Singlet oxygen and non-photochemical quenching contribute to oxidation of the plastoquinone-pool under high light stress in Arabidopsis

Pathways of Oxygen-Dependent Oxidation of the Plastoquinone Pool in the Dark After Illumination

The redox state of the plastoquinone (PQ) pool in thylakoids plays an important role in the regulation of chloroplast metabolism. In the light, the PQ pool is mostly reduced, followed by oxidation after light cessation. It has been believed for a long time that dark oxidation depends on oxygen, although the precise mechanisms of the process are still unknown and debated. In this work, we analyzed PQ pool oxidation kinetics in isolated pea (Pisum sativum) thylakoids by tracking the changes in the area above the OJIP fluorescence curve (Afl) over time intervals from 0.1 s to 10 min in the dark following illumination. Afl served as an indirect measure of the redox state of the PQ pool that enabled quantification of the rate of PQ pool oxidation. The results showed a two-phase increase in Afl. The "fast" phase appeared to be linked to electron flow from the PQ pool to downstream acceptors of the photosynthetic electron transport chain. The "slow" phase involved oxidation of PQH2 through oxygen-dependent mechanisms. Adding octyl gallate, an inhibitor of plastid terminal oxidase (PTOX), to isolated thylakoid suspensions decreased the rate of the "slow" phase of PQ pool oxidation in the dark after illumination. The addition of either H2O2 or catalase, an enzyme that decomposes H2O2, revealed that H2O2 accelerates oxidation of the PQ pool. This indicates that under conditions that favor H2O2 accumulation, H2O2 can contribute substantially to PQ pool oxidation in the dark after illumination. The contribution of PTOX and H2O2 to the modulation of the PQ pool redox state in plants in the dark after illumination is discussed.

High Light Acclimation Mechanisms Deficient in a PsbS-Knockout Arabidopsis Mutant

The photosystem II PsbS protein of thylakoid membranes is responsible for regulating the energy-dependent, non-photochemical quenching of excess chlorophyll excited states as a short-term mechanism for protection against high light (HL) stress. However, the role of PsbS protein in long-term HL acclimation processes remains poorly understood. Here we investigate the role of PsbS protein during long-term HL acclimation processes in wild-type (WT) and npq4-1 mutants of Arabidopsis which lack the PsbS protein. During long-term HL illumination, photosystem II photochemical efficiency initially dropped, followed by a recovery of electron transport and photochemical quenching (qL) in WT, but not in npq4-1 mutants. In addition, we observed a reduction in light-harvesting antenna size during HL treatment that ceased after HL treatment in WT, but not in npq4-1 mutants. When plants were adapted to HL, more reactive oxygen species (ROS) were accumulated in npq4-1 mutants compared to WT. Gene expression studies indicated that npq4-1 mutants failed to express genes involved in plastoquinone biosynthesis. These results suggest that the PsbS protein regulates recovery processes such as electron transport and qL during long-term HL acclimation by maintaining plastoquinone biosynthetic gene expression and enhancing ROS homeostasis.

Action spectrum of the redox state of the plastoquinone pool defines its function in plant acclimation

The plastoquinone (PQ) pool mediates electron flow and regulates photoacclimation in plants. Here we report the action spectrum of the redox state of the PQ pool in Arabidopsis thaliana, showing that 470-500, 560 or 650-660 nm light favors Photosystem II (PSII) and reduces the PQ pool, whereas 420-440, 520 or 690 nm light favors Photosystem I (PSI) and oxidizes PQ. These data were used to construct a model predicting the redox state of PQ from the spectrum of any polychromatic light source. Moderate reduction of the PQ pool induced transition to light state 2, whereas state 1 required highly oxidized PQ. In low-intensity PSI light, PQ was more oxidized than in darkness and became gradually reduced with light intensity, while weak PSII light strongly reduced PQ. Natural sunlight was found to favor PSI, which enables plants to use the redox state of the PQ pool as a measure of light intensity.

ROS-Responsive Polymeric Nanocarriers with Photoinduced Exposure of Cell-Penetrating Moieties for Specific Intracellular Drug Delivery

In situ modulation of the surface properties on the micellar drug delivery nanocarriers offers an efficient method to improve the drug delivery efficiency into cells while maintaining stealth and stability during blood circulation. Light has been demonstrated to be a temporally and spatially controllable tool to improve cellular internalization of nanoparticles. Herein, we develop reactive oxygen species (ROS)-responsive mixed polymeric micelles with photoinduced exposure of cell-penetrating moieties via photodynamic ROS production, which can facilitate cellular internalization of paclitaxel (PTX) and chlorin e6 (Ce6)-coloaded micelles for the synergistic effect of photodynamic and chemotherapy. The thioketal-bond-linked block polymers poly(ε-caprolactone)-TL-poly(N,N-dimethylacrylamide) (PCL-TL-PDMA) with a long PDMA block are used to self-assemble into mixed micelles with PCL-b-poly(2-guanidinoethyl methacrylate) (PCL-PGEMA) consisting of a short PGEMA block, which are further used to coencapsulate PTX and Ce6. After intravenous injection, prolonged blood circulation of the micelles guarantees high tumor accumulation. Upon irradiation by 660 nm light, ROS production of the micelles by Ce6 induces cleavage of PDMA to expose PGEMA shells for significantly improved cellular internalization. The combination of photodynamic therapy and chemotherapy inside the tumor cells achieves improved antitumor efficacy. The design of ROS-responsive mixed polymeric nanocarriers represents a novel and efficient approach to realize both long blood circulation and high-efficiency cellular internalization for combined photodynamic and chemotherapy under light irradiation.

Effect of oxygen on the non-photochemical quenching of vascular plants and potential oxygen deficiency in the stroma of PsbS-knock-out rice

The excessive and harmful light energy absorbed by the photosystem (PS) II of higher plants is dissipated as heat through a protective mechanism termed non-photochemical quenching (NPQ) of chlorophyll fluorescence. PsbS-knock-out (KO) mutants lack the trans-thylakoid proton gradient (ΔpH)-dependent part of NPQ. To elucidate the molecular mechanism of NPQ, we investigated its dependency on oxygen. The development of NPQ in wild-type (WT) rice under low-oxygen (LO) conditions was reduced to more than 50% of its original value. However, under high-oxygen (HO) conditions, the NPQ of both WT and PsbS-KO mutants recovered. Moreover, WT and PsbS-KO mutant leaves infiltrated with the ΔpH dissipating uncoupler nigericin showed increased NPQ values under HO conditions. The experiments using intact chloroplasts and protoplasts of Arabidopsis thaliana supported that the LO effects observed in rice leaves were not due to carbon dioxide deficiency. There was a noticeable 90% reduction in the half-time of P700 oxidation rate in LO-treated leaves compared with that of WT control leaves, but the HO treatment did not significantly change the half-time of P700 oxidation rate. Overall, the results obtained here indicate that the stroma of the PsbS-KO plants could be potentially under O₂ deficiency. Because the functions of PsbS in rice leaves are likely to be similar to those in other higher plants, our findings offer novel insights into the role of oxygen in the development of NPQ.

Plastoquinol generates and scavenges reactive oxygen species in organic solvent: Potential relevance for thylakoids

The present work reports reactions of plastoquinol (PQH₂-9) and plastoquinone (PQ-9) in organic solvents and summarizes the literature to understand similar reactions in thylakoids. In thylakoids, PQH₂-9 is oxidized by the cytochrome b₆/f complex (Cyt b₆/f) but some PQH₂-9 is also oxidized by reactions in which oxygen acts as an electron acceptor and is converted to reactive oxygen species (ROS). Furthermore, PQH₂-9 reacts with ROS. Light enhances oxygen-dependent oxidation of PQH₂-9. We examined the oxidation of PQH₂-9 via dismutation of PQH₂-9 and PQ-9 and scavenging of the superoxide anion radical (O₂^-) and hydrogen peroxide (H₂O₂) by PQH₂-9. Oxidation of PQH₂-9 via dismutation to semiquinone was slow and independent of pH in organic solvents and in solvent/buffer systems, suggesting that intramembraneous oxidation of PQH₂-9 in darkness mainly proceeds via reactions catalyzed by the plastid terminal oxidase and cytochrome b₅₅₉. In the light, oxidation of PQH₂-9 by singlet oxygen and by O₂^- formed in PSI contribute significantly. In addition, Cyt b_6/f forms H₂O₂ with a PQH₂-9 dependent mechanism. Measurements of the reaction of O₂^- with PQH₂-9 and PQ-9 in acetonitrile showed that O₂^- oxidizes PQH₂-9, forming PQ-9 and several PQ-9-derived products. The rate constant of the reaction between PQH₂-9 and O₂^- was found to be 10⁴ M^-1 s^-1. H₂O₂ was found to oxidize PQH₂-9 to PQ-9, but failed to oxidize all PQH₂-9, suggesting that the oxidation of PQH₂-9 by H₂O₂ proceeds via deprotonation mechanisms producing PQH^--9, PQ^2--9 and the protonated hydrogen peroxide cation, H₃O₂⁺.

Conserved Function of Fibrillin5 in the Plastoquinone-9 Biosynthetic Pathway in Arabidopsis and Rice

Plastoquinone-9 (PQ-9) is essential for plant growth and development. Recently, we found that fibrillin5 (FBN5), a plastid lipid binding protein, is an essential structural component of the PQ-9 biosynthetic pathway in Arabidopsis. To investigate the functional conservation of FBN5 in monocots and eudicots, we identified OsFBN5, the Arabidopsis FBN5 (AtFBN5) ortholog in rice (Oryza sativa). Homozygous Osfbn5-1 and Osfbn5-2 Tos17 insertion null mutants were smaller than wild type (WT) plants when grown on Murashige and Skoog (MS) medium and died quickly when transplanted to soil in a greenhouse. They accumulated significantly less PQ-9 than WT plants, whereas chlorophyll and carotenoid contents were only mildly affected. The reduced PQ-9 content of the mutants was consistent with their lower maximum photosynthetic efficiency, especially under high light. Overexpression of OsFBN5 complemented the seedling lethal phenotype of the Arabidopsis fbn5-1 mutant and restored PQ-9 and PC-8 (plastochromanol-8) to levels comparable to those in WT Arabidopsis plants. Protein interaction experiments in yeast and mesophyll cells confirmed that OsFBN5 interacts with the rice solanesyl diphosphate synthase OsSPS2 and also with Arabidopsis AtSPS1 and AtSPS2. Our data thus indicate that OsFBN5 is the functional equivalent of AtFBN5 and also suggest that the SPSs-FBN5 complex for synthesis of the solanesyl diphosphate tail in PQ-9 is well conserved in Arabidopsis and rice.

The diatom Phaeodactylum tricornutum adjusts nonphotochemical fluorescence quenching capacity in response to dynamic light via fine-tuned Lhcx and xanthophyll cycle pigment synthesis

Diatoms contain a highly flexible capacity to dissipate excessively absorbed light by nonphotochemical fluorescence quenching (NPQ) based on the light-induced conversion of diadinoxanthin (Dd) into diatoxanthin (Dt) and the presence of Lhcx proteins. Their NPQ fine regulation on the molecular level upon a shift to dynamic light conditions is unknown. We investigated the regulation of Dd + Dt amount, Lhcx gene and protein synthesis and NPQ capacity in the diatom Phaeodactylum tricornutum after a change from continuous low light to 3 d of sine (SL) or fluctuating (FL) light conditions. Four P. tricornutum strains with different NPQ capacities due to different expression of Lhcx1 were included. All strains responded to dynamic light comparably, independently of initial NPQ capacity. During SL, NPQ capacity was strongly enhanced due to a gradual increase of Lhcx2 and Dd + Dt amount. During FL, cells enhanced their NPQ capacity on the first day due to increased Dd + Dt, Lhcx2 and Lhcx3; already by the second day light acclimation was accomplished. While quenching efficiency of Dt was strongly lowered during SL conditions, it remained high throughout the whole FL exposure. Our results highlight a more balanced and cost-effective photoacclimation strategy of P. tricornutum under FL than under SL conditions.

Function of isoprenoid quinones and chromanols during oxidative stress in plants

Isoprenoid quinones and chromanols in plants fulfill both signaling and antioxidant functions under oxidative stress. The redox state of the plastoquinol pool (PQ-pool), which is modulated by interaction with reactive oxygen species (ROS) during oxidative stress, has a major regulatory function in both short- and long-term acclimatory responses. By contrast, the scavenging of ROS by prenyllipids affects signaling pathways where ROS play a role as signaling molecules. As the primary antioxidants, isoprenoid quinones and chromanols are synthesized under high-light stress in response to any increased production of ROS. During photo-oxidative stress, these prenyllipids are continuously synthesized and oxidized to other compounds. In turn, their oxidation products (hydroxy-plastochromanol, plastoquinol-C, plastoquinone-B) can still have an antioxidant function. The oxidation products of isoprenoid quinones and chromanols formed specifically in the face of singlet oxygen, can be indicators of singlet oxygen stress.

β-cyclocitral upregulates salicylic acid signalling to enhance excess light acclimation in Arabidopsis

β-cyclocitral (β-CC), a volatile oxidized derivative of β-carotene, can upregulate the expression of defence genes to enhance excess light (EL) acclimation. However, the signalling cascades underlying this process remain unclear. In this study, salicylic acid (SA) is involved in alleviating damage to promote β-CC-enhanced EL acclimation. In early stages of EL illumination, β-CC pretreatment induced SA accumulation and impeded reactive oxygen species (ROS) production in the chloroplast. A comparative analysis of two SA synthesis pathways in Arabidopsis revealed that SA concentration mainly increased via the isochorismate synthase 1 (ICS1)-mediated isochorismate pathway, which depended on essential regulative function of enhanced disease susceptibility 1 (EDS1). Further results showed that, in the process of β-CC-enhanced EL acclimation, nuclear localization of nonexpressor of pathogenesis-related genes 1 (NPR1) was regulated by SA accumulation and NPR1 induced subsequent transcriptional reprogramming of gluthathione-S-transferase 5 (GST5) and GST13 implicated in detoxification. In summary, β-CC-induced SA synthesis contributes to EL acclimation response by decreasing ROS production in the chloroplast, promoting nuclear localization of NPR1, and upregulating GST transcriptional expression. This process is a possible molecular regulative mechanism of β-CC-enhanced EL acclimation.

Experimental evidence suggesting that H2O2 is produced within the thylakoid membrane in a reaction between plastoquinol and singlet oxygen

Plastoquinol (PQH2-9) and plastoquinone (PQ-9) mediate photosynthetic electron transfer. We isolated PQH2-9 from thylakoid membranes, purified it with HPLC, subjected the purified PQH2-9 to singlet oxygen ((1)O2) and analyzed the products. The main reaction of (1)O2 with PQH2-9 in methanol was found to result in formation of PQ-9 and H2O2, and the amount of H2O2 produced was essentially the same as the amount of oxidized PQH2-9. Formation of H2O2 in the reaction between (1)O2 and PQH2-9 may be an important source of H2O2 within the lipophilic thylakoid membrane.

Hypothesis: increase of the ratio singlet oxygen plus superoxide radical to hydrogen peroxide changes stress defense response to programmed leaf death

The level of reactive oxygen species (ROS) increases under different stresses and, by destroying cellular components, may cause cell death. In addition, ROS are part of the complex network of transduction signals that induce defense reactions against stress or, alternatively, trigger programmed cell death, and key questions are the levels of each ROS that, respectively determine defense and death responses of the cell. The answer to those questions is difficult because there are several patterns of cell death that frequently appear mixed and are hardly distinguishable. Moreover, although considerable progresses have been achieved in the determination of the levels of specific ROS, critical questions remain on the ROS level in specific cell compartments. By considering chloroplasts as the main source of ROS in photosynthetic tissues at light, a comparison of the levels in stress and senescence of the chloroplastic activities involved in the generation and scavenging of ROS suggests plausible differences in the levels of specific ROS between stress defense and death. In effect, the three activities of the chlororespiratory chain increase similarly in stress defense response. However, in senescence, superoxide dismutase (SOD), that converts superoxide anion radical ([Formula: see text]) to hydrogen peroxide (H2O2,) decreases, while the thylakoid Ndh complex, that favors the generation of singlet oxygen ((1)O2) and [Formula: see text], and peroxidase (PX), that consumes H2O2, increase. The obvious inference is that, in respect to defense response, the ratio ((1)O2 plus [Formula: see text])/H2O2 is increased in the senescence previous to cell death. We hypothesize that the different ROS ratios, probably through changes in the jasmonic acid/H2O2 ratio, could determine the activation of the defense network or the death network response of the cell.

The role of the PsbS protein in the protection of photosystems I and II against high light in Arabidopsis thaliana

The PsbS protein is recognised in higher plants as an important component in dissipating excess light energy via its regulation of non-photochemical quenching. We investigated photosynthetic responses in the arabidopsis npq4 mutant, which lacks PsbS, and in a mutant over-expressing PsbS (oePsbS). Growth under low light led to npq4 and wild-type plants being visibly indistinguishable, but induced a phenotype in oePsbS plants, which were smaller and had shorter flowering spikes. Here we report that chloroplasts from npq4 generated more singlet oxygen ((1)O(2)) than those from oePsbS. This accompanied a higher extent of photosystem II photoinhibition of leaves from npq4 plants. In contrast, oePsbS was more damaged by high light than npq4 and the wild-type at the level of photosystem I. The plastoquinone pool, as measured by thermoluminescence, was more oxidised in the oePsbS than in npq4, whilst the amount of photo-oxidisable P(700), as probed with actinic light or saturating flashes, was higher in oePsbS compared to wild-type and npq4. Taken together, this indicates that the level of PsbS has a regulatory role in cyclic electron flow. Overall, we show that under high light oePsbS plants were more protected from (1)O(2) at the level of photosystem II, whereas lack of cyclic electron flow rendered them susceptible to damage at photosystem I. Cyclic electron flow is concluded to be essential for protecting photosystem I from high light stress.