Flavonoids and Isoflavonoids: From Plant Biology to Agriculture and Neuroscience

Abstract

Flavonoids represent one of the largest and most studied classes of phenylpropanoid-derived plant specialized metabolites, with an estimated 10,000 different members. Structurally, they consist of two main groups, the 2-phenylchromans (the flavonoids, including flavanones, flavones, flavonols, flavan-3-ols, and anthocyanidins) and the 3-phenylchromans (the isoflavonoids, including isoflavones, isoflavans, and pterocarpans; Fig. 1A). Flavonoid classes and an outline of their biosynthesis. A, The 2- and 3-phenylchroman skeletons. B, Schematic of flavonoid biosynthesis. Enzymes are CHS, chalcone isomerase (CHI), flavanone 3-β-hydroxyalse (F3H), flavonol synthase (FLS), and dihydroflavonol reductase (DFR). Anthocyanins are formed from leucoanthocyanidin (the product of DFR) and condensed tannins; PAs are formed from (epi)catechins, products of leucoanthocyanidin reductase or anthocyanidin reductase. 3-Phenylchromans (isoflavonoids) are formed via IFS. All classes of (iso)flavonoids can be further modified by substitution (e.g. O-methylation, glycosylation, C-prenylation). Flavonoids act as attractants to pollinators and symbionts, as sunscreens to protect against UV irradiation, as allelochemicals, and as antimicrobial and antiherbivory factors. Their importance in plant biology goes beyond their specific functions within the plant. For example, the early advances in floral genetics were primarily the result of the ease of screening for mutations impacting flavonoid-derived flower colors, and the first demonstration of epigenetic gene silencing in plants was likewise associated with flavonoid biosynthesis (Jorgensen, 1995). Flavonoids have been ascribed positive effects on human and animal health and are central to the current interest in “botanicals” for disease therapy and chemoprevention. 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). The distinction between in vitro activity and in vivo function in flavonoid biosynthesis is not always clear. Many flavonoid biosynthetic enzymes, particularly those catalyzing the substitution reactions, are quite promiscuous and are active with multiple flavonoid classes and even nonflavonoid specialized metabolites (e.g. triterpenes or coumarins). For example, the glucosyltransferase UGT78G1 was initially identified as an isoflavone glucosyltransferase in vitro, with anthocyanidins being the kinetically least preferred substrates among the multiple classes of flavonoids tested. However, the UGT78G1 gene is controlled by a MYB transcription factor that is a master regulator of anthocyanin biosynthesis, and gain- and loss-of-function studies confirmed that the enzyme is involved in anthocyanin glycosylation in Medicago truncatula in vivo (Peel et al., 2009). It is possible that the in vivo substrate specificity of flavonoid UGTs is determined in some cases by their association with other pathway enzymes in metabolic complexes rather than by their in vitro kinetic properties alone. Recent advances in live cell imaging in plants (Kwaaitaal et al., 2010) should help to address such possibilities Much remains to be learned about the intracellular and intercellular transport of (iso)flavonoids. Isoflavone secretion from legume roots, a process critical for the establishment of nodulation, may involve plasma membrane-localized ATP-binding cassette transporters, and vacuolar uptake of anthocyanins and precursors destined for PA biosynthesis can involve multidrug and toxin extrusion family transporters (Zhao et al., 2010). However, alternative mechanisms have been proposed for anthocyanin transport (Grotewold and Da