Rapid induction of multiple terpenoid groups by Ponderosa pine in response to bark beetle-associated fungi
Conifers are confronted with a number of biotic agents that can exploit their tissues, of which bark beetles and their associated microorganisms are the most threatening. Because these insects and fungi colonize subcortical tissues, they impair trees’ abilities to translocate water and nutrients, resulting in tree death. Conifers defend against bark beetle-fungal complexes in several ways, including xylem resin, which presents both a physical and chemical defense that can be constitutive and induced. Most research on mountain pine beetle has been with lodgepole pine, however, this insect also attacks ponderosa pine, sometimes causing high mortality. While several components of induced defense appear minimal in ponderosa pine, we currently lack data on chemical components of induced defense. To assess the constitutive and possible induced responses of ponderosa pine to bark beetle-vectored fungus, we collected initial phloem samples from 40 trees at a field site in south-central Montana then assigned trees to one of two treatments: mechanical damage or mechanical damage + inoculation with Grosmannia clavigera fungus. Approximately 17 days following treatment, we collected phloem samples from the damaged area and from an undamaged area on the opposite side of the tree to test for a systemic reaction. All phloem samples were then analyzed for mono-, sesqui-, and diterpenes using gas chromatography.
The total quantities of all three terpene classes varied with treatment and within-tree distance from treatment. In trees that were mechanically wounded only, terpene concentrations within the reaction zone rose to greater levels than those of constitutive samples (~5-18 times more, depending upon terpene class). In trees subjected to both mechanical wounding and fungal inoculation, concentrations within the reaction zone rose to much greater levels than those of constitutive samples (~24-85 times more, depending upon terpene class). We did not find evidence of systemic induction of total terpenes in response to either the mechanical wounding or fungal inoculation. Our results clearly show that ponderosa pine terpene chemistry responds both to the physical damage caused by bark beetle entry and an even greater amount to the fungus introduced by the insect. In addition, the induction of terpenes due to damage and fungus occurs in a relatively short time period. Thus, like other conifer species where trees with higher induced resin were less likely to be killed by mountain pine beetles, ponderosa pine individuals may be able to quickly overwhelm initial beetle attack and inhibit the spread of any introduced fungus.