| Summary: | Symbiotic bacteria form a mutualistic relationship with nematodes and are pathogenic to many insect pests. They kill insects using various strategies to evade or suppress their humoral and cellular immunity. Here we evaluate the toxic effects of these bacteria and their secondary metabolites on the survival and phenoloxidase (PO) activation of <i>Octodonta nipae</i> larvae using biochemical and molecular methods. The results show <i>P. luminescens</i> H06 and <i>X. nematophila</i> All treatments caused significant reductions in the number of <i>O. nipae</i> larvae in a dose-dependent manner. Secondly, the <i>O. nipae</i> immune system recognizes symbiotic bacteria at early and late stages of infection via the induction of C-type lectin. Live symbiotic bacteria significantly inhibit PO activity in <i>O. nipae</i> whereas heat-treated bacteria strongly increase PO activity. Additionally, expression levels of four <i>O. nipae</i> proPhenoloxidase genes following treatment with <i>P. luminescens</i> H06 and <i>X. nematophila</i> All were compared. We found that the expression levels of all proPhenoloxidase genes were significantly down-regulated at all-time points. Similarly, treatments of <i>O. nipae</i> larvae with metabolites benzylideneacetone and oxindole significantly down-regulated the expression of the PPO gene and inhibited PO activity. However, the addition of arachidonic acid to metabolite-treated larvae restored the expression level of the PPO gene and increased PO activity. Our results provide new insight into the roles of symbiotic bacteria in countering the insect phenoloxidase activation system.
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