The first step in engineering nitrogen fixing cereals; transferring the capability to perceive rhizobial bacteria (sLOLA)

发展中国家的小农，特别是撒哈拉以南非洲的小农，既没有购买无机肥料的资源，也没有生产和供应无机肥料的基础设施。再加上这些农民经常在养分极度枯竭的土壤上耕作，这意味着产量非常低，很少足以满足他们的需求。找到克服这些营养限制的机制将提高发展中国家农民的产量。即使氮素利用率略有提高，在每公顷25-50公斤的范围内，也可以显著提高作物产量，并为小股东农民带来重大经济效益。相比之下，英国的农民拥有世界上最高的每公顷产量，但维持这些产量需要高投入，特别是氮肥。无机肥料的成本已经占到小麦生产成本的近40%，而且随着能源价格的上涨，这一比例可能还会上升。氮肥的施用是英国农业高产的基础，但氮肥的使用对环境产生了重大的有害影响。寻找维持作物营养的替代手段是可持续和安全粮食生产系统的内在组成部分。豆科植物已经进化出了与固氮根瘤菌相互作用的能力，根瘤菌为植物提供了所需的氮。在根瘤内，细菌固氮是通过光合作用提供的糖和细菌所需的一系列宏量和微量营养物质来支持的。在共生状态下，细菌激活固氮并关闭氮同化，使细菌类似于一种新的细胞器，其灵魂目的是为植物提供氮。正是这种水平的整合，确保豆科植物-根瘤菌共生提供大量的固定氮。事实上，多种植物物种已经独立进化出与固氮细菌相互作用的能力，并产生了一个结节状器官，这为将这种共生能力转移到非豆科植物提供了希望。在本提案中，我们将通过工程结瘤信号向谷物转移生物固氮迈出第一步。这是一个复杂的问题。然而，从豆科植物中获得的知识表明，许多根瘤菌信号传递所需的机制存在于谷物中，而设计根瘤菌的感知可能比最初预期的要简单。结瘤的进化史似乎涉及到这个过程的效率和复杂性的逐渐提高。因此，原始共生与完全发育的根瘤或复杂的共生结构无关，但存在一定程度的固氮作用。因此，有可能的是，工程谷物感知固氮细菌可能允许某种程度的植物-细菌结合，可以提供一些固定的氮，而不需要完全分化的结节或复杂感染结构的发展。可以预见，谷物固氮工程可能会遵循一个逐步提高效率和效益的路径，但该工程过程的早期阶段可能提供有用的固定氮水平，但不是最大的水平。

Small-scale farmers in the developing world, particularly those in sub-Saharan Africa, neither have the resources to buy inorganic fertlisers, nor the infrastructure for their production and supply. This coupled with the fact that such farmers are often working with very nutrient deplete soils, means that yields are very low, rarely sufficient to sustain their needs. Finding mechanisms to overcome these nutrient limitations will enhance the yields of developing world farmers. Even slight increases in nitrogen availability, within the range of 25-50 kg per hectare, could significantly improve crop yields and have major economic benefits to small shareholder farmers. In contrast farmers in the UK have some of the highest yields per hectare in the world, but sustaining these yields requires high inputs, in particular nitrogenous fertilisers. The cost of inorganic fertlisers already accounts for nearly 40% of wheat production costs and this is likely to increase as energy prices rise. Application of nitrogen fertilisers underpins the high yields in UK agriculture, but their use comes with significant detrimental impacts on the environment. Finding alternative means to sustain crop nutrition is an intrinsic component of sustainable and secure food production systems. Legumes have evolved the capability to interact with nitrogen-fixing rhizobial bacteria that supply the plant with its nitrogen needs. Within the nodule bacterial nitrogen fixation is supported through the supply of sugars from photosynthesis and a range of macro and micronutrients that the bacteria need. In the symbiotic state the bacteria activates nitrogen-fixation and switches off nitrogen assimilation, making the bacteria analogous to a novel organelle, the soul purpose of which is the supply of nitrogen to the plant. It is this level of integration that ensures that the legume-rhizobial symbiosis delivers a high amount of fixed nitrogen. The fact that multiple plant species have independently evolved the capability to interact with nitrogen fixing bacteria with the result of a nodule-like organ, provides promise for the transfer of this symbiotic capability to non-leguminous plants. In this proposal, we will initiate the first steps towards the transfer of biological nitrogen fixation to cereals, through engineering nodulation signalling. This represents a complex problem. However, the knowledge gained in legumes reveals that much of the machinery necessary for nodulation signalling is present in cereals and engineering the perception of rhizobial bacteria is likely simpler than initially anticipated. The evolutionary history of nodulation appears to have involved a gradual improvement in the efficiency and complexity of this process. Thus primitive symbioses are not associated with fully developed nodules or complex symbiotic structures, yet a degree of nitrogen fixation occurs. It is therefore possible, that engineering cereals to perceive the nitrogen-fixing bacteria may allow some degree of plant-bacterial association that could provide some fixed nitrogen without the need for a fully differentiated nodule or the development of complex infection structures. It is anticipated that the engineering of nitrogen fixation in cereals may follow a gradual path of increasing efficiency and effectiveness, but that the early stages of this engineering process may provide a useful, but not maximal, level of fixed nitrogen.