Conifer stem defense mechanisms: Identification and localization of signaling pathways involved in phenylpropanoid and terpenoid responses

Conifers are some of the longest-lived organisms and have evolved defense strategies to resist the constant pressure from herbivore and microbial attack. In North America, the Pinaceae are the largest family and come under attack from bark beetles and pathogenic fungi carried by the beetles. Attacks can result in the death of individual trees and result in large-scale forest damage. Conifers have constitutive and inducible defense systems to combat beetle attacks and fungal growth. Terpenoid and phenylpropanoid compounds are known to be major components of defense systems but the signaling events controlling these defenses are not well understood. Identifying these signaling pathways is important to understanding resistance in conifers and to enhance resistance genetically or chemically. The objective of this study was to determine signaling pathways of inducible defenses in conifer stems. First I present a review on known signaling events that specifically occur in conifers compared with angiosperm model systems, from initial elicitation to the inducible anatomical and chemical changes that follow. It is hypothesized that the jasmonate and ethylene pathways are involved in defense signaling in the bark, and this was tested using conifer saplings. The jasmonate pathway is apparently involved in early signaling events and induces ethylene biosynthesis in key tissues of the bark and cambial zone. The temporal expression and spatial localization of key regulatory enzymes involved in the synthesis of jasmonate and ethylene are characterized with respect to induction of terpenoid and phenylpropanoid defense responses. Signaling pathway models are presented based on results from studies of jasmonate and ethylene leading to induced stem defense responses. The final chapter examines stem responses in Pinus monticola following natural infection by Cronartium ribicola, the non-necrophylactic fungal causal agent for white pine blister rust. Results from this study contrasts the well-studied necrophylactic blue stain fungi interactions in conifer bark, and illustrate how some fungi can circumvent the induced host defenses.