Plants Make Bilirubin, Too

Scientists have discovered that the pigment responsible for the brilliant orange seed arils of the bird-of-paradise tree is bilirubin, a molecule thought to exist only in animals (J. Am. Chem. Soc., DOI: 10.1021/ja809065g). How plants produce the pigment is a mystery. Both plants and animals generate the compound biliverdin from the oxidative degradation of heme. Biliverdin in animals is converted to bilirubin, but plant biliverdin goes on to form phytochromobilin, which is the precursor of the light-absorbing chromophore in the pigment phytochrome. Both phytochromobilin and bilirubin are bilins, molecules that contain a chain of four pyrrole rings. In people, bilirubin is sometimes observed as the yellowish hue associated with bruises and jaundice. Cary Pirone, David W. Lee, and colleagues at Florida International University identified the colorful compound in the seed arils via HPLC and NMR. The finding "likely necessitates the revision of the plant tetrapyrrole pathway since there is currently no known mechanism of bilirubin production in the plant kingdom," the researchers write. The group has now found bilirubin in two other bird-of-paradise species and is continuing the search for the pigment in related plant families.