NO Metabolism

Nitric oxide being present in the atmosphere and also in the gaseous phase of the soil can be diffused into plants through the stomatal pores and the roots, respectively. Beyond the uptake of environmental NO, plant cells produce endogenous NO. Among the multiple forms of nitrogen (N) in plants, NO is in the middle in terms of oxidation state. Therefore, its formation within the plant body is possible through the oxidation of reduced nitrogen compounds (e.g. amines) and through the reduction of higher oxides (e.g. nitrate, nitrite). Many studies confirm NO formation by enzymatic and non-enzymatic mechanisms, depending on the organism, the location, and the factors that stimulate its formation. Figure summarizes the pathways of NO production and conversion in plants.

Overview of NO production (black arrows) and conversion (red arrows) pathways in plants. Enzymatic and non-enzymatic reductive pathways using nitrate and nitrite reduction (left): NO is produced by the cytosolic nitrate reductase (NR), nitrite: NO reductase (NiNOR), nitrate reductase-nitric oxide forming nitrite reductase (NR-NOFNiR), xanthine oxidoreductase (XOR), and mitochondrial cytochrome c oxidase (COX). Oxidative pathways of NO synthesis include an NOS-like enzyme using L-Arg as a substrate, undescribed metabolism of hydroxylamine (HA) and polyamines (PAs), and the production of NO from oximes catalyzed by peroxidase (POD). Pathways of NO conversion (right): The reaction of NO with molecular oxygen (O2) leads to the formation of nitrate (NO3−) and nitrite (NO2−). Phytoglobins (Phytogb) can act as NO dioxygenases and metabolize NO to NO3−. Truncated hemoglobin (THB) modulates NO levels and NR activity. The transfer of the NO+ group to the cysteine residue of reduced glutathione (GSH) forms stable S-nitrosoglutathione (GSNO). The interaction of reactive nitrogen species with free sulfhydryl groups of protein Cys results in S-nitrosation. The enzyme S-nitrosoglutathione reductase (GSNOR) breaks down GSNO to form oxidized glutathione (GSSG) and ammonia (NH3). GSNO can be cleaved by the thioredoxin system consisting of thioredoxin reductase (TRXR) and thioredoxin h5 (TRXh5). Aldo-keto reductases (AKRs) form a new class of enzymes involved in NO homeostasis. During prolonged immune activation, GSNOR is regulated through reactive oxygen species (ROS) oxidation. NO reacts with the superoxide anion radical (O2−) to form peroxynitrite (ONOO−), which can cause nitration of tyrosine residues in proteins (from Sedlářová et al. 2025).

Reference: Sedlářová M, Jedelská T, Lebeda A, Petřivalský M (2025) Progress in Plant Nitric Oxide Studies: Implications for Phytopathology and Plant Protection. Int. J. Mol. Sci. 2025, 26(5), 2087; https://doi.org/10.3390/ijms26052087