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Due to their sessile nature, plants must alter their phenotype to respond to environmental stressors. Herbivory is the most ubiquitous ecological interaction, and plants have consequently evolved an extensive array of traits with which to mitigate fitness lost to herbivores. The response to herbivory typically involves deploying suites of chemical resistance traits, known as “inducible” traits, and altering metabolic processes, all of which are generated through large-scale restructuring of the plant transcriptome mediated by a small set of phytohormones. Despite evidence of genetic and ecologically-driven variation in induced responses, very little is known about the proximal mechanisms responsible for variation in chemical resistance traits. Using herbivore-induced volatile organic compound (VOC) emissions, which act as a within-plant signal of damage and indirect resistance trait, as markers of induced resistance, this dissertation explores the transcriptional basis of variation in phenotypic plasticity in the undomesticated perennial Datura wrightii. Using a non-targeted approach, RNA-Seq analyses were used to track differences in gene expression (1) over the course of a single, continuous herbivore attack in the laboratory, (2) at various intervals over the course of two growing seasons under field conditions, and (3) across inbred genetic lines grown under field conditions. The results of these studies indicate that transcriptional variation in damaged plants across time, ontogeny, and genotypes is far greater than VOC emissions would suggest. The overall number of genes induced was highly variable with respect to plant ontogeny and genotype, and showed no relation to VOC emissions. Instead, multivariate analyses show that phytohormone-related gene activity predicts VOC emissions, and has increased power to do so when phytohormone pathways are included that ostensibly have no relation to resistance induction. Taken together, these results suggest that a smaller number of “upstream” signaling molecules regulate VOC emissions independently of other, large-scale changes to the transcriptome following herbivore attack. Such a pattern implies that natural selection may act on components of the induced response individually, but contemporary variation among populations is likely the result of diffuse selection. |
