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, and metabolic engineering of flavonoid biosynthesis has therefore 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) or TANNIN-DEFICIENT SEED (tds) mutants from 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 Plant Physiology Preview. Published on April 13, 2010, as DOI:10.1104/pp.110.155432