Proanthocyanidin biosynthesis and postharvest seed coat darkening in pinto bean

Proanthocyanidins (PAs) are polyphenolic compounds present widely in the plant kingdom. These specialized metabolites are derived from the phenylpropanoid pathway and are known for producing brown pigments in different plant organs. PAs accumulate in the seed coat tissues of flowering plants and play a determinant role in seed germination and viability, protect seeds from biotic and abiotic stresses, and thus ensure the long-term storage potential of seeds. In addition, PAs are a rich source of antioxidants for the health of both human and livestock. Many of the commercially relevant dry beans (Phaseolus vulgaris) contain high levels of PAs, which when oxidized cause the beans to overdarken, a phenomenon known as postharvest darkening. These darker beans give the impression of oldness, and consumers tend to avoid buying them which, in turn, affects their market value. Pinto beans, one of the leading market classes of dry beans, are affected the most by the postharvest darkening. Therefore, exploring the regulation of PA biosynthesis and accumulation in bean seed coat tissues will help to develop strategy to manage the postharvest darkening effect in pintos. In this review, we discuss the PA biosynthesis and its regulation, connecting it to seed coat color genetics for a better understanding of the mechanism of seed coat darkening.