In most plants, growth rate is limited by the rate at which carbon dioxide from the atmosphere is taken up and converted to sugars in the process of photosynthesis. The enzyme responsible for the first step in this process, Rubisco, does not work at its potential maximum efficiency at the current levels of carbon dioxide present in the atmosphere. If levels were much higher, photosynthesis would be increased and plant productivity would be higher. There is an immediate requirement for increased crop productivity to provide food for the rising population of the planet. Our project addresses this problem. We are studying a mechanism present in unicellular green algae that results in high concentrations of carbon dioxide inside their photosynthesising cells (called a Carbon Concentrating Mechanism, or CCM), enabling Rubisco to work at maximum efficiency. During the initial CAPP1 programme, we discovered important new information about this mechanism, and using new and rapid methods we have identified novel algal genes and additional regulatory components which allow the CCM to operate in association with a specific micro-compartment called a pyrenoid. We have also successfully introduced some of these components into a model higher plant, Arabidopsis, and also successfully introduced a modified form of Rubisco which may facilitate aggregation into the pyrenoid. The ambitious goals of the CAPP2 extension will be to combine the expression of the CCM and pyrenoid in the Advanced Plant. Firstly, we will continue to identify genes required by the algae to achieve high concentrations of carbon dioxide inside the cells, and develop new markers and sensors to reveal the location and activity of these genes when expressed in the higher plant. Secondly, we will identify additional regulatory elements needed to form a pyrenoid, as well as exploring the impact on Rubisco enzyme efficiency and light utilisation. Thirdly, we will continue to introduce successive components into our model Advanced Plant so as to "stack" up the activities of CCM components and examine the extent of pyrenoid formation and enhanced productivity associated with the CCM. This work will provide new insights into how plants and algae acquire and use carbon dioxide from the atmosphere, of great importance in predicting and coping with the current rapid changes in the atmosphere and hence in climate. The work will also contribute to strategies to increase global food security, because it will indicate new ways in which crop productivity can be increased.