Gut bacterial‐mediated adaptation of Streltzoviella insularis to a constitutive defense compound in an ash tree ( Fraxinus sp.)

Gut microbiota play a key role in an insect's ability to overcome host tree defenses. Streltzoviella insularis (Lepidoptera: Cossidae) is a generalist wood-boring pest in China, and Fraxinus chinensis var. rhynchophylla is its preferred host in Xiong'an New Area. Previous mechanistic studies have demonstrated that S. insularis can tolerate and degrade host defense compounds via its gut microbes. However, whether hydroxytyrosol (a constitutive phenolic compound potentially involved in browning reactions) functions as a key defense of Fraxinus chinensis var. rhynchophylla against woodborers, and whether the gut microbiota of attacking S. insularis can degrade hydroxytyrosol, remains unclear. We determined the hydroxytyrosol content in S. insularis-infested and uninfested Fraxinus chinensis var. rhynchophylla. We further evaluated the effects of hydroxytyrosol on larval survival, tested the ability of larval gut bacteria to degrade hydroxytyrosol, and identified the resulting degradation products. Although hydroxytyrosol was toxic to S. insularis, its concentrations were lower in larval galleries than in uninfested trees, suggesting that S. insularis can degrade this compound. Gut bacteria from S. insularis degraded hydroxytyrosol both in vivo and in vitro, and Serratia sp. showed the strongest hydroxytyrosol degradation activity. Metabolomics further indicated that gut bacteria may degrade hydroxytyrosol via the tricarboxylic acid cycle pathway. Because this pathway yields ATP, the bacterial degradation products of hydroxytyrosol may provide an energy source for the insect hosts or their gut microorganisms.