Nitric oxide (NO) is involved in many physiological functions of higher plants. Under stress conditions, a family of NO-derived molecules, called reactive nitrogen species (RNS), can cause nitrosative stress. Reactive oxygen species (ROS) participate in the generation of oxidative stress. Recent data show that these two families of molecules are characterized by rigorous metabolic interplay that can regulate a nitro-oxidative stress response in higher plants.
ROS and RNS are necessary by-products of an oxygen- and nitrogen-rich environment. The concept of oxidative stress is very commonly used in plant biology to explain cellular damage caused by the imbalance between ROS production and scavenging antioxidant protective mechanisms. Thousands of research papers have studied their impact on higher plants from many different perspectives in order to elucidate the molecular networks involved in stress perception, signal transduction, and the expression of specific antioxidative stress-related genes and antioxidant metabolites.
At the end of the 1990s, some plant researchers began to study the potential involvement of the ‘old’ NO molecule, which was previously shown to be produced enzymatically in animal cells and to participate in a broad spectrum of functions in cardiovascular, immune, and nervous systems. It is now widely recognized that NO participates in important functions in higher plants through its involvement in physiological and stress-related processes. NO is a free radical associated with a family of molecules called RNS such as peroxyinitrite (ONOO−), nitrogen dioxide (.NO2), dinitrogen trioxide (N2O3) and S-nitrosoglutathione (GSNO) which can mediate post-translational modifications of different bio-molecules, mainly nitration (Corpas et al., 2013) and S-nitrosylation of proteins (Astier & Lindermayr, 2012; Astier et al., 2012).
Interplay between ROS and RNS metabolism
These two families of molecules (ROS and RNS) have been shown to be involved in signaling process of higher plants essentially when they are kept under cellular no-stress situations. However, under adverse environmental conditions, plant cells can be characterized by overproduction of these molecules that are capable of mediating damage to bio-molecules, with proteins being the most frequently studied; a fraction of these same molecules may also have a signaling function in order to attenuate this damage. A good example of interaction between ROS and RNS is the interplay between the superoxide radical (.−) and nitric oxide to generate ONOO−, a powerful oxidant that can mediate the tyrosine nitration of proteins which might be an effective biomarker of nitrosative stress in higher plants (Corpas et al., 2007). Experimental data later confirmed this hypothesis, which was previously observed in an animal system. Thus, it has been verified that diverse abiotic/biotic stresses in different model plants provoke an increase in protein tyrosine nitration (Romero-Puertas et al., 2007; Valderrama et al., 2007; Cecconi et al., 2009; Corpas et al., 2011) that usually involves a loss of function and consequently nitrosative stress. For example, ferredoxin-NADP reductase (FNR) and carbonic anhydrase (CA) are enzymes involved in photosynthesis carbon assimilation where, under high temperature stress, the activity of both enzymes is inhibited due to a process of tyrosine nitration (Chaki et al., 2011, 2013). Similarly, NADP-dependent isocitrate dehydrogenase enzyme is involved in nitrogen metabolism, which also supports the antioxidative system providing NADPH, during root senescence is down-regulated by nitration at Tyr392 (Begara-Morales et al., 2013).
Nevertheless, another set of data also indicates that protein S-nitrosylation needs to be regarded as involved in these processes which have signaling functions. For example, the protein NPR1 (nonexpresser of pathogenesis-related gene1 whose conformation is regulated by the cytosolic redox state) and the transcription factor TGA1 (also called a DNA-binding protein TGA1-like protein) are essential components for regulating salicylic acid-dependent gene expression during systemic acquired resistance (SAR). Both elements can be S-nitrosylated by GSNO which increases the DNA binding activity of TGA1 in the presence of NPR1 (Tada et al., 2008; Lindermayr et al., 2010). Additionally, the connection between ROS and RNS is also demonstrated in the case of the antioxidant enzyme peroxiredoxin which has the capacity to detoxify ONOO−, although its activity is inhibited by a process of S-nitrosylation (Romero-Puertas et al., 2007). In the same way, the Arabidopsis NADPH oxidase, also known as AtRBOHD (for respiratory burst oxidase homolog D), which is responsible for the superoxide radical in response to pathogens, is inhibited by S-nitrosylation at Cys890 (Yun et al., 2011).
Although an interrelationship could be expected to exist between ROS and RNS metabolisms under physiological and adverse environmental conditions, few studies carried out in plant biology have adopted this position (Capone et al., 2004; Zhao et al., 2007). Very recently, new data shows that both ROS and RNS metabolisms under specific environmental stress conditions can cause oxidative as well as nitrosative stress (Valderrama et al., 2007; Molassiotis & Fotopoulos, 2011; Bai et al., 2011). This phenomenon has been described in relation to plants under low temperature (Airaki et al., 2012), arsenic (Leterrier et al., 2012b), salinity (Valderrama et al., 2007; Leterrier et al., 2012a; Tanou et al., 2012), and drought (Signorelli et al., 2013) stress conditions, indicating that some stresses involve both oxidative and nitrosative stress. In other cases, the equilibrium in the production between NO and ROS could determine the final result. For example, in soybean cell suspensions, the generation of hypersensitive cell death requires an appropriate balance between ROS ( and H2O2) and NO production as, even when high levels of NO were observed, cell death did not occur in the absence of a correspondingly strong oxidative burst (Delledonne et al., 2001).
In this context, it is important to point out that S-nitrosoglutahtione (GSNO) and S-nitrosoglutathione reductase (GSNOR), the enzyme that catalyses its decomposition (Leterrier et al., 2011), could be key elements in the interplay between the ROS and RNS metabolisms (Malik et al., 2011). GSNO is considered to be an intracellular NO reservoir and a NO vehicle throughout the cell which is formed by NO's S-nitrosylation reaction with reduced glutathione (GSH). This is not a direct reaction as it appears to take place either through the formation of N2O3 or through the addition of NO to a glutathionyl radical formed during the reaction (Keszler et al., 2010; Broniowska et al., 2013). Glutathione is one of the major, soluble, low-molecular-weight antioxidants and the main nonprotein thiol in plant cells, which is therefore a regulator of the cell's redox state. The ratio of reduced to oxidized glutathione (GSH/GSSG) is therefore considered to be an effective indicator of the physiological state of the cell (Foyer & Noctor, 2011). And usually this ratio decreases under oxidative stress conditions. Recent data show that the content of GSNO is in the same range as oxidized glutathione content (GSSG; Airaki et al., 2011). Accordingly, the GSH/GSNO ratio should be regarded as a new element that contributes to the equilibrium of the cell's redox state and shows that there is a clear connection between both ROS and RNS metabolism. However, GSNOR catalyses the NADH-dependent reduction of GSNO to GSSG and NH3, this enzyme regulates the intracellular level of GSNO and consequently the effects of NO in cells, which is corroborated by experimental data (Feechan et al., 2005; Lee et al., 2008; Chaki et al., 2009; Yun et al., 2011; Espunya et al., 2012; Leterrier et al., 2012a,b).
Figure 1 depicts a simple model showing the interplay between oxidative and nitrosative stress as a consequence of adverse environmental stresses through the interaction between ROS and RNS (Lounifi et al., 2013) where both S-nitroglutathione (GSNO) and peroxynitrite can be regarded as key elements. Thus, the term nitro-oxidative stress might be a more appropriate term in the context of plant biology studies as this complex and emerging research field includes the relationship of ROS and RNS metabolisms under stress situations in higher plants. However, it is important to bear in mind that a fraction of these molecules clearly has a signaling function.
Work in the laboratories is supported by ERDF-cofinanced grants from the Ministry of Science and Innovation (BIO2012-33904) and Junta de Andalucía (groups BIO192 and BIO 286).