Chlorophyll breakdown in higher plants and algae

Leaf senescence is accompanied by the metabolism of chlorophyll (Chl) to nonfluorescent catabolites (NCCs). The pathway of Chi degradation comprises several reactions and includes the occurrence of intermediary catabolites. After removal of phytol and the central Mg atom from Chi by chlorophyllase and Mg dechelatase, respectively, the porphyrin macrocycle of pheophorbide (Pheide) a is cleaved. This two-step reaction is catalyzed by Pheide a oxygenase and RCC reductase and yields a primary fluorescent catabolite (pFCC). After hydroxylation and additional species-specific modifications, FCCs are tautomerized nonenzymically to NCCs inside the vacuole. Different subcellular compartments participate in Chi catabolism and, thus, transport processes across membranes are required. This review focuses on the catabolites and the individual reactions of Chi breakdown in higher plants. In addition, the pathway is compared to Chl conversion to red catabolites in an alga, Chlorella protothecoides. Finally, the significance and regulation of Chi degradation are discussed.