Search

Plastoquinone-9 (PQ-9) is an essential component of photosynthesis that carries electrons in the linear and alternative electron transport chains, and is also a redox sensor that regulates state transitions and gene expression. However, a large fraction of the PQ pool is located outside the thylakoid membranes, in the plastoglobules and the chloroplast envelopes, reflecting a wider range of functions beyond electron transport. This review describes new functions of PQ in photoprotection, as a potent antioxidant, and in chloroplast metabolism as a cofactor in the biosynthesis of chloroplast metabolites. It also focuses on the essential need for tight environmental control of PQ biosynthesis and for active exchange of this compound between the thylakoid membranes and the plastoglobules. Through its multiple functions, PQ connects photosynthesis with metabolism, light acclimation, and stress tolerance. Plastoquinone (PQ)-9 is an electron carrier that plays an essential role in photosynthesis where it is involved in linear and alternative electron flows. However, the functions of PQ go far beyond photosynthesis. PQ-9 is a multilocation compound that is partitioned in the chloroplast between thylakoids, plastoglobules, and envelopes. Its distribution is modulated dynamically by various regulatory enzymes as a function of the environmental conditions. The redox state of the PQ pool plays a crucial role in various functional and metabolic processes, such as light acclimation, biosynthesis of metabolites, and gene expression. PQ is a potent antioxidant that scavenges reactive oxygen species and mitigates lipid peroxidation in planta , supplementing the photoprotection provided by β-carotene in photosystem II and tocopherols in the thylakoid lipid phase. [ABSTRACT FROM AUTHOR]

β-cyclocitral is a volatile short-chain apocarotenoid generated by enzymatic or non-enzymatic oxidation of the carotenoid β-carotene. β-cyclocitral has recently emerged as a new bioactive compound in various organisms ranging from plants and cyanobacteria to fungi and animals. In vascular plants, β-cyclocitral and its direct oxidation product, β-cyclocitric acid, are stress signals that accumulate under unfavorable environmental conditions such as drought or high light. Both compounds regulate nuclear gene expression through several signaling pathways, leading to stress acclimation. In cyanobacteria, β-cyclocitral functions as an inhibitor of competing microalgae and as a repellent against grazers. As a volatile compound, this apocarotenoid plays also an important role in intra-species and inter-species communication. This review summarizes recent findings on the multiple roles of β-cyclocitral and of some of its derivatives. • β-cyclocitral is a volatile oxidation product of the carotenoid β-carotene. • β-cyclocitral is found in a large variety of living organisms ranging from cyanobacteria and plants to fungi and animals. • β-cyclocitral can be converted in vivo into various derivatives including β-cyclocitric acid. • β-cyclocitral and β-cyclocitric acid are plant signals triggering defense mechanisms against environmental stresses. • β-cyclocitral and its derivatives also function as allelochemicals and repellents in non-vascular plants and in animals. [ABSTRACT FROM AUTHOR]

Singlet oxygen (1O2) is a singular reactive oxygen species (ROS) that is produced constitutively in plant leaves in light via chlorophylls that act as photosensitizers. This 1O2 production is spatially resolved within thylakoid membranes and is enhanced under light stress conditions. 1O2 can also be produced by phytotoxins during plant–pathogen interactions. 1O2 is highly reactive, can be toxic to cells and can be involved in the signaling of programmed cell death or acclimation processes. Here, we summarize current knowledge on 1O2 management in plants and on the biological effects of this peculiar ROS. Compared with other ROS, 1O2 has received relatively little attention, but recent developments indicate that it has a crucial role in the responses of plants to light. [Copyright &y& Elsevier]

The xanthophyll cycle has a major role in protecting plants from photooxidative stress, although the mechanism of its action is unclear. Here, we have investigated Arabidopsis plants overexpressing a gene encoding β-carotene hydroxylase, containing nearly three times the amount of xanthophyll cycle carotenoids present in the wild-type. In high light at low temperature wild-type plants exhibited symptoms of severe oxidative stress: lipid peroxidation, chlorophyll bleaching, and photoinhibition. In transformed plants, which accumulate over twice as much zeaxanthin as the wild-type, these symptoms were significantly ameliorated. The capacity of non-photochemical quenching is not significantly different in transformed plants compared with wild-type and therefore an enhancement of this process cannot be the cause of the stress tolerant phenotype. Rather, it is concluded that it results from the antioxidant effect of zeaxanthin. 80-90% of violaxanthin and zeaxanthin in wild-type and transformed plants was localized to an oligomeric LHCII fraction prepared from thylakoid membranes. The binding of these pigments in intact membranes was confirmed by resonance Raman spectroscopy. Based on the structural model of LHCII, we suggest that the protein/lipid interface is the active site for the antioxidant activity of zeaxanthin, which mediates stress tolerance by the protection of bound lipids. [ABSTRACT FROM AUTHOR]

Living organisms spontaneously emit faint light, and this autoluminescence is stimulated in response to many stresses. This phenomenon is attributable to the endogenous production of excited states during oxidative reactions, particularly during peroxidation of lipids, which generates light-emitting molecules such as triplet carbonyls and singlet oxygen. Using highly sensitive cameras, it is now possible to remotely image spontaneous luminescence with a good spatial resolution, providing a new non-invasive tool for mapping oxidative stress and lipid peroxidation in plants. [Copyright &y& Elsevier]

Abstract: Far-red illumination of plant leaves for a few seconds induces a delayed luminescence rise, or afterglow, that can be measured with the thermoluminescence technique as a sharp band peaking at around 40–45 °C. The afterglow band is attributable to a heat-induced electron flow from the stroma to the plastoquinone pool and the PSII centers. Using various Arabidopsis and tobacco mutants, we show here that the electron fluxes reflected by the afterglow luminescence follow the pathways of cyclic electron transport around PSI. In tobacco, the afterglow signal relied mainly on the ferredoxin-quinone oxidoreductase (FQR) activity while the predominant pathway responsible for the afterglow in Arabidopsis involved the NAD(P)H dehydrogenase (NDH) complex. The peak temperature T m of the afterglow band varied markedly with the light conditions prevailing before the TL measurements, from around 30 °C to 45 °C in Arabidopsis. These photoinduced changes in T m followed the same kinetics and responded to the same light stimuli as the state 1–state 2 transitions. PSII-exciting light (leading to state 2) induced a downward shift while preillumination with far-red light (inducing state 1) caused an upward shift. However, the light-induced downshift was strongly inhibited in NDH-deficient Arabidopsis mutants and the upward shift was cancelled in plants durably acclimated to high light, which can perform normal state transitions. Taken together, our results suggest that the peak temperature of the afterglow band is indicative of regulatory processes affecting electron donation to the PQ pool which could involve phosphorylation of NDH. The afterglow thermoluminescence band provides a new and simple tool to investigate the cyclic electron transfer pathways and to study their regulation in vivo. [Copyright &y& Elsevier]

In green plants, the xanthophyll carotenoid zeaxanthin is synthesized transiently under conditions of excess light energy and participates in photoprotection. In the Arabidopsis lut2 npq2 double mutant, all xanthophylls were replaced constitutively by zeaxanthin, the only xanthophyll whose synthesis was not impaired. The relative proportions of the different chlorophyll antenna proteins were strongly affected with respect to the wild-type strain. The major antenna, LHCII, did not form trimers, and its abundance was strongly reduced as was CP26, albeit to a lesser extent. In contrast, CP29, CP24, LHCI proteins, and the PSI and PSII core complexes did not undergo major changes. PSII-LHCII supercomplexes were not detectable while the PSI-LHCI supercomplex remained unaffected. The effect of zeaxanthin accumulation on the stability of the different Lhc proteins was uneven: the LHCII proteins from lut2 npq2 had a lower melting temperature as compared with the wild-type complex while LHCI showed increased resistance to heat denaturation. Consistent with the loss of LHCII, light-state 1 to state 2 transitions were suppressed, the photochemical efficiency in limiting light was reduced and photosynthesis was saturated at higher light intensities in lut2 npq2 leaves, resulting in a photosynthetic phenotype resembling that of high light-acclimated leaves. Zeaxanthin functioned in vivo as a light-harvesting accessory pigment in lut2 npq2 chlorophyll antennae. As a whole, the in vivo data are consistent with the results obtained by using recombinant Lhc proteins reconstituted in vitro with purified zeaxanthin. While PSII photoinhibition was similar in wild type and lut2 npq2 exposed to high light at low temperature, the double mutant was much more resistant to photooxidative stress and lipid peroxidation than the wild type. The latter observation is consistent with an antioxidant and lipid protective role of zeaxanthin in vivo. [ABSTRACT FROM AUTHOR]

Peroxidation of polyunsaturated fatty acids is one of the main events triggered by oxidative stress in cells. Some lipid peroxidation products are light-emitting species, and their luminescence can be used as an internal marker of oxidative stress. However, this spontaneous chemiluminescence is weak and difficult to measure. Recent studies have shown that an alternative approach that involves measuring thermoluminescence bands at high temperature (in the range 80–150°C) is a simple way of detecting and quantifying lipid peroxidative damage and oxidative stress in plants. [Copyright &y& Elsevier]

The hli genes, present in cyanobacteria, algae and vascular plants, encode small proteins [high-light-inducible polypeptides (HLIPs)] with a single membrane-spanning α-helix related to the first and third helices of eukaryotic chlorophyll a/b-binding proteins. The HLIPs are present in low amounts in low light and they accumulate transiently at high light intensities. We are investigating the function of those polypeptides in a Synechocystis PCC6803 mutant lacking four of the five hli genes. Growth of the quadruple hli mutant was adversely affected by high light intensities. The most striking effect of the quadruple hli mutation was an alteration of cell pigmentation. Pigment changes associated with cell acclimation to increasing light intensity [i.e. decrease in light-harvesting pigments, accumulation of the carotenoid myxoxanthophyll and decrease in photosystem I (PSI)-associated chlorophylls] were strongly exacerbated in the quadruple hli mutant, resulting in yellowish cultures that bleached in high light and died as light intensities exceeded (>500 μmol photon m−2 s−1). However, these pigment changes were not associated with an inhibition of photosynthesis, as probed by in vivo chlorophyll fluorescence, photoacoustic and O2-evolution measurements. On the contrary, the HLIP deficiency was accompanied by a stimulation of the photochemical activity, especially in high-light-grown cells. Western blot analyses revealed that the PSI reaction center level (PsaA/B) was noticeably reduced in the quadruple hli mutant relative to the wild type, whereas the abundance of the PSII reaction center protein D1 was comparatively little affected. The hli mutations did not enhance photoinhibition and photooxidation when cells were exposed over a short term to a very high light intensity. Together, the results of this study indicate that HLIPs are critical in the adaptation of the cyanobacterium to variations in light intensity. The data are consistent with the idea that HLIPs are involved, through a direct or indirect means, in nonphotochemical dissipation of absorbed light energy. [Copyright &y& Elsevier]

Tocochromanols (tocopherols, tocotrienols and plastochromanol-8), isoprenoid quinone (plastoquinone-9 and plastoquinol-9) and carotenoids (carotenes and xanthophylls), are lipid-soluble antioxidants in the chloroplasts, which play an important defensive role against photooxidative stress in plants. In this study, the interplay between the antioxidant activities of those compounds in excess light stress was analyzed in wild-type (WT) Arabidopsis thaliana and in a tocopherol cyclase mutant (vte1), a homogentisate phytyl transferase mutant (vte2) and a tocopherol cyclase overexpressor (VTE1oex). The results reveal a strategy of cooperation and replacement between α-tocopherol, plastochromanol-8, plastoquinone-9/plastoquinol-9 and zeaxanthin. In the first line of defense (non-radical mechanism), singlet oxygen is either physically or chemically quenched by α-tocopherol; however, when α-tocopherol is consumed, zeaxanthin and plastoquinone-9/plastoquinol-9 can provide alternative protection against singlet oxygen toxicity by functional replacement of α-tocopherol either by zeaxanthin for the physical quenching or by plastoquinone-9/plastoquinol-9 for the chemical quenching. When singlet oxygen escapes this first line of defense, it oxidizes lipids and forms lipid hydroperoxides, which are oxidized to lipid peroxyl radicals by ferric iron. In the second line of defense (radical mechanism), lipid peroxyl radicals are scavenged by α-tocopherol. After its consumption, plastochromanol-8 overtakes this function. We provide a comprehensive description of the reaction pathways underlying the non-radical and radical antioxidant activities of α-tocopherol, carotenoids, plastoquinone-9/plastoquinol-9 and plastochromanol-8. The interplay between the different plastid lipid-soluble antioxidants in the non-radical and the radical mechanism provides step by step insights into protection against photooxidative stress in higher plants. Image 1 • Interplay between tocochromanols, isoprenoid quinones and carotenoids is crucial to prevent photooxidative stress. • In first line of defense, interplay of α-tocopherol, total plastoquinone-9 and carotenoids protects plant from singlet oxygen. • Singlet oxygen fled from first line of defense oxidize lipids to lipid hydroperoxides which decomposes to peroxyl radical. • In second line of defense, plastochromanol-8 substitutes α-tocopherol's function in its absence to scavenge peroxyl radicals. [ABSTRACT FROM AUTHOR]

Plant surfaces form the barrier between a plant and its environment. Upon damage, the wound healing process begins immediately and is accompanied by a rapid production of extracellular reactive oxygen species (ROS), essential in deterring pathogens, signalling responses and cell wall restructuring. Although many enzymes produce extracellular ROS, it is unclear if ROS-producing enzymes act synergistically. We characterised the oxidative burst of superoxide (O 2 − ) and hydrogen peroxide (H 2 O 2 ) that follows wounding in pea ( Pisum sativum L.) seedlings. Rates of ROS production were manipulated by exogenous application of enzyme substrates and inhibitors. The results indicate significant roles for di-amine oxidases (DAO) and peroxidases (Prx) rather than NADPH oxidase. The burst of O 2 − was strongly dependent on the presence of H 2 O 2 produced by DAO. Potential substrates released from wounded seedlings included linoleic acid that, upon exogenous application, strongly stimulated catalase-sensitive O 2 − production. Moreover, a 65 kD plasma membrane (PM) guaiacol Prx was found in the secretome of wounded seedlings and showed dependence on linoleic acid for O 2 − production. Lipoxygenases are suggested to modulate O 2 − production by consuming polyunsaturated fatty acids in the apoplast. Overall, a O 2 − -producing mechanism involving H 2 O 2 -derived from DAO, linoleic acid and a PM-associated Prx is proposed. [ABSTRACT FROM AUTHOR]

Abstract: Barley displays a great genetic diversity, constituting a valuable source to delineate the responses of contrasted genotypes to environmental constraints. Here, we investigated the level of oxidative stress and the participation of antioxidant systems in two barley genotypes: Express, a variety known to be sensitive to drought, and Saïda, an Algerian landrace selected for its tolerance to water deficit. Soil-grown 15-day-old plants were subjected to water deficit for 8 days and then rewatered. We observed that upon water stress Express exhibits compared to Saïda accelerated wilting and a higher level of oxidative stress evaluated by HPLC measurements of lipid peroxidation and by imaging techniques. In parallel, Express plants also display lower levels of catalase and superoxide dismutase activity. No great difference was observed regarding peroxiredoxins and methionine sulfoxide reductases, enzymes detoxifying peroxides and repairing oxidized proteins, respectively. In contrast, upon water stress and recovery, much higher contents and oxidation ratios of glutathione and ascorbate were measured in Express compared to Saïda. Express also shows during water deficit greater increases in the pools of lipophilic antioxidants like xantophyll carotenoids and α-tocopherol. Altogether, these data show that the differential behavior of the two genotypes involves distinct responses regarding antioxidant mechanisms. Indeed, the drought sensitivity of Express compared with Saïda is associated with oxidative damage and a lower enzymatic ROS-scavenging capacity, but in parallel with a much stronger enhancement of most mechanisms involving low-molecular weight antioxidant compounds. [Copyright &y& Elsevier]

Abstract: The cellular redox balance seems an important modulator under heavy metal stress. While for other heavy metals these processes are well studied, oxidative stress related responses are also known to be triggered under uranium stress but information remains limited. This study aimed to further unravel the mechanisms by which plants respond to uranium stress. Seventeen-day-old Arabidopsis thaliana seedlings, grown on a modified Hoagland solution under controlled conditions, were exposed to 0, 0.1, 1, 10 and 100 μM uranium for 1, 3 and 7 days. While in Part I of this study oxidative stress related responses in the roots were discussed, this second Part II discusses oxidative stress related responses in the leaves and general conclusions drawn from the results of the roots and the leaves will be presented. As several responses were already visible following 1 day exposure, when uranium concentrations in the leaves were negligible, a root-to-shoot signaling system was suggested in which plastids could be important sensing sites. While lipid peroxidation, based on the amount of thiobarbituric acid reactive compounds, was observed after exposure to 100 μM uranium, affecting membrane structure and function, a transient concentration dependent response pattern was visible for lipoxygenase initiated lipid peroxidation. This transient character of uranium stress responses in leaves was emphasized by results of lipoxygenase (LOX2) and antioxidative enzyme transcript levels, enzyme capacities and glutathione concentrations both in time as with concentration. The ascorbate redox balance seemed an important modulator of uranium stress responses in the leaves as in addition to the previous transient responses, the total ascorbate concentration and ascorbate/dehydroascorbate redox balance increased in a concentration and time dependent manner. This could represent either a slow transient response or a stable increase with regard to plant acclimation to uranium stress. [Copyright &y& Elsevier]

Abstract: The PsaE protein is located at the reducing side of photosystem I (PSI) and is involved in docking the soluble electron acceptors, particularly ferredoxin. However, deletion of the psaE gene in the cyanobacterium Synechocystis sp. strain PCC 6803 inhibited neither photoautotrophic growth, nor in vivo linear and cyclic electron flows. Using photoacoustic spectroscopy, we detected an oxygen-dependent, PSI-mediated energy storage activity in the ΔpsaE null mutant, which was not present in the wild type (WT). The expression of the genes encoding catalase (katG) and iron superoxide dismutase (sodB) was upregulated in the ΔpsaE mutant, and the increase in katG expression was correlated with an increase in catalase activity of the cells. When catalases were inhibited by sodium azide, the production of reactive oxygen species was enhanced in ΔpsaE relative to WT. Moreover, sodium azide strongly impaired photoautotrophic growth of the ΔpsaE mutant cells while WT was much less sensitive to this inhibitor. The katG gene was deleted in the ΔpsaE mutant, and the resulting double mutant was more photosensitive than the single mutants, showing cell bleaching and lipid peroxidation in high light. Our results show that the presence of the PsaE polypeptide at the reducing side of PSI has a function in avoidance of electron leakage to oxygen in the light (Mehler reaction) and the resulting formation of toxic oxygen species. PsaE-deficient Synechocystis cells can counteract the chronic photoreduction of oxygen by increasing their capacity to detoxify reactive oxygen species. [Copyright &y& Elsevier]

Summary: Thellungiella halophila seedlings grown on a solid substrate for 25 days on standard medium were challenged with NaCl. Growth, tissue hydration, ion accumulation, photosynthesis, lipid peroxidation and antioxidant enzymatic activities were studied on rosette leaves. Three accessions of Arabidopsis thaliana were cultivated under the same conditions. During the first two weeks of salt treatment, the growth of T. halophila leaves was restricted by NaCl. No significant difference appeared between T. halophila and A. thaliana concerning biomass deposition, or hydric and ionic parameters. However, all A. thaliana plants displayed foliar damage, and died during the third week of salt (50mM NaCl) treatment. Almost all (94%) T. halophila plants remained alive, but did not display any sign of altered physiological condition. Tissue hydration, chlorophyll content, stomatal conductance, photosynthetic quantum yield, and photosynthetic rate were very similar to those of control plants. Lipid peroxidation, estimated from thermoluminescence, was very low and insensitive to salt treatment. Only slight changes occurred in antioxidant enzymatic activities (SOD, several peroxidases, and catalase). From the absence of physiological disorder symptoms, we infer that salt was efficiently compartmentalized in leaf vacuoles. In salt-treated A. thaliana, the photosynthetic quantum yield was diminished, and lipid peroxidation was augmented. These observations reinforce the conclusion that T. halophila could accumulate salt in its leaves without damage, in contrast to A. thaliana. [Copyright &y& Elsevier]

When there is an imbalance between the light energy absorbed by a photosynthetic organism and that which can be utilized in photosynthesis, photo-oxidative stress can damage pigments, proteins, lipids, and nucleic acids. In this work we compared the wild type and a xanthophyll-deficient mutant of Chlamydomonas reinhardtii in their response to high amounts of light. Wildtype Chlamydomonas cells were able to acclimate to high amounts of light following transfer from low light conditions. In contrast, the npq1 lor1 double mutant, which lacks protective xanthophylls (zeaxanthin and lutein) in the chloroplast, progressively lost viability and photosynthetic capacity along with destruction of thylakoid membrane protein-pigment complexes and accumulation of reactive oxygen species and membrane lipid peroxides. Loss of viability was partially rescued by lowered oxygen tension, suggesting that the high sensitivity of the mutant to light stress is caused by the production of reactive oxygen species in the chloroplast. Cell death was not prevented by the addition of an organic carbon source to the growth medium, demonstrating that the photo-oxidative damage can target other essential chloroplast processes besides photosynthesis. From the differential sensitivity of the mutant to exogenously added pro-oxidants, we infer that the reactive oxygen species produced during light stress in npq1 lor1 may be singlet oxygen and/or superoxide but not hydrogen peroxide. The bleaching phenotype of npq1 lor1 was not due to enhanced photodamage to photosystem II but rather to a less localized phenomenon of accumulation of photo-oxidation products in chloroplast membranes. [ABSTRACT FROM AUTHOR]