AN ENTOMOPATHOGENIC BACTERIUM, Xenorhabdus nematophila , SUPPRESSES EXPRESSION OF ANTIMICROBIAL PEPTIDES CONTROLLED BY TOLL AND IMD PATHWAYS BY BLOCKING EICOSANOID BIOSYNTHESIS

Immune‐associated genes of the beet armyworm, S podoptera exigua, were predicted from 454 pyrosequencing transcripts of hemocytes collected from fifth instar larvae challenged with bacteria. Out of 22,551 contigs and singletons, 36% of the transcripts had at least one significant hit ( E ‐value cutoff of 1e‐20) and used to predict immune‐associated genes implicated in pattern recognition, prophenoloxidase activation, intracellular signaling, and antimicrobial peptides ( AMP s). Immune signaling and AMP genes were further confirmed in their expression patterns in response to different types of microbial challenge. To discriminate the AMP expression signaling between T oll and I md pathways, RNA interference was applied to specifically knockdown each signal pathway; the separate silencing treatments resulted in differential suppression of AMP genes. An entomopathogenic bacterium, Xenorhabdus nematophila, suppressed expression of most AMP genes controlled by T oll and I md pathways, while challenge with heat‐killed X . nematophila induced expression of all AMP s in experimental larvae. Benzylideneacetone ( BZA ), a metabolite of X. nematophila, suppressed the AMP gene inductions when it was co‐injected with the heat‐killed X . nematophila. However, arachidonic acid, a catalytic product of PLA 2 , significantly reversed the inhibitory effect of BZA on the AMP gene expression. This study suggests that X . nematophila suppresses AMP production controlled by T oll and I md pathways by inhibiting eicosanoid biosynthesis in S . exigua.