Biochemical models predict that photosynthesis in C₃ plants is most frequently limited by the slower of two processes, the maximum capacity of the enzyme Rubisco to carboxylate RuBP (V_c,max), or the regeneration of RuBP via electron transport (J). At current atmospheric [CO₂] levels Rubisco is not saturated; consequently, elevating [CO₂] increases the velocity of carboxylation and inhibits the competing oxygenation reaction which is also catalyzed by Rubisco. In the future, leaf photosynthesis (A) should be increasingly limited by RuBP regeneration, as [CO₂] is predicted to exceed 550 ppm by 2050. The C₃ cycle enzyme sedoheptulose-1,7 bisphosphatase (SBPase, EC 3.1.3.17) has been shown to exert strong metabolic control over RuBP regeneration at light saturation.

We tested the hypothesis that tobacco transformed to overexpressing SBPase will exhibit greater stimulation of A than wild type (WT) tobacco when grown under field conditions at elevated [CO₂] (585 ppm) under fully open air fumigation. Growth under elevated [CO₂] stimulated instantaneous A and the diurnal photosynthetic integral (A') more in transformants than WT. There was evidence of photosynthetic acclimation to elevated [CO₂] via downregulation of V_c,max in both WT and transformants. Nevertheless, greater carbon assimilation and electron transport rates (J and J_max) for transformants led to greater yield increases than WT at elevated [CO₂] compared to ambient grown plants.

These results provide proof of concept that increasing content and activity of a single photosynthesis enzyme can enhance carbon assimilation and yield of C₃ crops grown at [CO₂] expected by the middle of the 21st century.