ABSTRACT
EARLY SYMBIOTIC STEPS The establishment of nitrogen-fixing symbioses between Rhizobiutn, Bradyrhizobium and Azorhizobium (collectively referred to by the term Rhizobiutn) and their host plants is a complex phenomenon wherein rhizobia simultaneously induce two processes: infection of host plant roots and organogenesis induction leading to differentiation of a structure called the nodule. This symbiotic association also involves a specific recognition of the two partners, resulting in a well-defined spectrum of hosts (Table 8.1) for each species of Rhizobiutn. This spectrum can be very narrow, for example R. tneliloti which can form nodules only in the genera Medicago, Melilotus
Common nod Genes The nod ABC genes were the first nodulation genes to be identified. They are called the common nod genes because they are present in all species of Rhizobium, Azorhizobium and Bradyrhizobium, and are responsible for similar functions in these different species. A mutation in one of these genes results in the loss of all the symbiotic properties of the strain and can generally be complemented by a homologous gene of another species of Rhizobium (Denarie et ah, 1992; Fisher and Long, 1992; Schultze et ah, 1994).
