Over-expressing the C3 photosynthesis cycle enzyme Sedoheptulose-1-7 Bisphosphatase improves photosynthetic carbon gain and yield under fully open air CO2 fumigation (FACE)

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 Vc,max in both WT and transformants. Nevertheless, greater carbon assimilation and electron transport rates (J and Jmax) 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.Biochemical models of C3 photosynthesis (A) predict that A is limited by the slowest of three processes: the maximum carboxylation capacity of the enzyme Rubisco (Vc,max), the regeneration of Ribulose-5-phosphate (RuBP) via whole chain electron transport (J or Jmax), or the inorganic phosphate release from the utilization of triose phosphates (TPU or Pi limited) [1,2]. At current atmospheric [CO2], and under non stressed conditions, light saturated A operates at the transition between Rubisco and RuBP regeneration limitation. Globally, [CO2] is expected to increase from current levels of 390 ppm [3] to over 550 ppm by the middle of this century [4,5]. Elevating [CO2] stimulates C3 photosynthesis by increasing the substrate for carboxylation, CO2, and by reducing photorespiration [6,7]. Therefore, as atmospheric carbon dioxide concentration increases, the control of photosynthesis will shift away from Rubisco limitation toward RuBP regeneration limitation.Although photosynthetic stimulation at 550 ppm [CO2] could in theory increase production by 34%, the observed increase in fi