Glutathione reductase 2 maintains the function of photosystem II in Arabidopsis under excess light

Organelle-specific localization of glutathione in plants grown under different light intensities and spectra

Plant ascorbate and glutathione metabolism counteracts oxidative stress mediated, for example, by excess light. In this review, we discuss the properties of immunocytochemistry and transmission electron microscopy, redox-sensitive dyes or probes and bright-field microscopy, confocal microscopy or fluorescence microscopy for the visualization and quantification of glutathione at the cellular or subcellular level in plants and the quantification of glutathione from isolated organelles. In previous studies, we showed that subcellular ascorbate and glutathione levels in Arabidopsis are affected by high light stress. The use of light-emitting diodes (LEDs) is gaining increasing importance in growing indoor crops and ornamental plants. A combination of different LED types allows custom-made combinations of wavelengths and prevents damage related to high photon flux rates. In this review we provide an overview on how different light spectra and light intensities affect glutathione metabolism at the cellular and subcellular levels in plants. Findings obtained in our most recent study demonstrate that both light intensity and spectrum significantly affected glutathione metabolism in wheat at the transcriptional level and caused genotype-specific reactions in the investigated Arabidopsis lines.

Light Intensity- and Spectrum-Dependent Redox Regulation of Plant Metabolism

Both light intensity and spectrum (280–800 nm) affect photosynthesis and, consequently, the formation of reactive oxygen species (ROS) during photosynthetic electron transport. ROS, together with antioxidants, determine the redox environment in tissues and cells, which in turn has a major role in the adjustment of metabolism to changes in environmental conditions. This process is very important since there are great spatial (latitude, altitude) and temporal (daily, seasonal) changes in light conditions which are accompanied by fluctuations in temperature, water supply, and biotic stresses. The blue and red spectral regimens are decisive in the regulation of metabolism because of the absorption maximums of chlorophylls and the sensitivity of photoreceptors. Based on recent publications, photoreceptor-controlled transcription factors such as ELONGATED HYPOCOTYL5 (HY5) and changes in the cellular redox environment may have a major role in the coordinated fine-tuning of metabolic processes during changes in light conditions. This review gives an overview of the current knowledge of the light-associated redox control of basic metabolic pathways (carbon, nitrogen, amino acid, sulphur, lipid, and nucleic acid metabolism), secondary metabolism (terpenoids, flavonoids, and alkaloids), and related molecular mechanisms. Light condition-related reprogramming of metabolism is the basis for proper growth and development of plants; therefore, its better understanding can contribute to more efficient crop production in the future.

Dissecting the genetic control of natural variation in sorghum photosynthetic response to drought stress

Drought stress causes crop yield losses worldwide. Sorghum is a C4 species tolerant to moderate drought stress, and its extensive natural variation for photosynthetic traits under water-limiting conditions can be exploited for developing cultivars with enhanced stress tolerance. The objective of this study was to discover genes/genomic regions that control the sorghum photosynthetic capacity under pre-anthesis water-limiting conditions. We performed a genome-wide association study for seven photosynthetic gas exchange and chlorophyll fluorescence traits during three periods of contrasting soil volumetric water content (VWC): control (30% VWC), drought (15% VWC), and recovery (30% VWC). Water stress was imposed with an automated irrigation system that generated a controlled dry-down period for all plants, to perform an unbiased genotypic comparison. A total of 60 genomic regions were associated with natural variation in one or more photosynthetic traits in a particular treatment or with derived variables. We identified 33 promising candidate genes with predicted functions related to stress signaling, oxidative stress protection, hormonal response to stress, and dehydration protection. Our results provide new knowledge about the natural variation and genetic control of sorghum photosynthetic response to drought with the ultimate goal of improving its adaptation and productivity under water stress scenarios.

High-level production and extraction of C-phycocyanin from cyanobacteria Synechococcus sp. PCC7002 for antioxidation, antibacterial and lead adsorption

Global warming and climate change because carbon dioxide (CO₂) release to atmosphere is the forecasting challenges to human being. We are facing how to overcome the dilemma on the balance between economic and environment, thus taking more efforts on green processes to meet agreement of sustainable society are urgent and crucial. The absorption of CO₂ by microalgae reduces the impact of CO₂ on the environment. In this study, the CO₂ removal efficiency was the highest in the culture of Cyanobacterium Synechococcus sp. PCC7002 (also called blue-green algae), at 2% CO₂ to reach a value of 0.86 g-CO₂/g-DCW. The main product of PCC7002 is C-phycocyanin (C-PC) which regarding to phycobilisome complex in all cyanobacterial species. A 160% increasing C-PC was achieved in the cultivation under 100 μmol/m²/s light intensity, 12:12 light-period with 2% CO₂ at 30 °C. The mix-culture of nitric and ammonia ions had positive effect on the cell growth and C-PC accumulation, thus realized the highest yield of 0.439 g-CPC/g-DCW. Additionally, the partial purified C-PC displayed 89% antioxidant activity of 2,2-diphenyl-1-picryhydrazyl (DPPH) and 11% of superoxide free radical scavenging activity, respectively. The production of C-PC from PCC7002 reduced the CO₂ emission and exhibited antibacterial activity against Escherichia coli and lead ion adsorption at room temperature, which has the great potential for eco-friendly application.

Microalgal competence in urban wastewater management: phycoremediation and lipid production

The present study was conducted to assess the strain aptness, phycoremediation potential and lipid yield in microalgae Chloroccocum humicola and Oscillatoria sp. Results revealed that microalgae treated with different concentration of wastewater (25%, 50%, 75% and 100%) recovered nutrients (Nitrogen: 50.55-85.90%, Phosphorus: 69.98-93.72%) and removed heavy metals (24.17-88.10%) from wastewater. Microalgae C. humicola showed significant reduction in physico-chemical parameters of wastewater at 25% and 50% respectively with considerable increase in lipid production (1.61 folds) at 50% wastewater concentration. In order to counterbalance the wastewater induced toxicity, both microalgae exhibited stimulated antioxidants viz., proline (1.26-4.04 folds), ascorbic acid (1.01-9.21 folds), cysteine (1.44-4.92 folds), catalase (0.99-3.63 folds), superoxide dismutase (1.15-1.43 folds) and glutathione reductase (1.43-6.67 folds) at different wastewater concentrations. Further, Fourier transforms infrared spectroscopy spectral elucidation of Chloroccocum humicola at 50% reflected high lipid peak in the regions 3000-2800 cm^-1 as compared to Oscillatoria sp. Thus, growth characteristics, biochemical responses and lipid yield presented the suitability of the Chloroccocum humicola to be used in phycoremedation, resource recovery as well as lipid production, which may be further utilized as potent feedstock for third generation energy demand.

Novel Cinnamic Acid Derivatives Containing the 1,3,4-Oxadiazole Moiety: Design, Synthesis, Antibacterial Activities, and Mechanisms

There is a lack of effective antibacterial agents against rice bacterial leaf streak and leaf blight. Cinnamic acid derivatives containing the 1,3,4-oxadiazole moiety were synthesized, and their antibacterial activities against Xanthomonas oryzae pv. oryzicola (Xoc) and X. oryzae pv. oryzae (Xoo) were evaluated. Based on the three-dimensional quantitative structure-activity relationship (3D-QSAR) model, compound 31 with better antibacterial activity against Xoc was designed and synthesized, and the 50% effective concentration (EC₅₀) value was 0.2 mg/L. The curative and protective activities of compound 31 against rice bacterial leaf streak at 100 mg/L were 39.5 and 35.4%, respectively, which were higher than those of thiodiazole copper (28.4 and 20.7%, respectively). The antibacterial activity of compound 31 against rice bacterial leaf streak is closely associated with the activity of related defensive enzymes and the increase in glutathione metabolism.

Glutathione Metabolism in Plants under Stress: Beyond Reactive Oxygen Species Detoxification

Glutathione is an essential metabolite for plant life best known for its role in the control of reactive oxygen species (ROS). Glutathione is also involved in the detoxification of methylglyoxal (MG) which, much like ROS, is produced at low levels by aerobic metabolism under normal conditions. While several physiological processes depend on ROS and MG, a variety of stresses can dramatically increase their concentration leading to potentially deleterious effects. In this review, we examine the structure and the stress regulation of the pathways involved in glutathione synthesis and degradation. We provide a synthesis of the current knowledge on the glutathione-dependent glyoxalase pathway responsible for MG detoxification. We present recent developments on the organization of the glyoxalase pathway in which alternative splicing generate a number of isoforms targeted to various subcellular compartments. Stress regulation of enzymes involved in MG detoxification occurs at multiple levels. A growing number of studies show that oxidative stress promotes the covalent modification of proteins by glutathione. This post-translational modification is called S-glutathionylation. It affects the function of several target proteins and is relevant to stress adaptation. We address this regulatory function in an analysis of the enzymes and pathways targeted by S-glutathionylation.

Age-Dependent Abiotic Stress Resilience in Plants

Developmental age is a strong determinant of stress responses in plants. Differential susceptibility to various environmental stresses is widely observed at both the organ and whole-plant level. While it is clear that age determines stress susceptibility, the causes, regulatory mechanisms, and functions are only now beginning to emerge. Compared with concepts on age-related biotic stress resilience, advancements in the abiotic stress field are relatively limited. In this review, we focus on current knowledge of ontogenic resistance to abiotic stresses, highlighting examples at the organ (leaf) and plant level, preceded by an overview of the relevant concepts in plant aging. We also discuss age-related abiotic stress resilience mechanisms, speculate on their functional relevance, and outline outstanding questions.

Is protein carbonylation a biomarker of seed priming and ageing?

For a long time, it has been known that seed priming allows the improvement of plant production and tolerance to abiotic stresses. However, a negative effect on the longevity of the seeds thus primed was observed; these mechanisms are still poorly understood. In addition, it has been shown by several authors that seed ageing is associated with the oxidation and particularly with carbonylation of protein. Our work consisted in studying the AOPP and carbonyl protein at the different parts of the embryo from freshly primed seeds and from those that have been primed for 4 years (after storage). We subjected Vigna unguiculata (L.) Walp. seeds to a single or double hydropriming. Our study showed that hydropriming, and more particularly a double cycle of hydration-dehydration, makes it possible to attenuate the oxidation of the protein while it favours a certain threshold of carbonylation in the freshly dehydrated seeds in order to better trigger the germination process. On the other hand, after a storage period of 4 years, these dehydrated seeds are characterised by a strong accumulation of the products of oxidation and especially carbonylated protein, compared with the untreated seeds, which could explain the decrease of the longevity of these seeds.

Recent Developments in Enzymatic Antioxidant Defence Mechanism in Plants with Special Reference to Abiotic Stress

The stationary life of plants has led to the evolution of a complex gridded antioxidant defence system constituting numerous enzymatic components, playing a crucial role in overcoming various stress conditions. Mainly, these plant enzymes are superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferases (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR), which work as part of the antioxidant defence system. These enzymes together form a complex set of mechanisms to minimise, buffer, and scavenge the reactive oxygen species (ROS) efficiently. The present review is aimed at articulating the current understanding of each of these enzymatic components, with special attention on the role of each enzyme in response to the various environmental, especially abiotic stresses, their molecular characterisation, and reaction mechanisms. The role of the enzymatic defence system for plant health and development, their significance, and cross-talk mechanisms are discussed in detail. Additionally, the application of antioxidant enzymes in developing stress-tolerant transgenic plants are also discussed.

Comparison of Light Condition-Dependent Differences in the Accumulation and Subcellular Localization of Glutathione in Arabidopsis and Wheat

This study aimed to clarify whether the light condition-dependent changes in the redox state and subcellular distribution of glutathione were similar in the dicotyledonous model plant Arabidopsis (wild-type, ascorbate- and glutathione-deficient mutants) and the monocotyledonous crop species wheat (Chinese Spring variety). With increasing light intensity, the amount of its reduced (GSH) and oxidized (GSSG) form and the GSSG/GSH ratio increased in the leaf extracts of both species including all genotypes, while far-red light increased these parameters only in wheat except for GSH in the GSH-deficient Arabidopsis mutant. Based on the expression changes of the glutathione metabolism-related genes, light intensity influences the size and redox state of the glutathione pool at the transcriptional level in wheat but not in Arabidopsis. In line with the results in leaf extracts, a similar inducing effect of both light intensity and far-red light was found on the total glutathione content at the subcellular level in wheat. In contrast to the leaf extracts, the inducing influence of light intensity on glutathione level was only found in the cell compartments of the GSH-deficient Arabidopsis mutant, and far-red light increased it in both mutants. The observed general and genotype-specific, light-dependent changes in the accumulation and subcellular distribution of glutathione participate in adjusting the redox-dependent metabolism to the actual environmental conditions.

Improving light distribution and light/dark cycle of 900 L tangential spiral-flow column photobioreactors to promote CO2 fixation with Arthrospira sp. cells

The light distribution and light/dark cycle were improved in 900 L tangential spiral-flow column photobioreactors (TSCP) to promote CO₂ fixation with Arthrospira sp. cells. Solar irradiation model was employed in CFD simulation to investigate light distribution and light/dark cycle in flow field composed of culture medium, CO2 bubbles and Arthrospira sp. cells under actual sunlight irradiation considering geolocation and time. An accurate way to divide light/dark zone based on saturate light intensity and light intensity field was adopted for the first time. When Arthrospira sp. cell concentration increased from 0.1 to 0.9 g/L, light/dark cycle frequency of cells firstly increased from 0.650 Hz to 0.868 Hz and then decreased to 0.117 Hz. Intracellular chlorophyll a content and carotenoids content of Arthrospira sp. cells in TSCP were 6% and 41% higher than those in conventional bubble column photobioreactor. This promoted cellular photosynthesis and stress resistance, which contributed to increase CO₂ fixation rate of Arthrospira sp. cells by 59%. When CO₂ aeration rate, CO₂ volume concentration, and circulating pump power were 0.210 L/min, 15%, and 30 W, chlorophyll a content, helix pitch, and CO₂ fixation rate of Arthrospira sp. cells all reached peak values of 8.769 mg/L, 78.26 μm and 0.358 g/L/d, respectively.

Sulfonamides-induced oxidative stress in freshwater microalga Chlorella vulgaris: Evaluation of growth, photosynthesis, antioxidants, ultrastructure, and nucleic acids

Sulfadiazine (SD), sulfamerazine (SM1), and sulfamethazine (SM2) are widely used and disorderly discharged into surface water, causing contamination of lakes and rivers. However, microalgae are regard as a potential resource to alleviate and degrade antibiotic pollution. The physiological changes of Chlorella vulgaris in the presence of three sulfonamides (SAs) with varying numbers of –CH₃ groups and its SA-removal efficiency were investigated following a 7-day exposure experiment. Our results showed that the growth inhibitory effect of SD (7.9–22.6%), SM1 (7.2–45.9%), and SM2 (10.3–44%) resulted in increased proteins and decreased soluble sugars. Oxidative stress caused an increase in superoxide dismutase and glutathione reductase levels but decreased catalase level. The antioxidant responses were insufficient to cope-up with reactive oxygen species (hydrogen peroxide and superoxide anion) levels and prevent oxidative damage (malondialdehyde level). The ultrastructure and DNA of SA-treated algal cells were affected, as evident from the considerable changes in the cell wall, chloroplast, and mitochondrion, and DNA migration. C. vulgaris-mediated was able to remove up to 29% of SD, 16% of SM1, and 15% of SM2. Our results suggest that certain concentrations of specific antibiotics may induce algal growth, and algal-mediated biodegradation process can accelerate the removal of antibiotic contamination.

Characterization, bioavailability and protective effects of phenolic-rich extracts from almond hulls against pro-oxidant induced toxicity in Caco-2 cells

Almond hulls, the main by-product of almond production, are considered a valuable source of bioactive phenolic compounds. This study aimed to characterize the phenolic composition, bioavailability of the phenolic-rich extracts from almond hulls (PEAH), and their protective effect on oxidative stressed Caco-2 cells induced by tert-butylhydroperoxide (t-BOOH). The ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS) analysis detected 11 phenolic compounds in the PEAH with high total phenolic content and antioxidant activity. Oxidative Caco-2 cell damage was reduced by PEAH, especially at 5 μg/mL, through scavenging reactive oxygen species (ROS), modulating the cellular endogenous antioxidant system and cell redox at a predictable status. Also, in vitro digestion influenced the phenolic compounds' composition and antioxidant power of PEAH. These results suggested that almond hulls, rich in phenolic compounds, can meliorate the oxidative stressed Caco-2 cells and restore its impaired redox balance, and ultimately improve health benefits.

Arabidopsis glutathione reductase 2 is indispensable in plastids, while mitochondrial glutathione is safeguarded by additional reduction and transport systems

A highly negative glutathione redox potential (E_GSH ) is maintained in the cytosol, plastids and mitochondria of plant cells to support fundamental processes, including antioxidant defence, redox regulation and iron-sulfur cluster biogenesis. Out of two glutathione reductase (GR) proteins in Arabidopsis, GR2 is predicted to be dual-targeted to plastids and mitochondria, but its differential roles in these organelles remain unclear. We dissected the role of GR2 in organelle glutathione redox homeostasis and plant development using a combination of genetic complementation and stacked mutants, biochemical activity studies, immunogold labelling and in vivo biosensing. Our data demonstrate that GR2 is dual-targeted to plastids and mitochondria, but embryo lethality of gr2 null mutants is caused specifically in plastids. Whereas lack of mitochondrial GR2 leads to a partially oxidised glutathione pool in the matrix, the ATP-binding cassette (ABC) transporter ATM3 and the mitochondrial thioredoxin system provide functional backup and maintain plant viability. We identify GR2 as essential in the plastid stroma, where it counters GSSG accumulation and developmental arrest. By contrast a functional triad of GR2, ATM3 and the thioredoxin system in the mitochondria provides resilience to excessive glutathione oxidation.

A comparative study of wavelength-dependent photoinactivation in photosystem II of drought-tolerant photosynthetic organisms in Antarctica and the potential risks of photoinhibition in the habitat

BACKGROUND AND AIMS

All photosynthetic organisms are faced with photoinhibition, which would lead to death in severe environments. Because light quality and light intensity fluctuate dynamically in natural microenvironments, quantitative and qualitative analysis of photoinhibition is important to clarify how this environmental pressure has impacted ecological behaviour in different organisms.

METHODS

We evaluated the wavelength dependency of photoinactivation to photosystem II (PSII) of Prasiola crispa (green alga), Umbilicaria decussata (lichen) and Ceratodon purpureus (bryophyte) harvested from East Antarctica. For evaluation, we calculated reaction coefficients, Epis, of PSII photoinactivation against energy dose using a large spectrograph. Daily fluctuation of the rate coefficient of photoinactivation, kpi, was estimated from Epis and ambient light spectra measured during the summer season.

KEY RESULTS

Wavelength dependency of PSII photoinactivation was different for the three species, although they form colonies in close proximity to each other in Antarctica. The lichen exhibited substantial resistance to photoinactivation at all wavelengths, while the bryophyte showed sensitivity only to UV-B light (<325 nm). On the other hand, the green alga, P. crispa, showed ten times higher Epi to UV-B light than the bryophyte. It was much more sensitive to UV-A (325-400 nm). The risk of photoinhibition fluctuated considerably throughout the day. On the other hand, Epis were reduced dramatically for dehydrated compared with hydrated P. crispa.

CONCLUSIONS

The deduced rate coefficients of photoinactivation under ambient sunlight suggested that P. crispa needs to pay a greater cost to recover from photodamage than the lichen or the bryophyte in order to keep sufficient photosynthetic activity under the Antarctic habitat. A newly identified drought-induced protection mechanism appears to operate in P. crispa, and it plays a critical role in preventing the oxygen-evolving complex from photoinactivation when the repair cycle is inhibited by dehydration.

Sodium chloride incites reactive oxygen species in green algae Chlorococcum humicola and Chlorella vulgaris: Implication on lipid synthesis, mineral nutrients and antioxidant system

In the present study, microalgae Chlorococcum humicola and Chlorella vulgaris were grown in different concentrations of NaCl (25-1000 mM) to elucidate its impact on morphology, lipid synthesis, minerals status and antioxidative responses. Scanning Electron microscopy showed distorted cell morphology and increased cell size by 33.52% (C. humicola) and 27.79% (C. vulgaris) at 100 mM NaCl. Energy Dispersive Spectroscopy data revealed reduction in mineral contents (C, S, Fe, Mg, Si, Mn and Zn) by 14-54% in both algae. Further, C. humicola was found to have high lipid content than C. vulgaris under NaCl regime. The activities of superoxide dismutase, catalase and glutathione reductase were increased by 2.5-5 folds in both algae as compared to control. The increased level of ascorbate, cysteine and proline in both algae indicated tolerance against salinity. Thus, C. humicola and C. vulgaris may exhibit dual benefits viz., high lipid production and reclamation of sodic soil.

Promoting helix pitch and trichome length to improve biomass harvesting efficiency and carbon dioxide fixation rate by Spirulina sp. in 660 m2 raceway ponds under purified carbon dioxide from a coal chemical flue gas

The helix pitch and trichome length of Spirulina sp. were promoted to improve the biomass harvesting efficiency and CO₂ fixation rate in 660 m² raceway ponds aerated with food-grade CO₂ purified from a coal chemical flue gas. The CO₂ fixation rate was improved with increased trichome length of the Spirulina sp. in a raceway pond with double paddlewheels, baffles, and CO₂ aerators (DBA raceway pond). The trichome length has increased by 33.3 μm, and CO₂ fixation rate has increased by 42.3% and peaked to 51.3 g/m²/d in a DBA raceway pond. Biomass harvesting efficiency was increased with increased helix pitch. When the day-average greenhouse temperature was 33 °C and day-average sunlight intensity was 72,100 lu×, the helix pitch of Spirulina sp. was increased to 56.2 μm. Hence the biomass harvesting efficiency was maximized to 75.6% and biomass actual yield was increased to 35.9 kg in a DBA raceway pond.

SELENOPROTEIN O is a chloroplast protein involved in ROS scavenging and its absence increases dehydration tolerance in Arabidopsis thaliana

The evolutionary conserved family of Selenoproteins performs redox-regulatory functions in bacteria, archaea and eukaryotes. Among them, members of the SELENOPROTEIN O (SELO) subfamily are located in mammalian and yeast mitochondria, but their functions are thus far enigmatic. Screening of T-DNA knockout mutants for resistance to the proline analogue thioproline (T4C), identified mutant alleles of the plant SELO homologue in Arabidopsis thaliana. Absence of SELO resulted in a stress-induced transcriptional activation instead of silencing of mitochondrial proline dehydrogenase, and also high elevation of Δ(1)-pyrroline-5-carboxylate dehydrogenase involved in degradation of proline, thereby alleviating T4C inhibition and lessening drought-induced proline accumulation. Unlike its animal homologues, SELO was localized to chloroplasts of plants ectopically expressing SELO-GFP. The protein was co-fractionated with thylakoid membrane complexes, and co-immunoprecipitated with FNR, PGRL1 and STN7, all involved in regulating PSI and downstream electron flow. The selo mutants displayed extended survival under dehydration, accompanied by longer photosynthetic activity, compared with wild-type plants. Enhanced expression of genes encoding ROS scavenging enzymes in the unstressed selo mutant correlated with higher oxidant scavenging capacity and reduced methyl viologen damage. The study elucidates SELO as a PSI-related component involved in regulating ROS levels and stress responses.

The Phytol Phosphorylation Pathway Is Essential for the Biosynthesis of Phylloquinone, which Is Required for Photosystem I Stability in Arabidopsis

Phytyl-diphosphate, which provides phytyl moieties as a common substrate in both tocopherol and phylloquinone biosynthesis, derives from de novo isoprenoid biosynthesis or a salvage pathway via phytol phosphorylation. However, very little is known about the role and origin of the phytyl moiety for phylloquinone biosynthesis. Since VTE6, a phytyl-phosphate kinase, is a key enzyme for phytol phosphorylation, we characterized Arabidopsis vte6 mutants to gain insight into the roles of phytyl moieties in phylloquinone biosynthesis and of phylloquinone in photosystem I (PSI) biogenesis. The VTE6 knockout mutants vte6-1 and vte6-2 lacked detectable phylloquinone, whereas the phylloquinone content in the VTE6 knockdown mutant vte6-3 was 90% lower than that in wild-type. In vte6 mutants, PSI function was impaired and accumulation of the PSI complex was defective. The PSI core subunits PsaA/B were efficiently synthesized and assembled into the PSI complex in vte6-3. However, the degradation rate of PSI subunits in the assembled PSI complex was more rapid in vte6-3 than in wild-type. In vte6-3, PSI was more susceptible to high-light damage than in wild-type. Our results provide the first genetic evidence that the phytol phosphorylation pathway is essential for phylloquinone biosynthesis, and that phylloquinone is required for PSI complex stability.