Chlorophyll Degradation and its Physiological Function.

Research on chlorophyll degradation has progressed significantly in recent decades. In the 1990s, the structure of linear tetrapyrrole, which is unambiguously a chlorophyll degradation product, was determined. From the 2000s until the 2010s, the major enzymes involved in chlorophyll degradation were identified, and the pheophorbide a oxygenase/phyllobilin pathway was established. This degradation pathway encompasses several steps: (1) initial conversion of chlorophyll b to 7-hydroxymethyl chlorophyll a; (2) conversion of 7-hydroxymethyl chlorophyll a to chlorophyll a; (3) dechelation of chlorophyll a to pheophytin a; (4) dephytylation of pheophytin a to pheophorbide a; (5) opening of the macrocycle to yield a red chlorophyll catabolite; and (6) conversion of red chlorophyll catabolite to phyllobilins. This pathway converts potentially harmful chlorophyll into safe molecules of phyllobilins, which are stored in the central vacuole of terrestrial plants. The expression of chlorophyll-degrading enzymes is mediated by various transcription factors and influenced by light conditions, stress, and plant hormones. Chlorophyll degradation is differently regulated in different organs and developmental stages of plants. The initiation of chlorophyll degradation induces the further expression of chlorophyll-degrading enzymes, resulting in the acceleration of chlorophyll degradation. Chlorophyll degradation was initially considered the last reaction in senescence; however, chlorophyll degradation plays crucial roles in enhancing senescence, degrading chlorophyll-protein complexes, forming photosystem II, and maintaining seed quality. Therefore, controlling chlorophyll degradation has important agricultural applications.