PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 28

O -specifi c polysaccharide or O -chain) covalently linked to a lipophilic moiety
termed lipid A. LPS not possessing the O -chain are termed rough LPS or lipooligo-
saccharides (LOSs) (Silipo et al. 2005 ). The polysaccharide moiety contains a long-
chain polysaccharide, called O -antigen, which is highly variable with respect to
composition, length, and the branching of its carbohydrate subunits (Knirel 2009 ).
In contrast, the oligosaccharide core and the lipid A, which form the sheet of the
membrane, are highly conserved in different bacteria (Holst and Molinaro 2009 ).
LPS are present in the outer monolayer of the external membrane of almost all
Gram-negative bacteria. LPS contribute to the structural properties of the cell enve-
lope and play a vital role for bacterial growth (Silipo et al. 2010 ).
The lipid A part of LPS has been considered as the PAMP epitope in LPS. The
lipid A part of LPS is as effective as intact LPS in inducing defense response in
Arabidopsis (Zeidler et al. 2004 ). Phosphorylation and acylation of the lipid A moi-
ety seem to infl uence LPS elicitor activity (Silipo et al. 2008 ). The structure of LPS
of Xanthomonas campestris pv . campestris ( Xcc ) shows a strong accumulation of
negatively charged groups in the lipid A inner-core region and has a number of
novel features, including a galacturonyl phosphate attached at a 3-deoxy-D-manno-
oct-2-ulosonic acid residue and a unique phosphoramide group in the inner core
region. Dephosphorylated LPS molecule, which retains a single negative charge on
the inner core, does not induce any defense response in A . thaliana . It suggests a key
role for the charged phosphate, phosphoramide, and galacturonic residues in LPS
signaling (Silipo et al. 2005 ).
The lipid A moiety is not solely responsible for all of the effects of LPS in plants:
core oligosaccharide and O -antigen components can elicit specifi c responses
(Newman et al. 2007 ). O -chain in LPS may also act as a PAMP, besides the lipid
part. Synthetic oligorhamnans, which are common components of O -chain in LPS,
can trigger innate immune responses in Arabidopsis (Bedini et al. 2005 ). Besides
activating defenses, LPS can suppress defense responses, probably by chelating cal-
cium ions (Newman et al. 2007 ; Tellstrom et al. 2007; Aslam et al. 2008 ).
The O -antigen of the LPS from many phytopathogenic bacteria comprises a
rhamnan backbone with the trisaccharide repeating unit [ α -L-Rha-(1 → 3)- α -L-

Rha-(1 3)]. This trisaccharide was synthesized and oligomer-

ized to obtain hexa-and nona-saccharides. These rhamnans were effective in
elicitation of transcription of the defense-related genes PR1 and PR2 (Bedini et al.
2005 ). The results suggest that the coil structure containing O -antigen polysaccha-
rides may also be a plant-recognizable PAMP.
Both LPS and LOS have been described in Xcc , with LOS being the predomi-
nant form in some strains (Dow et al. 1995 ). Both lipid A and the core oligosac-
charide of Xcc LOS were able to trigger defense responses in A . thaliana (Silipo
et al. 2005 ). LOS induced defense responses in two temporal phases, while the
core oligosaccharide induced only the earlier phase and lipid A induced only the
later phase (Table 2.3 ; Silipo et al. 2005 ). The results suggest that plant cells can
recognize lipid A and core oligosaccharide structures within LPS to trigger
defense responses and that this may occur via two distinct recognition events
(Silipo et al. 2005 ) (Table 2.3 ).

→ 2)- α -L-Rha-(1 →