Changes in spinach thylakoid activity due to nitrite ions

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.

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.

The Acceptor Side of Photosystem II Is the Initial Target of Nitrite Stress in Synechocystis sp. Strain PCC 6803

Nitrite, a common form of inorganic nitrogen (N), can be used as a nitrogen source through N assimilation. However, high levels of nitrite depress photosynthesis in various organisms. In this study, we investigated which components of the photosynthetic electron transfer chain are targeted by nitrite stress in Synechocystis sp. strain PCC 6803 cells. Measurements of whole-chain and photosystem II (PSII)-mediated electron transport activities revealed that high levels of nitrite primarily impair electron flow in PSII. Changes in PSII activity in response to nitrite stress occurred in two distinct phases. During the first phase, which occurred in the first 3 h of nitrite treatment, electron transfer from the primary quinone acceptor (Q_A) to the secondary quinone acceptor (Q_B) was retarded, as indicated by chlorophyll (Chl) a fluorescence induction, S-state distribution, and Q_A^- reoxidation tests. In the second phase, which occurred after 6 h of nitrite exposure, the reaction center was inactivated and the donor side of photosystem II was inhibited, as revealed by changes in Chl fluorescence parameters and thermoluminescence and by immunoblot analysis. Our data suggest that nitrite stress is highly damaging to PSII and disrupts PSII activity by a stepwise mechanism in which the acceptor side is the initial target. IMPORTANCE In our previous studies, an alga-based technology was proposed to fix the large amounts of nitrite that are released from NO_X-rich flue gases and proved to be a promising industrial strategy for flue gas NO_X bioremediation (W. Chen et al., Environ Sci Technol 50:1620-1627, 2016, https://doi.org/10.1021/acs.est.5b04696; X. Zhang et al., Environ Sci Technol 48:10497-10504, 2014, https://doi.org/10.1021/es5013824). However, the toxic effects of high concentrations of nitrite on algal cells remain obscure. The analysis of growth rates, photochemistry, and protein profiles in our study provides important evidence that the inhibition by nitrite occurs in two phases: in the first phase, electron transfer between Q_A^- and Q_B is retarded, whereas in the second, the donor side of PSII is affected. This is an excellent example of investigating the "early" inhibitory effects (i.e., within the first 6 h) on the PSII electron transfer chain in vivo This paper provides novel insights into the mechanisms of nitrite inhibition of photosynthesis in an oxygenic phototrophic cyanobacterium.

Investigation of the inhibitory effect of nitrite on Photosystem II

The role of chloride in photosystem II (PSII) is unclear. Several monovalent anions compete for the Cl(-) site(s) in PSII, and some even support activity. NO2(-) has been reported to be an activator in Cl(-)-depleted PSII membranes. In this paper, we report a detailed investigation of the chemistry of NO2(-) with PSII. NO2(-) is shown to inhibit PSII activity, and the effects on the donor side as well as the acceptor side are characterized using steady-state O2-evolution assays, electron paramagnetic resonance (EPR) spectroscopy, electron-transfer assays, and flash-induced polarographic O2 yield measurements. Enzyme kinetics analysis shows multiple sites of NO2(-) inhibition in PSII with significant inhibition of oxygen evolution at <5 mM NO2(-). By EPR spectroscopy, the yield of the S2 state remains unchanged up to 15 mM NO2(-). However, the S2-state g = 4.1 signal is favored over the g = 2 multiline signal with increasing NO2(-) concentrations. This could indicate competition of NO2(-) for the Cl(-) site at higher NO2(-) concentrations. In addition to the donor-side chemistry, there is clear evidence of an acceptor-side effect of NO2(-). The g = 1.9 Fe(II)-QA(-•) signal is replaced by a broad g = 1.6 signal in the presence of NO2(-). Additionally, a g = 1.8 Fe(II)-Q(-•) signal is present in the dark, indicating the formation of a NO2(-)-bound Fe(II)-QB(-•) species in the dark. Electron-transfer assays suggest that the inhibitory effect of NO2(-) on the activity of PSII is largely due to the donor-side chemistry of NO2(-). UV-visible spectroscopy and flash-induced polarographic O2 yield measurements indicate that NO2(-) is oxidized by the oxygen-evolving complex in the higher S states, contributing to the donor-side inhibition by NO2(-).

Mechanism of action of anions on the electron transport chain in thylakoid membranes of higher plants

With an aim to improve our understanding of the mechanisms behind specific anion effects in biological membranes, we have studied the effects of sodium salts of anions of varying valency in thylakoid membranes. Rates of electron transport of PS II and PS I, 77K fluorescence emission and excitation spectra, cyclic electron flow around PS I and circular dichroism (CD) spectra were measured in thylakoid membranes in order to elucidate a general mechanism of action of inorganic anions on photosynthetic electron transport chain. Re-distribution of absorbed excitation energy has been observed as a signature effect of inorganic anions. In the presence of anions, such as nitrite, sulphate and phosphate, distribution of absorbed excitation energy was found to be more in favor of Photosystem I (PS I). The amount of energy distributed towards PS I depended on the valency of the anion. In this paper, we propose for the first time that energy re-distribution and its valence dependence may not be the effect of anions per se. The entry of negative charge (anion) is accompanied by influx of positive charge (protons) to maintain a balance of charge across the thylakoid membranes. As reflected by the CD spectra, the observed energy re-distribution could be a result of structural rearrangements of the protein complexes of PS II caused by changes in the ionic environment of the thylakoid lumen.

Nitrite regulates distribution of excitation energy between the two photosystems by causing state transition

The mechanism of distribution of absorbed excitation energy between the two photosystems in the presence of nitrite has been investigated in spinach (Spinacia oleracea L.) thylakoid membranes. Nitrite inhibited PS II activity (H(2)O --> DCPIP reaction) and enhanced PS I activity (DCPIPH(2) --> MV reaction). Nitrite decreased the F(v)/F(m) ratio measured at room temperature and increased the F(730)/F(685) ratio measured at low temperature (77 K). These results suggested that nitrite caused a decrease in the excitation energy available to PS II and transferred more energy to PS I by the mechanism of state transition. Measurement of fluorescence excitation spectra at 77 K showed that nitrite increased the absorption cross-section of PS I antenna at the expense of chlorophyll b and LHC II. Based on these observations we have suggested a role of nitrite in causing state transition.

Mechanism and importance of post-translational regulation of nitrate reductase

In higher plants, nitrate reductase (NR) is inactivated by the phosphorylation of a conserved Ser residue and binding of 14-3-3 proteins in the presence of divalent cations or polyamines. A transgenic Nicotiana plumbaginifolia line (S521) has been constructed where the regulatory, conserved Ser 521 of tobacco NR (corresponding to Ser 534 in Arabidopsis) was mutated into Asp. This mutation resulted in the complete abolition of activation/inactivation in response to light/dark transitions or other treatments known to regulate the activation state of NR. Analysis of the transgenic plants showed that, under certain conditions, when whole plants or cut tissues are exposed to high nitrate supply, post-translational regulation is necessary to avoid nitrite accumulation. Abolition of the post-translational regulation of NR also results in an increased flux of nitric oxide from the leaves and roots. In view of the results obtained from examining the different transgenic N. plumbaginifolia lines, compartmentation of nitrate into an active metabolic pool and a large storage pool appears to be an important factor for regulating nitrate reduction. The complex regulation of nitrate reduction is likely to have evolved not only to optimize nitrogen assimilation, but also to prevent and control the formation of toxic, and possibly regulatory, products of NR activities. Phos phorylation of NR has previously been found to influence the degradation of NR in spinach leaves and Arabidopsis cell cultures. However, experiments with whole plants of N. plumbaginifolia, Arabidopsis, or squash are in favour of NR degradation being the same in light and darkness and independent of phosphorylation at the regulatory Ser.

Nitric oxide: the versatility of an extensive signal molecule

Nitric oxide (NO) is a small highly diffusible gas and a ubiquitous bioactive molecule. Its chemical properties make NO a versatile signal molecule that functions through interactions with cellular targets via either redox or additive chemistry. In plants, NO plays a role in a broad spectrum of pathophysiological and developmental processes. Although nitric oxide synthase (NOS)-dependent NO production has been reported in plants, no gene, cDNA, or protein has been isolated to date. In parallel, precise and regulated NO production can be measured from the activity of the ubiquitous enzyme nitrate reductase (NR). In addition to endogenous NO formation, high NO emissions are observed from fertilized soils, but their effects on the physiology of plants are largely unknown. Many environmental and hormonal stimuli are transmitted either directly or indirectly by NO signaling cascades. The ability of NO to act simultaneously on several unrelated biochemical nodes and its redox homeostatic properties suggest that it might be a synchronizing molecule in plants.

Inorganic anions induce state changes in spinach thylakoid membranes

The role of cations in excitation energy distribution between the two photosystems of photosynthesis is well established. This paper provides evidence, for the first time, for an important role of anions in the regulation of distribution of absorbed light energy between the two photosystems. Inorganic anions caused redistribution of energy more in favour of photosystem I, as judged from measurements of chlorophyll a fluorescence transients, rates of electron transport in low light and 77 K fluorescence emission spectra: the Fv/Fm ratio was decreased by inorganic anions even in the presence of DCMU, the PS II electron transport was decreased whereas PS I electron transport was increased and the F735 (77 K emission from PS I)/F685 (77 K emission from PS II) ratio was increased. Such changes were observed with inorganic anions having different valencies (Cl- , SO4(2-), PO4(3-)): the higher the valency of the inorganic anion, the more the energy transferred towards PS I. Change in the valency of the inorganic anions thus regulates distribution of absorbed light energy between the two photosystems. However, organic anions like acetate, succinate, and citrate caused no significant changes in the Fv/Fm ratio, and in rates of PS I and PS II electron transport, showing their ineffectiveness in regulating light energy distribution.