ABSTRACT
The completion and initial analysis of the Populus trichocarpa (black cottonwood) genome (Tuskan et al. 2006) was a milestone in plant and tree biology. The foundation for this accomplishment was laid by years of research on the genetics, physiology, and molecular biology of members of the Populus genus (poplars, cottonwoods, aspens; referred to as “poplar”), and by the development of complementary genomic resources such as expressed sequence tag (EST) collections, microarrays, and a physical map (Jansson and Douglas 2007). In addition to its status as a model tree, poplars are commercially important for pulp and paper, solid wood, potentially for biofuel production, and are ecologically important in their native ranges (Brunner et al. 2004). Phenylpropanoid and phenolic metabolism strongly affects many poplar traits that infl uence their suitability for such uses as well as the biology of poplars, and is the focus of this chapter. Secondary wall formation is a hugely important process in forest ecosystems, because the bulk of the biomass of trees is cellulose and encrusting lignin in secondary walls. Lignin, whose monomeric constituents are derived from the phenylpropanoid pathway, is a major component of wood, and has a profound impact on the properties of wood with respect to its use in pulp and paper production and conversion of ligno-cellulose into ethanol. Phenolic and phenylpropanoid derived compounds appear to play important roles in poplar defenses against pathogens and insect pests, and provide a potentially rich source of renewable organic compounds. These pathways of phenylpropanoid and phenolic natural product biosynthesis require the coordinated activity of the shikimate pathway to supply pathway intermediates and, especially, phenylalanine as a starter molecule for phenylpropanoid metabolism. Completion of the poplar genome and continued genome-level, biochemical, genetic, and comparative genomics analyses are providing new insights into phenylpropanoids and phenolics and their biosynthesis in these trees. This information will be of critical importance in improvement of poplar genotypes for deployment for practical purposes, and in understanding the biology of poplar as an ecological keystone species.
