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

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.

The evolution of nodulation in legumes has involved the recruitment of a pre-exisiting signalling pathway, the symbiosis signalling pathway, to allow the recognition of rhizobial bacteria. The symbiosis signalling pathway is present in most species of plants and functions in the establishment of the mycorrhizal symbiosis. Cereals possess the symbiosis signalling pathway and it has been shown that the components of this pathway in cereals function in a manner analogous to their orthologs in legumes during nodulation signalling. The signalling molecules produced by rhizobial bacteria, so called Nod factors, are very similar to Myc factors produced by mycorrhizal fungi and recent work shows that non-legumes can perceive these lipo-chito oligosaccharide signals to activate the symbiosis signalling pathway. Thus there are close parallels between the mechanisms of recognition of mycorrhizal fungi in many plant species, including cereals, and the recognition of rhizobial bacteria by legumes. This means that it is feasible to engineer cereals to allow the recognition of rhizobial bacteria as a symbiont, with potential consequences for the accommodation of these nitrogen-fixing bacteria. This proposal is a partner to an international programme of research currently under consideration by The Bill and Melinda Gates Foundation. The two proposals build on the knowledge gleaned from years of research in nodulation signalling in legumes to engineer cereals for recognition of rhizobial bacteria and the initiation of nodule organogenesis. These two proposals provide the scale of investment necessary to begin a major new strategic initiative to engineer nitrogen-fixing cereals. This is an ambitious goal and one that cannot be achieved within a 5 year timeframe. However, it is anticipated that some level of fixed nitrogen in cereals could be achieved along the route to a fully functioning nitrogen-fixing nodule on a cereal root, with implications for sustainable food production.

In many developing nations, particularly those in sub-Saharan Africa, crop yields are very low due to poor crop nutrition. Improving crop nutrition in these resource poor regions is key to sustainable and secure food production systems that can provide sufficient yields to support the local population. In contrast developed world farming systems use large amounts of inorganic fertilisers, that underpin high yields, but lead to significant environmental degradation. In these resource rich systems, finding alternative means to deliver crop nutrients is important to enhance sustainability. The major nutrients that limit crop production are nitrogen, phosphorus and potassium and this proposal focuses on finding alternative mechanisms for the delivery of nitrogen. The proposal uses the wealth of knowledge generated in the study of legumes, to initiate the first stages of engineering nitrogen-fixing cereals. From the decades of research in rhizobia and in legumes, we can now approach this problem rationally, building on the knowledge already generated. This proposal and its sister proposal to the Bill and Melinda Gates Foundation represent a new international initiative to engineer nitrogen-fixing cereals, building on the first two steps in this process: engineering the symbiosis signalling pathway to allow recognition of rhizobial bacteria and initiating the first steps in nodule organogenesis. While this work may enhance the interaction between cereal roots and nitrogen-fixing bacteria, there is likely to be additional work required to generate an efficient and effective nitrogen-fixing symbiosis. However, it is possible that steps along the way to a fully functional nodule on a cereal root, may provide some level of fixed nitrogen, that is likely to be useful in enhancing yields of crops grown by resource poor farmers.