Although the loss of green color in senescent leaves and ripening fruits is a spectacular natural phenomenon, research on chlorophyll breakdown has been largely neglected until recently. This review summarizes knowledge about the fate of chlorophyll in degreening tissues that has been gained during the past few years. Structures of end- and intermediary products of degradation as well as the biochemistry of the porphyrin-cleaving reaction have been elucidated. The intracellular localization of the catabolic pathway is particularly important in the regulation of chlorophyll breakdown. None of the genes encoding the related catabolic enzymes has so far been isolated, which makes chlorophyll degradation an area of opportunity for future research.

regeneration were carried out as described.I7. Developing shoots were selected on 100 pgmL-' kanamycin.Fifteen kanamycin resistant plants were tested for cyanamide hydratase a~tivity!~] Enzyme activity was found in all plants, the level of expression ranging from 0.03 to 0.79 units per mg protein in the cell extract. The highest value corresponds to about a fifth of the specific activity measured in induced Myrothecium verrucaria extracts. Although the specific activity was highest in the roots, the total enzyme activity per gram fresh weight was distinctly higher in the leaves than in the other plant organs (Table 1). No activity was found in plants transgenic for cah-.

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