The Legume Root Nodule: From Symbiotic Nitrogen Fixation to Senescence

Biological nitrogen fixation (BNF) is the biological process by which the atmospheric nitrogen (N2) is converted to ammonia by an enzyme called nitrogenase. It is the major source of the biosphere nitrogen and as such has an important ecological and agronomical role, accounting for 65 % of the nitrogen used in agriculture worldwide. The most important source of fixed nitrogen is the symbiotic association between rhizobia and legumes. The nitrogen fixation is achieved by bacteria inside the cells of de novo formed organs, the nodules, which usually develop on roots, and more occasionally on stems. This mutualistic relationship is beneficial for both partners, the plant supplying dicarboxylic acids as a carbon source to bacteria and receiving, in return, ammonium. Legume symbioses have an important role in environment-friendly agriculture. They allow plants to grow on nitrogen poor soils and reduce the need for nitrogen inputs for leguminous crops, and thus soil pollution. Nitrogen-fixing legumes also contribute to nitrogen enrichment of the soil and have been used from Antiquity as crop-rotation species to improve soil fertility. They produce high protein-containing leaves and seeds, and legumes such as soybeans, groundnuts, peas, beans, lentils, alfalfa and clover are a major source of protein for human and animal consumption. Most research concentrates on the two legume-rhizobium model systems Lotus-Mesorhizobium loti and Medicago-Sinorhizobium meliloti, with another focus on the economically-important Glycine max (soybean) -Bradyrhizobium japonicum association. The legume genetic models Medicago truncatula and Lotus japonicus have a small genome size of ca. 450 Mbp while Glycine max has a genome size of 1,115 Mbp, and all are currently targets of large-scale genome sequencing projects (He et al., 2009; Sato et al., 2008; Schmutz et al., 2010). The complete genome sequence of their bacterial partners has been established (Galibert et al., 2001; Kaneko et al., 2000; Kaneko et al., 2002; Schneiker-Bekel et al., 2011).