Optimising Photosynthetic Efficiency via Leaf Structure

Almost all our food depends eventually on the process of photosynthesis by which carbon dioxide in the atmosphere is fixed, using the energy of sunlight, into simple sugars. This process occurs mainly in the leaves of plants. Although the biochemical process of photosynthesis is highly conserved (i.e., is very similar in different plants), leaf form is very variable, both between different species and even within a species, depending on environmental conditions.The form of the leaf is important for photosynthesis since it will directly influence the efficiency of the process. For example, carbon dioxide in the atmosphere must first enter aleaf by small pores on the leaf surface (stomata), then traverse the inside of the leaf via air spaces before reaching the cells where the chloroplasts are located, which is where photosynthesis occurs. Even though these distances may seem quite small, differences in this internal pathway of carbon dioxide movement can have a major affect on the efficiency of photosynthesis. The aim of this project is to understand more about the rules which link the efficiency of photosynthesis and the internal cellular architecture of a leaf. By knowing more about these rules, we will be in a stronger position to select new breeds of plant which can perform photosynthesis more efficiently.
To achieve this aim, we will initially use the model plant Arabidopsis. This is the most advanced lab plant and we have developed tools which allow us to manipulate the cellular architecture of this leaf is a precise and controlled fashion. We will thus generate plants with a variety of altered cellular architectures. We will then analyse these architectures using an advanced imaging technique called micro-computer tomography. This will provide us with quantitative data on a number of parameters which might influence the flow of carbon dioxide within a leaf. We will also analyse the same leaves to investigate how well they are performing photosynthesis. By combining these data, we will be able, firstly, to test ideas already put forward on the importance of cellular architecture for the efficiency of photosynthesis. We will also be able to analyse our data to look for novel architectures which could act to improve the efficiency of photosynthesis but which do not have adverse knock-on affects for the leaf, for example on the rate of water loss. This concerted and ordered analysis of cellular architecture and photosynthesis and physiology will help identify the ground rules relating these factors.
Finally, by linking up with colleagues working at the International Rice Research Institute (IRRI) in the Phillipines (a main centre for rice research, the most important crop in the world) we will investigate whether the data obtained from our lab plant can help in screening experiments which are being performed to try and identify improved rice varieties.

Previous research has identified mesophyll conductance (gm) as an important factor which limits the efficiency of photosynthesis and have proposed that optimising this parameter could lead to improved crop performance. Although many previous studies have characterised gm, there have been few methodical studies in which particular elements of leaf cellular architecture have been altered in a targetted fashion and the outcome on gm (and other photosynthetic and physiological parameters) measured. In this project we will use molecular genetic tools which we have developed to alter cellular architecture in a targetted fashion in the model plant Arabidopsis, creating a spectrum of leaf architectures. The outcome on cellular architecture will be quantified using a novel imaging approach (micro-computer tomography) which allows us to measure in intact, living leaves parameters such as mesophyll cell volume, surface area, intercellular void space and leaf thickness). We will analyse the same leaves with combined fluorescent imaging, gas exchange and isotope ratio techniques to gain data on a variety of photosynthetic and physiological parameters (e.g., Pmax, transpiration rate, gm). We will then use these data in modelling approaches to test hypotheses on the linkage of cellular architecture and photosynthesis and physiology, with a focus on mesophyll conductance. We will also explore these data sets to identify potential novel linkages between the different data sets, with the aim of identifying cellular architecture parameters that could be used as leads for screening for improved photosynthetic efficiency. Finally, we will introduce these lead parameters into ongoing screens for rice leaf architecture at IRRI aimed at identifying improved varieties of rice.

The project is aimed at discovering basic information on the relationship between leaf cellular architecture and the efficiency of photosynthesis. As part of the project, we will liaise with the leading centre for rice breeding in the world (IRRI) and explore the potential of using the information gained in this project to provide a novel approach to identifying plants with improved photosynthesis and physiology. If successful, this could provide a major impact for crop improvement. In addition, during the project we will liaise with a major European wheat breeder and explore the potential of implementing a similar approach to wheat. Again, this provides a potential pathway to impact. We will also use our research as an exemplar of a multidisciplinary approach to a major problem in food security in various outreach activities which we are involved in. The research outputs of the work will also have an impact on the academic community and we will disseminate the outputs of the project via scientific journals and conferences. Finally, our project will provide an excellent training environment for the young scientists employed to perform the research. They will gain training in advanced approaches of molecular biology, physiology, imaging and modelling, equipping them for a career in multidisciplinary research.
To summarise, the project will have impact by:
- Provision of data to IRRI to explore a novel screening approach to identify improved rice varieties.
- Delivery of knowledge of this screening approach to a UK/European wheat breeder and, thus, explore potential translation to this crop.
- Engagement of stakeholders, particularly school groups and the general public with our project and the current imperative to produce high yielding crops (highlighting the food security agenda).
- Dissemination of research findings via publication in scientific journals, and talks at scientific conferences and institute seminars.
- Provision of skilled researchers and staff for academic and non-academic positions