2 Bioefficacy of Endophytes in the Control of Plant Diseases
19
Several plants possess different compounds with antimicrobial activity; this is
the case of Allium sativum (garlic) that produces alliin, an alkyl cysteine sulphoxide
that is converted to allicin by the enzyme alliinase. Wang et al. (2019) isolated
endophytes from garlic and selected two strains showing antagonism against Sclerotium cepivorum, the causal agent of white rot disease. Also, these isolates reduced
the disease index by up to 66% in greenhouse assays. The authors proposed that
there is not only a direct antagonistic effect due to the production of extracellular
enzymes, but also an induction in the production of antimicrobial products in the
host. Thus, inoculation with one of the isolates primed the expression of alliinase
and the accumulation of alliin (Wang et al. 2019).
2.2.3 Parasitism
This kind of interaction is more commonly observed in bacteria-fungi and fungi-fungi
interactions. During these interactions, fungal cells are lysed due to breaking down to
mycelial cell wall. These effects are due to the action of extracellular enzymes such
as glucanases, chitinases, and proteases as well as antifungal compounds (Chauhan
et al. 2016; Whipps 2001). For instance, poplar canker provoked by Cytospora
chrysosperma, Phomopsis macrospora, and Fusicoccum aesculi can be controlled
with an efficiency up to 90% by bacterial endophytes with antagonistic activity
against these pathogens due to production of extracellular enzymes such as β-1,3glucanases, proteases, and chitinases (Ren et al. 2011). The important role of βglucanases was evidenced recently in the interaction between B. halotolerans, a
cotton endophytic strain, and the pathogen V. dahliae. This endophyte was selected
because of the ability to inhibit conidial germination and mycelial growth of the
pathogen in vitro and showed β-glucanase activity. Mutant and overexpressing strains
were generated to elucidate the role of β-glucanase. In vitro antagonism assays
using the mutant strain exhibited diminished antifungal activity against V. dahliae
compared to wild type or the complementary strain. In turn, bioassays using the
overexpressing strain showed a greater protective effect compared to wild type, as
the disease indexes diminished from 17.86 (wild type strain) to 8.33 (overexpressing
strain) after 45 days post-inoculation (Zhang et al. 2019a).
As mentioned earlier, the protective effects observed by endophyte inoculation
usually involved more than one mechanism. For instance, an Enterobacter strain
isolated from finger millet roots, has been found as an endophyte in other crops such
as maize and wheat showed antagonism against Fusarium graminearum among other
pathogens, and it was able to reduce disease symptoms up to 90% in greenhouse
trials (Mousa et al. 2016). Interestingly, during confrontation assays, Enterobacter
sp. seemed to be attracted to fungal cells, formed biofilms over fungal hyphae, and
finally destroy fungal cells. Moreover, when inoculated on roots, the isolates showed
the ability to induce proliferation of root hairs and establish a physicochemical barrier
to trap and degrade the pathogen hyphae. Biocontrol and antagonistic ability of these
endophytes require the production of phenazine, c-di-GMP-dependent signaling
19
Several plants possess different compounds with antimicrobial activity; this is
the case of Allium sativum (garlic) that produces alliin, an alkyl cysteine sulphoxide
that is converted to allicin by the enzyme alliinase. Wang et al. (2019) isolated
endophytes from garlic and selected two strains showing antagonism against Sclerotium cepivorum, the causal agent of white rot disease. Also, these isolates reduced
the disease index by up to 66% in greenhouse assays. The authors proposed that
there is not only a direct antagonistic effect due to the production of extracellular
enzymes, but also an induction in the production of antimicrobial products in the
host. Thus, inoculation with one of the isolates primed the expression of alliinase
and the accumulation of alliin (Wang et al. 2019).
2.2.3 Parasitism
This kind of interaction is more commonly observed in bacteria-fungi and fungi-fungi
interactions. During these interactions, fungal cells are lysed due to breaking down to
mycelial cell wall. These effects are due to the action of extracellular enzymes such
as glucanases, chitinases, and proteases as well as antifungal compounds (Chauhan
et al. 2016; Whipps 2001). For instance, poplar canker provoked by Cytospora
chrysosperma, Phomopsis macrospora, and Fusicoccum aesculi can be controlled
with an efficiency up to 90% by bacterial endophytes with antagonistic activity
against these pathogens due to production of extracellular enzymes such as β-1,3glucanases, proteases, and chitinases (Ren et al. 2011). The important role of βglucanases was evidenced recently in the interaction between B. halotolerans, a
cotton endophytic strain, and the pathogen V. dahliae. This endophyte was selected
because of the ability to inhibit conidial germination and mycelial growth of the
pathogen in vitro and showed β-glucanase activity. Mutant and overexpressing strains
were generated to elucidate the role of β-glucanase. In vitro antagonism assays
using the mutant strain exhibited diminished antifungal activity against V. dahliae
compared to wild type or the complementary strain. In turn, bioassays using the
overexpressing strain showed a greater protective effect compared to wild type, as
the disease indexes diminished from 17.86 (wild type strain) to 8.33 (overexpressing
strain) after 45 days post-inoculation (Zhang et al. 2019a).
As mentioned earlier, the protective effects observed by endophyte inoculation
usually involved more than one mechanism. For instance, an Enterobacter strain
isolated from finger millet roots, has been found as an endophyte in other crops such
as maize and wheat showed antagonism against Fusarium graminearum among other
pathogens, and it was able to reduce disease symptoms up to 90% in greenhouse
trials (Mousa et al. 2016). Interestingly, during confrontation assays, Enterobacter
sp. seemed to be attracted to fungal cells, formed biofilms over fungal hyphae, and
finally destroy fungal cells. Moreover, when inoculated on roots, the isolates showed
the ability to induce proliferation of root hairs and establish a physicochemical barrier
to trap and degrade the pathogen hyphae. Biocontrol and antagonistic ability of these
endophytes require the production of phenazine, c-di-GMP-dependent signaling