RNAi-mediated silencing of NlGRP3 augments the insecticidal virulence of Metarhizium anisopliae to the brown planthopper Nilaparvata lugens

The rapid development of insecticide resistance reinforces the urgent need to develop eco-friendly strategies for controlling Nilaparvata lugens (brown planthopper, BPH), the most destructive insect pest of rice. Both entomopathogens and RNA interference (RNAi) provide attractive alternatives to chemical insecticides. In this study, we demonstrated the synergistic potential of the combination use of entomopathogen- and RNAi-mediated approaches to control BPH. The β-1, 3-glucan recognition protein (βGRP) encoding gene NlGRP3 was identified and its potential role in immune defense was characterized in BPH. The open reading frame (ORF) of NlGRP3 is 1740 bp in length, encoding a 65.8 kDa protein with conserved CBM39 and GH16 domains that typically existed in the βGRP family members. NlGRP3 was shown to be differentially expressed across developmental stages and highly transcribed in the immune responsive tissues haemolymph and fat body. Topical infection with a fungal entomopathogen Metarhizium anisopliae could significantly up-regulate its expression level. RNAi-mediated silencing of NlGRP3 resulted in significantly decreased survival rate and increased susceptibility to fungal challenge in the fifth-instar BPH nymphs. The greatly enhanced mortality of NlGRP3-silenced BPH following fungal infection might be in part directly due to the immune suppression by down-regulating expressions of antimicrobial peptide genes and the imbalance of the bacterial community harboring in BPH body. Our results highly demonstrated that suppressing the insect innate immune defense through RNAi targeting the immune-related genes could effectively strengthen the biocontrol efficacy of fungal entomopathogens, providing clues to the combination use of RNAi and entomopathogens as a promising approach for BPH control.