PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 22

INF1 secreted by the pathogen P . infestans in potato (Bos et al. 2009, 2010).
Mutagenesis of the conserved C-terminal tyrosine residue at position 147 in AVR3a
retained the ability to trigger defense, but lost the ability to suppress the defense
responses. The results suggest that tyrosine residue at position 147 in the AVR3a
is the critical factor in suppression of defense responses. Distinct amino acids
appear to condition the two effector activities (induction and suppression of defense
responses) in AVR3a (Bos et al. 2009). In the effector protein AvrPtoB of P . syrin-
gae pv. tomato , the two activities (induction and suppression of plant immunity)
have been shown to be carried out by distinct domains within the protein
(Abramovitch et al. 2003 ; Janjusevic et al. 2006).

2.4 Toxins Acting as PAMPs

Microbial toxins constitute a separate class of elicitors produced by oomycete, fungal,
and bacterial pathogens. These cytolytic compounds function as key virulence
determinants of pathogens (van't Slot and Knogge 2002 ; Glazebrook 2005; Gijzen
and Nurnberger 2006 ) and these toxins can be called effectors. The same toxic com-
pounds also function as PAMPs by acting as nonself recognition determinants for
the activation of plant innate immune responses (Gijzen and Nürnberger 2006 ).
Nep1 (for Necrosis and ethylene-inducing peptide1) from Fusarium oxysporum
f. sp. erythroxyli and Nep1-like proteins (NLPs) detected in several oomycetes,
fungi, and bacteria are host nonselective toxins (Mattinen et al. 2004 ; Pemberton
and Salmond 2004 ; Qutob et al. 2006 ; Staats et al. 2007 ; Kufner et al. 2009 ; Cabral
et al. 2012 ). The NLPs exert cytolytic activity that causes cell maceration and cell
death in dicotyledonous plants in a manner that is similar to disease symptom devel-
opment (Kufner et al. 2009 ). NLPs act as positive virulence factors (effectors) during
infection of plants (Mattinen et al. 2004 ; Pemberton et al. 2005 ; Ottmann et al.
2009 ). The NLPs can also activate defense-related responses (Bae et al. 2006 ; Qutob
et al. 2006 ; Ottmann et al. 2009 ). The NLPs trigger immune responses similar to
that of classic PAMPs. They mediate the activation of MAPKs, induction of ion
fl uxes, production of reactive oxygen species, induction of defense-related genes,
production of phytoalexins and callose deposition. These responses resemble to a
great extent those triggered by classical PAMPs (Kufner et al. 2009 ). The NLPs act
like PAMPs in many instances, as they are detected in a wide range of pathogens
and not in plants and recognize nonself triggering defense responses (Qutob et al.
2006 ). However, NLPs differ from classical PAMPs in that the elicitor-active minimal
motif has not been detected in NLPs and the NLPs are transiently expressed pro-
teins (Qutob et al. 2006 ; Kufner et al. 2009 ; Ottmann et al. 2009 ).
The maize pathogen Fusarium moniliforme produces a phytotoxin, fumonisin
(FB1) that elicits cytolysis of plant cells (Gilchrist et al. 1995). FBI also triggers
accumulation of reactive oxygen species (ROS), deposition of callose, phytoalexin
synthesis, and defense-related genes (Asai et al. 2000 ). Fusarium graminearum and
F . culmorum produce trichothecene family phytotoxins (Nishiuchi et al. 2006 ). The