COLLABORATIVE PROJECT: MAGIC - A multi-tiered approach to generating increased carbon dioxide for photosynthesis

从粮食生产到工业建设，光合作用几乎是社会方方面面的核心。陆地光合作用与我们对其他自然资源的利用密切相关，它对世界上的水、矿物和碳循环起着重要的控制作用。例如，植物蒸腾被认为是导致最近与全球二氧化碳上升相关的淡水供应变化的原因之一，它是未来20-30年预计将出现的水资源供应危机的核心。在同一时期，估计全球粮食产量将需要增加50%，才能跟上人口增长的步伐。直到最近，作物产量一直与人口增长相匹配，但在十年前，在绿色革命中培育的谷物品种的收益完全实现了。因此，现在寻求进一步提高光合作用效率的途径是至关重要的。在大多数物种中，二氧化碳在Calvin-Benson循环中被二磷酸核酮糖羧化酶/加氧酶(Rubisco)固定，生成三碳化合物。Rubisco在底物选择性方面非常差，并且混杂地固定二氧化碳和O2，这一事实使Rubisco可以说是光合作用中效率最低的步骤。减少Rubisco使用O2的一种方法是提高二氧化碳分压(PCO2)。所谓的碳浓缩机制(CCM)在自然界中已经进化了多次，尽管不是大多数常见作物物种的特征。因此，比较表明，如果在作物中绕过Rubisco对O2的消耗，总体产量可能会增加约50%。大量的资源已经投入到设计Rubisco以提高二氧化碳选择性和在水稻中引入单细胞版本的C4光合作用，但这些方法还没有看到光合作用效率的逐步变化。一套有待探索的新战略是利用光驱泵浦、阴离子交换传输和底物通道向Rubisco供应二氧化碳。到目前为止，人们还不知道这些过程中的任何一个有助于光合作用，尽管这三个过程都是自然发生的，并已被综合应用于生物学。我们的目标是开发相当于‘两级泵’的东西：(1)串联放置一种运输机制，以在叶绿体中浓缩HCO3--由原生盐生盐生细菌的光驱动离子泵卤视紫质(HR)驱动，以及(2)底物在叶绿体内利用来自梭状芽孢杆菌或蓝藻的一个或多个分子‘积木’来携带HCO3-或四碳中间体至Rubisco。这一分两个阶段的战略预计将最大限度地利用HR吸收的光能独立驱动CCM增益，并且它还具有额外的潜力，使HR能够利用光合作用光谱以外的未使用的光资产。此外，这种方法的一个主要特点是其模块化性质：可以并行开发两级泵的每一级，并在分子、细胞和整个生物体水平上分别评估其功能，然后结合组件进行最终验证。这种模块化的方法确保了在三年内实现我们的目标的最大效率和速度。

Photosynthesis is at the core of virtually every aspect of society, from food production to industrial construction. Terrestrial photosynthesis is intimately connected with our use of other natural resources, and it exerts major controls on the water, mineral and carbon cycles of the world. For example, plant transpiration is thought to have contributed to recent changes in fresh-water availability associated with the global rise in CO2, and it is at the centre of a crisis in water availability expected over the next 20-30 years. Over this same period it is estimated that a 50% increase in global food production will be required to keep pace with the increase in human population. Crop yields have matched population growth until recently, but the gains from cereal cultivars bred in the Green Revolution were realised in full a decade ago. Thus it is vital that routes to further improvements in photosynthetic efficiency are sought now. In most species, CO2 is fixed by Ribulose Bisphosphate Carboxylase/Oxygenase (RuBisCO) in the Calvin-Benson cycle to generate a three-carbon compound. RuBisCO is remarkably poor in its substrate selectivity and promiscuously fixes both CO2 and O2, a fact that makes RuBisCO arguably the most inefficient step in photosynthesis. One way of reducing O2 use by RuBisCO is to raise the partial pressure of CO2 (pCO2). So-called carbon concentrating mechanisms (CCMs) have evolved multiple times in nature, albeit not as a feature of most common crop species. Thus, comparisons suggest roughly a 50% increase in overall yield might be realised if O2 use by RuBisCO were bypassed in crops. Significant resources have gone into engineering RuBisCO for increased CO2 selectivity and introducing a single-celled version of C4 photosynthesis in rice, but these approaches have yet to see a step change in photosynthetic efficiency. One new set of strategies yet to be explored is to co-opt light-driven pumps, anion exchange transport and substrate channelling to supply CO2 to RuBisCO. To date none of these processes is known to facilitate photosynthesis, although all three occur naturally and have been employed synthetically in biology. It is our goal to develop the equivalent of a 'two-stage pump': placing in series (1) a transport mechanism to concentrate HCO3- in the chloroplast powered by the light-driven ion pump halorhodopsin (hR) from the archeon Halobacterium halobium, and (2) substrate channelling within the chloroplast using one or more molecular 'building blocks' from Clostridium or cyanobacteria to carry HCO3- or a four-carbon intermediate to RuBisCO. This two-stage strategy is expected to maximise CCM gain driven independently with light energy absorbed by hR, and it has the added potential for engineering hR to tap the unused asset of light beyond the photosynthetic spectrum. Furthermore, an overarching feature of this approach is in its modular nature: it will be possible to develop each stage of the two-stage pump in parallel, and to assess its functionality separately at molecular, cellular and whole-organismal levels, combining the components thereafter for final validation. This modular approach ensures the maximum efficiency and speed in realising our goal within the three-year period.