PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 32

of biochemical features (Engelhardt et al. 2009 ). Pseudomonas syringae pv.
tomato DC3000 produces two harpins, HrpZ1 and HrpW1 (Kvitko et al. 2007 ).
The harpins HrpN Ea (Wei et al. 1992 ), HrpZ Pss (He et al. 1993 ), HrpZ Psph ,
(Tampakaki et al. 2000), HrpG Xoo (Wen and Wang 2001 ), and HpaG Xooc (Chen
et al. 2008 ) are produced by Erwinia amylovora , Pseudomonas syringae pv.
syringae , P . syringae pv. phaseolicola , and Xanthomonas oryzae pv. oryzae , and
X . oryzae pv. oryzicola , respectively.

2.6.7.2 PAMP May Reside Within the Harpin Structure

It has been demonstrated that harpins act as PAMPs triggering plant immune
responses in several plants (Alfano and Collmer 2004 ; Wu et al. 2011 ). The harpin
proteins HrpZ1 from P . syringae , HrpN from Erwinia amylovora , and PopA from
Ralstonia solanacearum , elicit innate immune responses in a non-cultivar-specifi c
manner in various plants (Wei et al. 1992 ; He et al. 1993 ; Lee et al. 2001a ; Racapé
et al. 2005 ; Wu et al. 2011 ). Transgenic Nicotiana benthamiana and sugar beet
plants expressing hrpG gene of P. syringae pv. phaseolicola triggered the activation
of several defense signaling genes (Pavli et al. 2011 ). The HrpN of E . amylovora
contributes directly or indirectly to callose elicitation on apple leaves (Boureau
et al. 2011 ). The harpin HrpZ1 triggers several defense signaling systems and
hypersensitive response (HR) in various plant species (Nürnberger et al. 2004 ;
Grant et al. 2006 ).
A C-terminal fragment of the HrpZ1 protein retained the ability of the harpin to
trigger plant immunity. Random insertion mutagenesis of HrpZ1 further revealed
that the C- terminus is important for the PAMP activity of the protein (Engelhardt
et al. 2009 ). The 24-amino-acid HrpG fragment found in the C-terminal regions
showed the PAMP activity (Haapalainen et al. 2011 ). These studies suggest that a
PAMP resides within in the harpin structure.

2.6.7.3 Harpin-Binding Sites in Plant Membranes

The harpin HrpZ1 binds to plant membranes with high affi nity and specifi city,
suggesting that the activation of plant immunity-associated responses by HrpZ1 is
receptor-mediated. The binding site found in the microsomes was protease- and
heat-resistant, suggesting that the binding site may therefore not be a protein at all
(Engelhardt et al. 2009 ). HrpN harpin from E . amylovora has been shown to bind
to a small 6.5-kDa plasma membrane-associated protein from apple (HrpN-
interacting protein from Malus , HIPM) and Arabidopsis (AtHIPM) (Oh and Beer
2007 ). These studies suggest that harpins possess all important characters of
PAMPs: wide occurrence in various bacterial species, binding with PRRs and trig-
gering innate immune responses.