Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

66j 4 A Review on the Taxonomy and Possible Screening Traits of Plant Growth Promoting Rhizobacteria
details of fatty acid composition; hence, they are considered to represent separate
genera. Nonetheless, the work of Sawada et al. [9] showed the genus Sinorhizobium
clustering with the genus Ensifer (a non-LNB), and further data on sequence comparison
of 16S–23S rDNA internal transcribed spacer regions has led to the proposal
to join the genera Ensifer and Sinorhizobium into one genus. Still, a consensus
over whether the genus should be named Sinorhizobium [38] or Ensifer [39] is yet to
be achieved.
The Mesorhizobium Group Mesorhizobium species cluster on the basis of 16S rDNA
sequence alignment with high bootstrap values [40], and on the basis of such an
analysis are distinct from other genera of the family Rhizobiaceae. This genus is also
distinguishable in terms of DNA homology and phenotypic differentiation from
other genera by a distinct fatty acid profile and a slower growth rate [41]. At present,
these characteristics are sufficient to justify Mesorhizobium as a distinct genus [34],
but it is no longer a member of the family Rhizobiaceae. It has been placed into the
family Phyllobacteriaceae along with the leaf-nodulating Phyllobacterium, and recent
works highlight its status as a group of three genera [9]. The Mesorhizobium group
contains also the non-LNB genera Aminobacter and Pseudoaminobacter .
The Bradyrhizobium Group The genus Bradyrhizobium, another of the LNB
genera outside the Rhizobiaceae, belongs to the family Bradyrhizobiaceae of the
Rhizobiales (http://www.cme.msu.edu/Bergeys). Strains located in this genus can
be distinguished from other legume symbionts on the basis of slow growth and
theproductionofanalkalinereactionbynoserumzoneinlitmusmilk.The
difficulties in performing DNA–DNA hybridizations in LNB are noted along with
the difficulty of obtaining DNA from Bradyrhizobium in quantity and quality to
perform extensive DNA–DNA hybridizations [42]. Doignon-Bourcier et al. [42]
chose to characterize Bradyrhizobium strains by intergenic spacer (IGS) PCR–
RFLP, amplified fragment length polymorphism (AFLP) and 16S ARDRA. For
especially diverse strains, the study of their tRNA ala gene sequence has also been
performed [38].
ThereviewofSawadaetal.[9]describestheBradyrhizobiumgroupasalsocontaining
strains of the genera Agromonas, Nitrobacter, Rhodopseudomonas and Afipia and the
species Blastoblaster denitrificans. A proposal to merge all the involved species of the
different genera would be justified; however, phenotypic differences and the convenience
of maintaining the current genera in their present form led to this proposal
being rejected [43]. At present, only the transfer of the species Blastobacter denitrificans
to the genus Bradyrhizobium as Bradyrhizobium denitrificans has been proposed [44].
Azorhizobium Azorhizobium nodulates the stem of Sesbania rostrata and molecular
data indicate it is distinct from other members of Rhizobiaceae. It does share many
molecular systematic characteristics with Xanthobacter and Aquabacter. Indeed, it
has been suggested that all three genera could be combined into a single genus –
Xanthobacter [45]. The genus is now found in the family Hyphomicrobiaceae of the
Rhizobiales.