downstream. Therefore, an additional LysM RLK protein OsCERK1 cooperates
with CEBiP and functions as a crucial component for chitin-triggered immunity in
rice. OsCERK1 and CEBiP heterodimers to form a plasma membrane receptor
complex (Shimizu et al. 2010). LYP4 and LYP6 also bind to chitin. As all four
chitin-binding proteins including CEBiP, OsCERK1, LYP4, and LYP6 contain at
least one LysM domain, it can be speculated that this domain is critical to the
perception of chitin oligosaccharides in rice (Liu et al. 2014). CERK1 probably
functions either as a shared coreceptor or signaling partner for LysM domaincontaining PRRs.
After the perception of chitin by CEBiP, CERK1 gets activated and
phosphorylates the guanine nucleotide exchange factor RacGEF1 which is involved
in the activation of the small GTPase OsRac1 (Akamatsu et al. 2013) (Fig. 12.1).
Subsequently, OsRac1 positively regulates RBOHB (RESPIRATORY BURST
OXIDASE HOMOLOGUE PROTEIN B), which is the NADPH oxidase responsible for ROS bursts in rice, showing a direct link between PRR complex activation
and ROS production (Torres et al. 2006; Wong et al. 2007). In addition to RacGEF1,
OsRac1 is also activated through directly interacting with the rice NB-LRR-type R
protein Pit and is necessary for Pit-mediated immunity to the rice blast fungus,
suggesting that OsRac1 plays a pivotal role in both PTI and ETI in rice (Kawano
et al. 2010).
In addition to OsRacGEF1, RLCK176, and RLCK185, which are members of the
rice RLCK family VII, both function downstream of OsCERK1 in chitin- and
peptidoglycan-induced plant immunity (Yamaguchi et al. 2013). Loss of
RLCK185 in rice specifically impaired MAPK activation in response to chitin.
Moreover, RLCK185 is not required for chitin-triggered ROS burst, suggesting
another branch downstream signaling of PRR complex independent of ROS production (Meng et al. 2019b; Yamaguchi et al. 2013).
12.2.3 MSP1-Triggered Immunity in Rice
In addition to the chitin, two additional PAMPs identified include an MSP1 and
MoHRIP1. Of these, MSP1 is a fungal secreted cerato-platanin family protein with
four conserved cysteine residues and is required for pathogen virulence (Wang et al.
2016). At first, it was shown that the deletion of the MSP1 gene in M. oryzae did not
show any phenotypic differences and developed normal appressoria, however, the
virulence of the mutants was greatly reduced as compared to the wild type (Jeong
et al. 2007). Further, it was observed that MSP1 is a secreted protein and is not
associated with the fungal cell wall (Jeong et al. 2007). Ectopic expression of MSP1
(also called as MoSM1) in Arabidopsis induced broad-spectrum disease resistance
against Botrytis cinerea, Alternaria brassicicola, and Pseudomonas syringae
pv. tomato (Yang et al. 2009). Further, the accumulation of reactive oxygen species
(ROS) was observed in the transgenic plants expressing MSP1 (Yang et al. 2009).
Exogenous application of recombinant MSP1 protein resulted in autophagic cell
death and the production of H 2 O 2 in both rice suspension-cultured cells and rice
12 Unraveling the Molecular Mechanism of Magnaporthe oryzae Induced. . .
367
with CEBiP and functions as a crucial component for chitin-triggered immunity in
rice. OsCERK1 and CEBiP heterodimers to form a plasma membrane receptor
complex (Shimizu et al. 2010). LYP4 and LYP6 also bind to chitin. As all four
chitin-binding proteins including CEBiP, OsCERK1, LYP4, and LYP6 contain at
least one LysM domain, it can be speculated that this domain is critical to the
perception of chitin oligosaccharides in rice (Liu et al. 2014). CERK1 probably
functions either as a shared coreceptor or signaling partner for LysM domaincontaining PRRs.
After the perception of chitin by CEBiP, CERK1 gets activated and
phosphorylates the guanine nucleotide exchange factor RacGEF1 which is involved
in the activation of the small GTPase OsRac1 (Akamatsu et al. 2013) (Fig. 12.1).
Subsequently, OsRac1 positively regulates RBOHB (RESPIRATORY BURST
OXIDASE HOMOLOGUE PROTEIN B), which is the NADPH oxidase responsible for ROS bursts in rice, showing a direct link between PRR complex activation
and ROS production (Torres et al. 2006; Wong et al. 2007). In addition to RacGEF1,
OsRac1 is also activated through directly interacting with the rice NB-LRR-type R
protein Pit and is necessary for Pit-mediated immunity to the rice blast fungus,
suggesting that OsRac1 plays a pivotal role in both PTI and ETI in rice (Kawano
et al. 2010).
In addition to OsRacGEF1, RLCK176, and RLCK185, which are members of the
rice RLCK family VII, both function downstream of OsCERK1 in chitin- and
peptidoglycan-induced plant immunity (Yamaguchi et al. 2013). Loss of
RLCK185 in rice specifically impaired MAPK activation in response to chitin.
Moreover, RLCK185 is not required for chitin-triggered ROS burst, suggesting
another branch downstream signaling of PRR complex independent of ROS production (Meng et al. 2019b; Yamaguchi et al. 2013).
12.2.3 MSP1-Triggered Immunity in Rice
In addition to the chitin, two additional PAMPs identified include an MSP1 and
MoHRIP1. Of these, MSP1 is a fungal secreted cerato-platanin family protein with
four conserved cysteine residues and is required for pathogen virulence (Wang et al.
2016). At first, it was shown that the deletion of the MSP1 gene in M. oryzae did not
show any phenotypic differences and developed normal appressoria, however, the
virulence of the mutants was greatly reduced as compared to the wild type (Jeong
et al. 2007). Further, it was observed that MSP1 is a secreted protein and is not
associated with the fungal cell wall (Jeong et al. 2007). Ectopic expression of MSP1
(also called as MoSM1) in Arabidopsis induced broad-spectrum disease resistance
against Botrytis cinerea, Alternaria brassicicola, and Pseudomonas syringae
pv. tomato (Yang et al. 2009). Further, the accumulation of reactive oxygen species
(ROS) was observed in the transgenic plants expressing MSP1 (Yang et al. 2009).
Exogenous application of recombinant MSP1 protein resulted in autophagic cell
death and the production of H 2 O 2 in both rice suspension-cultured cells and rice
12 Unraveling the Molecular Mechanism of Magnaporthe oryzae Induced. . .
367