PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 39

acid spanning domain (Pep-13), which is also essential for TGase activity (Brunner
et al. 2002 ). Mutational analysis within Pep-13 identifi ed the same amino acids
indispensable for both TGase and defense-eliciting activity. Several species
of Phytophthora including P . cactorum , P . capsici , P . cinnamomi , P . cryptogea ,
P . drechsleri , P . infestans , P . nicotianae , P . palmivora , P . parasitica , and P . sojae
possess a gene family encoding GP42 TGase-related proteins. GP42 TGase homo-
logs have been detected in all the 10 Phytophthora species tested. The peptide
fragment Pep-13 was found to be highly conserved among all Phytophthora species
tested (Brunner et al. 2002 ).
Pep-13 treatment triggered the accumulation of defense-related transcripts
encoding lipoxygenase, 4-coumarate::CoA ligase and PR protein 1 in potato cells
and in intact potato leaves (Brunner et al. 2002 ). Plants possess neither orthologs of
Phytophthora TGase nor proteins containing peptide with Pep-13 elicitor activity
(Brunner et al. 2002 ). Collectively these studies reveal that Pep-13 is a PAMP found
in Phytophthora spp. Pep-13 binds to its receptor, which is a protease- and heat-
sensitive 100-kDa protein (Nürnberger et al. 1994 ).

2.8.2 Elicitins as Oomycete PAMPs

Elicitins are small lipid binding proteins secreted by the oomycetes Phytophthora
and Pythium (Bonnet et al. 1996 ; Bourque et al. 1999 ; Baillieul et al. 2003 ;
Qutob et al. 2003 ; Kawamura et al. 2009 ; Kim et al. 2010 ). Elicitin genes have
been cloned from Phytophthora infestans (Kamoun et al. 1997a , b ), P . sojae
(Mao and Tyler 1996 ), P . parasitica (Kamoun et al. 1993a , b ), P . cinnamomi
(Duclos et al. 1998 ), P . cryptogea (Panabieres et al. 1995), and P . capsici (Kim
et al. 2010 ). Elicitins are highly conserved 10-kDa proteins that are secreted in
culture by all tested Phytophthora and Pythium species (Kamoun et al. 1993a , b ;
Pernollet et al. 1993 a, b). All the elicitins share a conserved elicitin domain from
amino acids 1 to 98.
The elicitins are grouped into fi ve classes based on their primary structure
(Vidhyasekaran 2007a ). Class I-A and class I-B comprise 98 amino acid-long
elicitin proteins. Class I-A elicitins have an acidic pI, while class I-B elicitins
have basic pI. Class II contains highly acidic elicitins, which possess a short (5-6
amino acids long) hydrophilic C-terminal tail. Class III comprises elicitins with a
long (65-101 amino acids long) amino acid C-terminal domain rich in Ser, Thr,
Ala, and Pro. Elicitins from Pythium spp. are classifi ed as a distinct group called
Py class (Ponchet et al. 1999). They trigger innate immunity in a narrow range of
plants, including Nicotiana species in the Solanaceae and some radish and rape
cultivars in the Brassicaceae (Ponchet et al. 1999). However, they trigger a wide
range of defense responses in most Nicotiana species and this response is suffi -
cient to protect against infection not only by Phytophthora but also by bacteria,
fungi, and viruses (Tyler 2002 ).