Nitric Oxide in Plants. Группа авторов

      Because of the presence of an unpaired electron, it is a highly reactive gaseous molecule that occurs with oxygen in a variety of reduced states such as nitroxyl ion (NO⁻), nitric oxide free radical (NO^•), and nitrosonium (NO⁺). These NO-derived molecules are referred to as reactive nitrogen species (RNS). NO influences signaling in biological systems through a variety of mechanisms. The interaction of NO^• with O₂ results in the formation of several redox compounds (including NO₂^•, N₂O₃, and N₂O₄), which may react with cellular amines and thiols or simply change to form the metabolites nitrogen dioxide radical (NO₂⁻) and nitrate (NO₃⁻) (Wendehenne et al. 2001). NO combines with dioxygen to form NO₂ or with reactive oxygen species (ROS) to form peroxynitrite (ONOO⁻), which triggers cellular damage. NO^• facilitates electrophilic assault on reactive sulfur, oxygen, nitrogen, and aromatic carbon centers, with thiols being the most reactive of the reactive teams. Nitrosation is the name given to this natural process. Nitrosation of numerous enzymes or proteins results in chemical change, which may affect the function of those entities. These alterations are reversible, and supermolecule nitrosation–denitrosation might be a crucial mechanism for controlling signal transduction (Hayat et al. 2010).

In contrast to the mammalian system, the cellular/subcellular localization of NO production in plants is exceedingly diverse and contentious. The production of NO in plants is determined by the plant’s physiological condition. This includes NO production during root development, stomatal movement control, blooming, plant component expansion, and leaf senescence (Neill et al. 2002; Mishina et al. 2007). NO is produced in plants through nonenzymatic and accelerator systems, depending on the plant species, organ, or tissue, as well as the plant’s state and ever-changing environmental circumstances. The most effective recognized NO sources in plants are as a substrate by cytosolic (cNR) and membrane-specific nitrate enzyme (PM-NR), and NO synthesis by many arginine-dependent gas synthase-like activities (NOS).

      According to studies, mitochondria are a major source of arginine- and nitrite-dependent NO synthesis in plants. Tischner et al. acquired the first evidence for mitochondrial NO synthesis in plants when they assessed NO production under anoxic conditions from the unicellular blue green alga Chlorella sorokiniana (Tischner et al. 2004). This green alga does not create NO when exposed to nitrate (NO₃), but it does create NO when exposed to nitrite (NO₂). NO generation was also inhibited by mitochondrial electron transport inhibitors. Shortly after, mitochondrial NO synthesis in higher plants was discovered. Gupta et al. discovered mitochondrial NO production in barley plants grown in anoxic conditions (Gupta and Kaiser 2010). Under anoxic circumstances, a tobacco Nia 1, 2 (nitrate reductase-deficient) cell suspension was able to manufacture NO from exogenous nitrite, despite the absence of nitrate reductase (which can also manufacture NO from nitrite) (Gupta et al. 2011). Other putative NO producers in plants include xanthine oxido-reductase, peroxidase, and cytochrome P450. NO is a ubiquitous chemical that is found in all eukaryotes. The NR system is by far the most effective and well-characterized mechanism for NO generation in plants. In this case, the cytosolic NR mostly catalyzes the reduction of nitrate to NADH as the predominant negatron donor. NR’s NAD(P)H-dependent NO production has been demonstrated in vitro and in vivo (Rockel et al. 2002). The biological significance of NR activity as a source of NO was first shown in Arabidopsis guard cells by Desikan et al. (2002). The peroxisomal catalyst organic compound enzyme can also catalyze group reduction to NO (XOR). XOR activity in pea (Pisum sativum) leaves is linked to peroxisomes, and as a result, the possibility of interaction between the construction of reactive chemical elements and reactive gas species (ROS and RNS, respectively) has been suggested (del Río et al. 2004). NO production in animals was demonstrated by a chemical reaction of arginine transforming into citrulline mediated by the enzyme NO synthase (Palmer et al. 1987). Following the discovery of a purpose for NO in plants in 1998 (Delledonne et al. 1998; Durner et al. 1998), several researchers began to look for NOS activity in plants, despite the fact that the Arabidopsis thaliana ordering failed to reveal any factor with significant similarity to animal NOS (Moreau et al. 2010).

      1.1.1 Historical Evidence and Biosynthesis of Nitric Oxide

Several studies have been conducted over the past several decades to investigate the presence and characteristics of NO gas in living beings. Gas, as a versatile molecule, has piqued curiosity and opened up new opportunities for research. NO gas is an atom gas with well-defined communication roles in mammalian systems, serving as a second messenger during vasorelaxation, neurotransmission, immunity, and toxicity. It is now clear that NO performs a critical role in animal physiology. Because of its extensive biological relevance, NO was designated “Molecule of the Year” in 1992 by Science, and Furchgott, Murad, and Ignarro were given the Nobel Prize in Physiology and Medicine in 1998. Furchgott discovered in 1980 that an unknown molecule found in animal tissue could relax smooth muscle cells, and he dubbed it EDRF (endothelium-derived reposeful factor). Murad discovered years ago that vasodilators activate guanylate cyclase (GC), which creates cyclic guanosine monophosphate and relaxes muscle fibers. This discovery begged the question of how a vasodilator outside the cell could influence a catalyst inside the cell. The solution was that the vasodilator was tainted with NO residues. Murad then bubbled NO gas across swish muscle cells, activating gigacycles per second. Thus, even before eukaryotes were thought to produce NO, he postulated that hormones may regulate swish muscles via NO. Years later, Ignarro demonstrated that NO has comparable chemical behavior to EDRF and is, in fact, a twin of EDRF. NO release from plants was initially suspected by Klepper in soybeans in 1975, much earlier than in mammals (Klepper 1979). The vast biological importance of gas in plants was established in the 1990s (Gouvea et al. 1997; Leshem et al. 1998).

      In animals, NO has since been recognized as an important signaling molecule in maintaining blood pressure within the circulatory system, stimulating host defenses within the system, controlling neural transmission within the brain, controlling organic phenomena, protoplasm aggregation, learning and memory, male sexual function, toxicity and cytoprotection, the development of artery hardening, and a variety of other functions. It functions as a secondary transmitter in mammalian systems during vasorelaxation, neurotransmission, immunity, and toxicity. As a result, they play critical roles in animal physiology.

      However, unlike animal physiology, the physiology and chemical chemistry of NO in plants is less well known. NO has the potential to be a dynamic bioactive molecule that plays an important physiological role in plants and animals.

      1.1.2 NO Biosynthesis in Plants

The process of NO production has been explored in a variety of organisms, including microorganisms, alga, lichens, gymnosperms, and angiosperms (Rőszer et al. 2014). NO synthesis utilizes both accelerator and nonaccelerator mechanisms. Body parts, plastids, mitochondria, and peroxisomes are important sites for NO production (Rőszer 2012a, 2012b). Furthermore, multiple organelles, including protoplasm, cell wall, endoplasmic reticulum, and apoplast, generate NO in higher plants (Fröhlich and Durner 2011). Chakraborty and Acharya (2017) distinguish between subtractive and aerobic NO production mechanisms. The protoplasm, mitochondria, plastid, peroxisomes, and apoplast are the primary sites of subtractive NO_X production, which is mediated by the nitrate enzyme or mitochondrial negatron transport chain and deoxygenated proteins containing heme. The aerobic route of NO production begins with l-arginine, which appears to include the enzyme NO synthase. Despite the fact that numerous genes and proteins coding for NOS enzymes are known in the class system, prokaryotes, and eukaryotes, the kingdom Plantae is still little characterized (Figure 1.1).

      Figure 1.1 Diagram representing NO synthesis. NO3−, nitrate; NiR, plastidial nitrate reductase; XOR, xanthine oxido-reductase; NiR-NOR, nitrite reductase; NR, nitrate reductase; PA, polyamines.

      1.2 The Function of Nitric Oxide in Plants

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