Strategies for improving C₄ photosynthesis

Abstract

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.