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and overexpressing AtMPK6 enhanced the ABA-dependent expression of
CAT1
and H
2
O
2
production. The activity of AtMPK6 was increased by ABA in an
AtMKK1-dependent manner. These results suggest an ABA-dependent signaling
pathway connecting
CAT1
expression through a phosphorylation system including
AtMKK1 and AtMPK6 (Xing et al.
2008
).
In another study, Xing et al. (
2007
) showed that ABA-induced expression of
CAT1
catalase is mediated by an
Arabidopsis
MAPK kinase, AtMEK1, by trigger-
ing H
2
O
2
signal production. The
mek1
mutant totally blocked stress-induced
CAT1
expression and H
2
O
2
production. Overexpression of
AtMEK1
signifi cantly induced
CAT1
expression and H
2
O
2
production (Xing et al.
2007
). These results suggest that
MAPK pathway acts upstream of H
2
O
2
signaling.
An
Arabidopsis
MAPKKK, MEKK1 has been shown to act downstream of H
2
O
2.
MEKK1 kinase activity and protein stability was regulated by H
2
O
2
in a proteasome-
dependent manner (Nakagami et al.
2006
). H
2
O
2
has been shown to activate two
MAPKs, AtMPK6 and AtMPK3 in
Arabidopsis
(Grant et al.
2000
; Kovtun et al.
2000
;
Desikan et al.
2001
), through a MAPKKK, AN1 (Kovtun et al.
2000
). In maize, H
2
O
2
has been reported to activate a 46 kDa MAPK (Zhang et al.
2006
). In tobacco, ROS
induced SIPK and WIPK (Samuel and Ellis
2002
).
7.14
MAP Kinases Positively or Negatively Regulate SA
Signaling System
Several MAP kinases are known to positively or negatively regulate SA-signaling
systems in plants. The
Arabidopsis MKK7
gene encoding MAP kinase kinase 7
positively regulates SA-signaling system (Zhang et al.
2007c
). The activation-
tagged
bud1
mutant, in which the expression of MKK7 is increased, accumulates
SA, exhibits constitutive PR gene expression, and displays enhanced resistance to
both the oomycete pathogen
Hyaloperonospora parasitica
and the bacterial
pathogen
Pseudomonas syringae
pv.
maculicola
(Zhang et al.
2007c
). Expression
of a WIPK-activated transcription factor results in increase of endogenous sali-
cylic acid in tobacco (Waller et al.
2006
). Salicylic acid levels were 50-fold higher
in the transgenic plants than those in wild-type plants. The levels of JA did not
signifi cantly differ (Waller et al.
2006
). SA pathway genes were more rapidly and
strongly induced in plants overexpressing a MAPK gene. These tobacco plants
showed enhanced defense responses against fungal and viral pathogens (Shi et al.
2010
). Collectively these results suggest that the MAPKs may act upstream of SA
biosynthesis.
MPK4 acts as a negative regulator of defense responses through a salicylic acid-
dependent signaling system (Petersen et al.
2000
). The
mpk4
knockout mutant
shows elevated SA levels and constitutively expresses pathogenesis-related (PR)
genes (Petersen et al.
2000
). Expression of the bacterial NahG salicylate hydroxy-
lase in
mpk4
plants abolishes PR gene expression, indicating the role of the MAPK
in SA-mediated signaling system (Petersen et al.
2000
; Brodersen et al.
2006
).
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