A role for ascorbate conjugates of (+)-catechin in proanthocyanidin polymerization

Proanthocyanidins (PAs) are natural polymers of flavan-3-ols, commonly (+)-catechin and (-)-epicatechin. However, exactly how PA oligomerization proceeds is poorly understood. Here we show, both biochemically and genetically, that ascorbate (AsA) is an alternative starter unit to flavan-3-ol monomers for leucocyanidin-derived (+)-catechin subunit extension in the Arabidopsis thaliana anthocyanidin synthase (ans) mutant. These (catechin)(n):ascorbate conjugates (AsA-[C](n)) also accumulate throughout the phase of active PA biosynthesis in wild-type grape flowers, berry skins and seeds. In the presence of (-)-epicatechin, AsA-[C](n) can further provide monomeric or oligomeric PA extension units for non-enzymatic polymerization in vitro, and their role in vivo is inferred from analysis of relative metabolite levels in both Arabidopsis and grape. Our findings advance the knowledge of (+)-catechin-type PA extension and indicate that PA oligomerization does not necessarily proceed by sequential addition of a single extension unit. AsA-[C](n) defines a new type of PA intermediate which we term sub-PAs. Proanthocyanidins are natural polymers of flavan-3-ols produced by plants. Here the authors provide genetic and biochemical evidence for the involvement of ascorbate linked flavan-3-ol oligomers in the extension of the proanthocyanidin chain.

Automated summary

The study demonstrates the involvement of ascorbate-linked flavan-3-ol oligomers in the extension of proanthocyanidins. These novel intermediates, termed sub-PAs, provide new insights into the mechanism of PA oligomerization.