The involvement of Apolipophorin-III in orchestrating prophenoloxidase activation and NO production in Ostrinia furnacalis larvae.

Apolipophorin-III (ApoLp-III), a multifunctional protein with lipid transport and immune defense functions, widely exists in insects. Although the function of ApoLp-III as a pattern recognition receptor (PRR) in immunity has been relatively studied, the immune response mediated by ApoLp-III is still vague. To understand whether ApoLp-III is involved in the activation of the prophenoloxidase-activating system (PPO-AS), we examined the production of nitric oxide (NO), and the synthesis of antimicrobial peptides after immune recognition. The larvae of lepidopteran pest Ostrinia furnacalis were used as a model to address these questions by detecting the changes of phenoloxidase (PO) activity and NO concentration after the knockdown of OfApoLp-III and bacterial infections. In the present study, we reported the cloning and characterization of the OfApoLp-III complementary DNA, and found that OfApoLp-III is mainly expressed in the larval fat body. These investigations revealed that OfApoLp-III was an immune-related gene, its knockdown reduced the PO activity by 41.9%, and NO concentration reached 2.7-fold higher level than that after double-stranded GFP treatment. Our data indicated that OfApoLp-III was involved in increased expression of Moricin, activation of PPO, and reduction of NO production in O. furnacalis larvae after different bacterial infections, which were required for innate immunity. ApoLp-III is a candidate target for an integrated pest control strategy using the combined application of double-stranded RNA and biocontrol bacteria.