2012). It was shown that ectopic expression of MoHrip1 in rice showed higher
resistance of transgenic plants to M. oryzae and enhanced tolerance to drought stress
than wild type (Wang et al. 2017b). Moreover, increased expression of SA and ABA
related genes was observed in the MoHrip1 expressing plants as compared to the
wild types, suggesting that MoHrip1 induced signaling is mediated by the functioning of these two phytohormones (Wang et al. 2017b). Recently, it was shown that
MoHrip1 can bind to the tobacco and rice plant plasma membrane, indicating that
MoHrip1 is a PAMP perceived by the plant immune system (Zhang et al. 2017). The
expression of MoHrip1 was found to be increased during penetration and colonization of M. oryzae (Nie et al. 2019). Moreover, it was shown that MoHrip1 is required
for fungal virulence as MoHrip1 deletion mutants showed significantly reduced
virulence on rice (Nie et al. 2019). However, as no or very fewer efforts have been
dedicated to understanding the molecular mechanism of MoHrip1-induced signaling
in rice, the exact mechanism of MoHrip1-induced PTI responses is still to be
deciphered. Moreover, the PRRs for both MSP1 and MoHrip1 is yet to be identified.
12.3 Downstream Responses of PTI Signaling
PTI signaling orchestrates a number of events including activation of MAP kinase
cascade, production of antimicrobial compounds, commonly termed as
phytoalexins, synthesis, production of ROS, and secretion of PR-proteins, among
others (Thomma et al. 2011). In addition, closure of stomata also takes place as a
downstream signaling event of PTI, however, it is majorly effective against bacterial
pathogens because fungal pathogens generally use mechanical pressure to rupture
the leaf surfaces (Bigeard et al. 2015). The majority of the PTI events are common
with the ETI events and there is no sharp distinction between these two signaling
cascades (Liu et al. 2013).
12.3.1 Activation of MAPK Cascade
Mitogen-activated protein kinase (MAPK) cascades are well established, highly
conserved signaling modules that play pivotal roles in regulating PTI. It is well
established that rapid and transient activation of MAPKs occurs during the activation
of PTI responses. MAPK cascades are activated as one of the earliest signaling
events after recognition of PAMPs by plant PRRs, which consist of a MAPKKKMEK-MPK module. For instance, Group A MAPKs including rice MPK3 and
MPK6 are involved in plant responses to biotic and abiotic stresses and are also
involved in growth and development. Similar phosphorylation of MPK3/6 was
observed upon MSP1 treatment in rice leaves (Gupta et al. 2019; Meng et al.
2019a). OsMPK5, also known as OsBIMK1 and OsMPK3, has been reported to
be involved in the disease resistance responses, positive regulation of the JA
signaling pathway, and plant resistance to a chewing herbivore in rice. Moreover,
MAPKKK1 is involved in the activation of ethylene biosynthesis and thereby
12 Unraveling the Molecular Mechanism of Magnaporthe oryzae Induced. . .
369
resistance of transgenic plants to M. oryzae and enhanced tolerance to drought stress
than wild type (Wang et al. 2017b). Moreover, increased expression of SA and ABA
related genes was observed in the MoHrip1 expressing plants as compared to the
wild types, suggesting that MoHrip1 induced signaling is mediated by the functioning of these two phytohormones (Wang et al. 2017b). Recently, it was shown that
MoHrip1 can bind to the tobacco and rice plant plasma membrane, indicating that
MoHrip1 is a PAMP perceived by the plant immune system (Zhang et al. 2017). The
expression of MoHrip1 was found to be increased during penetration and colonization of M. oryzae (Nie et al. 2019). Moreover, it was shown that MoHrip1 is required
for fungal virulence as MoHrip1 deletion mutants showed significantly reduced
virulence on rice (Nie et al. 2019). However, as no or very fewer efforts have been
dedicated to understanding the molecular mechanism of MoHrip1-induced signaling
in rice, the exact mechanism of MoHrip1-induced PTI responses is still to be
deciphered. Moreover, the PRRs for both MSP1 and MoHrip1 is yet to be identified.
12.3 Downstream Responses of PTI Signaling
PTI signaling orchestrates a number of events including activation of MAP kinase
cascade, production of antimicrobial compounds, commonly termed as
phytoalexins, synthesis, production of ROS, and secretion of PR-proteins, among
others (Thomma et al. 2011). In addition, closure of stomata also takes place as a
downstream signaling event of PTI, however, it is majorly effective against bacterial
pathogens because fungal pathogens generally use mechanical pressure to rupture
the leaf surfaces (Bigeard et al. 2015). The majority of the PTI events are common
with the ETI events and there is no sharp distinction between these two signaling
cascades (Liu et al. 2013).
12.3.1 Activation of MAPK Cascade
Mitogen-activated protein kinase (MAPK) cascades are well established, highly
conserved signaling modules that play pivotal roles in regulating PTI. It is well
established that rapid and transient activation of MAPKs occurs during the activation
of PTI responses. MAPK cascades are activated as one of the earliest signaling
events after recognition of PAMPs by plant PRRs, which consist of a MAPKKKMEK-MPK module. For instance, Group A MAPKs including rice MPK3 and
MPK6 are involved in plant responses to biotic and abiotic stresses and are also
involved in growth and development. Similar phosphorylation of MPK3/6 was
observed upon MSP1 treatment in rice leaves (Gupta et al. 2019; Meng et al.
2019a). OsMPK5, also known as OsBIMK1 and OsMPK3, has been reported to
be involved in the disease resistance responses, positive regulation of the JA
signaling pathway, and plant resistance to a chewing herbivore in rice. Moreover,
MAPKKK1 is involved in the activation of ethylene biosynthesis and thereby
12 Unraveling the Molecular Mechanism of Magnaporthe oryzae Induced. . .
369