Roles of Peroxinectin in PGE(2)-Mediated Cellular Immunity in Spodoptera exigua

BACKGROUND: Prostaglandins (PGs) mediate insect immune responses to infections and invasions. Although the presence of PGs has been confirmed in several insect species, their biosynthesis in insects remains a conundrum because orthologs of the mammalian cyclooxygenases (COXs) have not been found in the known insect genomes. PG-mediated immune reactions have been documented in the beet armyworm, Spodoptera exigua. The purpose of this research is to identify the source of PGs in S. exigua. PRINCIPAL FINDINGS: Peroxidases (POXs) are a sister group of COX genes. Ten putative POXs (SePOX-A ∼ SePOX-J) were expressed in S. exigua. Expressions of SePOX-F and -H were induced by bacterial challenge and expressed in the hemocytes and the fat body. RNAi of each POX was performed by hemocoelic injection of their specific double-stranded RNAs. dsPOX-F or, separately, dsPOX-H, but not the other eight dsRNA constructs, specifically suppressed hemocyte-spreading behavior and nodule formation; these two reactions were also inhibited by aspirin, a COX inhibitor. PGE(2), but not arachidonic acid, treatment rescued the immunosuppression. Sequence analysis indicated that both POX genes were clustered with peroxinectin (Pxt) and their cognate proteins shared some conserved domains corresponding to the Pxt of Drosophila melanogaster. CONCLUSIONS: SePOX-F and -H are Pxt-like genes associated with PG biosynthesis in S. exigua.