Phospholipase Cβ regulates midgut homeostasis and defends against Bacillus thuringiensis in Spodoptera exigua

Bacillus thuringiensis, an insect pathogen, is widely used for pest control. Phospholipase Cβ (PLCβ) is an important factor in activating the dual oxidase-reactive oxygen species (DUOX-ROS) pathway for defense against pathogens. However, the role of PLCβ in maintaining midgut homeostasis of Lepidopteran insects has not yet been fully elucidated during Bt infection. Here, a total of two PLCβ genes, SePLCβ1 and SePLCβ4, were identified and characterized in Spodoptera exigua. Both SePLCβ1 and SePLCβ4 can regulate the expression of SeDUOX to increase ROS levels, which restrains a high load of midgut symbiotic bacteria in S. exigua larvae, suggesting the role of SePLCβs in maintaining midgut homeostasis. Furthermore, the expression of SePLCβs was upregulated in response to short-term uracil induction and downregulated following long-term uracil induction and Bt treatment. After Bt oral infection, the knockdown of SePLCβs caused lower midgut ROS levels, higher load of midgut total bacteria, and increased loads of two midgut opportunistic pathogens, Bacillus cereus HB1 and Enterococcus mundtii HB1, which enhanced the Bt insecticidal activity. It was noteworthy that the knockdown of SePLCβ1 or SePLCβ4 led to a decrease in larval wet weight, indicating the potential regulatory function on larval development. Our results demonstrated the important contribution of PLCβ-regulated DUOX-ROS pathway in Bt insecticidal activity against Lepidopteran insects, providing more evidence for the development of management strategies of other pests.