Strategies for improving C4 photosynthesis

Recent activities to improve photosynthetic performance in crop plants has focused mainly on C-3 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 C-4 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 C-4 plants, work in transgenic Flaveria bidentis, a transformable model C-4 dicot, and recent transcriptional analysis of leaves from diverse C-4 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 C-4 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) b(6)/f complex, but in better sharing the harvesting of light energy between mesophyll and bundle sheath chloroplasts. Gene candidates for improvement of C-4 photosynthesis could be utilized either through transgenic approaches or via mining natural allelic variation in sequenced populations of crop species.