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

粮食安全被国际公认为21世纪的主要全球挑战之一。据预测，到2050年，世界粮食产量将不得不增加50%才能满足需求。这与全球气候变化和资源限制的压力背道而驰。迎接这一挑战将需要制定创新战略，以利用我们在后基因组时代对现代生物科学的前所未有的知识。由于大米是人类最重要的食物，世界上一半以上的人口消费大米，开发高产水稻新品种将是实现2050年目标的根本。虽然通过传统育种获得了水稻产量的大幅增长，但未来实现重大增长将需要新的方法。改善光合作用已被确定为提高C3作物产量的生物工程的主要目标。提高光合作用效率不仅增加了每公顷土地产生的碳水化合物的数量，而且还增加了每单位水和每单位氮素的生产量。这不仅意味着环境的可持续性，也意味着小农户更高的收入和更好的粮食安全。众所周知，固定碳酶Rubisco(核酮糖-1：5-二磷酸羧化酶/加氧酶)的性质限制了陆地植物的光合作用效率。Rubisco催化RuBP(核酮糖-1，5-二磷酸)与二氧化碳的结合，但也催化RuBP与氧气的反应，导致光呼吸，在这个过程中NH3和之前固定的CO2会丢失。一些陆地植物(C4植物)和细菌进化出了将二氧化碳浓缩到Rubisco附近的机制，从而既增加了光合作用对二氧化碳的同化，又减少了与氧气的竞争浪费反应。然而，水稻缺乏这种能力。因此，在玉米等具有碳浓缩机制的植物中，水稻利用的Rubisco酶具有更高的二氧化碳亲和力，但比Rubisco酶慢。因此，植物必须投入相当数量的蛋白质，从而投入大量的氮素，以使Rubisco能够进行足够的碳固定，从而减少产量和生物量生产。根据已发表的模型，用一种速度更快、二氧化碳专一性较低的酶来取代内源Rubisco，以及碳浓缩机制(CCM)，可以显著提高二氧化碳固定。我们建议改造水稻的核基因组，以表达蓝藻β-羧体的成分，包括速度更快的Rubisco酶。我们将鉴定具有所需水平的羧基体壳蛋白、内部蛋白和蓝藻Rubisco的转基因水稻品系，然后这些转基因品系可以杂交，在同一品系中表达所有新的蛋白质。，我们将安装一组碳酸氢盐转运体，以便为工程羧基提供二氧化碳。最后，我们还将通过RNA沉默技术降低叶绿体基质碳酸氢酶和内源水稻Rubisco的表达，基于对模式植物烟草的良好结果，我们认为现在是时候推进一个可能将蓝藻CCM引入水稻的项目了。

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.