Spodoptera Exigua Research Articles

Plant-Guided Microbiome Selection Produces Transient Effects on Insect Performance and Rhizosphere Community Assembly

Microbial communities are essential for plant health, but using these relationships to enhance growth and pest protection is challenging. Leveraging the natural mechanisms plants employ to manage relationships with microbes is one promising means to selectively engineer whole microbial communities with beneficial properties. This approach, known as host-guided selection, has been successful in some model species targeting performance traits (e.g., biomass). However, few studies use crop plants or focus on defensive traits (e.g., pest resistance). Our goal was to naturally engineer tomato root-associated microbiomes that increase resistance to insect pests. First, we used an iterative soil microbial inoculation process to engineer insect-suppressive rhizosphere soil microbiomes that reduce damage from aphid feeding ( Macrosiphum euphorbiae) or caterpillar defoliation ( Manduca sexta, Spodoptera exigua). We then characterized the bacterial and fungal microbial communities of soils associated with differences in insect performance using metabarcoding approaches. Overall, soil microbiome selection produced transient differences in aphid performance, but caterpillar growth was unaffected. In four of nine generations, the aphid population growth rate was significantly lower on plants with rhizospheres selected for “low” insect performance, where abundance was reduced by up to 20%. Correspondingly minor shifts in fungal and bacterial relative abundance occurred in insect-suppressive communities. However, network analysis indicated that aphid feeding disrupted rhizosphere microbiome assembly, resulting in lower community complexity and connectivity and fewer structurally important taxa compared with uninfested controls. Overall, our results highlight critical factors for successful engineering of beneficial microbiomes, particularly insect feeding guild and microbial community stability.

Suppression of a transcriptional regulator, HexA, is essential for triggering the bacterial virulence of the entomopathogen, Xenorhabdus hominickii

A nematode-symbiotic bacterium, Xenorhabdus hominickii, exhibits two distinct lifestyles. Upon infection of its host nematode into a target insect, X. hominickii is released into the insect hemocoel and becomes pathogenic. This study examines the critical transformation in bacterial life forms concerning the activity of a transcriptional regulator, HexA. When X. hominickii was cultured in tryptic soy broth, HexA was expressed during the stationary phase of bacterial growth. Conversely, HexA was expressed in the early growth stage within the insect host, Spodoptera exigua, when infected with X. hominickii. The transient expression of HexA was succeeded by the expression of another transcriptional regulator, Lrp, which led to the production of bacterial virulent factors. Expression of HexA was manipulated by replacing its promoter with an inducible promoter controlled by the inducer, l-arabinose. In the absence of the inducer, the mutant bacteria expressed HexA at a low level, resulting in a bacterial culture broth that was more effective at suppressing insect immune responses than the wild type. When the inducer was added, HexA was expressed at high levels, rendering the culture broth ineffective in immunosuppression. Interestingly, expression of HexA inhibited the expression of another transcriptional regulator, Lrp, which in turn induced the expression of a non-ribosomal peptide synthetase, gxpS, leading to the production of an immunosuppressive metabolite, GXP. Suppression of HexA expression in mutant bacteria augmented GXP levels in secondary metabolites. This indicates that infection of X. hominickii into the insect host represses HexA expression and upregulates Lrp expression, leading to GXP production. The GXP metabolites inhibit insect immunity, thus protecting the bacteria-nematode complex. Therefore, the suppression of HexA expression in the insect hemocoel is crucial for the bacteria’s transition from a symbiotic to a pathogenic life form.

PGE2 Binding Affinity of Hemocyte Membrane Preparations of Manduca sexta and Identification of the Receptor-Associated G Proteins in Two Lepidopteran Species.

Prostaglandin E2 (PGE2) is an eicosanoid that mediates a range of physiological actions in vertebrates and invertebrates, including reproduction and immunity. The PGE2 receptor was identified and functionally assessed in two lepidopteran insects, Manduca sexta and Spodoptera exigua. However, its binding affinity to the receptor has not been reported. The PGE2 receptor is a G-protein coupled receptor (GPCR) although its corresponding G-protein is not identified. PGE2 binding assays were performed with membrane preparations from hemocytes of M. sexta larvae. We recorded an optimal binding in 4 h reactions conducted at pH 7.5 with 12 nM tritium-labeled PGE2. We found that hemocytes express a single population of PGE2 binding sites with a high affinity (Kd = 35 pmol/mg protein), which are specific and saturable. The outcomes of experiments on the influence of purine nucleotides suggested these are functional GPCRs. A bioinformatics analysis led to a proposed trimeric G-protein in the S. exigua transcriptome, in which the Gα subunit is classified into five different types: Gα(o), Gα(q), Gα(s), Gα(12), and Gα(f). After confirming expressions of these five types in S. exigua, individual RNA interference (RNAi) treatments were applied to the larvae using gene-specific double-stranded RNAs. RNAi treatments specific to Gα(s) or Gα(12) gene expression significantly suppressed the cellular immune responses although the RNAi treatments specific to other three Gα components did not. While PGE2 treatments led to elevated hemocyte cAMP or Ca2+ levels, the RNAi treatments specific to Gα(s) or Gα(12) genes led to significantly reduced second messenger levels under PGE2, although the RNAi treatments specific to the other three Gα components did not. These results showed that the PGE2 receptor has high PGE2 affinity in the nanomolar range and binds G-proteins containing a Gα(s) or Gα(12) trimeric component in S. exigua and M. sexta, and likely, all lepidopteran insects.

Genome-wide analysis of detoxification genes conferring diamide insecticide resistance in Spodoptera exigua identifies CYP9A40

For over a decade, diamide insecticides have been effective against lepidopteran pests like beet armyworm, Spodoptera exigua (Hübner, 1808). However, the evolution of resistance poses a challenge to their sustainable use. We identified an I4790M mutation in the S. exigua ryanodine receptor (RyR) gene, but its correlation with resistance varied across the field-collected Korean populations of S. exigua. RNA sequencing and differential gene expression analysis were performed to investigate other resistance mechanisms. Diamide-resistant and susceptible strains and F1 hybrids were compared by mapping RNA-seq reads to the S. exigua reference genome. CYP9A40 was identified as a critical gene in diamide resistance due to its high expression in the resistant strains. Synergist bioassays with piperonyl butoxide supported the role of P450s in diamide metabolic resistance in S. exigua. A strong positive correlation between CYP9A40 over-expression levels (up to 80-fold) and diamide LC50 values was obtained for field-collected populations uniformly showing a 100% frequency of the RyR I4790M target-site resistance allele. To validate the function of CYP9A40 in diamide detoxification, we recombinantly expressed the gene and tested its ability to bind and degrade chlorantraniliprole as a substrate. The results confirmed its catalytic role in diamide metabolism. CYP9A40 has been identified and validated to confer metabolic resistance in Korean S. exigua populations. It works alongside the RyR target-site I4790M mutation to enhance diamide resistance. These mechanisms offer insights for resistance monitoring and support insecticide resistance management programs to improve control strategies for S. exigua.

Gut bacteria of lepidopteran herbivores facilitate digestion of plant toxins

Lepidopterans commonly feed on plant material, being the most significant insect herbivores in nature. Despite plant resistance to herbivory, such as producing toxic secondary metabolites, herbivores have developed mechanisms encoded in their genomes to tolerate or detoxify plant defensive compounds. Recent studies also highlight the role of gut microbiota in mediating detoxification in herbivores; however, convincing evidence supporting the significant contribution of gut symbionts is rare in Lepidoptera. Here, we show that the growth of various lepidopteran species was inhibited by a mulberry-derived secondary metabolite, 1-deoxynojirimycin (DNJ); as expected, the specialist silkworm Bombyx mori grew well, but interestingly, gut microbiota of early-instar silkworms was affected by the DNJ level, and several bacterial species responded positively to enriched DNJ. Among these, a bacterial strain isolated from the silkworm gut (Pseudomonas fulva ZJU1) can degrade and utilize DNJ as the sole energy source, and after inoculation into nonspecialists (e.g., beet armyworm Spodoptera exigua), P. fulva ZJU1 increased host resistance to DNJ and significantly promoted growth. We used genomic and transcriptomic analyses to identify genes potentially involved in DNJ degradation, and CRISPR-Cas9-mediated mutagenesis verified the function of ilvB, a key binding protein, in metabolizing DNJ. Furthermore, the ilvB deletion mutant, exhibiting normal bacterial growth, could no longer enhance nonspecialist performance, supporting a role in DNJ degradation in vivo. Therefore, our study demonstrated causality between the gut microbiome and detoxification of plant chemical defense in Lepidoptera, facilitating a mechanistic understanding of host-microbe relationships across this complex, abundant insect group.

Cytochrome P450 CYP6AE70 Confers Resistance to Multiple Insecticides in a Lepidopteran Pest, Spodoptera exigua.

Cytochrome P450 monooxygenases are associated with the detoxification of xenobiotics, and overexpression of P450 genes has been proven to be associated with insecticide resistance in insect species. Our previous study has revealed that multiple CYP6AE genes were significantly overexpressed in a resistant strain of Spodoptera exigua, and among these genes, CYP6AE70 was particularly highly expressed. However, the functional roles of the CYP6AE genes in insecticide resistance remain unknown in this pest. Here, we investigate the relationship between the CYP6AE genes and insecticide resistance by focusing on CYP6AE70. The expression of CYP6AE70 was increased after exposure to chlorpyrifos, cypermethrin, and deltamethrin. Ectopic overexpression of P450 in transgenic flies by the GAL4/UAS system dramatically enhanced the tolerance to these three insecticides. Furthermore, the recombinant CYP6AE70 was functionally expressed in Sf9 cells, and metabolic assays revealed that the recombinant P450 protein could efficiently metabolize chlorpyrifos, cypermethrin, and deltamethrin. Finally, molecular modeling and docking also showed that this P450 protein were tightly bound to the three insecticides. These results determine that the upregulation of CYP6AE genes results in resistance to multiple insecticides in S. exigua. This study improves our understanding of P450-mediated insecticide resistance and will help us to design more effective resistance management for pest control.

V1848I Mutation in the Voltage-Gated Sodium Channel Confers High-Level Resistance to Indoxacarb and Metaflumizone in Spodoptera exigua.

Spodoptera exigua is one of the most serious lepidopteran pests of global importance. With the intensive use of insecticides, S. exigua has evolved resistance to many insecticides, including the sodium channel blocker insecticides (SCBIs) indoxacarb and metaflumizone. In this study, we investigated the role of the V1848I mutation in the voltage-gated sodium channel (VGSC) in SCBI resistance and its inheritance patterns in S. exigua through the development and characterization of a near-isogenic resistant strain. The AQ-23 strain of S. exigua, collected in 2023 from Anqing, Anhui province of China, shows 165-fold resistance to indoxacarb compared with the susceptible WH-S strain. A frequency of 44.6% for the V1848I mutation was detected in the SeVGSC of the AQ-23 strain, while no F1845Y mutation was found. Through repeated backcrossing and marker-assisted selection, the V1848I mutation in the AQ-23 strain was introgressed into the susceptible WH-S strain, creating a near-isogenic strain named WH-1848I. This WH-1848I strain exhibits high levels of resistance to indoxacarb (146-fold) and metaflumizone (431-fold) but remains susceptible to broflanilide and spinosad compared with the WH-S strain. Inheritance analysis revealed that SCBI resistance in the WH-1848I strain is autosomal, nonrecessive, and genetically linked to the V1848I mutation. These findings establish a clear link between the V1848I mutation and SCBI resistance in S. exigua, offering valuable insights for developing molecular detection tools and resistance management strategies.

Identification and functional analysis of gut dsRNases in the beet armyworm Spodoptera exigua

RNA interference (RNAi)-based products have the potential to significantly contribute to insect pest control. However, RNAi efficiency varies widely among different insect orders, particularly in Lepidoptera, where it is often low. One key factor affecting RNAi efficiency is the presence of double-stranded ribonuclease (dsRNase) in the digestive tract, which can degrade dsRNA prior to uptake by gut cells. In this study, four dsRNases were identified in the beet armyworm, Spdoptera exigua, of which two were highly expressed gut dsRNases, SedsRNase1 and SedsRNase2. To assess their effect on dsRNA degradation activity via the oral route, CRISPR/Cas9-based gene editing was employed to knock out these gut dsRNases. The results indicate that all mutant strains, including SeKO1 (knockout SedsRNase1), SeKO2 (knockout SedsRNase2), and SeKO1KO2 (knockout SedsRNase1 and SedsRNase2), showed significantly decreased dsRNA degradation activity, particularly in the SeKO1KO2 mutant strain, where the weakest degradation occurred in both the gut and whole body. Additionally, we noticed that the lack of gut SedsRNases led to a slight extended developmental period and reduced reproductive capacity in S. exigua. Collectively, these findings deepen our understanding of gut SedsRNases and how they can impact the biology of the beet armyworm and can support the exploration dsRNA-based approaches for pest control.

Temperature and relative humidity mediated life processes of Spodoptera species (Lepidoptera: Noctuidae)

AbstractAnthropogenic-mediated climate change is expected to negatively affect pest management in agriculture. Hence, we investigated the oviposition, immature mortality, and developmental processes of Spodoptera species (Spodoptera exigua (Hübner) and Spodoptera litura (Fabricius)) under different temperatures (20, 25, and 30°C) and relative humidity (RH) (30–35, 50–55, 70–75, and 90–95%) conditions. For fecundity, mouths of each Spodoptera species were released into a rectangular box whose inner walls were covered with a sheet of white paper for each combination of temperature and RH. The mouths were kept inside the box to deposit eggs for 72 h. Temperature and RH significantly affected the fecundity, with the maximum number of eggs laid in 70–75% at 30°C. The highest egg and larval mortalities were recorded in 30–35 and 90–95% RH, respectively. Temperature and RH greatly affected the developmental period (egg–adult) and adult emergence rate. The rapid development was recorded in 70–75% RH at 30°C. Higher number of adults was found with an increase in temperature and RH. Adult longevity was significantly higher in 70–75% RH at 20°C. Based on the present study's findings, temperature and RH had an individual apparent effect on the developmental processes of Spodoptera species instead of an interactive effect. Therefore, there is need for an in-depth study of the influence of several climatic factors, including CO2, on the developmental modality and demographic changes of Spodoptera species to assess the impacts of climatic components and the sustainable development of management strategies.

Constitutive and inducible tomato defenses contribute to Bacillus thuringiensis lethality against Spodoptera exigua

In nature, insect herbivory exerts continuous selective pressure on plants that, in turn, have developed a wide array of constitutive and inducible defenses to fight against attackers. Since plant defenses may affect higher trophic levels, including entomopathogens, further research is required to understand how plant compounds influence insect-pathogens interactions and their implications for integrated pest management programs. Here, we evaluated the impact of tomato defenses on the lethality produced by the bacterial entomopathogen Bacillus thuringiensis (Bt) against second instar larvae of Spodoptera exigua. We first examined the effect of constitutive defenses from fifteen wild tomato species and forty cultivated varieties (Solanum lycopersicum) on S. exigua larval growth and susceptibility to Bt. The results showed larvae fed on wild tomato species had a reduced larval growth compared to larvae fed on cultivated varieties, whereas susceptibility to Bt was similar between both groups. We then selected six cultivated varieties, including those with high and low impacts on larval growth and Bt-induced mortality, to further explore the effect of inducible defenses. Elicitation of defenses by methyl jasmonate (MeJA) reduced larval growth and increased basal mortality. Additionally, when larvae were infected with Bt, MeJA treatment further increased their susceptibility to the entomopathogen. Metabolomic analysis confirmed a reprogramming of tomato leaf metabolism following MeJA elicitation, with an induced accumulation of bioactive compounds, such as saponins or flavonoids, known for their anti-herbivory properties in most tomato varieties. Overall, these data reveal that both constitutive and inducible tomato defenses not only protect the plant by directly affecting the insect pest but also enhance the efficacy of bacterial entomopathogens

Chemical Mechanism of Exogenous Jasmonic Acid-Induced Resistance in Rose Against Spodoptera exigua (Lepidoptera: Noctuidae)1

Abstract Jasmonic acid (JA) plays an important role in the indirect plant-mediated interactions between rose powdery mildew (Podosphaera pannosa [Wallr.: Fr.] de Bary) and Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) on their shared host Rosa chinensis Jacquin. Bioassays showed that the total number of eggs laid by S. exigua on rose twigs decreased significantly after the twigs were treated with exogenous JA. Gas chromatography coupled with mass spectrometry (GC-MS) showed that the volatile organic chemicals (VOCs) from roses, including alkanes, terpenes, aldehydes, ketones, alcohols, and esters, were significantly changed following treatment with JA. Based on gas chromatography-electroantennographic detection (GC-EAD) and GC-MS analysis, the electrophysiological responses of S. exigua moths could be elicited by 8 compounds from the JA-induced roses, including 3-carene, 1-dodecanol, methyl stearate, 1-tetradecanol, hexadecane, eucalyptol, β-myrcene, and 1-iodododecane. Among these chemicals, the first 6 exhibited significant repellent activity to ovipositional behaviors of the gravid moths, while the latter 2 were attractive. The inhibition index of methyl stearate at a concentration of 15 mg/ml reached 65.12%. On the other hand, the quantity of the first 6 chemicals increased significantly and the latter two decreased due to the JA induction. These results reveal a new mechanism for resistance in rose plants against S. exigua. After JA induction, the rose plants appear to up-regulate the biosynthesis of the chemicals with repellent activities against S. exigua and, in the meantime, down-regulate the attractant chemicals.

Host Species Affects Gut Microbial Community and Offspring Developmental Performances in the Pupal Parasitoid Chouioia cunea Yang (Hymenoptera: Eulophidae).

Chouioia cunea are known to exploit in varying degrees a wide range of lepidopteran species and its offspring development may vary with host species. This study examined its preimaginal development and larval gut microbiota in parasitizing five folivorous lepidopteran hosts including Hyphantria cunea (referred to thereafter as CcHc), Antherea pernyi (CcAp), Helicoverpa armigera (CcHa), Spodoptera exigua (CcSe), and Spodoptera frugiperda (CcSf). Though rates of parasitism and offspring eclosion did not change with host species, the development period and number of offspring eclosed varied with hosts, with the shortest period in CcSf and the highest number from CcAp. For offspring larval gut microbiota, though phylum Proteobacteria was dominant for attacking CcAp, Firmicutes was so for the other hosts. All microbial genera except Enterococcus were less abundant for CcSf than the other hosts. The database-based predictions indicate a significant positive correlation between Cutibacterium and Aureimonas with the relative number of wasp emergence, while Blastomonas exhibits a strong positive association with the developmental period. Our results imply the potential relevance of the gut microbial community in offspring larvae to host species attacked by C. cunea.

Application of Entomopathogenic Fungus Beauveria Bassiana on Shallot Plants Through Soaking the Bulbs and its Effect on the on the Growth of Spodoptera Exigua Larvae

Spodoptera exigua Hubner is a major pest of shallots. This pest attack can cause a decrease in shallot production. The use of entomopathogenic fungus Beauveria bassiana is an environmentally friendly pest control alternative. This study aims to study the development of S. exigua larvae in shallot plants applied with B. bassiana fungus. This study was arranged in a Completely Randomized Design (CRD) with 6 treatments and 5 replications. Five isolates of B. bassiana (BbWS, BbJG, PD114, TD312, PB211) were used in this study. The concentration of B. bassiana suspension used was 108 conidia/ml. B. bassiana was applied to shallots by soaking the bulbs for 15 minutes. The results showed that the mortality of larvae feeding on plants applied with B. bassiana reached 100%. The application of B. bassiana also extended the larval period, reducing the percentage of pupae and adults formed. BbWS and TD312 were isolated and were better at inhibiting the growth of S. exigua larvae

Allicin impacts the susceptibility of Spodoptera exigua to nucleopolyhedrovirus

Allicin impacts the susceptibility of Spodoptera exigua to nucleopolyhedrovirus

Juvenile hormone inhibits lipogenesis of Spodoptera exigua to response to Bacillus thuringiensis GS57 infection

The application of Bacillus thuringiensis (Bt) has brought environmental benefits and delayed resistance development of pests. Most studies focus on the Bt insecticidal activity against pests, however, the molecular mechanism of Bt on impairing the growth and development of Spodoptera exigua remains unknown. Here, we show that juvenile hormone (JH) inhibits the lipogenesis mediated by fatty acid synthases (Fas) of S. exigua in response to Bt infection. The weight and lipid accumulation of S. exigua larvae post Bt infection were less than those of larvae without Bt infection. We further demonstrated that Bt infection causes the JH titer with a significant increase, which downregulates the expression of lipogenesis-related genes, SeFas3, SeFas4, and SeFas5, resulting in the delayed development of S. exigua larvae. In addition, the expression levels of SeFas genes were regulated by SeACC, indicating that SeFas genes were modulated by multiple pathways. Our findings reveal that novel insights into the molecular mechanisms underlying the impaired development caused by Bt infection which can inform the development of strategies for the sustainable pest control in the future.

Design, Synthesis, and Antifeedant Activity Evaluation of 13/14-Arylthioether Matrine Derivatives.

Introducing a sulfur atom into active agricultural molecules is an important strategy for pesticide development. Matrine, an environmentally friendly botanical pesticide, has the advantage of being easily degraded and has drawn attention in the agricultural field. To explore the novel matrine-type pesticides, in this study, we designed and synthesized 13/14-arylthioether matrine derivatives by introducing various aryl sulfide motifs into bioactive matrine. Most of the synthesized arylthioether matrines exhibited good antifeedant activity against Spodoptera exigua. Among them, compound 2q showed the best antifeedant effect with an EC50 value of 0.038 mg/mL, which is approximately 125-fold more activity than matrine and reached the activity level of commercial standard azadirachtin A. Furthermore, compound 2q exhibited an inhibitory effect on antifeedant-related enzyme carboxylesterase (CarE) from S. exigua. In short, the high activity of arylthioether matrines offers new insights into developing new antifeedants.

An RdDM-independent function of Pol V transcripts in gene regulation and plant defence.

RNA polymerase V (Pol V) and Pol IV are known to be specialized for RNA-directed DNA methylation (RdDM). Here we report that Pol V, but not Pol IV, regulates hundreds of genes including jasmonic acid-responsive genes and confers plant defence to Botrytis cinerea and Spodoptera exigua. About half of the Pol V-regulated genes are associated with Pol V transcripts (PVTs). We thus hypothesized that some PVTs could regulate gene expression in an RdDM-independent manner. To test this hypothesis, we studied three PVTs, PVT-ERF5a/b and PVT-ERF6, as models. PVT-ERF5a/b and PVT-ERF6 are transcribed from the upstream regions of ERF5 and ERF6 and positively regulate their transcription, thereby regulating plant defence. Such regulation involves PVT-dependent H3K4me3 deposition and requires the DRD1-DMS3-RDM1 complex that mediates Pol V recruitment to the target loci. These findings highlight an unprecedented role for PVTs in regulating gene transcription, apart from serving as scaffold RNAs to direct DNA methylation.

Spraying dsRNA with chitosan formulation improves control of the western flower thrips, Frankliniella occidentalis, in a greenhouse.

The western flower thrips, Frankliniella occidentalis, is a serious pest causing both direct feeding damage and indirect harm by transmitting the tomato spotted wilt virus. A spraying double-stranded RNA (dsRNA) targeted at the vacuolar-type ATPase (vATPase) gene was developed and demonstrated high insecticidal activity in the laboratory but less effective in field applications. To improve control efficacy under field conditions, three strategies were explored in this study. First, to identify a more efficient RNA interference (RNAi) target, dsRNA specific to the Snf7 gene was tested alongside dsRNA targeting vATPase, and both were found to be similarly effective in controlling the thrips. Second, to elucidate the factors contributing to dsRNA resistance, dsRNA-degrading enzymes were annotated and their physiological roles in diminishing RNAi efficacy were investigated. Third, to suppress the dsRNA degradation from the dsRNase activities and protect it in field conditions, the dsRNA was encapsulated with chitosan. This formulation enhanced the dsRNA's resistance to environmental stressors such as ultraviolet light and the digestive enzymes in the thrips' gut. Additionally, the chitosan formulation specifically increased the RNAi efficacy, likely by facilitating more efficient entry into the target cells, thus bolstering the insecticidal activity of the dsRNA. The formulated dsRNA was applied on F. occidentalis infesting the hot peppers in a greenhouse at a concentration of 500 ppm, demonstrating an 82.4% control efficacy compared with 59.2% control efficacy observed with the application of naked dsRNA. This study further demonstrated an enhancement in the spectrum of control by combining dsRNAs specific to three distinct thrips species, while the mixture showed no adverse effects on non-target insects, such as the lepidopteran Spodoptera exigua. Collectively, these findings reveal that the chitosan formulation of dsRNA not only improves control efficacy under field conditions but also broadens the control spectrum against three different thrips pests.

Receptor interactions of protoxin and activated Vip3Aa structural conformations in Spodoptera exigua.

The Vip3Aa insecticidal protein, produced by Bacillus thuringiensis, has been effectively used in commercial Bt-crops to manage lepidopteran pests. Upon ingestion by larvae, the protoxin is processed by midgut proteases into the activated protein and binds specifically to its receptors in the midgut, leading to insect mortality. Cryo-EM resolution of the trypsin-processed Vip3Aa protein unveiled structural remodelling of the N-terminal region during the transition from protoxin to activated protein. This conformational change has been demonstrated to be crucial for toxicity against Spodoptera exigua larvae, a major global lepidopteran pest. In this study, we investigated the relevance of the structural remodelling for the specific binding to midgut receptors. We conducted in vitro binding assays with radiolabelled proteins and brush border membrane vesicles (BBMV) from S. exigua, employing structural mutants that lock the protein in either its protoxin or its activated conformation. Our results indicate that both structural stages of the protein share binding sites in the midgut epithelium. Moreover, in vivo competition assays revealed that Vip3Aa is able to bind to functional receptors in S. exigua larvae both as protoxin and as activated protein. Altogether, our findings point to both structural conformations contributing to receptor binding. In vivo, either spontaneous structural shift upon proteolytic cleavage or receptor-mediated remodelling could be occurring. However, the timing and context in which the conformational change occurs could influence membrane insertion and toxicity. Our results show the complex interplay between proteolytic processing, protein structure and receptor interactions in Vip3Aa's toxicity. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Expressional respones of hsp70 genes against abiotic and entomopathgenic stresses in four different noctuid larval species (Lepidoptera: Noctidae)

Heat shock proteins (Hsps) are stress response proteins. In a previous study, host larval Hsp70s were identified as the structural proteins of virions of Heliothis virescens ascovirus 3h (HvAV-3h), an insect virus that mainly infects noctuid larvae. To investigate the response of hsp70s of healthy Mythimna separata, Spodoptera exigua, Spodoptera frugiperda, and Spodoptera litura larvae to various abiotic or entomopathogenic stresses, quantitative PCR was used to detect larval hsp70s expression patterns. Results showed distinct expression patterns of hsp70s in response to different abiotic stresses. Notably, Mshsp70 expression pattern resembled Slhsp70 under most treatments. In healthy larvae, no tissue tropism was observed concerning the relative expression of Mshsp70, Sfhsp70, and Slhsp70. After infection with HvAV-3h, the expression of hsp70s in all dissected tissues of all tested larval species increased. Significant differences were found in the fat bodies of M. separata, S. exigua, and S. litura as well as in the hemolymph of S. exigua and S. litura. Subsequent silencing of Slhsp70, resulted in a significant decrease in DNA replication levels of HvAV-3h in S. litura larvae at 24 and 72 h post RNA interference, indicating that Slhsp70 is necessary for DNA replication in HvAV-3h. These data can provide references for the studying on the stress response of noctuid larvae to different environmental factors.