PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 26

as the full-length fl agellin. However, fl g22 is recognized by rice but this response
was weaker than with full-length fl agellin (Takai et al. 2008 ). The results suggest
that additional PAMP epitopes, besides fl g22 may be present in the full-length fl a-
gellin. A shortened version of fl g22 epitope derived from Escherichia coli , fl g15,
was highly active in triggering innate immunity in tomato, but not in Arabidopsis
(Robatzek et al. 2007 ). In contrast, fl g22 derived from Pseudomonas syringae is
active in both Arabidopsis and tomato (Meindl et al. 2000 ; Bauer et al. 2001 ), sug-
gesting existence of other forms of epitopes similar to fl g22.
The fl g22 region of fl agellin is normally buried in the assembled polymer's tertiary
structure (MacNab 1996 ). Hence it may be diffi cult for the plant pattern recognition
receptors (PRRs) to recognize the buried fl g22 epitope in fl agellin. It is not known
whether plants that do detect fl agellin recognize assembled fl agella shed from the
bacterium, free fl agella, or fragments of degraded fl agellin (Pfund et al. 2004 ). It is
suggested that alternative epitopes of fl agellin may be displayed by shed fl agella
compared with intact fl agellin (Pfund et al. 2004 ).

2.6.2.2 Flagellin from Different Bacteria May Differ in Their Action

Flagellin purifi ed from the incompatible Acidovorax avenae N1141 strain induced
immune responses, whereas fl agellin from the compatible K1 strain induced no
responses (Takai et al. 2008 ). Flagellin purifi ed from the K1 strain was identical to
that of the N141 fl agellin, suggesting that N1141 fl agellin has an epitope in addition
to the fl g22 region capable of eliciting immune responses (Takai et al. 2008 ).
Flagellins purifi ed from P . syringae pv. glycinea , an incompatible pathogen for
tobacco, induced immune responses in tobacco, whereas fl agellin from P . syringae
pv. tabaci , a compatible pathogen, does not, despite complete amino acid identity
(Taguchi et al. 2003b ). Flagellins derived from nonadapted bacteria but having iden-
tical protein sequences differentially induce strong defense responses in nonhost
plants, suggesting that other domains and/or posttranslational modifi cations of
fl agellin may be involved in triggering immune responses (Taguchi et al. 2003a , b ,
2006 ; Takeuchi et al. 2003 , 2007 ). The major difference between various fl agellins
has been suggested to be in the glycosylation sites in fl agellin (Ishiga et al. 2005 ;
Taguchi et al. 2006 ; Takai et al. 2008 ).

2.6.2.3 Flg22 Upregulates Several Signals and Signaling Systems Involved

in Plant Immune Responses

Flg22 trigger the upregulation of Arabidopsis thaliana genes involved in signal
perception ( FLS2 ), Ca 2+ infl ux ( CNGC4 , DND1 ), calmodulin-mediated signaling
( CML41 ), mitogen-activated kinase ( MKS1 , MPK3 , EDR1 ) signaling, phosphatase
( VSP1 ) signaling, reactive oxygen species (ROS) signaling ( RbohC , RbohD , RbohF ),
nitric oxide (NO) signaling ( NOS1 ), salicylic acid (SA) signaling ( NPR1 , SID2 ,
PAD4 , EDS1 , EDS5 ), jasmonate signaling ( LOX3 , OPR3 , ERF4 , CYP81F2 , ACX1 ), and