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

88j 5 Diversity and Potential of Nonsymbiotic Diazotrophic Bacteria in Promoting Plant Growth
acid is produced, so the bacterium is named acetic acid bacteria. It is phylogenetically
described as a-proteobacteria. Gluconoacetobacter diazotrophicus is isolated from
sugarcane (Saccharum officinarum) roots and stems and endophytically fixes
nitrogen.
The family Acetobacteraceae includes the genera Acetobacter, Gluconobacter,
Gluconoacetobacter and Acidomonas. Based on 16S rRNA sequence analysis, the
name Acetobacter diazotrophicus has been changed to Gluconoacetobacter diazotrophicus
[66]. In addition to G. diazotrophicus, two more diazotrophs, G. johannae and
G. azotocaptans have been included in the list [64]. The genetic diversity of
G. diazotrophicus isolated from various sources does not exhibit much variation
[67]. However, Suman et al. [68] found that the diversity of the isolates of G. diazotrophicus
by RAPD analysis was more conspicuous than that reported on the basis of
morphological and biochemical characters. On the basis of DNA fingerprinting
studies, the existence of genetically distinct G. diazotrophicus strains in sugarcane
cultivars has been reported from Louisiana.
Azoarcus Azoarcus gen. nov., an aerobic/microaerophilic nitrogen-fixing bacterium,
was isolated from surface-sterilized tissues of Kallar grass (Leptochloa fusca (L.)
Kunth) [69] and can infect roots of rice plants as well. Kallar grass is a salt-tolerant
grass used as a pioneer plant in Pakistan on salt-affected low-fertility soils. Repeated
isolation of one group of diazotrophic rods [70] from Kallar grass roots and the
results of polyphasic taxonomy led to the identification of genus Azoarcus, with two
species, A. indigens and A. communis, and three additional unnamed groups, which
were distinct at species level. Nitrogen fixation by Azoarcus is extremely efficient
(specific nitrogenase activity, one order of magnitude higher than that found for
bacteroids). Such hyperinduced cells contain tubular arrays of internal membrane
stacks that can cover a large proportion of the intercellular volume. These structures
are considered vital for high-efficiency nitrogen fixation [69].
Burkholderia Burkholderia is identified as b-proteobacteria. Some strains have the
ability to fix atmospheric nitrogen. Presently, the genus Burkholderia includes 30
species with valid published names [71], with Burkholderia cepacia as the typical
species. For a long time, nitrogen-fixing ability in bacteria of the genus Burkholderia
was recognized only in the species Burkholderia vietnamiensis [72]. Recently,
two nodulating strains recovered from legume plants were assigned to the genus
Burkholderia as two novel species, Burkholderia tuberum and Burkholderia phymatum
[73].
Bacillus Bacillus exists as Gram-positive rods. They are endospore formers that can
allow them to survive for extended periods under stressed environmental conditions.
Bacillus is considered a well-established PGPR [74]. Some Bacillus species
can fix nitrogen [75]. Many bacilli can produce antibiotics including bacitracin,
polymyxin, tyrocidin, gramicidin and circulin. In most cases, antibiotics are released
during sporulation, when the culture enters the stationary phase of growth and after
it is committed to sporulation [76].