Degradation of RuBisCO in the Leaves of Crop Plants During Senescence

点击翻译按钮获取中文摘要

Rubisco (ribulose-1, 5-bisphosphate carboxylase/oxygenase) is an important enzyme for photosynthetic CO_2 fixation, and its amount in green leaves accounted for 50% of the soluble protein. It has been suggested that Rubisco can be degraded within chloroplasts. In this International ScientificResearch, we examined on the degradation of Rubisco in the chloroplasts isolated mechanically and purified from wheat (us), barley (Huffaker's group) and Lorium (Thomas' group) leaves.Rubisco-degradation activity in the chloroplasts was very weak and its degradation could be detected only by applying Western blotting (Mae et al. 1989). This activity was stimulated by SDS (Yokota et al. 1990). However, the degradation products were very similar to those obtained when Rubisco was incubated with the vacuoles purified from like leaves. Chloroplasts, which had pretreated with thermolysin, had no proteolytic activity against Rubisco. These results show that the prteolytic activity against Rubisco found i … More n the chloroplasts was mostly due to the contamination of vacuole proteases adhering to the outer envelope of chloroplasts during isolation (Miyadai et al. 1990).By increasing the amount of chloroplast proteins applied for Western blotting, however, new polypeptides could be detected. Many spots of these polypeptides after incubation with V-8 protease corresponded to those from Rubisco, indicating that the new polypeptides are some of the degradation products of Rubisco. In addition, the existence of these peptides was independent of thermolysin treatment. Thus, these results strongly suggested the existence of protease (s) in the chloroplasts (Suzuki, in preparation).Leaf senescence under light stress had some of the regulatory changes in the thylakoid proteins similar to those of shade plants (Hidema et al. submitted, Mae et al. in preparation). However, declines in stromal enzymes during senescence always show a strong correlation one another irrespective of lght stress. These results may suggest that there is no selective degradation mechanism for each enzyme including Rubisco in chloroplast stroma (Hidema et al. submitted).Thus, we consider that Rubisco may be degraded by non-selective proteolytic enzyme (s) in the chloroplasts. Less