2011 ). Endophytic bacterial colonization sites in tree shoots and their potential
growth-promoting effects have been reviewed in detail by Pirttilä( 2011 ).
6.2.1 Diazotrophic Bacterial Endophytes in Forest Tree
Species
N-fixing bacteria also known as“diazotrophic bacteria”are well known for their
ability tofix N biologically. Apart from root nodule-forming diazotrophs living in
association with leguminous plants, there are bacterial species that canfixNin
association with non-leguminous plant species. The presence of diazotrophic bac-
teria in non-leguminous plants wasfirst detected by Brazilian researchers in the
rhizosphere of sugarcane (Döbereiner and Alvahydo 1959 ;Döbereiner 1961 ). In
subsequent studies, it was determined that diazotrophic bacteria in rhizosphere
contributed only small amounts of N to the sugarcane plants and diazotrophic
bacteria living in internal tissues of stem and rootsfix significant amounts of N from
the atmosphere (Cavalcante and Döbereiner 1988 ; Boddey et al. 1991 ; Stephan
et al. 1991 ). Cavalcante and Döbereiner ( 1988 ) isolated a diazotrophic bacteria,
Saccharobacter nitrocaptans(renamed toAcetobacter diazotrophicus(Gillis et al.
1989 ), and then toGluconacetobacter diazotrophicus(Yamada et al. 1997 )), from
internal tissues of sugarcane. Such diazotrophs were designated as diazotrophic
bacterial endophytes (Döbereiner 1992 ) and were detected in many other agricul-
tural crops like corn (Padda et al. 2017 ; Puri et al. 2015 ,2016b), rice (Baldani et al.
2000 ), wheat (Sabry et al. 1997 ), and canola (Padda et al.2016a,b; Puri et al.
2016a). Diazotrophic bacterial endophytes have been detected in stem tissues of
forest trees like poplar (Populus trichocarpa) (Ulrich et al.2008a; Scherling et al.
2009 ; Doty et al. 2009 ; Xin et al. 2009 ; Knoth et al. 2014 ), willow (Salix sitchensis)
(Doty et al. 2009 ), lodgepole pine (Pinus contorta) (Bal et al. 2012 ; Bal and
Chanway2012a; Anand et al. 2013 ; Tang et al. 2017 ; Yang et al. 2016 ), and
western red cedar (Thuja Plicata) (Bal and Chanway2012b; Anand and Chanway
2013b). Diazotrophic bacterial endophytes havefixed significant amounts of N
from the atmosphere (in some cases up to 79%) after establishing a symbiotic
relationship with these tree species.
6.3 Plant Growth Promotion by Bacterial Endophytes
in Forest Tree Species
In this section, studies highlighting the beneficial effects of bacterial endophytes in
forest trees have been reviewed. We have compiled an elaborative list of bacterial
endophytes that have been isolated from forest trees and have shown plant
6 Beneficial Effects of Bacterial Endophytes... 113