38
A. Dahiya et al.
3.3 Colonization of Endophytic Microbes in Tissues
The endophytic colonization of plants is a complex process that requires the capacity
of microorganisms to compete in the rhizosphere soil and find a niche to communicate
and interact with the plant tissues. Endophytic microbes have been isolated and characterized from a diverse type of plant hosts including agronomic crops, prairie plants,
plants growing in extreme environments, wild and perennial plants (Zinniel et al.
2002; Nair and Padmavathy 2014; Yuan et al. 2014). Different plant parts including
roots, stems, leaves, seeds, fruits, tubers, ovules, and nodules, have the presence of
endophytes but the population of endophytes is larger in the roots as compared to
above-ground tissues (Rosenblueth and Martínez-Romero 2006; Senthilkumar et al.
2011). Microorganisms residing in the rhizosphere have the potential to enter and
colonize the plant roots and this micro-ecosystem has been widely known as one of
the primary sources for endophytic colonization (Hallmann et al. 1997). Therefore,
organisms in the rhizosphere, either potentially beneficial or pathogenic, are highly
competitive in colonizing plant tissues and assimilate nutrients, that possibly affect
plant growth and development (Haas and Keel 2003).
Endophytes employ different mechanisms to gain entry into the plant tissues
(Truyens et al. 2014). They usually enter the plants through the root zone but the aerial
parts of the plants, including stems, leaves, flowers, and cotyledons, may also contain
endophytic microorganisms (Zinniel et al. 2002). The most common mode of entry
of endophytic microbes into plant tissues is through primary and lateral root cracks,
and diverse tissue wounds occurring as a result of plant growth (Sprent and de Faria
1998; Sorensen and Sessitsch 2015). The leaves (stomata) and young stems are the
other sites through which endophytes may enter (Roos and Hattingh 1983) through
lenticels, which are usually present in the periderm of stems and roots (Scott et al.
1996), and by germinating radicles (Gagné et al. 1987) (Fig. 3.1). Bacteria can also
enter via the emergence of lateral roots or root hair cells (Huang 1986). Hallmann et al.
(1997) demonstrated that entry of the endophytic bacterium Enterobacter asburiae
JM22 in cotton plants was assisted by the ability of the bacterium to hydrolyze plant
cell wall-bound cellulose.
The endophytic colonization of the host plant by the bacteria is determined by
various bacterial traits. These traits are collectively referred to as colonization traits.
Moreover, endophytic colonization involves a suite of environmental and genetic
factors that allow a bacterium to enter the plant endosphere (Compant et al. 2010).
The colonization process involves complex communication or signaling between
the microbe and the host plant particularly root. It requires recognition of specific
compounds in the root exudates by the endophytic bacteria (de Weert et al. 2002;
Rosenblueth and Martínez-Romero 2006). Plants produce these root exudates to
interact with beneficial bacteria for their ecological advantage (Compant et al. 2005).
Once inside the roots, endophytic bacteria can now systemically infect the adjacent
plant tissues.
A. Dahiya et al.
3.3 Colonization of Endophytic Microbes in Tissues
The endophytic colonization of plants is a complex process that requires the capacity
of microorganisms to compete in the rhizosphere soil and find a niche to communicate
and interact with the plant tissues. Endophytic microbes have been isolated and characterized from a diverse type of plant hosts including agronomic crops, prairie plants,
plants growing in extreme environments, wild and perennial plants (Zinniel et al.
2002; Nair and Padmavathy 2014; Yuan et al. 2014). Different plant parts including
roots, stems, leaves, seeds, fruits, tubers, ovules, and nodules, have the presence of
endophytes but the population of endophytes is larger in the roots as compared to
above-ground tissues (Rosenblueth and Martínez-Romero 2006; Senthilkumar et al.
2011). Microorganisms residing in the rhizosphere have the potential to enter and
colonize the plant roots and this micro-ecosystem has been widely known as one of
the primary sources for endophytic colonization (Hallmann et al. 1997). Therefore,
organisms in the rhizosphere, either potentially beneficial or pathogenic, are highly
competitive in colonizing plant tissues and assimilate nutrients, that possibly affect
plant growth and development (Haas and Keel 2003).
Endophytes employ different mechanisms to gain entry into the plant tissues
(Truyens et al. 2014). They usually enter the plants through the root zone but the aerial
parts of the plants, including stems, leaves, flowers, and cotyledons, may also contain
endophytic microorganisms (Zinniel et al. 2002). The most common mode of entry
of endophytic microbes into plant tissues is through primary and lateral root cracks,
and diverse tissue wounds occurring as a result of plant growth (Sprent and de Faria
1998; Sorensen and Sessitsch 2015). The leaves (stomata) and young stems are the
other sites through which endophytes may enter (Roos and Hattingh 1983) through
lenticels, which are usually present in the periderm of stems and roots (Scott et al.
1996), and by germinating radicles (Gagné et al. 1987) (Fig. 3.1). Bacteria can also
enter via the emergence of lateral roots or root hair cells (Huang 1986). Hallmann et al.
(1997) demonstrated that entry of the endophytic bacterium Enterobacter asburiae
JM22 in cotton plants was assisted by the ability of the bacterium to hydrolyze plant
cell wall-bound cellulose.
The endophytic colonization of the host plant by the bacteria is determined by
various bacterial traits. These traits are collectively referred to as colonization traits.
Moreover, endophytic colonization involves a suite of environmental and genetic
factors that allow a bacterium to enter the plant endosphere (Compant et al. 2010).
The colonization process involves complex communication or signaling between
the microbe and the host plant particularly root. It requires recognition of specific
compounds in the root exudates by the endophytic bacteria (de Weert et al. 2002;
Rosenblueth and Martínez-Romero 2006). Plants produce these root exudates to
interact with beneficial bacteria for their ecological advantage (Compant et al. 2005).
Once inside the roots, endophytic bacteria can now systemically infect the adjacent
plant tissues.