PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 19

also called as microbe-associated molecular patterns (MAMPs) (Bittel and Robatzek
2007 ; Viterbo et al. 2007 ; Denoux et al. 2008 ; Aslam et al. 2009 ; Boller and Felix
2009 ; Erbs and Newman 2009 ; Jeworutzki et al. 2010 ; Thomma et al. 2011 ).
The plant immune system uses several second messengers to encode information
generated by the PAMPs and deliver the information downstream of PRRs to proteins
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. 2010 ;
Boudsocq et al. 2010 ; Hwang and Hwang 2011 ). Highly complex networks of signal-
ing pathways are involved in transmission of the signals to induce distinctly different
defense-related genes to mount offence against different biotrophic, hemibiotrophic,
and necrotrophic pathogens (Zheng et al. 2007 ; Koornneef and Pieterse 2008 ; Gaige
et al. 2010 ; Gfeller et al. 2010 ; Leon-Reyes et al. 2010 ; Perchepied et al. 2010 ; Katagiri
and Tsuda 2010 ; Ahmad et al. 2011 ; Choi and Hwang 2011 ; Fernández-Calvo et al.
2011 ; Kobeasy et al. 2011 ; Zhu et al. 2011 ; Alkan et al. 2012 ; Cheng et al. 2012 ).
Several studies have provided evidences that plant innate immune systems have
high potential to fi ght against viral, bacterial, oomycete, and fungal pathogens and
protect the crop plants against wide range of diseases (Mandal et al. 2008 ; Zipfel
2008 ; Pitzschke et al. 2009a , b ; Véronési et al. 2008 ; D'Amelio et al. 2011 ; Knecht
et al. 2010 ; Lacombe et al. 2010 ; Molloy 2010 ; Hwang and Hwang 2011 ; Alkan
et al. 2012 ). Potential pathogens contain several PAMPs and they serve as alarm
signals to activate the plant innate immunity.

2.2 Effector-Like PAMPs

Pathogens have additional pathogen-only molecules called effectors, besides
PAMPs (Kwon 2010 ). Effectors are pathogen molecules that manipulate host cell
structure and function thereby facilitating infection and/or triggering defense
responses (Hogenhout et al. 2009 ). Effectors are double-edged swords; they enhance
virulence of pathogens and also trigger resistance in plants carrying cognate defense
receptors (Zong et al. 2008 ).
Effectors induce susceptibility, mostly by suppressing PAMP-induced immune
responses. The effector proteins target basic innate immunity in plants (Block et al.
2008 ; Bartetzko et al. 2009 ; Boller and He 2009 ; Song and Yang 2010 ; Szczesny
et al. 2010 ). The effector AvrBsT from Xanthomonas campestris pv. vesicatoria has
been identifi ed as a suppressor of specifi c plant defense in pepper plants (Szczesny
et al. 2010 ). The effector XopZ Pxo99 from the rice bacterial blight pathogen interferes
with host innate immunity during the pathogen infection process in rice (Song and
Yang 2010 ). The Pseudomonas syringae pv. tomato DC3000 effector HopF2 inter-
cepts PAMP signaling at the plasma membrane immediately of signal perception. It
acts as a potent suppressor of early immune-response gene transcription and mito-
gen-activated protein kinase signaling activated by multiple PAMPs (Wu et al.
2011 ). LysM domain-containing effector protein Ecp6 of the fungal plant pathogen
Cladosporium fulvum mediates virulence through perturbation of chitin-triggered