PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 29

Table 2.3 Relative ability of LOS and lipid A and core oligosaccharide structures within LPS of
Xanthomonas campestris pv. campestris in triggering PR1 gene expression in Arabidopsis thaliana

PR gene expression (fold regulated)
LOS Time after treatment Core oligosaccharide Lipid A

12 h 131 144 11

20 h 5 17 8

24 h 1192 11 448

Adapted from Silipo et al. ( 2005 )

2.6.4 Muropeptides and Sugar Backbone Structure

PAMPs in Peptidoglycans

Peptidoglycan, not found in eukaryotes, is an essential and unique component of
the bacterial envelope that provides rigidity and structure to the bacterial cell.
Peptidoglycan is found as thick outer layer in the cell wall of Gram-positive bacte-
ria, whereas a relatively thin layer is present in the cell wall of Gram-negative
bacteria, where it is overlaid with lipopolysaccharides (Erbs et al. 2008 ; Vollmer
and Born 2009 ). Peptidoglycan from both Gram-positive and Gram-negative
bacteria is composed of a network of glycan strands that are interlinked by short
peptides. The glycan chains are formed by alternating N -acetylmuramic acid
(MurNAc) and N -acetylglucosamine (GLcNAc) linked by

β →

4)-glycosidic
bonds (Cloud-Hansen et al. 2006 ; Erbs et al. 2008 ; Cirillo et al. 2010 ; Silipo et al.
2010 ). The presence of the lactyl group of the muramic acid allows for the covalent
attachment of a short peptide stem that typically contains alternating L- and
D-amino acids. The structure of the carbohydrate backbone is generally conserved
in all bacteria, but different degrees of acetylation are the major variations among
the bacteria (Silipo et al. 2010 ). Glycan strands are frequently deacetylated and/or
O -acetylated in bacterial species (Volmmer 2008). The peptide moiety also dis-
plays considerable diversity among the Gram-positive and Gram-negative bacteria.
In general, the third-position amino acid in Gram-positive bacteria is L-lysine
(Lys), whereas Gram-negative bacteria possess the meso -2,6-diaminopimelic
(DAP) as the third amino acid (McDonald et al. 2005 ). Gram-positive bacteria
have peptide stems that are usually cross-linked through an interpeptide bridge
(generally glycine), whereas gram-negative bacteria peptide stems are usually
directly crosslinked (Erbs et al. 2008 ).
Peptidoglycan is located on most bacterial surfaces, which constitute excellent
targets for recognition by the innate immune system. Peptidoglycan is considered as
a typical PAMP, because it is widely found in bacteria, structurally stable, displayed
on the cell surface and not found in plant cells (Gust et al. 2007 ). Peptidoglycans
from both gram-positive and gram-negative bacteria have been reported to be
PAMPs (Gust et al. 2007 ; Erbs et al. 2008 ). Perception of gram-positive peptidogly-
cans mostly depends on their sugar backbones (Gust et al. 2007 ), whereas

-(1