2 Bioefficacy of Endophytes in the Control of Plant Diseases
15
unique redox properties and broad-spectrum antibiotic activity, and playing a wide
variety of roles in nature (Mavrodi et al. 2006). Endophytes are essential to the
production of several secondary metabolites in grasses, in the process of gummosis
in trees, and the production of useful metabolites such as alkaloids, pestaloside,
cryptocandin, enfumafungin, subglutinols, etc., for the host plant (Maheshwari and
Annapurna 2017).
The transformation of P. fluorescens strain Q8r1-96 with the biosynthetic locus
leading to the production of phenazine-1-carboxylic acid, a precursor of several
phenazine compounds, conduces to an increase in the biocontrol efficacy against
Rhizoctonia root rot in wheat. In this trend, a lower dose of the transformant antagonist is required to exert the similar level of control exerted by the parental strain
(Huang et al. 2004). The presence of these traits in soils is thought to explain disease
decline in suppressive soils, in which specific soil-borne plant pathogens cause only
limited disease although the pathogen and susceptible host plants are both present.
However, the quantification of these characteristics in different types of soils has
no clear correlation between the presence of antibiotic synthetic genes and disease
suppression (Garbeva et al. 2004; Imperiali et al. 2017). Further analysis is required
to understand how these are expressed genetically and regulated in soil and/or inside
plants.
Other metabolites with direct action against bacteria and fungi are lipopeptides
such as iturin, surfactin, thanamycin, and fengycin (Ongena and Jacques 2008; Raaijmakers et al. 2010); and also the polyketide antibiotics bacillaene, difficidin, and
macrolactin produced mainly, but not exclusively, by different strains of the genus
Bacillus. The role of these compounds in biocontrol activity was also evidenced by the
use of mutant strains defective in their production. For instance, the biocontrol activity
of B. subtilis strain 6051 against P. syringae in Arabidopsis was impaired when a
mutant strain unable to produce surfactin was used (Bais et al. 2004). Similarly,
Pseudomonas strain SH-C52 reduces the incidence of stem rot disease of groundnut,
whereas a thanamycin-deficient mutant strain was less effective (Le et al. 2012).
Non-ribosomal peptides also contribute to the antagonism against bacteria and
fungi (Abdalla and Matasyoh 2014). For instance, Tontou et al. (2015) demonstrated
that an endophytic strain of P. synxantha isolated from Actinidia chinense showed
antagonism against P. syringae pv. actinidiae (Psa) in vitro. To find out the molecular mechanisms involved in the antagonism, a mini transposon-mutant library was
constructed and antagonism-deficient mutants were selected. Molecular characterization of these mutants showed that three genes could be involved in antagonistic
activity, an acyl-homoserine lactone acylase gene, a glucose-6-phosphate dehydrogenase gene, and an mbtH-like gene. As these genes are directly or indirectly involved
in the synthesis of non-ribosomal peptides, the authors claimed that these molecules
are involved in the antagonistic ability of P. synxantha (Tontou et al. 2015). However,
it is worthy to mention that these genes could also be affecting other antagonismassociated mechanisms. Thus, it has been shown that quorum sensing perturbation by the action of acyl-homoserine lactone degrading enzymes can interfere with
interspecies competition (Amara et al. 2011; Kusari et al. 2014).
15
unique redox properties and broad-spectrum antibiotic activity, and playing a wide
variety of roles in nature (Mavrodi et al. 2006). Endophytes are essential to the
production of several secondary metabolites in grasses, in the process of gummosis
in trees, and the production of useful metabolites such as alkaloids, pestaloside,
cryptocandin, enfumafungin, subglutinols, etc., for the host plant (Maheshwari and
Annapurna 2017).
The transformation of P. fluorescens strain Q8r1-96 with the biosynthetic locus
leading to the production of phenazine-1-carboxylic acid, a precursor of several
phenazine compounds, conduces to an increase in the biocontrol efficacy against
Rhizoctonia root rot in wheat. In this trend, a lower dose of the transformant antagonist is required to exert the similar level of control exerted by the parental strain
(Huang et al. 2004). The presence of these traits in soils is thought to explain disease
decline in suppressive soils, in which specific soil-borne plant pathogens cause only
limited disease although the pathogen and susceptible host plants are both present.
However, the quantification of these characteristics in different types of soils has
no clear correlation between the presence of antibiotic synthetic genes and disease
suppression (Garbeva et al. 2004; Imperiali et al. 2017). Further analysis is required
to understand how these are expressed genetically and regulated in soil and/or inside
plants.
Other metabolites with direct action against bacteria and fungi are lipopeptides
such as iturin, surfactin, thanamycin, and fengycin (Ongena and Jacques 2008; Raaijmakers et al. 2010); and also the polyketide antibiotics bacillaene, difficidin, and
macrolactin produced mainly, but not exclusively, by different strains of the genus
Bacillus. The role of these compounds in biocontrol activity was also evidenced by the
use of mutant strains defective in their production. For instance, the biocontrol activity
of B. subtilis strain 6051 against P. syringae in Arabidopsis was impaired when a
mutant strain unable to produce surfactin was used (Bais et al. 2004). Similarly,
Pseudomonas strain SH-C52 reduces the incidence of stem rot disease of groundnut,
whereas a thanamycin-deficient mutant strain was less effective (Le et al. 2012).
Non-ribosomal peptides also contribute to the antagonism against bacteria and
fungi (Abdalla and Matasyoh 2014). For instance, Tontou et al. (2015) demonstrated
that an endophytic strain of P. synxantha isolated from Actinidia chinense showed
antagonism against P. syringae pv. actinidiae (Psa) in vitro. To find out the molecular mechanisms involved in the antagonism, a mini transposon-mutant library was
constructed and antagonism-deficient mutants were selected. Molecular characterization of these mutants showed that three genes could be involved in antagonistic
activity, an acyl-homoserine lactone acylase gene, a glucose-6-phosphate dehydrogenase gene, and an mbtH-like gene. As these genes are directly or indirectly involved
in the synthesis of non-ribosomal peptides, the authors claimed that these molecules
are involved in the antagonistic ability of P. synxantha (Tontou et al. 2015). However,
it is worthy to mention that these genes could also be affecting other antagonismassociated mechanisms. Thus, it has been shown that quorum sensing perturbation by the action of acyl-homoserine lactone degrading enzymes can interfere with
interspecies competition (Amara et al. 2011; Kusari et al. 2014).