CHLOROPHYLL DEGRADATION DURING SENESCENCE^*

The catabolic pathway of chlorophyll (Chl) during senescence and fruit ripening leads to the accumulation of colorless breakdown products (NCCs). This review updates an earlier review on Chl breakdown published here in 1999 (69). It summarizes recent advances in the biochemical reactions of the pathway and describes the characterization of new NCCs and their formation inside the vacuole. Furthermore, I focus on the recent molecular identification of three chl catabolic enzymes, chlorophyllase, pheophorbide a oxygenase (PAO), and red Chl catabolite reductase (RCCR). The analysis of Chl catabolic mutants demonstrates the importance of Chl breakdown for plant development and survival. Mutants defective in PAO or RCCR develop a lesion mimic phenotype, due to the accumulation of breakdown intermediates. Thus, Chl breakdown is a prerequisite to detoxify the potentially phototoxic pigment within the vacuoles in order to permit the remobilization of nitrogen from Chl-binding proteins to proceed during senescence.