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2 Regulation of NO Biosynthesis Under Abiotic Stresses and Modulation Due to Osmolytes
Kaneez Fatima1, Fozia Sardar2, and Asma Imran2
1Department of Life Sciences, University of Management and Technology (UMT), Lahore, Pakistan
2National Institute for Biotechnology and Genetic Engineering-Campus-Pakistan Institute of Engineering and Applied Sciences (NIBGE-C-PIEAS), Faisalabad, Pakistan
2.1 Introduction
Nitrogen monoxide/nitric oxide (NO) is a small, fat-soluble gaseous radical that has been well recognized for a long period to be involved in the nitrogen cycle (Jedelská et al. 2021). Earlier it was considered a gaseous free radical and toxic air pollutant (Gupta et al. 2011; Astier et al. 2018). However, over the past 30 years, NO has turned out to be the center of many biological processes in animals and plants as it has been reported as a signaling molecule. This has shifted research interest and understanding of NO as a noxious by-product to it being seen as an important molecule. NO has multiple functions at diverse levels including chemical, cellular, organ, and tissue (Santolini et al. 2017; Corpas et al. 2019; Vishwakarma et al. 2019). In animals, NO acts as a vital regulator and plays an important role in reproduction; while, in plants, it is involved in various functional developments, for instance, germination, seed dormancy (Probert 2000; Bethke et al. 2006; Arc et al. 2013; Nagel et al. 2019), defense responses, flowering/leaf senescence, and protection against environmental stresses (Lora et al. 2019; Abedi et al. 2021).
Nitric oxide is available as three exchangeable species: the cation of nitrosonium (NO+), NO, and the anion of nitroxyl (NO−) (Butler et al. 1995). It is water soluble and by the addition of ferrous salts, its solubility is enhanced. Therefore, it can easily diffuse in the aqueous part of a cell including the cytoplasm, and also can freely move through the lipid bilayer. It is believed that once it is produced, it can move to the adjacent cells (Del Río et al. 2004; Delledonne 2005).
2.2 Biosynthesis of NO
There are numerous prospective sources of nitric oxide in plants (Figure 2.1) and its production is highly dependent on the type of plant, tissues/cells, plant growth environment, and stimulation of the signaling pathway under specific conditions (González-Moscoso et al. 2021). It is believed that the synthesis of NO can be achieved via two major routes: nitrate reductase and the oxidative pathway (Bethke et al. 2004; Besson-Bard et al. 2008). These are discussed in Sections 2.2.1 and 2.2.2.
Figure 2.1 Biosynthesis pathways of nitric oxide in plants.
2.2.1 Nitrate Reductase
In higher plants, nitrate reductase (NR, EC 1.7.1.1) is a cytosolic enzyme that facilitates the assimilation and metabolism of nitrogen (Ahmad et al. 2021). This enzyme utilizes nicotinamide adenine dinucleotide (NADH) (electron donor), molybdopterin, and heme/flavin adenine dinucleotide (FAD) (cofactors) to activate the reduction of nitrate into nitrite (Kaya et al. 2020b).Nitrate reductase is generally present as a homodimer in tetrameric form, depending upon the plant species. In addition to its primary role, nitrate reductase is involved in the production of nitrite: its activity level is quite low and makes up 1% of total nitrate-reducing ability (Mohn et al. 2019).
As well as cytoplasmic-based nitrate reductase, membrane-associated nitrite – NO reductase (Ni-NOR) – is also involved in NO production in plants. It is exclusively present in rhizome, which utilizes NAD(P)H (electron donor) to yield the NO from nitrite. It works at low oxygen pressure and in conjunction with membrane-linked nitrate reductase, which produces nitrite from nitrate. In their structural features, both cytoplasmic and membrane-bound reductase enzymes exhibit the presence of a cofactor named molybdenum (Moco) (Chamizo-Ampudia et al. 2017; Gao et al. 2019). In plants, some other Moco-containing enzymes exist, including (i) aldehyde oxidases (AOs), (ii) sulfite oxidases (SOs), and (iii) xanthine oxidases (XOs), all of which produce NO from nitrite (Bethke et al. 2004).
2.2.2 Mechanisms of Oxidative NO Synthesis
As plants can survive and grow without nitrite and nitrate, they should have nitrite-independent, oxidative mechanisms for NO synthesis. Indeed, analogous to animal nitric oxide synthase (NOS) (EC 1.14.13.39), the plant enzyme is not dependent on nitrite and is involved in the deamination of L-arginine into L-citrulline and NO via NADPH and O2 and necessitating Ca2+/calmodulin (Rőszer 2012). Enzymatic oxidation has been witnessed in the peroxisomes (leaf), chloroplasts of the tracheophytes, and in green algae. In chloroplasts, the oxidation of L-arginine to NO requires NADPH and is independent of Ca2+availability. While in the leaf peroxisomes, the conversion of L-arginine/L-citrulline requires Ca2+/calmodulin, flavin mononucleotide (FMN), FAD, and NADPH (Mur et al. 2013).
2.2.3 Nonenzymatic Synthesis of NO
In addition to enzymatic methods, a nonenzymatic pathway for the production of NO from NO2 has been reported. This sort of NO production is favored by low pH, e.g., in the apoplast of the growing seeds, where NO can be released from a protonated form of NO2, i.e., nitrous acid (HNO2). Another possible mechanism is the release of NO from S-nitrosoglutathione (GSNO). In the peroxisomes, both NO and the NO-derived peroxynitrite can interact with glutathione to synthesize GSNO (Neill et al. 2003; Courtois et al. 2008; Wilson et al. 2008; Palavan-Unsal and Arisan 2009).
2.3 NO Signaling and Gene Regulation Under Abiotic Stress
In animals, NO is involved in the regulation and the expression of different genes related to several different pathways, either directly by interacting with receptors involved in the signal transduction pathway, or indirectly by moderating the activity of transcription factors, or influencing the stability and translation of messenger RNA (mRNA) (Kolbert et al. 2021). Recently, a protein named “AtNOS1” exhibiting NOS activity has been recognized. This protein belongs to the NOS family (a novel family) and displays some similarity to previously reported mammalian NOS. It consists of a guanosine-5′-triphosphate (GTP)-binding domain and may have GTPase activity (Li et al. 2020). AtNOS1 activity is highly dependent upon Ca2+-calmodulin/NADPH−; however, it is independent of FAD, FMN, and BH4. Studies have provided a genetic basis for a role of AtNOS1 as a source of NO in abscisic acid (ABA)-induced closure of stomata, in the suppression of flowering, and the defense responses induced by lipopolysaccharides (Wang et al. 2020). Under abiotic stress, ABA production is the key stress response in plants. The signaling between ABA and NO governs the major molecular mechanisms that incorporate external signals to modify internal systems leading to plant adaptations against stress (Falak et al. 2021). NO interacts with ABA to modulate the gene expression and the protein function. For example, ABA prevents water loss through stomatal closure and contributes to the synthesis of osmolytes. It has been observed that in the guard cells of Arabidopsis thaliana, ABA increases H2O2 production, which in turn regulates the production of NO, which subsequently