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Pseudomonas solanacearum
. These
GhMPK16
overexpressing
Arabidopsis
plants
showed rapid accumulation of ROS and enhanced expression of
PR
genes (Shi et al.
2011
). The results suggest that MPK16 is another MAPK gene involved in plant
immune responses.
7.2.11
SIPK and WIPK Activates Plant Immune Responses
by Modulating SA and JA Signaling Systems
Two mitogen-activated protein kinases, salicylic acid-induced protein kinase (SIPK)
and wound-induced protein kinase (WIPK), are involved in plant immune responses
(Ren et al.
2006
; Kallenbach et al.
2010
; Meldau et al.
2012
). WIPK and SIPK play
an important role in JA production and they function cooperatively to control SA
biosynthesis (Seo et al.
2007
). Activation of SIPK and WIPK by their upstream
MAPK kinase (MAPKK), NtMEK2 leads to hypersensitive reaction-like cell death
in tobacco (Jin et al.
2003
). WIPK and SIPK have been implicated in TMV resistance
in tobacco (Kobayashi et al.
2010
). Silencing of WIPK/SIPK reduced TMV accumu-
lation in tobacco and was correlated with an increase in SA and a decrease in JA.
The reduction in viral accumulation was attenuated by expressing a gene for an
SA-degrading enzyme or by exogenously applying JA (Kobayashi et al.
2010
). These
results suggest that WIPK and SIPK function to negatively regulate local resistance
to TMV accumulation, partially through modulating accumulation of SA and JA.
7.3
MAPK Kinases (MAPKKs) in Plant Immune Responses
7.3.1
MKK1 in Plant Immune Responses
Arabidopsis
MAP kinase kinase MKK1 has been shown to be activated by PAMP
elicitors. MKK1 is activated in cells treated with fl g22, and it phosphorylates the
MAPK MPK4. MKK1 negatively regulates the activity of fl agellin-responsive
genes. The
mkk1
mutant is compromised in resistance to both virulent and avirulent
Pseudomonas syringae
strains (Mészáros et al.
2006
). The results suggest that
MKK1 plays an important role in plant immune responses.
A MAPKK, NbMKK1, identifi ed in
Nicotiana benthamiana
is a potent
inducer of hypersensitive response (HR)-like cell death (Takahashi et al.
2007b
).
NbMKK1-mediated cell death was compromised in leaves where NbSIPK
expression was silenced by virus-induced gene silencing. NbMKK1 and NbSIPK
physically interact.
Phytophthora infestans
INF1 elicitor-mediated HR was
delayed in NbMKK1-silenced plants, indicating NbMKK1 is involved in this
immune response pathway (Takahashi et al.
2007a
). NbMKK1 is also involved in
inducing resistance against the bacterial pathogen
Pseudomonas cichorii
(Takahashi et al.
2007b
).
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