Nitric oxide imbalance provokes a nitrosative response in plants under abiotic stress Francisco J. Corpasa, , , Marina Leterriera, Raquel Valderramab, Morad Airakia, Mounira Chakia, José M. Palmaa and Juan B. Barrosob
a Departamento de Bioquímica, Biología Celular y Molecular de Plantas, Estación Experimental del Zaidín (EEZ), CSIC, Granada, Spain
b Grupo de Señalización Molecular y Sistemas Antioxidantes en Plantas, Unidad Asociada al CSIC (EEZ), Área de Bioquímica y Biología Molecular, Universidad de Jaén, Spain Received 20 January 2011;
revised 11 March 2011;
accepted 12 April 2011.
Available online 22 April 2011.
Nitric oxide (NO), a free radical generated in plant cells, belongs to a family of related molecules designated as reactive nitrogen species (RNS). When an imbalance of RNS takes place for any adverse environmental circumstances, some of these molecules can cause direct or indirect damage at the cellular or molecular level, promoting a phenomenon of nitrosative stress. Thus, this review will emphasize the recent progress in understanding the function of NO and its production under adverse environmental conditions.
Full-size image (13K)
? Nitric oxide has a family of NO-related molecules designated as reactive nitrogen species (RNS) such as S-nitrosothiols or peroxynitrite. ? RNS participate in the mechanism of response against environmental stresses and can provoke post-translational modifications. ? A rise of
protein tyrosine nitration is a reliable footprint of nitrosative stress. Keywords: Abiotic stress; Nitric oxide; Nitrosative stress; Nitrotyrosine; Reactive nitrogen species; Salinity
Abbreviations: ABA, abscisic acid; CLSM, confocal laser scanning microscopy; cPTIO, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; DAF-2 DA, 4,5-diaminofluorescein diacetate; DAF-FM DA, 4-aminomethyl-2′,7′-difluorofluorescein diacetate;
DPI, diphenyleneiodonium; GSNO, nitrosoglutathione; JA, jasmonic acid; L-NAME, L-NG-nitroarginine methyl ester; L-NNA, NG-nitro-l-arginine; L-NMMA, L-NG-monomethyl
arginine acetate; NR, nitrate reductase; NO, nitric oxide; NOS, nitric oxide synthase; O2?,
superoxide radical; ONOO?, peroxynitrite ion; PTIO,
2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; RNS, reactive nitrogen species; ROS, reactive oxygen species; SA, salicylic acid; SNAP, S-nitroso-N-acetyl-penicillamine; SNOs, S-nitrosothiols; SOD, superoxide dismutase; SNP, sodium nitroprusside
Metabolism of NO in plant cells
2.1. Potential sources of NO
2.2. Reactive nitrogen species and nitrosative stress
Nitric oxide and environmental abiotic stress
3.2. Water stress
3.3. Extreme heat and cold
3.4. Mechanical wounding
3.5. UV radiation
3.7. Heavy metal
The gases nitric oxide (NO) and nitrogen dioxide (NO2) are commonly known under the term nitrogen oxides (NOx). The main interest of these gases concerns their involvement in air pollution because they contribute to acid rain, and the depletion of the ozone layer and they have harmful effects on human heath. However, the attention on NO suffered a significant change when it was identified, separately by Moncada and Ignarro, as the endothelium-derived relaxing factor (EDRF)  and . Later, it was demonstrated that this gas was generated in mammalian cells from the amino acid l-arginine by a family of enzymes designated as nitric oxide synthases (NOS)  and . Thus, in animals NO participates in a broad spectrum of functions in the cardiovascular, immune, and nervous systems . However, NO is also involved in a wide array of human pathologies such as tumours, heart disease, asthma, infection diseases, diabetes, hypertension, atopic dermatitis, Alzheimer's disease, Parkinson's disease, or amyotrophic lateral sclerosis, among others. In higher plants, NO also has an important function in plant growth and development, including seed germination, primary and lateral root growth, flowering, pollen-tube growth regulation, fruit ripening, senescence, defence response, and abiotic stress, in addition to being a key signalling molecule in different intracellular processes , , , ,  and . 2. Metabolism of NO in plant cells
Nitric oxide or nitrogen monoxide is a colourless gas which belongs to the free radical-type molecules because it has an unpaired electron in its π orbital, making NO a special molecule.
Among these properties, NO has a solubility of 1.9 mM in aqueous solutions at 1 atm pressure; it diffuses at a rate of 50 μm per s, its in vivo lifetime is relatively short (less than 10 s), and it is a
lipophilic radical that can diffuse across cell membranes as well as through the cytoplasm, reacting with a certain number of macromolecules (proteins, lipids, nucleic acids, etc.). Another relevant aspect of NO or NO-derived molecules is that directly or indirectly they may be involved in post-translational changes including binding to metal centres, S-nitrosylation of thiol groups, and the nitration of tyrosine, which may be involved in cell signalling under physiological and stress conditions.
2.1. Potential sources of NO
Even when the relevance of NO in plants is completely recognized, at present, a major question is still to determine which endogenous NO source(s) is (are) involved in a given physiological or stress process. Hence, plants can generate NO by non-enzymatic and enzymatic mechanisms , ,  and , the l-arginine and nitrite pathways being the most plausible routes. The production of NO for l-arginine-dependent nitric oxide synthase (NOS) activity has been reported in different plant species having similar cofactor requirements, identified in mammalian systems (for a review see Corpas et al. ). Fig. 1A shows the biochemical characterization of a NOS activity in sunflower, where NO production depends strictly on the l-Arg concentration, BH4, with calmodulin and calcium activity strongly being reduced (82–97%) in the absence of its cofactors
(NADPH, FAD and FMN). In addition, NOS activity is also reduced a 97% in the presence of the animal NOS inhibitors AG or L-NMMA . However, at this moment, search for the protein and gene of plant NOS continues  and . Very recently the sequences of two l-arginine dependent NOS enzymes have been described from two green algal species of the genus Ostreococcus . Furthermore, nitrate reductase (NR) is another NO producer, using NO2? and NADH as substrates , and it has been proposed that it is involved in NO production in some processes such as stomatal closure . However, there is little information on the direct production of NO from NR in plant stress and the available information came from NR mutants that are impaired in NO production .
Full-size image (16K)
Biochemical characterization of l-arginine-dependent nitric oxide synthase (NOS) activity in hypocotyls of sunflower (Helianthus annuus L). Reaction mixtures containing hypocotyl samples were incubated in the absence and presence of l-arginine (1 mM or 0.1 mM), NADPH (1 mM), EGTA (0.5 mM), cofactors (10 μM FAD, 10 μM FMN and 10 μM BH4), calmodulin (CaM),
L-NG-monomethyl arginine acetate (L-NMMA, competitive and irreversible inhibitor of all three animal NOS isoforms) and 1 mM aminoguanidine (AG, an animal NOS activity inhibitor). The
NOS activity was quantified from the NO produced, which was determined by the ozone chemiluminescence method. l-Arg, l-arginine .
View Within Article
Additionally, it is known that the non-enzymatic reduction of nitrite can lead to the formation of NO, and this reaction is favoured at acidic pH values, when nitrite can convert to NO and nitrate. Nitrite can also be chemically reduced by ascorbic acid at pH 3–6 to yield NO and
dehydroascorbic acid. This reaction could occur under microlocalized pH conditions in barley aleurone layers and in the chloroplast and apoplastic space where ascorbic acid is known to be present. Another non-enzymatic mechanism proposed for NO formation is the light-mediated reduction of NO2 by carotenoids  and .
Polyamines include molecules such as putrescine, spermidine or spermine, which are synthesised from l-arginine. They are aliphatic nitrogen compounds positively charged at physiological pH, allowing its interaction with molecules having a negative charge such as proteins, nucleic acids or phospholipids. Recently, it has been shown that polyamines can induce the production of NO , but the mechanism is unknown. Future analysis in this sense could clarify the new signalling pathway considering that polyamines are also involved in the response mechanism against stress conditions .
2.2. Reactive nitrogen species and nitrosative stress
The term reactive nitrogen species (RNS) was introduced in the biological literature to designate NO and other NO-related molecules, such as S-nitrosothiols (SNOs), S-nitrosoglutathione (GSNO), peroxynitrite (ONOO?), dinitrogen trioxide (N2O3) and nitrogen dioxide (NO2), which
have relevant roles in multiple physiological processes of animal and plant cells ,  and . S-Nitrosothiols, and particularly GSNO, seem to have a relevant role in the biochemistry of NO because they may function both as an intracellular NO reservoir and as a vehicle of NO throughout the cell . In this context, a reaction of S-transnitrosation is considered to be the main mechanism for the biological effects of SNOs . Consequently, they may be involved in post-translational changes in cell signalling under physiological and phytopathological conditions. However, in higher plants the information available on the metabolism of SNOs and RNS is still limited compared to that in animal systems.
The term “oxidative stress” describes the cellular damage caused by overproduction of ROS, and the term nitrosative stress was introduced to describe a similar process caused by RNS in plants under stress conditions. Consequently, it is considered that under a specific situation the plant undergoes nitrosative stress when there is a de-regulated synthesis or overproduction of NO and NO-derived products that can have toxic physiological consequences. In this context, it is also important to define a reliable marker or footprints of this type of stress . So far, a body of data has revealed that a rise of protein tyrosine nitration could be a good marker to evaluate a process of nitrosative stress ,  and .
S-Nitrosoglutathione is formed by the reaction of NO with reduced glutathione (GSH) in the presence of ox