Defining the Tyrosine Biosynthetic Pathways in Plants

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Non-technical summary:Plants synthesize an aromatic amino acid tyrosine (Tyr), which is an essential nutrient in human diets and a key precursor of numerous plant specialized compounds. These Tyr-derived plant natural products have profound impacts on plant growth, development and environmental responses, and also have important nutritional and pharmacological activities in humans (e.g., vitamin E, morphine). Despite their importance in both plant and human biology, we still do not know how Tyr is synthesized in plants. Therefore, this project will gain fundamental knowledge to enhance the nutritional quality or pharmaceutical values of crops and medicinal plants, and can potentially improve plant growth and stress responses for sustainable production of food and biomass. An outreach event, "Pigment-Art", will take advantage of attractive and beneficial plant pigments derived from aromatic amino acids (e.g., betalains). While younger children will appreciate the beautiful and complex colors in natural pigments, parents and older children will have opportunities to learn about how these pigments are synthesized by plants. Technical Description of the Project:Accumulating evidence suggests that many plants synthesize Tyr via the intermediate arogenate and therefore differ from microbial models that usually use 4-hydroxyphenylpyruvate as an intermediate. However, significant variations appear to exist for the Tyr biosynthetic routes among different plant species. The goals of this project are to define the Tyr biosynthetic route(s) in plants and determine their compartmentalization and crucial regulatory step(s). The proposed research will build upon PI's recent isolation of Arabidopsis and legume genes encoding arogenate and prephenate dehydrogenases (ADH and PDH), key enzymes of the Tyr pathway. Arabidopsis ADHs have strong substrate specificity toward arogenate, are localized in the plastids and inhibited by Tyr. In contrast, legume dehydrogenases specifically use prephenate, lack a plastid transit peptide, and are insensitive to Tyr. The primary hypothesis of this study is that Arabidopsis mainly uses the plastidic arogenate pathway that is regulated by Tyr, whereas legumes synthesize Tyr via both arogenate and 4-hydroxyphenylpyruvate with unknown localization and regulatory mechanism. In this project, kinetic analysis and localization studies are first planned for the newly identified legume PDHs (Aim 1). In planta functions of PDHs and ADHs will be investigated and compared using two contrasting plant models, Medicago truncatula (Aim 2) and Arabidopsis thaliana (Aim 3). Targeted metabolite profiling and stable-isotope feeding experiments will be further conducted in the generated mutants and transgenics to directly measure the distribution of carbon flow among the alternative Tyr pathways. Although primary metabolism is generally assumed to be conserved across plant species, our preliminary data indicate otherwise. This project will directly test this assumption and address how Tyr biosynthesis, a primary metabolic pathway, has evolved in different plants to support the enormous chemical diversity that exists in the downstream specialized metabolism.