Breakdown of Chlorophyll in Chloroplasts of Senescent Barley Leaves Depends on ATP

Abstract

Chloroplasts isolated from senescent mesophyll of barley primary leaves were tested for the production of one of the blue-fluorescent putative catabolites of chlorophyll, FC 2, of which a small pool is present in senescent organelles. When illuminated during incubation, intact chloroplasts produced this compound for up to 1h. The reaction was abolished by DCMU, an inhibitor of photosynthetic electron transport. In darkness, the reaction continued for at least 4h if the chloroplasts were supplemented with ATP. In presenescent chloroplasts the ATP-dependent accumulation of FC 2 occurred at a very low rate. The catalist(s) responsible for the reaction appears to be induced during the main period of chlorophyll breakdown: its activation, in leaves induced to senesce, was prevented by cycloheximide, an inhibitor of cytosolic protein synthesis. The activity described is likely to represent the key reaction in chlorophyll breakdown, namely the conversion of chlorophyllide into a product, FC 2, having an oxidatively opened tetrapyrrol ring system.