The role of 20-hydroxyecdysone and juvenile hormone in insecticidal activity of Bacillus thuringiensis regulated by DUOX-ROS immunity in Spodoptera exigua

Insect midgut bacteria can be transferred to the blood cavity due to Bt Infection and proliferate, becoming pathogens and enhancing Bt insecticidal activity. Dual oxidase (DUOX)-reactive oxygen species (ROS) signaling pathway plays a key role in regulating microbial homeostasis and resisting pathogen Infection. However, the functions of MEKK and MKK associated with DUOX-ROS immunity are rarely studied in insects, moreover, the regulatory mechanisms underlying DUOX-ROS immunity via 20-Hydroxyecdysone (20E) and juvenile hormone (JH) are underexplored. In this study, we investigated that Spodoptera exigua MAPK kinase kinase 4 (SeMEKK4) and MAPK kinase 6 (SeMKK6) were required for Sep38β expression, and RNAi-mediated knockdown of SeMEKK4 and SeMKK6 significantly decreased ROS level and increased Bacterial load in the midgut of S. exigua larvae, thereby enhancing Bt insecticidal activity. Furthermore, 20E and JH titers were elevated in insects infected with Bt. 20E upregulated the expression of SeMEKK4, SeMKK6, and Sep38β through SeEcR and SeUSP receptors, and activated the expression of SeDUOX to increase ROS level and decrease Bacterial load in the midgut, which was not conducive to the enhancement of Bt Insecticide activity. JH showed an opposite effect on midgut-related DUOX-ROS immunity via SeMet1 and SeMet2, and it was noteworthy that JH played a dominant role in negatively regulating DUOX-ROS immunity post Bt Infection, which enhanced Bt insecticidal activity. This is an adjustment strategy for insects to cope with Bt Infection, providing a new perspective for pest management.

20-Hydroxyecdysone; Bacillus thuringiensis; Juvenile hormone; Midgut bacteria; ROS level; Spodoptera exigua.