et al. 1997 , 1998 ). The major mechanism of endophytic bacteria in plant disease
control is—(i) to assist nutrient availability and uptake (ii) to enhance stress tol-
erance and (iii) to provide disease resistance (Ryan et al. 2008 ; Hamilton et al.
2012 ).
Endophytic bacteria are correlated with the enhanced plant growth by the pro-
duction of hormones that increase accessibility of nutrients, such as nitrogen,
potassium and phosphorus (Glick 2012 ). While induced disease resistance activities
are allied with the abilities to produce secondary metabolites, such as antibiotics or
chitinase enzyme, which can inhibit growth of plant pathogens. Hence they act as
biocontrol agents (Christina et al. 2013 ; Wang et al. 2014 ). Endophytic bacteria can
also induce seedling emergence and stimulate plant growth (Chanway 1997 ) under
stress conditions (Bent and Chanway 1998 ). Further, endophytic bacteria have the
capacity to cope with phytopathogenic fungi with induced systemic resistance
(ISR) (Pieterse et al. 2014 ). Due to their beneficial function such as plant growth
promotion and disease control, endophytes can be used in the form of
bio-formulations (seed treatment, soil application and seedling dip) in agriculture.
7.2 Nature and Occurrence
Various groups of endophytic bacteria signify their role in ecosystems and plant
physiology. These bacteria colonize all plant compartments, generally the inter-
cellular and intracellular spaces of inner tissues. Initial studies on diversity of
endophytic bacteria were mostly based on characterization of isolates obtained from
the plants either from rhizosphere/phyllosphere region after surface disinfection.
Lodewyckx et al. ( 2002 ) characterized methods for the isolation and he found that
81 bacterial species which form endophytic associations with plants. The endo-
phytic bacteria and plant association include a vast diversity of bacterial taxa and
host plant. The early studies on composition of endophytic communities revealed
that different plant hosts harbour similar community of bacterial endophytes (Mundt
and Hinkle 1976 ). The genera ofBacillusandPseudomonasare identified as fre-
quently occurring bacteria in agricultural crops (Seghers et al. 2006 ; Souza et al.
2013 ). The presence of different endophytic species depends mostly on plants biotic
and abiotic environmental factors. A single host plant species comprises several
genera and species of endophytes but the tissue type of plant or season of isolation
may determine the extent of the endophytic population (Kuklinsky-Sobral et al.
2004 ; Rosenblueth and Martinez-Romero 2006 ). An extensive research work
conducted on bacterial endopyte communities revealed that although endophytic
bacteria colonize entire plant, the roots usually contain higher number of species.
Endophytic species mostly belong to thea,b, andc-proteobacteria subgroups and
are closely related to epiphytic species (Kuklinsky-Sobral et al. 2004 ). Interestingly,
thec-proteo bacteria group is the most diverse and dominant. It has been reported
that most of Gram-negative endophytes act as agents of biological control
(Kobayashi and Palumbo 2000 ), while among the Gram-positive bacteria, the
7 Role of Bacterial Endophytes in Plant Disease Control 135