Product Line 1.4. C4 rice
In the majority of plants, including rice, CO2 is first fixed into a compound with three carbons (C3) by the photosynthetic enzyme ribulose bisphosphate carboxylase oxygenase (Rubisco)—this is known as C3 photosynthesis. In contrast, the more efficient C4 pathway involves the initial fixation of atmospheric CO2 into C4 acids using an enzyme that is insensitive to O2. In the next stage of the pathway, CO2 is released from the C4 acids for fixation by Rubisco. The two stages are spatially separated, allowing a high concentration of CO2 in the vicinity of Rubisco. The buildup of CO2 by this “CO2 pump” requires extra energy from sunlight and therefore it operates most optimally in bright warm climates.
A fully functional C4 pathway requires a coordinated change in tissue structure and metabolic biochemistry. In nature, this has occurred more than 50 times in a wide range of flowering plants, indicating that, despite being complex, it is a relatively easy pathway to evolve. This provides hope that it is possible to replicate the process using a combination of genetic engineering and breeding. The aim of the C4 Rice Project is to produce a large (approximately 50%) and sustainable increase in rice productivity in all ecosystems by increasing the efficiency of solar energy capture by photosynthesis. To achieve this, the C4 photosynthetic pathway will be introduced into rice. C4 rice will not only bring about very high productivity but also far more efficient use of limited resources, such as water and nitrogen fertilizers; thus, it is particularly relevant to solving the problems faced by resource-poor farmers in developing countries.
To achieve these objectives requires a sustained collaborative effort with multiple disciplines. An international team of scientists with the necessary range of skills and experience has been assembled to form a C4 Rice Consortium (http://beta.irri.org/projects15/en/consortium-c4rice). The team is multidisciplinary and contains molecular biologists, geneticists, physiologists, breeders, biochemists, and mathematicians; all of this expertise is essential to achieve these objectives. Partners of the C4 Rice Consortium conduct research in four areas: genetic screening for gain or loss of C4-function, physiological phenotyping, molecular toolkit development, and dissection of biochemical pathways and Kranz anatomy through comparative genomic and molecular approaches.
In the first phase of the project, the primary focus is on understanding the genetic regulation of C4 leaf anatomy. Large-scale screening for C4-like characters in rice mutants and for reversion to C3 characters in sorghum mutants is now in progress. The consortium will identify genes controlling mesophyll cell number between veins, chloroplast number, and the size and distribution of bundle sheath cells. A transgenic approach is being used to decipher the biochemical network of regulatory genes for C4-function. Molecular toolkits are being developed to define elements that allow specific expression of transgenes in mesophyll cells and bundle sheath cells of rice.
PL 1.4 has only one product: the creation of C4 rice with improved photosynthetic efficiency and productivity. Valuable intermediate products for understanding photosynthetic efficiency are expected in the pursuit of this research agenda. In the medium term (4–11 years), we expect to have identified the genes responsible for Kranz-type C4 anatomy and biochemistry and constructed prototypes of C4 rice. In the long term (12–15 years), we expect to have optimized C4 photosynthesis in cultivated rice lines and evaluated its benefits in some farmers’ fields.
The C4 project is conducted by a consortium of partners contributing their unique skills toward the engineering of C4 rice. Currently, the consortium includes IRRI, the University of Cambridge, University of Oxford, Cornell University, University of Sheffield, University of Nottingham, University of Toronto, CSIRO, Washington State University, University of Düsseldorf, Kyung Hee University, Australian National University, Shanghai Institutes for Biological Sciences, Academia Sinica-Taiwan, and Simon Fraser University. These partners play distinct roles in genetic screening, the production of transgenic plants, comparative analysis of C3/C4 pathways, biochemical and physiological characterization of transgenes, and the development of molecular toolkits. We expect more to join over the course of the project.
The next users of products from C4 rice research will be global plant scientists interested in C3- C4 photosynthesis, evolution, plant development, and biochemistry, who are largely in advanced research institutes and larger national research systems. They will gain from having new technologies for screening for alterations in photosynthetic efficiency, C4 mutants that have reverted toward C3 in their leaf anatomy and physiology, activation-tagged rice showing C4 characteristics, transgenic rice plants with genes associated with photosynthesis silenced in a cell-specific manner, and a molecular toolbox specific to the manipulation of the factors regulating cell-specific photosynthetic activity.
The intermediate users will be researchers and breeders in national systems and the private sector who are interested in applying genes and genetic materials generated from the C4 Rice Project. The final users will be farmers interested in growing rice cultivars with increased photosynthetic capability and higher productivity.
Within theme 1, C4 rice is linked to PL 1.2 (characterizing genetic diversity). Novel genotypic variation created by recombination of the cultivated and wild rice gene pools will provide new resources for screening C4 photosynthetic properties. A new initiative on nitrogen-fixing rice under discussion has the potential to reduce the dependency on high-nitrogen input in productive C4 rice and to enhance the use of increased photosynthetic capacity in rice. With other CRPs, C4 rice is linked to research on the impacts of climate change on yield via high CO2 in CRP Climate Change.
Long-term sustained funding is needed to conduct very large scale experiments to recover interesting variants with altered photosynthesis and leaf anatomy. Scientific facilities and institutional infrastructure need to be available to accommodate the long-term research agenda. IRRI provided startup funds to initiate the C4 Rice Project in 2008. The project is currently supported by the BMGF for the first 3-year phase ($11.1 million for 2009-11). New funding will be required from 2012 onward. We expect the same level of annual funding for the next phase. Discussion is ongoing to engage research organizations in China and India to leverage co-investments. Co-investments are also expected from the European Union and advanced institutions with interest in agricultural productivity in the long term. We will also explore linkages with the private sector.