COS 180-2 - The role of local and transported non-structural carbohydrate reserves for conifer defense synthesis during drought

Abstract: Central to plant function is the ability to transport carbon from sites of production to sites of demand. Tree carbon transport is particularly important for understanding conifer terpene defense responses to biotic agents such as bark beetles because the sites of attack (i.e., branches and bole) require nonstructural carbohydrates (NSC) like sugar and starch that originate from distal source organs. While local NSC availability tends to be positively correlated with biotically-induced terpene concentrations, the relative contribution of transported NSC towards induced defenses remains unclear, but potentially critical, for tree resistance to biotic agents during drought. This is because drought reduces water in the xylem that is essential for driving phloem transport of sugars via pressure-driven bulk flow. It follows that if defense metabolism relies heavily on transported NSC for metabolite synthesis, trees faced with concurrent drought and biotic challenge may be unable to mobilize NSC to sites of attack, leaving trees unable to mount an effective defense response. To assess the role of local and transported NSC for conifer defense synthesis during drought, mature, potted Pinus edulis were either well-watered or exposed to a gradient of drought stress. Once trees reached an ecologically relevant range of stress (-0.31 to -3.61 MPa), a bark-girdle treatment was applied to a segment of the bole to inhibit carbon translocation from the canopy and roots. We then elicited a defense response by inoculating trees with a bark beetle fungal symbiont (Ophiostoma spp.) and measured shifts in phloem terpenes, metabolites with important defensive roles against bark beetles.

Results show that transported NSC are critical for the synthesis of biotically-induced terpenes. Inoculation alone increased terpene concentration four-fold compared to non-inoculated trees. However, trees that were both girdled and inoculated increased terpene concentration by only two-fold compared to non-inoculated trees, highlighting that the induced terpene defense depends on both transported and locally-stored NSC. We found that the degree of drought stress influenced the magnitude of terpene induction in non-girdled trees. At more negative water potentials, terpene concentrations of non-girdled trees were increasingly similar to girdled trees. This suggests that carbon transport is reduced in drought-stressed trees and that during biotic attack, induced defense synthesis is constrained by local carbon limitation. A mechanistic understanding of how drought weakens tree defense is lacking and these findings identify reduced carbon transport as a key physiological process that constrains induced defenses in the face of biotic attack.