Introduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 3

PAMPs have been detected in a wide range of plant pathogens (Shinya et al. 2007 ;
Boller and Felix 2009 ; Silipo et al. 2010 ; Tsuda and Katagiri 2010 ; Nürnberger and
Kufner 2011 ). One of the common features of PAMPs is their presence in a broad
range of microbial species (Brunner et al. 2002 ). The general structure of lipopoly-
saccharides (LPS) is shared by all gram-negative bacteria (Medzhitov 2001 ) and the
protein PAMP fl agellin is highly conserved among bacterial taxa (Felix et al. 1999 ).
Chitin is the widespread, conserved, and intrinsic structure detected in fungi
(Thomma et al. 2011 ). CBEL (for C ellulose- B inding E licitor L ectin) is a glycopro-
tein PAMP and it occurs widely in the oomycete Phytophthora species (Khatib et al.
2004 ). The PAMP double-stranded RNA is a structural signature of several groups
of viruses (Medzhitov 2001 ; Ding 2010 ).
PAMPs are exclusively recognized as the molecules involved in triggering innate
immunity. PAMPs are actually defi ned as the molecules, which bind to plant PRRs
and induce defense responses (Nicaise et al. 2009 ; Tsuda and Katagiri 2010 ).
However, most of the PAMPs also have virulence functions besides eliciting defense
responses (Thomma et al. 2011 ). The well characterized PAMP fl agellin also has a
role in virulence. Glycosylation of the fl agellin molecule has been shown to be
required for virulence in Pseudomonas syringae pv. tabaci (Taguchi et al. 2010 ).
P . syringae pv. tabaci fl agellin mutants affected in their elicitor activity also showed
reduced virulence in plants due to reduced motility (Naito et al. 2008 ; Taguchi et al.
2010 ). The bacterial lipopolysaccharide (LPS) generally acts as PAMP inducing
defenses (Tellström et al. 2007 ; Aslam et al. 2008 ; Silipo et al. 2008 ; Thomma et al.
2011 ). However, changes in composition of LPS affect bacterial virulence, suggesting
a role for LPS in virulence of pathogens (Newman et al. 2007 ). When the PAMP
chitin synthesis was disrupted in the fungal pathogen Botrytis cinerea , virulence of
the pathogen was drastically reduced (Soulie et al. 2006 ). Mutation of peptidoglycan
(PGN) genes reduces the virulence of Ralstonia solanacearum and of Erwinia
amylovora (Cloud-Hansen et al. 2006 ), suggesting the role of the PAMP peptido-
glycan in virulence of pathogens.
PAMPs are detected not only in pathogens, but also in several nonpathogens, and
saprophytes. Since the PAMPs are detected in all microbes, the PAMPs are better
called as microbe-associated molecular patterns (MAMPs) (Viterbo et al. 2007 ; Zhang
et al. 2007 ; Denoux et al. 2008 ; Aslam et al. 2009 ; Jeworutzki et al. 2010 ; Thomma
et al. 2011 ; de Freitas and Stadnik 2012 ). MAMPs are molecular signatures typical
of whole classes of microbes, and their recognition plays a key role in innate immunity
(Boller and Felix 2009 ).

1.2 Plant Pattern Recognition Receptors (PRRs)

PAMPs are perceived as alarm/danger signals by cognate plant pattern recognition
receptors (PRRs) and the PAMP-PRR complex activates the plant immune system
(Takakura et al. 2004 ; Jones and Dangl 2006 ; Altenbach and Robatzek 2007 ; He
et al. 2007 ; Wan et al. 2008 ; Iriti and Faoro 2009 ). Several receptors for the PAMPs