Abstract
Experimental elevation of [CO₂] around C₃ crops in the field has been shown to increase yields by suppressing the Rubisco oxygenase reaction and, in turn, photorespiration. Bioengineering a cyanobacterial carbon-concentrating mechanism (CCM) into C₃ crop species provides a potential means of elevating [CO₂] at Rubisco, thereby decreasing photorespiration and increasing photosynthetic efficiency and yield. The cyanobacterial CCM is an attractive alternative relative to other CCMs, because its features do not require anatomical changes to leaf tissue. However, the potential benefits of engineering the entire CCM into a C₃ leaf are unexamined. Here, a CO₂ and HCO₃⁻ diffusion-reaction model is developed to examine how components of the cyanobacterial CCM affect leaf light-saturated CO₂ uptake (A(sat)) and to determine whether a different Rubisco isoform would perform better in a leaf with a cyanobacterial CCM. The results show that the addition of carboxysomes without other CCM components substantially decreases A(sat) and that the best first step is the addition of HCO₃⁻ transporters, as a single HCO₃⁻ transporter increased modeled A(sat) by 9%. Addition of all major CCM components increased A(sat) from 24 to 38 µmol m⁻² s⁻¹. Several Rubisco isoforms were compared in the model, and increasing ribulose bisphosphate regeneration rate will allow for further improvements by using a Rubisco isoform adapted to high [CO₂]. Results from field studies that artificially raise [CO₂] suggest that this 60% increase in A(sat) could result in a 36% to 60% increase in yield.
Highlights
Experimental elevation of [CO2] around C3 crops in the field has been shown to increase yields by suppressing the Rubisco oxygenase reaction and, in turn, photorespiration
concentrating mechanism (CCM) to a C3 crop leaf suggests that a nearly 60% improvement in net leaf CO2 uptake could be achieved without any modification of leaf anatomy
By analogy to the artificial elevation of [CO2] in crop fields driving increased leaf CO2 uptake, this could lead to a 36% to 60% increase in C3 crop yield
Summary
Experimental elevation of [CO2] around C3 crops in the field has been shown to increase yields by suppressing the Rubisco oxygenase reaction and, in turn, photorespiration. C3 crop species provides a potential means of elevating [CO2] at Rubisco, thereby decreasing photorespiration and increasing photosynthetic efficiency and yield. The potential benefits of engineering the entire CCM into a C3 diffusion-reaction model is developed to examine how components of the cyanobacterial CCM affect leaf light-saturated CO2 uptake (Asat) and to determine whether a different Rubisco isoform would perform better in a leaf with a cyanobacterial CCM. The results show that the addition of carboxysomes without other CCM components substantially decreases Asat and that the best transporter increased modeled Asat by 9%. Addition of all first step is major CCM the addition components of HCO32 transporters, as a increased Asat from 24 to 38. The yield of many crop species has been substantially improved through breeding and agronomy, but advancement in yield has substantially slowed in many of the major C3 crops in the last decade, suggesting that limits on yield improvement using these techniques are being reached and that other approaches are needed (Long and Ort, 2010; Ray et al, 2012)
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