Progress and prospects of arthropod chitin pathways and structures as targets for pest management

Chitosan-based insecticide formulations for insect pest control management: A review of current trends and challenges

Future agricultural practices necessitate green alternatives to replace hazardous insecticides while distinguishing between pests and beneficial insects. Chitosan, as a biological macromolecule derived from chitin, is biodegradable and exhibits low toxicity to non-target organisms, making it a sustainable alternative to synthetic pesticides. This review identifies chitosan-derivatives for insecticidal activity and highlights its efficacy including genotoxicity, defense mechanism, and disruption of insect's exoskeleton at different concentrations against several insect pests. Similarly, synergistic effects of chitosan in combination with natural extracts, essential oils, and plant-derived compounds, enhances insecticidal action against various pests was evaluated. The chitosan-based insecticide formulations (CHIF) in the form of emulsions, microcapsules, and nanoparticles showed efficient insecticide action on the targeted pests with less environmental impact. The current challenges associated with the field-trial application were also recognized, by optimizing potent CHIF-formulation parameters, scaling-up process, and regulatory hurdles addressed alongside potential solutions. These findings will provide insight into achieving the EU mission of reducing chemical pesticides by 50 %.

The moulting arthropod: a complete genetic toolkit review

Exoskeletons are a defining character of all arthropods that provide physical support for their segmented bodies and appendages as well as protection from the environment and predation. This ubiquitous yet evolutionarily variable feature has been instrumental in facilitating the adoption of a variety of lifestyles and the exploitation of ecological niches across all environments. Throughout the radiation that produced the more than one million described modern species, adaptability afforded by segmentation and exoskeletons has led to a diversity that is unrivalled amongst animals. However, because of the limited extensibility of exoskeleton chitin and cuticle components, they must be periodically shed and replaced with new larger ones, notably to accommodate the growing individuals encased within. Therefore, arthropods grow discontinuously by undergoing periodic moulting events, which follow a series of steps from the preparatory pre-moult phase to ecdysis itself and post-moult maturation of new exoskeletons. Each event represents a particularly vulnerable period in an arthropod's life cycle, so processes must be tightly regulated and meticulously executed to ensure successful transitions for normal growth and development. Decades of research in representative arthropods provide a foundation of understanding of the mechanisms involved. Building on this, studies continue to develop and test hypotheses on the presence and function of molecular components, including neuropeptides, hormones, and receptors, as well as the so-called early, late, and fate genes, across arthropod diversity. Here, we review the literature to develop a comprehensive overview of the status of accumulated knowledge of the genetic toolkit governing arthropod moulting. From biosynthesis and regulation of ecdysteroid and sesquiterpenoid hormones, to factors involved in hormonal stimulation responses and exoskeleton remodelling, we identify commonalities and differences, as well as highlighting major knowledge gaps, across arthropod groups. We examine the available evidence supporting current models of how components operate together to prepare for, execute, and recover from ecdysis, comparing reports from Chelicerata, Myriapoda, Crustacea, and Hexapoda. Evidence is generally highly taxonomically imbalanced, with most reports based on insect study systems. Biases are also evident in research on different moulting phases and processes, with the early triggers and late effectors generally being the least well explored. Our synthesis contrasts knowledge based on reported observations with reasonably plausible assumptions given current taxonomic sampling, and exposes weak assumptions or major gaps that need addressing. Encouragingly, advances in genomics are driving a diversification of tractable study systems by facilitating the cataloguing of putative genetic toolkits in previously under-explored taxa. Analysis of genome and transcriptome data supported by experimental investigations have validated the presence of an "ultra-conserved" core of arthropod genes involved in moulting processes. The molecular machinery has likely evolved with elaborations on this conserved pathway backbone, but more taxonomic exploration is needed to characterise lineage-specific changes and novelties. Furthermore, linking these to transformative innovations in moulting processes across Arthropoda remains hampered by knowledge gaps and hypotheses based on untested assumptions. Promisingly however, emerging from the synthesis is a framework that highlights research avenues from the underlying genetics to the dynamic molecular biology through to the complex physiology of moulting.

Curcumin interferes with chitin synthesis in Aedes aegypti: a computational and experimental investigation

Throughout history, vector-borne diseases have consistently posed significant challenges to human health. Among the strategies for vector control, chemical insecticides have seen widespread use since their inception. Nevertheless, their effectiveness is continually undermined by the steady growth of insecticide resistance within these vector populations. As such, the demand for more robust, efficient, and cost-effective natural insecticides has become increasingly pressing. One promising avenue of research focuses on chitin, a crucial structural component of mosquitoes' exoskeletons and other insects. Chitin not only provides protection and rigidity but also lends flexibility to the insect body. It undergoes substantial transformations during insect molting, a process known as ecdysis. Crucially, the production of chitin is facilitated by an enzyme known as chitin synthase, making it an attractive target for potential novel insecticides. Our recent study delved into the impacts of curcumin, a natural derivative of turmeric, on chitin synthesis and larval development in Aedes aegypti, a mosquito species known to transmit dengue and yellow fever. Our findings demonstrate that even sub-lethal amounts of curcumin can significantly reduce overall chitin content and disrupt the cuticle development in the 4th instar larvae of Aedes aegypti. Further to this, we utilized computational analyses to investigate how curcumin interacts with chitin synthase. Techniques such as molecular docking, pharmacophore feature mapping, and molecular dynamics (MD) simulations helped to illustrate that curcumin binds to the same site as polyoxin D, a recognized inhibitor of chitin synthase. These findings point to curcumin's potential as a natural, bioactive larvicide that targets chitin synthase in mosquitoes and potentially other insects.

Chitin-induced disease resistance in plants: A review

Chitin is composed of N-acetylglucosamine units. Chitin a polysaccharide found in the cell walls of fungi and exoskeletons of insects and crustaceans, can elicit a potent defense response in plants. Through the activation of defense genes, stimulation of defensive compound production, and reinforcement of physical barriers, chitin enhances the plant's ability to defend against pathogens. Chitin-based treatments have shown efficacy against various plant diseases caused by fungal, bacterial, viral, and nematode pathogens, and have been integrated into sustainable agricultural practices. Furthermore, chitin treatments have demonstrated additional benefits, such as promoting plant growth and improving tolerance to abiotic stresses. Further research is necessary to optimize treatment parameters, explore chitin derivatives, and conduct long-term field studies. Continued efforts in these areas will contribute to the development of innovative and sustainable strategies for disease management in agriculture, ultimately leading to improved crop productivity and reduced reliance on chemical pesticides.

Gene Cloning, Heterologous Expression, and In Silico Analysis of Chitinase B from Serratia marcescens for Biocontrol of Spodoptera frugiperda Larvae Infesting Maize Crops

Spodoptera frugiperda, the fall armyworm (FAW), is a highly invasive polyphagous insect pest that is considered a source of severe economic losses to agricultural production. Currently, the majority of chemical insecticides pose tremendous threats to humans and animals besides insect resistance. Thus, there is an urgent need to develop new pest management strategies with more specificity, efficiency, and sustainability. Chitin-degrading enzymes, including chitinases, are promising agents which may contribute to FAW control. Chitinase-producing microorganisms are reported normally in bacteria and fungi. In the present study, Serratia marcescens was successfully isolated and identified from the larvae of Spodoptera frugiperda. The bacterial strain NRC408 displayed the highest chitinase enzyme activity of 250 units per milligram of protein. Subsequently, the chitinase gene was cloned and heterologously expressed in E. coli BL21 (DE3). Recombinant chitinase B was overproduced to 2.5-fold, driven by the T7 expression system. Recombinant chitinase B was evaluated for its efficacy as an insecticidal bioagent against S. frugiperda larvae, which induced significant alteration in subsequent developmental stages and conspicuous malformations. Additionally, our study highlights that in silico analyses of the anticipated protein encoded by the chitinase gene (ChiB) offered improved predictions for enzyme binding and catalytic activity. The effectiveness of (ChiB) against S. frugiperda was evaluated in laboratory and controlled field conditions. The results indicated significant mortality, disturbed development, different induced malformations, and a reduction in larval populations. Thus, the current study consequently recommends chitinase B for the first time to control FAW.

Discovery of Rhodanine Inhibitors Targeting Of ChtI Based on the π-Stacking Effect and Aqueous Solubility

The application of some reported inhibitors against the chitinolytic enzyme Of ChtI was limited due to their unsatisfactory insecticidal activities. Hence, we first performed a synergetic design strategy combining the π-stacking effect with aqueous solubility to find novel rhodanine analogues with inhibitory activities against Of ChtI. Novel rhodanine compounds IAa-f and IBa-f have weak aqueous solubility, but they (IAd: Ki = 4.0 μM; IBd: Ki = 2.2 μM) showed better inhibitory activities against Of ChtI and comparable insecticidal efficiency toward Ostrinia furnacalis compared to the high aqueous solubility compounds IIAa-f and IIBa-f (IIAd: Ki = 21.6 μM; IIBd: Ki = 14.3 μM) without a large conjugate plane. Further optimized compounds IIIAa-j with a conjugate plane as well as a higher aqueous solubility exhibited similar good inhibitory activities against Of ChtI (IIIAe: Ki = 2.4 μM) and better insecticidal potency (IIIAe: mortality rate of 63.33%) compared to compounds IAa-f and IBa-f, respectively. Molecular docking studies indicated that the conjugate planarity with the π-stacking effect for rhodanine analogues is responsible for their enzyme inhibitory activity against Of ChtI. This study provides a new strategy for designing insect chitinolytic enzyme inhibitors as insect growth regulators for pest control.

Identification of Triazolo-quinazolinone Derivatives as Novel and Potent Chitinase OfChi-h Inhibitors Based on Structure-Based Virtual Screening

Insect chitinase, OfChi-h, from Ostrinia furnacalis, is considered as a promising target for the development of green pesticides. On the basis of the crystal structure of OfChi-h, we successfully obtained a triazolo-quinazolinone scaffold as the novel class of OfChi-h inhibitor via a structure-based virtual screening approach. Rational compound screening enabled us to acquire a potent OfChi-h inhibitor TQ19 with a Ki value of 0.33 μM. Furthermore, the in vivo biological activity of target compounds was assayed. The results showed that compounds TQ8 and TQ19 could dramatically inhibit the growth and development of Ostrinia nubilalis larvae, and most of the compounds showed higher insecticidal activity than hexaflumuron. This present work reveals that triazolo-quinazolinone derivatives can serve as novel candidates for insect growth regulators.

Silencing of Glutamine: Fructose-6-Phosphate Aminotransferase Impairs Growth and Development in Sogatella furcifera (Hemiptera: Delphacidae)

Glutamine: fructose-6-phosphate aminotransferase (GFAT), the fourth enzyme in the chitin synthesis pathway, exerts wide-ranging effects on the growth and development of organisms. However, the role of GFAT in Sogatella furcifera remains unknown. In this study, the functional significance of the GFAT gene of S. furcifera was analyzed using a reverse transcription-polymerase chain reaction and RNA interference (RNAi) analyses. The complementary DNA sequence of SfGFAT was 3162 bp in length and contained a 2067 bp open reading frame encoding 688 amino acid residues. Structural domain analysis indicated that the SfGFAT protein consisted of one glutamine aminotransferase class 2 domain and two sugar isomerase domains. Expression profile analysis revealed that SfGFAT was expressed throughout the egg, nymph, and adult phases and was strongly expressed on the first day of each nymph stage and in the integuments of five tissues. RNAi results revealed that SfGFAT gene silencing significantly inhibited the mRNA expression of the target gene and resulted in severe mortality among S. furcifera. In summary, these findings demonstrate that SfGFAT plays a critical role in the development of S. furcifera. Moreover, these results may aid in the development of methods to control the spread of S. furcifera.

An entomopathogenic fungus exploits its host humoral antibacterial immunity to minimize bacterial competition in the hemolymph

BACKGROUND

The insect hemolymph (blood-equivalent fluid), composed of a large number of hemocytes (blood cells) and a variety of soluble immune effectors, is hostile for pathogens including fungi. In order to survive in the insect hemocoel (body cavity), the entomopathogenic fungus (EPF) has evolved two classical coping strategies, namely evasion and suppression of the host immune reactions. However, it remains unclear whether EPF has other ways of coping with host immunity.

RESULTS

In this study, we demonstrated that Metarhizium rileyi (an EPF) infection by injection of blastospores into the hemocoel enhanced the plasma antibacterial activity of cotton bollworm (Helicoverpa armigera), which was partially due to the enhanced expression of antimicrobial peptides (AMPs). The early stage of M. rileyi infection induced the translocation of gut bacteria into the hemocoel, where they were subsequently cleared due to the enhanced plasma antibacterial activity. Further, we showed that the enhanced plasma antibacterial activity and AMP expression were attributable to M. rileyi but not the invasive gut bacteria (opportunistic bacteria). Elevated ecdysone (major steroid hormone in insects) levels in the hemolymph at 48 h post-M. rileyi infection might contribute to the enhanced expression of AMPs. The fungus-elicited AMPs, such as cecropin 3 or lebocin, exhibited potent inhibitory activity against the opportunistic bacteria but not against hyphal bodies. In addition, the opportunistic bacteria competed with hyphal bodies for amino acid nutrients.

CONCLUSIONS

M. rileyi infection induced the translocation of gut bacteria, and then the fungi activated and exploited its host humoral antibacterial immunity to eliminate opportunistic bacteria, preventing them from competing for nutrients in the hemolymph. Unlike the classical strategies, EPF utilizes to evade or suppress host immunity, our findings reveal a novel strategy of interaction between EPF and host immunity. Video Abstract.

Eco-friendly deacetylated chitosan base siRNA biological-nanopesticide loading cyromazine for efficiently controlling Spodoptera frugiperda

Spodoptera frugiperda is a serious threat to various crops, such as corn and rice, and results in severe economic losses. Herein, a chitin synthase sfCHS highly expressed in the epidermis of S. frugiperda was screened, and when interfered by an sfCHS-siRNA nanocomplex, most individuals could not ecdysis (mortality rate 53.3 %) or pupate (abnormal pupation 80.6 %). Based on the results of structure-based virtual screening, cyromazine (CYR, binding free energy -57.285 kcal/mol) could inhibit ecdysis (LC₅₀, 19.599 μg/g). CYR-CS/siRNA nanoparticles encapsulating CYR and SfCHS-siRNA with chitosan (CS) were successfully prepared, as confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and 74.9 mg/g CYR was characterized in the core of CYR-CS/siRNA by high-performance liquid chromatography and Fourier transform infrared spectroscopy. Small amounts of prepared CYR-CS/siRNA containing only 1.5 μg/g CYR could better inhibit chitin synthesis in the cuticle and peritrophic membrane (mortality rate 84.4 %). Therefore, chitosan/siRNA nanoparticle-loaded pesticides were useful for pesticide reduction and comprehensive control of S. frugiperda.

1,2-Dibenzoylhydrazine as a Multi-Inhibitor Compound: A Morphological and Docking Study

In the framework of the multitarget inhibitor study, we report an in silico analysis of 1,2-dibenzoylhydrazine (DBH) with respect to three essential receptors such as the ecdysone receptor (EcR), urease, and HIV-integrase. Starting from a crystallographic structural study of accidentally harvested crystals of this compound, we performed docking studies to evaluate the inhibitory capacity of DBH toward three selected targets. A crystal morphology prediction was then performed. The results of our molecular modeling calculations indicate that DBH is an excellent candidate as a ligand to inhibit the activity of EcR receptors and urease. Docking studies also revealed the activity of DBH on the HIV integrase receptor, providing an excellent starting point for developing novel inhibitors using this molecule as a starting lead compound.

Potential dsRNAs can be delivered to aquatic for defense pathogens

The use of antibiotics to facilitate resistance to pathogens in aquatic animals is a traditional method of pathogen control that is harmful to the environment and human health. RNAi is an emerging technology in which homologous small RNA molecules target specific genes for degradation, and it has already shown success in laboratory experiments. However, further research is needed before it can be applied in aquafarms. Many laboratories inject the dsRNA into aquatic animals for RNAi, which is obviously impractical and very time consuming in aquafarms. Therefore, to enable the use of RNAi on a large scale, the methods used to prepare dsRNA need to be continuously in order to be fast and efficient. At the same time, it is necessary to consider the issue of biological safety. This review summarizes the key harmful genes associated with aquatic pathogens (viruses, bacteria, and parasites) and provides potential targets for the preparation of dsRNA; it also lists some current examples where RNAi technology is used to control aquatic species, as well as how to deliver dsRNA to the target hydrobiont.

Knockdown of GFAT disrupts chitin synthesis in Hyphantria cunea larvae

Glutamine-fructose-6-phosphate transaminase (GFAT) has been reported to regulate the hexosamine biosynthetic pathway as the first rate-limiting enzyme. As a key enzyme that catalyzes the substrate of glycosylation modification, which has a wide-ranging effect on cellular functions. However, there are few studies on the relationship between GFAT and chitin metabolism in insects. In the present study, the GFAT gene from Hyphantria cunea was identified based on transcriptome and bioinformatic analysis. The role of HcGFAT in regulating development and chitin synthesis was analyzed by RNA interference (RNAi) in H. cunea larvae. The full-length HcGFAT gene (2028 bp) encodes a 676 amino acid (aa) polypeptide had typical structural features of the SIS and Gn_AT_II superfamily. Phylogenetic analyses showed that GFAT of H. cunea shares the highest homology and identity with GFAT of Ostrinia furnacalis. Expression profiles indicated that HcGFAT was expressed throughout larval, pupal and three tissues (midgut, fat body, epidermis), and highly expressed in the last instar of larvae and strongly expressed in epidermis among three tissues. Bioassay results showed that knockdown of HcGFAT repressed larval growth and development, resulting in a significant loss of larval body weight. Meanwhile, HcGFAT knockdown also significantly caused larval developmental deformity. Knockdown of HcGFAT regulated the expression of four other critical genes in the chitin synthesis pathway (HcGNA, HcPAGM, HcUAP, HcCHSA), and ultimately resulted in decreased chitin content in the epidermis. In summary, these findings indicated that GFAT plays a critical role in larval growth and development, as well as chitin synthesis in H. cunea.

Chitinase (CHI) of Spodoptera frugiperda affects molting development by regulating the metabolism of chitin and trehalose

Chitin is the main component of insect exoskeleton and midgut peritrophic membrane. Insect molting is the result of the balance and coordination of chitin synthesis and degradation in chitin metabolism under the action of hormones. In this study, a 678 bp dsRNA fragment was designed and synthesized according to the known CHI (Chitinase) sequence of Spodoptera frugiperda. It was injected into the larvae to observe the molting and development of S. frugiperda. At the same time, the activities of trehalase and chitinase, the contents of trehalose, chitin and other substances were detected, and the expression of related genes in the chitin synthesis pathway was determined. The results showed that CHI gene was highly expressed at the end of each instar, prepupa and pupal stage before molting; At 12 and 24 h after dsRNA injection of CHI gene of S. frugiperda, the expression of CHI gene decreased significantly, and the chitinase activity decreased significantly from 12 to 48 h. The expression of chitin synthase (CHSB) gene decreased significantly, and the chitin content increased significantly. Some larvae could not molt normally and complete development, leading to certain mortality. Secondly, after RNAi of CHI gene, the content of glucose and glycogen increased first and then decreased, while the content of trehalose decreased significantly or showed a downward trend. The activities of the two types of trehalase and the expression levels of trehalase genes decreased first and then increased, especially the trehalase activities increased significantly at 48 h after dsCHI injection. And trehalose-6-phosphate synthase (TPS), glutamine: fructose-6-phosphate amidotransferase (GFAT), UDP-N-acetylglucosamine pyrophosphorylases (UAP), hexokinase (HK), glucose-6-phosphate isomerase (G6PI) and phosphoacetylglucosamine mutase (PAGM) all decreased significantly at 24 h, and then increased or significantly increased at 48 h. These results indicated that when the expression of chitinase gene of S. frugiperda was inhibited, it affected the degradation of chitin in the old epidermis and the formation of new epidermis, and the content of chitin increased, which led to the failure of larvae to molt normally. Moreover, the chitin synthesis pathway and trehalose metabolism were also regulated. The relevant results provide a theoretical basis for screening target genes and developing green insecticides to control pests by using the chitin metabolism pathway.

Identification and RNAi-based functional analysis of chitinase family genes in Agrotis ipsilon

BACKGROUND

Chitin is a major component in the extracellular matrix of insects, and its metabolism largely affects insect development and molting. As essential degradative enzymes, chitinases are encoded by multiple genes that differ in size, expression pattern and function in insects. However, our limited knowledge on the functions of different chitinases in Agrotis ipsilon has prevented our application of new technologies to target these genes as new pest management strategies.

RESULTS

We revealed 11 full-length complementary DNA sequences of chitinase genes (AiChts) from A. ipsilon transcriptome. Although the domain architecture of these chitinases varied greatly, they all contained at least one chitinase catalytic domain. Developmental stage- and tissue-dependent expression profiles showed that most AiChts had the highest expression in the pupal stage. Furthermore, AiCht2, AiCht6, AiCht7 and AiCht10 were mainly expressed in the integument, whereas AiCht8 and AiCht-h had the highest expression in the midgut. The RNA interference (RNAi) experiment revealed that knockdown of AiCht10 or the imaginal disc growth factor gene (AiIDGF) induced high larval mortality. Larvae failed to shed the old cuticle during molting after the injection of double-stranded RNA targeting AiCht10 (dsAiCht10), whereas the larval bodies shrunk and blackened after the injection of dsRNA targeting AiIDGF (dsAiIDGF).

CONCLUSION

Our results revealed for the first time the important functions of AiCht10 and AiIDGF in A. ipsilon. These genes are essential for larval development, and can potentially serve as new targets for RNAi-based pest management. © 2022 Society of Chemical Industry.

Discovery of Azo-Aminopyrimidines as Novel and Potent Chitinase OfChi-h Inhibitors via Structure-Based Virtual Screening and Rational Lead Optimization

Chitinase OfChi-h, from the destructive agricultural pest Ostrinia furnacalis, is considered as a promising target for green pest control and management. In this study, structure-based virtual screening and rational molecular optimization led to the synthesis of a series of azo-aminopyrimidine derivatives as a novel class of OfChi-h inhibitors. Among them, the most potent compound 8f, with a benzyl on the amino group at the 4-position of pyrimidine, exhibited a K_i value of 64.7 nM against OfChi-h. In addition, molecular docking studies were carried out to investigate the basis for the potency of the aminopyrimidines against OfChi-h. Furthermore, the insecticidal activity of the target compounds against Plutella xylostella and Ostrinia nubilalis was assessed, and the potent OfChi-h inhibitors 8f and 8i showed higher insecticidal activity than the control pesticide hexaflumuron. The present work revealed that the azo-aminopyrimidine skeletons characterized by concise chemical structure and high efficiency could be further developed as potential pesticides for the control of lepidopteran pests.

Rational Design and Identification of Novel Piperine Derivatives as Multichitinase Inhibitors

Asian corn borer (Ostrinia furnacalis) is one of the most destructive pests in agriculture. Three chitinases OfChtI, OfChtII, and OfChi-h are regarded as potential targets for discovering novel agrochemicals to control O. furnacalis. In this study, piperine (K_i = 43.78∼83.03 μM) was first shown to exhibit inhibitory activities against all three chitinases. Subsequently, 19 novel piperine derivatives were rationally designed based on the conserved aromatic residues of three chitinases and then synthesized. Among them, Compound 5k (K_i = 11.78∼22.82 μM) was identified as the most effective multichitinase inhibitor and indeed displayed higher insecticidal activity against O. furnacalis than dual- or single-chitinase inhibitors. Molecular mechanism studies clarified that Compound 5k interacted with two conserved TRP and TYR of three chitinases in identical modes through hydrogen bonds, hydrophobic, and π-π interactions. Moreover, the microinjection experiment indicated that Compound 5k exhibited substantial sublethal effects against O. furnacalis by regulating its growth and development. This study provides evidence of multichitinase inhibitors to be applied in the control of O. furnacalis.

Knockdown of UDP-N-acetylglucosamine pyrophosphorylase and chitin synthase A increases the insecticidal efficiency of Lufenuron to Spodoptera exigua

Spodoptera exigua (Lepidoptera, Noctuidae) has been responsible for causing considerable and widespread agricultural losses worldwide. Owing to strong selective pressure, S. exigua showed increased resistance to Lufenuron (LUF). Consequently, RNA interference (RNAi)-based insecticides had more benefits than chemical insecticides. Therefore, to enhance the insecticidal activity of LUF to S. exigua, in the present study, we aimed to elucidate the impact of double-stranded RNAs (dsRNAs) on S. exigua larval susceptibility to LUF. First, the transcriptome of S. exigua was sequenced following the treatment with LUF. By comparing the upregulated and downregulated GO enrichment, chitin binding and chitin metabolic processes were the significantly enriched pathways. According to transcriptome sequencing, 8 genes associated with chitin biosynthesis, 8 chitin degradation genes, and 17 cuticle protein genes were obtained. UDP-N-acetylglucosamine pyrophosphorylase (UAP) and Chitin synthase A (CHSA) showed significantly downregulated expression after treatment with different sublethal doses of LUF. Downregulation of UAP increased mortality from 31.97% to 47.91% when the larvae were exposed to LUF. A significant increase in the mortality of S. exigua from 30.63% to 50.19% was observed following LUF administration after dsCHSA. In addition, the expression analysis of genes associated with chitin biosynthesis was significantly changed after LUF treatment, dsRNAs-RNAi, and their combination (LUF-dsRNAs). Significant differences were observed in the chitin content between the control group at 72 h after treatments. Results of the present study can help further elucidate the understanding of the combined effects of RNAi and LUF on S. exigua. Additionally, this research provides a suitable foundation for future studies with the aim to develop an efficient method of delivery for large-scale pest control in the fields.

Identification and Functional Characterization of Argonaute (Ago) Proteins in Insect Genomes

RNA processing is a vital process in all organisms. In eukaryotes, the RNA induced silencing complex (RISC) mediates this function during development and physiological processes and, at least in arthropods, during RNA-viral infections. Argonaute-like RNA-binding proteins are central components of this complex. RNA-based insecticides are gaining more and more a central role in pest control. Understanding of the underlying molecular mechanisms including Ago-like proteins is crucial in designing powerful, species-specific and environmental-friendly insecticides. This chapter describes a protocol for identification and genetic functional analyses of insect Ago-like proteins in the fruit fly Drosophila melanogaster that serves as a living test tube.

Discovery of Natural Products as Multitarget Inhibitors of Insect Chitinolytic Enzymes through High-Throughput Screening

Small-molecule inhibitors of insect chitinolytic enzymes are potential insecticides. However, the reported inhibitors that target one enzyme usually exhibit unsatisfactory bioactivity. On the basis of the multitarget strategy, we performed a high-throughput screening of a natural product library to find insecticide leads against four chitinolytic enzymes from the Asian corn borer Ostrinia furnacalis (OfChtI, OfChtII, OfChi-h, and OfHex1). Several phytochemicals were discovered to be multitarget inhibitors of these enzymes and were predicted to occupy the -1 substrate-binding subsite and engage in polar interactions with catalytically important residues. Shikonin and wogonin, which had good inhibitory activities toward all four enzymes, also exhibited significant insecticidal activities against lepidopteran agricultural pests. This study provides the first example of using a multitarget high-throughput screening strategy to exploit natural products as insecticide leads against chitin biodegradation during insect molting.

A Piperine-Based Scaffold as a Novel Starting Point to Develop Inhibitors against the Potent Molecular Target OfChtI

The insect chitinase OfChtI from the agricultural pest Ostrinia furnacalis (Asian corn borer) is a promising target for green insecticide design. OfChtI is a critical chitinolytic enzyme for the cuticular chitin degradation at the stage of molting. In this study, piperine, a natural amide compound isolated from black pepper, Piper nigrum L., was discovered for the first time to have inhibitory activity toward OfChtI. The compound-enzyme interaction was presumed to take place between the piperine benzo[d][1,3] dioxole skeleton and subsite -1 of the substrate-binding pocket of OfChtI. Hence, on the basis of the deduced inhibitory mechanism and crystal structure of the substrate-binding cavity of OfChtI, compounds 5a-f were designed and synthesized by introducing a butenolide scaffold into the lead compound piperine. The enzymatic activity assay indicated that compounds 5a-f (K_i = 1.03-2.04 μM) exhibited approximately 40-80-fold higher inhibitory activity than the lead compound piperine (I) (K_i = 81.45 μM) toward OfChtI. The inhibitory mechanism of the piperonyl butenolide compounds was elucidated by molecular dynamics, which demonstrated that the introduced butenolide skeleton improved the binding affinity to OfChtI. Moreover, the in vivo activity assay indicated that these compounds also displayed moderate insecticidal activity toward O. furnacalis. This work introduces the natural product piperine as a starting point for the development of novel insecticides targeting OfChtI.

In silico characterization of chitin deacetylase genes in the Diaphorina citri genome

Chitin deacetylases (CDAs) are one of the least understood components of insect chitin metabolism. The partial deacetylation of chitin polymers appears to be important for the proper formation of higher order chitin structures, such as long fibers and bundles, which contribute to the integrity of the insect exoskeleton and other structures. Some CDAs may also be involved in bacterial defense. Here, we report the manual annotation of four CDA genes from the Asian citrus psyllid, Diaphorina citri, laying the groundwork for future study of these genes.

Annotation of chitin biosynthesis genes in Diaphorina citri, the Asian citrus psyllid

The polysaccharide chitin is critical for the formation of many insect structures, including the exoskeleton, and is required for normal development. Here we report the annotation of three genes from the chitin synthesis pathway in the Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), the vector of Huanglongbing (citrus greening disease). Most insects have two chitin synthase (CHS) genes but, like other hemipterans, D. citri has only one. In contrast, D. citri is unusual among insects in having two UDP-N-acetylglucosamine pyrophosphorylase (UAP) genes. One of the D. citri UAP genes is broadly expressed, while the other is expressed predominantly in males. Our work helps pave the way for potential utilization of these genes as pest control targets to reduce the spread of Huanglongbing.

Molecular Characterization of Chitin Synthase Gene in Tetranychus cinnabarinus (Boisduval) and Its Response to Sublethal Concentrations of an Insecticide

The carmine spider mite, Tetranychus cinnabarinus (Boisduval), is one of the most important acarine pest species. At present, its control remains primarily dependent on using various chemical insecticides/acaricides in agricultural crops worldwide. To clarify the mechanism whereby T. cinnabarinus responds to insecticide exposure, we identified the chitin synthase 1 gene (TcCHS1) and then explored the gene expression levels of TcCHS1 at different developmental stages of T. cinnabarinus. We also investigated the effects of sublethal concentrations of diflubenzuron on the toxicities and survivals of T. cinnabarinus eggs and larvae as well as TcCHS1 expression levels. The full-length cDNA sequence contains an open reading frame (ORF) of 4881 nucleotides that encoded for a 1474 amino acid residues protein. The predicted TcCHS1 protein had a molecular mass of 168.35 kDa and an isoelectric point of 6.26, and its amino acid sequence contained all the signature motifs (EDR, QRRRW and TWGTR) of chitin synthases. The results of phylogenetic analyses demonstrated that the putative CHS1 amino acid sequence of T. cinnabarinus revealed high similarities with chitin synthases in other insects and mites. Additionally, at the molecular level, transcriptional analysis by real-time quantitative PCR in different developmental stages of T. cinnabarinus revealed that TcCHS1 mRNA was expressed in all stages, and highest in eggs and female adults, but lowest in deutonymphs. Furthermore, the results of toxicity bioassays indicated that diflubenzuron treatment resulted in high mortality rates in eggs and larvae of T. cinnabarinus. The mRNA expression levels of TcCHS1 from the eggs and larvae of T. cinnabarinus were up-regulated in response to sublethal concentrations of diflubenzuron exposures. Together, all these results demonstrate that diflubenzuron has ovicidal and larvicidal effects and TcCHS1 may play an important role in the growth and development of T. cinnabarinus and may disrupt the chitin biosynthesis, thereby controlling T. cinnabarinus populations.

Drosophila, Chitin and Insect Pest Management

Insects are a great menace in agriculture and vectors of human diseases. Hence, controlling insect populations is an important issue worldwide. A common strategy to control insects is the application of insecticides. However, insecticides entail three major problems. First, insecticides are chemicals that stress ecosystems and may even be harmful to humans. Second, insecticides are often unspecific and also eradicate beneficial insect species like the honeybee. Third, insects are able to develop resistance to insecticides. Therefore, the efficient generation of new potent insecticides and their intelligent delivery are the major tasks in agriculture. In addition, acceptance or refusal in society is a major issue that has to be considered in the application of a pest management strategy. In this paper, we unify two issues: 1) we illustrate that our molecular knowledge of the chitin synthesis and organization pathways may offer new opportunities to design novel insecticides that are environmentally harmless at the same time being specific to a pest species; and 2) we advocate that the fruit fly Drosophila melanogaster may serve as an excellent model of insect to study the effects of insecticides at the genetic, molecular and histology level in order to better understand their mode of action and to optimize their impact. Especially, chitin synthesis and organization proteins and enzymes are excellently dissected in the fruit fly, providing a rich source for new insecticide targets. Thus, D. melanogaster offers a cheap, efficient and fast assay system to address agricultural questions, as has been demonstrated to be the case in bio-medical research areas.

Novaluron prevents oogenesis and oviposition by inducing ultrastructural changes in ovarian tissue of young adult Lygus lineolaris

BACKGROUND

The tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), has emerged as a major pest of cotton, Gossypium hirsutum L, in the mid-southern USA. In the early 1990s L. lineolaris populations developed resistance to several classes of conventional insecticides, increasing the need for insecticides with alternative modes of action such as insect growth regulators (IGRs) for integrated pest management (IPM). The benzoylphenyl urea (BPU) class of IGRs acts by disrupting the growth and development of immature stages of insects, but little is known about its impact on adult stages.

RESULTS

The effect of novaluron (Diamond™ 0.83EC), a BPU with known chitin synthesis inhibitor activity, was investigated on adult females of L. lineolaris. Treatment of 1-day-old adults with 600 ppm of novaluron in the diet prevented oviposition, while treatment of older females had no impact on oviposition. Oral novaluron exposure of adults of all ages reduced the viability of eggs laid. Novaluron treatment caused ultrastructural changes in the ovaries of 1-day-old adults (48 h post exposure), distorting the follicular epithelial cell architecture of developing oocytes. Additionally, novaluron treatment decreased the chitin content in ovarian tissue.

CONCLUSION

Our results suggest that chitin or chitin-like components in the developing ovaries of adult L. lineolaris are a target of IGRs such as novaluron, but its activity is specific to a critical time during development. This enhances our understanding of the effects of BPUs on adult insects and could lead to incorporation of IGRs in IPM for controlling adult insect pest populations in the field. © 2020 Society of Chemical Industry.

Identification of Peritrophins and Antiviral Effect of Bm01504 against BmNPV in the Silkworm, Bombyx mori

The insect midgut secretes a semi-permeable, acellular peritrophic membrane (PM) that maintains intestinal structure, promotes digestion, and protects the midgut from food particles and pathogenic microorganisms. Peritrophin is an important PM protein (PMP) in the PM. Here, we identified 11 peritrophins with 1–16 chitin binding domains (CBDs) comprising 50–56 amino acid residues. Multiple CBDs in the same peritrophin clustered together, rather than by species. The CBD contained six highly conserved cysteine residues, with the key feature of amino acids between them being CX_11-15CX₅CX_9-14CX_11-12CX_6-7C. Peritrophins with 2 and 4 CBDs (Bm09641 and Bm01504, respectively), and with 1, 8, and 16 CBDs (Bm11851, Bm00185, and Bm01491, respectively) were mainly expressed in the anterior midgut, and throughout the midgut, respectively. Survival rates of transgenic silkworms with Bm01504 overexpression (Bm01504-OE) and knockout (Bm01504-KO) infected with B. morinucleopolyhedrovirus (BmNPV) were significantly higher and lower, whereas expression of the key viral gene, p10, were lower and higher, respectively, compared with wild type (WT). Therefore, Bm01504-OE and Bm01504-KO transgenic silkworms were more and less resistant, respectively, to BmNPV. Bm01504 plays important roles in resisting BmNPV invasion. We provide a new perspective for studying PM function, and reveal how the silkworm midgut resists invasive exogenous pathogenic microorganisms.

Effect of RNAi-mediated silencing of two Knickkopf family genes (LmKnk2 and LmKnk3) on cuticle formation and insecticide susceptibility in Locusta migratoria

BACKGROUND

Knickkopf (Knk) proteins play crucial roles in the formation of insect cuticle. Recent studies in the holometabolous insect red flour beetle (Tribolium castaneum) have shown that three Knk genes encoding TcKnk, TcKnk2 and TcKnk3 play different but essential roles at different developmental stages and in different tissues. However, the functions of Knk genes had not been fully explored in hemimetabolous insects such as the migratory locust Locusta migratoria.

RESULTS

We identified three transcripts of LmKnk-like genes LmKnk2 and LmKnk3 with the full-length cDNA sequences, which were named as LmKnk2, LmKnk3-FL and LmKnk3-5'. These three transcripts were highly expressed before molting and mainly expressed in the integument. Among these genes, silencing only LmKnk3-5' by RNA interference (RNAi) caused molting defects and high mortality of the locusts. Injection of dsLmKnk3-5' dramatically decreased chitin content, but did not affect cuticle laminar ultra-structures in the integument. After the knockdown of LmKnk3-5' transcript, lipid deposition on the cuticle surface was impeded, and locusts exhibited increased susceptibility to each of four insecticides in three different classes. However, no visible phenotypic changes were observed after LmKnk2 or LmKnk3-FL was silenced by RNAi.

CONCLUSION

We demonstrate that LmKnk3-5' is essential for cuticle formation in L. migratoria. This contrasts the findings that the cognate protein in T. castaneum TcKnk3-5' is dispensable for cuticle formation and survival. Hence, we provide some evidence that the function of Knk-type proteins may be species-specific. We therefore think that LmKnk3-5' may be a good target for the application of RNAi-based technologies for species-specific insect pest management. © 2020 Society of Chemical Industry.

Feeding on tea GH19 chitinase enhances tea defense responses induced by regurgitant derived from Ectropis grisescens

Multiple isoforms of chitinases participate in plant defense against outside invaders. However, the functions of hydrolase family 19 (GH19) chitinases on pest control remain largely unknown. Here we reported the isolation and functional analysis of a gene CsChi19, which encodes a GH19 endochitinase protein of 332 amino acid residues from tea plant (Camellia sinensis). CsChi19 expression levels were upregulated in response to mechanical wounding, infestation by two important pests: the tea geometrid Ectropis grisescens and the tea green leafhopper Empoasca (Matsumurasca) onukii, a fungal pathogen Colletotrichum fructicola, and treatment with two phytohormones: jasmonic acid (JA) and salicylic acid. CsChi19 was heterologously expressed in Escherichia coli, and its catalytic function was further elucidated. The protein could hydrolyze colloidal chitin, and the optimum temperature and pH for its activity was 40°C and pH 5.0. CsChi19 were found to be toxic to tea pests when they were fed on artificial diets containing this protein. Interestingly, the regurgitant derived from E. grisescens fed with artificial diets containing CsChi19 protein induced stronger expression of CsMPK3, more JA burst, more accumulation of defense-related secondary metabolites, and more emission of volatiles than the regurgitant derived from E. grisescens fed only with artificial diets. Our results provide first evidence that CsChi19 is involved in mediating a novel defense mechanism of tea plant through altering the composition of the regurgitant.

Identification of critical enzymes in the salmon louse chitin synthesis pathway as revealed by RNA interference-mediated abrogation of infectivity

Treatment of infestation by the ectoparasite Lepeophtheirus salmonis relies on a small number of chemotherapeutant treatments that currently meet with limited success. Drugs targeting chitin synthesis have been largely successful against terrestrial parasites where the pathway is well characterised. However, a comparable approach against salmon lice has been, until recently, less successful, likely due to a poor understanding of the chitin synthesis pathway. Post-transcriptional silencing of genes by RNA interference (RNAi) is a powerful method for evaluation of protein function in non-model organisms and has been successfully applied to the salmon louse. In the present study, putative genes coding for enzymes involved in L. salmonis chitin synthesis were characterised after knockdown by RNAi. Nauplii I stage L. salmonis were exposed to double-stranded (ds) RNA specific for several putative non-redundant points in the pathway: glutamine: fructose-6-phosphate aminotransferase (LsGFAT), UDP-N-acetylglucosamine pyrophosphorylase (LsUAP), N-acetylglucosamine phosphate mutase (LsAGM), chitin synthase 1 (LsCHS1), and chitin synthase 2 (LsCHS2). Additionally, we targeted three putative chitin deacetylases (LsCDA4557, 5169 and 5956) by knockdown. Successful knockdown was determined after moulting to the copepodite stage by real-time quantitative PCR (RT-qPCR), while infectivity potential (the number of attached chalimus II compared with the initial number of larvae in the system) was measured after exposure to Atlantic salmon and subsequent development on their host. Compared with controls, infectivity potential was not compromised in dsAGM, dsCHS2, dsCDA4557, or dsCDA5169 groups. In contrast, there was a significant effect in the dsUAP-treated group. However, of most interest was the treatment with dsGFAT, dsCHS1, dsCHS1+2, and dsCDA5956, which resulted in complete abrogation of infectivity, despite apparent compensatory mechanisms in the chitin synthesis pathway as detected by qPCR. There appeared to be a common phenotypic effect in these groups, characterised by significant aberrations in appendage morphology and an inability to swim. Ultrastructurally, dsGFAT showed a significantly distorted procuticle without distinct exo/endocuticle and intermittent electron dense (i.e. chitin) inclusions, and together with dsUAP and dsCHS1, indicated delayed entry to the pre-moult phase.