Pathway of chlorophyll breakdown in citrus fruit peel, as compared with arabidopsis leaves and other plant systems

The pigment changes of citrus fruit peel [='colorbreak'] are the best visual markers of citrus fruit maturation. Pigment changes of most citrus cultivars consist of breakdown of chlorophyll and buildup of carotenoids, both of which are enhanced by ethylene and delayed by gibberellins. The degreening of citrus fruit by ethylene has served for more than 30 years as a model system for the study of chlorophyll catabolism. The abrupt upsurge of chlorophyllase enzyme activity in response to ethylene prompted further, molecular investigations. Citrus Chlase 1 overexpression was shown to initiate chlorophyll catabolism in several heterologous systems, supporting the notion that chlorophyllase is a rate limiting enzyme. Confocal microscopy localization has shown that both forms of chlorophyllase [ the longer precursor and the shorter, mature form] reside in citrus' plastids. In conclusion, the strong correlation between ethylene induction of citrus fruit colorbreak and chlorophyllase gene expression and protein maturation indicate that chlorophyllase initiates the chlorophyll catabolic pathway in this system. In Arabidopsis (Arabidopsis thaliana), on the other hand, recent work based on analysis of mutants questions the role of chlorophyllase in chlorophyll breakdown during senescence. Moreover, the Hortensteiner group identified an alternative enzyme from Arabidopsis leaves involved in dephytilation, pheophytin pheophorbide hydrolase (PPH), which exhibits specificity to the Mg-free form of chlorophyll (phein), and must, therefore, be preceded by a Mg removal step. This finding raises doubts regarding the role of chlorophyllase as the universal initiator of chlorophyll catabolism and points to the existence of two, alternative catabolic pathways. Examination of the actual roles of chlorophyllase and PPH in various plant systems is presently underway. The hypothesis that ripening fruits and senescing leaves represent distinct modes of chlorophyll catabolism deserves consideration.