PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 43

(HAMPs). Fragments of pectic polysaccharide homogalacturonan (HGA) called
oligogalacturonides (OGs) function as danger signals activating plant innate
immune responses and these OGs are called endogenous elicitors/host-associated
molecular patterns (HAMPs) (Ryan et al. 2007 ; Caffali and Mohnen 2009 ; Galletti
et al. 2009 ; Zipfel 2009 ; Huffaker et al. 2011 ). Oligogalacturonides are generated
by the action of endopolygalacturonases (PGs) secreted by pathogens during the
infection process. The PGs generate OGs by hydrolyzing pectin. The activity of
OGs depends on the degree of polymerization (DP). The OG oligomers with a
degree of polymerization (DP) between 9 and 15 have been shown to be most
potent inducers of defense responses in several plants (Poinssot et al. 2003 ; Aziz
et al. 2004 ; Moscatiello et al. 2006 ; Huang et al. 2007). However, in some reports,
small-sized oligomers have also been shown to induce the accumulation of prote-
ase inhibitor proteins and ethylene synthesis in tomato involved in defense
responses, while larger oligomers were ineffective (Simpson et al. 1998 ).
The formation of a Ca 2+ -dependent “egg box” conformation of OGs has been
shown to be necessary for the biological activity of oligogalacturonide fragments
(Cabrera et al. 2008 ). The term “egg box” is used to designate the calcium-induced
association between chains of homogalacturonides of DP > 8 (Cabrera et al. 2008 ).
In the egg box model, dimerization of two homogalacturonide rigid chains occurs
by cooperative bridging of parallel facing chains through Ca 2+ ions. The binding of
a fi rst calcium cation by two pectin chains facilitates their alignment with respect to
each other, which in turn allows the easier binding of a next calcium ion, and so on
along the sequence (Cabrera et al. 2008 ). It has been shown that a minimum DP of
9 is critical for OGs to form stable egg boxes (Liners et al. 1992 ). At least fi ve
calcium ions are needed between two pectin chains to allow egg box dimer forma-
tion (Liners et al. 1989 ).
Once the generated OGs are of right size, they dimerize, and bind putative
receptors, which trigger production of extracellular polygalacturonase inhibiting
proteins (PGIP) that specifi cally recognize and inhibit PGs produced by pathogens
(Di Matteo et al. 2003 ; Cabrera et al. 2008 ). PGIPs are plant extracellular leucine-
rich repeat proteins that specifi cally bind and inhibit fungal polygalacturonases.
The interaction with PGIP limits the destructive potential of polygalacturonases
and might trigger the plant defense responses induced by oligogalacturonides
(Di Matteo et al. 2006 ).
PG inhibition by PGIP delays OG hydrolysis by 24 h (D'Ovidio et al. 2004a , b ).
The egg box formation by OGs progressively increases with time and it needs
about 10 h for OGs to form fully associated egg boxes (Cabrera et al. 2008 ). The
major function of PGIP may therefore be to delay OGs hydrolysis enough for the
oligomers not only to accumulate but also to mature into more bioactive egg boxes
(Cabrera et al. 2008 ).
The egg box conformation of OGs may be specifi cally recognized by the OG
perception system (Messiaen and Van Cutsem 1999 ). It has been shown that the
extracellular domain of the transmembrane receptor-like wall-associated kinase 1
(WAK1) could ionically bind calcium-associated homogalacturonans (Decreux and
Messiaen 2005 ). Any condition that impaired the formation of egg boxes also