disease. Even thefirst microorganism reported to colonize internal tissue of plant
leaves was a pathogenic fungus (de Bary 1866 ). But now, it is widely known that
microorganisms can colonize internal tissues of plants and establish beneficial
symbiotic interaction with the host plant. Such microbes are known as endophytes.
Literally taken, endophyte means“within plant”(Chanway 1996 ). Although many
authors have defined the term endophyte, but in this chapter, we will use the term
defined by Chanway et al. ( 2014 ). According to Chanway et al. ( 2014 ), bacterial
endophytes are“bacteria that can be detected at a particular moment within the
tissue of apparently healthy plant hosts without inducing disease or organogenesis.”
The occurrence of bacterial endophytes wasfirst reported in internal tissues of
healthy potato plant (Trevet and Hollis 1948 ). Since then, most studies have been
focused on isolating and evaluating the benefits of bacterial endophytes in agri-
cultural plants (reviewed by Hallmann et al. 1997 ; Kobayashi and Palumbo 2000 ;
Sturz et al. 2000 ; Suman et al. 2016 ). Although there is huge literature about
endophytic fungi in forest ecosystem (reviewed by Doty 2011 ), studies of bacterial
endophytes in forest tree species are rather limited but their importance should not
be underrated.
6.2 Bacterial Endophytes in Forest Tree Species
Forest trees can provide unique ecological conditions for bacterial endophytes since
they have larger biomass and exist for a longer period in terrestrial ecosystems as
compared to agricultural plants (Izumi 2011 ). Bacterial endophytes have only been
reported in very limited host tree species including pine, spruce, poplar, oak, cedar,
and willow. The most common bacterial endophytes isolated from forest trees
belong to the genus Acinetobacter, Burkholderia, Bacillus, Enterobacter,
Methylobacterium, Microbacterium, Pseudomonas, Paenibacillus, Rahnella,
Sphingomonas, andXanthomonas(Izumi 2011 ; Pirttilä 2011 ). The diversity of
bacterial endophytes found in forest ecosystem has been reviewed by Izumi ( 2011 ).
Endophytes enhance the growth of forest tree species by various direct and indirect
mechanisms. Direct mechanisms involve production of phytohormones like cy-
tokinins (Pirttilä 2011 ), auxins (Taghavi et al. 2005 ; Madmony et al. 2005 ), gib-
berellins (Bottini et al. 2004 ), and nitrogen (N)fixation (Bal and Chanway2012a,b;
Anand and Chanway 2013b; Anand et al. 2013 ; Tang et al. 2017 ). Indirect
mechanisms involve suppression of pathogens and improvement of the mutualistic
relationship of a mycorrhizae and plant host (Anand et al. 2006 ). It is believed that
most interactions between plants and beneficial bacteria occur in roots of host plant
but shoots represent a unique ecological niche where endophytes can carry out
major plant-beneficial activities. Generally, bacterial endophytes in forest trees have
been isolated from shoot tips,flowers, pollens or seeds, and seedlings (Pirttilä
2011 ). Although shoot endophytes provide similar benefits as provided by root
endophytes, they have also been reported to induce plant growth through other
mechanisms like the production of adenine derivatives and vitamin B 12 (Pirttilä
112 A. Puri et al.