association as symbiotic or pathogenic (Zeilinger et al. 2016). Scientists believe that,
similar to AM association with plants, N-acetylglucosamine-based oligomers,
lipochito-oligosaccharides, and other chitin signals play a key role in differentiating
beneficial fungi from phytopathogens (Genre et al. 2013; Maillet et al. 2011). Similar
molecules may be involved in differentiating EIPF from pathogens (Heinz et al.
2018; Razinger et al. 2018). Strigolactones are phytohormones used as chemical
signals by plants to attract and facilitate establishment of symbiotic association with
AM by promoting hyphal branching and increasing the probability of contact with
plant roots (López-Ráez et al. 2017; Massalha et al. 2017; Mori et al. 2016). When
EIPF come into contact with host plants, the early events of association may involve
similar molecules for recognition (Hu and Bidochka 2019).
Different phytohormones are involved in specific defense tasks and plants are
able to switch pathways on or off depending upon the challenges they face. For
example, abscisic acid (ABA) is involved in the modulation of plant growth and
defense against abiotic stress (Herrera-Medina et al. 2007). Jasmonic acid (JA) and
salicylic acid (SA) defense pathways respond in many plant systems where
biotrophic pathogens and phloem-feeding insects induce the SA-dependent pathway
and necrotrophic pathogens and chewing insects induce the JA-dependent pathway
(De Vos et al. 2005; Glazebrook 2005; Pieterse et al. 2012, 2014). However,
eliciting a plant defense response can be associated with a trade-off between growth
and defense (Karasov et al. 2017) and the antagonistic crosstalk of SA and JA
pathways can be an adaptive strategy by plants to improve resource allocation to
integrate plant growth and defense (Thaler et al. 2012).
EIPF in the genus Metarhizium play a role in plant growth and defense modulation under stress conditions through phytohormone-mediated pathways. For successful establishment of EIPF as endophytes, signaling cascades involving different
defense pathways may become activated. For example, Liao et al. (2017) detected
IAA in the filtrates of M. robertsii, M. brunneum, and M. acridum that promoted root
proliferation, possibly through auxin-dependent mechanisms in early signaling
events of root colonization. In plants, IAA plays a critical role in cell division,
differentiation, and elongation (Woodward and Bartel 2005). Changes in the concentration of signaling molecules and phytohormones in plant–EIPF interactions can
indicate changes in the status of plant defense. M. anisopliae-colonized plants
showed a decreased concentration of SA compared with uninoculated plants,
suggesting a suppression of plant defense against biotrophic phytopathogens (Hao
et al. 2017). Ahmad et al. (2020b) reported down-regulation of gene expression of
pathogenesis-related protein 5 (pr5) from the SA pathway in M. robertsii-colonized
maize plants that suggests plants may have suppressed their defense in the absence of
stress. Compared with uninoculated plants, Metarhizium-inoculated soybean plants
showed a decrease in plant ABA and increase in JA levels under salinity stress
suggesting a stress ameliorating effect (Khan et al. 2012). M. anisopliae-inoculated
peanut plants differentially expressed genes responsible for various transcription
factors, signaling effectors, and binding proteins compared to uninoculated plants
(Hao et al. 2017).
Beauveria spp. modulate defense reactions under stress conditions through
defense-related pathways. The strategy of B. bassiana to colonize roots and evade
386
I. Ahmad et al.
similar to AM association with plants, N-acetylglucosamine-based oligomers,
lipochito-oligosaccharides, and other chitin signals play a key role in differentiating
beneficial fungi from phytopathogens (Genre et al. 2013; Maillet et al. 2011). Similar
molecules may be involved in differentiating EIPF from pathogens (Heinz et al.
2018; Razinger et al. 2018). Strigolactones are phytohormones used as chemical
signals by plants to attract and facilitate establishment of symbiotic association with
AM by promoting hyphal branching and increasing the probability of contact with
plant roots (López-Ráez et al. 2017; Massalha et al. 2017; Mori et al. 2016). When
EIPF come into contact with host plants, the early events of association may involve
similar molecules for recognition (Hu and Bidochka 2019).
Different phytohormones are involved in specific defense tasks and plants are
able to switch pathways on or off depending upon the challenges they face. For
example, abscisic acid (ABA) is involved in the modulation of plant growth and
defense against abiotic stress (Herrera-Medina et al. 2007). Jasmonic acid (JA) and
salicylic acid (SA) defense pathways respond in many plant systems where
biotrophic pathogens and phloem-feeding insects induce the SA-dependent pathway
and necrotrophic pathogens and chewing insects induce the JA-dependent pathway
(De Vos et al. 2005; Glazebrook 2005; Pieterse et al. 2012, 2014). However,
eliciting a plant defense response can be associated with a trade-off between growth
and defense (Karasov et al. 2017) and the antagonistic crosstalk of SA and JA
pathways can be an adaptive strategy by plants to improve resource allocation to
integrate plant growth and defense (Thaler et al. 2012).
EIPF in the genus Metarhizium play a role in plant growth and defense modulation under stress conditions through phytohormone-mediated pathways. For successful establishment of EIPF as endophytes, signaling cascades involving different
defense pathways may become activated. For example, Liao et al. (2017) detected
IAA in the filtrates of M. robertsii, M. brunneum, and M. acridum that promoted root
proliferation, possibly through auxin-dependent mechanisms in early signaling
events of root colonization. In plants, IAA plays a critical role in cell division,
differentiation, and elongation (Woodward and Bartel 2005). Changes in the concentration of signaling molecules and phytohormones in plant–EIPF interactions can
indicate changes in the status of plant defense. M. anisopliae-colonized plants
showed a decreased concentration of SA compared with uninoculated plants,
suggesting a suppression of plant defense against biotrophic phytopathogens (Hao
et al. 2017). Ahmad et al. (2020b) reported down-regulation of gene expression of
pathogenesis-related protein 5 (pr5) from the SA pathway in M. robertsii-colonized
maize plants that suggests plants may have suppressed their defense in the absence of
stress. Compared with uninoculated plants, Metarhizium-inoculated soybean plants
showed a decrease in plant ABA and increase in JA levels under salinity stress
suggesting a stress ameliorating effect (Khan et al. 2012). M. anisopliae-inoculated
peanut plants differentially expressed genes responsible for various transcription
factors, signaling effectors, and binding proteins compared to uninoculated plants
(Hao et al. 2017).
Beauveria spp. modulate defense reactions under stress conditions through
defense-related pathways. The strategy of B. bassiana to colonize roots and evade
386
I. Ahmad et al.