Engineering nitrogen fixing symbiotic associations in cereals

The ability to take up mineral nutrients, particularly nitrogen and phosphorus, is generally the major limitation to plant growth. Because of this farmers apply nitrogen and phosphorus through fertiliser application to promote crop growth. Sustained yields are dependent on this fertiliser application, but it comes at a high price, both in the cost of the fertiliser and the environmental damage that results from its use. A number of plant species have evolved beneficial interactions with micro-organisms that facilitate the uptake of these nutrients. Legumes form symbiotic interactions with mycorrhizal fungi that facilitate phosphate uptake and with rhizobial bacteria that provide the plant with a source of nitrogen. The establishment of these symbioses involves a molecular communication between the plant and the symbiotic micro-organisms in the soil. Mycorrhizal fungi and rhizobial bacteria release signals that are recognised by the host plant and lead to developmental changes associated with the accommodation of the symbionts.  Genetic dissection in the legume Medicago truncatula has defined the signalling pathways involved in these symbioses.  A number of the genes required for the mycorrhizal interaction are also necessary for the rhizobial interaction, indicating a conserved symbiosis signalling pathway. This implies that the evolution of nodulation involved the recruitment of a signalling pathway already functioning in mycorrhizal signalling.  This signalling pathway is present in most plant species, including cereals suggesting that engineering the perception of rhizobial bacteria in cereals is simplified and requires an understanding of the legume specific components that activate and are activated by the common symbiosis signalling pathway. We are in the process of engineering this signalling pathway in cereals to promote the recognition of rhizobial bacteria as the first step in engineering biological nitrogen fixation into cereal crops.