PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 80

et al. 2010 ). The PRR EFR accumulation was signifi cantly reduced when synthe-
sized without N -glycans (Häweker et al. 2010 ). A single N -glycan plays a critical
role for receptor abundance and ligand recognition during plant-pathogen interactions
at the cell surface (Häweker et al. 2010 ).
PRRs such as EFR, FLS2, and the co-receptor BAK1 carry multiple putative
N -glycosylation sites in their ectodomains. Successful folding in the ER and migra-
tion through the Golgi apparatus occur by N -glycosylation. N -glycosylation is
important for folding and subsequent transport of the PRRs EFR and FLS2 to the
cell surface (Häweker et al. 2010 ).

2.19 Signifi cance of PRRs in Innate Immunity

Mutations in PRRs often compromise PAMP-induced defense responses and overall
resistance to pathogens. For example, plants lacking FLS2 are completely defective
in fl g22-induced ROS accumulation, MAPK activation, and defense gene expres-
sion (Gómez-Gómez et al. 1999 ; Asai et al. 2002 ). Arabidopsis plants mutated in
FLS2 are more susceptible to the pathogen Pseudomonas syringae pv. tomato
(Zipfel et al. 2004 ). The FLS2-mediated resistance to this strain is largely attributed
to PAMP-induced guard cell closure that limits bacterial entry into the leaf tissue
(Melotto et al. 2006 ). The fl agellin gene fl i C -induced defenses partially account for
Arabidopsis non-host resistance to P . syringae pv. tabaci strain, a non-adapted
pathogen on Arabidopsis (Li et al. 2005b ).
The efr mutants are completely abolished in all responses to elf18 and show
enhanced susceptibility to Agrobacterium tumefaciens (Zipfel et al. 2006 ). Cerk1
mutants not only are insensitive to chitin treatment and display enhanced suscepti-
bility to fungal pathogens (Miya et al. 2007 ; Wan et al. 2008b ), but also are more
susceptible to P . syringae bacteria (Gimenez-Ibanez et al. 2009a ). Sustained acti-
vation of PRR signaling is important for mounting robust PAMP-triggered immu-
nity (Saijo 2010 ). Collectively these studies demonstrate that PRR function is
essential in triggering immune responses.

2.20 PAMPs-Induced Early Signaling Events

Downstream of PRRs

2.20.1 PAMPs Trigger Complex Networks

of Signaling Pathways

The plant immune system uses several second messengers to encode information
generated by the PAMPs and deliver the information downstream of PRRs to pro-
teins which decode/interpret signals and initiate defense gene expression (Snedden
and Fromm 2001 ; Lecourieux et al. 2006 ; van Verk et al. 2008 ; Mersmann et al.