dominant endophytic species areBacillusspecies (Gupta et al. 2002 ; Bacon and
Hinton 2007 ).
Most culturable endophytic species belongs to the phylum Proteobacteria, class
Firmicutes, Gram-negative and also Bacteroides are less common (Reinhold-Hurek
and Hurek 2011 ). This suggests that 50% of endophytic bacterial communities
could be identified and others are over represented based on their capability to grow
on synthetic medium. To obtain clear picture of the diversity of endophytic
microorganisms, recently a number of studies have been concentrated on identifi-
cation of unculturable endophytes using novel metagenomic analysis approaches
(Akinsanya et al. 2015 ). To this, direct amplification of microbial DNA from plant
tissue samples and application of modern bioinformatics tools allow analysis of a
bacterial community composition and its phylogenetic structure inside plant organs
or tissues (Chun et al. 2007 ; Manter et al. 2010 ; Sessitsch et al. 2012 ) examined the
structure and functions of genes of bacterial endophytes colonizing rice roots
in vivo. The results showed the population was superior by members of
c-proteobacteria, comprising mostly of enterobacter-related endophytes. Whereas
(Tsurumaru et al. 2015 ) studied that endophytic colonization on tap root of sugar
beet (Beta vulgariL.) is a metagenome, who observed that alphaproteobacteria are
dominant, followed by the actinobacteria and the betaproteobacteria. Maropola
et al. ( 2015 ) analysed metagenomic study of the sorghum root and stem micro-
biome and revealed that both were dominated by bacterial pathogens such as
Agrobacterium, Erwinia, Herbaspirillum, Microbacterium, Pseudomonas,
SphingobacteriumandStenotrophomonasspecies.
7.3 Plant Colonization with Endophytes
The apical root zone having thin-walled surface of root cells includes cell elon-
gation and the root hair zone (zone of active penetration), and the basal root zone
with small cracks are the preferable sites of bacterial attachment and subsequent
entry caused by the emergence of lateral roots (zone of passive penetration)
(Fig.7.1). For active invasion, endophytic bacteria must bear the abilities of pro-
duction of cellulolytic enzymes to hydrolyze exodermal cell walls of plants.
As earlier, the density of bacteria in the rhizosphere and rhizoplane is always
higher than in the soil which lacks substances secreted from the roots of plants
(Rosenblueth and Martinez-Romero 2006 ) for example, with seed germination,
amount of carbon and nitrogen compounds are excreted into the surrounding
environment that invites a large population of microorganisms (Okon and
Labandera-Gonzales 1994 ). The root exudates contain that colonize different bac-
terial genera and they differ normally according to plant species (Bisseling et al.
2009 ).
Root colonization or rhizospheric beneficial microorganisms are familiar bio-
control agents and plant growth promoters. They have indirect positive effects on
plants with their mechanistic behaviour that mainly includes antagonism against
136 A. Muthukumar et al.