Engineering the C4 pathway into a C3 plant requires manipulation of both anatomical and biochemical traits and our project is organized into two workstreams around these objectives.
To introduce Kranz anatomy into rice, we need to change vein spacing patterns so that veins are closer together in the leaf, and we need to activate chloroplast development in the bundle sheath cells (rice bundle sheath cells are non-photosynthetic). At this point, it is not known how the development of Kranz anatomy is regulated in a C4 plant and therefore our current work is aimed at identifying the regulatory genes in maize. Once identified, engineering in rice can begin. This workstream is being carried out by groups at the University of Cambridge, Donald Danforth Center, Heinrich Heine University, University of Oxford and University of Toronto.
Manipulation of the biochemistry is more straightforward, in that the genes encoding the C4 pathway enzymes and most of the metabolite transporters have been identified. However, there are at least 12 genes involved (all enzymes labelled in blue and all transporters depicted by blue circles in Figure 1), and they all need to be switched on at the right time, to the right level, and in the correct cell-type.
For now, we are introducing the biochemistry into rice with C3 anatomy, with the goal of having a C4 cycle in a two cell radius of existing veins. This workstream is being carried out by groups at the Australian National University, Chinese Academy of Sciences, Heinrich Heine University, Max Planck Institute, IRRI and Washington State University.
Ultimately, the two workstreams will come together and the biochemistry will be introduced into a rice leaf with Kranz anatomy. But that is a long way off, and some might say an unrealistic goal. Why do we think we can do it? Essentially we are being guided by evolution – the C4 pathway has evolved from the C3 pathway over 60 times independently and therefore, although the changes are quite complex, the transition must be relatively simple. We just need to work out the underlying mechanism.