Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 355

The mpk4 mutant is impaired in the induction of JA- and ET-responsive genes. The
SA-repressing and the ET/JA-activating functions depend on the defense regulators
EDS1 and PAD4 (Brodersen et al. 2006 ). EDS1 and PAD4 participate in defense
amplifi cation loop that responds to SA and reactive oxygen species (ROS)-derived
signals (Rustérucci et al. 2001 ). Mutations in EDS1 / PAD4 affect SA-ET/JA signal
antagonism as activators of SA but as repressors of ET/JA defenses, and MPK4
negatively regulates both of these functions (Brodersen et al. 2006 ).
MAP kinase acts as a negative regulator of SA signaling and a positive regulator of
JA signaling in Arabidopsis (Petersen et al. 2000 ). Inactivation of MPK4 in mutant
mpk4 Arabidopsis plants resulted in increased expression of SA-responsive genes
and suppression of JA-responsive genes (Petersen et al. 2000 ). MAP KINASE4
SUBSTRATE 1 (MKS1) is the target of MPK4 and phosphorylation of MKS1 by
MPK4 has been reported to repress SA signaling. MSK1 interacts with the WRKY
transcription factors WRKY25 and WRKY33 and both of them can be phosphory-
lated by MPK4 (Andreasson et al. 2005 ). Overexpression of both WRKY25 and
WRKY33 results in repression of SA signaling responses, suggesting that these tran-
scription factors, after activation by phosphorylation by the action of MPK4, suppress
SA signaling system (Zheng et al. 2006 , 2007 ). By contrast, wrky33 mutant plants
showed reduced expression of JA-responsive genes, suggesting that WRKY33 after
phosphorylation by MPK4 activates JA signaling system (Zheng et al. 2006 ). These
studies suggest that MPK4 suppresses SA signaling system, while it activates JA sig-
naling system and it is involved in cross-talk between SA and JA signaling systems.

7.18 MAPK Phosphatases as Negative Regulators

of MAP Kinases

MAPK cascades include both phosphorylation and dephosphorylation events result-
ing in transient increases in MAPK activity. MAP kinases are dephosphorylated and
inactivated by protein phosphatases, including tyrosine-specifi c phosphatases, ser-
ine/threonine-specifi c phosphatases, and dual-specifi city MAPK phosphatases
(MKPs), which are highly specifi c to MAPKs (Keyse 2000 ; Theodosiou and
Ashworth 2002 ). MAPK phosphatases (MKPs) are negative regulators of MAPKs
(Katou et al. 2005 ). Overexpression of the tobacco MKP NtMKP1 compromises
wound-induced SIPK and WIPK, suggesting a role for the MKP in regulating these
MAPKs in vivo (Yamakawa et al. 2004 ). In Arabidopsis , the transmission of ROS
and pathogen signaling by MAPKs involves the coordinated activation of MPK6
and MPK3. MAPK phosphatase 2 (MKP2) regulates plant immune responses and
functionally interacts with MPK3 and MPK6. Plants lacking a functional MKP2
gene show defense response against the bacterial pathogen Ralstonia solanacearum
and by contrast, these plants show enhanced susceptibility against the fungal patho-
gen Botrytis cinerea (Lumbreras et al. 2010 ). This MKP2 function appears to be
linked to differential regulation of MPK3 and MPK6 networks by different types of
pathogens (Lumbreras et al. 2010 ).