Redox reactions are evolutionarily a very old signaling principle, which occur in prokaryotes and eukaryotes. The most important redox molecules are reactive oxygen species (ROS), such as singlet oxygen, hydroxyl radical, superoxide anion, hydrogen peroxide, and nitric oxide (·NO). In plants, they have very important signaling functions and are involved in the regulation of transpiration, gas exchange, plant defense response, cell death, germination, and plant growth and development. The diverse functions may be explained by the fact that ROS and ·NO interact rapidly to form a number of reactive nitrogen species, such as ONOO−, NO2, N2O3, and other NOx species. Besides the direct interactions of these redox molecules, both molecules can act as oxidizing agents on proteins, and in this way they can modify the activity or function of proteins involved in ·NO and ROS signaling as well as metabolism and homeostasis.
The first evidence for a physiological interplay between ·NO and ROS was provided by Delledonne et al. (2001). They demonstrated that hypersensitive cell death is only triggered by balanced production of ·NO and ROS and that interaction of ·NO with hydrogen peroxide is required. Additionally, as part of the innate immune response in Arabidopsis (Arabidopsis thaliana), ·NO inhibits NADPH oxidase and regulates cell death (Yun et al., 2011). Moreover, in guard cells, ·NO and ROS act in concert with abscisic acid during stomatal closure (Bright et al., 2006).
Physiologically, ROS and ·NO have both beneficial and deleterious effects, depending upon the concentration and exposure time. Plants have developed effective mechanisms to control ROS levels, protecting themselves from oxidative damage on one side, and they also use ROS as signaling molecules on the other side. ROS are detoxified by the glutathione-ascorbate cycle. The cycle involves the antioxidant metabolites: ascorbate, glutathione, and NADPH and the enzymes linking these metabolites, among them ascorbate oxidase (Noctor and Foyer, 1998). In this issue, Yang et al. (2015) add a new and important aspect to the interplay of ·NO and ROS metabolism and control: the regulation of the ROS-degrading enzyme ascorbate peroxidase by nitric oxide. They demonstrated that ·NO positively regulates the activity of cytosolic ASCORBATE PEROXIDASE1 by S-nitrosylation of Cys-32. S-Nitrosylation of this residue results in an enhanced resistance to oxidative stress and positively affects the immune response. Interestingly, pea (Pisum sativum) cytosolic ASCORBATE PEROXIDASE1 is regulated by ·NO in a dual way. While S-nitrosylation enhances its activity, Tyr nitration results in inhibition (Begara-Morales et al., 2014), demonstrating the complexity of the ·NO and ROS interplay.
A connection between ·NO and ROS has also been demonstrated for other antioxidant enzymes, such as peroxiredoxin II E and superoxide dismutase. S-Nitrosylation of Arabidopsis peroxiredoxin II E inhibits its hydrogen peroxide-reducing and peroxinitrite-detoxifying activities, and in this way, ·NO regulates and fine-tunes the effects of its own radicals but also that of ROS (Romero-Puertas et al., 2007). Superoxide dismutase converts superoxide anions to hydrogen peroxide, which further can be degraded to water by catalase and peroxidase. Thus, this enzyme is an important regulator of cellular redox homeostasis and signaling. Interestingly, Arabidopsis superoxide dismutases are differentially inhibited by peroxinitrite, suggesting a regulatory function under stress conditions (Holzmeister et al., 2015).
All these studies demonstrate the importance of the interplay between ·NO and ROS. They illustrate that ·NO and ROS signaling can underlay synergistic or antagonistic mechanisms or function in parallel. Moreover, the observations by Yang et al. (2015) support the notion that ·NO-based modifications of antioxidant enzymes play a key role in the regulation of oxidative stress and ROS homeostasis and provide evidence for a molecular mechanism of the sensor-effector relationship between ·NO and ROS. Although the ·NO research starts to focus on oxidative stress and redox signaling mediated by ROS, we will probably never be able to solve all ·NO and ROS interactions, but we should and we can identify critical interfaces, such as the interaction of ·NO with ascorbate peroxidase.
Glossary
- ROS
reactive oxygen species
- ·NO
nitric oxide
References
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