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subfamily XII of LRR-RK and consists of an extracellular domain with 28 LRR
motifs, a transmembrane domain, and a cytoplasmic Ser/Thr kinase domain
(Gómez-Gómez and Boller 2000 ). The A. thaliana FLS2 (AtFLS2) LRR domains
9-15 contribute signifi cantly to fl g22 binding (Dunning et al. 2007 ). The exact
fl g22-binding site is unknown. Flg22 directly binds to FLS2 and contributes to
recognition specifi city (Chinchilla et al. 2006 ). FLS2 physically interacts with the
fl g22 epitope and determines ligand specifi city (Chinchilla et al. 2006 ).
The genes similar to Arabidopsis FLS2 genes have been detected in tomato
(Robatzek et al. 2007 ), tobacco (Hann and Rathjen 2007 ; Hann et al. 2010 ), and
Brassica species (Dunning et al. 2007 ). FLS2 with extracellular LRR domain has
been detected in barley, tomato, tobacco, and Arabidopsis (Chinchilla et al. 2006 ,
2007a ; Shen et al. 2007 ). The LRR kinase FLS2, which is homologous to
Arabidopsis FLS2 has been detected in rice (Takai et al. 2008 ; Shinya et al. 2010 )
and an FLS2 - like gene has also been identifi ed in rice (Takai et al. 2008 ). These
results suggest that both monocotyledonous and dicotyledonous plants may pos-
sess a fl g22 perception system.
Flagellin perception is required for full immunity against bacteria, because
plants defi cient in FLS2 are more susceptible to adapted and nonadapted bacterial
pathogens (Zipfel 2009 ). In Arabidopsis , pretreatment with fl g22 restricts growth
of the pathogenic bacterium Pseudomonas syringae pv. tomato DC3000 and fl s 2
mutants are more susceptible to this bacterium (Zipfel et al. 2004 ). Lack of fl agellin
recognition allows more growth of the nonadapted bacteria P . syringae pv. tabaci
and P . syringae pv. phaseolicola (Li et al. 2005b ; de Torres et al. 2006 ). Successful
bacterial pathogens need to avoid or suppress PAMP-triggered immunity induced
by fl agellin (Zipfel 2009 ). Some virulence effectors from phytopathogenic bacteria
do so by directly targeting FLS2 (Göhre et al. 2008 ).
2.11.3
EFR, the PRR for the PAMP EF-Tu
EFR (for EF-Tu RECEPTOR) is the PRR detected in Arabidopsis for binding the
PAMP EF-Tu. It belongs to the same subfamily (LRR-RK XII) as FLS2 (Zipfel
et al. 2006 ). EFR is a receptor-like kinase (Albert et al. 2010 ) and its structure is
highly similar to FLS2, with a 21-LRR extracellular domain, a transmembrane
domain, and a cytoplasmic Ser/Thr kinase domain (Zipfel et al. 2006 ; Albert et al.
2010 ). ERF physically interacts with the fi rst 18 amino acids of the N-terminus of
EF-Tu, the elf peptide (Zipfel et al. 2006 ). No binding of ligand was found with the
ectodomain lacking the transmembrane domain or with EFR lacking the fi rst 5 of its
21 LRRs (Albert et al. 2010 ). It suggests that the transmembrane domain and LRR
ectodomain of EFR are essential for ligand binding.
It is suggested that the EFR receptor, which presumably resides in the plasma
membrane of the plant cells, is exposed to bacterial EF-Tu during infection. It is still
not known how EF-Tu gets released from the bacterial cells; however, EF-Tu has
been detected in the secretome of Xanthomonas campestris and Erwinia chrysan-
themi (Kazemi-Pour et al. 2004 ; Watt et al. 2005 ).
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