In wheat (Triticum aestivum L.), leaf senescence can be initiated by different factors. Depending on the plant system (intact plants or detached leaves) or the environmental conditions (light, nutrient availability), the symptoms of senescence differ. The aim of this work was to elucidate the catabolism of ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco, EC. 4.1.1.39) under various senescence-inducing conditions. Leaf senescence was initiated in intact plants by darkness or by N-deprivation and in leaf segments by exposure to light or darkness. Depending on the treatment, a 50 kDa fragment of Rubisco was observed. The formation of this fragment was enhanced by leaf detachment and low light. In segments exposed to high light and in intact plants induced to senesce by N-deprivation, the fragment was essentially absent. Since an antibody against the N-terminus of a large subunit of Rubisco (LSU) did not cross-react with the fragment, it appears likely that a smaller fragment was removed from the Nterminus of LSU. Inhibitor studies suggest that a cysteine endopeptidase was involved in the formation of the 50 kDa fragment. Non-denaturing-PAGE followed by SDS-PAGE revealed that the fragment was produced while LSU was integrated in the holoenzyme complex, and that it remained there after being produced. It remains open how the putative endopeptidase reaches the stromal protein Rubisco. The results indicate that depending on the senescence-inducing conditions, different proteolytic enzymes may be involved. The involvement of vacuolar proteases must be considered as occurring during LSU degradation, which takes place in darkness, low light or under carbon limitation.

Chloroplast proteins can be degraded in the intact organelle, but the relative rates of degradation vary considerably. To investigate regulatory aspects of glutamine synthetase (GS) degradation, isolated pea (Pisum sativum L.) chloroplasts were used as a suitable system. Since chloroplasts were re-isolated after incubation, only chloroplasts remaining intact throughout the incubation and processes occurring therein were analysed. Net changes of nuclear-encoded proteins indicate degradation because protein synthesis is no longer possible under these conditions. Incubation of intact chloroplasts in a medium which promotes CO2 -assimilation led to the stabilisation of GS and several other chloroplast proteins. This general effect suggests that photosynthetic metabolism is relevant for the stability of stromal proteins. In addition, GS was specifically stabilised by methionine sulfoximine (MSO), a potent inhibitor of GS. However, other enzymes tested were not affected by MSO. When chloroplasts were incubated with methyl viologen, the degradation of several stromal enzymes including GS was accelerated, most likely by active oxygen species. Again, MSO specifically delayed the degradation of GS. As MSO is a substrate analogue, it appears likely that substrates or other ligands influence the susceptibility of enzymes against degradation systems present in the same compartment.