PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 44

impaired the interaction between WAK1 and homogalacturonides (Decreux and
Messiaen 2005 ). The results suggest that egg box conformation of OGs is the criti-
cal factor recognized by the PRR WAK1 to activate the innate immune system.
Cabrera et al. ( 2008 ) suggested that there may be at least two different perception
systems for egg box dimers. One binds egg box junctions and the other binds egg
box ends. Perception system 1 may be able to bind OGs in a size-and conformation-
dependent way. The egg box junctions may be the ligand of this perception system
1. Perception system 2 may also bind OGs in a size- and conformation-dependent
way but the ends of the egg boxes may constitute the ligand. Many of the early
defense responses induced by OGs in Arabidopsis cells would depend on this type
2 perception system. According to the hypothesis of Cabrera et al. ( 2008 ), the modi-
fi cation of the reducing end of the OGs does not hinder egg box formation but it
prevents egg box binding to the type 2 perception system.

2.10.2 Cellodextrins as HAMPs

Cellulose-derived oligosaccharides may also play an important role in triggering
innate immune responses (Vidal et al. 1998 ). Cellodextrins (CD), water-soluble
derivatives of cellulose composed of -1,4 glucoside residues, have been shown to

induce a variety of defense responses in grapevine cells (Aziz et al. 2007 ). Degree
of polymerization determines effi cacy of the CD in inducing defense responses.
Oligomers of DP 3 and 4 induced a slight production of H 2 O 2 , those of DP5 and 6
were unable to induce any signifi cant response, while CD of DP > 6 were active.
DP7 oligomer was 2-fold more active than CD 8 and 9 (Aziz et al. 2007 ). The CD
oligomers stimulated chitinase and

β

-1,3-glucanase activities in grapevine cells.
Overall, the high values were obtained with CD of DP 7-9 (Aziz et al. 2007 ).

β

2.10.3 Arabidopsis At Pep Peptides

Besides the oligosaccharides, some peptides of host plant origin have been reported
as endogenous elicitors, which are also called HAMPs. At Pep1 ( Arabidopsis thali-
ana elicitor peptide 1), a 23-aa peptide, was identifi ed in soluble extracts of
Arabidopsis leaves as an endogenous elicitor (Huffaker et al. 2006 ; Yamaguchi et al.
2006 ; Huffaker and Ryan 2007 ; Krol et al. 2010 ; Shinya et al. 2010 ). Two other
endogenous elicitors, At Pep2 and At Pep3, have also been identifi ed in Arabidopsis
and they are distant homologues of At Pep1. All three elicitors are recognized by the
PRRs PEPR1 and PEPR2 (Krol et al. 2010 ). Several other homologs of At Pep1,
including At Pep3, At Pep4, At Pep5, At Pep6, and At Pep7 have been identifi ed in
Arabidopsis (Huffaker et al. 2006 ).
At Pep1 and its homologs, At Pep2-7, are derived from the C-terminal portion of
their precursor proteins PROPEP1-7, respectively (Huffaker et al. 2006 ). PROPEP1
is a 92-aa precursor protein. PROPEP1 belongs to a seven-member gene family