Breakdown of Chlorophyll in Higher Plants-Phyllobilins as Abundant, Yet Hardly Visible Signs of Ripening, Senescence, and Cell Death

Fall colors have always been fascinating and are still a remarkably puzzling phenomenon associated with the breakdown of chlorophyll (Chl) in leaves. As discovered in recent years, nongreen bilin‐type Chl catabolites are generated, which are known as the phyllobilins. Collaborative chemical‐biological efforts have led to the elucidation of the key Chl‐breakdown processes in senescent leaves and in ripening fruit. Colorless and largely photoinactive phyllobilins are rapidly produced from Chl, apparently primarily as part of a detoxification program. However, fluorescent Chl catabolites accumulate in some senescent leaves and in peels of ripe bananas and induce a striking blue glow. The structural features, chemical properties, and abundance of the phyllobilins in the biosphere suggest biological roles, which still remain to be elucidated. The time is ripe: The seasonal disappearance of chlorophyll has been a long‐standing and striking biological enigma. In higher plants, chlorophyll is broken down by a largely common, regulated pathway to linear tetrapyrroles—phyllobilins—which are related to heme catabolites. They accumulate in degreened leaves and vegetables, as well as in ripening fruit, and are presumed to have relevant physiological functions.