10.8 Microbe-Assisted Mitigation of Biotic Stresses
Several management practices have been suggested and implemented for management of biotic stresses in plants, though none are completely effective because of the
wide distribution and the high diversity of pathogens and their host range (Muthoni
et al. 2013). Most of these practices are dependent on the use of chemical fertilizers
and pesticides to ensure food security however, use of such synthetic agrochemicals
causes environmental problems and obliteration of soil structure and soil microorganism, and possible diminishing of food quality (Ward 2016). One of the alternative approaches to have sustainable agriculture along with environment preservation
is the development and implementation of biopesticides and biofertilizers containing
agro-friendly microorganisms, which enhance the growth and development of plants
and suppress different plant diseases (Youssef and Eissa 2014; Thomas and Singh
2019). The dynamic interplay of the diverse rhizospheric microflora through different synergistic and antagonistic interactions within the limits of the available
nutrients helps plants in their development and acclimation to variety of stresses
(Van Loon and Glick 2004). Studies on plant rhizosphere, mycorrhizosphere, and
endorhiza have revealed presence of a varied microbial community of PGPM (Nion
and Toyota 2015). The use of rhizospheric, free-living PGPR that colonize plant
roots, as biocontrol agents (BCAs) of plant diseases can be an ecological means to
manage agricultural disease complications along with other beneficial effects on
plant (Bouizgarne 2013). PGPR can specifically or non-specifically suppress plant
diseases by either antagonism or inducing a plant systemic resistance against
multiple root and foliar pathogens. The biocontrol mechanisms of most BCAs are
discoursed to involve biosynthesis of antibiotics, siderophores, surfactants and
phytohormones, niche and nutrient competition, mycoparasitism, ISR, phage therapy, and quorum quenching (Diallo et al. 2011; Thakur and Singh 2018).
Several microorganisms that display natural antagonism to pathogens have been
identified. This microbe mediated biotic stress tolerance has been reported in many
plants, and is thus implemented as BCAs of several pathogens. PGPR produce
different secondary metabolites such as lipopeptides and polyketides that antagonize
other microorganisms including phytopathogens. For instance, the widely
distributed, resistant endospore forming Bacilli demonstrate various forms of biocontrol mechanisms against different plant pathogens including antagonism, competition for niche space and nutrients, and induction of host resistance (Stein 2005;
Aleti et al. 2015; Villarreal-Delgado et al. 2018). Bacillus subtilis is one of the most
commercialized BCAs that produces various bioactive compounds, particularly
cyclic lipopeptides having antibacterial, antifungal, and antiviral activities that
develop an ISR, including the surfactin, iturin, fengycin bacillomycin, bacilysin,
lichenysin, and mycobacillin families, against a wide range of pathogens (Deleu
et al. 2008; Ongena and Jacques 2008; Jourdan et al. 2009; Falardeau et al. 2013;
Cawoy et al. 2014; Farace et al. 2015). Besides, Bacillus sp. can promote tolerance in
rice plants to a leaf blight (caused by Xanthomonas oryzae), mediated by increased
accumulation of phenylalanine ammonia lyase, peroxidase and polyphenol oxidase
(Udayashankar et al. 2011).
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L. Thomas and I. Singh
Several management practices have been suggested and implemented for management of biotic stresses in plants, though none are completely effective because of the
wide distribution and the high diversity of pathogens and their host range (Muthoni
et al. 2013). Most of these practices are dependent on the use of chemical fertilizers
and pesticides to ensure food security however, use of such synthetic agrochemicals
causes environmental problems and obliteration of soil structure and soil microorganism, and possible diminishing of food quality (Ward 2016). One of the alternative approaches to have sustainable agriculture along with environment preservation
is the development and implementation of biopesticides and biofertilizers containing
agro-friendly microorganisms, which enhance the growth and development of plants
and suppress different plant diseases (Youssef and Eissa 2014; Thomas and Singh
2019). The dynamic interplay of the diverse rhizospheric microflora through different synergistic and antagonistic interactions within the limits of the available
nutrients helps plants in their development and acclimation to variety of stresses
(Van Loon and Glick 2004). Studies on plant rhizosphere, mycorrhizosphere, and
endorhiza have revealed presence of a varied microbial community of PGPM (Nion
and Toyota 2015). The use of rhizospheric, free-living PGPR that colonize plant
roots, as biocontrol agents (BCAs) of plant diseases can be an ecological means to
manage agricultural disease complications along with other beneficial effects on
plant (Bouizgarne 2013). PGPR can specifically or non-specifically suppress plant
diseases by either antagonism or inducing a plant systemic resistance against
multiple root and foliar pathogens. The biocontrol mechanisms of most BCAs are
discoursed to involve biosynthesis of antibiotics, siderophores, surfactants and
phytohormones, niche and nutrient competition, mycoparasitism, ISR, phage therapy, and quorum quenching (Diallo et al. 2011; Thakur and Singh 2018).
Several microorganisms that display natural antagonism to pathogens have been
identified. This microbe mediated biotic stress tolerance has been reported in many
plants, and is thus implemented as BCAs of several pathogens. PGPR produce
different secondary metabolites such as lipopeptides and polyketides that antagonize
other microorganisms including phytopathogens. For instance, the widely
distributed, resistant endospore forming Bacilli demonstrate various forms of biocontrol mechanisms against different plant pathogens including antagonism, competition for niche space and nutrients, and induction of host resistance (Stein 2005;
Aleti et al. 2015; Villarreal-Delgado et al. 2018). Bacillus subtilis is one of the most
commercialized BCAs that produces various bioactive compounds, particularly
cyclic lipopeptides having antibacterial, antifungal, and antiviral activities that
develop an ISR, including the surfactin, iturin, fengycin bacillomycin, bacilysin,
lichenysin, and mycobacillin families, against a wide range of pathogens (Deleu
et al. 2008; Ongena and Jacques 2008; Jourdan et al. 2009; Falardeau et al. 2013;
Cawoy et al. 2014; Farace et al. 2015). Besides, Bacillus sp. can promote tolerance in
rice plants to a leaf blight (caused by Xanthomonas oryzae), mediated by increased
accumulation of phenylalanine ammonia lyase, peroxidase and polyphenol oxidase
(Udayashankar et al. 2011).
312
L. Thomas and I. Singh