Rice Research Newton Fund: Exploiting a Cyanobacterial CO2 Concentrating Mechanism to Increase Photosynthesis and Yield in Rice

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Food security is internationally recognised as one of the major global challenges of the 21st century. By 2050, it is predicted that world food production will have to increase by 50% to meet demand. This is against the pressures of global climate change and resource limitations. Meeting this challenge is going to require the development of innovative strategies to make use of our unprecedented knowledge of modern bioscience in the post genomic era. Since rice is the most important human food, consumed by more than half of the world's population, developing new highly-productive rice varieties will be fundamental to meeting the 2050 goal.Whilst substantial gains in rice yield have been obtained by traditional breeding, achievement of major increases in the future will require novel approaches. Improved photosynthesis has been identified as a major target for bioengineering for enhanced yield of C3 crop plants. Improving photosynthetic efficiency does not simply increase the amount of carbohydrate produced per hectare of land, but it also increases the amount produced per unit of water and per unit of nitrogen. This means not only environmental sustainability, but also higher income and better food security for small farmers.The properties of the carbon-fixing enzyme Rubisco (ribulose-1:5-bisphosphate carboxylase/oxygenase) are known to limit the efficiency of photosynthesis in land plants. Rubisco catalyses the combination of RuBP (ribulose-1,5-bisphophate) with CO2, but also catalyses the reaction of RuBP with oxygen, leading to photorespiration, a process in which NH3 and previously fixed CO2 are lost. Some land plants (C4 plants) and bacteria have evolved mechanisms that concentrate CO2 near Rubisco and thereby both increase photosynthetic CO2 assimilation and decrease the competing wasteful reaction with oxygen. However, rice lacks this ability. As a result, rice utilizes a Rubisco enzyme that has higher CO2 affinity but is slower than Rubisco enzymes in plants with carbon-concentrating mechanisms such as maize. Consequently, the plants must devote considerable amounts of protein, and thereby, nitrogen, to allow Rubisco to carry out adequate carbon fixation, reducing yield and biomass production. Replacing endogenous Rubisco with a faster enzyme with less CO2 specificity, along with a carbon concentrating mechanism (CCM), could significantly improve CO2 fixation, according to published models.We propose to engineer the nuclear genome of rice to express components of the cyanobacterial beta-carboxysome, including a faster Rubisco enzyme. We will identify transgenic lines of rice with the required levels of carboxysome shell proteins, internal proteins, and cyanobacterial Rubisco, which can then be crossed to express all of the novel proteins in the same line. , we will install a combination of bicarbonate transporters in order to supply CO2 to the engineered carboxysome. Finally we will also reduce the expression of the chloroplast stromal carbonic anhydrase and endogenous rice Rubisco through RNA silencing technology Based on promising results with the model plant tobacco, we believe it is time for forge ahead with a project that could bring the cyanobacterial CCM into rice for the future.