Strategies for improving photosynthetic electron transport in C4 plants
- ARC Centre of Excellence for Translational Photosynthesis, Australian National University, Canberra, Australia.
- CSIRO Agriculture, Canberra, Australia.
Recent activities to improve photosynthetic performance in crop plants have focused primarily 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. However, C4 plants that utilise the C4 photosynthetic pathway also play a key role in world agriculture and strategies to manipulate and enhance C4 photosynthesis thus have potential for major agricultural impacts. The C4 photosynthetic pathway is a biochemical CO2 concentrating mechanism that requires the coordinated functioning of mesophyll (M) and bundle sheath cells (BS) of leaves and species have evolved a complex blend of anatomy and biochemistry to achieve this. Chloroplast electron transport in C4 plants is shared between these two cell types and the diversity of thylakoid protein complexes of each cell type is defined by the requirements of the metabolic sub-type of C4 photosynthesis. Our recent work with the model monocot C4 species Setaria viridis (green foxtail millet) and transgenic S.viridis plants with altered amount of cytochrome (Cyt) b6f complex demonstrates the link between electron transport capacity of the leaves and CO2 assimilation. Overexpression of the Cyt b6f in both M and BS allows higher rates of assimilation in transgenic plants without affecting Rubisco content. However, increasing the amount of the Cyt b6f only in M, surprisingly, leads to a reduced rate of CO2 assimilation at low CO2. We link this observation to measurements of electron transport components and light harvesting capacity of BS.