The Role of GIGANTEA in mediating metabolic input in to the Arabidopsis circadian clock

Most living organisms measure time. We are familiar with our human 'body clock' which acts as an internal watch allowing us to sleep and wake up at regular time intervals. In particular, we experience the physiological effects of disruption of the clock when we cross time zones and get jet-lag. Plants also measure time and use this information to measure the length of the seasons and to control flowering. Many plant behaviours only occur at specific times of the day, for example water lily flowers are only open in the middle of the day and close long before dusk. We have recently found that plants with a functional clock grow faster than plants that do not have a working clock. The time-keeper in plants is present in every cell and is known as a circadian clock. We have new data that demonstrate that sugar can regulate the behaviour of the circadian clock. This is important because in plants, there are rhythmic changes in the content of sugar in the cells. In the day, using the energy from sunlight, plants make sugar by photosynthesis. At night the sugar content of cells is low because there is no photosynthesis. Our finding that sugar regulates the behaviour of the circadian clock suggests a mechanism by which the metabolism of plants regulates daily rhythms of other activities. We have identified one of the genes required for the sensing of sugar by the plant circadian clock. This is GIGANTEA (GI), a gene already known to be part of the clock. We now want to understand how GI responds to sugar and how this affects the function of the clock. We will measure how sugar affects the GI gene and interactions between the GI protein with other proteins. Sugar does not only regulate the circadian clock. Sugar affects the development of the plant and regulates many genes. We will find out if GI has a role in sugar-mediated changes in development and gene activity, in addition to its role in sugar regulation of the clock. This will find out if GI is part of a general sugar sensing mechanism. We hope to find other genes important for sugar sensing by the circadian clock. We will look for effects of other genes of the circadian clock on sugar sensing. We also will investigate whether other genes already known to have a role in other sugar-sensing pathways also have an effect on sugar sensing by the clock. A major goal of our project is to find out why sugar affects the plant circadian clock. We will do this by studying the growth and physiology of plants with genetically-manipulated levels of the GI gene in environments that affect the sugar content of the plant. This understanding of the physiological role of GI and sugar sensing is an essential step in planning how to produce crop plants that grow faster, are healthier, larger or produce new products, such as biofuels.

We have found that sucrose has profound effects on the circadian clock of Arabidopsis. We have demonstrated that GIGANTEA (GI) is required for sugar-sensing by the clock. This identifies a mechanism by which metabolism can regulate circadian function and suggests that GI might be a major regulator of yield. We identified a role for GI using predictive modelling of the circadian oscillator. This was confirmed by insensitivity of the oscillator to sucrose in the gi-11 null. We will determine the mechanism and physiological significance of GI-mediated sugar signalling. Mutant analyses will test whether other clock genes also mediate sucrose input in to the circadian oscillator. The effects of other sugars and mutations in sugar signalling networks will characterize the mechanisms by which sugars regulate the circadian clock. We will identify the effects of sucrose on GI transcription, GI protein stability and GI-protein interactions. We will test if GI contributes to sugar signalling outside of the circadian clock by performing microarray analysis and network reconstruction in wild type and gi-11. This will determine if GI is required for sucrose-induced transcriptional changes. Phenotypic screens will identify if GI is required for the regulation of development by sugars. The functional significance of GI in regulating carbon metabolism and growth will be quantified by measuring plant growth and gas exchange in wild types and GI mutants in a range of light intensities, photoperiods and atmospheric CO2 concentrations. Our systems approach in the model plant Arabidopsis will address a question of relevance to crop performance and yield. Daily rhythms of carbon metabolism are controlled to maximise growth. This study will provide a mechanistic and physiological understanding of how rhythmic behaviour and carbon metabolism are integrated.

Our research addresses the integration of plant metabolism with the circadian clock. Both the clock and metabolic pathways are major determinants of yield. An increase in crop yield is essential for Food Security. We recently demonstrated that the circadian clock increases yield in Arabidopsis (Dodd et al., 2005 Science 309, 630 - 633). This has been extended by others who have provided convincing evidence that the yield advantage seen in alloploidy is due to the expression of clock genes (Ni et al. (2009) Nature 457, 327-331). Since polyploidy is common in cereals and many modern crop plants are significantly larger than the founding varieties, it is possible that the expression of plant circadian clock genes has had major impact on modern farming. In the short term our research will be of use to colleagues in the commercial private sector interested in engineering plants to improve metabolism and yield and generate crops for biofuels. In the longer-term it is likely that elements of our research will identify candidate gene targets for plant breeders. To ensure that our findings are freely available to colleagues in the commercial and public sectors, we aim to publish our research in the highest quality journals with a broad readership. We have a good track record of this. This makes our findings widely-available and is the essential mechanism for the free-flow of scientific ideas. Additionally, our work will be presented at international research meetings where both commercial and public-sector academics will be present. Dr Webb will present at least four international meetings as an invited speaker in 2009. The Webb laboratory collaborates with the National Institute of Agricultural Botany to identify 'nearer-market' applications for our research. This formal link provides ample opportunity to explore application of our findings in an agronomic environment. Additionally, Dr Webb holds a BBSRC-Industrial Case award with a very large international biotechnology company. This formal working relationship permits discussion and potential exploitation of our research with the biotechnology sector. To continue to develop links with a broad community and seek avenues for wider impact of the research, the Department of Plant Sciences has established several facilitating partnerships to promote discourse and working relationship within the plant biology and agricultural community. The Cambridge Partnership for Plant Sciences (CPPS) is a forum that includes 500 members from the Cambridge area, including academic scientists, researchers in biotech start-ups, crop breeders and agronomists. Joint scientific meetings and social events promote scientific exchange and collaborations (as exemplified by our relationship with NIAB). On a wider scale, funding from the East of England Development Agency to the Department of Plant Sciences supports 'InCrops' to promote the development of networks in the region and to further plant and agronomic sciences. An administrator has been funded to organize and facilitate networking activities. Dr Webb is the Plant Science's Enterprise Champion. He and attends events organized by Cambridge Enterprise, a wholly-owned subsidiary of the University of Cambridge that promotes commercialisation of discoveries. This provides numerous opportunities to meet with industrialists and venture capitalists.