PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 21

These observations suggest that it may be diffi cult to distinguish effectors from
PAMPs based on their elicitor function or wide/narrow occurrence.
Another major difference between PAMPs and effectors is in induction of
hypersensitive response (HR). Effectors often induce HR (Wei et al. 1992 ; Ron and
Avni 2004 ; Zhang and Zhou 2010 ), while the PAMPs do not (Mishina and Zeier
2007 ; Thomma et al. 2011 ). However, some of the PAMPs do induce HR. It has
been demonstrated that the PAMP fl g22 induces an HR in Arabidopsis , rice, and
tobacco (Naito et al. 2008 ; Taguchi et al. 2003; Hann and Rathjen 2007 ). CBEL,
the glycoprotein PAMP from Phytophthora parasitica var. nicotianae induces HR
in tobacco and Arabidopsis (Khatib et al. 2004 ).
To call an elicitor as a PAMP, it should be present in a broad range of microbial
species but not in plants, directly bind to plant pattern recognition receptor, and
activate defense responses (Nicaise et al. 2009 ; Tsuda and Katagiri 2010 ). Based on
this defi nition of PAMPs, several elicitors previously classifi ed as effectors may
have to be reclassifi ed as PAMPs. These include harpins produced by bacterial
pathogens (Tampakaki et al. 2010 ; Boureau et al. 2011 ), Nep1-like proteins from
bacterial, fungal, and oomycete pathogens (Gijzen and Nürnberger 2006 ; Kamoun
2006 ), crinkers produced by oomycete pathogens (Haas et al. 2009 ), avrXa21
detected in Xanthomonas species (Lee et al. 2009 ), Avr4 protein from Cladosporium
fulvum (Sterigopoulos et al. 2010 ; Thomma et al. 2011 ), and Ecp2 from different
fungal pathogens (Sterigopoulos et al. 2010 ). All these effector proteins have been
now reclassifi ed as PAMPs (Thomma et al. 2011 ).

2.3 PAMPs Found Within Effectors

Some of the PAMPs may be contained within effectors. The fungal effector ethylene
inducing xylanase (EIX) is an important factor for the success of Trichoderma
viride as an invasive pathogen (Rotblat et al. 2002 ). EIX is not recognized by its
enzyme activity as an elicitor. Instead a PAMP composed of fi ve amino acids of a
surface-exposed

- strand of EIX is essential for its defense response triggering
activity (Rotblat et al. 2002 ). EIX is a fungal effector that contains a PAMP that is
recognized by PAMP-receptors (Mackey and McFall 2006 ). The fungal effector
'AvrPita' from the rice blast pathogen Magnaporthe oryzae contains a PAMP that
interacts directly with the LRRs of the rice R-protein, Pi-ta (Jia et al. 2000 ). AvrL567
effectors from the fl ax rust fungus Melampsora lini contain a PAMP (Dodds et al.
2006 ). A PAMP is also found within a cell wall transglutaminase from the oomycete
Phytophthora sojae (Brunner et al. 2002 ). A mutation was identifi ed that disrupted
the defense eliciting activity without altering the enzymatic activity of the protein.
A 13-aminoacid peptide, called Pep-13, contains the PAMP from within the
transglutaminase (Brunner et al. 2002 ). A bacterial effector from Ralstonia sola-
nacearum , POP2, contains a PAMP that directly interacts with the host R-proteins
RRS1-RRS1-S (Deslandes et al. 2003).
The RXLR effector AVR3a of Phytophthora infestans has dual function; it activates
immune system and also suppresses the defense responses induced by the PAMP

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