the activities of anti-oxidative defense-related enzymes like glutathione reductase
and superoxide dismutase in hybrid poplar seedlings (Weyens et al. 2012 ),
GFP-tagging of W619 had negative effects on plant growth and health since the
W619 +gfpstrain significantly lagged in the before-mentioned parameters. They
also found that GFP-tagging effects the endophytic colonization ability of W619.
These results are contradictory to what was observed whenAzospirillum brasilense
8-I (Rodriguez et al. 2006 ) andPaenibacillus polymyxaP2b-2R were tagged with
GFP (discussed in Sect.6.3.1), thus casting doubts on this phenomenon of reduced
microbial efficiency after GFP-tagging. In a recent study, a novel endophytic
bacterium,Pseudomonas populisp. nov. (KBL-4-9T), was isolated from stem tis-
sues ofPopulus euphraticatrees (Anwar et al. 2016 ). The plant growth-promoting
traits of this bacterial endophyte have not been fully determined yet. Since the
complete genome cottonwood was sequenced (Tuskan et al. 2006 ), there has been
an increased interest in elucidating the interaction of bacterial endophytic and
rhizospheric communities withPopulustrees at the molecular and genetic level
(Schaefer et al. 2013 , 2016 ), which will obviously help in understanding their
interaction with other tree species.
6.3.3 Picea
Piceagenus is most closely related toPinussince they belong to the same family,
Pinaceae. Tree species belonging toPiceaare usually found in northern temperate
and boreal regions and are commonly known as spruce trees. Commercially
important species like black spruce (Picea mariana), engelmann spruce
(P. engelmannii), sitka spruce (P. sitchensis), white spruce (P. glauca), Norway or
alpine spruce (P. alpestrisandP. abies), and Siberian spruce (P. obovataand
P. omorika) are generally found in northern hemisphere (North America, North
Europe, and Eurasia) (Parish and Thomson 1994 ). Evidence of plant
growth-promoting rhizobacteria (PGPR) in spruce (hybrid spruce) was reported by
O’neill et al. ( 1992 ) and Chanway and Holl (1993a,b). But thefirst reported spruce
endophyte,Pseudomonassp. Ss2, was isolated from roots of hybrid spruce (Picea
glauca(Moench) Voss xPicea engelmanniiParry) seedlings naturally regenerating
near Salmon Arm, BC, Canada (51°04′N, 119°26′W, 1250 m elevation) (Shishido
and Chanway 1999 ). Rifamycin-resistant derivative of Ss2 was generated and the
resulting strain was designated as Ss2-RN. Shishido et al. (1996b) inoculated
Ss2-RN into hybrid spruce (Picea glaucaxP. engelmannii) seedlings and grew
them in the greenhouse for 15 weeks. Inoculate hybrid spruce seedlings increased
root weight by 19%, shoot weight by 10%, and seedling height by 6% in com-
parison to the non-treated (control) seedlings. In this study, it was also observed that
bacterial inoculation had no effect on the mycorrhizal status of seedlings and
growth promotion achieved by bacterial inoculation was similar in mycorrhizal and
non-mycorrhizal spruce seedlings. It was also reported that strain Ss2-RN performs
better when inoculated into spruce ecotype that originated from the same
6 Beneficial Effects of Bacterial Endophytes... 123