Regulation of Ribulose-l,5-bisphosphate Carboxylase Activity in Response to Changes in the Source/Sink Balance in Single-Rooted Soybean Leaves: The Role of Inorganic Orthophosphate in Activation of the Enzyme

摘要

The results of our previous study Sawada et al. (1989)Plant Cell Physiol. 30: 691 implied that, under sink-limited conditions, a decrease in the activity of ribulose-l,5-bisphosphate carboxylase (EC 4.1.1.39) caused a reduction in the rate of photosynthetic fixation of CO2in single-rooted leaves of soybean (Glycine maxL. Merr. cv. Tsurunoko). This reduction in the rate of photosynthesis in source leaves seemed to correspond to a decrease in the demand by sink tissues for photoassimilates. In the present study, the activity of RuBPcase in vivo was estimated by measuring the“initial”activity immediately after extraction from standard leaves (grown under a regime of 10 h of light and 14 h of darkness) and from sink-limited leaves (exposed for 6 or 7 d to continuous light to alter the source/sink balance). The rate of photosynthesis in the sink-limited leaves decreased to 47of that in the standard leaves. The“initial”activity of RuBPcase was 4.3 in the standard leaves but only 1.6μmole CO2.(mg Chl)−1.min−1in the sink-limited leaves. These results appear to indicate that the reduction in photosynthetic activity under sink-limited conditions was mostly due to a deactivation of RuBPcase. The activity of deactivated RuBPcase in the sink-limited leaves was restored to 4.1μmole CO2.(mg Chl)−1.min−1by incubation of the enzyme in a medium that contained only Na2HPO4. This result suggests that free Piin chloro-plasts plays an important role in the activation of the enzyme. The level of Piin the sink-limited leaves was 62of that in the standard leaves. On the basis of these various results, it appears that the deactivation of RuBPcase in the sink-limited leaves is the result of a decrease in the level of Pi. The role of free Piin the activation of RuBPcase, in particular at atmospheric concentrations of CO2, was