Molecular and Functional Analysis of Phenolic Metabolism in Poplar

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Forest health is of major significance to Canada. Understanding how trees are adapted to stresses in the environment is thus an important research aim. This proposal describes a program of research on the biochemical adaptations of Populus trees (collectively called poplars). Poplar has become a model tree for molecular biology and genomics. Importantly, poplars also contain high concentrations of bioactive phenolic phytochemicals, in particular condensed tannins. These are polymeric flavonoids, also known as proanthocyanidins, which can constitute up to 25 % of poplar leaf dry weight. Condensed tannins are the most widely distributed secondary metabolite in the plant kingdom and are very prevalent among woody plants. Poplars also make salicinoid phenolic glycosides, which are unique to the Salicaceae and whose biosynthesis is unknown. Both classes of phenolics help adapt poplar to its environment. The phenolic glycosides are established as effective defenses against insects, whereas the role of condensed tannins appears to be much broader. *** The aim of the proposed research is to determine how the synthesis of poplar phenolic phytochemicals, in particular of the tannins, is regulated, and how these chemicals contribute to adaptation of trees. In poplars, tannins are inducible by stress. We previously showed that this induction is transcriptionally regulated and identified a transcription factor, MYB134, that stimulates condensed tannin synthesis in poplar. Recent results indicate that MYB134 is part of a tannin gene regulatory network that also includes at least four MYB repressor-like proteins. We hypothesize that these repressors can fine-tune tannin and flavonoid synthesis. The proposed research will investigate the functions of these MYB repressors, and determine how they interact to form a regulatory network. In parallel, we will investigate the functions of tannins in poplar. While there are many studies on the potential role of tannins in defense against insect herbivores, recent evidence suggests their role includes tolerance to other types of stresses. Here we will test the idea that condensed tannins play a role in abiotic stress tolerance, making use of our suite of transgenic poplars with altered tannin levels. A further objective is to dissect the biosynthesis of the salicinoid phenolic glycosides. We have recently identified a gene for an enzyme that is required for their synthesis, and we will use this gene as a probe to dissect the rest of the pathway.*** The results of this research can be applied directly to other trees, many of which accumulate high concentrations of tannin. How tannins affect tree health as well as the larger ecosystem will be important for both forest management and ecology. Furthermore, the importance of tannins to the human diet in reducing the risk of cardiovascular disease makes understanding their regulation important for human health. **