possessednifH gene and were able to reduce significant amounts of acetylene in
ARA. One of the strains WPB (Burkholderia vietnamiensis) was able to produce
ethylene (concentration = 68.4 ppm) enormously higher than other strains when
exposed to acetylene for 72 h. Phylogenetic analysis ofrecAgene andnifHDKgene
cluster of WPB strain was performed in a subsequent study (Xin et al. 2009 ). In this
study, it was also determined that WPB strain can produce 5.7 mg IAA/g dry cell
after 7-day incubation with 0.1% L-tryptophan but does not produce IAA when
L-tryptophan was not applied. Tryptophan generally acts as a precursor for the
release of IAA (Omay et al. 1993 ; Hung et al. 2007 ; Taghavi et al. 2009 ), but some
microbes lack the ability to synthesize tryptophan, essential for protein synthesis,
and must obtain it from the plant (Radwanski and Last 1995 ). Thus, it can be
inferred that a mutually advantageous plant–microbe interaction occurs in this case,
where the plant provides tryptophan for WPB and WPB, in return, converts extra
tryptophan to IAA for promoting the plant growth. Strain WPB and other dia-
zotrophic strains isolated by Doty et al. ( 2005 , 2009 ) were used in a glasshouse and
afield experiment on black cottonwood and hybrid cottonwood (Knoth et al. 2014 ).
They tested each strain individually as well as collectively by making consortiums.
Diazotrophic bacterial endophytes significantly increased the biomass of black
cottonwood and cottonwood hybrid in both glasshouse andfield experiments and
inoculation with microbial consortia made of many strains was more successful
than single-strain inoculation. Inoculated cottonwood seedlingsfixed up to 65% of
N directly from the atmosphere in this study, clearly exhibiting the role of dia-
zotrophic bacterial endophytes in promoting poplar tree growth by providing
substantial N nutrition.
Pseudomonasspp. are one of the most common bacterial endophytes found in
Populustrees (Ulrich et al.2008b; Doty et al. 2009 ; Gottel et al. 2011 ; Izumi 2011 ).
Taghavi et al. ( 2005 , 2009 ) isolated several endophytes from the root and shoot
tissues of hybrid poplar and used three representative strains,Serratia protea-
maculans568,Enterobactersp. strain 638, andPseudomonas putidaW619, for
further evaluation of their endophytic and growth-promoting properties.
GFP-tagged strains colonized root surface and interior of hybrid poplar and one of
the strains,Enterobactersp. strain 638, significantly increased shoot biomass in
greenhouse experiment (Taghavi et al. 2009 ). In a subsequent study,P. putida
W619 was used to construct a trichloroethene (TCE)-degrading strain W619-TCE
(Taghavi et al. 2005 ; Weyens et al. 2009 , 2010 ).P. putidaW619-TCE inoculation
of hybrid polar trees reduced TCE evapotranspiration significantly and promoted
plant growth (Weyens et al. 2010 ). Infield conditions (TCE contaminated sites),
TCE evapotranspiration from hybrid poplar was reduced by 90% by inoculation
with this strain (Weyens et al. 2009 ), thus clearly establishing its phytoremediation
characteristics. Weyens et al. ( 2012 ) used the GFP-tagged derivative of strain W619
(Taghavi et al. 2009 ) to compare the colonization ability of wild-type and
GFP-tagged strain W619 along with investigating the morphological, physiological,
and biochemical parameters so as to compare the PGP ability of the two strains.
Although wild-type W619 was able to promote plant growth by producing IAA and
cytokinins, increasing root and shoot mass, reducing stomatal resistance, decreasing
122 A. Puri et al.