PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 50

kinase activity (Miya et al. 2007 ). These serine/threonine kinases have been considered
good candidates for playing a role in fungal chitin reception (Eckardt 2008 ;
Lohmann et al. 2010 ).
CERK1 has a higher affi nity for chitin having a longer residue of N -acetyl
glucosamine (Lizasa et al. 2010 ). CERK1 is autophosphorylated in vitro and chitin
does not affect the phosphorylation of CERK1 (Lizasa et al. 2010 ). CERK1 binds
specifi cally and directly to chitin. LysM RLK1 was shown to bind only to chitin,
and not to colloidal chitosan and peptidoglycan (PGN), although all of them have a
common backbone, GlcNAc. The results suggest that LysM RLK1 may recognize
the acetyl group of N-acetylglucosamine residues of chitin and it may be inhibited
by the bulky peptide group cross-linked to N-acetyl-muramic acid residues of
peptidoglycan (Lizasa et al. 2010 ).
The knock-out mutants for CERK1 completely lost the ability to respond to
the chitin elicitor, including MAPK activation, ROS generation, and gene expres-
sion (Miya et al. 2007 ). The complete loss of the gene responses induced by the
chitin elicitor indicates that CERK1 serves as the 'master switch' of the signaling
cascade. The mutation in CERK1 gene blocked the induction of almost all
chitooligosaccharide-responsive genes and led to more susceptibility to fungal
pathogens but had no effect on infection by a bacterial pathogen (Wan et al.
2008b ). Arabidopsis cerk1 mutants are more susceptible to fungal pathogens
(Miya et al. 2007 ; Wan et al. 2008b ). Exogenously applied chitooligosaccharides
enhanced resistance against both fungal and bacterial pathogens in the wild-type
plants but not in the mutant. These results suggest that CERK1 (LysM RLK1) is
essential for chitin signaling in plants as part of the receptor complex and is
involved in chitin-mediated plant innate immunity (Wan et al. 2008a ).
CERK1 was also involved in bacterial recognition, as cerk1 mutants are more
susceptible to P . syringae pv. tomato DC3000 (Gimenez-Ibanez et al. 2009a ).
cerk1 mutants, however, were not impaired in their responsiveness to fl g22,
elf18, LPS, or PGN (Gimenez-Ibanez et al. 2009a ), suggesting that CERK1 is
involved in the recognition of yet unknown bacterial PAMP. Gimenez-Ibanez
et al. ( 2009b ) reported reduced activation of a PAMP-induced defense response
on plants lacking the CERK1 gene after treatment with crude extracts of the
bacterial pathogen P . syringae pv. tomato DC3000. This strengthens the earlier
fi ndings that CERK1 mediates perception of an unknown bacterial PAMP in
Arabidopsis .
A LysM receptor-like kinase similar to Arabidopsis CERK1 has been detected
in rice. It was designated OsCERK1 and it showed high homology with Arabidopsis
CERK1 (Shimizu et al. 2010 ). OsCERK1 encoded a receptor-like kinase consist-
ing of 624 amino acid residues, containing a signal peptide, an extracellular
domain, a transmembrane region and an intracellular Ser/Thr kinase domain. Motif
analysis indicated the presence of one LysM in the OsCERK1 extracellular domain,
while CERK1 contained three LysM motifs in its extracellular domain (Shimizu
et al. 2010 ). The expression of OsCERK1 was up-regulated by elicitor treatment
(Shimizu et al. 2010 ).