Sublethal Effects of vATPase-A and Snf7 dsRNAs on Biology of Southern Corn Rootworm, Diabrotica undecimpunctata howardi Barber

Vacuolar (H+)-ATPase Genes Are Essential for Cuticle and Wing Development in Locusta migratoria

BACKGROUND/OBJECTIVES

Vacuolar (H+)-ATPases (V-ATPases) are crucial in several significant biological processes, including intracellular transport, endocytosis, autophagy and protein degradation. However, their role in the growth and development of insects remains largely unknown. This study aimed to explore the molecular and functional properties of V-ATPases in Locusta migratoria.

METHODS

LmV-ATPase genes were identified based on the locust transcriptome database and bioinformatics analysis. Quantitative reverse-transcription polymerase chain reaction was used to assess the relative expression of LmV-ATPases in different tissues and developmental stages. RNA interference combined with hematoxylin-eosin staining and transmission electron microscopy was used to explore the functions of LmV-ATPases.

RESULTS

Ten V-ATPase genes were identified in L. migratoria and were named LmV-ATPase A, B, C, D, E, F, G, c″, d and e, respectively. These genes were highly expressed in the head, integument, gastric caecum, midgut, hindgut, fat body, trachea and ovary. The transcripts of LmV-ATPases were expressed in the developmental stages examined (from the 3rd to 5th instar nymphs). The injection of double-stranded RNA (dsRNA) against each LmV-ATPase induced high silencing efficiency in the 3rd instar nymphs. Knockdown of LmV-ATPases resulted in lethal phenotypes, with visible defects of the wing and cuticle. We further demonstrated that the deformation was caused by the defects of epidermal cells and fewer new cuticles.

CONCLUSIONS

These findings suggest that LmV-ATPases are required for the wing and cuticle development of L. migratoria, which could be potential targets for the control of locusts.

Apprehending siRNA Machinery and Gene Silencing in Brinjal Shoot and Fruit Borer, Leucinodes orbonalis

RNA interference (RNAi) technology is widely used in gene functional studies and has been shown to be a promising next generation alternative for insect pest management. To understand the efficiency of RNAi machinery in Leucinodes orbonalis (L. orbonalis) Guenee, a destructive pest of eggplant, core RNAi pathway genes Argonaute-2, Dicer-2, Loquacious, and Sid-1 were mined from the transcriptome and characterized. The transcript abundance of these genes was studied after exposure to exogenous double-stranded RNA (dsRNA). Domain structure analysis revealed that these genes have conserved domains required for the definite protein function in the siRNA pathway. The protein sequences when subjected to phylogenetic analysis showed a close relation with homologs obtained from Ostrinia sp. The insects fed with dsRNA designed for vacuolar sorting protein SNF7 gene showed significant downregulation at 48 h post treatment and about 79% larval mortality. The expression study of genes showed a significant spike in transcript abundance of Dicer-2, Argonatute-2, and downregulation of Loquacious at 24 and 48 h post dsRNA exposure. The results on siRNA machinery genes expression and target gene knockdown implies L. orbonalis has an ample response to exogenous dsRNA.

Suppression of Thrips palmi population by spray-on application of dsRNA targeting V-ATPase-B

The RNA interference (RNAi)-based gene silencing technique has enormous potential as a non-chemical and eco-friendly alternative to hazardous pesticides. This study reports a spray-induced gene silencing (SIGS) approach for managing Thrips palmi by lowering survival and offspring development. Vacuolar ATP synthases (V-ATPases) are responsible for survival, egg-laying, and viability of eggs in insects. In the current study, T. palmi V-ATPase-B was targeted to suppress the pest population by spray-on application of double-stranded RNA (dsRNA). Silencing of V-ATPase-B was first validated by oral administration of dsV-ATPase-B. The expression of V-ATPase-B was reduced by 5.40-fold post-dsRNA feeding leading to increased mortality (57.03 %) and reduced reproductive fitness (67.73 %). Spray-on application of naked dsV-ATPase-B at concentrations of 3.0 μg/mL and 5.0 μg/mL effectively suppressed the population by 30.00 % and 43.33 %, respectively. The expression of the target gene was downregulated by up to 4.24-fold. Two consecutive sprays at a concentration of 5.0 μg/mL provided substantial protection against the fresh release of T. palmi for up to 10 days. The spray-on application of dsV-ATPase-B would be an eco-friendly alternative for managing T. palmi populations thereby reducing crop damage and limiting the spread of orthotospoviruses.

Bacteria-mediated RNAi for managing fall webworm, Hyphantria cunea: screening target genes and analyzing lethal effect

BACKGROUND

The fall webworm, Hyphantria cunea, an invasive forest pest found worldwide, causes serious ecological and economic damage. Currently, the application of chemical pesticides is the most widely used strategy for H. cunea management. However, long-term pesticide use leads to pest resistance, phytotoxicity, human poisoning, and environmental deterioration. RNA interference (RNAi) technology may provide an environmentally friendly and cost-effective option for H. cunea control. However, effective RNAi targets and application methods for H. cunea are lacking.

RESULTS

We screened and obtained two highly effective RNAi targets, vATPase A (V-type proton ATPase catalytic subunit A) and Rop (Ras opposite), from 23 candidate genes, using initial and repeat screening tests with the double-stranded RNA (dsRNA) injection method. RNAi against these two genes was effective in suppressing each target messenger RNA level and interfering with larval growth, leading to significant larval mortality and pupal abnormality. For massive production of dsRNA and practical application of RNAi technology in H. cunea, transformed bacteria expressing dsRNAs of these two genes were prepared using the L4440 expression vector and HT115 strain of Escherichia coli. Oral administration of bacterially expressed dsRNA targeting vATPase A and Rop genes showed high mortality and the same malformed phenotype as the injection treatment. To further investigate the lethal effects of targeting these two genes on larval development, transcriptome sequencing (RNA-seq) was performed on RNAi samples. The results demonstrated disorders in multiple metabolic pathways, and the expression levels of most genes related to insect cuticle metabolism were significantly different, which may directly threaten insect survival. In addition, some new findings were obtained via RNA-seq analysis; for example, the progesterone-mediated oocyte maturation and oocyte meiosis processes were significantly different after silencing vATPase A, and the insect olfactory protein-related genes were significantly downregulated after dsHcRop treatment.

CONCLUSION

vATPase A and Rop are two highly effective RNAi-mediated lethal genes in H. cunea that regulate insect growth via multiple metabolic pathways. Oral delivery of bacterially expressed dsRNA specific to vATPase A and Rop can potentially be used for RNAi-based H. cunea management. This is the first study to apply bacteria-mediated RNAi for the control of this invasive pest, which is a major step forward in the application of the RNAi technology in H. cunea. © 2022 Society of Chemical Industry.

RNA interference of vATPase subunits A and E affects survival of larvae and adults in Plagiodera versicolora (Coleoptera: Chrysomelidae)

Vacuolar-type H⁺-ATPases (vATPases) are ATP-driven proton pumps and play essential roles in many physiological functions. Plagiodera versicolora (Coleoptera: Chrysomelidae) is a leaf-eating forest pest found in salicaceous trees worldwide. RNA interference (RNAi) is a powerful tool for functional identify and pest control. In this study, we used RNAi as an approach to knock down subunits A and E of the vATPase gene. The phylogenetic analysis showed that vATPase-A and vATPase-E from the same order were clustered together to form Coleoptera subclades, respectively. The expression levels of vATPase-A and vATPase-E were higher in gut, Malpighian tubules and 1st instar larvae. Ingest the dsvATPase-A and dsvATPase-E significantly inhibited the development of 1st to 3th instar larvae, incapacitated of mating and oviposition in adults. In addition, knockdown of vATPase subunit genes caused higher mortality in larvae and adults. The results demonstrate that RNAi efficiencies both vATPase-A and vATPase-E genes at various larvae stages and adults. Moreover, this research suggested that silencing of two vATPase subunits A and E offers a potential strategy to control P. versicolora.

Characterizing the sublethal effects of SmartStax PRO dietary exposure on life history traits of the western corn rootworm, Diabrotica virgifera virgifera LeConte

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is an economically important pest of field corn (Zea mays L.) across the United States (U.S.) Corn Belt. Repeated use of transgenic hybrids expressing Bacillus thuringiensis (Bt) proteins has selected for field-evolved resistance to all current rootworm-active Bt proteins. The newest product available for WCR management is SmartStax^® PRO, a rootworm-active pyramid containing Cry3Bb1, Cry34/35Ab1 [now reclassified as Gpp34Ab1/Tpp35Ab1] and a new mode of action, DvSnf7 dsRNA. Understanding the fitness of adult WCR after dietary exposure to SmartStax^® PRO will identify potential impacts on WCR population dynamics and inform efforts to optimize resistance management strategies. Therefore, the objective of the present study was to characterize the effect of SmartStax^® PRO dietary exposure on WCR life history traits. Adult WCR were collected during 2018 and 2019 from emergence tents placed over replicated field plots of SmartStax^® PRO or non-rootworm Bt corn at a site with a history of rootworm-Bt trait use and suspected resistance to Cry3Bb1 and Cry34/35Ab1. Adult survival was reduced by 97.1–99.7% in SmartStax^® PRO plots relative to the non-rootworm Bt corn plots during the study. Individual male/female pairs were fed different diets of ear tissue to simulate lifetime or adult exposure. Life history parameters measured included adult longevity, adult head capsule width, lifetime female egg production, and egg viability. Results indicate that lifetime or adult exposure to SmartStax^® PRO significantly reduced adult longevity and lifetime egg production. Larval exposure to SmartStax^® PRO significantly reduced WCR adult size. Results from this study collectively suggest that SmartStax^® PRO may negatively impact WCR life history traits, which may lead to reduced population growth when deployed in an area with WCR resistance to Bt traits.

Vacuolar ATPase subunit F is critical for larval survival in Henosepilachna vigintioctopunctata

Vacuolar ATPase (vATPase) is an important proton pump in insect tissues including gut and Malpighian tubule. Subunit F, one of the 16 subunits of the vATPase holoenzyme, is not well characterized. Here, we found that two HvvATPaseF isoforms were highly expressed in the hindgut and Malpighian tubules (MT) in the 28-spotted lady-beetle Henosepilachna vigintioctopunctata, an agricultural pest that feeds on Solanaceae and Cucurbitaceae. Knockdown of both HvvATPaseF variants by RNA interference (RNAi) delayed larval growth and negatively affected ecdysis and adult emergence. In the midgut, RNAi treatment resulted in the disappearance of peritrophic membrane, the reduction in the size and the impaired integrity of the gut, which was associated with sparse principle cells and an increase in TUNEL- and EdU-positive cells. Whereas the MT were opaque and the tubule lumens were full of urine in dsegfp-fed larvae, the tubules were clear and the tubule lumens were empty in the dsvATPaseF-fed larvae. HvvATPaseF knockdown was also associated with a decrease in the abundance of the fat body and the levels of glucose, trehalose, triglyceride, total soluble amino acids and proteins, and an increase in glycogen. Consistent with the known effects of sugars on chitin formation, both the expression level of a chitin biosynthesis gene and the thickness of the head capsule cuticle were reduced in the HvvATPaseF-depleted beetles. Our results demonstrated that subunit F plays an essential role in H. vigintioctopunctata development.

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.

EFSA Panel on Plant Health (PLH), Bragard C, Dehnen‐Schmutz K, Di Serio F, Gonthier P, Jacques M, Jaques Miret JA, Justesen AF, MacLeod A, Magnusson CS, Milonas P, Navas‐Cortes JA, Parnell S, Potting R, Reignault PL, Thulke H, Van der Werf W, Civera AV, Yuen J, Zappalà L, Kertész V, Maiorano A, Streissl F, MacLeod A. Pest categorisation of Diabrotica undecimpunctata howardi

The EFSA Panel on Plant Health performed a pest categorisation of Diabrotica undecimpunctata howardi (Coleoptera: Chrysomelidae) for the EU. This subspecies occurs in North and Central America. Adults oviposit on annual plants in the families Asteraceae, Chenopodiaceae, Cucurbitaceae, Fabaceae, Poaceae, Polygonaceae and Solanaceae. Adults feed on tender plant parts in hosts in 40 additional botanical families. Preimaginal development takes place on the roots of the host plant, where larvae feed and pupate. D. undecimpunctata howardi is a multivoltine species. Overwintering adults, which may enter a facultative diapause, abandon crops in autumn and reinvade them in spring. D. undecimpunctata howardi is not known to occur in the EU and is regulated in Annex IIA of Commission Implementing Regulation 2019/2072. This species is a competent vector of Erwinia tracheiphila (Smith) Bergey et al., which can cause bacterial wilt, a serious disease of cucurbits. The bacterium, which is restricted to temperate midwestern and eastern North America, is not regulated in the EU. Within Commission Implementing Regulation 2019/2072, potential entry pathways for D. undecimpunctata howardi, such as Asteraceae, Poaceae and Solanaceae plants for planting with foliage and soil/growing medium, and soil/growing media by themselves can be considered as closed. However, plants for planting of the families Chenopodiaceae, Cucurbitaceae, Fabaceae and Polygonaceae are not specifically regulated. Should D. undecimpunctata howardi arrive in the EU, climatic conditions and availability of susceptible hosts provide conditions suitable for establishment and further spread. Economic impact is anticipated in maize and outdoor cucurbit production. D. undecimpunctata howardi satisfies the criteria that are within the remit of EFSA to assess for this species to be regarded as a potential Union quarantine pest. This species does not meet the criteria of being present in the EU, nor plants for planting being the main pathway for spread, for it to be regarded as a potential regulated non-quarantine pest.

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

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

Control of western corn rootworm via RNAi traits in maize: lethal and sublethal effects of Sec23 dsRNA

BACKGROUND

RNA interference (RNAi) triggered by maize plants expressing RNA hairpins against specific western corn rootworm (WCR) transcripts have proven to be effective at controlling this pest. To provide robust crop protection, mRNA transcripts targeted by double-stranded RNA must be sensitive to knockdown and encode essential proteins.

RESULTS

Using WCR adult feeding assays, we identified Sec23 as a highly lethal RNAi target. Sec23 encodes a coatomer protein, a component of the coat protein (COPII) complex that mediates ER-Golgi transport. The lethality detected in WCR adults was also observed in early instar larvae, the life stage causing most of the crop damage, suggesting that WCR adults can serve as an alternative to larvae for dsRNA screening. Surprisingly, over 85% transcript inhibition resulted in less than 40% protein knockdown, suggesting that complete protein knockdown is not necessary for Sec23 RNAi-mediated mortality. The efficacy of Sec23 dsRNA for rootworm control was confirmed in planta; T₀ maize events carrying rootworm Sec23 hairpin transgenes showed high levels of root protection in greenhouse assays. A reduction in larval survival and weight were observed in the offspring of WCR females exposed to Sec23 dsRNA LC₂₅ in diet bioassays.

CONCLUSION

We describe Sec23 as RNAi target for in planta rootworm control. High mortality in exposed adult and larvae and moderate sublethal effects in the offspring of females exposed to Sec23 dsRNA LC₂₅ , suggest the potential for field application of this RNAi trait and the need to factor in responses to sublethal exposure into insect resistance management programs. © 2019 Society of Chemical Industry.

Feeding Delivery of dsHvSnf7 Is a Promising Method for Management of the Pest Henosepilachna vigintioctopunctata (Coleoptera: Coccinellidae)

RNA interference (RNAi) techniques have emerged as powerful tools in the development of novel management strategies for the control of insect pests, such as Henosepilachna vigintioctopunctata, which is a major solanaceous pest in Asia. Our results showed that levels of HvSnf7 expression were greater in larval midguts than in other tissues. Silencing of HvSnf7 led to greater H. vigintioctopunctata mortality rates and appeared to be time- and partially dose-dependent. Bacterially expressed dsHvSnf7 that was applied to detached plant leaves caused 98, 88, and 60% mortality in 1st and 3rd instars, and adults after 10, 12, and 14 d, respectively; when applied to living plants, bacterially expressed dsHvSnf7 led to mortality in 1st and 3rd instars, with no effect on adults. Bacterially expressed dsHvSnf7 led to improved plant protection against H. vigintioctopunctata. Ultrastructural changes caused by HvSnf7-RNAi in larval midguts showed extensive loss of cellular contents that indicate loss of membrane integrity. This study indicate that HvSnf7 potentially can be used as RNAi target gene for controlling of H. vigintioctopunctata.

Evaluation of reference genes for real-time quantitative PCR analysis in southern corn rootworm, Diabrotica undecimpunctata howardi (Barber)

Quantitative reverse transcription PCR (RT-qPCR) is one of the most efficient, reliable and widely used techniques to quantify gene expression. In this study, we evaluated the performance of six southern corn rootworm, Diabrotica undecimpunctata howardi (Barber), housekeeping genes (HKG), β-actin (Actin), β-tubulin (Tubulin), elongation factor 1 alpha (EF1α), glyceraldehyde-3 phosphate dehydrogenase (GAPDH), 40 S ribosomal protein S9 (RpS9) and ubiquitin-conjugating protein (Ubi), under different experimental conditions such as developmental stage, exposure of neonate and adults to dsRNA, exposure of adults to different temperatures, different 3^rd instar larva tissues, and neonate starvation. The HKGs were analyzed with four algorithms, including geNorm, NormFinder, BestKeeper, and delta-CT. Although the six HKGs showed a relatively stable expression pattern among different treatments, some variability was observed. Among the six genes, EF1α exhibited the lowest Ct values for all treatments while Ubi exhibited the highest. Among life stages and across treatments, Ubi exhibited the least stable expression pattern. GAPDH, Actin, and EF1α were among the most stable HKGs in the majority of the treatments. This research provides HKG for accurate normalization of RT-qPCR data in the southern corn rootworm. Furthermore, this information can contribute to future genomic and functional genomic research in Diabrotica species.

Characterization of the Spectrum of Insecticidal Activity for IPD072Aa: A Protein Derived from Pseudomonas chlororaphis with Activity Against Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae)

Western corn rootworm (Diabrotica virgifera virgifera LeConte) presents significant pest management challenges for farmers in both North America and Europe. IPD072Aa, a protein derived from Pseudomonas chlororaphis, has previously been shown to have activity against western corn rootworm. In the current study, the spectrum of activity of IPD072Aa was evaluated in controlled laboratory diet bioassays. IPD072Aa was fed at high concentrations in subchronic or chronic bioassays to 11 different insect species, representing 4 families within Coleoptera, and an additional 4 species representing four families of Lepidoptera. No adverse effects were noted in the Lepidoptera species. Within the order Coleoptera, western corn rootworm was the most sensitive species tested. A range of responses was observed within each of the four families of Coleoptera evaluated that included either no-observed effects or reduced growth, developmental delays, and/or reduced survival. These data will help inform the environmental risk assessment of genetically modified plants that express the IPD072Aa protein for western corn rootworm control.

Southern Corn Rootworm (Coleoptera: Chrysomelidae) Adult Emergence and Population Growth Assessment After Selection With Vacuolar ATPase-A double-stranded RNA Over Multiple Generations

The southern corn rootworm, Diabrotica undecimpunctata howardi Barber (Coleoptera: Chrysomelidae), was exposed over multiple generations to vacuolar (v)ATPase-A double-stranded (ds)RNA, first as adults and later, as neonate larvae. During adult selection, high mortality and lower fecundity were observed in the RNAi-selected cages after beetles were exposed to sublethal dsRNA concentrations that varied between LC40 and LC75. During larval selection, a delay in adult emergence and effects on population growth parameters were observed after neonates were exposed to sublethal dsRNA concentrations that varied between LC50 and LC70. Some of the parameters measured for adult emergence such as time to reach maximum linear adult emergence, time elapsed before attaining linear emergence, termination point of the linear emergence, and total days of linear emergence increase, were significantly different between RNAi-selected and control colonies for at least one generation. Significant differences were also observed in population growth parameters such as growth rate, net reproductive rate, doubling time, and generation time. After seven generations of selection, there was no indication that resistance evolved. The sublethal effects caused by exposures of southern corn rootworm to dsRNAs can affect important life history traits and fitness especially through delays in adult emergence and reduction in population growth. Although changes in susceptibility did not occur, the observation of sublethal effects suggests important responses to potential selection pressure. Assuming resistance involves a recessive trait, random mating between susceptible and resistant individuals is an important factor that allows sustainable use of transgenic plants, and delays in adult emergence observed in our studies could potentially compromise this assumption.