The cyanobacterial CCM as a source of genes for improving photosynthetic CO₂ fixation in crop species

Crop yields need to nearly double over the next 35 years to keep pace with projected population growth. Improving photosynthesis, via a range of genetic engineering strategies, has been identified as a promising target for crop improvement with regard to increased photosynthetic yield and better water-use efficiency (WUE). One approach is based on integrating components of the highly efficient CO2-concentrating mechanism (CCM) present in cyanobacteria (blue-green algae) into the chloroplasts of key C₃ crop plants, particularly wheat and rice. Four progressive phases towards engineering components of the cyanobacterial CCM into C3 species can be envisaged. The first phase (1a), and simplest, is to consider the transplantation of cyanobacterial bicarbonate transporters to C₃ chloroplasts, by host genomic expression and chloroplast targeting, to raise CO₂ levels in the chloroplast and provide a significant improvement in photosynthetic performance. Mathematical modelling indicates that improvements in photosynthesis as high as 28% could be achieved by introducing both of the single-gene, cyanobacterial bicarbonate transporters, known as BicA and SbtA, into C₃ plant chloroplasts. Part of the first phase (1b) includes the more challenging integration of a functional cyanobacterial carboxysome into the chloroplast by chloroplast genome transformation. The later three phases would be progressively more elaborate, taking longer to engineer other functional components of the cyanobacterial CCM into the chloroplast, and targeting photosynthetic and WUE efficiencies typical of C₄ photosynthesis. These later stages would include the addition of NDH-1-type CO₂ pumps and suppression of carbonic anhydrase and C₃ Rubisco in the chloroplast stroma. We include a score card for assessing the success of physiological modifications gained in phase 1a.