Improving crop yield through enhanced photosynthesis
Julian Hibberd, Head of Molecular Physiology Group, and Head of Department
The problem we are aiming to solve:
Research in my group is focussed on leveraging advances in understanding photosynthesis to guide crop improvement. Because of irrigation, disease control, and fertilisers, in efficient agricultural systems maximum crop yields are now limited by photosynthesis. Enhancing photosynthetic capacity in crops is widely recognised as one solution to sustainably feed our growing population, which is estimated to be 11.2 billion by 2100.
The solution:
Billions of years ago photosynthetic bacteria elaborated a process allowing CO2 to be made into sugars. They developed a pathway known as C3 photosynthesis, and most plants still use this ancient metabolism. However, from around 30 million years ago, evolution repeatedly led to the appearance of species with a 50% more efficient photosynthetic pathway. This is possible because leaves of such species use the C4 pathway, and it involves separating the reactions of photosynthesis into two compartments, typically mesophyll and bundle sheath cells that lie adjacent to one another. In C3 plants photosynthesis takes place almost exclusively in mesophyll cells. These changes to photosynthesis itself are superimposed on alterations to leaf development so that C4 leaves also contain more bundle sheath cells, with more chloroplast volume in them. The net effect is that compared with C3 species, in which about 25% of the bundle sheath area is occupied by chloroplasts, this increases to around 60% in C4 plants. Thus, these cells are more numerous and activated to allow efficient photosynthesis. Rice and wheat represent staples for about 3.5 and 3 billion people respectively, but use the ancestral, less productive photosynthetic pathway, and so our vision is to understand and install C4 photosynthesis into such plants.
What we have achieved:
Our strategy is three-pronged. First, we use the natural diversity of C3 and C4 plants to understand molecular switches associated with the inner workings of C4 photosynthesis – in other words, we study multiple lineages containing both C3 and C4 species to understand what allowed evolution from C3 to C4. One factor that makes our work globally distinct has been a focus on how processes important for C4 operate in the ancestral C3 leaf, and this has allowed us to define what mesophyll and bundle sheath cells of crops such as rice are already capable of. Second, we are early adopters of new technologies. My current favourite is single nuclei sequencing - this has proved immensely powerful as it allows unprecedented insight into changes in gene expression in each tissue of the leaf. As the C4 pathway is built on reconfiguring the process between cells, this approach has provided the first true insights into the extent to which this has taken place. Third, we have long sought to engineer C4 photosynthesis into C3 crops to test the limits of our understanding.
From our work we have established a number of principles important to our long-term aim. For example, we established that C3 plants operate C4-like metabolism, implying that the evolution of the C4 pathway has been based on tweaking existing processes. We also know that evolution has repeatedly re-wired existing gene regulatory networks, i.e. C4 has emerged because existing parts found in C3 species have been modified to allow gene expression to be changed in space and time. These changes in gene expression allow the C4 pathway to be partitioned between the mesophyll and bundle sheath cells. Sometimes, this is founded on changes in cis-regulatory mechanisms that decode an existing transcription factor network, in others the patterning of transcription factors has changed.
What happens next?
In the last year we have made a significant breakthrough in engineering the C4 pathway into rice. We have been working on this challenge for ~15 years, and our recent analysis of what governs gene expression in bundle sheath cells allowed us to define the blueprint enabling this. From this design we have activated the rice bundle sheath so that it now resembles that of a C4 plant. More needs to be done to generate a C4 rice, but we have a powerful springboard, an exciting opportunity, and a real-world problem to solve that all help to fulfil our long-term goal. A key next step is to combine this chassis containing the enhanced chloroplast compartment in the bundle sheath with the C4 biochemical pathway to bring together these elements required for the process to operate. Components allowing this biochemistry are known, but a major challenge likely remains, which is to fine tune activities to optimise function of the pathway such that it delivers a 50% increase in yield.