PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 20

host immunity. During infection, Ecp6 sequesters chitin oligosaccharides that are
released from the cell walls of invading hyphae to prevent elicitation of host immu-
nity. Since LysM effectors are widely conserved in the fungal kingdom, this type of
effector action may be a common strategy of fungal pathogens in suppressing host
immune systems (de Jonge et al. 2010 ).
The PWL proteins are the effectors detected in the rice blast pathogen
Magnaporthe oryzae (Thomma et al. 2011 ) and the effectors were shown to
have virulence function and suppress host defense responses (Khang et al. 2010 ;
Valent and Khang 2010 ). The effector ATR13 of the downy mildew pathogen
Hyaloperonospora arabidopsidis suppresses callose deposition triggered by the
bacterial pathogen P . syringae (Sohn et al. 2007 ). Circumvention of innate immunity
is crucial for pathogenesis in plants and effectors play important role in suppression
of plant immunity (Block et al. 2008 ; Schornack et al. 2009 ).
While PAMPs are essential for microbial fi tness and survival, effectors specifi -
cally contribute to virulence by targeting host plant innate immunity (Thomma et al.
2011 ). However, this type of differentiation of effectors from PAMPs is only blurred
one, as most of the effectors are also known to trigger innate immunity (Thomma
et al. 2011 ; Gassmann and Bhattacharjee 2012 ). The effector AvrPto from P . syrin-
gae pv. tomato inhibits immune responses in Arabidopsis but triggers immune
responses in some tomato plants carrying resistance proteins Pto, a serine/threonine
kinase, and Prf, a nucleotide-binding leucine-rich repeat (LRR) protein (Zong et al.
2008 ). Plant innate immunity triggered by the effectors is called “effectors-triggered
immunity (ETI)” (Nürnberger and Kemmerling 2009; Gassmann and Bhattacharjee
2012 ), whereas the immunity triggered by PAMP is called “PAMP-triggered immunity
(PTI)” (Tsuda et al. 2009 ; Tsuda and Katagiri 2010 ; Thomma et al. 2011 ). It is
diffi cult to distinguish between PAMPs and effectors based on virulence and elicitor
functions as both of them have the dual virulence and elicitor functions (Cloud-
Hansen et al. 2006 ; Soulie et al. 2006 ; Newman et al. 2007 ; Naito et al. 2008 ;
Taguchi et al. 2010 ; Thomma et al. 2011 ). The bacterial PAMPs fl agellin (Naito
et al. 2008 ; Taguchi et al. 2010 ), lipopolysaccharide (LPS) (Newman et al. 2007 ),
and peptidoglycan (Cloud-Hansen et al. 2006 ) and the fungal PAMP chitin (Soulie
et al. 2006 ) have been reported to have a function in virulence of the pathogens.
Other differences between PAMPs and effectors include the wide occurrence of
PAMPs in microbes as against narrow occurrence of effectors in specifi c pathogens.
PAMPs are generally considered as molecules which are widely conserved across
genera of microbes, while effectors are specifi c to single or a few related species of
pathogens (Jones and Dangl 2006 ; Erbs et al. 2008 ). However, it is now known that
several groups of effector proteins are also widespread (Thomma et al. 2011 ). LysM
effectors widely occur in the fungal kingdom (Bolton et al. 2008 ; de Jonge and
Thomma 2009 ; Thomma et al. 2011 ). The effectors harpins are produced by several
Gram-negative bacteria (Tampakaki et al. 2010 ). The Nep1 - like proteins are the
effectors that are conserved among oomycetes, fungi, and bacteria (Gijzen and
Nürnberger 2006 ; Kamoun 2006 ). In contrast with groups of widely conserved
effectors, some PAMPs are only narrowly conserved (Lee et al. 2009 ). The PAMP
AxYS22 has been detected only in a few Xanthomonas species (Lee et al. 2009 ).
The PAMP Pep-13 is conserved only in Phytophthora species (Brunner et al. 2002 ).