Chemistry Reference
In-Depth Information
compounds, alkaloids, non-protein amino acids and a-tocopherols) antioxidant defense
systems cooperatively working on controlling the cascades of uncontrolled oxidation and
protecting plant cells from oxidative damage by scavenging of ROS. Eventually, the
equilibrium has maintained between ROS production and antioxidant defense systems.
However, this balance will always be perturbed by various biotic and abiotic stress factors
such as salinity, UV radiation, drought, heavy metals, temperature extremes, nutrient
deficiency, air pollution, herbicides and pathogen attacks. Once it has been challenged,
various signals pathways start to be proceeded to mediate the disturbances to protect cells
from harm brought by extra ROS. For example, when osmotic stress comes, various plant
species show an obviously reduced assimilation rate due to stomatal closure (Huchzermeyer
and Koyro 2005). This result can be owed to an excessive production of reactive oxygen
species (ROS) who are highly destructive to lipids, nucleic acids, and proteins (Kant et al.
2006; Türkan and Demiral 2009; Geissler et al. 2010).
First and foremost, having been identified as second messengers how does ROS affect stress
signal transduction? Several enzymes which are involved in cell signaling mechanisms are
also potential targets of ROS. These include guanylyl cyclase (E. Vranova, S.
Atichartpongkul, 2002), phospholipase C (C.H. Foyer, G. Noctor, 2003), phospholipase A2
(I.M. Moller, 2001) and phospholipase D (A.G. Rasmusson, K.L. Soole, 2004). Ion channels
may be targets as well (G. Noctor, R.D. Paepe, 2006), among which calcium channels was
mentioned (D.M. Rhoads, A.L. Umbach, 2006). Since calcium has ubiquitous functions in
plant stress signal transduction pathway, we are interested in the relationship between ROS
and calcium. Before dive into calcium, let's back to NADPH oxidases that are an important
ROS-generating system. RBOHs shorting for respiratory burst oxidase homologs is always
an eye-catching topic. Recent evidence points out RBOHs relate to heavy-metal induced
accumulation of ROS (Pourrut et al. 2008) and early response to salt stress (Leshem et al.
2007). Subsequently, ROS produced by Rbohs are thought to activate Ca
2+
channels leading
to further increases in cytosolic Ca
2+
(Foreman et al. 2003) and downstream signaling. In
general, it has been suggested that ROS took part in the regeneration of Ca
2+
signals by
activating Ca
2+
channels. Then additional signal transduction was triggered through Ca
2+
-
mediated pathways (reviewed in Mori, I.C. and Schroeder, J.I. 2004).
Except the interaction with calcium, another route for ROS to work is that ROS themselves
can directly modify signaling molecules through redox regulation. Redox status inside a cell
is essential to the correct functioning of many enzymes, which can be used to alter enzyme
activity; thus alteration of the redox status could be treated as a signaling mechanism
(Gamaley and Klyubin, 1999). One of the most important and well-known redox-sensitive
molecules in this respect is glutathione (GSH), which can form the GSH/GSSG couple. The
balance between the GSH and GSSG takes the central position to maintain cellular redox
state (C.H. Foyer, G. Noctor, 2005). But ROS like H2O2 can affect the process of lowering the
cells' GSH content to alter the redox status. Meanwhile, it also has been suggested that
enzymes such as ribonucleotide reductase and thioredoxin reductase, as well as
transcription factors, might be among the targets for altered redox status. In detail, cysteine
