DISSERTATION RESEARCH: Supply-Side Regulation of Herbivore-Induced Phenolics in Plants

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DISSERTATION RESEARCH: Supply-side Regulation of Herbivore-Induced Phenolics in PlantsJack Schultz and Toni SchaefferPhenolics are carbon-based chemicals synthesized within all plants. Lignin, the major component of wood, is a phenolic. In addition to the structural role of lignin, many other phenolics play diverse roles in the lives of all plants as both structural and defensive elements. Changes in phenolic production comprise the most-studied class of "defenses" in research on plant pests. Much ecological theory on how plants allocate their carbon and nutrient resources presumes that plants, in response to changes in their environment, are able to vary the proportion of their resources that they use to synthesize phenolics. The majority of the world's carbon passes through the biosynthetic pathways leading to phenolic synthesis (once it is taken from the atmosphere by the plant during photosynthesis). Hence, these pathways are critical components of global carbon cycling. All evidence to date suggests that global change will enhance the importance of these biosynthetic pathways. However, plant regulation of phenolic production is still poorly understood. This may be partly because the two biosynthetic pathways that plants use to synthesize phenolics have traditionally been studied separately. The shikimate pathway (SP) has been studied as the source of aromatic amino acids, and the phenylpropanoid pathway (PP) as the source of defenses and lignin. Most plant scientists have focused on the detailed regulation of the PP, the second pathway in this pipeline, ignoring earlier steps in these pathways. The core hypothesis of the proposed research is that regulation of phenolics is accomplished at the "supply-side" of synthesis within the SP, the first pathway, as well as downstream within the PP. Using a series of greenhouse experiments, this research aims to determine how plants coordinately regulate the "top" and "bottom" of this synthetic pipeline, namely two regulatory enzymes (DAHPS and PAL). Also being explored is the idea that plants are able to exploit stored pools of carbon-based pathway intermediates when they change their phenolic production. By comparing the phenolic regulation strategy of a model herbaceous plant, tobacco, to a model woody plant, poplar, this research will help explain how carbon resources are allocated to these important pathways and products in very different plants. The results will clarify the regulation and evolution of carbon allocation patterns in plants. This information will point to ways in which plants may be bred or engineered to maximize carbon allocation for various purposes, and will facilitate better understanding of plant growth, defense, and productivity.