Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 344

mpk6 mutants (Beckers et al. 2009 ). These results suggest that prestress deposition
of the signaling components MPK3 and MPK6 is a critical step in priming plants for
full induction of defense responses during induced resistance.

7.8 PAMP Signals Activate MAP Kinases

PAMP and DAMP/HAMP elicitors activate several MAP kinases in plants. A typi-
cal array of early defense responses induced by PAMPs includes Ca 2+ infl ux and the
generation of ROS, nitric oxide, and ethylene. Much of this follows the activation of
mitogen-activated protein kinase cascades, leading to transcriptional changes of
many defense-related genes (Aslam et al. 2009 ; Boller and He 2009 ; Boller and
Felix 2009 ). PAMP-triggered immunity requires a signal transduction from recep-
tors to downstream components via the MAPK cascade and many of the known
PAMPs were shown to activate MAP kinases (Pitzschke et al. 2009b ). Four specifi c
MAPK pathways involving MMK2 (for M edicago MAPK2), MMK3, SAMK (for
stress-activated MAPK), and SIPK (for salicylate-induced protein kinase) in alfalfa
were found to be activated to different levels and with different kinetics by four dif-
ferent elicitors, chitin, -glucan, ergosterol, and yeast extract (Cardinale et al. 2000 ).

The tomato MAPKs, LeMPK1 and LeMPK2, were activated in response to four
different oligosaccharide elicitors (Holley et al. 2003 ). The tobacco MAP kinase
SIPK is activated both by the oomycete elicitor,

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-megaspermin and the bacterial
elicitor hrpZ psph (Hall et al. 2007 ). However, SIPK activation induced by the oomy-
cete elicitor required external calcium infl ux, whereas that induced by the bacterial
elicitor does not. It suggests that SIPK activation is involved in different elicitor-
initiated signaling pathways (Hall et al. 2007 ).
The Arabidopsis MKK1 has been shown to be activated by fl agellin or laminarin
(Teige et al. 2004 ). Arabidopsis MPK3 and MPK6 have been shown to be positive
regulators of plant immune responses. These MAPKs are activated by PAMP elici-
tors and DAMPs (endogenous elicitors) (Asai et al. 2002 ; Bethke et al. 2012 ). The
bacterial PAMP fl g22 and the oligogalacturonides elicitor of host plant origin acti-
vated MPK3 and MPK6 in Arabidopsis (Galletti et al. 2011 ). Phytophthora infestans
INF1 elicitor activates NbMKK1 in Nicotiana benthamiana (Takahashi et al. 2007b ).
The MAPK module MEKK1-MKK4/MKK5-MPK3/MPK6 has been proposed
to be responsible for fl g22 signal transmission (Asai et al. 2002 ; Bethke et al.
2009b ). The PAMP fl g22 triggers a rapid and strong activation of MPK3, MPK4 and
MPK6 (Droillard et al. 2004 ). The PAMP fl g22 treatment activated MPK11 in
Arabidopsis and MPK11 constitutes a fourth MAPK activated by fl g22, in addition
to MPK3, MPK4, and MPK6 in Arabidopsis (Bethke et al. 2012 ). Flagellin-derived
fl g22 peptide strongly activated MPK6 but it only poorly activated MPK7 in
Arabidopsis (Dóczi et al. 2007 ). Arabidopsis MAP kinase kinase MKK1 is acti-
vated in cells treated with fl g22, and it phosphorylates the MAPK MPK4 (Mészáros
et al. 2006 ). Thus several studies have shown that MAPK signaling is activated by
fl agellin in Arabidopsis (Tsuda et al. 2009 ; Wu et al. 2011 ). Although such studies

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