Chemistry Reference
In-Depth Information
wounding, high salinity, UV radiation, ozone, ROS, heavy metals), as well as biotic stress
reactions (Jonak C et al., 2002; Xiong L et al., 2003; Raman, M. et al., 2007; Gohar Taj et al.,
2010; Alok Krishna Sinha et al., 2011).
According to the researches on MAPK pathways, we can see that regulated expression of
MAPK components shows effects on stress sensitivity. Here are some examples. Expression
of an active form of a tobacco MAP3K, NPK1, increases freezing tolerance of transgenic
tobacco or maize plants (Kovtun, Y et al., 2000; Shou, H et al., 2004). Meanwhile, MAP2K1
shows transcriptional induction under salt stress, drought and cold, as well as activated by
wounding and drought stress. And MAP2K1 can phosphorylate MAPK4. An unsurprised
fact is that MAPK4 and MAPK6 are found to be activated by cold, salt and drought
(Ichimura K et al., 2000). Indeed, a MAPK module composed of MAP3K1-
MAP2K1/MAP2K2-MAPK4/MAPK6 has been confirmed in cold and salt stress by yeast two
hybrid analyses and yeast complementation (Teige M et al., 2004). So we can say different
MAPKs are activated at different times after the onset of stress and the activities of these
MAPKs are activated within different time periods. By the way, during osmolarity signaling
MAPKs module seems to be widely involved (reviewed by Gohar Taj et al., 2010).
Due to the interlink between osmotic stress and oxidative stress, we are informed of the
relationship between ROS, hormone signaling and MAPKs. ROS like H
2
O
2
is closely
associated to MAPKs' activities. In Arabidopsis, H
2
O
2
activates AtMPK6 and the related
AtMPK3 via the MAP3K ANP1(Desikan R et al., 1999), and AtMPK6 are involved in cold
stress as we knew before. Additionally, in tobacco, under H
2
O
2
and ozone treatment, the
ortholog of AtMPK, SIPK1 will be activated as well (Samuel MA et al., 2000). These findings
imply that multiple MAPK modules mediate oxidative stress responses and that MAPK
cascades are not only induced by ROS but may also regulate ROS levels. Meanwhile,
activating SIPK1 (salicylic acid-induced protein kinase), who is an NO-activated protein
kinase in tobacco, can not process without SA, which brings a suggestion for the existence of
cross-talk between ROS, hormone signaling and MAPKs. Here is some other evidence
coming from studies on stomatal movement (Eckardt NA., 2009). In guard cells of Vicia
faba, MAP2K is believed to regulate stomatal movement through mediating H
2
O
2
generation induced by ABA (Song XG et al., 2008). Later, in guard cells studies, MAPK9 and
MAPK12 have been proved to serve as positive regulators acting downstream of reactive
oxygen species and calcium signaling in ABA signaling. In 2.2.4, we talk about ABA- and
Ca
2+
-induced stomatal closure, so we can't help wonder is there any link between MAPK
and ABA- and Ca
2+
-induced pathways? Yes, it has been found that ABA and Ca
2+
signals
cannot activate anion channels in mpk9/12 mutants, thus indicating that these two MAPKs
act between the ABA and Ca
2+
signals and the anion channels. (Jammes F et al., 2009).
Even if the role of MAPK in ABA signaling has not yet been directly confirmed, the attempts
to clarify the relationship between hormone and MAPKs never stop. Not just for ABA, the
role of MAPK signaling cascades in auxin signaling and ethylene biosynthesis has been
documented in numerous studies (Dai Y et al., 2006; Xu J et al., 2008). Eventually, the genes
in hormone biosynthesis (ethylene) and responses (auxin) are altered, it is of great
significance to distinguish the direct targets of MAPK cascade from those that are regulated
