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

72j 4 A Review on the Taxonomy and Possible Screening Traits of Plant Growth Promoting Rhizobacteria
4.4.2.7 Pantoea
The genus Pantoea contains strains previously allocated in the Enterobacter agglomerans
complex and species formerly named as Erwinia (E. herbicola, E. lathyri, E. ananas,
E.uredovora, E.milletiae andE.stewartii),andfutureprospectspoint tothepossibilityof
more species within the genera Enterobacter and Erwinia being relocated to this genus.
4.4.2.8 Serratia
Most species currently known in the genus Serratia have been shown to be homogeneous
and discrete genomospecies: S. entomophila, S. ficaria, S. marcescens, S.
odorifera, S. plymuthica and S. rubidaea. S. liquefaciens sensu lato was shown to be
heterogeneous and later found to be composed of three genomospecies: S.
liquefaciens sensu stricto, S. proteamaculans and S. grimessi. Another genomospecies
composed of a group of strains referred to as Citrobacter-like was described as S.
fonticola; however, this species does not possess the key characteristics of the genus
Serratia, Serratia phages that are active on strains of any Serratia species are inactive
on all S. fonticola strains tested, and so are bacteriocins of Serratia [80]. Despite these
data, the 16S rRNA gene sequence of S. fonticola branches within the psychrotolerant
Serratia cluster (S. liquefaciens, S. proteamaculans, S. grimesii and S. plymuthica), and
this justifies the inclusion of S. fonticola in the genus Serratia [80].
4.4.2.9 Pseudomonas
Strains belonging to the genus Pseudomonas might be plant pathogenic or PGPR.
The latter do not form nodules but proliferate in the surroundings of the roots, using
the root exudates as sources of carbon and energy. In exchange, these strains protect
the plant from other plant pathogenic microorganisms due to their production of
siderophores andother molecules[5,18]. The taxonomyof thestrains allocatedtothe
genus Pseudomonas was reviewed by De Vos et al. [35], Anzai et al.[81]andPicardand
Bosco [6]. All these studies uncovered groups of strains belonging to the Alphaproteobacteria,
Betaproteobacteria and Gammaproteobacteria. The difficulty of obtaining
sound groupings using the 16S rDNA gene as a phylogenetic marker seems to be
due to the presence of several different copies of this gene in the same strain [82,83].
Notwithstanding, a division of the genus into five RNA similarity groups has been
proposed and confirmed by workers in many different laboratories [83].
4.4.2.10 Azotobacter (Azomonas, Beijerinckia and Derxia)
The genus Azotobacter, inthefamilyPseudomonadaceae of the Gammaproteobacteria,
is composed of bacteria that promote plant growth mainly due to their ability
to fix dinitrogen from the atmosphere and do not nodulate plants [20,84]. The genus
has undergone revision between the last two editions of the Bergey s Manual of
Systematic Bacteriology andhasbeenmovedfromitspreviousfamily,Azotobacteraceae
[85] to the family Pseudomonadaceae after studies on the phylogeny of its
members. The genus Azomonas, included in the former family Azotobacteraceae,
has also been moved into the family Pseudomonadaceae with a comment on the
phylogenetic heterogeneity of its members, which will probably result in their
separation into more than one genus [86]. It was once proposed that the genera