Polyphenol oxidase genes in barley ( L.): functional activity with respect to black grain pigmentation.

Polyphenol oxidase (PPO) is an oxidoreductase. In damaged plant tissues, it catalyzes enzymatic browning by oxidizing -diphenols to highly reactive -quinones, which polymerize producing heterogeneous dark polymer melanin. In intact tissues, functions of PPO are not well understood.

Melanin Pigment in Plants: Current Knowledge and Future Perspectives.

The word "melanin" refers to a group of high molecular weight, black, and brown pigments formed through the oxidation and polymerization of phenolic compounds. This pigment is present in all kingdoms of living organisms, but it remains the most enigmatic pigment in plants. The poor solubility of melanin in particular solvents and its complex polymeric nature significantly constrain its study.