Suppressive activity of benzylideneacetone, a metabolite of Xenorhabdus nematophila, against expression of antimicrobial peptide genes derived from hemocyte transcriptome of Spodoptera exigua

Hemocytes are the major tissue to express immune responses consisting of both cellular and humoral reactions. Transcriptome analysis of hemocytes of the beet armyworm, Spodoptera exigua, would be expected to propose unidentified immune-associated genes and their signal pathways. Total RNAs were extracted from the hemocytes of the fifth instar larvae challenged with Xenorhabdus nematophila and used for constructing cDNAs. GS FLX 454 pyrosequencing of the transcripts provided 10,589 unique contigs and 11,962 singletons. Among them, only 18% clones were matched to functionally known genes, which were classified according to genetic ontology. About 5.92% clones matched to known genes were classified to immune-associated categories nonself recognition, intracellular immune signal mediation, antimicrobial peptides, phenoloxidase-associated. Two immune signal genes (Toll, Relish), and several antimicrobial peptide (AMP) genes were further analyzed in their expression profiles in response to various microbial pathogens. An entomopathogenic bacterium, Xenorhabdus nematophila, significantly inhibited some AMP genes. Benzylideneacetone, a monoterpenoid compound identified from the culture broth of X. nematophila, also inhibited the target AMP gene expression.