On track for enhanced photosynthesis in certain crops Not all plants are made equal. Whilst they all use an enzyme called Rubisco to capture carbon, some can only count on the inefficient C3 photosynthesis pathway to do so, resulting in the loss of already-fixed carbon in a process called photorespiration. The 3TO4 project aimed to enhance photosynthesis in these plants, drawing inspiration from the more efficient C4 photosynthesis. Food and Natural Resources © K.Narloch-Liberra, Shutterstock Plants use photosynthesis to convert carbon dioxide and water into carbohydrates using energy from light. Central to this carbon fixation process is an enzyme called Rubisco, which first evolved 3.5 billion years ago in photosynthetic bacteria. Many crop plants, including wheat, barley, rice, soybean and potatoes, use Rubisco in an inefficient pathway – known as C3 photosynthesis – to fix carbon. More recently evolved grasses like maize have altered their leaf structure and biochemistry to concentrate CO2 around Rubisco in the more efficient C4 photosynthesis pathway. In general, plants with C4 photosynthesis account for about 50 % of known grasses, 3 % of flowering plant species, and 40 % of the world’s grain harvest. But what if these percentages could be raised by means of green biotech? If photorespiration, a process that works against photosynthesis, could be reduced in C3 crops, or if C3 crops could be converted to use C4 photosynthesis, large economic and environmental benefits would be ensured because of the reduced inputs per unit yield associated with the C4 pathway,’ says Richard Leegood, coordinator of 3TO4 and Professor of Plant Biochemistry at the University of Sheffield. C4 photosynthesis results in improved rates of carbon fixation, improved nitrogen use and improved water use, but bringing it to C3 plants is far from easy. ‘Efficient C4 photosynthesis is associated with alterations to leaf development, cell biology and biochemistry,’ Prof Leegood explains. ‘Transferring these traits into C3 crops is a long-term undertaking, but even partial long-term success would already have significant economic and environmental benefits.’ 3TO4 has been laying the groundwork by trying to uncover the fundamental aspects of C4 biology. The team’s ultimate aim is to use the C4 mechanism to reduce the extent of photorespiration. ‘The work proposed largely proceeded according to plan,’ says Prof Leegood. ‘However, although rape lines with a photorespiratory by-pass were generated, the plants transformed with the by-pass did not show a sufficiently strong phenotype to warrant the extensive programme of work originally proposed.’ To overcome this problem, the team refocused their work on C3-C4 intermediate plants such as Moricandia arvensis, which is closely related to rape and features a natural photorespiratory by-pass. Another of the project’s main ambitions was to contribute to the C4 Rice Project funded by the Bill & Melinda Gates Foundation. ‘C4 rice is destined to increase food production in its major markets in South-East Asia and Africa, but once the development of a C4 crop (or any crop with reduced photorespiration) has been achieved, it should be relatively straightforward to apply to technology to other crops, including C3 crops in Europe, such as wheat,’ says Prof Leegood. Although the project has now been completed, work is continuing in partners’ labs in areas such as C4 leaf development and anatomy, the photorespiratory by-pass, post-translational regulation of C4 proteins, function of transcription factors and regulation of gene expression. If all goes according to plan, Prof Leegood believes C4 crops could become a reality in 15 to 20 years. Keywords 3TO4, photosynthesis, C4 rice, photorespiration, CO2 fixation, Rubisco, C3 photosynthesis, C4 photosynthesis
