Figure 2.

Model for PA transport and polymerization. PA regulatory transcription factors (TT factors) activate PA biosynthesis structural genes (TTs) in the nuclei of seed coat endothelial cells under appropriate conditions. PA pathway proteins are translocated to the cytosolic side of the ER for synthesis of epicatechin (white circles) and anthocyanins (red circles). These flavonoids can be compartmentalized in different ways. Epicatechin and anthocyanins are readily glycosylated, and the conjugates are transported into the vacuole by MATE (TT12) transporters. They could also be loaded into the ER membrane system or derived membrane vesicles, which are transported to the central vacuole through prevacuole compartment (PVC)-dependent vesicle trafficking, or else they could be bound to the TT19 GST, which facilitates their transport into the ER, vacuole, or other compartments. The acidic vacuolar conditions may facilitate nonenzymatic condensation of PA units, or the units may undergo enzymatic condensation catalyzed by TT10, with E3′G as a possible extension unit, or by yet unidentified proteins (red cylinders). After synthesis on the ER and modification by glycosylation in the Golgi, TT10 could be sorted and targeted within membrane vesicles that also contain epicatechin and other PA biosynthetic units. TT10 may also catalyze the condensation of PA units into oligomers (such as procyanidin B1 and B2) in the vesicles, which are transported to the vacuole, where PA chain elongation could be further catalyzed by TT10 using epicatechin glucoside and PA oligomers as substrates. PAs can also be transported through membrane vesicles or other mechanisms to the apoplastic space, where they are subjected to oxidative polymerization and further cross-linked with other cell wall components, catalyzed by apoplastic TT10-like polyphenol oxidases. Blue hexagons represent glycosylated moieties (epicatechin, membrane sterols, and TT10 protein).