Root Nodule Bacteria and Symbiotic Nitrogen Fixation

Symbiotic nitrogen fixation in plants occurs in root nodules of legumes and nonlegumes. The bacterium rhizobium is one of the most studied symbiotic nitrogen-fixing bacteria because it nodulates legumes, which are e in soil N fertility management of cultivated lands. The majority of nonleguminous nodules belong to the Alnus-type symbiosis, in which the actinomycete Frankia is the microsymbiont. The cyanobacteria Nostoc or Anabaena nodulate the Cycadales, while the bacterium rhizobium forms Parasponia-type symbioses. and phenotypic approaches, applied to isolates obtained from a large number of legume species and from different region n of known rhizobial species andin an increased number of new species. Symbiotic rhizobia belong to the a-subclass of Proteobacteria (a-rhizobia). However, some tropical legumes are nodulated by strains ofBurkholderia andRalstonia species belonging to the b-subclass of Proteobacteria. These strains evolved from diazotrophs through multiple lateral nod gene transfers, and this phenomenon seems to be widespread in nature (Chen et al. 2003). The current taxonomy of rhizobia (Rhizobia_Taxonomy 2006) includes the genera Rhizobium 2006 by Taylor & Francis Group, LLC.s, resulted in reclassificatioThis chapter focuses on the methodology developed to study the rhizobium–legume symbi-osis. The global success of legume production is due to the development of inoculation technologies and cropping systems by multidisciplinary teams. Microbiologists, soil scien-tists, plant physiologists, plant breeders, and agronomists contributed to this breakthrough. Increased knowledge in rhizobial ecology is mainly due to the development of molecular techniques. Moreover, the taxonomy of the microsymbiont rhizobium has considerably changed since the last edition of this chapter (Rice and Olsen 1993). The use of genotypicnvironmentally significantChapter 3