Enhancing leaf transient carbon stores - role of fumarate as a possible storage compound

Human society finds itself facing a great challenge. Populations around the world have grown massively over the last century and are set to continue growing well into the next. Rising standards of living in countries like China and India and increases in production of biofuels are further increasing demand. This growing demand, combined with crop failures possibly linked to global climate change, have resulted in food prices rising substantially in recent years. It is estimated that food production needs to increase by 50% by the middle of this century to keep pace with demand.

Photosynthesis is the process by which plants absorb light and use the energy to make sugars. This process forms the basis of life on this planet - all the food we eat originally comes from photosynthesis. An obvious way to increase the productivity of plants is to increase their photosynthetic capacity. Plants are however already very good at capturing light, so if we wish to improve photosynthesis we need to identify where the limiting steps in this process are. During the day, as the plant makes sugars, the concentration of these rise in the leaf. If the concentration of sugars becomes too high, this can slow down or stop photosynthesis. To overcome this effect, plants can convert sugars into different substances, in particular starch. Starch is relatively unreactive and does not inhibit photosynthesis. The rate at which starch can accumulate and the amount that the leaf can hold is currently metabolically limited. So, if a plant does more photosynthesis, starch accumulation cannot keep up and sugar concentrations will increase and photosynthesis will then become inhibited.

Work in a weed called Thale Cress has identified, in addition to starch, a novel storage compound fumaric acid. Fumaric acid is a natural substance, found in all organisms including humans. It is widely used as a food additive to control the acidity of food and drink products. Although all plants contain fumaric acid, only a few are able to store high concentrations. In most crop species, only low amounts are found. If we could breed crops that, in addition to starch, are able to store fumaric acid in their leaves, , we may be able to increase plants' ability to carry out photosynthesis. The aim of this project is to test this idea and see whether it leads to an increase in the plants overall growth. The results obtained will be used to direct future plant breeding strategies in crops and with future development will play an important role in feeding the world's population.

Increasing the efficiency of CO2 fixation offers potential to increase crop yield but studies show that short-term gains may not be converted into long term growth. The rate of carbon assimilation is limited and sugar accumulation in the leaf leads to short term inhibition of photosynthesis and long term down regulation of photosynthetic capacity. Any strategy to increase overall photosynthesis that does not address storage and export of carbon is likely to fail.

The novelty of this application is that we aim to explore strategies to manipulate transient leaf storage. We will use physiology, quantitative metabolomics and modelling to examine how the ability to store carbon in the leaf impacts on photosynthetic capacity. Plants of Arabidopsis will be grown under a range of conditions (altered temperature, light, CO2 and day length) that give rise to differing levels of accumulation of fixed carbon in the leaf -. The impact of these conditions on photosynthesis and respiration will be assayed in situ through the day and the capacity for photosynthesis also measured. Leaf samples will be assayed for metabolite accumulation (using mass spectrometry linked to chromatography) and selected samples assayed for gene expression using microarray analysis. Comparisons will be made of plants deficient in leaf carbon storage. Modellung of Arabidopsis metabolism, will identify bottlenecks in the accumulation of storage compounds. In particular, we will investigate the hypothesis that fumarate accumulation may be increased as a transient carbon store. Plants overexpressing cytosolic fumarase and the tonoplast dicarboxylate transporter will be produced and their ability to accumulate fumarate assayed. Plants lacking each or both of these proteins will be examined to establish the extent to which fumarate represents a complementary carbon store, alongside starch. The results obtained will be used to direct plant breeding strategies into the future.

The key stakeholders in this project are (1) other scientists working on improving photosynthesis or on systems biology (2) plant breeders and biotechnologists (3) policy makers and (4) the general public. Activities will be undertaken to ensure that outcomes of the project are communicated to each of these groups.

(1) Other researchers will be reached using normal methods of scientific communication, i.e. publication of scientific papers and active participation in relevant conferences. We will ensure that both PDRAs will present our findings and models at at least one conference during the course of the project. Where PIs are participating in conferences, this work will also, where appropriate, be highlighted. Funding to allow staff to attend conferences and to support publications in high impact journals are included in the costings.

(2) and (3) To address groups who may be interested in exploiting the findings of the project, beyond the lifetime of the research, we will establish an advisory panel, inviting the involvement of representatives of plant seed companies and advisory bodies. Specifically, we have recently been in discussion with representatives of Syngenta and of Monsanto, both of whom have expressed an interest in the work we are carrying out. We will invite representatives of industry and of other stakeholders (e.g. the Home Grown Cereal Association) to form part of this panel and to attend discussion meetings in Manchester at the beginning of the project, after 12 months and in the final 2 months. This will allow us to better understand the needs of these groups as we carry out the research and also allow us to communicate our findings directly to interested parties. In addition, as we prepare work for publication, we will also produce press releases, in collaboration with the Faculty of Life Sciences press officer, for circulation to specialist and general press organisations, publicising our findings.

(4) To address the wider public and to increase understanding of the issues surrounding food security and the role of this and related research in addressing these issues, we will undertake activities to promote public understanding. To this end, the PDRAs will be encouraged to take part in media training sessions. We will, in collaboration with the Manchester Museum, organise an interactive exhibition, in which we will showcase our own research and invite participation from colleagues working on related topics. Manchester Museum is located in and belongs to the University of Manchester. Its location, close to some of the most deprived areas in Greater Manchester, means that it attracts a broad range of visitors from different backgrounds and acts to attract, in particular, young people from inner city areas to visit the University and so to consider pursuing higher education. The PDRAs will be encouraged to take an active part in such activities, bringing in participation from postgraduate and undergraduate students from within the University. The aim will be to produce material for displays which can then be offered for exhibition elsewhere. We have previously been involved in successful events that have gone from Manchester to the Royal Society, Natural History Museum etc.