Engineering a synthetic symbiosis to solve the nitrogen crisis

The growth of the world's population is not followed by a proportionate increase in available arable land for food production. Therefore, more efficient ways to optimise food production are needed. At the same time, we must minimise the environmental impacts of modern agriculture.

Finding novel and more sustainable ways to support the nitrogen requirements of economically important cereal crops is essential to guarantee global food security in the future. Although synthetic nitrogen fertilisers support extensive agriculture, their overuse is related to a series of environmental problems. Firstly, their production and distribution require large amounts of fossil fuel. Secondly, excess use of synthetic nitrogen leads to the emission of nitrous oxide (N2O), a greenhouse gas 300-fold more potent than carbon dioxide (CO2).

On the other hand, smallholder farmers in developing countries have limited access to nitrogen fertiliser due to its high cost. Some soil bacteria associated with plants can convert atmospheric nitrogen (N2), an inert form of nitrogen to most living organisms, to ammonia (NH3), which plants can readily assimilate. Using these bacteria to supply nitrogen to cereal crops is a viable alternative to increase food production while reducing the impact of agriculture on global warming.

This project aims to understand how naturally occurring nitrogen-fixing bacteria can support the nitrogen fertilisation of crops. To do this, new insights into the mechanisms that regulate biological nitrogen fixation (the reduction of N2 to NH3 by the nitrogenase enzyme) will guide the modification and optimisation of these microorganisms. Determining how the modified bacteria interact with cereal crops to fulfil their nitrogen requirements is also fundamental to the success of this strategy. The knowledge generated in this research programme will be readily translatable into new technologies to help the establishment of more sustainable and equitable agriculture. The outcomes of this programme will also provide a better understanding of fundamental signal transduction mechanisms that may lead to new broadly applicable biological insights in microbial research.

Engineering soil diazotrophs to release fixed nitrogen while engaging in specific associations with cereal crops will provide an alternative to reduce the use of synthetic nitrogen fertilizers that harm our environment. This project aims to understand how nitrogen fixation and ammonia release can be controlled in response to specific environmental cues to generate an efficient and specific synthetic symbiosis.

The research programme is divided into three objectives using cutting-edge methodologies to gain novel fundamental and applied knowledge in the regulation of biological nitrogen fixation. In the first objective, I will obtain novel structural insights into the molecular mechanisms driving regulation of nitrogen fixation by the dedicated NifL-NifA two-component system. This will uncover how the properties of this regulatory system can be tweaked to manipulate the levels of expression of the nitrogen fixation machinery. In the second objective, I will investigate the physiological and genetic requirements to support bacterial fitness, nitrogen fixation and ammonia secretion in response to different carbon and nitrogen regimes both in free-living conditions and in the rhizosphere. By expressing engineered activators as part of tightly controlled regulatory circuits, I will define further requirements for the generation of optimized strains able to release ammonia for the benefit of cereals. Finally, in the third objective, I will determine the potential of the engineered ammonia releasing strains to promote plant growth and support the N-requirements of economically important cereal crops, such as wheat and barley.

Besides providing a blueprint for future engineering efforts with broad applicability, the results will enlighten new fundamental aspects of bacterial physiology required for efficient adaptation to the rhizosphere.