RNA Interference of Phenoloxidases of the Fall Armyworm, , Enhance Susceptibility to Protein Vip3Aa19.

Phenoloxidase (PO) is an important enzyme in the cellular immune system and is involved in defense against a wide range of pathogens, including . Vip3Aa19 is secreted and expressed by (Bt) at the middle exponential growth phase and is a kind of protein with efficient insecticidal activity against . However, immune responses of the target insects have been regarded as a hindrance to Bt pathogenicity. This paper reports two phenoloxidase (PO) genes (PAE and PO2) identified from the hemocyte transcriptome data of the fall armyworm, qRT-PCR validation results showed that the expression levels of two PO genes were significantly upregulated after Vip3Aa19 (LC = 4.98 µg/g) toxin treatment compared with those of fed an insecticide-free artificial diet. Meanwhile, two PO genes were expressed from the egg to adult stages even without an immune challenge. We noticed that at all developmental stages investigated in the , PAE was generally expressed at a higher level than PO2. However, after Vip3Aa19 treatment, the PO2 gene mRNA expression level was significantly elevated in response to the toxin challenge. An injection of a specific double-stranded RNA (dsRNA) against POs could suppress its expression. The third instar larvae of treated with dsRNA were much more susceptible to Vip3Aa19 toxin than the control larvae were. Notably, the mortality rate was nearly 90% after a dsPO2 injection. These results proved that PO2 was more important for the survival of . Finally, RNA interference and then PO activity detection revealed that PO genes mainly existed in the hemolymph and played an important role in immune defense against Bt toxin.