Symbiosis of rhizobia with legume plants (Fabaceae)

Biological nitrogen fixation is one of the most important processes in which atmospheric nitrogen is reduced to ammonia by symbiotic bacteria called rhizobia, which belong to alpha- and beta-proteobacteria. Legume plants (Fabaceae) have the capacity to enter into mutualistic symbiosis with nitrogen-fixing bacteria, enabling them to grow in nitrogen-limited agricultural soils. In these specific associations, new organs called root or shoot nodules are developed. Infection of plants by rhizobia is a series of sequential stages in which rhizobial lipochitooligosaccharide, called the Nod factor, activates plant transmission signaling and initiates nodule development. In the nodules the bacteria multiply and differentiate into nitrogen-fixing bacteroids. In return for reduced nitrogen compounds provided to the plants, rhizobia are supplied with plant carbon photosynthetic products and are protected from environmental stresses. Genomes of rhizobia and other soil bacteria are large and multipartite, composed of the chromosome and plasmids (megaplasmids), which may comprise up to 50% of the genome. A common feature of the rhizobial genomes is that genes responsible for nodulation and nitrogen fixation are clustered on symbiotic plasmids (pSym) or incorporated into the chromosome as symbiotic islands. Plasmids are heterogeneous in size and gene content even in closely related rhizobia. Recently described extrachromosomal replicons, named "chromids", have some chromosome and plasmid properties and are essential for bacterial growth under natural and laboratory conditions. The special architecture of rhizobial genomes may cause their dynamic state and plasticity, leading to significant diversity of rhizobia on the genetic and metabolic levels.