Two bacterial genera, Xenorhabdus and Photorhabdus, are mutually symbiotic to the entomopathogenic nematodes, Steinernema and Heterorhabditis, respectively. Success parasitism of the nematode-bacterial complex depends on the host immunosuppression by the bacteria via their secondary metabolites. Lrp (Leucine-responsive regulatory protein) is a global transcriptional factor of the bacteria and play a crucial role in the parasitism. However, its regulatory targets to suppress the insect immunity were not clearly determined. This study investigated the regulatory target genes and subsequent secondary metabolites by Lrp in Xenorhabdus hominickii. Lrp expression occurred at the early infection stage in a target insect, Spodoptera exigua. Among eight non-ribosomal peptide synthetase (NRPS1-NRPS8) genes, six gene (NRPS3-NRPS8) expressions were positively correlated with Lrp expression in the infected larvae of S. exigua. Exchange of the Lrp promoter with an inducible promoter altered the production of the secondary metabolites along with alteration of the NRPS expression levels. The immunosuppressive activities of X. hominickii depended on the Lrp expression level. The metabolites produced by Lrp expression possessed the eicosanoid-biosynthesis inhibitors and hemolytic factors. A cyclic dipeptide (= cPF) was produced under Lrp control and identified to inhibit phospholipase A2 activity of S. exigua in a competitive inhibitory manner. These results suggest that Lrp is a global transcriptional factor of X. hominickii and plays crucial role in insect immunosuppression by modulating NRPS expressions.

Entomopathogenic nematodes (EPNs) of the genus Steinernema are pathogenic to the insects and well known as ideal models for understanding parasite-host interaction. EPNs harbor a number of bacterial symbionts in their gut belonging to the noble genus Xenorhabdus which are capable of killing insects by themselves or by combination with nematodes by suppressing insect immune defense. Here, we report host range of Steinernema monticolum and its symbiont Xenorhabdus hominickii. S. monticolum has a diverse host range including lepidopteran and coleopteran insects although they showed higher pathogenicity to the lepidopteran insects. Especially, X. hominickii suppressed insect immune responses. A target insect, Spodoptera exigua, exhibited both cellular and humoral immune responses by expressing antimicrobial peptides and forming nodules in response to heat-killed X. hominickii. However, live bacteria significantly suppressed the immune responses. An addition of arachidonic acid to the bacterial infection significantly rescued the immune responses, suggesting eicosanoid biosynthetic pathway as a pathogenic target of X. hominickii.