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MAPKs also have been found to play important role in the stomatal closure
immune response. AtMPK9 and AtMPK12 are mainly localized in guard cells, and
plants silenced in both MAPKs have strong defects in ABA-induced stomatal
closure (Jammes et al.
2009
). AtMPK9 and AtMPK12 redundantly and positively
control stomatal closure in response to ABA and H
2
O
2
treatment (Jammes et al.
2009
,
2011
). Silencing AtMPK3 in
Arabidopsis
guard cells reduces H
2
O
2
-induced
inhibition of stomatal opening or the promotion of stomatal closure (Gudesblat
et al.
2007
).
A
Nicotiana attenuata
MAPK, NaMPK4, plays an important role in guard cell-
mediated defense against the bacterial pathogen
Pseudomonas syringae
pv.
tomato
DC3000. NaMPK4 appears to act downstream of ABA in regulating stomatal
closure (Hettenhausen et al.
2012
). NaMPK4 also has been reported to function
downstream of ROS to mediate stomatal closure. Knocking down
NaMPK4
of
N
.
attenuata
compromises the stomatal closure response after supplying H
2
O
2
(Hettenhausen et al.
2012
). The results suggest that NaMPK4 acts downstream of
ROS in stomatal closure response. The S-type anion channel-associated 1 is impor-
tant for stomatal closure in response to ABA, H
2
O
2
, and Ca
2+
(Vahisalu et al.
2008
;
Kim et al.
2010
). Downstream of ABA/ROS, NaMPK4 modulates the activity of
S-type anion channels, which regulate the stomatal closure response (Hettenhausen
et al.
2012
). Supplying irNaMPK4 guard cells with Ca
2+
induced the normal stoma-
tal closure, suggesting the importance of Ca
2+
in stomatal closure (Hettenhausen
et al.
2012
). Collectively these studies suggest that ABA- and H
2
O
2
-induced cyto-
solic Ca
2+
([Ca
2+
]cyt) signature activates the anion channels, which regulate stoma-
tal closure.
Arabidopsis thaliana
MPK3 also has been shown to be involved in
stomatal guard cell signaling. It probably acts in signaling downstream of H
2
O
2
in
the signaling system. MPK3 is activated by abscisic acid and H
2
O
2
, which control
stomatal closure (Gudesblat et al.
2007
). These studies suggest that MAP kinase
signaling cascades are involved in PAMPs-triggered stomatal closure immune
responses.
7.23
Effectors Inhibit PAMP-Triggered MAPK Signaling
to Suppress Plant Immune Responses
Plant innate immune systems have high potential to fi ght against a wide range of
viral, bacterial, oomycete, and fungal pathogens (Lacombe et al.
2010
; Hwang and
Hwang
2011
; Alkan et al.
2012
). However, potential pathogens produce several
effectors to nullify the defense responses induced by the innate immune system (Wu
et al.
2011
; Cheng et al.
2012
). To avoid or suppress or delay the expression of the
defense gene-activating signaling systems, the pathogens secrete several effectors
into the host cell (Göhre et al.
2008
; Kim et al.
2010
; Wu et al.
2011
; Cheng et al.
2012
). MAP kinase signaling cascades constitute a major immune response system.
Activation of the MAPK signaling system confers resistance against viral (Shi et al.
2010
,
2011
; Zhang et al.
2011
), bacterial (Brader et al.
2007
; Dóczi et al.
2007
;
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