Can CO2 fixation be increased by raising levels of the Calvin cycle enzyme, sedoheptulose-1,7-bisphosphatase?

Abstract

Sedoheptulose-1,7-bisphosphatase (SBPase) catalyses the dephosphorylation of sedoheptulose-1,7-bisphosphate, in the phase of the photosynthetic carbon reduction (Calvin) cycle which results in the regeneration of the CO2 acceptor molecule ribulose-1,5-bisphosphate. We have shown, using antisense plants, that SBPase has a high flux control coefficient, up to 0.75, over photosynthetic CO2 assimilation. We have also shown, more recently, that allocation to sucrose and starch, growth rates and yield are sensitive to small reductions (20%) in SBPase activity. These results lead to the hypothesis that it may possible to increase carbon assimilation rates and plant growth by increasing SBPase activity in transgenic plants. We have produced transgenic lines in which the wild type SBPase content has been increased by expression of i) an Arabidopsis thaliana SBPase cDNA and ii) a mutant wheat SBPase, no longer requiring thiol activation. Preliminary analysis of transgenic tobacco lines with increased levels of SBPase indicated that photosynthetic rates were increased by 10-15% above wild type and changes in carbohydrate partitioning were also evident in these plants.