Beauveria bassiana ERL836-mediated suppression of oxidative phosphorylation and immune response in fat body of Japanese pine sawyer beetle

Japanese pine sawyer beetle (JPSB), Monochamus alternatus Hope, is a vector of the pine wilt nematode that causes pine wilting. Chemicals have been used to control JPSB, but they often cause environmental pollution and insect resistance. Alternatively, Beauveria bassiana ERL836 could be used to control JPSB and RNA-seq of the full body of JPSB was conducted to investigate host defense mechanisms. However, little interest was given to the defense mechanisms of the JPSB fat body which is a key organ of host defense and energy metabolism. ERL836 showed high insecticidal activity against JPSB adults and active sporulation on the cadavers, which potentially enables active spreading to other JPSB populations. From the RNA-seq of infected JPSB fat body, genes encoding for oxidative phosphorylation for energy metabolism were up-regulated 2 days but finally down-regulated 4 days compared to non-treated control, suggesting a reduction of energy metabolism during fungal infection. Simultaneously in the immune response of the fat body, serine protease inhibitor (serpin) genes which adversely regulate host defense response were continuously up-regulated and consequently most immune response were suppressed. The JPSB serpins including Spn42 and SRPN2 were phylogenetically close to ERL836 serpins. In fungal pathogenesis, JPSB defense pathways were strongly suppressed with the over-expression of serpin genes, although energy metabolism pathways were down-regulated. This work suggests that active expression of fat body serpin genes is possibly related to the speedy fungal infection, which could create a more favorable environment for fungal infection. In the next work, factors inducing the expression of JPSB serpins need to be clarified and the roles of fungal serpins would be investigated.