Over-expressing the C3 photosynthesis cycle enzyme Sedoheptulose-1-7 Bisphosphatase improves photosynthetic carbon gain and yield under fully open air CO2fumigation (FACE)

<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>Biochemical models predict that photosynthesis in C<jats:sub>3</jats:sub>plants is most frequently limited by the slower of two processes, the maximum capacity of the enzyme Rubisco to carboxylate RuBP (V<jats:sub>c,max</jats:sub>), or the regeneration of RuBP via electron transport (J). At current atmospheric [CO<jats:sub>2</jats:sub>] levels Rubisco is not saturated; consequently, elevating [CO<jats:sub>2</jats:sub>] increases the velocity of carboxylation and inhibits the competing oxygenation reaction which is also catalyzed by Rubisco. In the future, leaf photosynthesis (<jats:italic>A</jats:italic>) should be increasingly limited by RuBP regeneration, as [CO<jats:sub>2</jats:sub>] is predicted to exceed 550 ppm by 2050. The C<jats:sub>3</jats:sub>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.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>We tested the hypothesis that tobacco transformed to overexpressing SBPase will exhibit greater stimulation of<jats:italic>A</jats:italic>than wild type (WT) tobacco when grown under field conditions at elevated [CO<jats:sub>2</jats:sub>] (585 ppm) under fully open air fumigation. Growth under elevated [CO<jats:sub>2</jats:sub>] stimulated instantaneous<jats:italic>A</jats:italic>and the diurnal photosynthetic integral (<jats:italic>A</jats:italic>') more in transformants than WT. There was evidence of photosynthetic acclimation to elevated [CO<jats:sub>2</jats:sub>] via downregulation of V<jats:sub>c,max</jats:sub>in both WT and transformants. Nevertheless, greater carbon assimilation and electron transport rates (J and J<jats:sub>max</jats:sub>) for transformants led to greater yield increases than WT at elevated [CO<jats:sub>2</jats:sub>] compared to ambient grown plants.</jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p>These results provide proof of concept that increasing content and activity of a single photosynthesis enzyme can enhance carbon assimilation and yield of C<jats:sub>3</jats:sub>crops grown at [CO<jats:sub>2</jats:sub>] expected by the middle of the 21st century.</jats:p></jats:sec>