6.3.2 Populus
Populusis a genus of deciduousflowering plants mostly native to the northern
hemisphere. It includes commercially important species like poplar, cottonwood,
and aspen. Thefirst tree species whose full genome was sequenced belong to this
genus (Black cottonwood). The genome of black cottonwood (Populus tri-
chocarpa) is four times larger than the genome of the first plant sequenced,
Arabidopsis thaliana(Tuskan et al. 2006 ). Apart from traditional varieties of poplar
and cottonwood trees, many hybrid varieties have been developed.Populusspp. are
known for rapid growth, deep root network, and ability to grow in nutrient-poor
sites (Stettler et al. 1996 ). They are economically important and are grown in
short-rotation plantations for the production of pulp and paper, lumber, and fuel
(Doty et al. 2009 ). Thefirst discovery about the presence of a diazotrophic bacterial
endophyte in aPopulussp. was reported by Doty et al. ( 2005 ). In this study, clones
of hybrid cottonwood (Populus trichocarpaP. deltoides) were evaluated for the
presence of bacterial endophytes in stem tissues.Rhizobium tropiciwas the most
common bacterial endophyte found in all clones of hybrid cottonwood. Although
this bacterial species is well known for its nodule-forming ability and diazotrophic
trait in legumes (Perret et al. 2000 ), this study reported its endophytic nature in a
non-legume host.Populusspp. are also known for their phytoremediation capa-
bilities. Endophytic bacteria belonging to genusPseudomonaswere isolated from
xylem sap of hybrid poplar trees growing on a phytoremediation site near a motor
factory in Genk, Belgium (Germaine et al. 2004 ). Isolated strains were tested for
their ability to solubilize phosphorus, produce IAA, act as biocontrol agents, and
resist heavy metal. Selected strains were tagged with GFP to visualize endophytic
colonization sites. GFP-tagged strains were found to colonize inner cortex and
xylem tracheid cells in the root and intercellular spaces of root xylem cells when
visualized with CLSM. Although stem and leaf colonization was not observed with
CLSM but through culture-based technique, it was determined that these
Pseudomonasstrains colonize root, stem/sap, and leaf tissues with population
density ranging from 10^3 to 10^6.
Poplar trees harbor diverse bacterial endophytes in different parts and each
bacterial community plays its own role in enhancing the growth and protecting the
tree against pathogens. The diversity of endophytic bacterial communities residing
insidefield-grown poplar trees was evaluated by Ulrich et al. (2008b). Aerial parts
(leaves and branch sections) of four hybrid poplar clones were evaluated for the
presence of endophytic bacteria. Bacterial endophytes belonging to 53 different
genera were isolated including Curtobacterium, Plantibacter, Pseudomonas,
Xanthomonas,Sphingomonas,Methylobacterium,Pedobacter, andPaenibacillus
and the most abundant genera among all clones of hybrid poplar were
CurtobacteriumandPseudomonas. Several bacterial endophytes were also isolated
from young poplar seedlings (black cottonwood) growing in Three Forks Park
alongside the Snoqualmie River in Western Washington state, USA (Doty et al.
2009 ). Isolates belonging to the genusBurkholderia,Rahnella, andAcinetobacter
6 Beneficial Effects of Bacterial Endophytes... 121