Differential abilities of nitrogen dioxide and nitrite to nitrate proteins in thylakoid membranes isolated from Arabidopsis leaves

Nitrogen Dioxide at Ambient Concentrations Induces Nitration and Degradation of PYR/PYL/RCAR Receptors to Stimulate Plant Growth: A Hypothetical Model

Exposing Arabidopsis thaliana (Arabidopsis) seedlings fed with soil nitrogen to 10-50 ppb nitrogen dioxide (NO₂) for several weeks stimulated the uptake of major elements, photosynthesis, and cellular metabolisms to more than double the biomass of shoot, total leaf area and contents of N, C P, K, S, Ca and Mg per shoot relative to non-exposed control seedlings. The ¹⁵N/¹⁴N ratio analysis by mass spectrometry revealed that N derived from NO₂ (NO₂-N) comprised < 5% of the total plant N, showing that the contribution of NO₂-N as N source was minor. Moreover, histological analysis showed that leaf size and biomass were increased upon NO₂ treatment, and that these increases were attributable to leaf age-dependent enhancement of cell proliferation and enlargement. Thus, NO₂ may act as a plant growth signal rather than an N source. Exposure of Arabidopsis leaves to 40 ppm NO₂ induced virtually exclusive nitration of PsbO and PsbP proteins (a high concentration of NO₂ was used). The PMF analysis identified the ninth tyrosine residue of PsbO1 (⁹Tyr) as a nitration site. ⁹Tyr of PsbO1 was exclusively nitrated after incubation of the thylakoid membranes with a buffer containing NO₂ and NO₂^- or a buffer containing NO₂^- alone. Nitration was catalyzed by illumination and repressed by photosystem II (PSII) electron transport inhibitors, and decreased oxygen evolution. Thus, protein tyrosine nitration altered (downregulated) the physiological function of cellular proteins of Arabidopsis leaves. This indicates that NO₂-induced protein tyrosine nitration may stimulate plant growth. We hypothesized that atmospheric NO₂ at ambient concentrations may induce tyrosine nitration of PYR/PYL/RCAR receptors in Arabidopsis leaves, followed by degradation of PYR/PYL/RCAR, upregulation of target of rapamycin (TOR) regulatory complexes, and stimulation of plant growth.

Nitrate, but not nitrite, derived from nitrogen dioxide accumulates in Arabidopsis leaves following exposure to 15N-labeled nitrogen dioxide

It is known that when plant leaves are exposed to exogenously applied nitrogen dioxide (NO2), nitrogen derived from NO2 is reduced to amino acid nitrogen. However, whether this is the sole metabolic fate of exogenously applied NO2 is unclear. In this study, Arabidopsis leaves were exposed to 4 ppm 15N-labeled NO2 for 4 h in light, followed by capillary ion analysis and elemental analysis-mass spectrometry with an elemental analyzer connected directly to a mass spectrometer. We found that leaf cells exposed to 15N-labeled NO2 accumulated a large amount of 15N-labeled nitrate. Neither 15N-labeled nitrite nor endogenous nitrite was present in exposed leaves. It is likely that exogenously applied NO2 is first converted to nitrite, and that nitrite is oxidized to nitrate in Arabidopsis leaf cells. The complete disappearance of nitrite derived from exogenously applied NO2 and endogenous nitrite supports this mechanism.

A novel role for PsbO1 in photosynthetic electron transport as suggested by its light-triggered selective nitration in Arabidopsis thaliana

Exposure of Arabidopsis leaves to nitrogen dioxide (NO2) results in the selective nitration of specific proteins, such as PsbO1. The 9th tyrosine residue (9Tyr) of PsbO1 has been identified as the nitration site. This nitration is triggered by light and inhibited by photosynthetic electron transport inhibitors. During protein nitration, tyrosyl and NO2 radicals are formed concurrently and combine rapidly to form 3-nitrotyrosine. A selective oxidation mechanism for 9Tyr of PsbO1 is required. We postulated that, similar to 161Tyr of D1, 9Tyr of PsbO1 is selectively photo-oxidized by photosynthetic electron transport in response to illumination to a tyrosyl radical. In corroboration, after reappraising our oxygen evolution analysis, the nitration of PsbO1 proved responsible for decreased oxygen evolution from the thylakoid membranes. NO2 is reportedly taken into cells as nitrous acid, which dissociates to form NO2-. NO2- may be oxidized into NO2 by the oxygen-evolving complex. Light may synchronize this reaction with tyrosyl radical formation. These findings suggest a novel role for PsbO1 in photosynthetic electron transport.

Selective nitration of PsbO1, PsbO2, and PsbP1 decreases PSII oxygen evolution and photochemical efficiency in intact leaves of Arabidopsis

Exposure of intact Arabidopsis leaves to 40 ppm nitrogen dioxide (NO₂) in light resulted almost exclusively in nitration of PsbO1, PsbO2, and PsbP1 of photosystem II (PSII), with minor nitration of four non-PS II proteins, including peroxiredoxin II E, as reported previously. Our previous findings that light-triggered selective nitration of PsbO1 decreased oxygen evolution and that inhibition of photoelectric electron transport inhibited nitration of PsbO1 implied that the nitratable tyrosine residue of PsbO1 is redox-active. However, whether the nitratable tyrosine residues of PsbO2 and PsbP1 are redox-active is unknown. In this study, we determined the oxygen evolution and maximal photochemical efficiency of PSII in intact Arabidopsis leaves following exposure to 40 ppm NO₂ in light and found that these parameters were decreased to 60 and 70% of the non-exposed control, respectively. Because PsbO1, PsbO2, and PsbP1 accounted for > 80% of anti-3-nitrotyrosine antibody signal intensities, observed decreases in the oxygen evolution and maximal photochemical efficiency of PSII were mainly attributable to nitration of the tyrosine residues of these PSII proteins. Thus, it is postulated that nitratable tyrosine residues of PsbO2 and PsbP1 are redox-active, as in the case of PsbO1. A new hypothetical model is proposed.

Melatonin and nitric oxide regulate sunflower seedling growth under salt stress accompanying differential expression of Cu/Zn SOD and Mn SOD

Salinity results in significant reduction in sunflower (Helianthus annuus L.) seedling growth and excessive generation of reactive oxygen species (ROS). Present work highlights the possible role of melatonin as an antioxidant through its interaction with nitric oxide (NO), and as an early and long distance NaCl-stress sensing signaling molecule in seedling cotyledons. Exogenous melatonin (15µM)±NaCl (120mM) inhibit seedling growth, which is also correlated with NO availability, accumulation of potential superoxide anion (O₂^•-) and peroxynitrite anion (ONOO^-), extent of tyrosine-nitration of proteins, spatial localization and activity of superoxide dismutase (SOD) isoforms. NO acts as a positive modulator of melatonin accumulation in seedling cotyledons as a long-distance signaling response. Modulation of superoxide anion and peroxynitrite anion content by melatonin highlights its crucial role in combating deleterious effects of ROS and reactive nitrogen species (RNS). Present findings provide evidence for an interaction between melatonin and NO in their effect on seedling growth under salt stress accompanying differential modulation of two SOD isoforms, i.e. Cu/Zn SOD and Mn SOD.

Selective nitration of PsbO1 inhibits oxygen evolution from isolated Arabidopsis thylakoid membranes

Treatment of isolated Arabidopsis thaliana thylakoid membranes with nitrogen dioxide (NO₂) induces selective nitration of the tyrosine residue at the ninth amino acid (⁹Tyr) of PsbO1. This selective nitration is triggered by light and is inhibited by photosynthetic electron transport inhibitors. Therefore, we postulated that, similar to ¹⁶¹Tyr of D1 (Y_Z), ⁹Tyr of PsbO1 is redox active and is selectively oxidized by photosynthetic electron transport in response to illumination to a tyrosyl radical that is highly susceptible to nitration. This tyrosyl radical may combine rapidly at diffusion-controlled rates with NO₂ to form 3-nitrotyrosine. If this postulation is correct, the nitration of ⁹Tyr of PsbO1 should decrease oxygen evolution activity. We investigated the effects of PsbO1 nitration on oxygen evolution from isolated thylakoid membranes, and found that nitration decreased oxygen evolution to ≥ 0% of the control. Oxygen evolution and nitration were significantly negatively correlated. This finding is consistent with redox active properties of the ⁹Tyr gene of PsbO1, and suggests that PsbO1 ⁹Tyr acts as an electron relay, such as Y_Z in the photosystem II oxygenic electron transport chain.

Light-triggered selective nitration of PsbO1 in isolated Arabidopsis thylakoid membranes is inhibited by photosynthetic electron transport inhibitors

PsbO1 is exclusively nitrated when isolated thylakoid membranes are incubated in a buffer bubbled with nitrogen dioxide (NO₂) containing NO₂ and nitrite. NO₂ is the primary intermediate for this selective nitration. Isolated thylakoid membranes were incubated in NO₂-bubbled buffer at 25°C in the light or dark. Protein analysis confirmed the selective nitration of PsbO1. Illumination was found to be essential in PsbO1 nitration. A nitration mechanism whereby nitratable tyrosine residues of PsbO1 are, prior to nitration, selectively photo-oxidized by photosynthetic electron transport to tyrosyl radicals to combine with NO₂ to form 3-nitrotyrosine was hypothesized. We tested the electron transport inhibitors 3-(3,4-dichlorophenyl)-1,1- dimethylurea, sodium azide, and 1,5-diphenylcarbazide and found distinct inhibition of nitration of PsbO1. We also propose a possible nitration mechanism.