Strategies for improving C₄ photosynthesis

Recent activities to improve photosynthetic performance in crop plants has focused mainly on C₃ photosynthesis where there are clear identified targets such as improving Rubisco kinetics, installation of a CO₂ concentrating mechanism and alleviating limitations in chloroplast electron transport. Here we address strategies to improve photosynthetic performance in C₄ plants, which utilize a CO₂ concentrating mechanism, having evolved a complex blend of anatomy and biochemistry to achieve this. While the limitations to photosynthetic flux are not as well studied in C₄ plants, work in transgenic Flaveria bidentis, a transformable model C₄ dicot, and recent transcriptional analysis of leaves from diverse C₄ plants, provides several gene candidates for improvement of carbon metabolism (such as pyruvate orthophosphate dikinase, phosphoenolpyruvate carboxylase and Rubisco) and for access of CO₂ to phosphoenolpyruvate carboxylase in the mesophyll cells (such as carbonic anhydrase and CO₂ porins). Chloroplast electron transport in C₄ plants is shared between the two cell types, providing opportunities not only to alleviate limitations to flux through intersystem electron transport by targeting nuclear encoded proteins in the cytochrome (Cyt) b₆/f complex, but in better sharing the harvesting of light energy between mesophyll and bundle sheath chloroplasts. Gene candidates for improvement of C₄ photosynthesis could be utilized either through transgenic approaches or via mining natural allelic variation in sequenced populations of crop species.