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

70j 4 A Review on the Taxonomy and Possible Screening Traits of Plant Growth Promoting Rhizobacteria
4.4.1.2 Azospirillum
Only the genus Azospirillum contains species reported as plant growth promoters of all
genera described within the family Rhodospirillaceae. Strains belonging to this genus
occurasfreecellsinthesoilorassociatedwiththeroots,stems,leavesandseedsmainly
of cereals and forage grasses, although they have also been isolated from coconut
plants,vegetables,fruits,legumesandtuberplants.Notwithstanding,rootnodulesare
not inducedbystrains inthisgenus.16SrDNAsequencingstudies showahighdegree
of relatedness among Azospirillum species and that they form a cohesive phylogenetic
cluster within the Alphaproteobacteria [70]. Notwithstanding, strains of Azospirillum
may still be misnamed in other genera, as shown by the recent reclassification of
Rosseomonas fauriae as Azospirillum brasilense [71].
4.4.2
Gammaproteobacteria
4.4.2.1 Enterobacteria
The family Enterobacteriaceae, in the class Gammaproteobacteria, encompass a wide
range of microorganisms including 42 genera in the last edition of Bergey s Manual
of Bacteriology [30], but half of the isolates of new or unusual Enterobacteriaceae seem
to be misidentified [72]. To avoid misidentification of species in future, Paradis et al.
[73] propose the classification of species within Enterobacteriaceae by studying the
genes encoding the elongation factor Tu and F-ATPase-b-subunit additionally to the
gene encoding the 16S small ribosomal subunit.
The genera within the family Enterobacteriaceae that feature members described
as plant growth promoting bacteria are Citrobacter, Enterobacter, Erwinia, Klebsiella,
Kluyvera, Pantoea and Serratia although some of these genera also contain species
reported to be plant pathogens, for example Erwinia carotovora. The seven genera
mentioned above have undergone changes in their taxonomy in the time elapsed
between the two most recent releases of Bergey s Manual of Systematic Bacteriology.
These seven genera are reviewed below.
4.4.2.2 Citrobacter
Eight new Citrobacter species have been described out of already known species
within this same genus, and two other species have been found to be synonyms of C.
koseri [74].
4.4.2.3 Enterobacter
This genus contains the Enterobacter agglomerans group, which was extremely heterogeneous.
Strains previously included in the Enterobacter agglomerans group have
been proposed to be relocated into the genera Erwinia, Leclercia and Pantoea. Another
species, Enterobacter intermedius, was first described as a senior subjective
synonym for the species Kluyvera cochleae as shown by DNA–DNA hybridization [74]
but was later transferred to the genus Kluyvera as the species K. intermedia comb.
nov., and K. cochleae was demonstrated to be a later synonym of K. intermedia [75].