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AtMPK3 orthologue), since the WIPK activity induced by ozone was significantly
reduced in SIPK overexpression lines, whereas the opposite was true for the SIPK
suppression line (Samuel and Ellis
2002
). Moreover, the suppression of OsSIPK
(OsMAPK6) in rice was shown to increase the abundance of OsMAPK5a transcripts
(an orthologue of NtWIPK) (Reyna and Yang
2006
). These findings indicate that
SIPK negatively regulates WIPK activation in response to ozone and other abiotic
stresses. It is highly likely that such regulation is conserved in plants under diverse
environmental conditions (Cho et al.
2009
).
Over the past years, various MAPKs have been identified and characterized in
several plant species. SIPK and its orthologue in other plants are of particular
interest, as a large body of evidence demonstrated their involvement in the fine-
tuned regulation of plant responses to ozone, wounding, SA and JA. Their function
also appears to be conserved across reference plants, such as rice, tobacco and
Arabidopsis (Cho et al.
2009
). The first observation of the OsSIPK response to JA
was the transcriptional upregulation of OsSIPK by JA treatment (Rakwal and
Agrawal
2003
). It should be noted that the JA-induced OsSIPK transcript was
significantly stronger than that induced by wounding. So far, most MAPKs reported
were found to be JA-responsive. It was also shown that a rice gene encoding an
MAPK kinase kinase, OsEDR1, is constitutively expressed in seedling leaves and is
up-regulated within a few minutes upon wounding, or treatment with JA, SA,
ethylene, ABA or H
2
O
2
(Kim et al.
2003
b). Two other rice (Oryza sativa L.)
MAPKs, OsMSRMK3 (multiple stress responsive) and OsWJUMK1 (wound- and
JA-uninducible), which most likely exist as single copy genes in the genome, were
also isolated (Agrawal et al.
2003
). The steady state mRNA analysis of these
MAPKs revealed that OsMSRMK3 was up-regulated by wounding, JA, SA, eth-
ylene, ABA, H
2
O
2
, protein phosphatase inhibitors, chitosan, high salt/sugar and
heavy metals, whereas OsWJUMK1 was not induced by either wounding, JA or SA,
and only showed up-regulation as a result of H
2
O
2
, heavy metals and cold stress.
MPK4 also acts as a negative regulator of SA-mediated defence against bio-
trophic pathogens, while it is essential for the JA- and ET-mediated defence against
necrotrophic pathogens (Brodersen et al.
2006
).It has been shown that mpk4 mutants
constitutively express SA-dependent stress genes, and this was found to be due to
elevated levels of SA, as the dwarfed phenotype of these mutants could be rescued
by the expression of the bacterial nahG gene (Petersen et al.
2000
).
MAPK signalling cascades were reported to be induced by cold/salt, drought,
O
3
-induced ROS, osmotic stress and wounding or ABA-induced H
2
O
2
production.
Besides abiotic-induced functions, a number of studies have also revealed the role
of MAPK pathways in other processes, including stomatal patterning and auxin
signalling (Colcombet and Hirt
2008
).
Oxidative stress also causes Ca
2+
influx into the cytoplasm from the extracel-
lular environment and from the endoplasmic reticulum or sarcoplasmic reticulum
(Ermak and Davies
2001
). Ca
2+
may control the activity of plant protein kinases
throughindirect or direct interaction with the enzymes. Indirect interactions
involve calmodulin, a calcium-binding protein. Six Arabidopsis genes designated
as AtSR1-6, related to a tobacco early ethylene-responsive gene encoding a
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