transcript level vis-à-vis ingestion as demonstrated in dsRNA Serine Protease 33 (ds-SP33)-fed larvae. Over-expression of key RNAi component DICER and detection of siRNA authenticated the presence of RNAi in M. vitrata. Additionally, we have identified inhibitor molecules like morpholine, piperidine, carboxamide and piperidine-carboxamide through in silico analysis for blocking the function of SP33 to demonstrate the utility of functional genomics. Thus, the present study establishes the usefulness of injection and ingestion approaches for exogenous dsRNA delivery into M. vitrata larvae for effective RNAi.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02741-8.

Engineering pest tolerance through plant-mediated RNA interference

Expression of insect-targeted RNA interference (RNAi) constructs in transgenic plants is a promising approach for agricultural pest control. Compared to conventional chemical insecticides, RNAi target specificity is high and the potential for negative environmental effects is low. However, although numerous laboratory studies show insect growth inhibition by double stranded RNA or artificial microRNA, few of these constructs have been moved into commercial application as genetically engineered plants. Variation in RNA degradation, uptake, processing, and systemic transport in insects can influence interspecific and intraspecific differences in RNAi efficacy and the development of resistance to RNAi in agricultural settings. Further research is needed, both to identify optimal gene targets for efficient RNAi in pest species and to reduce the potential for off-target effects in beneficial species.

RNA interference of trehalose-6-phosphate synthase and trehalase genes regulates chitin metabolism in two color morphs of Acyrthosiphon pisum Harris

Trehalose-6-phosphate synthase (TPS) and trehalase (TRE) directly regulate trehalose metabolism and indirectly regulate chitin metabolism in insects. Real-time quantitative PCR (RT-qPCR) and RNA interference (RNAi) were used to detect the expressions and functions of the ApTPS and ApTRE genes. Abnormal phenotypes were found after RNAi of ApTRE in the Acyrthosiphon pisum. The molting deformities were observed in two color morphs, while wing deformities were only observed in the red morphs. The RNAi of ApTPS significantly down-regulated the expression of chitin metabolism-related genes, UDP-N-acetyglucosamine pyrophosphorylase (ApUAP), chitin synthase 2 (Apchs-2), Chitinase 2, 5 (ApCht2, 5), endo-beta-N-acetylglucosaminidase (ApENGase) and chitin deacetylase (ApCDA) genes at 24 h and 48 h; The RNAi of ApTRE significantly down-regulated the expression of ApUAP, ApCht1, 2, 8 and ApCDA at 24 h and 48 h, and up-regulated the expression of glucose-6-phosphate isomerase (ApGPI) and Knickkopf protein (ApKNK) genes at 48 h. The RNAi of ApTRE and ApTPS not only altered the expression of chitin metabolism-related genes but also decreased the content of chitin. These results demonstrated that ApTPS and ApTRE can regulate the chitin metabolism, deepen our understanding of the biological functions, and provide a foundation for better understanding the molecular mechanism of insect metamorphosis.

Epigenetic Molecular Mechanisms in Insects

Insects are the largest animal group on Earth both in biomass and diversity. Their outstanding success has inspired genetics and developmental research, allowing the discovery of dynamic process explaining extreme phenotypic plasticity and canalization. Epigenetic molecular mechanisms (EMMs) are vital for several housekeeping functions in multicellular organisms, regulating developmental, ontogenetic trajectories and environmental adaptations. In Insecta, EMMs are involved in the development of extreme phenotypic divergences such as polyphenisms and eusocial castes. Here, we review the history of this research field and how the main EMMs found in insects help to understand their biological processes and diversity. EMMs in insects confer them rapid response capacity allowing insect either to change with plastic divergence or to keep constant when facing different stressors or stimuli. EMMs function both at intra as well as transgenerational scales, playing important roles in insect ecology and evolution. We discuss on how EMMs pervasive influences in Insecta require not only the control of gene expression but also the dynamic interplay of EMMs with further regulatory levels, including genetic, physiological, behavioral, and environmental among others, as was earlier proposed by the Probabilistic Epigenesis model and Developmental System Theory.

Transcriptomic changes in the pea aphid, Acyrthosiphon pisum: Effects of the seasonal timer and photoperiod

Many insect species use photoperiod as a cue for induction of seasonal responses, including seasonal polyphenism. Although most aphid species viviparously produce parthenogenetic and sexual morphs under long and short days, respectively, a seasonal timer suppresses the sexual morph production over several successive generations during a few months following hatching of a sexually produced diapause egg. To reveal the relative influences of photoperiod and the seasonal timer on the reproductive polyphenism at the gene expression level, we performed RNA sequencing-based transcriptome analyses in the pea aphid, Acyrthosiphon pisum (Hemiptera: Aphididae). Under short days, aphids with an expired seasonal timer showed a higher expression level in hundreds of genes than those with an operative seasonal timer. In contrast, aphids with an operative seasonal timer did not show upregulation in most of these genes. Functional annotations based on gene ontology showed that histone modifications and small non-coding RNA pathways were enriched in aphids with an expired seasonal timer under short-day conditions, suggesting that these epigenetic regulations on gene expression might be involved in a mechanism of maternal switching from the parthenogenetic to sexual morph production.

Big Genes, Small Effectors: Pea Aphid Cassette Effector Families Composed From Miniature Exons

Aphids secrete proteins from their stylets that evidence indicates function similar to pathogen effectors for virulence. Here, we describe two small candidate effector gene families of the pea aphid, Acyrthosiphon pisum, that share highly conserved secretory signal peptide coding regions and divergent non-secretory coding sequences derived from miniature exons. The KQY candidate effector family contains eleven members with additional isoforms, generated by alternative splicing. Pairwise comparisons indicate possible four unique KQY families based on coding regions without the secretory signal region. KQY1a, a representative of the family, is encoded by a 968 bp mRNA and a gene that spans 45.7 kbp of the genome. The locus consists of 37 exons, 33 of which are 15 bp or smaller. Additional KQY members, as well as members of the KHI family, share similar features. Differential expression analyses indicate that the genes are expressed preferentially in salivary glands. Proteomic analysis on salivary glands and saliva revealed 11 KQY members in salivary proteins, and KQY1a was detected in an artificial diet solution after aphid feeding. A single KQY locus and two KHI loci were identified in Myzus persicae, the peach aphid. Of the genes that can be anchored to chromosomes, loci are mostly scattered throughout the genome, except a two-gene region (KQY4/KQY6). We propose that the KQY family expanded in A. pisum through combinatorial assemblies of a common secretory signal cassette and novel coding regions, followed by classical gene duplication and divergence.

Ento(o)mics: the intersection of 'omic' approaches to decipher plant defense against sap-sucking insect pests

Plants are constantly challenged by insect pests that can dramatically decrease yields. Insects with piercing-sucking mouthparts, for example, aphids, whiteflies, and leaf hoppers, seemingly cause less physical damage to tissues, however, they feed on the plant's sap by piercing plant tissue and extracting plant fluids, thereby transmitting several plant-pathogenic viruses as well. As a counter-defense, plants activate an array of dynamic defense machineries against insect pests including the rapid reprogramming of the host cell processes. For a holistic understanding of plant-sap-sucking insect interactions, there is a need to call for techniques with the capacity to concomitantly capture these dynamic changes. Recent progress with various 'omic' technologies possess this capacity. In this review, we will provide a concise summary of application of 'omic' technologies and their utilization in plant and sap-sucking insect interaction studies. Finally, we will provide a perspective on the integration of 'omics' data in uncovering novel plant defense mechanisms against sap-sucking insect pests.

AcDCXR Is a Cowpea Aphid Effector With Putative Roles in Altering Host Immunity and Physiology

Cowpea, Vigna unguiculata, is a crop that is essential to semiarid areas of the world like Sub-Sahara Africa. Cowpea is highly susceptible to cowpea aphid, Aphis craccivora, infestation that can lead to major yield losses. Aphids feed on their host plant by inserting their hypodermal needlelike flexible stylets into the plant to reach the phloem sap. During feeding, aphids secrete saliva, containing effector proteins, into the plant to disrupt plant immune responses and alter the physiology of the plant to their own advantage. Liquid chromatography tandem mass spectrometry (LC-MS/MS) was used to identify the salivary proteome of the cowpea aphid. About 150 candidate proteins were identified including diacetyl/L-xylulose reductase (DCXR), a novel enzyme previously unidentified in aphid saliva. DCXR is a member of short-chain dehydrogenases/reductases with dual enzymatic functions in carbohydrate and dicarbonyl metabolism. To assess whether cowpea aphid DCXR (AcDCXR) has similar functions, recombinant AcDCXR was purified and assayed enzymatically. For carbohydrate metabolism, the oxidation of xylitol to xylulose was tested. The dicarbonyl reaction involved the reduction of methylglyoxal, an α-β-dicarbonyl ketoaldehyde, known as an abiotic and biotic stress response molecule causing cytotoxicity at high concentrations. To assess whether cowpea aphids induce methylglyoxal in plants, we measured methylglyoxal levels in both cowpea and pea (Pisum sativum) plants and found them elevated transiently after aphid infestation. Agrobacterium-mediated transient overexpression of AcDCXR in pea resulted in an increase of cowpea aphid fecundity. Taken together, our results indicate that AcDCXR is an effector with a putative ability to generate additional sources of energy to the aphid and to alter plant defense responses. In addition, this work identified methylglyoxal as a potential novel aphid defense metabolite adding to the known repertoire of plant defenses against aphid pests.

Critical view on RNA silencing-mediated virus resistance using exogenously applied RNA

In almost all eukaryotes, RNA interference (RNAi) is a natural defence mechanism against foreign nucleic acids, including transposons and viruses. It is generally triggered by long double stranded RNA molecules (dsRNA, >50bp) that are processed into small interfering RNAs (siRNAs). RNAi can be artificially activated by the expression of RNAi triggers through viruses (virus-induced gene silencing, VIGS) and transgenes. Moreover, for almost 10 years, exogenous RNA application methods are developed as tools to induce RNAi in plants. In this review, exogenous RNA application techniques having the potential to activate RNAi with a focus on RNAi-mediated virus resistance will be discussed. Limitations of exogenous RNA applications, targeting of virus vectors and open questions related to mechanistic details that still require further investigation will be pointed out.

Biological characterization of Euscelidius variegatus iflavirus 1

Virus-based biocontrol technologies represent sustainable alternatives to pesticides and insecticides. Phytoplasmas are prokaryotic plant pathogens causing severe losses to crops worldwide. Novel approaches are needed since insecticides against their insect vectors and rogueing of infected plants are the only available strategies to counteract phytoplasma diseases. A new iflavirus, named EVV-1, has been described in the leafhopper phytoplasma vector Euscelidius variegatus, raising the potential to use virus-based application strategies against phytoplasma disease. Here transmission routes of EVV-1 are characterized, and localization within the host reveals the mechanism of insect tolerance to virus infection. Both vertical and horizontal transmission of EVV-1 occur and vertical transmission was more efficient. The virus is systemic and occurs in all life-stages, with the highest loads measured in ovaries and first to third instar nymphs. The basic knowledge gained here on the biology of the virus is crucial for possible future application of iflaviruses as biocontrol agents.

Research advances in plant-insect molecular interaction

Acute and precise signal perception and transduction are essential for plant defense against insects. Insect elicitors-that is, the biologically active molecules from insects' oral secretion (which contains regurgitant and saliva), frass, ovipositional fluids, and the endosymbionts-are recognized by plants and subsequently induce a local or systematic defense response. On the other hand, insects secrete various types of effectors to interfere with plant defense at multiple levels for better adaptation. Jasmonate is a main regulator involved in plant defense against insects and integrates with multiple pathways to make up the intricate defense network. Jasmonate signaling is strictly regulated in plants to avoid the hypersensitive defense response and seems to be vulnerable to assault by insect effectors at the same time. Here, we summarize recently identified elicitors, effectors, and their target proteins in plants and discuss their underlying molecular mechanisms.

New Technologies for Studying Negative-Strand RNA Viruses in Plant and Arthropod Hosts

The plant viruses in the phylum Negarnaviricota, orders Bunyavirales and Mononegavirales, have common features of single-stranded, negative-sense RNA genomes and replication in the biological vector. Due to the similarities in biology, comparative functional analysis in plant and vector hosts is helpful for understanding host-virus interactions for negative-strand RNA viruses. In this review, we will highlight recent technological advances that are breaking new ground in the study of these recalcitrant virus systems. The development of infectious clones for plant rhabdoviruses and bunyaviruses is enabling unprecedented examination of gene function in plants and these advances are also being transferred to study virus biology in the vector. In addition, genome and transcriptome projects for critical nonmodel arthropods has enabled characterization of insect response to viruses and identification of interacting proteins. Functional analysis of genes using genome editing will provide future pathways for further study of the transmission cycle and new control strategies for these viruses and their vectors.

Suppressing tawny crazy ant (Nylanderia fulva) by RNAi technology

The tawny crazy ant (Nylanderia fulva) is a new invasive pest in the United States. At present, its management mainly relies on the use of synthetic insecticides, which are generally ineffective at producing lasting control of the pest, necessitating alternative environmentally friendly measures. In this study, we evaluated the feasibility of gene silencing to control this ant species. Six housekeeping genes encoding actin (NfActin), coatomer subunit β (NfCOPβ), arginine kinase (NfArgK), and V-type proton ATPase subunits A (NfvATPaseA), B (NfvATPaseB) and E (NfvATPaseE) were cloned. Phylogenetic analysis revealed high sequence similarity to homologs from other ant species, particularly the Florida carpenter ant (Camponotus floridanus). To silence these genes, vector L4440 was used to generate six specific RNAi constructs for bacterial expression. Heat-inactivated, dsRNA-expressing Escherichia coli were incorporated into artificial diet. Worker ants exhibited reduced endogenous gene expression after feeding on such diet for 9 d. However, only ingestion of dsRNAs of NfCOPβ (a gene involved in protein trafficking) and NfArgK (a cellular energy reserve regulatory gene in invertebrates) caused modest but significantly higher ant mortality than the control. These results suggest that bacterially expressed dsRNA can be orally delivered to ant cells as a mean to target its vulnerabilities. Improved efficacy is necessary for the RNAi-based approach to be useful in tawny crazy ant management.

An RNAi supplemented diet as a reverse genetics tool to control bluegreen aphid, a major pest of legumes

Aphids are important agricultural pests causing major yield losses worldwide. Since aphids can rapidly develop resistance to chemical insecticides there is an urgent need to find alternative aphid pest management strategies. Despite the economic importance of bluegreen aphid (Acyrthosiphon kondoi), very few genetic resources are available to expand our current understanding and help find viable control solutions. An artificial diet is a desirable non-invasive tool to enable the functional characterisation of genes in bluegreen aphid and discover candidate target genes for future use in RNA interference (RNAi) mediated crop protection against aphids. To date no artificial diet has been developed for bluegreen aphid, so we set out to develop a suitable diet by testing and optimising existing diets. Here, we describe an artificial diet for rearing bluegreen aphid and also provide a proof of concept for the supplementation of the diet with RNAi molecules targeting the salivary gland transcript C002 and gap gene hunchback, resulting in bluegreen aphid mortality which has not yet been documented in this species. Managing this pest, for example via RNAi delivery through artificial feeding will be a major improvement to test bluegreen aphid candidate target genes for future pest control and gain significant insights into bluegreen aphid gene function.

Topical dsRNA delivery induces gene silencing and mortality in the pea aphid

BACKGROUND

With the growing number of available aphid genomes and transcriptomes, an efficient and easy-to-adapt tool for gene function study is urgently required. RNA interference (RNAi), as a post-transcriptional gene silencing mechanism, is important as a research tool for determining gene functions and has potential as a novel insect control strategy. However, these applications have been hampered by the lack of effective dsRNA delivery approaches in aphids.

RESULTS

Here, we developed a convenient and efficient dsRNA delivery method, topical RNAi, in aphids. An investigation of its dose and time-dependent RNAi efficiencies revealed that with as little as 60 ng dsRNA per adult pea aphid (Acyrthosiphon pisum), the indicator gene, Aphunchback, could be significantly silenced within 2 h of exposure. The method was further validated by successfully silencing other different genes, and it was also efficient toward two other aphid species, Aphis citricidus and Myzus persicae. Furthermore, a noticeable mortality was also observed in pea aphids using topical RNAi-mediated gene silencing, within 4 days post-dsRNA application for four out of seven tested genes.

CONCLUSION

Compared with the currently used dsRNA delivery methods in aphids, microinjection and ingestion, topical RNAi is time- and cost-effective, which could greatly influence RNAi-based gene functional studies and potential candidate gene selection for developing RNAi-based aphid control strategies in the future. © 2019 Society of Chemical Industry.