16
F. M. Romero et al.
Even though the identity of the antimicrobial molecules has not been determined
so far, several other studies proved that endophytic bacteria produce compounds with
antimicrobial activity. These studies typically used cell-free supernatants from the
cultures of these isolates and to inhibit the in vitro growth of phytopathogens. For
instance, bacterial endophytes identified as Pseudomonas, Bacillus, and Pantoea
isolated from field-grown tomato leaves showed antagonism against bacterial (P.
syringae) and fungal (Botrytis cinerea) pathogens in vitro and planta (Romero et al.
2016). Moreover, their cell-free supernatants were able to inhibit the germination of
conidia from B. cinerea for about 30–60% and also stopped the growth of P. syringae
when added to growth media. Additional pieces of evidence of antibiotic production were demonstrated using a semi-purified ethyl acetate extract of the endophyte
B. velezensis EB-39 against Xanthomonas campestris subsp. citri (Rabbee et al.
2019). Interestingly, this extract showed similar inhibitory activity to that observed
in confrontation assays between EB-39 and X. campestris. Purification and identification of new compounds from new isolates will increase the possibility to develop
new biocontrol strategies using whole microbes or their cell free metabolites alone.
The presence of genes involved in the biosynthesis of different antimicrobial
compounds was also used as an indication of the ability to produce this kind of
metabolites. An analysis performed on cultivable bacterial endophytes from mulberry
cultivars having different resistance to sclerotiniosis showed that endophytic communities from resistant genotypes are more diverse than those from the sensible ones (Xu
et al. 2019). In this work, dual-culture assays were performed with these endophytes
against S. sclerotiorum, B. cinerea, and Colletotrichum gloeosporioide and most of
the isolates that inhibit fungal growth were positive for the presence of genes involved
in the biosynthesis of antimicrobial compounds, such as polyketides, non-ribosomal
peptides, surfactin, iturin, and fengycin (Xu et al. 2019). Following this approach,
Cui et al. (2019) isolated a B. amyloliquefaciens strain from Chinese cabbage with
antagonistic activity against Pectobacterium carotovorum subsp. carotovorum, the
causal agent of soft rot, possess genes involved in polyketides and dipeptide biosynthesis and showed a level of protection up to 75% in greenhouse experiments (Cui
et al. 2019). It is worthy to mention that the mere presence of these biosynthetic
genes is not sufficient to confirm the production of the antimicrobial molecules.
For instance, Hazarika et al. (2019) demonstrated that a B. subtilis strain isolated
from sugarcane as well as cell-free supernatants obtained from its culture showed
antagonism against several pathogens. Moreover, it was positive for the presence of
different genes involved in the synthesis of antimicrobial compounds, even though
only one of them was detected in supernatants (surfactin) (Hazarika et al. 2019). This
observation indicates that gene expression in combination with gene presence would
be a more accurate indicator of antagonistic potential.
Volatile organic compounds (VOCs) are also responsible for the ability of certain
isolates to inhibit the in vitro growth of different pathogens. For instance, an endophytic isolate from black pepper roots identified as P. putida inhibits the growth
of several plant pathogens due to the production of volatile compounds as revealed
by Gas Chromatography/Mass Spectrometry (GC/MS) (Sheoran et al. 2015). Moreover, the application of some of these chemically synthesized VOCs showed a high
F. M. Romero et al.
Even though the identity of the antimicrobial molecules has not been determined
so far, several other studies proved that endophytic bacteria produce compounds with
antimicrobial activity. These studies typically used cell-free supernatants from the
cultures of these isolates and to inhibit the in vitro growth of phytopathogens. For
instance, bacterial endophytes identified as Pseudomonas, Bacillus, and Pantoea
isolated from field-grown tomato leaves showed antagonism against bacterial (P.
syringae) and fungal (Botrytis cinerea) pathogens in vitro and planta (Romero et al.
2016). Moreover, their cell-free supernatants were able to inhibit the germination of
conidia from B. cinerea for about 30–60% and also stopped the growth of P. syringae
when added to growth media. Additional pieces of evidence of antibiotic production were demonstrated using a semi-purified ethyl acetate extract of the endophyte
B. velezensis EB-39 against Xanthomonas campestris subsp. citri (Rabbee et al.
2019). Interestingly, this extract showed similar inhibitory activity to that observed
in confrontation assays between EB-39 and X. campestris. Purification and identification of new compounds from new isolates will increase the possibility to develop
new biocontrol strategies using whole microbes or their cell free metabolites alone.
The presence of genes involved in the biosynthesis of different antimicrobial
compounds was also used as an indication of the ability to produce this kind of
metabolites. An analysis performed on cultivable bacterial endophytes from mulberry
cultivars having different resistance to sclerotiniosis showed that endophytic communities from resistant genotypes are more diverse than those from the sensible ones (Xu
et al. 2019). In this work, dual-culture assays were performed with these endophytes
against S. sclerotiorum, B. cinerea, and Colletotrichum gloeosporioide and most of
the isolates that inhibit fungal growth were positive for the presence of genes involved
in the biosynthesis of antimicrobial compounds, such as polyketides, non-ribosomal
peptides, surfactin, iturin, and fengycin (Xu et al. 2019). Following this approach,
Cui et al. (2019) isolated a B. amyloliquefaciens strain from Chinese cabbage with
antagonistic activity against Pectobacterium carotovorum subsp. carotovorum, the
causal agent of soft rot, possess genes involved in polyketides and dipeptide biosynthesis and showed a level of protection up to 75% in greenhouse experiments (Cui
et al. 2019). It is worthy to mention that the mere presence of these biosynthetic
genes is not sufficient to confirm the production of the antimicrobial molecules.
For instance, Hazarika et al. (2019) demonstrated that a B. subtilis strain isolated
from sugarcane as well as cell-free supernatants obtained from its culture showed
antagonism against several pathogens. Moreover, it was positive for the presence of
different genes involved in the synthesis of antimicrobial compounds, even though
only one of them was detected in supernatants (surfactin) (Hazarika et al. 2019). This
observation indicates that gene expression in combination with gene presence would
be a more accurate indicator of antagonistic potential.
Volatile organic compounds (VOCs) are also responsible for the ability of certain
isolates to inhibit the in vitro growth of different pathogens. For instance, an endophytic isolate from black pepper roots identified as P. putida inhibits the growth
of several plant pathogens due to the production of volatile compounds as revealed
by Gas Chromatography/Mass Spectrometry (GC/MS) (Sheoran et al. 2015). Moreover, the application of some of these chemically synthesized VOCs showed a high