Detoxification of conifer antimicrobial defenses promotes entomopathogenic fungus infection of bark beetles.

Abstract

After consumption by herbivores, plant antimicrobial defense compounds may enhance herbivore immunity to pathogenic microbes. In conifer-bark beetle interactions, beetles ingest large quantities of phloem tissue containing high concentrations of antimicrobial phenolic glucosides, such as stilbenes and flavonoids. It is not known, however, if these compounds increase bark beetle resistance to pathogens. We showed that Eurasian spruce bark beetles (<i>Ips typographus</i>) attacking Norway spruce (<i>Picea abies</i>) hydrolyze phenolic glucosides to their corresponding aglucones increasing their antifungal activity. However, the entomopathogen <i>Beauveria bassiana</i>, a natural fungal parasite of these beetles, detoxifies stilbene and flavonoid aglucones by forming methylglucoside derivatives. A two-step pathway involving a UDP-glycosyltransferase and an <i>O</i>-methyltransferase produces phenolic <i>O</i>-methylglucosides that are no longer toxic to <i>B. bassiana</i> and are stable to β-glucosidase action. Compared to wild-type strains of <i>B. bassiana</i>, mutant strains knocked out in the genes of this pathway exhibited decreased methylglucoside formation, slower growth on medium containing phenolic compounds, and reduced virulence toward bark beetles. Hence, methylglucosylation of plant-derived phenolics is a detoxification process that significantly increases the ability of <i>B. bassiana</i> to parasitize host insects consuming plant tissue high in phenolics, such as conifer phloem. This is one of the few examples of an entomopathogen that is able to resist the plant-derived defenses of an insect host.