2 Bioefficacy of Endophytes in the Control of Plant Diseases
17
percentage of protection on black pepper shoots against Phytophthora capsici, especially when these compounds were applied at low concentrations (Agisha et al.
2019). The production of VOCs was also evidenced and identified in two isolates
from cocoa that showed antagonism against the causal agent of a black pod, Phytophthora palmivora, both in vitro and in planta (Alsultan et al. 2019). The role of VOCs
in plant protection is not only due to the direct effect on pathogen’s growth as there is
evidence that 2,3-butanediol and acetoin can induce systemic resistance to pathogens
(Ryu et al. 2004).
2.2.2 Induction Disease Resistance in Plants
The recognition of microbial cell components and/or their metabolites can induce
in the host’s physiological state allowing them to respond faster and/or to a greater
extent to future pathogenic attacks. This phenomenon is called induced systemic
resistance (ISR) and shares characteristics with another type of systemic resistance
triggered by a previous attack of the necrosis-producing pathogen (SAR, systemic
acquired resistance). There are two possible molecular mechanisms activated during
ISR. Thus, endophyte-inoculation can induce the expression of defense-related genes
per se, or on the other hand, the presence of beneficial microorganisms primes plants
for enhanced defense responses. In primed plants, defense responses are not activated
directly but are potentiated upon pathogen attack, resulting in enhanced resistance
(van Wees et al. 2008).
Both ISR and SAR contribute to resistance to a wide range of pathogens
in systemic host´s tissues. However, the molecular mechanisms underlying both
processes may differ. It was initially proposed that ISR is independent of salicylic
acid (SA) signaling pathways but dependant of jasmonic acid (JA) and ethylene (ET),
while SAR is dependant of SA and variable dependant of JA and ethylene (van Loon
et al. 1998). However, a great number of systems studied afterward demonstrated that
beneficial microorganisms induce resistance by activating both SA- and JA-signaling
pathways (Mathys et al. 2012; Niu et al. 2011, 2012) or the SA-signaling pathway
alone (Tjamos et al. 2005; van de Mortel et al. 2012). Thus, it is probable that nature
and the molecular mechanisms underlying ISR depends on particular combinations
of plant-beneficial microorganism-pathogen.
Regarding endophyte-ISR induction, it would be strictly necessary to test that
endophyte and pathogen are physically separated in the plant to ensure that the
mechanism involved in protection is ISR (Kloepper and Ryu 2006). This is difficult
to perform with microbial endophytes that colonize the entire plant. In this section,
we will discuss some examples of bacterial endophytes inducing defense responses in
their host independently of their colonization pattern. Changes in gene expression due
to endophyte inoculation can be both local (at the site of inoculation) and systematic
(in inoculated and not inoculated tissues). A clear example of systematic responses
is the interaction between olives and the endophytic bacterium P. fluorescens PICF7,
which conduce to the overexpression of defense-related genes in both roots and
17
percentage of protection on black pepper shoots against Phytophthora capsici, especially when these compounds were applied at low concentrations (Agisha et al.
2019). The production of VOCs was also evidenced and identified in two isolates
from cocoa that showed antagonism against the causal agent of a black pod, Phytophthora palmivora, both in vitro and in planta (Alsultan et al. 2019). The role of VOCs
in plant protection is not only due to the direct effect on pathogen’s growth as there is
evidence that 2,3-butanediol and acetoin can induce systemic resistance to pathogens
(Ryu et al. 2004).
2.2.2 Induction Disease Resistance in Plants
The recognition of microbial cell components and/or their metabolites can induce
in the host’s physiological state allowing them to respond faster and/or to a greater
extent to future pathogenic attacks. This phenomenon is called induced systemic
resistance (ISR) and shares characteristics with another type of systemic resistance
triggered by a previous attack of the necrosis-producing pathogen (SAR, systemic
acquired resistance). There are two possible molecular mechanisms activated during
ISR. Thus, endophyte-inoculation can induce the expression of defense-related genes
per se, or on the other hand, the presence of beneficial microorganisms primes plants
for enhanced defense responses. In primed plants, defense responses are not activated
directly but are potentiated upon pathogen attack, resulting in enhanced resistance
(van Wees et al. 2008).
Both ISR and SAR contribute to resistance to a wide range of pathogens
in systemic host´s tissues. However, the molecular mechanisms underlying both
processes may differ. It was initially proposed that ISR is independent of salicylic
acid (SA) signaling pathways but dependant of jasmonic acid (JA) and ethylene (ET),
while SAR is dependant of SA and variable dependant of JA and ethylene (van Loon
et al. 1998). However, a great number of systems studied afterward demonstrated that
beneficial microorganisms induce resistance by activating both SA- and JA-signaling
pathways (Mathys et al. 2012; Niu et al. 2011, 2012) or the SA-signaling pathway
alone (Tjamos et al. 2005; van de Mortel et al. 2012). Thus, it is probable that nature
and the molecular mechanisms underlying ISR depends on particular combinations
of plant-beneficial microorganism-pathogen.
Regarding endophyte-ISR induction, it would be strictly necessary to test that
endophyte and pathogen are physically separated in the plant to ensure that the
mechanism involved in protection is ISR (Kloepper and Ryu 2006). This is difficult
to perform with microbial endophytes that colonize the entire plant. In this section,
we will discuss some examples of bacterial endophytes inducing defense responses in
their host independently of their colonization pattern. Changes in gene expression due
to endophyte inoculation can be both local (at the site of inoculation) and systematic
(in inoculated and not inoculated tissues). A clear example of systematic responses
is the interaction between olives and the endophytic bacterium P. fluorescens PICF7,
which conduce to the overexpression of defense-related genes in both roots and