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

diversity of the cycad cyanobionts, and only few studies used molecular techniques
for discriminating between different cyanobionts [63]. In addition, the molecular
techniques used by different authors are very diverse, and it is difficult to compare
such methods, for example RFLP and PCR amplification techniques of different
segments of the genome. Additionally, several studies analyzed cultured symbionts
which, in the case of Azolla, hasbeenshowntobeproblematic,sincethe
cyanobiont obtained in culture is not the same organism as the main strain in
symbiosis [63].
The molecular techniques most recently used to classify cyanobacteria have been
DNA–DNA hybridization and hybridization of highly repetitive (STRR) DNA sequences
[62] and amplification of the tRNA Leu (UAA) intron from the cyanobacterial
symbionts of cycads.
4.3.2.3 Gluconacetobacter
This genus, included in the family Acetobacteraceae of the class Alphaproteobacteria
is composed of obligate endophytic bacteria. Gluconacetobacter diazotrophicus colonizes
sugarcane roots, stem and leaves, where it is present in the intercellular space
of parenchyma, and is considered an obligate endophyte [57]. G. diazotrophicus,
originally described as Acetobacter diazotrophicus and later transferred to the genus
Gluconoacetobacter [64] which was subsequently corrected to Gluconacetobacter [65],
was the first nitrogen-fixing Acetobacteraceae species described [66].
Two other nitrogen-fixing species have been described in this genus, G. johannae
and G. azotocaptans [58]. The distribution of these species ranges from sugar-rich
plants such as sugarcane, sweet sorghum, sweet potato and pineapple to sugar-poor
plants such as coffee and ragi [66] and more recently from Kombucha tea [67].
4.4
Asymbiotic Plant Growth Promoting Bacteria
4.4 Asymbiotic Plant Growth Promoting Bacteriaj69
4.4.1
Alphaproteobacteria: Genera Acetobacter, Swaminathania and Azospirillum
4.4.1.1 Acetobacter and Swaminathania
The six nitrogen-fixing bacterial species so far described in the family Acetobacteraceae
belong to the genera Acetobacter, Gluconacetobacter and Swaminathania. The
genus Gluconacetobacter has been briefly described above. Of the two species of
Acetobacter, one is A. nitrogenifigens [68], a new species isolated from Kombucha tea.
The other nitrogen-fixing species in this genus is A. peroxydans [69] and this has
lately been reported as a diazotroph species after a study on strains associated with
wetland rice [66].
Recently, a novel genus has been described from strains isolated from the rhizosphere,
roots and stems of salt-tolerant, mangrove-associated wild rice (Porteresia
coarctata Tateoka) [22]. The isolates were able to fix nitrogen and solubilize phosphate
in the presence of NaCl and belonged to a well-defined taxon, for which the species
name proposed was Swaminathania salitolerans gen. nov., sp. nov. [22].