Zeaxanthin deficiency enhances the high light sensitivity of an ascorbate-deficient mutant of Arabidopsis

Tiller number is altered in the ascorbic acid-deficient rice suppressed for L-galactono-1,4-lactone dehydrogenase

The tiller of rice (Oryza sativa L.), which determines the panicle number per plant, is an important agronomic trait for grain production. Ascorbic acid (Asc) is a major plant antioxidant that serves many functions in plants. L-Galactono-1,4-lactone dehydrogenase (GLDH, EC 1.3.2.3) is an enzyme that catalyzes the last step of Asc biosynthesis in plants. Here we show that the GLDH-suppressed transgenic rices, GI-1 and GI-2, which have constitutively low (between 30% and 50%) leaf Asc content compared with the wild-type plants, exhibit a significantly reduced tiller number. Moreover, lower growth rate and plant height were observed in the Asc-deficient plants relative to the trait values of the wild-type plants at different tillering stages. Further examination showed that the deficiency of Asc resulted in a higher lipid peroxidation, a loss of chlorophyll, a loss of carotenoids, and a lower rate of CO(2) assimilation. In addition, the level of abscisic acid was higher in GI-1 plants, while the level of jasmonic acid was higher in GI-1 and GI-2 plants at different tillering stages. The results we presented here indicated that Asc deficiency was likely responsible for the promotion of premature senescence, which was accompanied by a marked decrease in photosynthesis. These observations support the conclusion that the deficiency of Asc alters the tiller number in the GLDH-suppressed transgenics through promoting premature senescence and changing phytohormones related to senescence.

L-ascorbic Acid: a multifunctional molecule supporting plant growth and development

L-Ascorbic acid (vitamin C) is as essential to plants as it is to animals. Ascorbic acid functions as a major redox buffer and as a cofactor for enzymes involved in regulating photosynthesis, hormone biosynthesis, and regenerating other antioxidants. Ascorbic acid regulates cell division and growth and is involved in signal transduction. In contrast to the single pathway responsible for ascorbic acid biosynthesis in animals, plants use multiple pathways to synthesize ascorbic acid, perhaps reflecting the importance of this molecule to plant health. Given the importance of ascorbic acid to human nutrition, several technologies have been developed to increase the ascorbic acid content of plants through the manipulation of biosynthetic or recycling pathways. This paper provides an overview of these approaches as well as the consequences that changes in ascorbic acid content have on plant growth and function. Discussed is the capacity of plants to tolerate changes in ascorbic acid content. The many functions that ascorbic acid serves in plants, however, will require highly targeted approaches to improve their nutritional quality without compromising their health.

L-ascorbic Acid: a multifunctional molecule supporting plant growth and development

L-Ascorbic acid (vitamin C) is as essential to plants as it is to animals. Ascorbic acid functions as a major redox buffer and as a cofactor for enzymes involved in regulating photosynthesis, hormone biosynthesis, and regenerating other antioxidants. Ascorbic acid regulates cell division and growth and is involved in signal transduction. In contrast to the single pathway responsible for ascorbic acid biosynthesis in animals, plants use multiple pathways to synthesize ascorbic acid, perhaps reflecting the importance of this molecule to plant health. Given the importance of ascorbic acid to human nutrition, several technologies have been developed to increase the ascorbic acid content of plants through the manipulation of biosynthetic or recycling pathways. This paper provides an overview of these approaches as well as the consequences that changes in ascorbic acid content have on plant growth and function. Discussed is the capacity of plants to tolerate changes in ascorbic acid content. The many functions that ascorbic acid serves in plants, however, will require highly targeted approaches to improve their nutritional quality without compromising their health.

Influence of nine antibiotics on key secondary metabolites and physiological characteristics in Triticum aestivum: leaf volatiles as a promising new tool to assess toxicity

Extensive worldwide use of antibiotics has resulted in significant diffuse pollution of antibiotics in environment, but understanding the effects of many important antibiotics on plant physiological activity is still limited, especially at realistic environmental concentrations of antibiotics. To gain insight into influences of different antibiotics on plant performance and identify the most promising traits for fast assessment of toxicity, we studied impacts of nine antibiotics (amoxicillin, ampicillin, penicillin G, ceftazidime, ceftriaxone, tetracycline, doxycycline, ciprofloxacin and erythromycin) on foliage photosynthesis, photosynthetic pigment content and emissions of secondary volatile metabolites in wheat (Triticum aestivum L. cv. "Lovrin"). The antibiotics were applied at concentrations of 0.5 mg L⁻¹ or 1.5 mg L⁻¹ either by watering for one week (0.5 L solution) or for two weeks (1 L solution) with given solution. Net assimilation rate was inhibited most strongly by ciprofloxacin and cephalosporins, but the inhibition was mainly due to reductions in stomatal conductance. Photosynthetic electron transport rate was affected by penicillins, cephalosporins and tetracyclines. Contents of photosynthetic pigments, chlorophylls and carotenoids, were most strongly reduced in treatments with tetracyclines, ciprofloxacin and erythromycin. The magnitude of plant photosynthetic and pigment responses generally increased with increasing the antibiotics' dose, but the overall effects were moderate, 10-20 percent in most cases. Emissions of the lipoxygenase pathway products (LOX, green leaf volatiles) were most sensitive indicators of antibiotic treatments. LOX emissions were increased by five to six fold in response to antibiotic treatments and the emissions were quantitatively associated with the treatment dose. Monoterpene emissions were also strongly enhanced by antibiotic treatments, but the dose dependence was weaker. These data collectively suggest that analysis of leaf volatiles can provide a novel sensitive assay to gauge the toxicity of different antibiotics.

Constitutive accumulation of zeaxanthin in tomato alleviates salt stress-induced photoinhibition and photooxidation

Zeaxanthin (Z) has a role in the dissipation of excess excitation energy by participating in non-photochemical quenching (NPQ) and is essential in protecting the chloroplast from photooxidative damage. To investigate the physiological effects and functional mechanism of constitutive accumulation of Z in the tomato at salt stress-induced photoinhibition and photooxidation, antisense-mediated suppression of zeaxanthin epoxidase transgenic plants and the wild-type (WT) tomato were used. The ratio of Z/(V + A + Z) and (Z + 0.5A)/(V + A + Z) in antisense transgenic plants were maintained at a higher level than in WT plants under salt stress, but the value of NPQ in WT and transgenic plants was not significantly different under salt stress. However, the maximal photochemical efficiency of PSII (Fv/Fm) and the net photosynthetic rate (Pn) in transgenic plants decreased more slowly under salt stress. Furthermore, transgenic plants showed lower level of hydrogen peroxide (H(2)O(2)), superoxide anion radical (O(2)(•-)) and ion leakage, lower malondialdehyde content. Compared with WT, the content of D1 protein decreased slightly in transgenic plants under salt stress. Our results suggested that the constitutive accumulation of Z in transgenic tomatoes can alleviate salt stress-induced photoinhibition because of the antioxidant role of Z in the scavenging quenching of singlet oxygen and/or free radicals in the lipid phase of the membrane.

The influence of ascorbate on anthocyanin accumulation during high light acclimation in Arabidopsis thaliana: further evidence for redox control of anthocyanin synthesis

Ascorbate and anthocyanins act as photoprotectants during exposure to high light (HL). They accumulate in Arabidopsis leaves in response to HL on a similar timescale, suggesting a potential relationship between them. Flavonoids and related metabolites were identified and profiled by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The ascorbate-deficient mutants vtc1, vtc2 and vtc3 accumulated less anthocyanin than wild-type (WT) during HL acclimation. In contrast, kaempferol glycoside accumulation was less affected by light and not decreased by ascorbate deficiency, while sinapoyl malate levels decreased during HL acclimation. Comparison of six Arabidopsis ecotypes showed a positive correlation between ascorbate and anthocyanin accumulation in HL. mRNA-Seq analysis showed that all flavonoid biosynthesis transcripts were increased by HL acclimation in WT. RT-PCR analysis showed that vtc1 and vtc2 were impaired in HL induction of transcripts of anthocyanin biosynthesis enzymes, and the transcription factors PAP1, GL3 and EGL3 that activate the pathway. Abscisic acid (ABA) and jasmonic acid (JA), hormones that could affect anthocyanin accumulation, were unaffected in vtc mutants. It is concluded that HL induction of anthocyanin synthesis involves a redox-sensitive process upstream of the known transcription factors. Because anthocyanins accumulate in preference to kaempferol glycosides and sinapoyl malate in HL, they might have specific properties that make them useful in HL acclimation.

Plastidic isoprenoid biosynthesis in tomato: physiological and molecular analysis in genotypes resistant and sensitive to drought stress

Isoprenoid compounds synthesised in the plastids are involved in plant response to water deficit. The functionality of the biosynthetic pathway of these compounds under drought stress has been analysed at the physiological and molecular levels in two related species of tomato (Solanum chilense and Solanum lycopersicum) that differ in their tolerance to abiotic challenge. Expression analysis of the genes encoding enzymes of these pathways (DXS, IPI, GGPPS, PSY1, NCED and HPT1) in plants at different RWC values shows significant differences for only GGPPS and HPT1, with higher expression in the tolerant S. chilense. Chlorophyll, carotenoids, α-tocopherol and ABA content was also determined in both species under different drought conditions. In agreement with HPT1 transcriptional activity, higher α-tocopherol content was observed in S. chilense than in S. lycopersicum, which correlates with a lower degree of lipoperoxidation in the former species. These results suggest that, in addition to lower stomatal conductance, α-tocopherol biosynthesis is part of the adaptation mechanisms of S. chilense to adverse environmental conditions.

Overlapping photoprotective function of vitamin E and carotenoids in Chlamydomonas

Tocopherols (vitamin E) and carotenoids are the two most abundant groups of lipid-soluble antioxidants in the chloroplast. Carotenoids are well known for their roles in protecting against photooxidative stress, whereas the photoprotective functions of tocopherols have only recently been examined experimentally. In addition, little is known about the functional overlap of carotenoids and tocopherols in vivo. To investigate this possible overlap, Chlamydomonas reinhardtii strains were engineered to overproduce tocopherols by chloroplast transformation with non-codon-optimized and codon-optimized versions of the homogentisate phytyltransferase vitamin E2 (VTE2) from Synechocystis and by nuclear transformation with VTE2 from C. reinhardtii, which resulted in 1.6-fold, 5-fold to 10-fold, and more than 10-fold increases in total tocopherol content, respectively. To test if tocopherol overproduction can compensate for carotenoid deficiency in terms of antioxidant function, the nuclear VTE2 gene from C. reinhardtii was overexpressed in the npq1 lor1 double mutant, which lacks zeaxanthin and lutein. Following transfer to high light, the npq1 lor1 strains that overaccumulated tocopherols showed increased resistance for up to 2 d and higher efficiency of photosystem II, and they were also much more resistant to other oxidative stresses. These results suggest an overlapping functions of tocopherols and carotenoids in protection against photooxidative stress.

The transcription factor ABI4 Is required for the ascorbic acid-dependent regulation of growth and regulation of jasmonate-dependent defense signaling pathways in Arabidopsis

Cellular redox homeostasis is a hub for signal integration. Interactions between redox metabolism and the ABSCISIC ACID-INSENSITIVE-4 (ABI4) transcription factor were characterized in the Arabidopsis thaliana vitamin c defective1 (vtc1) and vtc2 mutants, which are defective in ascorbic acid synthesis and show a slow growth phenotype together with enhanced abscisic acid (ABA) levels relative to the wild type (Columbia-0). The 75% decrease in the leaf ascorbate pool in the vtc2 mutants was not sufficient to adversely affect GA metabolism. The transcriptome signatures of the abi4, vtc1, and vtc2 mutants showed significant overlap, with a large number of transcription factors or signaling components similarly repressed or induced. Moreover, lincomycin-dependent changes in LIGHT HARVESTING CHLOROPHYLL A/B BINDING PROTEIN 1.1 expression were comparable in these mutants, suggesting overlapping participation in chloroplast to nucleus signaling. The slow growth phenotype of vtc2 was absent in the abi4 vtc2 double mutant, as was the sugar-insensitive phenotype of the abi4 mutant. Octadecanoid derivative-responsive AP2/ERF-domain transcription factor 47 (ORA47) and AP3 (an ABI5 binding factor) transcripts were enhanced in vtc2 but repressed in abi4 vtc2, suggesting that ABI4 and ascorbate modulate growth and defense gene expression through jasmonate signaling. We conclude that low ascorbate triggers ABA- and jasmonate-dependent signaling pathways that together regulate growth through ABI4. Moreover, cellular redox homeostasis exerts a strong influence on sugar-dependent growth regulation.

The physiological role of ascorbate as photosystem II electron donor: protection against photoinactivation in heat-stressed leaves

Previously, we showed that ascorbate (Asc), by donating electrons to photosystem II (PSII), supports a sustained electron transport activity in leaves in which the oxygen-evolving complexes were inactivated with a heat pulse (49°C, 40 s). Here, by using wild-type, Asc-overproducing, and -deficient Arabidopsis (Arabidopsis thaliana) mutants (miox4 and vtc2-3, respectively), we investigated the physiological role of Asc as PSII electron donor in heat-stressed leaves (40°C, 15 min), lacking active oxygen-evolving complexes. Chlorophyll-a fluorescence transients show that in leaves excited with trains of saturating single-turnover flashes spaced 200 ms apart, allowing continual electron donation from Asc to PSII, the reaction centers remained functional even after thousands of turnovers. Higher flash frequencies or continuous illumination (300 μmol photons m(-2) s(-1)) gradually inactivated them, a process that appeared to be initiated by a dramatic deceleration of the electron transfer from Tyr(Z) to P680(+), followed by the complete loss of charge separation activity. These processes occurred with half-times of 1.2 and 10 min, 2.8 and 23 min, and 4.1 and 51 min in vtc2-3, the wild type, and miox4, respectively, indicating that the rate of inactivation strongly depended on the Asc content of the leaves. The recovery of PSII activity, following the degradation of PSII proteins (D1, CP43, and PsbO), in moderate light (100 μmol photons m(-2) s(-1), comparable to growth light), was also retarded in the Asc-deficient mutant. These data show that high Asc content of leaves contributes significantly to the ability of plants to withstand heat-stress conditions.

Ecophysiology of photosynthesis in bryophytes: major roles for oxygen photoreduction and non-photochemical quenching?

CO(2) fixation in mosses saturates at moderate irradiances. Relative electron transport rate (RETR) inferred from chlorophyll fluorescence saturates at similar irradiance in shade species (e.g. Plagiomnium undulatum, Trichocolea tomentella), but many species of unshaded habitats (e.g. Andreaea rothii, Schistidium apocarpum, Sphagnum spp. and Frullania dilatata) show non-saturating RETR at high irradiance, with high non-photochemical quenching (NPQ). In P. undulatum and S. apocarpum, experiments in different gas mixtures showed O(2) and CO(2) as interchangeable electron sinks. Nitrogen + saturating CO(2) gave high RETR and depressed NPQ. In S. apocarpum, glycolaldehyde (inhibiting photosynthesis and photorespiration) depressed RETR in air more at low than at high irradiance; in CO(2) -free air RETR was maintained at all irradiances. Non-saturating electron flow was not suppressed in ambient CO(2) with 1% O(2) . The results indicate high capacity for oxygen photoreduction when CO(2) assimilation is limited. Non-saturating light-dependent H(2) O(2) production, insensitive to glycolaldehyde, suggests that electron transport is supported by oxygen photoreduction, perhaps via the Mehler-peroxidase reaction. Consistent with this, mosses were highly tolerant to paraquat, which generates superoxide at photosystem I (PSI). Protection against excess excitation energy in mosses involves high capacity for photosynthetic electron transport to oxygen and high NPQ, activated at high irradiance, alongside high reactive oxygen species (ROS) tolerance.

Expression, in vivo localization and phylogenetic analysis of a pyridoxine 5'-phosphate oxidase in Arabidopsis thaliana

Pyridoxal phosphate (PLP), a vitamin B(6) vitamer, is an essential cofactor for numerous enzymes. Pyridoxine/pyridoxamine phosphate oxidase (PPOX) catalyzes the synthesis of pyridoxal phosphate from pyridoxine phosphate (PNP) and/or pyridoxamine phosphate (PMP). The At5g49970 locus in Arabidopsis thaliana encodes an AtPPOX, a PNP/PMP oxidase. The expression of the AtPPOX gene varied in different tissues of Arabidopsis examined, being up-regulated by light, heat shock, ABA, and ethylene treatments, and down-regulated by exposure to drought and NaCl. Monoclonal antibodies raised against two different domains of AtPPOX recognized different sizes of AtPPOX, suggesting that AtPPOX proteins are produced as splice variants of the AtPPOX gene in Arabidopsis. Phylogenetic analysis of AtPPOX across all domains of life demonstrated that plant AtPPOX homologs have an additional Yjef_N domain preceding the Pyridox_Oxidase domain at the C-terminal end of the protein, while AtPPOX homologs from bacteria, fungi and animals have only Pyridox_Oxidase domain. The presence of the Yjef_N domain in plant AtPPOX homologs suggests that acquisition of this domain, and its fusion with the pyridox_oxidase domain began with the endosymbiotic acquisition of the chloroplast. Bioinformatic analysis suggested that AtPPOX is localized in chloroplast, but the monoclonal antibody could not be used for subcellular localization of this protein. A GFP-AtPPOX fusion construct introduced into the Arabidopsis protoplast confirmed localization of AtPPOX into the chloroplast. An RNAi mutant of AtPPOX showed sensitivity to high light suggesting a role for PPOX in resistance to photooxidative damage, and alteration in root growth in the presence of sucrose suggests a role for PPOX in root development.

Dynamics of zeaxanthin binding to the photosystem II monomeric antenna protein Lhcb6 (CP24) and modulation of its photoprotection properties

Lhcb6 (CP24) is a monomeric antenna protein of photosystem II, which has been shown to play special roles in photoprotective mechanisms, such as the Non-Photochemical Quenching and reorganization of grana membranes in excess light conditions. In this work we analyzed Lhcb6 in vivo and in vitro: we show this protein, upon activation of the xanthophyll cycle, accumulates zeaxanthin into inner binding sites faster and to a larger extent than any other pigment-protein complex. By comparative analysis of Lhcb6 complexes violaxanthin or zeaxanthin binding, we demonstrate that zeaxanthin not only down-regulates chlorophyll singlet excited states, but also increases the efficiency of chlorophyll triplet quenching, with consequent reduction of singlet oxygen production and significant enhancement of photo-stability. On these bases we propose that Lhcb6, the most recent addition to the Lhcb protein family which evolved concomitantly to the adaptation of photosynthesis to land environment, has a crucial role in zeaxanthin-dependent photoprotection.

Enhanced photoprotection by protein-bound vs free xanthophyll pools: a comparative analysis of chlorophyll b and xanthophyll biosynthesis mutants

When light absorbed by plants exceeds the capacity of photosynthesis, the xanthophyll violaxanthin is reversibly de-epoxidized to zeaxanthin in the so-called xanthophyll cycle. Zeaxanthin plays a key role in the protection of photosynthetic organisms against excess light, by promoting rapidly reversible (qE) and long-term (qI) quenching of excited chlorophylls, and preventing lipid oxidation. The photoprotective role of zeaxanthin, either free or bound to light-harvesting complexes (Lhcs), has been investigated by using mutants lacking Chl b (ch1) and/or specific xanthophyll species (npq, lut2). The ch1 mutation causes (1) the absence of Lhcb proteins; (2) strong reduction of the feedback de-excitation (qE); and (3) accumulation of xanthophylls as free pigments into thylakoids. Ch1 mutants showed extreme sensitivity to photo-oxidative stress in high light, due to higher singlet oxygen (¹O₂) release. The double mutant ch1npq1 was more sensitive to photo-oxidation than ch1, showing that zeaxanthin does protect lipids even when free in the membrane. Nevertheless, lack of zeaxanthin had a much stronger impact on the level of lipid peroxidation in Lhcs-containing plants (WT vs npq1) with respect to Lhc-less plants (ch1 vs ch1npq1), implying that its protective effect is enhanced by interaction with antenna proteins. It is proposed that the antioxidant capacity of zeaxanthin is empowered in the presence of PSII-LHCs-Zea complexes, while its effect on enhancement of qE only provides a minor contribution. Comparison of the sensitivity of WT vs npq1 plants to exogenous ¹O₂ suggests that besides the scavenging of ¹O₂, at least one additional mechanism is involved in chloroplast photoprotection.

The role of reactive oxygen species in signalling from chloroplasts to the nucleus

The coordination of chloroplast function with the rest of cellular activity requires a continual stream of communication from this organelle to the nucleus. Chloroplasts are major sites of the production of reactive oxygen species (ROS) as either by-products of the reduction of molecular oxygen (O(2)) or its excitation in the presence of highly energised pigments. Such ROS, while potentially damaging to the cell, are also important initiators or transducers of signals from these organelles to the nucleus in response to environmental cues. ROS can initiate such retrograde signalling pathways that trigger either programmed cell death or adjustment to changed conditions. Such different outcomes have implications for the way in which signal transduction by ROS is accomplished and is the subject of this review. In response to mild-stress situations, and as a consequence of their reactivity or because of their containment by cellular antioxidant systems, it is proposed that ROS engage with or initiate signalling at or very near their site of production. In contrast, under more extreme conditions, ROS are proposed to diffuse away from their site of production and consequently elicit a different set of signalling events.

Vitamin B6 deficient plants display increased sensitivity to high light and photo-oxidative stress

BACKGROUND

Vitamin B6 is a collective term for a group of six interconvertible compounds: pyridoxine, pyridoxal, pyridoxamine and their phosphorylated derivatives. Vitamin B6 plays essential roles as a cofactor in a range of biochemical reactions. In addition, vitamin B6 is able to quench reactive oxygen species in vitro, and exogenously applied vitamin B6 protects plant cells against cell death induced by singlet oxygen (1O2). These results raise the important question as to whether plants employ vitamin B6 as an antioxidant to protect themselves against reactive oxygen species.

RESULTS

The pdx1.3 mutation affects the vitamin B6 biosynthesis enzyme, pyridoxal synthase (PDX1), and leads to a reduction of the vitamin B6 concentration in Arabidopsis thaliana leaves. Although leaves of the pdx1.3 Arabidopsis mutant contained less chlorophyll than wild-type leaves, we found that vitamin B6 deficiency did not significantly impact photosynthetic performance or shoot and root growth. Chlorophyll loss was associated with an increase in the chlorophyll a/b ratio and a selective decrease in the abundance of several PSII antenna proteins (Lhcb1/2, Lhcb6). These changes were strongly dependent on light intensity, with high light amplifying the difference between pdx1.3 and the wild type. When leaf discs were exposed to exogenous 1O2, lipid peroxidation in pdx1.3 was increased relative to the wild type; this effect was not observed with superoxide or hydrogen peroxide. When leaf discs or whole plants were exposed to excess light energy, 1O2-mediated lipid peroxidation was enhanced in leaves of the pdx1.3 mutant relative to the wild type. High light also caused an increased level of 1O2 in vitamin B6-deficient leaves. Combining the pdx1.3 mutation with mutations affecting the level of 'classical' quenchers of 1O2 (zeaxanthin, tocopherols) resulted in a highly photosensitive phenotype.

CONCLUSION

This study demonstrates that vitamin B6 has a function in the in vivo antioxidant defense of plants. Thus, the antioxidant activity of vitamin B6 inferred from in vitro studies is confirmed in planta. Together with the finding that chloroplasts contain vitamin B6 compounds, the data show that vitamin B6 functions as a photoprotector that limits 1O2 accumulation in high light and prevents 1O2-mediated oxidative damage.

The chloroplastic lipocalin AtCHL prevents lipid peroxidation and protects Arabidopsis against oxidative stress

Lipocalins are small ligand-binding proteins with a simple tertiary structure that gives them the ability to bind small, generally hydrophobic, molecules. Recent studies have shown that animal lipocalins play important roles in the regulation of developmental processes and are involved in tolerance to oxidative stress. Plants also possess various types of lipocalins, and bioinformatics analyses have predicted that some lipocalin members may be present in the chloroplast. Here we report the functional characterization of the Arabidopsis thaliana chloroplastic lipocalin AtCHL. Cellular fractionation showed that AtCHL is a thylakoid lumenal protein. Drought, high light, paraquat and abscisic acid treatments induce AtCHL transcript and protein accumulation. Under normal growth conditions, knockout (KO) and over-expressing (OEX) lines do not differ from wild-type plants in terms of phenotype and photosynthetic performance. However, KO plants, which do not accumulate AtCHL, show more damage upon photo-oxidative stress induced by drought, high light or paraquat. In contrast, a high level of AtCHL allows OEX plants to cope better with these stress conditions. When exposed to excess light, KO plants display a rapid accumulation of hydroxy fatty acids relative to the wild-type, whereas the lipid peroxidation level remains very low in OEX plants. The increased lipid peroxidation in KO plants is mediated by singlet oxygen and is not correlated with photo-inhibition of the photosystems. This work provides evidence suggesting that AtCHL is involved in the protection of thylakoidal membrane lipids against reactive oxygen species, especially singlet oxygen, produced in excess light.

Impact of clock-associated Arabidopsis pseudo-response regulators in metabolic coordination

In higher plants, the circadian clock controls a wide range of cellular processes such as photosynthesis and stress responses. Understanding metabolic changes in arrhythmic plants and determining output-related function of clock genes would help in elucidating circadian-clock mechanisms underlying plant growth and development. In this work, we investigated physiological relevance of PSEUDO-RESPONSE REGULATORS (PRR 9, 7, and 5) in Arabidopsis thaliana by transcriptomic and metabolomic analyses. Metabolite profiling using gas chromatography-time-of-flight mass spectrometry demonstrated well-differentiated metabolite phenotypes of seven mutants, including two arrhythmic plants with similar morphology, a PRR 9, 7, and 5 triple mutant and a CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1)-overexpressor line. Despite different light and time conditions, the triple mutant exhibited a dramatic increase in intermediates in the tricarboxylic acid cycle. This suggests that proteins PRR 9, 7, and 5 are involved in maintaining mitochondrial homeostasis. Integrated analysis of transcriptomics and metabolomics revealed that PRR 9, 7, and 5 negatively regulate the biosynthetic pathways of chlorophyll, carotenoid and abscisic acid, and alpha-tocopherol, highlighting them as additional outputs of pseudo-response regulators. These findings indicated that mitochondrial functions are coupled with the circadian system in plants.

Singlet oxygen in plants: production, detoxification and signaling

Singlet oxygen ((1)O(2)) is a singular reactive oxygen species (ROS) that is produced constitutively in plant leaves in light via chlorophylls that act as photosensitizers. This (1)O(2) production is spatially resolved within thylakoid membranes and is enhanced under light stress conditions. (1)O(2) can also be produced by phytotoxins during plant-pathogen interactions. (1)O(2) 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 (1)O(2) management in plants and on the biological effects of this peculiar ROS. Compared with other ROS, (1)O(2) has received relatively little attention, but recent developments indicate that it has a crucial role in the responses of plants to light.

Mechanism and regulation of the violaxanthin cycle: the role of antenna proteins and membrane lipids

The violaxanthin cycle describes the reversible conversion of violaxanthin to zeaxanthin via the intermediate antheraxanthin. This light-dependent xanthophyll conversion is essential for the adaptation of plants and algae to different light conditions and allows a reversible switch of photosynthetic light-harvesting complexes between a light-harvesting state under low light and a dissipative state under high light. The photoprotective functions of zeaxanthin have been intensively studied during the last decade, but much less attention has been directed to the mechanism and regulation of xanthophyll conversion. In this review, an overview is given on recent progress in the understanding of the role of (i) xanthophyll binding by antenna proteins and of (ii) the lipid properties of the thylakoid membrane in the regulation of xanthophyll conversion. The consequences of these findings for the mechanism and regulation of xanthophyll conversion in the thylakoid membrane will be discussed.

An expression analysis of the ascorbate biosynthesis enzyme VTC2

The vtc2 mutant of Arabidopsis thaliana is vitamin C deficient and is defective in the ascorbate biosynthesis enzyme GDP-L-Galactose phosphorylase/L -Galactose guanylyltransferase. The connection between the VTC2 gene and this enzyme has only recently been established, and little is known about the molecular characteristics of the VTC2 gene. In this paper, the expression of the VTC2 gene was characterized on both the RNA and the protein level. The VTC2 gene was expressed in all developmental stages, and the mRNA was expressed at a higher level in green tissues than in the root. VTC2 mRNA expression was induced strongly by exposing dark-grown seedlings to light, to levels higher than found in light-grown seedlings. A VTC2:GUS fusion protein was detected only in green tissues. Unexpectedly, a VTC2:YFP fusion protein was found not only in the cytosol, but also in the nucleus, which suggests that GDP-L-Galactose phosphorylase/L: -Galactose guanylyltransferase might be a dual-function protein, which has both enzymatic and regulatory function.

Chloroplast signaling and LESION SIMULATING DISEASE1 regulate crosstalk between light acclimation and immunity in Arabidopsis

Plants are simultaneously exposed to abiotic and biotic hazards. Here, we show that local and systemic acclimation in Arabidopsis thaliana leaves in response to excess excitation energy (EEE) is associated with cell death and is regulated by specific redox changes of the plastoquinone (PQ) pool. These redox changes cause a rapid decrease of stomatal conductance, global induction of ASCORBATE PEROXIDASE2 and PATHOGEN RESISTANCE1, and increased production of reactive oxygen species (ROS) and ethylene that signals through ETHYLENE INSENSITIVE2 (EIN2). We provide evidence that multiple hormonal/ROS signaling pathways regulate the plant's response to EEE and that EEE stimulates systemic acquired resistance and basal defenses to virulent biotrophic bacteria. In the Arabidopsis LESION SIMULATING DISEASE1 (lsd1) null mutant that is deregulated for EEE acclimation responses, propagation of EEE-induced programmed cell death depends on the plant defense regulators ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and PHYTOALEXIN DEFICIENT4 (PAD4). We find that EDS1 and PAD4 operate upstream of ethylene and ROS production in the EEE response. The data suggest that the balanced activities of LSD1, EDS1, PAD4, and EIN2 regulate signaling of programmed cell death, light acclimation, and holistic defense responses that are initiated, at least in part, by redox changes of the PQ pool.

Identification and functional analysis of light-responsive unique genes and gene family members in rice

Functional redundancy limits detailed analysis of genes in many organisms. Here, we report a method to efficiently overcome this obstacle by combining gene expression data with analysis of gene-indexed mutants. Using a rice NSF45K oligo-microarray to compare 2-week-old light- and dark-grown rice leaf tissue, we identified 365 genes that showed significant 8-fold or greater induction in the light relative to dark conditions. We then screened collections of rice T-DNA insertional mutants to identify rice lines with mutations in the strongly light-induced genes. From this analysis, we identified 74 different lines comprising two independent mutant lines for each of 37 light-induced genes. This list was further refined by mining gene expression data to exclude genes that had potential functional redundancy due to co-expressed family members (12 genes) and genes that had inconsistent light responses across other publicly available microarray datasets (five genes). We next characterized the phenotypes of rice lines carrying mutations in ten of the remaining candidate genes and then carried out co-expression analysis associated with these genes. This analysis effectively provided candidate functions for two genes of previously unknown function and for one gene not directly linked to the tested biochemical pathways. These data demonstrate the efficiency of combining gene family-based expression profiles with analyses of insertional mutants to identify novel genes and their functions, even among members of multi-gene families.

Dehydroascorbate reductase affects non-photochemical quenching and photosynthetic performance

Ascorbic acid (Asc) is a major antioxidant involved in photoprotection and photosynthetic function in plants. Dehydroascorbate reductase (DHAR) catalyzes the regeneration of Asc from its oxidized state and serves as an important regulator of Asc recycling. In this work, we used a molecular biochemical approach to investigate how the efficiency of Asc recycling affects non-photochemical quenching (NPQ). Suppression of DHAR expression resulted in a lower induction of NPQ that correlated with reductions in chlorophyll and xanthophyll pigments, quantum yield of photosystem II, and CO(2) assimilation, whereas the level of reactive oxygen species increased. The quickly reversible component of NPQ decreased and the slowly reversible or irreversible component of NPQ increased following a reduction in DHAR expression. Significant photoinhibition was also observed following exposure to high light. Direct feeding with Asc restored the appropriate induction of NPQ in DHAR-suppressed leaves. In contrast, increasing DHAR expression increased the pool size of xanthophyll and chlorophyll pigments as well as the rate of CO(2) assimilation, particularly at high light intensities, whereas the level of reactive oxygen species was reduced. Leaves with increased DHAR expression experienced less photoinhibition than did wild-type plants following exposure to high light. DHAR activity, therefore, can affect the appropriate induction of NPQ and level of photoprotection during exposure to high light.

Vitamin E is essential for the tolerance of Arabidopsis thaliana to metal-induced oxidative stress

Arabidopsis (Arabidopsis thaliana) plants were grown in a hydroponic culture system for 7 to 14 d in the absence or presence of 75 microM Cd or 75 microM Cu. The Cu treatment resulted in visual leaf symptoms, together with anthocyanin accumulation and loss of turgor. Pronounced lipid peroxidation, which was detected by autoluminescence imaging and malondialdehyde titration, was observed in Cu-treated leaves. The Cd treatment also resulted in loss of leaf pigments but lipid peroxidation and oxidative stress were less pronounced than in the leaves exposed to Cu. Analysis of low-molecular-weight chloroplast and cytosolic antioxidants (ascorbate, glutathione, tocopherols, carotenoids) and antioxidant enzymes (thiol-based reductases and peroxidases) revealed relatively few responses to metal exposure. However, there was a marked increase in vitamin E (alpha-tocopherol) in response to Cd and Cu treatments. Ascorbate increased significantly in Cu-exposed leaves. Other antioxidants either remained stable or decreased in response to metal stress. Transcripts encoding enzymes of the vitamin E biosynthetic pathway were increased in response to metal exposure. In particular, VTE2 mRNA was enhanced in Cu- and Cd-treated plants, while VTE5 and hydroxylpyruvate dioxygenase (HPPD) mRNAs were only up-regulated in Cd-treated plants. Consistent increases in HPPD transcripts and protein were observed. The vitamin E-deficient (vte1) mutant exhibited an enhanced sensitivity towards both metals relative to the wild-type (WT) control. Unlike the vte1 mutants, which showed enhanced lipid peroxidation and oxidative stress in the presence of Cu or Cd, the ascorbate-deficient (vtc2) mutant showed WT responses to metal exposure. Taken together, these results demonstrate that vitamin E plays a crucial role in the tolerance of Arabidopsis to oxidative stress induced by heavy metals such as Cu and Cd.

Antioxidant status, peroxidase activity, and PR protein transcript levels in ascorbate-deficient Arabidopsis thaliana vtc mutants

Ascorbate is the most abundant small molecule antioxidant in plants and is proposed to function, along with other members of an antioxidant network, in controlling reactive oxygen species. A biochemical and molecular characterization of four ascorbate-deficient (vtc) Arabidopsis thaliana mutants has been carried out to determine if ascorbate deficiency is compensated by changes in the other major antioxidants. Seedlings grown in vitro were used to minimize stress and longer term developmental differences. Comparison was made with the low glutathione cad2 mutant and vtc2-1 treated with D,L-buthionine-[S,R]-sulphoximine to cause combined ascorbate and glutathione deficiency. The pool sizes and oxidation state of ascorbate and glutathione were not altered by deficiency of the other. alpha-Tocopherol and activities of monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and catalase were little affected. Ascorbate peroxidase activity was higher in vtc1, vtc2-1, and vtc2-2. Ionically bound cell wall peroxidase activity was increased in vtc1, vtc2-1, and vtc4. Supplementation with ascorbate increased cell wall peroxidase activity. 2,6-Dichlorobenzonitrile, an inhibitor of cellulose synthesis, increased cell wall peroxidase activity in the wild type and vtc1. The transcript level of an endochitinase, PR1, and PR2, but not GST6, was increased in vtc1, vtc2-1, and vtc-2-2. Endochitinase transcript levels increased after ascorbate, paraquat, salicylic acid, and UV-C treatment, PR1 after salicylic acid treatment, and PR2 after paraquat and UV-C treatment. Camalexin was higher in vtc1 and the vtc2 alleles. Induction of PR genes, cell wall peroxidase activity, and camalexin in vtc1, vtc2-1, and vtc2-2 suggests that the mutants are affected in pathogen response signalling pathways.

Zeaxanthin has enhanced antioxidant capacity with respect to all other xanthophylls in Arabidopsis leaves and functions independent of binding to PSII antennae

The ch1 mutant of Arabidopsis (Arabidopsis thaliana) lacks chlorophyll (Chl) b. Leaves of this mutant are devoid of photosystem II (PSII) Chl-protein antenna complexes and have a very low capacity of nonphotochemical quenching (NPQ) of Chl fluorescence. Lhcb5 was the only PSII antenna protein that accumulated to a significant level in ch1 mutant leaves, but the apoprotein did not assemble in vivo with Chls to form a functional antenna. The abundance of Lhca proteins was also reduced to approximately 20% of the wild-type level. ch1 was crossed with various xanthophyll mutants to analyze the antioxidant activity of carotenoids unbound to PSII antenna. Suppression of zeaxanthin by crossing ch1 with npq1 resulted in oxidative stress in high light, while removing other xanthophylls or the PSII protein PsbS had no such effect. The tocopherol-deficient ch1 vte1 double mutant was as sensitive to high light as ch1 npq1, and the triple mutant ch1 npq1 vte1 exhibited an extreme sensitivity to photooxidative stress, indicating that zeaxanthin and tocopherols have cumulative effects. Conversely, constitutive accumulation of zeaxanthin in the ch1 npq2 double mutant led to an increased phototolerance relative to ch1. Comparison of ch1 npq2 with another zeaxanthin-accumulating mutant (ch1 lut2) that lacks lutein suggests that protection of polyunsaturated lipids by zeaxanthin is enhanced when lutein is also present. During photooxidative stress, alpha-tocopherol noticeably decreased in ch1 npq1 and increased in ch1 npq2 relative to ch1, suggesting protection of vitamin E by high zeaxanthin levels. Our results indicate that the antioxidant activity of zeaxanthin, distinct from NPQ, can occur in the absence of PSII light-harvesting complexes. The capacity of zeaxanthin to protect thylakoid membrane lipids is comparable to that of vitamin E but noticeably higher than that of all other xanthophylls of Arabidopsis leaves.

Zeaxanthin has enhanced antioxidant capacity with respect to all other xanthophylls in Arabidopsis leaves and functions independent of binding to PSII antennae

The ch1 mutant of Arabidopsis (Arabidopsis thaliana) lacks chlorophyll (Chl) b. Leaves of this mutant are devoid of photosystem II (PSII) Chl-protein antenna complexes and have a very low capacity of nonphotochemical quenching (NPQ) of Chl fluorescence. Lhcb5 was the only PSII antenna protein that accumulated to a significant level in ch1 mutant leaves, but the apoprotein did not assemble in vivo with Chls to form a functional antenna. The abundance of Lhca proteins was also reduced to approximately 20% of the wild-type level. ch1 was crossed with various xanthophyll mutants to analyze the antioxidant activity of carotenoids unbound to PSII antenna. Suppression of zeaxanthin by crossing ch1 with npq1 resulted in oxidative stress in high light, while removing other xanthophylls or the PSII protein PsbS had no such effect. The tocopherol-deficient ch1 vte1 double mutant was as sensitive to high light as ch1 npq1, and the triple mutant ch1 npq1 vte1 exhibited an extreme sensitivity to photooxidative stress, indicating that zeaxanthin and tocopherols have cumulative effects. Conversely, constitutive accumulation of zeaxanthin in the ch1 npq2 double mutant led to an increased phototolerance relative to ch1. Comparison of ch1 npq2 with another zeaxanthin-accumulating mutant (ch1 lut2) that lacks lutein suggests that protection of polyunsaturated lipids by zeaxanthin is enhanced when lutein is also present. During photooxidative stress, alpha-tocopherol noticeably decreased in ch1 npq1 and increased in ch1 npq2 relative to ch1, suggesting protection of vitamin E by high zeaxanthin levels. Our results indicate that the antioxidant activity of zeaxanthin, distinct from NPQ, can occur in the absence of PSII light-harvesting complexes. The capacity of zeaxanthin to protect thylakoid membrane lipids is comparable to that of vitamin E but noticeably higher than that of all other xanthophylls of Arabidopsis leaves.

Two genes in Arabidopsis thaliana encoding GDP-L-galactose phosphorylase are required for ascorbate biosynthesis and seedling viability

Plants synthesize ascorbate from guanosine diphosphate (GDP)-mannose via L-galactose/L-gulose, although uronic acids have also been proposed as precursors. Genes encoding all the enzymes of the GDP-mannose pathway have previously been identified, with the exception of the step that converts GDP-L-galactose to L-galactose 1-P. We show that a GDP-L-galactose phosphorylase, encoded by the Arabidopsis thaliana VTC2 gene, catalyses this step in the ascorbate biosynthetic pathway. Furthermore, a homologue of VTC2, At5g55120, encodes a second GDP-L-galactose phosphorylase with similar properties to VTC2. Two At5g55120 T-DNA insertion mutants (vtc5-1 and vtc5-2) have 80% of the wild-type ascorbate level. Double mutants were produced by crossing the loss-of-function vtc2-1 mutant with each of the two vtc5 alleles. These show growth arrest immediately upon germination and the cotyledons subsequently bleach. Normal growth was restored by supplementation with ascorbate or L-galactose, indicating that both enzymes are necessary for ascorbate generation. vtc2-1 leaves contain more mannose 6-P than wild-type. We conclude that the GDP-mannose pathway is the only significant source of ascorbate in A. thaliana seedlings, and that ascorbate is essential for seedling growth. A. thaliana leaves accumulate more ascorbate after acclimatization to high light intensity. VTC2 expression and GDP-L-galactose phosphorylase activity rapidly increase on transfer to high light, but the activity of other enzymes in the GDP-mannose pathway is little affected. VTC2 and At5g55120 (VTC5) expression also peak in at the beginning of the light cycle and are controlled by the circadian clock. The GDP-L-galactose phosphorylase step may therefore play an important role in controlling ascorbate biosynthesis.

Arabidopsis thaliana deficient in two chloroplast ascorbate peroxidases shows accelerated light-induced necrosis when levels of cellular ascorbate are low

Arabidopsis chloroplasts have a multi-layered defense against hydrogen peroxide (H(2)O(2)) that includes a stromal and thylakoid ascorbate peroxidase (sAPX and tAPX). Single and double null mutants in SAPX and TAPX (sapx and tapx) were each crossed with ascorbate deficient vtc2. The single, double and triple mutants did not show visual light stress phenotypes when grown at control or high light intensities (CL and HL; 120 and 1,000 micromol photons m(-2) s(-1)). Upon shift from CL to HL, mesophyll of expanded leaves of the triple mutant bleached within hours, with exclusion of the major vein areas; this contrasts to reported patterns of cell death under ozone treatment and calatase deficiency. tapx-vtc2 and sapx-vtc2, but not tapx-sapx or single mutants, showed limited bleaching. Bleaching and necrosis were accompanied by accumulation of H(2)O(2). Cellular concentrations of alpha-tocopherol, ascorbate and glutathione showed dramatic increase in response to HL in all eight genotypes and the four vtc2 genotypes accumulated more glutathione under CL than the others. Transcript analysis of other ROS responsive genes in vtc2 and the triple mutant showed up to 20-fold induction after transition to HL, generally irrespective of genotype. We conclude that chloroplast APX proteins in Arabidopsis can be effectively compensated by other endogenous H(2)O(2) detoxification systems, but that low cellular ascorbate levels in absence of chloroplast APX activity are detrimental to the cell during excess light.

Photosynthesis and resource distribution through plant canopies

Plant canopies are characterized by dramatic gradients of light between canopy top and bottom, and interactions between light, temperature and water vapour deficits. This review summarizes current knowledge of potentials and limitations of acclimation of foliage photosynthetic capacity (A(max)) and light-harvesting efficiency to complex environmental gradients within the canopies. Acclimation of A(max) to high light availability involves accumulation of rate-limiting photosynthetic proteins per unit leaf area as the result of increases in leaf thickness in broad-leaved species and volume: total area ratio and mesophyll thickness in species with complex geometry of leaf cross-section. Enhancement of light-harvesting efficiency in low light occurs through increased chlorophyll production per unit dry mass, greater leaf area per unit dry mass investment in leaves and shoot architectural modifications that improve leaf exposure and reduce within-shoot shading. All these acclimation responses vary among species, resulting in species-specific use efficiencies of low and high light. In fast-growing canopies and in evergreen species, where foliage developed and acclimated to a certain light environment becomes shaded by newly developing foliage, leaf senescence, age-dependent changes in cell wall characteristics and limited foliage re-acclimation capacity can constrain adjustment of older leaves to modified light availabilities. The review further demonstrates that leaves in different canopy positions respond differently to dynamic fluctuations in light availability and to multiple environmental stresses. Foliage acclimated to high irradiance respond more plastically to rapid changes in leaf light environment, and is more resistant to co-occurring heat and water stress. However, in higher light, co-occurring stresses can more strongly curb the efficiency of foliage photosynthetic machinery through reductions in internal diffusion conductance to CO(2). This review demonstrates strong foliage potential for acclimation to within-canopy environmental gradients, but also highlights complex constraints on acclimation and foliage functioning resulting from light x foliage age interactions, multiple environmental stresses, dynamic light fluctuations and species-specific leaf and shoot structural constraints.

Elevated zeaxanthin bound to oligomeric LHCII enhances the resistance of Arabidopsis to photooxidative stress by a lipid-protective, antioxidant mechanism

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 beta-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.

Tocopherol functions in photosynthetic organisms

During the past decade, the genes required for tocopherol (vitamin E) synthesis in plants and cyanobacteria have been identified. A series of mutants in which specific pathway steps are disrupted have been generated, providing new insights into tocopherol functions in photosynthetic organisms. Tocopherols are essential for controlling non-enzymatic lipid peroxidation during seed dormancy and seedling germination. Their absence results in elevated levels of malondialdehyde and phytoprostanes, and in inappropriate activation of plant defense responses. Surprisingly, tocopherol deficiency in mature leaves has limited consequences under most abiotic stresses, including high intensity light stress. The cell wall development of phloem transfer cells under cold conditions is, however, severely impaired in mature leaves of tocopherol-deficient mutants, indicating that tocopherols are required for proper adaptation of phloem loading at low temperatures.

Characterization of a beta-carotene hydroxylase of Adonis aestivalis and its expression in Arabidopsis thaliana

Carotenoids are plant secondary metabolites that comprise two main groups: carotenes and xanthophylls. The latter group includes zeaxanthin which is synthesized by beta-carotene hydroxylase catalyzing the hydroxylation of the beta-rings of beta-carotene molecules. To develop tools to alter carotenoid biosynthesis in plants, we isolated a cDNA clone encoding a candidate beta-carotene hydroxylase, CrtH1, from the flower petals of Adonis aestivalis. CrtH1 protein has homology to beta-carotene hydroxylases from other organisms, and possesses the four histidine motifs conserved in this family of enzymes. Sequence analysis predicted the presence of a putative plastid transit peptide at the amino terminus and four transmembrane helical regions. Southern-blot analysis showed CrtH1 to be encoded by a multicopy gene family with at least three members in A. aestivalis. Analysis of CrtH1 transcript abundance by Northern blotting indicates it is highly expressed in flower petals, roots and stems, with relatively low expression in leaves and developing seeds. CrtH1 was able to catalyze the formation of zeaxanthin and its intermediate precursor beta-cryptoxanthin from beta-carotene in functional assays conducted in E. coli. Expression of CrtH1 in Arabidopsis thaliana wild type and a mutant deficient for endogenous beta-carotene hydroxylases enhanced the biosynthesis of violaxanthin in the seeds.

Specific roles of alpha- and gamma-tocopherol in abiotic stress responses of transgenic tobacco

Tocopherols are lipophilic antioxidants that are synthesized exclusively in photosynthetic organisms. In most higher plants, alpha- and gamma-tocopherol are predominant with their ratio being under spatial and temporal control. While alpha-tocopherol accumulates predominantly in photosynthetic tissue, seeds are rich in gamma-tocopherol. To date, little is known about the specific roles of alpha- and gamma-tocopherol in different plant tissues. To study the impact of tocopherol composition and content on stress tolerance, transgenic tobacco (Nicotiana tabacum) plants constitutively silenced for homogentisate phytyltransferase (HPT) and gamma-tocopherol methyltransferase (gamma-TMT) activity were created. Silencing of HPT lead to an up to 98% reduction of total tocopherol accumulation compared to wild type. Knockdown of gamma-TMT resulted in an up to 95% reduction of alpha-tocopherol in leaves of the transgenics, which was almost quantitatively compensated for by an increase in gamma-tocopherol. The response of HPT and gamma-TMT transgenics to salt and sorbitol stress and methyl viologen treatments in comparison to wild type was studied. Each stress condition imposes oxidative stress along with additional challenges like perturbing ion homeostasis, desiccation, or disturbing photochemistry, respectively. Decreased total tocopherol content increased the sensitivity of HPT:RNAi transgenics toward all tested stress conditions, whereas gamma-TMT-silenced plants showed an improved performance when challenged with sorbitol or methyl viologen. However, salt tolerance of gamma-TMT transgenics was strongly decreased. Membrane damage in gamma-TMT transgenic plants was reduced after sorbitol and methyl viologen-mediated stress, as evident by less lipid peroxidation and/or electrolyte leakage. Therefore, our results suggest specific roles for alpha- and gamma-tocopherol in vivo.

The Arabidopsis aba4-1 mutant reveals a specific function for neoxanthin in protection against photooxidative stress

The aba4-1 mutant completely lacks neoxanthin but retains all other xanthophyll species. The missing neoxanthin in light-harvesting complex (Lhc) proteins is compensated for by higher levels of violaxanthin, albeit with lower capacity for photoprotection compared with proteins with wild-type levels of neoxanthin. Detached leaves of aba4-1 were more sensitive to oxidative stress than the wild type when exposed to high light and incubated in a solution of photosensitizer agents. Both treatments caused more rapid pigment bleaching and lipid oxidation in aba4-1 than wild-type plants, suggesting that neoxanthin acts as an antioxidant within the photosystem II (PSII) supercomplex in thylakoids. While neoxanthin-depleted Lhc proteins and leaves had similar sensitivity as the wild type to hydrogen peroxide and singlet oxygen, they were more sensitive to superoxide anions. aba4-1 intact plants were not more sensitive than the wild type to high-light stress, indicating the existence of compensatory mechanisms of photoprotection involving the accumulation of zeaxanthin. However, the aba4-1 npq1 double mutant, lacking zeaxanthin and neoxanthin, underwent stronger PSII photoinhibition and more extensive oxidation of pigments than the npq1 mutant, which still contains neoxanthin. We conclude that neoxanthin preserves PSII from photoinactivation and protects membrane lipids from photooxidation by reactive oxygen species. Neoxanthin appears particularly active against superoxide anions produced by the Mehler's reaction, whose rate is known to be enhanced in abiotic stress conditions.

The Arabidopsis thaliana sulfiredoxin is a plastidic cysteine-sulfinic acid reductase involved in the photooxidative stress response

The 2-cysteine peroxiredoxins (2-Cys-Prxs) are antioxidants that reduce peroxides through a thiol-based mechanism. During catalysis, these ubiquitous enzymes are occasionally inactivated by the substrate-dependent oxidation of the catalytic cysteine to the sulfinic acid (-SO2H) form, and are reactivated by reduction by sulfiredoxin (Srx), an enzyme recently identified in yeast and in mammal cells. In plants, 2-Cys-Prxs constitute the most abundant Prxs and are located in chloroplasts. Here we have characterized the unique Srx gene in Arabidopsis thaliana (AtSrx) from a functional point of view, and analyzed the phenotype of two AtSrx knockout (AtSrx-) mutant lines. AtSrx is a chloroplastic enzyme displaying sulfinic acid reductase activity, as shown by the ability of the recombinant AtSrx to reduce the overoxidized 2-Cys-Prx form in vitro, and by the accumulation of the overoxidized Prx in mutant lines lacking Srx in vivo. Furthermore, AtSrx mutants exhibit an increased tolerance to photooxidative stress generated by high light combined with low temperature. These data establish that, as in yeast and in mammals, plant 2-Cys-Prxs are subject to substrate-mediated inactivation reversed by Srx, and suggest that the 2-Cys-Prx redox status and sulfiredoxin are parts of a signaling mechanism participating in plant responses to oxidative stress.

Lutein is needed for efficient chlorophyll triplet quenching in the major LHCII antenna complex of higher plants and effective photoprotection in vivo under strong light

BACKGROUND

Lutein is the most abundant xanthophyll in the photosynthetic apparatus of higher plants. It binds to site L1 of all Lhc proteins, whose occupancy is indispensable for protein folding and quenching chlorophyll triplets. Thus, the lack of a visible phenotype in mutants lacking lutein has been surprising.

RESULTS

We have re-assessed the lut2.1 phenotypes through biochemical and spectroscopic methods. Lhc proteins from the lut2.1 mutant compensate the lack of lutein by binding violaxanthin in sites L1 and L2. This substitution reduces the capacity for regulatory mechanisms such as NPQ, reduces antenna size, induces the compensatory synthesis of Antheraxanthin + Zeaxanthin, and prevents the trimerization of LHCII complexes. In vitro reconstitution shows that the lack of lutein per se is sufficient to prevent trimerization. lut2.1 showed a reduced capacity for state I-state II transitions, a selective degradation of Lhcb1 and 2, and a higher level of photodamage in high light and/or low temperature, suggesting that violaxanthin cannot fully restore chlorophyll triplet quenching. In vitro photobleaching experiments and time-resolved spectroscopy of carotenoid triplet formation confirmed this hypothesis. The npq1lut2.1 double mutant, lacking both zeaxanthin and lutein, is highly susceptible to light stress.

CONCLUSION

Lutein has the specific property of quenching harmful 3Chl* by binding at site L1 of the major LHCII complex and of other Lhc proteins of plants, thus preventing ROS formation. Substitution of lutein by violaxanthin decreases the efficiency of 3Chl* quenching and causes higher ROS yield. The phenotype of lut2.1 mutant in low light is weak only because rescuing mechanisms of photoprotection, namely zeaxanthin synthesis, compensate for the ROS production. We conclude that zeaxanthin is effective in photoprotection of plants lacking lutein due to the multiple effects of zeaxanthin in photoprotection, including ROS scavenging and direct quenching of Chl fluorescence by binding to the L2 allosteric site of Lhc proteins.

Chloroplast NAD kinase is essential for energy transduction through the xanthophyll cycle in photosynthesis

Photosynthetic parameters of the nadk2 mutant of Arabidopsis thaliana, which is defective in chloroplast NAD kinase, were investigated. In this plant, the effective efficiency of photosynthetic electron transport (PhiII) and the quantum yield of open reaction centers of photosystem II (Fv'/Fm') were decreased. Furthermore, an increase in non-photochemical quenching attributed to energy dissipation from the xanthophyll cycle was observed. The mutant showed an aberrant de-epoxidation state of xanthophyll cycle carotenoids and had a high level of zeaxanthin even under low light conditions. These results indicate that chloroplast NAD kinase, catalyzing phosphorylation of NAD, is essential for the proper photosynthetic machinery of PSII and the xanthophyll cycle.

Integrated analysis of metabolite and transcript levels reveals the metabolic shifts that underlie tomato fruit development and highlight regulatory aspects of metabolic network behavior

Tomato (Solanum lycopersicum) is a well-studied model of fleshy fruit development and ripening. Tomato fruit development is well understood from a hormonal-regulatory perspective, and developmental changes in pigment and cell wall metabolism are also well characterized. However, more general aspects of metabolic change during fruit development have not been studied despite the importance of metabolism in the context of final composition of the ripe fruit. In this study, we quantified the abundance of a broad range of metabolites by gas chromatography-mass spectrometry, analyzed a number of the principal metabolic fluxes, and in parallel analyzed transcriptomic changes during tomato fruit development. Metabolic profiling revealed pronounced shifts in the abundance of metabolites of both primary and secondary metabolism during development. The metabolite changes were reflected in the flux analysis that revealed a general decrease in metabolic activity during ripening. However, there were several distinct patterns of metabolite profile, and statistical analysis demonstrated that metabolites in the same (or closely related) pathways changed in abundance in a coordinated manner, indicating a tight regulation of metabolic activity. The metabolite data alone allowed investigations of likely routes through the metabolic network, and, as an example, we analyze the operational feasibility of different pathways of ascorbate synthesis. When combined with the transcriptomic data, several aspects of the regulation of metabolism during fruit ripening were revealed. First, it was apparent that transcript abundance was less strictly coordinated by functional group than metabolite abundance, suggesting that posttranslational mechanisms dominate metabolic regulation. Nevertheless, there were some correlations between specific transcripts and metabolites, and several novel associations were identified that could provide potential targets for manipulation of fruit compositional traits. Finally, there was a strong relationship between ripening-associated transcripts and specific metabolite groups, such as TCA-cycle organic acids and sugar phosphates, underlining the importance of the respective metabolic pathways during fruit development.

Dehydroascorbate reductase affects leaf growth, development, and function

Ascorbic acid (Asc) is a major antioxidant in plants that detoxifies reactive oxygen species (ROS) and maintains photosynthetic function. Expression of dehydroascorbate reductase (DHAR), responsible for regenerating Asc from an oxidized state, regulates the cellular Asc redox state, which in turn affects cell responsiveness and tolerance to environmental ROS. Because of its role in Asc recycling, we examined whether DHAR is important for plant growth. Suppression of DHAR expression resulted in a preferential loss of chlorophyll a, a lower steady state of Rubisco as measured by the amount of the large subunit of Rubisco (RbcL), and a lower rate of CO(2) assimilation. As a consequence, a slower rate of leaf expansion and reduced foliar dry weight were observed. In addition, an accelerated rate of loss of chlorophyll, RbcL, light-harvesting complex II, and photosynthetic functioning was observed in mature leaves, resulting in premature leaf aging. Reduced growth rate as measured by plant height and leaf number was consistent with the DHAR-mediated reduction of photosynthetic function. Increasing DHAR expression maintained higher levels of chlorophyll, RbcL, light-harvesting complex II, and photosynthetic functioning, resulting in delayed leaf aging. The effect of DHAR expression on leaf aging inversely correlated with the level of lipid peroxidation, indicating that DHAR functions to protect against ROS-mediated damage. These observations support the conclusion that through its Asc recycling function, DHAR affects the level of foliar ROS and photosynthetic activity during leaf development and as a consequence, influences the rate of plant growth and leaf aging.

Regulation of peroxiredoxin expression versus expression of Halliwell-Asada-Cycle enzymes during early seedling development of Arabidopsis thaliana

During early seedling development of oil seed plants, the transition from lipid based heterotrophic to photoautotrophic carbohydrate metabolism is accompanied with a biphasic control of the chloroplast antioxidant system. In continuous light, organellar peroxiredoxins (Prx) and thylakoid-bound ascorbate peroxidase (tAPx) are activated early in seedling development, while stromal ascorbate peroxidase (sAPx), Cu/Zn-superoxide dismutase-2 (Csd2) and monodehydroascorbate reductase (MDHAR) and the cytosolic peroxiredoxins PrxIIB, PrxIIC and PrxIID are fully activated between 2.5 and 3 days after radicle emergence (DARE). Discontinuous light synchronized the expression of chloroplast antioxidant enzymes, but defined diurnally specific typeII-Prx-patterns in the cytosol and initiated chloroplast senescence around 2.5 DARE. Carbohydrate feeding uncoupled sAPx expression from the light pattern. In contrast, sucrose-feeding did not significantly impact on Prx transcript amounts. It is concluded that upon post-germination growth Prxs are activated endogenously to provide early antioxidant protection, which is supported by the Halliwell-Asada-Cycle, whose expressional activation depends on metabolic signals provided only later in development or in day-night-cycles.