Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 331

2002 ; Teige et al. 2004 ). MPK4 is activated by both biotic and abiotic stresses
(Droillard et al. 2004 ; Teige et al. 2004 ). MAPKKs can also activate multiple
MAPKs in Medicago , tobacco, and Arabidopsis (Yang et al. 2001 ; Asai et al. 2002 ;
Cardinale et al. 2002 ; Jin et al. 2003 ). The tobacco MAPK kinase, NtMEK2, activates
three different MAP kinases, SIPK, WIPK, and Ntf4 (Ren et al. 2006 ).
The MAPKK pair MKK4 and MKK5 seems to mediate elicitor-induced MPK3
and MPK6 activation (Asai et al. 2002 ). Another MAPKK pair of MKK1 and
MKK2 was found to interact, phosphorylate, and activate MPK4, while only MKK2
was able to target also MPK6 (Teige et al. 2004 ). MAPKKs are multifunctional
entry routes for upstream signal integration as well as bifurcation points for activa-
tion of downstream MAPKs (Teige et al. 2004 ). Different kinases are assembled
into distinct modules by scaffold proteins. Scaffold proteins are important for
preventing cross-talk between different cascades and allow a given kinase in more
than one module without affecting the specifi city of the response (Hirt 2000 ).
Several distinct MAPK pathways have been detected in plants (Cardinale et al.
2000 ; Pedley and Martin 2005 ; Mészáros et al. 2006 ). Various stimuli differentially
induce the highly varying MAPK pathways. For example, the Arabidopsis MAPKK
MKK2 is activated by salt and cold stress, but not by the MAMP elicitors fl agellin,
or laminarin. In contrast, the Arabidopsis MKK1 is activated by fl agellin or lami-
narin, but not by salt or cold stress (Teige et al. 2004 ). SA strongly induced p48 and
p44 MAPKs in pea, while these kinases were not activated by JA (Uppalapati et al.
2004 ). Four specifi c MAPK pathways involving MMK2 (for M edicago MAPK2),
MMK3, SAMK (for stress-activated MAPK), and SIMK (for salicylate-induced pro-
tein kinase) in alfalfa were found to be activated to different levels and with different
kinetics by four different elicitors, chitin,

-glucan, ergosterol, and yeast extract
(Cardinale et al. 2000 ). Bacterial elicitor strongly activated MPK6 but resulted in
poor activation of MPK7. MPK6 and MPK7 were both activated by H 2 O 2 (D

β

czi
et al. 2007 ). The tomato MAPKs, LeMPK1 and LeMPK2, were activated in response
to systemin, four different oligosaccharide elicitors, and UV-B radiation. However,
another tomato MAPK LeMPK3 was only activated by UV-B radiation (Holley et al.
2003 ). The tobacco MAP kinase SIPK is activated both by the oomycete elicitor,
β

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-megaspermin and the bacterial elicitor hrpZ psph (Hall et al. 2007 ). However, SIPK
activation induced by the oomycete 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 ).

7.2 MAP Kinases Involved in Plant Immune Responses

7.2.1 Arabidopsis thaliana MPK3 and MPK6 Positively

Regulate Plant Immune Responses

The Arabidopsis MAP kinases, MPK3 and MPK6, have been implicated in positive
plant immune responses (Petersen et al. 2000 ; Innes 2001 ; Zhang and Klessig 2001 ;