Update on Photosynthetic Carbon Assimilation Increasing Photosynthetic Carbon Assimilation in C 3 Plants to Improve Crop Yield: Current and Future Strategies Christine A. Raines* Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, United Kingdom The realization that crop yields are reaching a pla- teau, while population increases continue at pace, has placed manipulation of photosynthesis in a central position to achieve increases in yield. Increasing flux through the C 3 cycle will be a major focus of this effort. Through application of new technologies together with novel modeling approaches, increased yield through improved photosynthetic carbon fixation should be an attainable goal in the near to mid term. This Update article reviews the past approaches and successes in this area. An outline of some of the known targets for future work is given, and approaches to identify novel targets for exploitation are outlined. Improving crop yield to meet the demands of an increasing world population for food and fuel is a central challenge for plant biology (Edgerton, 2009). This goal must be achieved in a sustainable manner (i.e. with minimal agricultural inputs and environ- mental impacts) in the face of elevated levels of CO 2 and more extreme conditions of water availability and temperature. Agricultural yields have generally kept pace with demand in the recent past as a result of the gains made through breeding programs and farming practice, but crops yields are now reaching a plateau. One fundamental component of plant productivity that has not been used to select for increased yield is photosynthesis. There is now the opportunity to ex- ploit our extensive knowledge of this fundamental process for the benefit of humankind (von Caemmerer and Evans, 2010; Zhu et al., 2010a). In the plant kingdom, there are three pathways of photosynthetic, atmospheric CO 2 fixation. However, the vast majority of plant species fix atmospheric CO 2 using the enzyme Rubisco in the Calvin-Benson cycle. The first stable product of this cycle is a three-carbon compound, phosphoglycerate (3-PGA), and for this reason this process is referred to as the C 3 cycle. Plants utilizing this pathway are often referred to as C 3 spe- cies. A major problem with the C 3 cycle is the enzyme Rubisco. This is because Rubisco is not only an ineffi- cient enzyme with a low turnover number, but it also catalyzes two competing reactions: carboxylation and oxygenation (Portis and Parry, 2007). The oxygenation reaction directs the flow of carbon through the photo- respiratory pathway (Fig. 1), and this can result in losses of between 25% and 30% of the carbon fixed. Environmental variables, such as high temperature and drought, can result in an increase in the oxygenase reaction. Therefore, reducing the Rubisco oxygenase reaction has the potential to increase carbon assimila- tion significantly and would represent a step change in photosynthesis (up to 100% depending on tempera- ture; Long et al., 2006). Two metabolic pathways have evolved to over- come this, the C 4 (the first stable compound synthe- sized is a C 4 acid, oxaloacetate) and crassulacean acid metabolism pathways. Both the C 4 and crassulacean acid metabolism pathways are additional to the C 3 cycle and increase the supply of CO 2 to Rubisco, thereby reducing the oxygenation reaction and flux to the photorespiratory pathway. This review will focus on progress and future prospects to improve the C 3 cycle. THE C 3 CYCLE The C 3 cycle is the primary pathway of carbon assimilation in the majority of photosynthetic organ- isms. It is the single largest flux of organic carbon in the biosphere and assimilates about 100 billion tons of carbon a year (15% of the carbon in the atmosphere). Understanding the responses of the Calvin cycle to altered demands for photosynthate within the plant and to external environmental conditions is essential for attempts to increase yield and to redirect carbon into important products. The C 3 cycle utilizes the prod- ucts of the light reactions of photosynthesis, ATP and NADPH, to fix atmospheric CO 2 into carbon skeletons that are used to fuel the rest of plant metabolism (Fig. 1; Stitt et al., 2010). The C 3 cycle is initiated by the enzyme Rubisco that catalyzes the carboxylation of the CO 2 acceptor molecule ribulose-1,5-bisP (RuBP). 3-PGA formed by this reaction is used to form the triose phosphates glyceraldehyde phosphate (G-3-P) and dihydroxyacetone phosphate via two reactions that consume ATP and NADPH. The regenerative phase of the cycle involves a series of reactions that convert triose phosphates into the CO 2 acceptor molecule RuBP (Fig. 1). Carbon compounds produced in this * E-mail rainc@essex.ac.uk. www.plantphysiol.org/cgi/doi/10.1104/pp.110.168559 36 Plant Physiology Ò , January 2011, Vol. 155, pp. 36–42, www.plantphysiol.org Ó 2010 American Society of Plant Biologists