PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 36

chitin-degrading enzymes (Vidhyasekaran 2007a ). Fungal infection induces the
expression of chitinases in plant cells, and these chitin-degrading enzymes accumu-
late at the sites of invasion. The chitinases release chitin fragments (chitin oligomers
or chitooligosaccharides) from fungal cell walls (Eckardt 2008 ).
The chitooligosaccharides are the classical PAMPs detected in fungi (Miya et al.
2007 ; Hamel and Beaudoin 2010 ; Lizasa et al. 2010 ) and they are known to trigger
the plant innate immune responses in a wide range of plants including both monocots
and dicots (Silipo et al. 2010 ; Shimizu et al. 2010 ; Son et al. 2012 ). Plant cells
perceive the chitin fragments and the elicitor activity of chitin fragments increases
with the increase of degree of polymerization up to the octasaccharide.
N -acetylchitoheptaose generated from cell walls of the rice blast pathogen
Magnaporthe oryzae has been identifi ed as a potential elicitor triggering defense
responses in rice cells (Yamaguchi et al. 2002 ). Chitooctaose has been reported to be
the most potential chitin fragment in eliciting defense responses in plants (Wan et al.
2008a , b ). However, Petutschnig et al. ( 2010 ) provided evidences that insoluble poly-
meric chitin may also be a potential elicitor. They showed that the chitin receptor
CERK1 (for Chitin Elicitor Receptor Kinase 1) binds to polymeric chitin more
strongly than to chitin oligomers. It suggests that the polymeric chitin is potentially
an active molecule in chitin signaling and generation of short chitooligomers by
apoplastic chitinases might not be an absolute prerequisite for chitin recognition.
Chitin treatment of rice induced transient membrane depolarization (Kuchitsu
et al. 1993a ), ion effl ux, cytoplasmic acidifi cation (Kuchitsu et al. 1997 ), transient
generation of ROS (Kuchitsu et al. 1995 ), protein phosphorylation (Kuchitsu et al.
1993b ), and jasmonic acid biosynthesis (Nojiri et al. 1996 ). Chitin oligomers
induced medium alkalinization and ROS generation in suspension-cultured soybean
cells (Day et al. 2001 ). Chitin treatment caused an immediate oxidative burst in
Physcomitrella patens (Lehtonen et al. 2012 ). The chitin-induced oxidative burst
was associated with the induction of alternative oxidase (AOX), lipoxygenase
(LOX), and NADPH oxidase (Lehtonen et al. 2012 ).
Chitin elicits phosphorylation of various proteins. Peck et al. ( 2001 ) identifi ed a
number of proteins that were phosphorylated within minutes after chitin treatment
of Arabidopsis tissue culture cells. Calcium-dependent protein kinase (CDPK) was
transiently induced upon chitin elicitation (Zhang et al. 2002a ). The phosphoryla-
tion event necessary for transmission of the chitin signal was completed within the
fi rst 20 min of chitin addition in Arabidopsis (Zhang et al. 2002a ).
Mitogen-activated protein kinases play important role in chitin signaling (Zhang
et al. 2002a ; Wan et al. 2004 ). Perception of PAMPs by receptors leads to the rapid
activation of MAP kinases including MPK3, MPK4, and MPK6. In particular, MAP
kinase 3 and 6 (MPK3/MPK6) were shown to be rapidly activated by chitin in
Arabidopsis and their activation depended on upstream MAPK kinases (MKK4 and
MKK5) (Wan et al. 2004 ).
Transcription factors (TF) are critical in reprogramming gene expression in plant
cells in response to various stimuli. Plant cells reprogram gene expression in
response to chitin elicitation and 118 TF genes have been shown to be induced by
chitin in Arabidopsis (Wan et al. 2008b ).