Update on Biosynthesis of Proanthocyanidins The Mysteries of Proanthocyanidin Transport and Polymerization

Flavonoids, including proanthocyanidins (PAs; also called condensed tannins), play a multitude of roles in plants (Winkel-Shirley, 2001). The presence of certain types of flavonoids in crops is associated with desirable and important agronomic traits; therefore, metabolic engineering of flavonoid biosynthesis has attracted considerable interest (Dixon, 2005). PAs are oligomers or polymers of flavan-3-ol units and are prominent flavonoid compounds in seed coats (where they become oxidized and confer a brownish color to the testa), leaves, fruits, flowers, and bark (Dixon et al., 2005). Characterization of a series of transparent testa (tt) and tannin-deficient seed (tds) mutants from Arabidopsis (Arabidopsis thaliana) has led to an in-depth understanding of flavonoid biosynthesis at the molecular level (Shirley et al., 1995; Winkel-Shirley, 2001; Abrahams et al., 2002; Lepiniec et al., 2006; Fig. 1). These studies clearly suggest that, in addition to structural enzymes and regulatory factors, transport proteins are also essential for flavonoid biosynthesis. Ablation of single transporter genes can result in defects in flavonoid and PA production associated with dysfunction of the central vacuole (Baxter et al., 2005; Marinova et al., 2007a, 2007b). With increasing evidence of such links between biochemistry and cell biology, transport and trafficking of plant secondary metabolites is emerging as an important, but technically challenging, research frontier (Grotewold, 2004). In this Update, we briefly review the status of our knowledge concerning PA biosynthesis and assembly, with particular emphasis on the still controversial mechanisms for transport and polymerization of the PA monomers.