RNAi-mediated suppression of toll-like receptor NlToll1 enhances Nilaparvata lugens susceptibility to entomopathogenic fungal infection

The brown planthopper (BPH), Nilaparvata lugens, is the most devastating insect pest of rice. Biological control with fungal entomopathogens is an attractive alternative to chemical pesticides for controlling BPH. However, the efficacy of fungal bioagents is often limited by the immune defense responses of host insects. Toll-like receptors (TLRs) are a family of crucial pattern recognition proteins that are responsible for the immunologic recognition of invading pathogen in insects. Here, we demonstrated that the suppression of a key TLR-encoding gene NlToll1 in BPH through RNA interference (RNAi) could significantly improve the insecticidal efficacy of an entomopathogenic fungus Metarhizium anisopliae. The open reading frame (ORF) of NlToll1 was 3351 bp in length encoding a 126.7 kDa protein with a typical intracellular Toll/interleukin-1 receptor (TIR) domain, two transmembrane domains and 14 extracellular leucine-rich repeats (LRRs). The qRT-PCR results showed that the expression of NlToll1 had obvious spatiotemporal preferences and strongly induced by topical infection with M. anisopliae. Silencing of NlToll1 using RNAi led to a significantly reduced survival rate and an increased susceptibility of BPH fifth-instar nymphs to fungal infection, which might be partially owing to the immunosuppressive effects by repressing expression of antimicrobial peptide genes and the dysbiosis of gut bacterial community caused by NlToll1 silencing. Our results manifested that the combination use of RNAi and fungal entomopathogens would be an effective avenue for exploiting pesticide-free BPH management strategies.