14
F. M. Romero et al.
production of antimicrobial compounds, competition for nutrients and space, induction of plant defense mechanisms, and parasitism. However, other modes of action
remain to be explored in-depth, as the inactivation of pathogen germination factors
or even degradation of virulence factors such as toxins produced by phytopathogens
(Whipps 2001). Further, endophytes possess significant efficiency of protection since
it is possible to prepare bioformulations with more than one microorganism for
multiple trait benefits to combine desired traits and/or efficacy (Aeron et al. 2011;
Baliyan et al. 2018; Pandey and Maheshwari 2007). For instance, Varo et al. (2016)
tested several BCAs alone or in combinations, which showed great levels of protection against wilt in olives caused by Verticillium dahlia, reducing the incidence and
mortality up to 90% (Varo et al. 2016).
2.2.1 Antagonism
Since endophytes share a similar niche as of many phytopathogens colonizing plant
cells and tissues, with different degrees of association direct antagonism between
them is a reliable screening technique to screen potential BCAs from a collection
of endophytic isolates. Microbial balance in the tissue of plant, as a micro-niche
microbial homeostasis is also important to notice, where microbe-microbe interactions interplay. The native microbe of endophytic nature sometimes competes with
the other invading microbe under natural conditions. The in vitro proves of this
phenomenon are constant and yet few.
Direct inhibition of pathogens is mainly mediated by the synthesis of antibiotics, volatile production of hydrogen cyanide (HCN), and antifungal metabolites
(Raaijmakers et al. 2002, 2010). Antibiotics encompass a chemically heterogeneous
group of organic, low-molecular-weight compounds. At low concentrations, these
are deleterious to the growth or metabolic activities of other microorganisms, and
most BCAs bacteria produce multiple antibiotics with different degrees of efficacy
against pathogens, some of them with overlapping activity. Several compounds have
been purified and identified from biocontrol bacteria. For example, pyrrolnitrin is
produced by bacteria from the genus Pseudomonas and Burkholderia, which has
been proven to be effective against a wide variety of plant pathogens such as Rhizoctonia solani, Botrytis cinerea, V. dahliae, and Sclerotinia sclerotiorum (Raaijmakers
et al. 2002). Moreover, mutant strains unable to produce this compound lost the
in vitro and in planta ability to control R. solani (Hill et al. 1994). In turn, 2,4diacetylphloroglucinol (DAPG) is a phenolic antibiotic produced by BCAs from the
genus Pseudomonas that exhibits antibacterial, antifungal, and anthelminthic activity
(Haas and Defago 2005; Weller et al. 2007). DAPG not only was proven to inhibit
the growth of pathogens directly but also, it has been demonstrated that Arabidopsis
thaliana inoculated with Pseudomonas sp. mutant strains unable to produce DAPG
are impaired in developing induced systemic resistance (ISR) against Pseudomonas
syringae pv. tomato (Weller et al. 2012). Phenazines are also a class of well-studied
natural antibiotics that are produced by diverse plant-associated bacteria, exhibiting
F. M. Romero et al.
production of antimicrobial compounds, competition for nutrients and space, induction of plant defense mechanisms, and parasitism. However, other modes of action
remain to be explored in-depth, as the inactivation of pathogen germination factors
or even degradation of virulence factors such as toxins produced by phytopathogens
(Whipps 2001). Further, endophytes possess significant efficiency of protection since
it is possible to prepare bioformulations with more than one microorganism for
multiple trait benefits to combine desired traits and/or efficacy (Aeron et al. 2011;
Baliyan et al. 2018; Pandey and Maheshwari 2007). For instance, Varo et al. (2016)
tested several BCAs alone or in combinations, which showed great levels of protection against wilt in olives caused by Verticillium dahlia, reducing the incidence and
mortality up to 90% (Varo et al. 2016).
2.2.1 Antagonism
Since endophytes share a similar niche as of many phytopathogens colonizing plant
cells and tissues, with different degrees of association direct antagonism between
them is a reliable screening technique to screen potential BCAs from a collection
of endophytic isolates. Microbial balance in the tissue of plant, as a micro-niche
microbial homeostasis is also important to notice, where microbe-microbe interactions interplay. The native microbe of endophytic nature sometimes competes with
the other invading microbe under natural conditions. The in vitro proves of this
phenomenon are constant and yet few.
Direct inhibition of pathogens is mainly mediated by the synthesis of antibiotics, volatile production of hydrogen cyanide (HCN), and antifungal metabolites
(Raaijmakers et al. 2002, 2010). Antibiotics encompass a chemically heterogeneous
group of organic, low-molecular-weight compounds. At low concentrations, these
are deleterious to the growth or metabolic activities of other microorganisms, and
most BCAs bacteria produce multiple antibiotics with different degrees of efficacy
against pathogens, some of them with overlapping activity. Several compounds have
been purified and identified from biocontrol bacteria. For example, pyrrolnitrin is
produced by bacteria from the genus Pseudomonas and Burkholderia, which has
been proven to be effective against a wide variety of plant pathogens such as Rhizoctonia solani, Botrytis cinerea, V. dahliae, and Sclerotinia sclerotiorum (Raaijmakers
et al. 2002). Moreover, mutant strains unable to produce this compound lost the
in vitro and in planta ability to control R. solani (Hill et al. 1994). In turn, 2,4diacetylphloroglucinol (DAPG) is a phenolic antibiotic produced by BCAs from the
genus Pseudomonas that exhibits antibacterial, antifungal, and anthelminthic activity
(Haas and Defago 2005; Weller et al. 2007). DAPG not only was proven to inhibit
the growth of pathogens directly but also, it has been demonstrated that Arabidopsis
thaliana inoculated with Pseudomonas sp. mutant strains unable to produce DAPG
are impaired in developing induced systemic resistance (ISR) against Pseudomonas
syringae pv. tomato (Weller et al. 2012). Phenazines are also a class of well-studied
natural antibiotics that are produced by diverse plant-associated bacteria, exhibiting