Storing away for a (not so) rainy day: The role of non-structural carbohydrate reserve status and mobilization during bark beetle attack

The interdependent response of carbon, water, and defense to drought stress has been suggested to play a critical role in tree resiliency or susceptibility bark beetle mortality, yet empirical evidence demonstrating the mechanistic links between these pools is lacking. Namely, the relative contribution of water status and its effects on non-structural carbohydrate (NSC) transport from distal organs to sustain the induced chemical response during drought remains unclear, but potentially critical. In addition, drought also induces starch hydrolysis, which increases levels of soluble sugars available for successful bark beetle larval development, which can be attractive for bark beetle initiation. In both defensive and nutritive contexts, the importance of NSC mobilization remains to be elucidated.
We exposed mature, potted pinon pine (Pinus edulis) to either well-watered or a gradient of drought conditions. A bark-girdle treatment was then applied to inhibit carbon translocation and a defense response elicited via inoculation with a bark beetle fungal symbiont (Ophiostoma spp.). We measured shifts in NSC, phloem terpene defenses, and cell turgor. Results show that transported NSC are critical for the synthesis of biotically-induced terpenes. Specifically, trees that were both girdled and inoculated increased terpene concentration by only two-fold compared to non-girdled trees. These findings identify reduced carbon transport as a key physiological process that constrains induced defenses in the face of biotic attack. The link between a lowered sustained induced response and a potentially enhanced nutritive value due to increased soluble sugars that cannot be mobilized will also be discussed.