Plant Nitric Oxide (NO)| Synthesis, Regulation, Reactions, Functions| Location, organelles, pathways
1) Plant nitric oxide? In plants, NO serves as a highly versatile signaling molecule with a plethora of vital roles. Its discovery in plants came much later than in animals.
2) NO synthesis in plants: NO synthesis in plants involves a combination of enzymatic and non-enzymatic pathways and oxidative and reductive reactions.
a) Enzymatic Synthesis Pathways:
(i) Nitrate reductase pathway. NR is a key enzyme involved in nitrogen assimilation, where it converts nitrate absorbed from the soil into nitrite. Nitrite is then further reduced to nitric oxide in a reaction catalyzed by the enzyme nitrite reductase.
(ii) nitric oxide synthase-like enzymes. While plants lack true NOS enzymes found in animals, they possess a family of enzymes called NOSL enzymes. These NOSL enzymes are structurally similar to animal NOS enzymes but differ in their catalytic activity. NOSL enzymes can use L-arginine, a common substrate for NOS enzymes, to produce nitric oxide and citrulline.
b) Non-enzymatic synthesis pathways.
(i) Ntrite-dependent non-enzymatic pathway. In addition to the enzymatic pathways, NO can be generated non-enzymatically through reactions involving nitrite and various reducing agents, such as ascorbic acid or vitamin C or glutathione. This nitrite-dependent non-enzymatic pathway can operate in different subcellular compartments, including the cytosol, mitochondria, and chloroplasts.
(ii) Nitrate-dependent non-enzymatic pathway. Apart from nitrite, nitrate can also contribute to the non-enzymatic synthesis of nitric oxide in plants. Under low-oxygen or hypoxic conditions, nitrate can undergo a series of chemical reactions leading to the formation of NO.
Reductive or oxidative mechanisms:
(a) NO reductive synthesis mechanism mostly occurs in plasma membrane, chloroplasts, apoplast, peroxisomes, cytoplasm, and mitochondria through reduction of nitrate or nitrite by nitrate reductase. NO production through nitrate substrate is reduced by cytosolic nitrate reductase and plasma membrane-bound nitrate reductase, whereas NO production through nitrite substrate is reduced by plasma membrane-bound nitrite: NO reductase, mitochondrial electron transfer chain-dependent enzymatic nitrite: NO reductase, and peroxisomal xanthine oxidoreductase. In an acidic environment, nitrite produces nitric oxide through several reversible reactions. Polyamines induce NO production through reductive and oxidative mechanisms.
(b) Oxidation of aminated molecules, such as L-arginine by the L-arginine -dependent NOSL enzyme or by using polyamines, and hydroxylamine is the second category. Oxidation of L-arginine takes place in chloroplasts and leaf peroxisomes. In chloroplasts, the oxidation of L-arginine requires nicotinamide adenine dinucleotide phosphate and calcium ions, whereas in leaf peroxisomes, the oxidation of L-arginine requires flavin mononucleotide, flavin adenine dinucleotide, calmodulin, and calcium ions. Hydroxylamine also acts as a substrate in the oxidative mechanism. NOSL enzyme is also involved in NO production by using polyamine as a substrate.
3) Spatial and Temporal Regulation of NO Synthesis: Synthesis of NO in plants is tightly regulated both spatially and temporally, allowing for precise signaling in response to various developmental and environmental cues.
4) Reactions and Interactions of NO: NO can interact with various molecules, resulting in the generation of reactive nitrogen species and leading to a wide range of physiological responses. One of the significant interactions involves the formation of peroxynitrite, which results from the rapid reaction between NO and superoxide. NO can also react with various metals of metalloproteins, such as iron (Fe) and copper (Cu) or with amino acids.
5) Functions of NO in Plants:
a) Regulation of Plant Growth and Development. NO participates in numerous aspects of plant growth and development, including seed germination, root development, shoot elongation, and flowering.
b) Defense Against Abiotic Stress. NO plays a crucial role in plants’ responses to abiotic stressors, such as drought, salinity, and extreme temperatures.
c) Response to Biotic Stress: NO is involved in the plant’s defense against pathogens and pests. It can trigger the synthesis of defense-related compounds.
d) Regulation of Stomatal Movements: NO influences stomatal movements, regulating gas exchange and water loss in plants.
e) Signal Transduction: NO acts as a secondary messenger in various signal transduction pathways, allowing plants to integrate and respond to multiple external and internal signals effectively.
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