A role for leucoanthocyanidin reductase in the extension of proanthocyanidins

Proanthocyanidins (PAs) are the second most abundant plant polyphenolic compounds after lignin. PAs affect taste, mouth feel and astringency of many fruits, wines and beverages(1,2), have been associated with reduced risks of cardiovascular disease, cancer and Alzheimer's disease(3-5), can improve nutrition and prevent bloat in ruminant animals(6) and enhance soil nitrogen retention(7). PAs are oligomers and polymers of flavan-3-ols, primarily (-)-epicatechin and (+)-catechin, but the mechanism by which the monomers polymerize and become insoluble is currently unknown. Leucoanthocyanidin reductase (LAR) has been shown to convert leucocyanidin to (+)-catechin(8,9). Here, we report that loss of function of LAR in the model legume Medicago truncatula leads unexpectedly to loss of soluble epicatechin-derived PAs, increased levels of insoluble PAs, and accumulation of 4 beta-(S-cysteinyl)-epicatechin, which provides the 4 -> 8 linked extension units during non-enzymatic PA polymerization. LAR converts 4 beta-(S-cysteinyl)epicatechin back to epicatechin, the starter unit in PAs, thereby regulating the relative proportions of starter and extension units and consequently the degree of PA oligomerization.