Plants continually confront a multitude of environmental challenges that can impede their growth, development, and survival, and thereby have evolved a remarkable array of responses to environmental stresses to ensure their persistence on the landscape and optimize growth. These stress responses are remarkably plastic and adaptable to a changing environment and have been the subject of intense research interest. The study of plant stress responses provides critical insights into fundamental physiological processes and has practical implications for agriculture, conservation, and ecosystem management. Understanding the intricate signaling cascades and molecular components that underlie plant stress responses is essential for developing strategies to enhance stress tolerance in crops, mitigate the impact of climate change on ecosystems, and conserve plant biodiversity.
In recurring encounters of tree species with both abiotic stress and pathogenic invasions, delimiting stress responses will be instrumental for conservation and management practices. Building on current understanding of induced resistance in the Pinus nigra - Diplodia spp. pathosystem, we hypothesized that, (1) predisposition of Austrian pine to abiotic stress such as climate change (CC) leads to increased susceptibility to pathogenic infections by Diplodia spp. and this heightened susceptibility is explainable by a detailed analysis of the transcriptional regulation of both the host and pathogen, (2) attack of Austrian pine by D. pinea results in a systemic induced resistance (SIR) phenotype that intensifies over time, and (3) this phenotype is mediated by the accumulation of terpenoids and is explainable by a detailed analysis of signaling pathways involving phytohormones in specific patterns.
The test of the first hypothesis is described in Chapter 2. We subjected Austrian pine trees to simulated CC conditions of high temperatures and prolonged water scarcity, followed by infection with either D. pinea, or the less aggressive sister pathogen, D. scrobiculata, followed by incubation for two weeks. We found that CC impacted trees had similar disease severity, regardless of pathogen aggression, while D. pinea remained the more aggressive pathogen compared to D. scrobiculata under normal conditions. This was supported by evidence of suppressed primary metabolism and defense in pines infected by D. pinea, while infection by D. scrobiculata results in elevated levels of defense response, amidst an unaltered primary metabolism compared to D. pinea infected trees under control temperature and water availability. Under CC conditions, suppressed primary and secondary metabolism along with phytohormone signaling and defense responses in the host coupled with enhanced primary metabolism in D. scrobiculata leads to increased aggressiveness of the pathogen and enhanced host susceptibility.
In Chapter 3, we used a factorial design to test hypothesis 2. SIR potency was measured at 0.5 days, 3 days, and 10 days post-induction by D. pinea infection. This was accompanied by a detailed characterization of the accumulation patterns of individual and co-regulated terpenoids and other volatile compounds at the different time points. We found that some individual compounds, as well as clusters of coordinated compounds, were strongly correlated with the strength of SIR. The role of several of these compounds in SIR was supported by their fungistatic activity, demonstrated at in vivo relevant concentrations.
To test the third hypothesis (Chapter 4), we used the same experimental approach used in Chapter 3, to investigate the effects of induction by pathogen attack or wounding alone on systemic elicitation of defense related gene expression, as well as triggering of stress hormones and their associated signal transduction pathways after 0.5, 1, 1.5, 2, 3, and 7 days. We found evidence of systemic induction of pathogen recognition within 0.5 days post-induction, along with significantly higher levels of abscisic acid (ABA) and activity of ROS-detoxifying enzyme genes, followed by significant and progressively stronger jasmonic acid (JA) pathway-mediated responses within a day and up to a week of induction.
Taken together, the dissertation highlights key mechanisms of conifer tree resistance/susceptibility against aggressive pathogens of worldwide concern like D. pinea, and the deleterious effects that CC can have on these relationships. It also confirms that SIR is expressed during early stages of induction and demonstrates that terpenoids play an important role in the early expression of SIR. Finally, this research demonstrates that SIR is largely mediated by the JA pathway, in concert with an early involvement of ABA. Therefore, the foundation of this model pathosystem is further cemented for future work in this area of science in conifers, which will help inform better breeding efforts and management practices in the future.