Nitroplast: A light-driven, synthetic nitrogen-fixing organelle

Plants require nitrogen in a form that can be assimilated, i.e. as ammonia and its many inorganic and organic derivatives. Certain microbes can fix nitrogen (N₂) and a few plant species engage in symbiotic relationships with them to access the fixed N₂. The Haber-Bosch process is used to convert N₂ to ammonia for fertilizers that are used extensively in agriculture. This process consumes a huge amount of fossil fuels, making agriculture itself unsustainable and can also result in runoff that contaminates aquifers and estuaries. It is thus desirable for crop plants to fix their own N₂. This however will impose an increased energy cost on the plant and thus reduce yields. Our concept to introduce N₂ fixation to plants based on the cyanobacterial model in which solar energy contributes more directly to the energy needs of N₂ fixation: we wish to make a novel synthetic N₂-fixing organelle, a ‘nitroplast’. As a first step towards this aim, we shall use the tools of synthetic biology to engineer N₂ fixation into a non-N₂ fixing cyanobacterium. This will require not only the gene cluster for N₂ fixation but also multiple changes in other cell processes, notably changes to the photosystems and nitrogen metabolism. We have chosen a cyanobacterial host (chassis) because of its similarity and evolutionary relevance to plastids. In this chassis our engineering goals should be tractable and will constitute technological stepping-stones leading to future research in which algal and (crop) plant plastids are re-engineered for N₂ fixation.

For this project to be successful, several objectives need to be met.

1. Gene clusters required for nitrogen fixation need to be reengineered using synthetic biology techniques as tunable nitrogen fixing gene modules, which can be precisely controlled.
2. New synthetic biology tools for engineering cyanobacteria will be developed.
3. The synthetic nitrogen fixing gene modules will be moved into a model non-N₂ fixing cyanobacterium.
4. In order to deal with the high metabolic energy costs of the process, a novel strategy will be employed by which extra light absorption capacity is engineered into cyanobacteria to power the process.