Silencing the ecdysone synthesis and signaling pathway genes disrupts nymphal development in the whitefly

Molecular characterization of the cytochrome P450 enzyme CYP18A1 in Henosepilachna vigintioctopunctata

In insects, the expression of 20E response genes that initiate metamorphosis is triggered by a pulse of 20-hydroxyecdysone (20E). The 20E pulse is generated through two processes: synthesis, which increases its level, and inactivation, which decreases its titer. CYP18A1 functions as an ecdysteroid 26-hydroxylase and plays a role in 20E removal in several representative insects. However, applying 20E degradation activity of CYP18A1 to other insects remains a significant challenge. In this study, we discovered high levels of Hvcyp18a1 during the larval and late pupal stages, particularly in the larval epidermis and fat body of Henosepilachna vigintioctopunctata, a damaging Coleopteran pest of potatoes. RNA interference (RNAi) targeting Hvcyp18a1 disrupted the pupation. Approximately 75% of the Hvcyp18a1 RNAi larvae experienced developmental arrest and remained as stunted prepupae. Subsequently, they gradually turned black and eventually died. Among the Hvcyp18a1-depleted animals that successfully pupated, around half became malformed pupae with swollen elytra and hindwings. The emerged adults from these deformed pupae appeared misshapen, with shriveled elytra and hindwings, and were wrapped in the pupal exuviae. Furthermore, RNAi of Hvcyp18a1 increased the expression of a 20E receptor gene (HvEcR) and four 20E response transcripts (HvE75, HvHR3, HvBrC, and HvαFTZ-F1), while decreased the transcription of HvβFTZ-F1. Our findings confirm the vital role of CYP18A1 in the pupation, potentially involved in the degradation of 20E in H. vigintioctopunctata.

Knockout of ecdysis triggering hormone receptor (ETHr) gene adversely affects the nymphal molting and adult reproduction in Bemisia tabaci

Bemisia tabaci (Gennadius) is one of the most dangerous polyphagous pests in the world causing damage to various crops by sucking sap during the nymphal and adult stages. Chemical management of whiteflies is challenging because of the emergence of pesticide resistance. RNA interference has been well established in whitefly to study the functions of various genes. G-protein coupled receptors (GPCRs) are important targets for development of new generation insecticides. In this study, Ecdysis triggering hormone receptor (ETHr) gene expression was recorded in different stages of whitefly and its function has been studied through RNAi. The expression of ETHr is highest in third-instar nymphs followed by other nymphal instars, pupae and newly emerged adults. Silencing of ETHr resulted in significantly higher adult mortality (68.88%), reduced fecundity (4.46 eggs /female), reduced longevity of male and female (1.05 and 1.40 days, respectively) when adults were fed with dsETHr @ 1.0 μg/μl. Silencing of ETHr in nymphs lead to significantly higher mortality (81.35%) as compared to control. This study confirms that ETHr gene is essential for growth and development of whitefly nymphs and adults. Hence, it can be future target for developing dsRNA based insecticides for management of whitefly.

The neuroendocrine and endocrine systems in insect - Historical perspective and overview

A complex cascade of events leads to the initiation and maintenance of a behavioral act in response to both internally and externally derived stimuli. These events are part of a transition of the animal into a new behavioral state, coordinated by chemicals that bias tissues and organs towards a new functional state of the animal. This form of integration is defined by the neuroendocrine (or neurosecretory) system and the endocrine system that release neurohormones or hormones, respectively. Here we describe the classical neuroendocrine and endocrine systems in insects to provide an historic perspective and overview of how neurohormones and hormones support plasticity in behavioral expression. Additionally, we describe peripheral tissues such as the midgut, epitracheal glands, and ovaries, which, whilst not necessarily being endocrine glands in the pure sense of the term, do produce and release hormones, thereby providing even more flexibility for inter-organ communication and regulation.

Identification and functional analysis of isopentenyl pyrophosphate isomerase genes in the whiteflies Bemisia tabaci (Hemiptera: Aleyrodidae)

The juvenile hormone (JH) plays a vital role in the regulation of a number of physiological processes, including development, reproduction, and ovarian maturation. Isopentenyl pyrophosphate isomerase (IPPI) is a key enzyme in the biosynthetic pathway of JH. In this study, we identified an isopentenyl pyrophosphate isomerase protein from Bemisia tabaci and named it BtabIPPI. The open reading frame (ORF) of BtabIPPI is 768 bp and encodes a protein of 255 amino acids that contains a conserved domain of the Nudix family. The temporal and spatial expression profiles showed that BtabIPPI was highly expressed in the female adults.RNA interference (RNAi)-mediated silencing of BtabIPPI reduced JH titers and the relative expression of vitellogenin receptor (VgR) and JH signaling pathway genes, resulting in a dramatic reduction in fecundity and hatchability. These results indicate that the BtabIPPI gene plays an important role in the female fecundity of B. tabaci. This study will broaden our understanding of the function of IPPI in regulating insect reproduction and provide a theoretical basis for targeting IPPI for pest control in the future.

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.

Effect of silencing Bemisia tabaci TLR3 and TOB1 on fitness and begomovirus transmission

Bemisia tabaci (Hemiptera: Aleyrodidae) is one of the most important invasive pests worldwide. It infests several vegetables, legumes, fiber, and ornamental crops. Besides causing direct damage by sucking plant sap, B. tabaci is the principal vector of begomoviruses. Chilli leaf curl virus (ChiLCV, Begomovirus) transmitted by B. tabaci is a major constraint in chilli production. B. tabaci genes associated with metabolism, signaling pathways, cellular processes, and organismal systems are highly enriched in response to ChiLCV infection. The previous transcriptome study suggested the association of B. tabaci Toll-like receptor 3 (TLR3) and transducer of erbB2.1 (TOB1) in ChiLCV infection. In the present study, B. tabaci TLR3 and TOB1 were silenced using double-stranded RNA (dsRNA) and the effect on fitness and begomovirus transmission has been reported. Oral delivery of dsRNA at 3 µg/mL reduced the expression of B. tabaci TLR3 and TOB1 by 6.77 and 3.01-fold, respectively. Silencing of TLR3 and TOB1 induced significant mortality in B. tabaci adults compared to untreated control. The ChiLCV copies in B. tabaci significantly reduced post-exposure to TLR3 and TOB1 dsRNAs. The ability of B. tabaci to transmit ChiLCV also declined post-silencing TLR3 and TOB1. This is the first-ever report of silencing B. tabaci TLR3 and TOB1 to induce mortality and impair virus transmission ability in B. tabaci. B. tabaci TLR3 and TOB1 would be novel genetic targets to manage B. tabaci and restrict the spread of begomovirus.

A Hepatocyte Nuclear Factor BtabHNF4 Mediates Desiccation Tolerance and Fecundity in Whitefly (Bemisia tabaci)

Hepatocyte nuclear factor 4 (HNF4) is essential for glucose homeostasis and lipid metabolism in insects. However, little is known about the role of HNF4 in whiteflies. In the present study, we identified a hepatocyte nuclear factor protein from Bemsia tabaci (Diptera: Drosophilidae) and named it BtabHNF4. The full-length of BtabHNF4 was 3,006 bp, encoding a sequence of 434 amino acids that contains a conserved zinc-finger DNA-binding domain (DBD) and a well-conserved ligand-binding domain (LBD). The temporal and spatial expression showed that BtabHNF4 was highly expressed in the female adult stage and abdominal tissues of B. tabaci. A leaf-mediated RNA interference method was used to explore the function of BtabHNF4 in whiteflies. Our results showed that the knockdown of BtabHNF4 influences the desiccation tolerance, egg production, and egg hatching rate of whiteflies. Additionally, BtabHNF4 silencing significantly inhibited the expression level of vitellogenin. These results expand the function of HNF4 and pave the way for understanding the molecular mechanisms of HNF4 in regulating multiple physiological processes.

RNAi-Mediated Silencing of Putative Halloween Gene Phantom Affects the Performance of Rice Striped Stem Borer, Chilo suppressalis

The physiological and biochemical characterization of the "Halloween" genes has fundamental importance in the biosynthesis pathway of ecdysteroids. These genes were found to catalyze the final phases of ecdysteroid biosynthesis from dietary cholesterol to the molting hormone 20-hydroxyecdysone. We report the characterization of the Cs-Phm in a major insect pest in agriculture, the rice striped stem borer, Chilo suppressalis (C. suppressalis). A full-length transcript of Cs-Phm was amplified with an open reading frame (ORF) of 478 amino acids through 5' and 3' RACE. Cs-Phm shows five insect-conserved P450 motifs: Helix-C, Helix-I, Helix-K, PERF, and heme-binding motifs. Phylogenetic analysis clearly shows high similarity to Lepidoptera and evolutionary conservation in insects. The relative spatial and temporal transcript profile shows that Cs-Phm is highly expressed in the prothoracic glands and appears throughout the larval development, but with low expression at the start of the larval instar. It seems to peak in 3-4 days and decreases again before the larvae molt. Double-stranded RNA (dsRNA) injection of Cs-Phm at the larval stage efficiently knocked down the target gene and decreased its expression level. The dsRNA-treated group showed significantly decreased ecdysteroid titers, which leads to delayed larval development and higher larval mortality. Negative effects of larval development were rescued by treating 20E in the dsRNA-treated group. Thus, in conclusion, our results suggest that Cs-Phm is functionally conserved in C. suppressalis and encodes functional CYP that contributes to the biogenesis of 20E.

MiR-3017b contributes to metamorphosis by targeting sarco/endoplasmic reticulum Ca2+ ATPase in Tribolium castaneum

In recent years, increasing numbers of microRNAs (miRNAs) have been reported to regulate insect metamorphosis. One thousand, one hundred fifty-four miRNAs have been previously identified from Tribolium castaneum by high-throughput sequencing; however, little is known about which miRNAs can participate in metamorphosis, leaving the role of miRNAs in regulating the underlying mechanism elusive. Here, we report the participation of miR-3017b in the metamorphosis of T. castaneum. Temporal profiles revealed that miR-3017b was highly expressed at the late larval stage, but significantly decreased at the early pupal stage. Overexpression of miR-3017b caused larval to pupal to adult metamorphosis arrested. Dual-luciferase reporter assay and miRNA-mRNA interaction assay illustrated that miR-3017b interacts with the coding sequence of sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) and suppresses its expression. Knockdown of SERCA caused metamorphosis arrested, similar to that observed in miR-3017b overexpression beetles. Further functional mechanism analyses revealed that 20-hydroxyecdysone application downregulates miR-3017b and up-regulates SERCA expression. The expression level of downstream genes in the 20E pathway was disrupted after overexpressing miR-3017 and the knockdown of SERCA. These results provided evidence miR-3017b-SERCA contributes to metamorphosis by regulating the 20E pathway in T. castaneum. It could advance our understanding of the coordination of 20E and miRNA regulation in insect metamorphosis.

The role of 20E biosynthesis relative gene Shadow in the reproduction of the predatory mirid bug, Cyrtorhinus lividipennis (Hemiptera: Miridae)

Cytorhinus lividipennis is a natural enemy of rice planthoppers and leafhoppers. Improving the fecundity of C. lividipennis will be helpful to improve its control effect on pests. However, little is known about the hormonal regulatory mechanism of reproduction in C. lividipennis. In the current study, we examined the role of 20-hydroxyecdysone (20E) biosynthesis relative gene Shadow in the reproduction of C. lividipennis. The complementary DNA sequence of ClSad is 2018 -bp in length with an open reading frame of 1398-bp encoding 465 amino acid residues. ClSad was readily detected in nymphal and adult stages, and highly expressed in the adult stage. ClSad was highly expressed in the midgut and ovaries of adult females. Moreover, RNA interference-mediated knockdown of ClSad reduced the 20E titers and ClVg transcript level, resulting in fewer fully developed eggs and a decrease in the number of eggs laid by dsSad-injected adult females within 15 days. These results suggest that ClSad plays a critical role in the reproduction of C. lividipennis. The present study provides insights into the molecular mechanism of the ClSad gene for the reproduction of C. lividipennis.

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.

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.

Combined Transcriptomic Analysis and RNA Interference Reveal the Effects of Methoxyfenozide on Ecdysone Signaling Pathway of Spodoptera exigua

Spodoptera exigua is a worldwide pest afflicting edible vegetables and has developed varying levels of resistance to insecticides. Methoxyfenozide (MET), an ecdysteroid agonist, is effective against lepidopteran pests such as S. exigua. However, the mechanism of MET to S. exigua remains unclear. In this study, we analyzed the expression patterns of genes related to the ecdysone signaling pathway in transcriptome data treated with sublethal doses of MET and analyzed how expression levels of key genes affect the toxicity of MET on S. exigua. Our results demonstrated that 2639 genes were up-regulated and 2512 genes were down-regulated in S. exigua treated with LC₃₀ of MET. Of these, 15 genes were involved in the ecdysone signaling pathway. qPCR results demonstrated that ecdysone receptor A (EcRA) expression levels significantly increased in S. exigua when treated with different doses of MET, and that the RNAi-mediated silencing of EcRA significantly increased mortality to 55.43% at 72 h when L3 S. exigua larvae were exposed to MET at the LC₃₀ dose. Additionally, knocking down EcRA suppressed the most genes expressed in the ecdysone signaling pathway. The combination of MET and dsEcRA affected the expression of E74 and enhanced the expression of TREA. These results demonstrate that the adverse effects of sublethal MET disturb the ecdysone signaling pathway in S. exigua, and EcRA is closely related to MET toxic effect. This study increases our collective understanding of the mechanisms of MET in insect pests.

Spotlight on the Roles of Whitefly Effectors in Insect-Plant Interactions

The Bemisia tabaci species complex (whitefly) causes enormous agricultural losses. These phloem-feeding insects induce feeding damage and transmit a wide range of dangerous plant viruses. Whiteflies colonize a broad range of plant species that appear to be poorly defended against these insects. Substantial research has begun to unravel how phloem feeders modulate plant processes, such as defense pathways, and the central roles of effector proteins, which are deposited into the plant along with the saliva during feeding. Here, we review the current literature on whitefly effectors in light of what is known about the effectors of phloem-feeding insects in general. Further analysis of these effectors may improve our understanding of how these insects establish compatible interactions with plants, whereas the subsequent identification of plant defense processes could lead to improved crop resistance to insects. We focus on the core concepts that define the effectors of phloem-feeding insects, such as the criteria used to identify candidate effectors in sequence-mining pipelines and screens used to analyze the potential roles of these effectors and their targets in planta. We discuss aspects of whitefly effector research that require further exploration, including where effectors localize when injected into plant tissues, whether the effectors target plant processes beyond defense pathways, and the properties of effectors in other insect excretions such as honeydew. Finally, we provide an overview of open issues and how they might be addressed.

Capsicum-infecting begomoviruses as global pathogens: host-virus interplay, pathogenesis, and management

Whitefly-transmitted begomoviruses are among the major threats to the cultivation of Capsicum spp. (Family: Solanaceae) worldwide. Capsicum-infecting begomoviruses (CIBs) have a broad host range and are commonly found in mixed infections, which, in turn, fuels the emergence of better-adapted species through intraspecies and interspecies recombination. Virus-encoded proteins hijack host factors to breach the well-coordinated antiviral response of plants. Epigenetic modifications of histones associated with viral minichromosomes play a critical role in this molecular arms race. Moreover, the association of DNA satellites further enhances the virulence of CIBs as the subviral agents aid the helper viruses to circumvent plant antiviral defense and facilitate expansion of their host range and disease development. The objective of this review is to provide a comprehensive overview on various aspects of CIBs such as their emergence, epidemiology, mechanism of pathogenesis, and the management protocols being employed for combating them.

Tiny Flies: A Mighty Pest That Threatens Agricultural Productivity-A Case for Next-Generation Control Strategies of Whiteflies

Simple Summary

Despite being a pest of global importance, effective management of whiteflies by the implication of environmentally friendly approaches is still a far-reaching task. In this review, we have tried to bring the readers’ attention to next-generation control strategies such as RNA interference and genetic modifications of plants for the expression of anti-whitefly proteins. These strategies offer huge promise to provide an effective and sustainable solution to the problem of whiteflies, either in isolation or in combination with other widely used practices under the regimes of integrated pest management. Focus has also been given to advanced technologies such as nanotechnology and genome editing, with promising prospects for field applications. The importance, applicability, and demand of these technologies for the control of whiteflies have been highlighted. We have also attempted to present the holistic picture of challenges in the path of commercial application of these promising technologies. To underline the pest status of whiteflies concisely, we have enlisted all economically important species of the pest along with their host plants/crops across the world. A comprehensive list of various insecticides of chemical, microbial, and botanical origin, applied in the field for the control of sweetpotato whitefly along with their resistance status, ecotoxicities, and effects on biological control agents, has been provided for readers.

Abstract

Whiteflies are a group of universally occurring insects that are considered to be a serious pest in their own way for causing both direct and indirect damages to crops. A few of them serve as vectors of plant viruses that are detrimental to the crop in question and cause an actual loss in productivity. A lot of attention is focused on pest control measures under the umbrella of IPM. In this review, we attempt to summarize the existing literature on how and why whiteflies are a serious concern for agriculture and society. We reviewed why there could be a need for fresh insight into the ways and means with which the pest can be combated. Here, we have emphasized next-generation strategies based on macromolecules, i.e., RNA interference and genetic engineering (for the expression of anti-whitefly proteins), as these strategies possess the greatest scope for research and improvement in the future. Recent scientific efforts based on nanotechnology and genome editing, which seem to offer great potential for whitefly/crop pest control, have been discussed. Comprehensive apprehensions related to obstacles in the path of taking lab-ready technologies into the farmers’ field have also been highlighted. Although the use of RNAi, GM crops, nanotechnologies, for the control of whiteflies needs to be evaluated in the field, there is an emerging range of possible applications with promising prospects for the control of these tiny flies that are mighty pests.

RNAi-based gene silencing in Phenacoccus solenopsis and its validation by in planta expression of a double-stranded RNA

BACKGROUND

Cotton is a cash crop majorly affected by many hemipteran pests, among them the cotton mealybug, Phenacoccus solenopsis. Cotton mealybug attack has a devastating effect on cotton production and causes huge yield losses.

RESULTS

In this study, 25 potential RNA interference (RNAi) target genes were selected from the iBeetle database and a transcriptome data set for P. solenopsis. To assess the effectiveness of the selected target genes, three methods were utilized to deliver double-stranded (ds)RNA (ingestion, artificial diet bioassay and transient gene silencing). dsRNA molecules at different concentrations were fed to insects and insect mortality was recorded for each target gene. Based on the mortality data, three genes, Krüppel homologue-1, ADP-ATP/Translocase and IDGF-1, were selected for further gene expression studies using a reduced concentration of dsRNA (5 μg/ml). Of the three genes, Krüppel homologue-1 showed significantly downregulated expression (by 70.81% and 84.33%) at two different time points (8 and 14 days). An RNAi silencing construct was designed for Krüppel homologue-1 under control of the double enhancer CamV35S promoter in the plant binary vector. Significant downregulation of gene expression, by 66.69% and 81.80%, was found for Krüppel homologue-1 using transient gene silencing at the same time intervals.

CONCLUSION

This work provides the first evidence for targeting the Krüppel homologue-1 gene in a hemipteran pest, P. solenopsis, using RNAi technology through oral delivery and in planta-based transient gene silencing methods. © 2020 Society of Chemical Industry.

Bemisia tabaci on Vegetables in the Southern United States: Incidence, Impact, and Management

Simple Summary

The sweetpotato whitefly, Bemisia tabaci, was initially discovered in the United States in 1894 but was not considered an economic insect pest on various agricultural crops across the southern and western states. After the introduction of B. tabaci Middle East-Asia Minor 1 (MEAM1) into the United States around 1985, the insect rapidly spread throughout the Southern United States to Texas, Arizona, and California. Extreme field outbreaks occurred on vegetable and other crops in those areas. The sweetpotato whitefly is now regarded as one of the most destructive insect pests in vegetable production systems in the Southern United States. The direct and indirect plant damage caused by B. tabaci has led to substantial economic losses in vegetable crops. Bemisia tabaci outbreaks on vegetables in Georgia resulted in significant economic losses of 132.3 and 161.2 million US dollars (USD) in 2016 and 2017, respectively. Therefore, integrated pest management (IPM) tactics are warranted, including cultural control by manipulation of production practices, resistant vegetable varieties, biological control using various natural enemies, and the judicious use of insecticides.

Abstract

Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) is among the most economically important insect pests of various vegetable crops in the Southern United States. This insect is considered a complex of at least 40 morphologically indistinguishable cryptic species. Bemisia tabaci Middle East-Asia Minor 1 (MEAM1) was initially introduced in the United States around 1985 and has since rapidly spread across the Southern United States to Texas, Arizona, and California, where extreme field outbreaks have occurred on vegetable and other crops. This pest creates extensive plant damage through direct feeding on vegetables, secreting honeydew, causing plant physiological disorders, and vectoring plant viruses. The direct and indirect plant damage in vegetable crops has resulted in enormous economic losses in the Southern United States, especially in Florida, Georgia, and Texas. Effective management of B. tabaci on vegetables relies mainly on the utilization of chemical insecticides, particularly neonicotinoids. However, B. tabaci has developed considerable resistance to most insecticides. Therefore, alternative integrated pest management (IPM) strategies are required, such as cultural control by manipulation of production practices, resistant vegetable varieties, and biological control using a suite of natural enemies for the management of the pest.

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

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

Broad-complex transcription factor mediates opposing hormonal regulation of two phylogenetically distant arginine kinase genes in Tribolium castaneum

The phosphoarginine-arginine kinase shuttle system plays a critical role in maintaining insect cellular energy homeostasis. Insect molting and metamorphosis are coordinated by fluctuations of the ecdysteroid and juvenile hormone. However, the hormonal regulation of insect arginine kinases remain largely elusive. In this report, we comparatively characterized two arginine kinase genes, TcAK1 and TcAK2, in Tribolium castaneum. Functional analysis using RNAi showed that TcAK1 and TcAK2 play similar roles in adult fertility and stress response. TcAK1 was detected in cytoplasm including mitochondria, whereas TcAK2 was detected in cytoplasm excluding mitochondria. Interestingly, TcAK1 expression was negatively regulated by 20-hydroxyecdysone and positively by juvenile hormone, whereas TcAK2 was regulated by the opposite pattern. RNAi, dual-luciferase reporter assays and electrophoretic mobility shift assay further revealed that the opposite hormonal regulation of TcAK1 and TcAK2 was mediated by transcription factor Broad-Complex. Finally, relatively stable AK activities were observed during larval-pupal metamorphosis, which was generally consistent with the constant ATP levels. These results provide new insights into the mechanisms underlying the ATP homeostasis in insects by revealing opposite hormonal regulation of two phylogenetically distant arginine kinase genes.

Zhang et al. characterize the functions of two distinct arginine kinase genes in flour beetles. Using RNA interference and electophoretic mobility shift assays, they identify Broad-Complex transcription factor as the mediator of opposing hormonal regulation in these genes.

The mechanism of sublethal chlorantraniliprole exposure causing silkworm pupation metamorphosis defects

BACKGROUND

Chlorantraniliprole (CAP) is widely used in agriculture and forestry to prevent and control pests. The effects of environmental CAP residue on non-target insect metamorphosis have not been reported. Our research aimed to investigate the sublethal effect of CAP on larva-pupa transformation in silkworm, and explore the mechanism of sublethal CAP exposure-mediated pupation metamorphosis defects.

RESULT

Sublethal CAP exposure affected the growth and development of silkworm larvae and caused defects in pupation metamorphosis. After CAP exposure, formation the of prepupa procuticle, ecdysial membrane and new epidermis was inhibited. Also, the level of 20-hydroxyecdysone (20E) and mRNA levels of the 20E signaling pathway-related genes EcR, USP, E74, E75 and Ftz-f1 were significantly reduced. Moreover, genes involved in chitin synthesis, such as ChsA, CDA1 and CDA2, were downregulated. Injection of 20E led to the upregulation of chitin synthesis-related genes and increased formation of new epidermis in CAP-treated silkworm. However, injection of 20E failed to prevent downregulation of Ftz-f1 and the defects in pupation metamorphosis.

CONCLUSION

Our results suggested that 20E is a target hormone of CAP exposure-mediated epidermis formation phenotype. Ftz-f1 was silenced by CAP and might be a direct target gene of sublethal CAP exposure. Our study provided new evidence of the effects of sublethal CAP exposure on insect development and metamorphosis. © 2020 Society of Chemical Industry.

Functional analysis of ecdysteroid biosynthetic enzymes of the rice planthopper, Nilaparvata lugens

Ecdysteroids, insect steroid hormones, play key roles in regulating insect development and reproduction. Hemipteran insects require ecdysteroids for egg production; however, ecdysteroid synthesis (ecdysteroidogenesis) details have not been elucidated. We identified all known genes encoding ecdysteroidogenic enzymes in Nilaparvata lugens and clarified their necessity during nymphal and ovarian development. We confirmed that N. lugens utilized 20-hydroxyecdysone as an active hormone. Assays using heterologous expression of enzymes in Drosophila S2 cells showed conserved functions of enzymes Neverland, CYP306A2, CYP314A1 and CYP315A1, but not CYP302A1. RNA interference and rescue analysis using 20-hydroxyecdysone demonstrated that most of the genes were necessary for nymphal development. The identified N. lugens enzymes showed conserved functions and pathways for ecdysteroidogenesis. Knockdown of ecdysteroidogenic enzyme genes in newly molted females caused failure of egg production: less vitellogenic and mature eggs in ovaries, fewer laid eggs and embryonic development deficiency of laid eggs. Considering the high expressions of ecdysteroidogenic enzyme genes in adults and ovaries, ecdysteroidogenesis in ovaries was critical for N. lugens ovarian development. Our study presents initial evidence that hemipteran insects require ecdysteroidogenesis for ovarian development.

Nitric Oxide Boosts Bemisia tabaci Performance Through the Suppression of Jasmonic Acid Signaling Pathway in Tobacco Plants

The intimate relationships between plants and insects start with herbivory, which can be traced back to approximately 420 million year ago. Like many other relationships, a plant–insect interaction can be mutualistic, commensalistic, or antagonistic. Within antagonistic relationships, plants deploy inducible defense to insect phytophagy. Insects, however, can evade/suppress effectual plant defenses by manipulating plant defense signaling. Previously, we showed that the sweet potato whitefly, Bemisia tabaci, a global invasive insect pest, can suppress jasmonic acid (JA)-dependent defenses, thereby enhancing their performance on host plants. Given that nitric oxide (NO), a multifunctional signaling molecule, interacts closely with JA signaling pathway, we hypothesized that NO is involved in the suppression of JA defensive responses. Equipped with an integrated approach, we comprehensively examined this overarching hypothesis. We showed that: (1) tobacco plants responded to B. tabaci infestation by accumulating high levels of NO, (2) the exogenous application of sodium nitroprusside, a NO donor, in tobacco plants attracted B. tabaci adults and accelerated nymphal development, whereas plants treated with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a NO scavenger, repelled B. tabaci adults and prolonged nymphal development, and, more importantly, (3) silencing of NO-associated protein 1, a gene associated with NO accumulation, and cPTIO application disrupted the B. tabaci-mediated suppression of JA in plants. Collectively, these results suggest that: (1) NO signaling is activated by B. tabaci infestation, (2) NO is involved in the suppression of JA-dependent plant defense, and, consequently, (3) NO improves B. tabaci performance on host plants. Our study reflects the remarkable arm race that co-evolved for millions of years between plants and insects and offers a potential novel target (nitric oxide) for the long-term sustainable management of this global invasive pest.

Molecular characterization and functional analysis of the Halloween genes and CYP18A1 in Bemisia tabaci MED

The ecdysteroid hormone 20-hydroxyecdysone (20E), a critical hormone in arthropods, plays an essential role in insect growth, molting and reproduction. A previous study showed that 20E is actually regulated by six P450 genes (five P450 genes belonging to the Halloween family and a CYP18A1 gene) in model insects. However, the role of the six P450 genes in Bemisia tabaci Q (also call Mediterranean, MED), an important pest of field crops, remains unclear. Here, six P450 genes were cloned by RT-PCR, and the phylogenetic tree indicated a close orthologous relationship of these P450 genes between MED and other insects. Spatiotemporal expression profiling revealed that five P450 genes (CYP18A1, CYP306A1, CYP307A2, CYP314A1 and CYP315A1) were expressed at significantly higher levels in the head than in the abdomen and thorax. Four P450 genes (CYP302A1, CYP307A2, CYP314A1 and CYP315A1) were expressed at the highest levels in males, and CYP18A1 was expressed at the highest levels in the 4th nymph stage. The molting process was delayed by approximately 1-3 days after knockdown of these genes at the 4th nymph stage, and the mean proportion of shriveled or dead insects reached 8.3% (CYP18A1), 20.8% (CYP302A1), 7.0% (CYP307A2), 31.8% (CYP306A1), 28.6% (CYP314A1) and 24.1% (CYP315A1). In addition, 20E rescued the negative effect of ds-CYP306A1, ds-CYP314A1 and ds-CYP315A1 on the eclosion rate. We concluded that these Halloween genes and CYP18A1 likely participate in the development of MED, and in particular, CYP306A1 could be used as a putative insecticide target for controlling this piercing-sucking insect.

Biotic stress triggered small RNA and RNAi defense response in plants

The yield of crops is largely affected by different types of biotic stresses. To minimize the damage, crop plants adapted themselves to overcome the stress conditions through gene expression reprogramming at transcriptional and post-transcriptional levels. With a better knowledge of plants' responses in adverse environments, new methodologies and strategies have been applied to develop better stress-tolerant plants. In this manner, small RNAs (micro RNA and small-interfering RNA) are reported to play a central role to combat biotic stresses in plants. Depending upon the stress stimuli, these small RNAs can up or down regulate the genes expression, that indicate their potential role in overcoming the stress. These stress-induced small RNAs may reduce the expression of the target gene(s) that might negatively influence plants' response to the adverse conditions. Contrariwise, miRNA, a class of small RNA, can downregulate its expression to upregulate the expression of the target gene(s), which might positively aid to the stress adaptation. Along with this, benefits of RNA interference (RNAi) have also been stated in functional genomic research on insects, fungi and plant pathogens. RNAi is involved in the safe transport of dsRNA to the targeted mRNA(s) in the biotic stress-causing agents (for example fungi and insects) and saves the plant from damage, which is a safer approach compared to use of chemical pesticides. The current review summarizes the role of small RNAs and the use of RNAi to save the plants from biotic stress conditions.

Genome-wide identification and analysis of nuclear receptors genes for lethal screening against Bemisia tabaci Q

BACKGROUND

Nuclear receptors (NRs) play an essential role in diverse biological processes, such as insect metamorphosis. Here, transcriptome analysis and functional studies were used to determine whether NRs are involved in metamorphosis of whitefly Bemisia tabaci Q, a serious pest to crops, and to find some potential insecticide targets.

RESULTS

Twenty NRs were identified in the Bemisia tabaci Q genome and categorized into the NR0-NR6 subfamilies. The phylogenetic tree of NRs from Bemisia tabaci Q and other representative species was constructed, which provided evolutionary insight into their genetic distances. The results of spatiotemporal gene expression indicated that the majority of NR gene expression was higher in the head than the abdomen and higher in eggs than adults. Further functional analysis using RNA interference (RNAi) showed that NR genes play an important role in Bemisia tabaci Q pupation and eclosion. With respect to high mortality and effects on growth, this was reflected in the unable to become pupa when the third-stage nymph treated with double-stranded RNA (dsRNA) and the developmental time delay (4-7 days) when pupae were treated with dsRNA for the 12 NR genes during molting compared with the development time in the control.

CONCLUSION

This study provides insight into NR functions during the metamorphosis stages of Bemisia tabaci Q. Several candidate genes could be potential insecticide targets for whitefly pest control due to their important roles in insect development. © 2020 Society of Chemical Industry.

Role of Chitin Deacetylase 1 in the Molting and Metamorphosis of the Cigarette Beetle Lasioderma serricorne

Chitin deacetylases (CDAs) are chitin-modifying enzymes known to play vital roles in insect metamorphosis and development. In this study, we identified and characterized a chitin deacetylase1 gene (LsCDA1) from the cigarette beetle Lasioderma serricorne. LsCDA1 contains a 1614 bp open reading frame encoding a protein of 537 amino acids that includes domain structures typical of CDAs. LsCDA1 was mainly expressed in the late larval and late pupal stages. In larval tissues, the highest level of LsCDA1 was detected in the integument. The expression of LsCDA1 was induced by 20-hydroxyecdysone (20E) in vivo, and it was significantly suppressed by knocking down the expression of ecdysteroidogenesis genes and 20E signaling genes. RNA interference (RNAi)-aided silencing of LsCDA1 in fifth-instar larvae prevented the larval–pupal molt and caused 75% larval mortality. In the late pupal stage, depletion of LsCDA1 resulted in the inhibition of pupal growth and wing abnormalities, and the expression levels of four wing development-related genes (LsDY, LsWG, LsVG, and LsAP) were dramatically decreased. Meanwhile, the chitin contents of LsCDA1 RNAi beetles were significantly reduced, and expressions of three chitin synthesis pathway genes (LsTRE1, LsUAP1, and LsCHS1) were greatly decreased. The results suggest that LsCDA1 is indispensable for larval–pupal and pupal–adult molts, and that it is a potential target for the RNAi-based control of L. serricorne.

Improving RNAi efficiency for pest control in crop species

The application of RNAi promotes the development of novel approaches toward plant protection in a sustainable way. Genetically modified crops expressing dsRNA have been developed as commercial products with great potential in insect pest management. Alternatively, some nontransformative approaches, including foliar spray, irrigation and trunk injection, are favorable in actual utilization. In this review, we summarize the recent progress and successful cases of RNAi-based pest management strategy, explore essential implications and possibilities to improve RNAi efficiency by delivery of dsRNA through transformative and nontransformative approaches, and highlight the remaining challenges and important issues related to the application of this technology.

Enhancing RNAi Efficiency to Decipher the Functional Response of Potential Genes in Bemisia tabaci AsiaII-1 (Gennadius) Through dsRNA Feeding Assays

Whitefly Bemisia tabaci is a global invasive pest that causes substantial losses to agricultural crops worldwide either by direct feeding or vectoring numerous plant viruses. Management with insecticides remains a big challenge due to its rapid resistance development potential as well as the impact of these chemicals on non-target organisms. Thus, in search of alternate and novel pest management strategies RNA interference (RNAi) has come up as potential future tool in this direction. The present study targets nine potential genes (Aquaporin (AQP), Calcitonin (CAL), CyclophilinB (CYCP), Knottin-1 (k-1), Heat shock proteins (Hsp20, Hsp40 and Hsp70), SWItch/Sucrose Non-fermentable (SNF7) and inhibitor of apoptosis (IAP) of whitefly that have been implicated to play a role in various vital physiological functions and virus transmission. The RNAi mediated knockdown efficiency of these genes has been improved through the conjugation of respective target gene dsRNA with CQD (carbon quantum dots) nanoparticles or simultaneous knockdown of dsRNA specific gut nucleases. The studies revealed that feeding of dsRNA (40 μg/ml sucrose diet) of the target gene(s) either conjugated with CQD or along with dsRNA against dsRNase (dsdsRNase) (40 μg/ml sucrose diet) enhanced the RNAi efficiency by 24-89% compared to whiteflies fed with naked dsRNA of the same target gene. The studies provide insights about the functional role of various genes in whitefly, which can possibly be exploited for the management of this pest in the future.

Characteristic and Functional Study of Intersex, a Gene Related to Female Fertility in Bemisia tabaci

The intersex (ix) gene acts in concert with doublesex (dsx) at the end of the sex determination hierarchy to control somatic sexual differentiation in Drosophila melanogaster. Here, we report the Drosophila ix homolog in Bemisia tabaci (Btix) with differential splicing events. Four isoforms were found in B. tabaci adults, including two sex-specific transcripts (Btix^F and Btix^M). Knockdown of Btix had no measurable effects on female morphological phenotypes but reduced the expression of the vitellogenin gene and resulted in the production of significantly fewer eggs, a lower eclosion rate, and a shorter body size of female progeny in comparison with control females. These results increase our understanding of the genes underlying sex determination in B. tabaci and reveal a potential target for RNA interference-based pest management.

Evaluation of a cuticle protein gene as a potential RNAi target in aphids

BACKGROUND

RNA interference (RNAi) has potential as a pest insect control technique. One possible RNAi target is the cuticle protein, which is important in insect molting and development. As an example, here we evaluate the possibility of designing double-stranded RNA (RNA) that is effective for silencing the cuticle protein 19 gene (CP19) in aphids but is harmless to non-target predator insects.

RESULTS

The sequences of CP19s were similar (86.6-94.4%) among the tested aphid species (Aphis citricidus, Acyrthosiphon pisum, and Myzus persicae) but different in the predator Propylaea japonica. Ingestion of species-specific dsRNAs of CP19 by the three aphids produced 39.3-64.2% gene silencing and 45.8-55.8% mortality. Ingestion of non-species-specific dsRNA (dsAcCP19) by Ac. pisum and M. persicae gave gene silencing levels ranging from 40.4% to 50.3% and 43.3-50.8% mortality. The dsApCP19 did not affect PjCP19 expression or developmental duration in P. japonica.

CONCLUSION

The results demonstrate that CP19 is a promising RNAi target for aphid control via one dsRNA design. The targeting of genes that are conserved in insect pests but not present in beneficial insects is a useful RNAi-based pest control strategy. © 2019 Society of Chemical Industry.

Downregulation of dystrophin expression in pupae of the whitefly Bemisia tabaci inhibits the emergence of adults

The whitefly Bemisia tabaci is a major pest to agriculture. Adults are able to fly for long distances and to colonize staple crops, herbs and ornamentals, and to vector viruses belonging to several important taxonomic groups. During their early development, whiteflies mature from eggs through several nymphal stages (instars I to IV) until adults emerge from pupae. We aim at reducing whitefly populations by inhibiting the emergence of adults from nymphs. Here we targeted dystrophin, a conserved protein essential for the development of the muscle system in humans, other animals and insects. We have exploited the fact that whitefly nymphs developing on tomato leaves feed from the plant phloem via their stylets. Thus, we delivered dystrophin-silencing double-stranded RNA to nymphs developing on leaves of tomato plantlets with their roots bathing in the silencing solution. Downregulation of dystrophin expression occurred mainly in pupae. Dystrophin silencing induced also the downregulation of the dystrophin-associated protein genes actin and tropomyosin, and disrupted F-actin. Most significantly, the treatment inhibited the emergence of adults from pupae, suggesting that targeting dystrophin may help to restrain whitefly populations. This study demonstrates for the first time the important role of dystrophin in the development of a major insect pest to agriculture.

Identification of Halloween Genes and RNA Interference-Mediated Functional Characterization of a Halloween Gene shadow in Plutella xylostella

Ecdysteroids play an essential role in controlling insect development and reproduction. Their pathway is regulated by a group of enzymes called Halloween gene proteins. The relationship between the Halloween genes and ecdysteroid synthesis has yet to be clearly understood in diamondback moth, Plutella xylostella (L.), a worldwide Lepidoptera pest attacking cruciferous crops and wild plants. In this study, complete sequences for six Halloween genes, neverland (nvd), shroud (sro), spook (spo), phantom (phm), disembodied (dib), shadow (sad), and shade (shd), were identified. Phylogenetic analysis revealed a strong conservation in insects, including Halloween genes of P. xylostella that was clustered with all other Lepidoptera species. Three Halloween genes, dib, sad, and shd were highly expressed in the adult stage, while nvd and spo were highly expressed in the egg and pupal stages, respectively. Five Halloween genes were highly expressed specifically in the prothorax, which is the major site of ecdysone production. However, shd was expressed predominantly in the fat body to convert ecdysone into 20-hydroxyecdysone. RNAi-based knockdown of sad, which is involved in the last step of ecdysone biosynthesis, significantly reduced the 20E titer and resulted in a longer developmental duration and lower pupation of fourth-instar larvae, as well as caused shorter ovarioles and fewer fully developed eggs of P. xylostella. Furthermore, after the knockdown of sad, the expression levels of Vg and VgR genes were significantly decreased by 77.1 and 53.0%. Meanwhile, the number of eggs laid after 3 days was significantly reduced in sad knockdown females. These results suggest that Halloween genes may play a critical role in the biosynthesis of ecdysteroids and be involved in the development and reproduction of P. xylostella. Our work provides a solid basis for understanding the functional importance of these genes, which will help to screening potential genes for pest management of P. xylostella.

Isoform specific roles of Broad-Complex in larval development in Leptinotarsa decemlineata

Broad-Complex (BrC) is a downstream target of both 20-hydroxyecdysone and juvenile hormone signalling. BrC regulates morphogenetic changes between nymphal instars in hemimetabolans, whereas it controls pupal commitment, pupal morphogenesis and inhibits adult differentiation in holometabolans. Among five BrC cDNAs (Z1-Z4 and Z6) identified in the Colorado potato beetle, we found in this work that Z1, Z2 and Z6 were mainly expressed at the last (fourth) instar and prepupal stages, whereas the levels of Z3 and Z4 increased during the penultimate (third) instar stage, peaked at the last instar larval phase and gradually decreased at the prepupal and pupal periods. When knocking down all BrC isoforms by RNA interference (RNAi) at the penultimate instar stage, around 20% of the resultant larvae remained as moribund beetles. These moribund BrC RNAi larvae were completely or partially wrapped in old cuticle. Likewise, a portion of larvae treated for a single double-stranded RNA of Z3, Z4 or Z6 displayed a degree of similar aberrancies, increasing in the order of isoforms Z6 < Z3 < Z4. When silencing all BrC isoforms at the last instar period, most of the RNAi larvae did not normally pupate or emerge as adults. Separately silencing each of the five zinc finger domains revealed that approximately 70% of the Z1 RNAi larvae remained as prepupae, around 60% of the Z6 RNAi specimens formed aberrant prepupae or pupae and about 60% of the Z2 RNAi beetles became deformed pupae. After removal of the old exuviae, these deformed larvae in which either Z1, Z2 or Z6 was depleted possessed adult prothorax and mesothorax, developing antenna, mouthparts and wing discs. Moreover, less than 50% of the resultant pupae finally emerged as adults when either of Z1, Z2 or Z6 was knocked down. Therefore, our findings reveal, for the first time, that the two roles of BrC in insect groups (ie directing morphogenetic changes during juvenile development and regulating larval-pupal-adult metamorphosis) are played by different BrC isoforms in Leptinotarsa decemlineata.

Potential of RNA interference in the study and management of the whitefly, Bemisia tabaci

Whiteflies cause considerable losses to crops, directly by feeding, and indirectly by transmission of viruses. The current control methods consist of a combination of different control tactics, mainly still relying on unsafe and non-ecofriendly chemical control. RNA interference (RNAi) is a post-transcriptional gene-silencing strategy in which double-stranded RNA (dsRNA), corresponding specifically to a target gene, is introduced in a target organism. Research on RNAi in the previous decade has shown its success as a potential insect control strategy, which can be highly species-specific and environment friendly. In whiteflies, the success of dsRNA delivery through the oral route opened possibilities for its management through plant-mediated RNAi. To date, several genes have been targeted in whiteflies through RNAi and these assays demonstrated its potential to manage whiteflies at lab level. However, further research and investments are needed to move toward an application at field level. In this review, for the first time, we collected the literature on genes targeted for silencing via RNAi in whiteflies and discuss the potential of RNAi in whitefly pest control. We also discuss likely delivery methods, including transgenic in planta delivery and symbiont-mediated delivery, and its potential for studying and interfering with insecticide resistance mechanisms and virus transmission by whiteflies.

Transgenic microRNA-14 rice shows high resistance to rice stem borer

Rice stem borer (RSB, Chilo suppressalis) is an insect pest that causes huge economic losses every year. Control efforts rely heavily on chemical insecticides, which leads to serious problems such as insecticide resistance, environment pollution, and food safety issues. Therefore, developing alternative pest control methods is an important task. Here, we identified an insect-specific microRNA, miR-14, in RSB, which was predicted to target Spook (Spo) and Ecdysone receptor (EcR) in the ecdysone signalling network. In-vitro dual luciferase assays using HEK293T cells confirmed the interactions of Csu-miR-14 with CsSpo and with CsEcR. Csu-miR-14 exhibited high levels of expression at the end of each larval instar stage, and its expression was negatively correlated with the expression of its two target genes. Overexpression of Csu-miR-14 at the third day of the fifth instar stage led to high mortality and developmental defects in RSB individuals. We produced 35 rice transformants to express miR-14 and found that three lines had a single copy with highly abundant miR-14 mature transcripts. Feeding bioassays using both T0 and T1 generations of transgenic miR-14 rice indicated that at least one line (C#24) showed high resistance to RSB. These results indicated that the approach of miRNAs as targets has potential for improving pest control methods. Moreover, using insect-specific miRNAs rather than protein-encoding genes for pest control may prove benign to non-insect species, and thus is worthy of further exploration.

Comparative transcriptomic analysis of larval and adult Malpighian tubules from the cotton bollworm Helicoverpa armigera

Malpighian tubules (MTs) are usually considered the key excretory and osmoregulatory organs of insects. However, increasing evidence has suggested that MTs perform many more functions than just osmoregulation. Until now, the molecular and physiological functions of MTs in the cotton bollworm (Helicoverpa armigera), a very important agricultural pest, are largely unknown. In this study, the transcriptomes of H. armigera MTs from larvae, male adults and female adults were sequenced using RNA-Seq technology, and comparative analyses of transcriptomes between two life stages (larval and adult) and between adult sexes were conducted. We generated a total of 84 643 high-quality unigenes, and identified a large number of abundant transcripts putatively encoding proteins involved in diuresis, detoxification, immunity, carbohydrate transport and metabolism, development and reproduction. We found that the expression pattern of unigenes was relatively similar between female and male adult MTs, but different between larval and adult MTs. Our data suggest that insect MTs may take multiple physiological functions as versatile organs. The extensive alterations in gene expression in MTs occurred from larvae to adults reflect an ecological adaptation to different feeding habits. Sexual dimorphism in the cotton bollworm is somewhat indicated by the transcriptional difference of genes related to carbohydrate metabolism, detoxification, immunity and reproduction in the MTs of male and female adults.

Antioxidant Role of PcGSTd1 in Fenpropathrin Resistant Population of the Citrus Red Mite, Panonychus citri (McGregor)

The citrus red mite, Panonychus citri, a major citrus pest distributed worldwide, has evolved severe resistance to various classes of chemical acaricides/insecticides including pyrethroids. It is well known that the resistance to pyrethroids is mainly caused by point mutations of voltage-gated sodium channel gene in a wide range of pests. However, increasing number of evidences support that pyrethroids resistance might also be resulted from the integrated mechanisms including metabolic mechanisms. In this study, firstly, comparative analysis of RNA-seq data showed that multiple detoxification genes, including a GSTs gene PcGSTd1, were up-regulated in a fenpropathrin-resistant population compared with the susceptible strain (SS). Quantitative real time-PCR results showed that the exposure of fenpropathrin had an induction effect on the transcription of PcGSTd1 in a time-dependent manner. In vitro inhibition and metabolic assay of recombinant PcGSTd1 found that fenpropathrin might not be metabolized directly by this protein. However, its antioxidant role in alleviating the oxidative stress caused by fenpropathrin was demonstrated via the reversely genetic experiment. Our results provide a list of candidate genes which may contribute to a multiple metabolic mechanisms implicated in the evolution of fenpropathrin resistance in the field population of P. citri. Furthermore, during the detoxification process, PcGSTd1 plays an antioxidant role by detoxifying lipid peroxidation products induced by fenpropathrin.

Leptinotarsa hormone receptor 4 (HR4) tunes ecdysteroidogenesis and mediates 20-hydroxyecdysone signaling during larval-pupal metamorphosis

Hormone receptor 4 (HR4) is involved in the regulation of 20-hydroxyecdysone (20E) biosynthesis and the mediation of 20E signaling during larval-pupal transition in a holometabolan Drosophila melanogaster, whereas it acts as a repressor in 20E-responsive transcriptional cascade in a hemimetabolan, Blattella germanica. Here we characterized two HR4 splicing variants, LdHR4X1 and LdHR4X2, in a coleopteran Leptinotarsa decemlineata. LdHR4X1 was highly expressed in the prothoracic gland and epidermis while LdHR4X2 was abundantly transcribed in the nervous system. In vivo results showed that both prothoracicotropic hormone and 20E pathways transcriptionally regulated LdHR4, in an isoform-dependent pattern. RNA interference of LdHR4 at the final (fourth) larval instar, in contrast to the second- and third-instar periods, enhanced the expression of two ecdysteroidogenesis genes, increased 20E titer, upregulated transcription of five 20E-response genes, and reduced the mRNA level of Fushi tarazu-factor 1 (FTZ-F1). As a result, the fourth-instar LdHR4 RNAi larvae exhibited accelerated development and reduced body weight. Moreover, knockdown of LdHR4 at the fourth instar resulted in larval lethality and impaired pupation. Feeding of pyriproxyfen (a mimic of juvenile hormone) or silencing of a juvenile hormone degrading enzyme gene restored the normal course of ecdysteroidogenesis, duration of larval development, and body weight in fourth-instar LdHR4 RNAi larvae. The treatment partially suppressed the larval mortality but not the failure to pupate. The dual role of HR4 during larval-pupal metamorphosis appears to be evolutionarily conserved among holometabolans.

Maternal Inheritance of a Single Somatic Animal Cell Displayed by the Bacteriocyte in the Whitefly Bemisia tabaci

Bacteriocytes are insect cells harboring symbiotic bacteria that are required by their insect host and are transmitted vertically via the female ovary [1]. In most insect groups, the bacteria are released from the bacteriocytes and transferred to the ovary [2, 3], but in whiteflies, maternal bacteriocytes migrate to each egg [4, 5, 6], where they have been reported to lyse, releasing the symbionts [1]. To investigate bacteriocyte inheritance in whiteflies further, we applied microsatellite genotyping and genomic analysis to a genetically diverse population of Bemisia tabaci, and we observed the fate of the bacteriocyte in embryos. Surprisingly, the microsatellite profile of the bacteriocytes was uniform, and insect cross experiments demonstrated that the bacteriocytes have a stable genotype that differs from the genotype of the insect head (which lacks bacteriocytes). Comparative genomic analysis indicates that genomes of the bacteriocyte and whitefly head are distinct. Interestingly, the bacterioyte genome contains the canonical arthropod telomere repeats TTAGG, and the bacteriocytes express telomere maintenance genes that may underlie cellular immortality in animal cells [7]. Microscopy observations confirmed that a single bacteriocyte transmitted to each egg is retained and divides once just before egg hatch, yielding two bacteriocytes in the neonate insect. These data demonstrate the maternal inheritance of an absolutely required somatic insect cell, violating the developmental separation of germline and soma [8, 9]. Future investigation on the mechanism and phylogenetic distribution of maternally inherited bacteriocytes will shed light on the developmental origins and evolutionary diversification of bacteriocytes [10] and the processes underlying cellular immortality [11].

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Whitefly bacteriocytes have different microsatellite alleles from other somatic cells

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The bacteriocyte genotype is stable over three sexual generations of the insect

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Bacteriocyte genomes in different insects are genetically very similar

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The single bacteriocyte transmitted to the sexual egg persists through embryogenesis

Hormone Signaling Regulates Nymphal Diapause in Laodelphax striatellus (Hemiptera: Delphacidae)

Diapause is a physiological adaptation that allows an organism to survive adverse environmental conditions. Diapause occurs at a specific developmental stage in each species. There are few reports regarding the molecular regulatory mechanism of nymphal diapause in Laodelphax striatellus, which is an important graminaceous crop pest. Our previous studies identified the conditions for nymphal diapause in this species. Here, we combined RNA sequencing transcriptomics and quantitative proteomic analyses to identify nymphal diapause-related genes and proteins. The analysis of differentially regulated genes identified four gene/protein pairs that were synchronously up-regulated, and six gene/protein pairs that were synchronously down-regulated, suggesting that these genes may regulate nymphal diapause. The up-regulated gene juvenile hormone acid methyl transferase (JHAMT) and the down-regulated gene cytochrome P450 monooxygenase (CYP314A1, Shd) were chosen for further functional studies. After knocking-down of LsJHAMT and LsShd in vivo by RNA interference, the titer of JH III and 20E decreased significantly, and the duration of the nymphal development period was severely altered. Thus LsJHAMT and LsShd regulated JH III and 20E titers in the hemolymph to control the nymphal diapause status. This study may lead to new information on the regulation nymphal diapause of this important agricultural insect pest.

RNAi-mediated resistance to whitefly (Bemisia tabaci) in genetically engineered lettuce (Lactuca sativa)

RNA interference (RNAi)-based transgenic technologies have evolved as potent biochemical tools for silencing specific genes of plant pathogens and pests. The approach has been demonstrated to be useful in silencing genes in insect species. Here, we report on the successful construction of RNAi-based plasmid containing an interfering cassette designed to generate dsRNAs that target a novel v-ATPase transcript in whitefly (Bemisia tabaci), an important agricultural pest in tropical and sub-tropical regions. The presence of the transgene was confirmed in T0 and T1 generations of transgenic lettuce lines, segregating in a Mendelian fashion. Seven lines were infested with whiteflies and monitored over a period of 32 days. Analysis of mortality showed that within five days of feeding, insects on transgenic plants showed a mortality rate of 83.8-98.1%. In addition, a reduced number of eggs (95 fold less) was observed in flies feeding on transgenic lettuce plants than insects on control lines. Quantitative reverse transcription PCR showed decreased expression level of endogenous v-ATPase gene in whiteflies feeding on transgenic plants. This technology is a foundation for the production of whitefly-resistant commercial crops, improving agricultural sustainability and food security, reducing the use of more environmentally aggressive methods of pest control.

Towards an understanding of the molecular basis of effective RNAi against a global insect pest, the whitefly Bemisia tabaci

In planta RNAi against essential insect genes offers a promising route to control insect crop pests, but is constrained for many insect groups, notably phloem sap-feeding hemipterans, by poor RNAi efficacy. This study conducted on the phloem-feeding whitefly Bemisia tabaci reared on tomato plants investigated the causes of low RNAi efficacy and routes to ameliorate the problem. Experiments using tomato transgenic lines containing ds-GFP (green fluorescent protein) revealed that full-length dsRNA is phloem-mobile, ingested by the insects, and degraded in the insect. We identified B. tabaci homologs of nuclease genes (dsRNases) in other insects that degrade dsRNA, and demonstrated that degradation of ds-GFP in B. tabaci is suppressed by administration of dsRNA against these genes. dsRNA against the nuclease genes was co-administered with dsRNA against two insect genes, an aquaporin AQP1 and sucrase SUC1, that are predicted to protect B. tabaci against osmotic collapse. When dsRNA constructs for AQP1, SUC1, dsRNase1 and dsRNase2 were stacked, insect mortality was significantly elevated to 50% over 6 days on artificial diets. This effect was accompanied by significant reduction in gene expression of the target genes in surviving diet-fed insects. This study offers proof-of-principle that the efficacy of RNAi against insect pests can be enhanced by using dsRNA to suppress the activity of RNAi-suppressing nuclease genes, especially where multiple genes with related physiological function but different molecular function are targeted.

RNA interference of an antimicrobial peptide, Btdef, reduces Tomato yellow leaf curl China virus accumulation in the whitefly Bemisia tabaci

BACKGROUND

The whitefly Bemisia tabaci (Gennadius) is considered one of the main pests for agriculture. One important problem with the whitefly is its notorious status as a vector for plant viruses, primarily begomoviruses. We have previously identified a defensin-like antimicrobial peptide, Btdef, from the whitefly B. tabaci MEAM1. However, the function of Btdef in the immune system of the insect vector and begomovirus transmission has yet to be explored.

RESULTS

To explore the role of Btdef during begomovirus transmission, we firstly investigated the transcriptional response of Btdef following acquisition of Tomato yellow leaf curl China virus (TYLCCNV). The expression of Btdef was up-regulated in the viruliferous whiteflies. After RNA silencing of the Btdef gene in adult whiteflies fed with dsRNA, they were allowed to feed on TYLCCNV-infected plants and then quantified for TYLCCNV DNA titre. Unexpectedly, silencing the Btdef gene reduced both the abundance and expressions of TYLCCNV genes in the whiteflies. In the meantime, the density of the endosymbiont Rickettsia was significantly reduced in dsBtdef-fed whiteflies.

CONCLUSION

Our data provide evidence that Btdef is involved in begomovirus infection, possibly through symbiont-mediated alteration of begomovirus-whitefly interactions. These findings indicate that Btdef may be targeted for the development of new technology for the control of whitefly-transmitted begomoviruses. © 2016 Society of Chemical Industry.

Leptinotarsa cap 'n' collar isoform C/Kelch-like ECH associated protein 1 signaling is critical for the regulation of ecdysteroidogenesis in the larvae

Drosophila cap 'n' collar isoform C (CncC) and Kelch-like ECH associated protein 1 (Keap1) regulate metamorphosis by transcriptional control of a subset of genes involved in ecdysteroidogenesis, 20-hydroxyecdysone (20E) signaling, and juvenile hormone (JH) degradation. In the present paper, we found that prothoracicotropic hormone signal was required for the activation of LdCncC and LdKeap1 in Leptinotarsa decemlineata. Moreover, RNA interference of LdCncC or LdKeap1 in the fourth-instar larvae delayed development. As a result, the treated larvae obtained heavier larval and pupal fresh weights and had larger body sizes than the controls. Furthermore, knockdown of LdCncC or LdKeap1 significantly reduced the mRNA levels of four ecdysone biosynthetic genes (Ldspo, Ldphm, Lddib and Ldsad), lowered 20E titer and decreased the transcript levels of five 20E response genes (LdEcR, LdUSP, LdE75, LdHR3 and LdFTZ-F1). However, the expression of two JH epoxide hydrolase genes and JH contents were not affected in the LdCncC and LdKeap1 RNAi larvae. Dietary supplementation with 20E shortened the developmental period to normal length, rescued the larval and pupal body mass rises, and recovered or even overcompensated the expression levels of the five 20E response genes in either LdCncC or LdKeap1 RNAi hypomorphs. Therefore, LdCncC/LdKeap1 signaling regulates several ecdysteroidogenesis genes, and consequently 20E pulse, to modulate the onset of metamorphosis in L. decemlineata.

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

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

Ras-like family small GTPases genes in Nilaparvata lugens: Identification, phylogenetic analysis, gene expression and function in nymphal development

Twenty-nine cDNAs encoding Ras-like family small GTPases (RSGs) were cloned and sequenced from Nilaparvata lugens. Twenty-eight proteins are described here: 3 from Rho, 2 from Ras, 9 from Arf and 14 from Rabs. These RSGs from N.lugens have five conserved G-loop motifs and displayed a higher degree of sequence conservation with orthologues from insects. RT-qPCR analysis revealed NlRSGs expressed at all life stages and the highest expression was observed in hemolymph, gut or wing for most of NlRSGs. RNAi demonstrated that eighteen NlRSGs play a crucial role in nymphal development. Nymphs with silenced NlRSGs failed to molt, eclosion or development arrest. The qRT-PCR analysis verified the correlation between mortality and the down-regulation of the target genes. The expression level of nuclear receptors, Kr-h1, Hr3, FTZ-F1 and E93 involved in 20E and JH signal pathway was impacted in nymphs with silenced twelve NlRSGs individually. The expression of two halloween genes, Cyp314a1 and Cyp315a1 involved in ecdysone synthesis, decreased in nymphs with silenced NlSar1 or NlArf1. Cyp307a1 increased in nymphs with silenced NlArf6. In N.lugens with silenced NlSRβ, NlSar1 and NlRab2 at 9^th day individually, 0.0% eclosion rate and almost 100.0% mortality was demonstrated. Further analysis showed NlSRβ could be served as a candidate target for dsRNA-based pesticides for N.lugens control.

Silencing of Two Insulin Receptor Genes Disrupts Nymph-Adult Transition of Alate Brown Citrus Aphid

Insulin receptors play key roles in growth, development, and polymorphism in insects. Here, we report two insulin receptor genes (AcInR1 and AcInR2) from the brown citrus aphid, Aphis (Toxoptera) citricidus. Transcriptional analyses showed that AcInR1 increased during the nymph–adult transition in alate aphids, while AcInR2 had the highest expression level in second instar nymphs. AcInR1 is important in aphid development from fourth instar nymphs to adults as verified by dsRNA feeding mediated RNAi. The silencing of AcInR1 or/and AcInR2 produced a variety of phenotypes including adults with normal wings, malformed wings, under-developed wings, and aphids failing to develop beyond the nymphal stages. Silencing of AcInR1 or AcInR2 alone, and co-silencing of both genes, resulted in 73% or 60%, and 87% of aphids with problems in the transition from nymph to normal adult. The co-silencing of AcInR1 and AcInR2 resulted in 62% dead nymphs, but no mortality occurred by silencing of AcInR1 or AcInR2 alone. Phenotypes of adults in the dsInR1 and dsInR2 were similar. The results demonstrate that AcInR1 and AcInR2 are essential for successful nymph–adult transition in alate aphids and show that RNAi methods may be useful for the management of this pest.

Identification and expression patterns of Halloween genes encoding cytochrome P450s involved in ecdysteroid biosynthesis in the cotton bollworm Helicoverpa armigera

20-Hydroxyecdysone (20E) is a key hormone which regulates growth, development and reproduction in insects. Although cytochrome P450 enzymes (P450s) participating in the ecdysteroid biosynthesis of 20E have been characterized in a few model insects, no work has been published on the molecular entity of their orthologs in the cotton bollworm Helicoverpa armigera, a major pest insect in agriculture worldwide. In this study, four cytochrome P450 homologs, namely HarmCYP302A1, HarmCYP306A1, HarmCYP314A1 and HarmCYP315A1 from H. armigera, were identified and evolutional conservation of these Halloween genes were revealed among lepidopteran. Expression analyses showed that HarmCYP302A1 and HarmCYP315A1 were predominantly expressed in larval prothoracic glands, whereas this predominance was not always observed for HarmCYP306A1 and CYP314A1. The expression patterns of Halloween genes indicate that the fat bodies may play an important role in the conversion of ecdysone into 20E in larval-larval molt and in larval-pupal metamorphosis, and raise the possibility that HarmCYP315A1 plays a role in tissue-specific regulation in the steroid biosynthesis in H. armigera. These findings represent the first identification and expression characterization of four steriodogenic P450 genes and provide the groundwork for future functional and evolutionary study of steroid biosynthesis in this agriculturally important pest.