Grain legumes are crops with excellent dietary characteristics as a remarkably high level of protein and fibre. Legumes also increase the amount of nitrogen in soil available for subsequent crops, thereby reducing mineral fertilization and emission of greenhouse gases. These unique characteristics are provided by their ability to establish symbiotic interactions with soil nitrogen-fixing bacteria which convert plant-inaccessible atmospheric nitrogen into plant-available ammonia. Despite the fact that improvement of nitrogen fixation is an ultimate goal of symbiosis research, as the most important aspect for agriculture, relatively little is known about the mechanisms controlling symbiotic efficiency. The aim of this proposal is to discover plant defence-related mechanisms with new and specific roles in regulation of nitrogen fixation, which modification or removal has the potential to enhance the symbiotic performance of legume crops.