Partnership and predation in plant-biotic interactions : theory, methods, and evidence

As sessile organisms, plants face an endless series of encounters with other living organisms in their environment. Ranging from the beneficial to the harmful, the pressures of these biotic interactions force plants to rapidly adapt to survive and thrive. This dissertation is aimed at addressing questions about the metabolism of plant interactions with herbivores and nutritional symbionts in legumes. Despite spanning different systems and methods, these questions reflect my interest in understanding the biochemistry underlying ecological and evolutionary function and my desire to develop tools that enable the investigation of such questions. To this end, the work in this dissertation is aimed at building conceptual and methodological tools to enable more thorough investigations of plant symbioses and, more broadly, plant-biotic interactions across levels of biological organization. First, I present a framework for making predictions on evolutionary trajectories and origins of plant--microbe communication systems. By highlighting the prevalence of coercive interactions in plant--microbe interactions, I demonstrate the plausibility of such interactions types to be an evolutionary precursor to seemingly stable signaling mechanisms. This work aims to provide useful evolutionary context for investigations concerning the evolutionary stability and exploitation of signaling mechanisms in established biological relationships. Second, I present collaborative work in which we developed and applied a cost-effective, high-throughput protocol for quantifying multiple biochemical defense responses from small quantities of plant tissue using spectrophotometric techniques. This protocol was then applied on two distinct populations of the legume Medicago polymorpha to investigate how changes defensive traits in responses to altered selective pressures have manifested over the course of novel range expansion. Our work demonstrated the feasibility and potential of assessing defense responses across plant populations. Further, the work documents a shift in herbivore preference of plants from familiar and unfamiliar ranges by demonstrating a herbivore preference for plant tissues with pre-induced defenses over uninduced tissues from an unfamiliar geographic range. Finally, I demonstrate the creation, construction, and validity of a novel microcosm system for assessing nutrient exchange in the symbiotic mutualism between plants and arbuscular mycorrhizal fungi (AMF). The novel system is reliable, biologically-relevant, durable, and sufficiently simple and cost-effective to deploy. I demonstrate the validity of the microcosm system and discuss in-progress work which demonstrates its potential to rigorously investigate unknown aspects of the plant-AMF mutualism. Taken together, these developments and suggestions contribute to the growing set of methods and frameworks developed for improving our understanding a various plant-biotic interactions.