PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 27

ethylene (ET) signaling ( ACS2 , ACS7 , ACS8 , CTR1 , ETR1 , ETR2 , EIN2 , EIN4 ,
EBF1 ) systems (Denoux et al. 2008 ). PAMPs generally do not induce hypersensitive
resistance. However, it has been shown that fl g22, as well as fl agellin, induces the
hypersensitive response (Naito et al. 2008 ).

2.6.2.4 Some Peptides Derived from Flagellin May

Not Have Elicitor Activity

Several studies suggest that the N-terminal fl g22 region may not be the sole deter-
minant of fl agellin recognition by plants (Che et al. 2000 ; Taguchi et al. 2003b ;
Tanaka et al. 2003 ). A peptide derived from Ralstonia solanacearum fl g22 region
contains at least one signifi cant change from a consensus sequence derived from
the fl agellin of many bacteria and it did not trigger defense responses (Pfund et al.
2004 ). The Gly to Ala change at position 18 has been shown to reduce the elicitor
activity of fl g22 in tomato cells by 96 % (Felix et al. 1999 ). Felix et al. ( 1999 )
demonstrated that peptides derived from the fl agellins of Agrobacterium spp. also
had no elicitation activity.

2.6.2.5 Flagellin May Not Be a Major Defense Elicitor in Some Bacteria

Flagellin may not be a major defense elicitor in R. solanacearum cells applied
to Arabidopsis thaliana . Flagellin also was not the primary elicitor of responses
in tobacco (Pfund et al. 2004 ). Boiled extracts from R . solanacearum con-
tained a strong elicitor of defense-associated responses. However, R . sola-
nacearum flagellin was not that elicitor, because extracts from wild-type
bacteria and mutants defective in flagellin production all elicited similar plant
responses. The primary eliciting activity in boiled R . solanacearum extracts
applied to Arabidopsis was attributable to one or more proteins other than
flagellin (Pfund et al. 2004 ).

2.6.3 Lipopolysaccharide Components Acting as PAMPs

Liposaccharides (LPS) of bacterial pathogens act as PAMPs triggering immune
responses in both dicots and monocots (Dow et al. 2000 ; Newman et al. 2001 , 2002 ,
2007 ; Gerber et al. 2004 , 2006 ; Desaki et al. 2006 ; Desender et al. 2006 ; Munford
and Varley 2006 ; Nicaise et al. 2009 ; Erbs et al. 2010 ). LPS induces local and sys-
temic resistance and mobilization/translocation occurs through the xylem in
Arabidopsis (Zeidler et al. 2010 ). LPS from Gram-negative bacterial pathogens are
a great source of novel monosaccharides with unusual and occasionally astounding
chemical structures, never found in plants and hence qualify to be recognized as
PAMPs (Molinaro et al. 2009 ). LPS are amphiphilic macromolecules composed of
a hydrophilic heteropolysaccharide (comprising the core oligosaccharide and