Chemistry Reference
In-Depth Information
residues of molecules may be key active sites as targets for redox regulation. And these
molecules can act as potential sensors for ROS (Xiong, L. and Zhu, J.K. 2002).
Secondly, ROS are very likely to play a significant role in the activation of stress-responsive
genes, especially those who encode enzymes responsible for antioxidants biosynthesis or
enzymes directly detoxify reactive oxidative radicals. For example, H
2
O
2
production is
thought to be raised under various abiotic stresses, which can enhance gene expression of
active oxygen scavenging (AOS) enzymes. NO, produced under salt stress, could serve as a
second messenger for the induction of PM H-ATPase genes' expression, which promote PM
H-ATPase activity (Liqun Zhao, Feng Zhang et al., 2004).
Thirdly, we are going to stress a little more on H
2
O
2
and NO. In maize, H
2
O
2
production
grows up induced by chilling stress, and exogenously applied H
2
O
2
lifted up chilling
tolerance (T.K. Prasad, M.D. Anderson, 1994). Increased H
2
O
2
production has been detected
occurring gradually responding to salt stress in rice plants (N.M. Fadzilla, R.P. Finch, 1997).
Moreover, H
2
O
2
was also reported to induce small heat shock proteins (HSP26) in tomato
and rice (J. Liu, M. Shono, 1999; B.H. Lee, S.H. Won, H.S. Lee, 2000). However, it was
recently shown that H
2
O
2
produced by apoplastic polyamine oxidase can influence the
salinity stress signaling in tobacco and can play a role in balancing the plant response
between stress tolerance and cell death (Moschou et al. 2008). NO has also been suggested to
act as a signal molecular mediating responses to biotic and abiotic stresses. Under salt stress,
NO could serve as a second messenger for the induction of PM H-ATPase expression, which
may account for the enhanced PM H-ATPase activity. Thus, ion homeostasis is reestablished
so as to adapt to salt stress (Liqun Zhao, Feng Zhang et al., 2004).
Furthermore, researches on ABA give us much more information on H
2
O
2
and NO in signal
transduction. So let's take a look at how they work in ABA signaling and other signal
transduction pathways. The process of stomata closure regulated by ABA in a large sense
require the generation of H
2
O
2
. Moreover, H
2
O
2
production may be a prerequisite for ABA-
induced stomatal closure (Zhang, X., Zhang, L. et al. 2001). Experiments have found out
mutations in genes encoding catalytic subunits of NADPH oxidase, known as the major
source for H
2
O
2
production, will impair ABA-induced ROS production, as well as the
activation of guard cell Ca
2+
channels and stomata closure (Kwak J.M., Mori, I.C. et al. 2003).
In plants, both nitrate reductases and NO synthases (NOS) can contribute to NO generation.
Loss-of-function mutations in Arabidopsis NOS, AtNOS1, impair ABA-induced NO
production and stomata closure (Guo, F.Q., Okamoto, M. and Crawford, N.M., 2003).
On the other hand, accumulated evidence indicate ROS seem to play a central role in
regulating Mitogen-activated protein kinase (MAPK or MPK) cascades (discussed later in
detail). However, only the functions of MPK4, MPK3 and MPK6, out of the 20 Arabidopsis
MAPKs, have been thoroughly characterized. What really counts is they can all be activated
by ROS and abiotic stress. In addition, activities of MPK1 and MPK2 have been shown to be
provoked by H
2
O
2
and ABA (Ortiz-Masia et al. 2007). Also MPK7 was found to be activated
by H
2
O
2
under specific circumstances (Dόczi et al. 2007). Furthermore, the authors found
that H
2
O
2
may probably have a generally stabilizing impact on MAPKKs (MAP kinase
