ABSTRACT
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 environmentally significant 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.
This 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 genotypic
and phenotypic approaches, applied to isolates obtained from a large number of legume
species and from different regions, resulted in reclassification of known rhizobial species and
in an increased number of new species.
