A C-type lectin NlCTL2 mediates immune defense against the fungal entomopathogen Metarhizium anisopliae in brown planthopper

The rapid rise in insecticide resistance necessitates environmentally sustainable strategies to control the primary rice pest Nilaparvata lugens Stål (brown planthopper, BPH). Entomopathogenic fungi (EPF) offer a promising alternative, but their insecticidal efficacy is limited by the antifungal immunity of host insects. C-type lectins (CTLs) play vital roles in modulating insect development and immune responses, holding significant promise for the development of EPF-based BPH biocontrol strategies. In the present study, we demonstrated that inhibition of a key CTL-encoding gene NlCTL2 in BPH using RNA interference (RNAi) significantly strengthened the insecticidal efficacy of a fungal entomopathogen Metarhizium anisopliae . The complete open reading frame (ORF) of NlCTL2 encoded a typical CTL with a secretion signal peptide and a single carbohydrate-recognition domain (CRD). Expression profiling revealed that NlCTL2 was highly expressed in hemolymph and gut tissues and was markedly induced by infection with M. anisopliae . Recombinant NlCTL2 exhibited strong agglutination activity to M. anisopliae conidia in a Ca 2+ dependent manner. RNAi of NlCTL2 resulted in significant reductions in survival and reproduction and inhibited the ovarian development and oocyte maturation. In addition, silencing of NlCTL2 markedly decreased antimicrobial peptide expression and phenoloxidase activity, thereby greatly increasing susceptibility to fungal infection. The median lethal times (LT 50 ) of NlCTL2 -silenced BPH nymphs and adults after M. anisopliae challenge decreased by an average of 46.4% relative to the control groups. These results underscored the crucial roles of NlCTL2 in BPH development and antifungal immunity, highlighting that NlCTL2 could be considered as a promising candidate target for developing BPH control strategies that combined EPF with RNAi.