The peritrophic membrane as a target of proteins that play important roles in plant defense and microbial attack

Insecticidal action of mammalian galectin-1-transfected Arabidopsis thaliana

BACKGROUND

Galectins (GALs) are a family of mammalian sugar-binding proteins specific for β-galactosides. Our previous studies have shown that the larval development of the diamondback moth (Plutella xylostella) is significantly disturbed when fed with recombinant mammalian galectin 1 (GAL1) derived from Escherichia coli. To further explore its applicability, two GAL1-overexpressed Arabidopsis [GAL1-Arabidopsis (whole plant) and GAL1-Arabidopsis-vas (vascular bundle-specific)] lines were established for insecticidal activity and mechanism studies.

RESULTS

The expression level of GAL1 in transgenic Arabidopsis is 1-0.5% (GAL1-Arabidopsis) and 0.08-0.01% (GAL1-Arabidopsis-vas) of total leaf soluble protein. Survival, body weight, and food consumption significantly decreased in a time-dependent manner in P. xylostella larvae (with chewing mouthparts) fed on GAL1-Arabidopsis. The mortality of Kolla paulula (with piercing-sucking mouthparts and xylem feeder) fed on GAL1-Arabidopsis-vas was also significantly higher than that fed on wild-type Arabidopsis (WT-Arabidopsis), but was lower than that fed on GAL1-Arabidopsis. The histochemical structure and results of immunostaining suggested that the binding of GAL1 to the midgut epithelium of P. xylostella fed on GAL1-Arabidopsis was dose- and time-dependent. Ultrastructural studies further showed the disruption of microvilli, abnormalities in epithelial cells, and fragments of the peritrophic membrane (PM) in P. xylostella larvae fed on GAL1-Arabidopsis.

CONCLUSION

The insecticidal mechanism of GAL1 involves interference with PM integrity and suggests that GAL1 is a potential candidate for bioinsecticide development. © 2024 Society of Chemical Industry.

Potential mechanisms implied in tick infection by arboviruses and their transmission to vertebrate hosts

Ticks can transmit many pathogens, including arboviruses, to their vertebrate hosts. Arboviruses must overcome or evade defense mechanisms during their passage from the tick gut to the hemolymph, salivary glands, and the feeding site in the host skin. This review summarizes current knowledge of defense mechanisms in specific tick tissues and at the feeding site in the host skin. We discuss the possible roles of these defense mechanisms in viral infection and transmission. The responses of tick salivary proteins to arbovirus infection are also discussed. This review provides information that may help accelerate research on virus-tick interactions.

Preliminary Study on the Pathogenic Mechanism of Jujube Flower Disease in Honeybees (Apis mellifera ligustica) Based on Midgut Transcriptomics

Honeybees are prone to poisoning, also known as jujube flower disease, after collecting nectar from jujube flowers, resulting in the tumultuous demise of foragers. The prevalence of jujube flower disease has become one of the main factors affecting the development of the jujube and beekeeping industries in Northern China. However, the pathogenic mechanisms underlying jujube flower disease in honeybees are poorly understood. Herein, we first conducted morphological observations of the midgut using HE-staining and found that jujube flower disease-affected honeybees displayed midgut damage with peritrophic membrane detachment. Jujube flower disease was found to increase the activity of chitinase and carboxylesterase (CarE) and decrease the activity of superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), and the content of CYP450 in the honeybee midgut. Transcriptomic data identified 119 differentially expressed genes in the midgut of diseased and healthy honeybees, including CYP6a13, CYP6a17, CYP304a1, CYP6a14, AADC, and AGXT2, which are associated with oxidoreductase activity and vitamin binding. In summary, collecting jujube flower nectar could reduce antioxidant and detoxification capacities of the honeybee midgut and, in more severe cases, damage the intestinal structure, suggesting that intestinal damage might be the main cause of honeybee death due to jujube nectar. This study provides new insights into the pathogenesis of jujube flower disease in honeybees.

Tick symbiosis

As obligate blood-feeders, ticks serve as vectors for a variety of pathogens that pose threats on both human and livestock health. The microbiota that ticks harbor play important roles in influencing tick nutrition, development, reproduction, and vector. These microbes also affect the capacity of ticks to transmit pathogens (vector competence). Therefore, comprehending the functions of tick microbiota will help in developing novel and effective tick control strategies. Here, we summarize the effects of main tick symbiotic bacteria on tick physiology and vector competency.

Three-dimensional correlative microscopy of the Drosophila female reproductive tract reveals modes of communication in seminal receptacle sperm storage

In many taxa, females store sperm in specialized storage organs. Most insect sperm storage organs have a tubular structure, typically consisting of a central lumen surrounded by epithelial cells. These specialized tubules perform the essential tasks of transporting sperm through the female reproductive tract and supporting long-term sperm survival and function. Little is known about the way in which female sperm storage organs provide an environment conducive to sperm survival. We address this using a combined light microscopy, micro computed tomography (microCT), and Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) approach for high-resolution correlative three-dimensional imaging to advance our understanding of sperm-female interactions in Drosophila melanogaster. Using this multimodal approach, we were able to scan the lower female reproductive tract and distal portion of the seminal receptacle at low magnification, and to subsequently zoom in for further analysis on an ultrastructural level. Our findings highlight aspects of the way in which the seminal receptacle keeps sperm viable in the lumen, and set the stage for further studies. The methods developed are suitable not only for Drosophila but also for other organisms with soft, delicate tissues.

Effects of non-lethal Cry1F toxin exposure on the growth, immune response, and intestinal microbiota of silkworm (Bombyx mori)

Bt (Bacillus thuringiensis) maize is expected to be commercial cultivated widely in China. When Bt maize is planted near mulberry trees, it renders silkworms (Bombyx mori) vulnerable, as they belong to the same class as the Lepidoptera insects targeted by Bt maize. Cry1F has been found to be highly toxic to silkworms, particularly in their early larval stages. In this study, we aimed to assess the effects of non-lethal Cry1F exposure on the growth, immune response, and intestinal microbiota in silkworms. The results showed that feeding silkworms with mulberry leaves soaked in 100 μg/mL Cry1F for 96 h had an impact on larval body weight acquisition, leading to a decrease in cocoon and pupae weight. Cry1F exposure disrupted the intestinal integrity of silkworms by affecting the columnar cells of the midgut. The activity of detoxification enzymes (CarE, AChE, and GST) as well as antioxidant enzymes (SOD, CAT, and POD) were also affected by Cry1F. After 96 h Cry1F exposure, the evenness of the bacterial community was disrupted, resulting in alterations in the structure of the intestinal microbiota. Additionally, Cry1F exposure affected the relative expression levels of the peritrophic membrane (PM) protein and the corresponding immune pathways genes of silkworms. Most of the immune-related gene expressions were inhibited after exposure to Cry1F toxin but increased with prolonged treatment. This study demonstrates that non-lethal Cry1F exposure can affect the growth, immune response, and intestinal microbiota of silkworm.

The pivotal roles of gut microbiota in insect plant interactions for sustainable pest management

The gut microbiota serves as a critical "organ" in the life cycle of animals, particularly in the intricate interplay between herbivorous pests and plants. This review summarizes the pivotal functions of the gut microbiota in mediating the insect-plant interactions, encompassing their influence on host insects, modulation of plant physiology, and regulation of the third trophic level species within the ecological network. Given these significant functions, it is plausible to harness these interactions and their underlying mechanisms to develop novel eco-friendly pest control strategies. In this context, we also outline some emerging pest control methods based on the intestinal microbiota or bacteria-mediated interactions, such as symbiont-mediated RNAi and paratransgenesis, albeit these are still in their nascent stages and confront numerous challenges. Overall, both opportunities and challenges coexist in the exploration of the intestinal microbiota-mediated interactions between insect pests and plants, which will not only enrich the fundamental knowledge of plant-insect interactions but also facilitate the development of sustainable pest control strategies.

RNAi-mediated CHS-2 silencing affects the synthesis of chitin and the formation of the peritrophic membrane in the midgut of Aedes albopictus larvae

BACKGROUND

Mosquitoes are an important vector of viral transmission, and due to the complexity of the pathogens they transmit, vector control may be the most effective strategy to control mosquito-borne diseases. Chitin is required for insect growth and development and is absent in higher animals and plants, so regulating the chitin synthesis pathway can serve as a potentially effective means to control vector insects. Most of the current research on the chitin synthase (CHS) gene is focused on chitin synthase-1 (CHS-1), while relatively little is known about chitin synthase-2 (CHS-2).

RESULTS

The CHS-2 gene of Ae. albopictus is highly conserved and closely related to that of Aedes aegypti. The expression of CHS-2 in the third-instar larvae and pupal stage of Ae. albopictus was relatively high, and CHS-2 expression in adult mosquitoes reached the highest value 24 h after blood-feeding. In the fourth-instar larvae of Ae. albopictus, CHS-2 expression was significantly higher in the midgut than in the epidermis. Silencing CHS-2 in Ae. albopictus larvae had no effect on larval survival and emergence. The expression of four genes related to chitin synthesis enzymes was significantly upregulated, the expression level of three genes was unchanged, and only the expression level of GFAT was significantly downregulated. The expression of chitin metabolism-related genes was also upregulated after silencing. The level of chitin in the midgut of Ae. albopictus larvae was significantly decreased, while the chitinase activity was unchanged. The epithelium of the midgut showed vacuolization, cell invagination and partial cell rupture, and the structure of the peritrophic membrane was destroyed or even absent.

METHODS

The expression of CHS-2 in different developmental stages and tissues of Aedes albopictus was detected by real-time fluorescence quantitative PCR (qPCR). After silencing CHS-2 of the fourth-instar larvae of Ae. albopictus by RNA interference (RNAi), the expression levels of genes related to chitin metabolism, chitin content and chitinase activity in the larvae were detected. The structure of peritrophic membrane in the midgut of the fourth-instar larvae after silencing was observed by paraffin section and hematoxylin-eosin (HE) staining.

CONCLUSION

CHS-2 can affect midgut chitin synthesis and breakdown by regulating chitin metabolic pathway-related genes and is involved in the formation of the midgut peritrophic membrane in Ae. albopictus, playing an important role in growth and development. It may be a potential target for enhancing other control methods.

Peritrophin-like Genes Are Associated with Delousing Drug Response and Sensitivity in the Sea Louse Caligus rogercresseyi

Caligus rogercresseyi is the main ectoparasite that affects the salmon industry in Chile. The mechanisms used by the parasite to support its life strategy are of great interest for developing control strategies. Due to the critical role of insect peritrophins in host–parasite interactions and response to pest control drugs, this study aimed to identify and characterize the peritrophin-like genes present in C. rogercresseyi. Moreover, the expression of peritrophin-like genes was evaluated on parasites exposed to delousing drugs such as pyrethroids and azamethiphos. Peritrophin genes were identified by homology analysis among the sea louse transcriptome database and arthropods peritrophin-protein database obtained from GenBank and UniProt. Moreover, the gene loci in the parasite genome were located. Furthermore, peritrophin gene expression levels were evaluated by RNA-Seq analysis in sea louse developmental stages and sea lice exposed to delousing drugs deltamethrin, cypermethrin, and azamethiphos. Seven putative peritrophin-like genes were identified in C. rogercresseyi with high homology with other crustacean peritrophins. Differences in the presence of signal peptides, the number of chitin-binding domains, and the position of conserved cysteines were found. In addition, seven peritrophin-like gene sequences were identified in the C. rogercresseyi genome. Gene expression analysis revealed a stage-dependent expression profile. Notably, differential regulation of peritrophin genes in resistant and susceptible populations to delousing drugs was found. These data are the first report and characterization of peritrophin genes in the sea louse C. rogercresseyi, representing valuable knowledge to understand sea louse biology. Moreover, this study provides evidence for a deeper understanding of the molecular basis of C. rogercresseyi response to delousing drugs.

Analysis of intact prophages in genomes of Paenibacillus larvae: An important pathogen for bees

Paenibacillus larvae is the etiological agent of American Foulbrood (AFB), a highly contagious and worldwide spread bacterial disease that affects honeybee brood. In this study, all complete P. larvae genomes available on the NCBI database were analyzed in order to detect presence of prophages using the PHASTER software. A total of 55 intact prophages were identified in 11 P. larvae genomes (5.0 ± 2.3 per genome) and were further investigated for the presence of genes encoding relevant traits related to P. larvae. A closer look at the prophage genomes revealed the presence of several putative genes such as metabolic and antimicrobial resistance genes, toxins or bacteriocins, potentially influencing host performance. Some of the coding DNA sequences (CDS) were present in all ERIC-genotypes, while others were only found in a specific genotype. While CDS encoding toxins and antitoxins such as HicB and MazE were found in prophages of all bacterial genotypes, others, from the same category, were provided by prophages particularly to ERIC I (enhancin-like toxin), ERIC II (antitoxin SocA) and ERIC V strains (subunit of Panton-Valentine leukocidin system (PVL) LukF-PV). This is the first in-depth analysis of P. larvae prophages. It provides better knowledge on their impact in the evolution of virulence and fitness of P. larvae, by discovering new features assigned by the viruses.

Diversity and Functional Roles of the Gut Microbiota in Lepidopteran Insects

Lepidopteran insects are one of the most widespread and speciose lineages on Earth, with many common pests and beneficial insect species. The evolutionary success of their diversification depends on the essential functions of gut microorganisms. This diverse gut microbiota of lepidopteran insects provides benefits in nutrition and reproductive regulation and plays an important role in the defence against pathogens, enhancing host immune homeostasis. In addition, gut symbionts have shown promising applications in the development of novel tools for biological control, biodegradation of waste, and blocking the transmission of insect-borne diseases. Even though most microbial symbionts are unculturable, the rapidly expanding catalogue of microbial genomes and the application of modern genetic techniques offer a viable alternative for studying these microbes. Here, we discuss the gut structure and microbial diversity of lepidopteran insects, as well as advances in the understanding of symbiotic relationships and interactions between hosts and symbionts. Furthermore, we provide an overview of the function of the gut microbiota, including in host nutrition and metabolism, immune defence, and potential mechanisms of detoxification. Due to the relevance of lepidopteran pests in agricultural production, it can be expected that the research on the interactions between lepidopteran insects and their gut microbiota will be used for biological pest control and protection of beneficial insects in the future.

Concerted impacts of antiherbivore defenses and opportunistic Serratia pathogens on the fall armyworm (Spodoptera frugiperda)

Insects frequently confront different microbial assemblages. Bacteria inhabiting an insect gut are often commensal, but some can become pathogenic when the insect is compromised from different stressors. Herbivores are often confronted by various forms of plant resistance, but how defenses generate opportunistic microbial infections from residents in the gut are not well understood. In this study, we evaluated the pathogenic tendencies of Serratia isolated from the digestive system of healthy fall armyworm larvae (Spodoptera frugiperda) and how it interfaces with plant defenses. We initially selected Serratia strains that varied in their direct expression of virulence factors. Inoculation of the different isolates into the fall armyworm body cavity indicated differing levels of pathogenicity, with some strains exhibiting no effects while others causing mortality 24 h after injection. Oral inoculations of pathogens on larvae provided artificial diets caused marginal (< 7%) mortality. However, when insects were provided different maize genotypes, mortality from Serratia increased and was higher on plants exhibiting elevated levels of herbivore resistance (< 50% mortality). Maize defenses facilitated an initial invasion of pathogenic Serratia into the larval hemocoel¸ which was capable of overcoming insect antimicrobial defenses. Tomato and soybean further indicated elevated mortality due to Serratia compared to artificial diets and differences between plant genotypes. Our results indicate plants can facilitate the incipient emergence of pathobionts within gut of fall armyworm. The ability of resident gut bacteria to switch from a commensal to pathogenic lifestyle has significant ramifications for the host and is likely a broader phenomenon in multitrophic interactions facilitated by plant defenses.

Characteristics of the Peritrophic Matrix of the Silkworm, Bombyx mori and Factors Influencing Its Formation

Simple Summary

The insect midgut is an important digestive organ with the peritrophic matrix (PM) being a semi-permeable membrane secreted by the midgut cells. The PM plays an important role in improving midgut digestion efficiency and protecting the midgut from food particles and exogenous pathogens. The silkworm, Bombyx mori, is an economically important insect. Understanding the structure of the PM is necessary for studying its function, but characteristics of PM in B. mori have been rarely reported. In this study, we conducted a comprehensive study on the PM structure of the PM in silkworms and found its thickness increased gradually during growth, but there was no difference in the thickness comparing the anterior, middle, and posterior regions. Permeability of the PM gradually decreased from the anterior to posterior regions. In addition, we found the formation of the PM was influenced by food ingestion and the gut microbiota.

Abstract

The peritrophic matrix (PM) secreted by the midgut cells of insects is formed by the binding of PM proteins to chitin fibrils. The PM envelops the food bolus, serving as a barrier between the content of the midgut lumen and its epithelium, and plays a protective role for epithelial cells against mechanical damage, pathogens, toxins, and other harmful substances. However, few studies have investigated the characteristics and synthesis factors of the PM in the silkworm, Bombyx mori. Here, we examined the characteristics of the PM in the silkworms. The PM thickness of the silkworms increased gradually during growth, while there was no significant difference in thickness along the entire PM region. Permeability of the PM decreased gradually from the anterior to posterior PM. We also found that PM synthesis was affected by food ingestion and the gut microbiota. Our results are beneficial for future studies regarding the function of the PM in silkworms.

Interaction of Viruses with the Insect Intestine

In nature, insects face a constant threat of infection by numerous exogeneous viruses, and their intestinal tracts are the predominant ports of entry. Insects can acquire these viruses orally during either blood feeding by hematophagous insects or sap sucking and foliage feeding by insect herbivores. However, the insect intestinal tract forms several physical and immunological barriers to defend against viral invasion, including cell intrinsic antiviral immunity, the peritrophic matrix and the mucin layer, and local symbiotic microorganisms. Whether an infection can be successfully established in the intestinal tract depends on the complex interactions between viruses and those barriers. In this review, we summarize recent progress on virus-intestinal tract interplay in insects, in which various underlying mechanisms derived from nutritional status, dynamics of symbiotic microorganisms, and virus-encoded components play intricate roles in the regulation of virus invasion in the intestinal tract, either directly or indirectly. Expected final online publication date for the Annual Review of Virology, Volume 8 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Expression of a gene for an MLX56 defense protein derived from mulberry latex confers strong resistance against a broad range of insect pests on transgenic tomato lines

Insect pests cause serious damage in crop production, and various attempts have been made to produce insect-resistant crops, including the expression of genes for proteins with anti-herbivory activity, such as Bt (Bacillus thuringiensis) toxins. However, the number of available genes with sufficient anti-herbivory activity is limited. MLX56 is an anti-herbivory protein isolated from the latex of mulberry plants, and has been shown to have strong growth-suppressing activity against the larvae of a variety of lepidopteran species. As a model of herbivore-resistant plants, we produced transgenic tomato lines expressing the gene for MLX56. The transgenic tomato lines showed strong anti-herbivory activities against the larvae of the common cutworm, Spodoptera litura. Surprisingly, the transgenic tomato lines also exhibited strong activity against the attack of western flower thrips, Frankliniera occidentalis. Further, growth of the hadda beetle, Henosepilachna vigintioctopunctata, fed on leaves of transgenic tomato was significantly retarded. The levels of damage caused by both western flower thrips and hadda beetles were negligible in the high-MLX56-expressing tomato line. These results indicate that introduction of the gene for MLX56 into crops can enhance crop resistance against a wide range of pest insects, and that MLX56 can be utilized in developing genetically modified (GM) pest-resistant crops.

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.

Complex Relationships at the Intersection of Insect Gut Microbiomes and Plant Defenses

Insect herbivores have ubiquitous associations with microorganisms that have major effects on how host insects may interact in their environment. Recently, increased attention has been given to how insect gut microbiomes mediate interactions with plants. In this paper, I discuss the ecology and physiology of gut bacteria associated with insect herbivores and how they may shape interactions between insects and their various host plants. I first establish how microbial associations vary between insects with different feeding styles, and how the insect host physiology and ecology can shape stable or transient relationships with gut bacteria. Then, I describe how these relationships factor in with plant nutrition and plant defenses. Within this framework, I suggest that many of the interactions between plants, insects, and the gut microbiome are context-dependent and shaped by the type of defense and the isolates present in the environment. Relationships between insects and plants are not pairwise, but instead highly multipartite, and the interweaving of complex microbial interactions is needed to fully explore the context-dependent aspects of the gut microbiome in many of these systems. I conclude the review by suggesting studies that would help reduce the unsureness of microbial interactions with less-defined herbivore systems and identify how each could provide a path to more robust roles and traits.