Laboratory of Microbiology, Dept. Biochemistry, Physiology and
Microbiology, Fac. Science, Ghent University. K.L. Ledeganckstraat 35, B-9000
Gent, BELGIUM
E-mail: anne.willems@rug.ac.be
__________________
The taxonomy of rhizobia, bacteria capable of fixing nitrogen in symbiosis with leguminous plants, has changed considerably over the last 20 years. As in many other bacterial groups, this was to a large extent influenced by 16S rDNA phylogenetic information. In addition, the continued exploration of new host plants, all over the world, has been a source of many new species and is expected to yield many more.
The original genus Rhizobium, a member of the
alpha-Proteobacteria, has been divided up in several genera. In a taxonomic sense, rhizobia are seen
as those taxa that at one point in their history belonged to the genus Rhizobium and thus they
comprise the genera Rhizobium, Bradyrhizobium, Sinorhizobium, and Mesorhizobium. However, other
genera with similar symbiotic properties that are phylogenetically close to the
genus Rhizobium are now also included in the rhizobia: Allorhizobium and Azorhizobium. Furthermore, it is well accepted that Agrobacterium, a related group
of mostly phythopathogenic bacteria, is entwined with the rhizobia. Symbiotic
and phytopathogenic strains can be highly related and current classification
does not adequately reflect this.
A recent radical proposal to assimilate Agrobacterium and Allorhizobium into Rhizobium tried to remedy
this, but is biologically as well as taxonomically not very satisfactory. More
data are needed to provide a detailed insight into these complex
relationships. Such information
may come from the complete genome sequences that are now becoming available (A.
tumefaciens, M. loti and S. meliloti are available) and that will
allow a more thorough comparison that can eventually lead to a stable taxonomy.
Recently bacteria
capable of nitrogen fixation with legume hosts, were discovered outside the
traditional rhizobial phylogenetic lineages. So far they include species of Methylobacterium and Devosia in the
alpha-Proteobacteria and of Burkholderia and Ralstonia in the
beta-Proteobacteria. These
organisms may have acquired symbiotic genes through lateral transfer and in
future other such taxa may be discovered.