Nicotine, a naturally occurring alkaloid found in tobacco, is a potent neurotoxin extensively used to control Nilaparvata lugens (St & aring;l), a destructive insect pest of rice crops. The insect gut harbors a wide array of resident microorganisms that profoundly influence several biological processes, including host immunity. Maintaining an optimal gut microbiota and immune homeostasis requires a complex network of reciprocal regulatory interactions. However, the underlying molecular mechanisms driving these symbiotic exchanges, particularly between specific gut microbe and immunity, remain largely unknown in insects. Our previous investigations identified and isolated a nicotine-degrading Burkholderia cepacia strain (BsNLG8) with antifungal properties. Building on those findings, we found that nicotine intake significantly increased the abundance of a symbiotic bacteria BsNLG8, induced a stronger bacteriostatic effect in hemolymph, and enhanced the nicotine tolerance of N. lugens. Additionally, nicotine-induced antimicrobial peptides (AMPs) exhibited significant antibacterial effects against Staphylococcus aureus. We adopted RNA-seq to explore the underlying immunological mechanisms in nicotine-stressed N. lugens. Bioinformatic analyses identified numerous differentially expressed immune genes, including recognition/immune activation (GRPs and Toll) and AMPs (i.e., Defensin, Lugensin, lysozyme). Temporal expression profiling (12, 24, and 48 hours) of immune genes revealed pattern recognition proteins and immune effectors as primary responders to nicotine-induced stress. Defensin A, a broad-spectrum immunomodulatory cationic peptide, exhibited significantly high expression. RNA interference-mediated silencing of Defensin A reduced the survival, enhanced nicotine sensitivity of N. lugens to nicotine, and decreased the abundance of BsNLG8. The reintroduction of BsNLG8 improved the expression of immune genes, aiding nicotine resistance of N. lugens. Our findings indicate a potential reciprocal immunomodulatory interaction between Defensin A and BsNLG8 under nicotine stress. Moreover, this study offers novel and valuable insights for future research into enhancing nicotine-based pest management programs and developing alternative biocontrol methods involving the implication of insect symbionts.

Bacterial symbionts are widespread in insects and other animals. These microbes play crucial roles in many aspects of insect physiology and biology, including immunity, nutrition and confronting plant defenses. In the present study, we isolated and identified the bacterium Burkholderia cepacia BsNLG8 from the brown planthopper (BPHs), Nilaparvata lugens, a devastating pest of rice crops worldwide. Plate confrontation assay indicated that BsNLG8 significantly inhibited the growth of phytopathogenic fungi. In addition, the BsNLG8 strain demonstrated the ability to produce siderophores, which explains its antagonistic mechanism. Lastly, we explored the nicotine degradation ability of BsNLG8 using in vitro and in vivo methods. In vitro, HPLC analysis results showed that BsNLG8 could significantly reduce the concentration of nicotine in the medium at 36 h. Moreover, microinjection of BsNLG8 in axenic BPHs increased the survival rate of the host on nicotine-containing rice seedlings. These findings could serve as the basis of future research in deciphering the interaction between host and symbionts.