It is easy to predict that future research on Chl breakdown will only lead to a better insight if genes encoding catabolic enzymes become available. As yet, attempts at the molecular cloning have not been overly successful. Nothing has been published so far and only personal communications announce that the genes of Citrus chlorophyllase (E.E. Goldschmidt) and barley RCCR (K. Wuthrich and S. Hortensteiner) have recently been isolated. Unfortunately, the key enzyme of the catabolic pathway, PaO, has not yet been purified and tracked down to the molecular level. The gene of PaO will probably turn out to be among the few genes that are exclusively expressed in green tissues induced to catabolize Chl. In any case, the availability of the relevant genes will open marvelous opportunities to study gene expression as associated with leaf yellowing and color changes in ripening fruits. It may even provide opportunities for producing genetically engineered stay-greens which can be used for turfs and other instances where degreening during development is undesirable.

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