Biochemical and Functional Analysis of Phenolic Secondary Metabolites in Populus

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Trees and forests are essential components of Canada's landscape and economy. Canada's land area contains 10% of the world's forests, and represent a significant long-term carbon sink. Forest health is therefore a priority for policy and research. As climate change impacts our forests and pests and pathogens expand their ranges, understanding the biochemical basis of tree adaptation is critical. My research program addresses fundamental questions in plant biochemistry and physiology, using poplar (Populus) as a model system. The proposed research focuses on the soluble phenolic secondary metabolites that accumulate to extraordinary levels in poplars and many other forest trees. The phenolics are a large family of plant compounds; they include the broadly distributed condensed tannins and other flavonoids, as well as diverse phenolic glycosides and esters. The objective of my research is to investigate how key phenolic compounds are synthesized, how their accumulation is regulated, and how such compounds can help trees adapt to environment stresses. I outline a program of research that covers both biochemical and functional aspects of poplar phenolic metabolism, and which will provide diverse training opportunities. First, we will characterize enzymes and elucidate the biosynthetic pathway for the poplar phenolic glycosides. These compounds, also known as salicinoids, are key anti-insect defenses in poplar and may also have therapeutic potential; however, their biosynthesis is still unknown. Second, we will study the molecular biology and biochemistry of poplar leaf bud resin formation, with the aim of identifying novel enzymes for the biosynthesis of antimicrobial flavonoids and phenolic esters. Third, we will investigate the physiological functions of the condensed tannins, the most widespread phenolic chemicals. We will make use of unique transgenic poplar lines with modified tannin content, and will test a novel hypothesis that condensed tannins function as antioxidants that facilitate efficient nitrogen reallocation during leaf senescence. Fourth, we will test for novel functions of condensed tannins in roots, focusing on their proposed interactions with microbes such as mycorrhizal fungi and pathogens. This will require the generation of new genome-edited transgenic poplars. This research will lead to the discovery of new enzymes, pathways, and functions for several groups of poplar secondary metabolites. It will provide the first molecular analysis of poplar leaf bud resin, known for its bioactive components and for its importance to the ecology of bees. The work will also lead to a deeper understanding of the functions of condensed tannins, and the physiological and ecological roles they play in trees and forests. Our forests produce large quantities of tannins and other phenolics. The impact of these molecules on forest and tree health is of fundamental importance.