Riptortus pedestris insect indiscriminately acquires not only the symbiotic bacterium Burkholderia insecticola, but also entomopathogens that are abundant in the soil via feeding. However, it is unclear how the host insect survives oral infections of entomopathogens. A previous study suggested that serralysin, a potent virulence factor produced by Serratia marcescens, suppresses cellular immunity by degrading adhesion molecules, thereby contributing to bacterial pathogenesis. Here, we observed that S. marcescens orally administered to R. pedestris stably colonized the insect midgut, while not exhibiting insecticidal activity. Additionally, oral infection with S. marcescens did not affect the host growth or fitness. When co-incubated with the midgut lysates of R. pedestris, serralysin was remarkably degraded. The detoxification activity against serralysin was enhanced in the midgut extract of gut symbiont-colonizing insects. The mRNA expression levels of serralysin genes were negligible in M3-colonizing S. marcescens. M3-colonizing S. marcescens did not produce serralysin toxin. Immunoblot analyses revealed that serralysin was not detected in the M3 midgut region. The findings of our study suggest that orally infected S. marcescens lose entomopathogenicity through host-derived degrading factors and suppression of serralysin.

Thanatin is an antimicrobial peptide (AMP) generated by insects for defense against bacterial infections. In the present study, we performed cDNA cloning of thanatin and found the presence of multiple precursor proteins from the bean bug, Riptortus pedestris. The cDNA sequences encoded 38 precursor proteins, generating 13 thanatin isoforms. In the phylogenetic analysis, thanatin isoforms were categorized into two groups based on the presence of the membrane attack complex/perforin (MACPF) domain. In insect-bacterial symbiosis, specific substances are produced by the immune system of the host insect and are known to modulate the symbiont’s population. Therefore, to determine the biological function of thanatin isoforms in symbiosis, the expression levels of three AMP genes were compared between aposymbiotic insects and symbiotic R. pedestris. The expression levels of the thanatin genes were significantly increased in the M4 crypt, a symbiotic organ, of symbiotic insects upon systemic bacterial injection. Further, synthetic thanatin isoforms exhibited antibacterial activity against gut-colonized Burkholderia symbionts rather than in vitro-cultured Burkholderia cells. Interestingly, the suppression of thanatin genes significantly increased the population of Burkholderia gut symbionts in the M4 crypt under systemic Escherichia coli K12 injection. Overgrown Burkholderia gut symbionts were observed in the hemolymph of host insects and exhibited insecticidal activity. Taken together, these results suggest that thanatin of R. pedestris is a host-derived symbiotic factor and an AMP that controls the population of gut-colonized Burkholderia symbionts.

Symbiotic bacteria have to overcome stressful conditions present in association with the host. In the Burkholderia -bean bug symbiosis, we speculated that a stress-resistant function of Burkholderia is important and that trehalose, known as a stress protectant, plays a role in the symbiotic association.

Background/Objectives: The mechanisms of action of phosphine are diverse and include neurotoxicity, metabolic inhibition, and oxidative stress; however, its efficacy at low temperatures is unclear. Methods: Comparative metabolomics is suitable for investigating the response of the spotted-wing fly Drosophila suzukii to exposure toward a combination of cold stimuli and fumigant PH₃. Results: Under this combined exposure, 52 metabolites exhibiting significant differences in stress were identified and their physiological roles were analyzed in the Drosophila metabolic pathway. Most metabolites were involved in amino acids, TCA cycle, and nucleic acids. In addition, the alteration levels of cell membrane lipids, such as glycerophospholipids, sphingolipids, and glycerolipids, clearly showed changes in the combined treatment compared to PH₃ and low temperatures alone. Aconitic acid, a component of the TCA cycle, was completely inhibited by the combined treatment. Conclusions: These results suggest that treatment-specific indicators could be useful biomarkers to indicate the synergistic effects of PH₃ and low temperature on energy metabolism.

Bacterial DegP has dual functions as a protease and a chaperone and supports bacterial growth under stressful conditions. In symbioses involving bacteria, bacterial symbionts encounter various stressors and may need functional DegP for symbiotic association with the host.

Ethyl formate (EF), a fumigant as an alternative to methyl bromide, is highly valued for its efficacy and low environmental risk of destroying the ozone layer. In this study, comparative metabolomics and lipidomic analyses were conducted to investigate the physiological and biochemical effects of EF exposure on adults of Planococcus citri. EF was exposed at lethal concentrations (LCt₁₀ and LCt₅₀), and metabolic changes were analyzed by liquid chromatography tandem quadrupole time-of-flight mass spectrometry. EF-inducing metabolic changes contained central carbon metabolism, amino acid metabolism, and nucleotide metabolism. Notably, EF exposure activated energy mobilization pathways, including glycolysis/gluconeogenesis and the tricarboxylic acid cycle, and promoted the up-regulation of detoxification-related pathways, such as cytochrome P450 and glucuronate interconversion. EF-inducing lipidomics further exhibited the inhibition of glycerolipid, glycerophospholipid, and sphingolipid metabolism, highlighting the disruption of cell membrane integrity and redox homeostasis. This study demonstrates that EF exposure induces physiological and biochemical disruptions in P. citri by activating energy metabolism and detoxification pathways while suppressing lipid metabolism.