PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 49

EF-Tu responsiveness was found only in Brassicaceae species (Kunze et al.
2004 ). Transient heterologous expression of AtEFR in Nicotiana benthamiana , a
plant that normally lacks elf18 responsiveness, restores elf18 binding and responses
(Zipfel et al. 2006 ). It suggests that downstream signaling components are con-
served between Brassicaceae and Solanaceae (Nicaise et al. 2009 ). Arabidopsis efr
mutants are more susceptible to Agrobacterium tumefaciens (Zipfel et al. 2006 ).
EF-Tu is recognized by EFR at the host plasma membrane (Zipfel 2008 ). EFR auto-
phosphorylation has also been reported, suggesting that EFR carries active kinase
domain (Xiang et al. 2008 ).

2.11.4 XA21, the PRR for the PAMP Ax21

XA21 is a receptor kinase, which consists of LRR, transmembrane, juxtamembrane
(JM) and intracellular kinase domains (Song et al. 1995 ). XA21 belongs to subfam-
ily XII of the LRR-RKs and is highly similar to EFR. Similar to FLS2 and EFR,
XA21 possesses a non-RD kinase, whose presence has been correlated with a role
in innate immunity across kingdoms (Dardick and Ronald 2006 ).

2.11.5 CERK1, the PRR for the PAMP Chitin

Miya et al. ( 2007 ) identifi ed a receptor-like kinase, designated CERK1 (for Chitin
E licitor R eceptor K inase 1 ) in Arabidopsis as a PRR for the fungal PAMP chitooli-
gosaccharides. Unlike FLS2, EFR, and XA21, CERK1 possesses three extracellular
Lysine Motif (LysM) domains instead of LRRs (Miya et al. 2007 ; Wan et al. 2008a ).
The Lysine motif is a ubiquitous protein module found in prokaryotes as well as
eukaryotes. LysM proteins were fi rst described in bacteria and shown to have bind-
ing capacity for peptidoglycan (PGN), a linear form of alternatively β -1,4-linked

N-acetyl-muramic acid and GlcNAc ( β -1,4 linked N-acetyl-glucosamine) (Zhang

et al. 2009a ).
CERK1 is a plasma membrane protein containing three LysM motifs in the
extracellular domain and an intracellular Ser/Thr kinase domain with autophos-
phorylation/myelin basic protein (MBP) kinase activity, suggesting that CERK1
plays a critical role in fungal PAMP perception in plants (Miya et al. 2007 ; Iriti and
Faoro 2009 ; Petutschnig et al. 2010 ). The CERK1 ectodomain binds chitin and
partially de-acetylated chitosan directly without any requirement for interacting
proteins (Petutschnig et al. 2010 ). The three LysM domains have been shown to be
necessary for chitin binding (Petutschnig et al. 2010 ).
CERK1 contains an intracellular serine/threonine kinase domain, which makes it
an excellent candidate for the Arabidopsis chitin receptor. CERK1 may be involved
in the perception of the chitin oligosaccharide elicitor at the cell surface and the
transduction of the signal into the cytoplasm via its intracellular serine/threonine