The basic pathways to the core (iso)flavonoid skeletons have been established both enzymatically and genetically. The entry point enzymes are the polyketide synthase chalcone synthase (CHS) and isoflavone synthase (IFS), more correctly termed 2-hydroxyisoflavanone synthase, a cytochrome P450 that catalyzes the aryl migration reaction that converts a 2-phenylchroman to a 3-phenylchroman ( Fig. 1B ). The structural diversity of (iso)flavonoids is derived by substitution of these basic carbon skeletons through further hydroxylation, glycosylation, methylation, acylation, and prenylation as well as, in the case of the proanthocyanidins (PAs; also known as condensed tannins; Fig. 1B ) and phlobaphenes, by polymerization. The enzymes that catalyze the substitution reactions are often encoded by large gene families, which can be recognized in EST and genome data sets through family-specific conserved sequence motifs. Several areas of flavonoid biosynthesis still require clarification or elucidation. These include in vivo substrate specificity of potentially multifunctional enzymes (e.g. 2-oxoglutarate-dependent dioxygenases such as anthocyanidin synthase or the many family I glycosyltransferases that are active with flavonoids), transport of flavonoids into and out of the central vacuole and other cellular compartments, and the oxidation of flavonoids associated with polymerization to PAs and phlobaphenes ( Pourcel et al., 2006 ).
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