Populus, the New Model System for Investigating Phenyl- propanoid Complexity

Plant secondary metabolism affects ecosystem diversity and the yield and quality of feedstocks for biomass and biofuel, through an elaborate network of pathways that share com- mon precursors. Until recently, functional dissection of these networks has depended largely on molecular information stored in the genome of Arabidopsis, an annual herb. Now that the Popu- lus genome sequence is available, the potential for understanding and exploiting secondary me- tabolism in tree species comes closer to realization. In the present overview, genomic informa- tion pointing to greatly expanded gene complexity and function of the phenylpropanoid pathway in Populus is summarized. Phenylpropanoid-derived flavonoid and salicylate phenolics occur in numerous functionally distinct forms, and can account for 50% of leaf biomass in Populus and other fast-growing tree taxa. Their potential effects on tree growth, and their documented im- pacts on ecosystem diversity and productivity justify molecular dissection of secondary metabo- lism in Populus. Biosynthesis of salicylate phenolics remains poorly understood. By contrast, in silico promoter analysis of flavonoid genes, and in situ flavonoid localization in Populus re- ported here, augment published gene expression data, and illustrate that intra and intercellular regulatory components dramatically affect secondary carbon partitioning in this woody peren- nial.