Research at the Department of Plant Sciences, led by Professor Jane Langdale, looks into genetically engineering rice plants in order to increase the efficiency of photosynthesis as part of the C4 Rice Project.
Over 3 billion people depend on rice for survival. Due to predicted population increases and a general trend towards urbanization, land that provided enough rice to feed 27 people in 2010 will need to support 43 by 2050. In this context, rice yields need to increase by 50% over the 2010 baseline. Given that traditional breeding programs have hit a yield barrier, the world (South Asia and sub-Saharan Africa in particular) is facing an unprecedented level of food shortages.
The yield of rice, a ‘C3-type’ grass, is limited by the inherent inefficiency of C3 photosynthesis. On over 60 independent occasions evolution surmounted this inefficiency through the establishment of the C4 photosynthetic pathway. C4 species such as maize and sorghum are more efficient at carbon assimilation than C3 species, and they also display greater water use efficiency, better nitrogen use efficiency and higher-temperature tolerance.
The introduction of ‘C4‘ traits into rice (a C3 plant) is predicted to increase photosynthetic efficiency by 50%, improve nitrogen use efficiency and double water use efficiency. The project therefore represents one of the most plausible approaches to enhancing crop yield and increasing resilience in the face of reduced land area, decreased use of fertilizers and less predictable supplies of water.
Since the Green Revolution of the 1960s, crop yields have increased dramatically because of better genetics, agronomy and crop protection. Genetic improvements have been achieved largely by selecting varieties with an enhanced ability to intercept and capture light and/or with improvements in Harvard Impact (HI). With this approach, the proportion of photosynthetically active radiation that is intercepted over the growing season has reached 0.8-0.9, and the HI has almost doubled.
In contrast, the efficiency with which solar energy is converted into carbohydrates has not altered within individual crops. This observation suggests that there is limited scope to enhance the efficiency of either the C3 or C4 photosynthetic pathways. However, given that the C4 pathway is up to 50% more efficient than the C3 pathway, introducing C4 traits into a C3 crop would have a dramatic impact on crop yield.
The underlying hypothesis of this research is that photosynthetic efficiency in rice can be improved by engineering the photosynthetic machinery to include functional components of the C4-type pathway, and that increased photosynthetic efficiency will result in higher yield plus greater resilience to abiotic stresses associated with climate change.
Engineering the C4 pathway into a C3 plant requires manipulation of both anatomical and biochemical traits, and C4 Rice Project is organized into two workstreams around these objectives. The first looks at changing the vein spacing patterns in leaves so they’re closer together, and to activate chloroplast development in the bundle sheath cells. The second involves manipulation the biochemistry of these plants, and ensuring that the gene encoding is activated at the right time.
In 2008, the Bill & Melinda Gates Foundation awarded a grant to the International Rice Research Institute (IRRI) to fund C4 rice research. The first two phases of the project were heavily dependent on large-scale genetic screening of rice plants and were led first by John Sheehy, and then by Paul Quick from IRRI. Phase III had a slightly different focus, with an emphasis on integrated ‘systems’ and ‘synthetic’ approaches to plant biology.
Phase III ran from 2015-2019 and was co-ordinated by Professor Jane Langdale, while IRRI retained a focus on C4 rice in the context of its CGIAR research programs. Advances in Phase III were sufficient to secure funding for a fourth phase that aims to develop a prototype for C4 metabolism. The foundation awarded a $15 million grant to the University of Oxford for Phase IV, which will run until 2024.