Chlorophyll breakdown-Regulation, biochemistry and phyllobilins as its products

Chlorophyll breakdown occurs throughout the life cycle of a plant, but is most obvious during leaf senescence and fruit ripening when respective tissues visually degreen. Important milestones in elucidating the fate of chlorophyll, which was considered a "biological enigma" more than 30 years ago, were, first, the initial decryption of the structures of chlorophyll catabolites, now collectively termed phyllobilins, in 1991, and, second, the identification of pheophorbide a oxygenase (PAO), a key enzyme of breakdown that determines the structural backbone of the phyllobilins, in 2003. Hence, nowadays the biochemical pathway of chlorophyll breakdown is termed the "PAO/phyllobilin" pathway. This chapter summarizes the current knowledge on the diversity of phyllobilins with regard to structural characteristics and distribution within plant species, and presents biochemical, cellular and molecular, as well as regulatory and evolutionary aspects of the enzymes of the PAO/phyllobilin pathway.