PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 25

Table 2.2 PAMPs detected in bacterial pathogens
Bacterial PAMPs Elicitor-active domain(s)

Flagellin fl g22, fl g15

Lipopolysaccharides Lipid A part of lipopolysaccharides,

O-antigen polysaccharides

Peptidoglycans Muropeptide

Elongation factors elf18

Cold shock proteins CSP22

Harpins C-terminal fragment

Ax21 Tyrosine-sulfated protein

Bacterial DNA Nonmethylated CpG sequence

Rhamnolipids -

been detected in several bacterial proteins including elongation factor proteins,
cold-shock proteins, harpin proteins, sulfated proteins, and the bacterial superoxide
dismutase enzyme. PAMPs have also been detected in bacterial rhamnolipids and
bacterial DNA structure (Table 2.2 ).

2.6.2 PAMPs Detected in Flagella

2.6.2.1 Flagellin Proteins May Contain Several Distinct PAMPs

Bacterial fl agella have been found to be potential sources for the PAMPs, which trig-
ger innate immunity in both plants and animals. The bacterial extracellular fl agella
are involved in motility of bacteria. The structural components of fl agella include a
basal body capable of rotary motion, a hook apparatus, and thousands of fl agellin
monomers that polymerize to form the fl agellar fi lament (Schuster and Khan 1994 ).
Flagellin is the structural protein that forms the major portion of fl agellar fi laments.
The protein fl agellin contains PAMPs that can be recognized by some plants, leading
to activation of defense responses (Felix et al. 1999 ; Che et al. 2000 ; Gómez-Gómez
and Boller 2002 ; Boller and Felix 2009 ; Nicaise et al. 2009 ). The fl agellin consists of
a conserved elicitor-active domain that is widespread in bacterial species. Synthetic
peptides comprising 15-22 aminoacids of the highly conserved domain within N
terminus of fl agellin acted as elicitors of defense responses at sub-nanomolar con-
centrations in cells of tomato and several other plant species. Peptides comprising
only the central 8-11 amino acids of the active domain had no elicitor activity but
acted as specifi c, competitive inhibitors of fl agellin elicitor activity in tomato cells
(Felix et al. 1999 ). These antagonists suppressed the plant's response to fl agellin,
crude bacterial extracts and living bacterial cells (Felix et al. 1999 ).
A peptide fl g22, the stretch of 22 amino acids in the N terminus of bacterial
fl agellin has been identifi ed as the bacterial PAMP epitope (Naito et al. 2007 ; Boller
and Felix 2009 ). The peptide fl g22 elicits responses in most plant species and is active