Determining how the circadian clock increases chlorophyll content

Nearly all living things have an internal clock that is set by the daily cycle of light and dark. This clock is called a circadian clock from the latin 'about a day'. The circadian clock in plants has been shown to control many of the functions in the plant and can cause daily rhythms in amongst other things, growth, movements of leaves and the process of making sugar from sunlight, which is called photosynthesis. Until recently, we have thought that the circadian clock must be important because so many process in the plant are controlled by it, but we did not know why the clock is important. We have new data that show that the circadian clock increases the rate of photosynthesis in plants and therefore makes the plant grow faster and do better than its neighbours. Our data suggest that the increase in photosynthesis is due to the circadian clock helping the plant to have more of the green pigment, chlorophyll. It is chlorophyll that traps the sun's energy to drive photosynthesis. We will find out how the circadian control of the activity of genes involved in chlorophyll production increases the amount of chlorophyll in the plant. We will look to see if the circadian clock increases the amount of chlorophyll made, increases the repair of chlorophyll if it is damaged by light or increases the protection of the plant from too much light, by for example making more pigments that act as a sunscreen. These experiments are important because they will help us understand how the circadian clock improves the efficiency of other major metabolic pathways in the plant. This information is important for understanding how plants have evolved and adapted and could help produce plants that grow better and are more nutritious to eat.

We have new data that demonstrate that the plant circadian clock increases chlorophyll content, assimilation, growth and competitive advantage. These findings raise important questions about the mechanisms by which the rhythmic control of gene expression can increase the flux through metabolic pathways. To answer these questions we will determine how the circadian clock increases chlorophyll content. We will investigate the accumulation, repair and photoprotection of the chlorophyll pigment and associated apoproteins in wild type plants and circadian period mutants, growing in light and dark cycles that either match or do not match the endogenous period of the circadian oscillator and in plants in which the circadian oscillator is arrhythmic. Our new data demonstrate that chlorophyll content is reduced by approximately 25 % in wild type and photoperiod mutants when the period of the light and dark cycles does not match the period of the endogenous circadian oscillator and in plants in which the circadian oscillator is arrhythmic. Specifically we will measure whether matching the circadian and environmental cycle periods (so called circadian resonance) increases 1. the accumulation of chlorophyll and associated apoproteins (PsaE, PsbO1, LHCA1, and LHCB1.3). We will also determine if the loss of chlorophyll phenotype in plants in which the endogenous and environmental periods are not matched can be rescued by over expression of ChlH and GUN4. 2. the turnover of chlorophyll and the associated apoproteins. We will measure turnover by transferring plants from environments that are resonant with the circadian oscillator to environments that do not match the period of the endogenous oscillator. Apoprotein concentrations will measured using imuunopreciptation of radiolabelled proteins. 3. photoprotection by measuring the levels of the photoprotection proteins PsbS, ELIP, and HLIP in wild types and circadian period mutants in environmental rhythms both matched and not matched to the period of the endogenous oscillator and in circadian arrhythmic mutants. Similarly, components of the xanthophyll cycle will be measured in these plants using HPLC. Our unpublished data demonstrate that circadian arrhythmic plants are more prone to photodamage in high light. Additionally, we will investigate whether the reduction in chlorophyll in dissonant plants is signalled via the sugar sensing pathway using gin2 mutants and whether precursor availability limits chlorophyll accumulation. These studies will provide a molecular and biochemical understanding of the processes by which the circadian clock enhances chlorophyll content.