Melanization induced by Heliothis virescens ascovirus 3h promotes viral replication

Flexible changes to the Heliothis virescens ascovirus 3h (HvAV-3h) virion components affect pathogenicity against different host larvae species

IMPORTANCE

Different pathogenic processes of a virus in different hosts are related to the host individual differences, which makes the virus undergoes different survival pressures. Here, we found that the virions of an insect virus, Heliothis virescens ascovirus 3h (HvAV-3h), had different protein composition when they were purified from different host larval species. These "adaptive changes" of the virions were analyzed in detail in this study, which mainly included the differences of the protein composition of virions and the differences in affinity between virions and different host proteins. The results of this study revealed the flexible changes of viruses to help themselves adapt to different hosts. Also, these interesting findings can provide new insights to improve our understanding of virus adaptability and virulence differentiation caused by the adaptation process.

Insight into the regulation of the Nrf2 pathway in response to ascovirus infection in Spodoptera exigua

BACKGROUND

Ascoviruses are a type of entomopathogenic microorganism with high biological pest control potential and are expected to contribute to the natural control of lepidopteran pests. However, knowledge of the molecular mechanism underlying the biocidal activity of ascovirus on its host insects remains limited.

RESULTS

In this study, the relative enzyme activity of superoxide dismutase and peroxidase, as well as the expression level of Spodoptera exigua peroxidase (SePOD), were found to be significantly increased at 6 h post infection with Heliothis virescens ascovirus 3h (HvAV-3h). H₂ O₂ accumulation and enhanced expression of NADPH Oxidase (SeNOX) were also observed. In addition, Nuclear Factor erythroid 2-Related Factor 2 (SeNrf2) and muscle aponeurosis fibromatosis (SeMaf) were overexpressed following infection with HvAV-3h. Silencing of SeNrf2 decreased the expression of SePOD, whereas the mortality of SeNrf2-silenced larvae and viral genome copy number also increased. Further RNA interference of SeNOX significantly decreased expression of SeNrf2 and SePOD and therefore increased the mortality and viral genome copy number of the ascovirus-infected host.

CONCLUSION

The HvAV-3h activated Nrf2/ARE pathway of S. exigua and reactive oxygen species were found to respond to ascovirus infection by regulating alterations in antioxidant enzyme genes mediated by the host Nrf2/ARE pathway. These findings enhance our knowledge of ascovirus-host interactions and lay the foundation for the application of ascoviruses in biological pest control. © 2022 Society of Chemical Industry.

Prophenoloxidase of Odontotermes formosanus (Shiraki) (Blattodea: Termitidae) Is a Key Gene in Melanization and Has a Defensive Role during Bacterial Infection

Melanization mediated by the prophenoloxidase (PPO)-activating system is an important innate immunity to fight pathogens in insects. In this study, the in vitro time-dependent increase in the intensity of melanization and phenoloxidase (PO) activity from the hemolymph of Odontotermes formosanus (Shiraki) challenged by pathogenic bacteria was detected. PPO is one of the key genes in melanization pathway, whereas the molecular characteristics and functions of O. formosanus PPO are unclear. The OfPPO gene was cloned and characterized. The open reading frame of OfPPO is 2085 bp in length and encodes a 79.497 kDa protein with 694 amino acids. A BLASTx search and phylogenetic analyses revealed that OfPPO shares a high degree of homology to the Blattodea PPOs. Moreover, real-time fluorescent quantitative PCR analysis showed that OfPPO is ubiquitously expressed in all castes and tissues examined, with the highest expression in workers and variable expression patterns in tissues of different termite castes. Furthermore, the expression of OfPPO was significantly induced in O. formosanus infected by pathogenic bacteria. Intriguingly, in combination with silencing of OfPPO expression, pathogenic bacteria challenge caused greatly increased mortality of O. formosanus. These results suggest that OfPPO plays a role in defense against bacteria and highlight the novel termite control strategy combining pathogenic bacteria application with termite PPO silencing.

Antimicrobial potential of a ponericin-like peptide isolated from Bombyx mori L. hemolymph in response to Pseudomonas aeruginosa infection

The main effectors in the innate immune system of Bombyx mori L. are antimicrobial peptides (AMPs). Here, we infected B. mori with varied inoculum sizes of Pseudomonas aeruginosa ATCC 25668 cells to investigate changes in morpho-anatomical responses, physiological processes and AMP production. Ultraviolet-visible spectra revealed a sharp change in λ_max from 278 to 285 nm (bathochromic shift) in the hemolymph of infected B. mori incubated for 24 h. Further, Fourier Transform InfraRed studies on the hemolymph extracted from the infected B. mori showed a peak at 1550 cm^-1, indicating the presence of α-helical peptides. The peptide fraction was obtained through methanol, acetic acid and water mixture (90:1:9) extraction, followed by peptide purification using Reverse Phase High Performance Liquid Chromatography. The fraction exhibiting antibacterial properties was collected and characterized by Matrix-Assisted Laser Desorption/Ionization-Time of Flight. A linear α-helical peptide with flexible termini (LLKELWTKMKGAGKAVLGKIKGLL) was found, corresponding to a previously described peptide from ant venom and here denominated as Bm-ponericin-L1. The antibacterial activity of Bm-ponericin-L1 was determined against ESKAPE pathogens. Scanning electron microscopy confirmed the membrane disruption potential of Bm-ponericin-L1. Moreover, this peptide also showed promising antibiofilm activity. Finally, cell viability and hemolytic assays revealed that Bm-ponericin-L1 is non-toxic toward primary fibroblasts cell lines and red blood cells, respectively. This study opens up new perspectives toward an alternative approach to overcoming multiple-antibiotic-resistance by means of AMPs through invertebrates' infection with human pathogenic bacteria.

Early in vivo transcriptome of Trichoplusia ni ascovirus core genes

Ascoviruses are large double-stranded DNA insect viruses that destroy the nucleus and transform each cell into 20 or more viral vesicles for replication. In the present study we used RNA-sequencing to compare the expression of Trichoplusia ni ascovirus 6a1 (TnAV-6a1) core genes during the first week of infection, with emphasis on the first 48 h, comparing transcript levels in major somatic tissues (epidermis, tracheal matrix and fat body), the sites infected initially, with those of the haemolymph, where viral vesicles circulate and most replication occurs. By 48 h post-infection (p.i.), only 26 genes were expressed in somatic tissues at ≥5 log2 reads per kilobase per million, whereas in the haemolymph 48 genes were expressed at a similar level by the same time. Early and high expression of TnAV caspase-2-like gene occurred in all tissues, implying it is required for replication, but that it is probably not associated with apoptosis induction, which occurs in infections of Spodoptera frugiperda ascovirus 1 a (SfAV-1a), the ascovirus type species. Other highly expressed viral genes at 48 h p.i. in viral vesicles included a dynein-like beta chain and lipid-modifying enzymes, suggesting their importance to vesicle formation and growth as well as virion synthesis. Finally, as occurs in SfAV expression, we found bicistronic and tricistronic mRNA messages produced by TnAV.

Chinese Cabbage Changes Its Release of Volatiles to Defend against Spodoptera litura

Simple Summary

Biological control is an important direction for pest control in the future, and chemical ecology is an indispensable part of biological control. Therefore, we tested the selection of Spodoptera litura and parasitic wasps on the volatiles of different treatments of cabbage and collected and analyzed the volatiles of different treatments of cabbage. This study found that cabbage was fed by Spodoptera litura to produce volatiles to avoid Spodoptera litura while also attracting Microplitis similis. As a result, some compounds were found to be related to the behavior of Spodoptera litura and Microplitis similis. These results provide a theoretical basis for searching for biological control resources and chemical control.

Abstract

Plants respond to herbivorous insect attacks by releasing volatiles that directly harm the herbivore or that indirectly harm the herbivore by attracting its natural enemies. Although the larvae of Spodoptera litura (the tobacco cutworm) are known to induce the release of host plant volatiles, the effects of such volatiles on host location by S. litura and by the parasitoid Microplitis similis, a natural enemy of S. litura larvae, are poorly understood. Here, we found that both the regurgitate of S. litura larvae and S. litura-infested cabbage leaves attracted M. similis. S. litura had a reduced preference for cabbage plants that had been infested with S. litura for 24 or 48 h. M. similis selection of plants was positively correlated with the release of limonene; linalool and hexadecane, and was negatively correlated with the release of (E)-2-hexenal and 1-Butene, 4-isothiocyanato. S. litura selection of plants was positively correlated with the release of (E)-2-hexenal, 1-Butene, 4-isothiocyanato, and decanal, and was negatively correlated with the release of limonene, nonanal, hexadecane, heptadecane, and octadecane. Our results indicate that host plant volatiles can regulate the behavior of S. litura and M. similis.

Novel strategies for the biocontrol of noctuid pests (Lepidoptera) based on improving ascovirus infectivity using Bacillus thuringiensis

Identifying novel biocontrol agents and developing new strategies are urgent goals in insect pest biocontrol. Ascoviruses are potential competent insect viruses that may be developed into bioinsecticides, but this aim is impeded by their poor oral infectivity. To improve the per os infectivity of ascovirus, Bacillus thuringiensis kurstaki (Btk) was employed as a helper to damage the midgut of lepidopteran larvae (Helicoverpa armigera, Mythimna separata, Spodoptera frugiperda, and S. litura) in formulations with Heliothis virescens ascovirus isolates (HvAV-3h and HvAV-3j). Btk and ascovirus mixtures (Btk/HvAV-3h and Btk/HvAV-3j) were fed to insect larvae (3rd instar). With the exception of S. frugiperda larvae, which exhibited low mortality after ingesting Btk, the larvae of the other tested species showed three types of response to feeding on the formulas: type I, the tested larvae (H. armigera) were killed by Btk infection so quickly that insufficient time and resources remained for ascoviral invasion; type II, both Btk and the ascovirus were depleted by their competition, such that neither was successfully released or colonized the tissue; type III, Btk was eliminated by the ascovirus, and the ascovirus achieved systemic infection in the tested larvae. The feeding of Btk/ascovirus formulas led to a great reduction in larval diet consumption and resulted in a significant decrease in the emergence rate of H. armigera, M. separata, and S. litura larvae, which suggested that the formulas exerted marked oral control effects on both the contemporary individuals and the next generation of these tested pest species.

3H-31, A Non-structural Protein of Heliothis virescens ascovirus 3h, Inhibits the Host Larval Cathepsin and Chitinase Activities

3h-31 of Heliothis virescens ascovirus 3h (HvAV-3h) is a highly conserved gene of ascoviruses. As an early gene of HvAV-3h, 3h-31 codes for a non-structural protein (3H-31) of HvAV-3h. In the study, 3h-31 was initially transcribed and expressed at 3 h post-infection (hpi) in the infected Spodoptera exigua fat body cells (SeFB). 3h-31 was further inserted into the bacmid of Autographa californica nucleopolyhedrovirus (AcMNPV) to generate an infectious baculovirus (AcMNPV-31). In vivo experiments showed that budded virus production and viral DNA replication decreased with the expression of 3H-31, and lucent tubular structures were found around the virogenic stroma in the AcMNPV-31-infected SeFB cells. In vivo, both LD₅₀ and LD₉₀ values of AcMNPV-31 were significantly higher than those of the wild-type AcMNPV (AcMNPV-wt) in third instar S. exigua larvae. An interesting finding was that the liquefaction of the larvae killed by the infection of AcMNPV-31 was delayed. Chitinase and cathepsin activities of AcMNPV-31-infected larvae were significantly lower than those of AcMNPV-wt-infected larvae. The possible regulatory function of the chitinase and cathepsin for 3H-31 was further confirmed by RNAi, which showed that larval cathepsin activity was significantly upregulated, but chitinase activity was not significantly changed due to the RNAi of 3h-31. Based on the obtained results, we assumed that the function of 3H-31 was associated with the inhibition of host larval chitinase and cathepsin activities, so as to restrain the hosts in their larval stages.