Bacteria Associated with a Tree-Killing Insect Reduce Concentrations of Plant Defense Compounds

Abstract

Bark beetles encounter a diverse array of constitutive and rapidly induced terpenes when attempting to colonize living conifers. Concentrations of these compounds at entry sites can rapidly reach levels toxic to beetles, their brood, and fungal symbionts. Large numbers of beetles can overwhelm tree defenses via pheromone-mediated mass attacks, but the mechanisms are poorly understood. We show that bacteria associated with mountain pine beetles can metabolize monoterpenes and diterpene acids. The abilities of different symbionts to reduce concentrations of different terpenes appear complementary. Serratia reduced concentrations of all monoterpenes applied to media by 55-75 %, except for α-pinene. Beetle-associated Rahnella reduced (-)- and (+)-α-pinene by 40 % and 45 %, respectively. Serratia and Brevundimonas reduced diterpene abietic acid levels by 100 % at low concentrations. However, high concentrations exhausted this ability, suggesting that opposing rates of bacterial metabolism and plant induction of terpenes are critical. The two major fungal symbionts of mountain pine beetle, Grosmannia clavigera and Ophiostoma montium were highly susceptible to abietic acid. Grosmannia clavigera did not reduce total monoterpene concentrations in lodgepole pine turpentine. We propose the ability of bark beetles to exert landscape-scale impacts may arise partly from micro-scale processes driven by bacterial symbionts.