Participation of Inorganic Orthophosphate in Regulation of the Ribulose-1,5-bisphosphate Carboxylase Activity in Response to Changes in the Photosynthetic Source-Sink Balance

摘要

Sink-limited conditions, defined as treatment with continuous illumination, cause a reduction in the rate of photosynthetic fixation of CO2in single-rooted leaves of soybean (Glycine max.Merr. cv. Turunoko). We suggested previously that this reduction is due to a deactivation of ribulose-1,5-bisphosphate carboxylase (RuBPcase, E.C. 4.1.1.39) that is caused by a decrease in the level of Piin the leaves Sawada et al. (1989)Plant Cell Physiol. 30: 691, Sawada et al. (1990)Plant Cell Physiol. 31: 697. In the present study, the mechanism of regulation of RuBPcase activity by Piwas examined. The activity of RuBPcase in the sink-limited leaves, exposed for 6 or 7 d to continuous illumination to alter the source/sink balance, was enhanced with increasing concentrations of Pi, in a CO2-free preincubation medium in the presence of 5 mM MgCl2The maximum value 6.3μmole CO2(mg Chl)−1min−1 was obtained at approximately 5 mM Piafter a 5 min incubation, being 3 times of the activity without the preincubation. The activity of a crude preparation of RuBPcase that had been deactivated by removal of CO2and Mg2+ions by the gel filtration was 5.2–9.3 nmole CO2(mg protein)−1min−1and was also enhanced by Piplus Mg2+ions. The maximum value 147–151 nmole CO2(mg protein)−1min−1 was attained at 5 mM Piafter a 5 min incubation. The cycle of activation and inactivation of deactivated crude RuBPcase was perfectly reversible by addition of Pito the enzyme and removal of Pifrom the enzyme. Levels of free Piand of esterified phosphate in the sink-limited leaves were 69and 31of the total phosphate, respectively. By contrast, in the control leaves, these values were 87and 13, respectively. These results support our previously stated hypothesis and indicate an important role for free Piin the regulation of RuBPcase activity, in particular in sin