plants (Badri and Vivanco 2009; Gaiero et al. 2013). Further, plants have the ability
to modulate the composition of the communities of commensal, mutualistic, and
pathogenic microorganisms that live in close association with its roots (root
microbiome). Secretion of certain specific compounds that selectively stimulate
and enrich beneficial microbes while repressing other microbes can provide important fitness benefits to the plants (Berendsen et al. 2012). This constitutes the
rhizosphere effect of plant root secretions in the rhizosphere on microbial biomass,
activity, and community composition compared with the majority soil. For instance,
maize plants secrete antimicrobial compounds of benzoxazinoids that inhibit most
microbes, but are selective for Pseudomonas putida KT2440 (Neal et al. 2012). The
plant-growth promoting microbes (PGPM) that include plant-growth promoting
bacteria (PGPB), such as rhizobia, and plant-growth-promoting fungi (PGPF),
such as mycorrhizal fungi, establish a mutually beneficial relationship with plants
and promote growth and development of plants by making them tolerant to environmental stresses of mineral deficiency, water scarcity, and phytopathogens (Drogue
et al. 2013). The PGPF arbuscular mycorrhizal fungi (AMF) develop mutual beneficial relationships with over 90% of terrestrial plants including agricultural and
horticultural crops, and improve the acquisition of less soluble or immobile nutrients
like phosphate (Singh and Giri 2017). Similarly, the PGPB rhizobia fix atmospheric
nitrogen in root nodules of plants belonging to family, Fabaceae (Geurts et al. 2012).
The disease suppressing capabilities of many of the PGPM are because of their
antagonistic activities such as antibiosis, competition, parasitism, and induction of
host-defense system against different pathogens (Thakur and Singh 2018). PGPM
can hormonally modulate and activate defensive reaction mechanisms within plants
by the systemic acquired resistance (SAR) and the induced systemic resistance
(ISR). Under biotic stress, the SAR comprises accumulation of SA and PR proteins,
while ISR involves pathways regulated by JA and ET (Bari and Jones 2009; SalasMarina et al. 2011). However, the induction of resistance in plants depends on the
released non-pathogenic microbial elicitor.
10.5 SAR Signaling
The SAR develops in plants as a fully active defense mechanism, involving recognition of molecular patterns of pathogen followed by detoxification utilizing altered
gene expression, and production of hormones and metabolites. There is an increase
in the local endogenous SA levels upon elicitation, which then concomitantly
generates a mobile signal that after transportation within the plant to distant leaves
initiates an additional distal or systemic SA production (Durrent and Dong 2004).
The SA provides this SAR against a wide variety of plant pathogens including
bacteria, fungi, and viruses (Malamy et al. 1990; Ryals et al. 1996; Durner et al.
1997). This can induce a hypersensitive response (HR), which involves localized
cell death at the point of pathogen entry, caused by interactions of a host confrontation gene product with a specific pathogen-produced elicitor (Staskawicz et al.
1995).
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
303
to modulate the composition of the communities of commensal, mutualistic, and
pathogenic microorganisms that live in close association with its roots (root
microbiome). Secretion of certain specific compounds that selectively stimulate
and enrich beneficial microbes while repressing other microbes can provide important fitness benefits to the plants (Berendsen et al. 2012). This constitutes the
rhizosphere effect of plant root secretions in the rhizosphere on microbial biomass,
activity, and community composition compared with the majority soil. For instance,
maize plants secrete antimicrobial compounds of benzoxazinoids that inhibit most
microbes, but are selective for Pseudomonas putida KT2440 (Neal et al. 2012). The
plant-growth promoting microbes (PGPM) that include plant-growth promoting
bacteria (PGPB), such as rhizobia, and plant-growth-promoting fungi (PGPF),
such as mycorrhizal fungi, establish a mutually beneficial relationship with plants
and promote growth and development of plants by making them tolerant to environmental stresses of mineral deficiency, water scarcity, and phytopathogens (Drogue
et al. 2013). The PGPF arbuscular mycorrhizal fungi (AMF) develop mutual beneficial relationships with over 90% of terrestrial plants including agricultural and
horticultural crops, and improve the acquisition of less soluble or immobile nutrients
like phosphate (Singh and Giri 2017). Similarly, the PGPB rhizobia fix atmospheric
nitrogen in root nodules of plants belonging to family, Fabaceae (Geurts et al. 2012).
The disease suppressing capabilities of many of the PGPM are because of their
antagonistic activities such as antibiosis, competition, parasitism, and induction of
host-defense system against different pathogens (Thakur and Singh 2018). PGPM
can hormonally modulate and activate defensive reaction mechanisms within plants
by the systemic acquired resistance (SAR) and the induced systemic resistance
(ISR). Under biotic stress, the SAR comprises accumulation of SA and PR proteins,
while ISR involves pathways regulated by JA and ET (Bari and Jones 2009; SalasMarina et al. 2011). However, the induction of resistance in plants depends on the
released non-pathogenic microbial elicitor.
10.5 SAR Signaling
The SAR develops in plants as a fully active defense mechanism, involving recognition of molecular patterns of pathogen followed by detoxification utilizing altered
gene expression, and production of hormones and metabolites. There is an increase
in the local endogenous SA levels upon elicitation, which then concomitantly
generates a mobile signal that after transportation within the plant to distant leaves
initiates an additional distal or systemic SA production (Durrent and Dong 2004).
The SA provides this SAR against a wide variety of plant pathogens including
bacteria, fungi, and viruses (Malamy et al. 1990; Ryals et al. 1996; Durner et al.
1997). This can induce a hypersensitive response (HR), which involves localized
cell death at the point of pathogen entry, caused by interactions of a host confrontation gene product with a specific pathogen-produced elicitor (Staskawicz et al.
1995).
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
303