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

Carotenoids, versatile components of oxygenic photosynthesis

Carotenoids (CARs) are a group of pigments that perform several important physiological functions in all kingdoms of living organisms. CARs serve as protective agents, which are essential structural components of photosynthetic complexes and membranes, and they play an important role in the light harvesting mechanism of photosynthesizing plants and cyanobacteria. The protection against reactive oxygen species, realized by quenching of singlet oxygen and the excited states of photosensitizing molecules, as well as by the scavenging of free radicals, is one of the main biological functions of CARs. X-ray crystallographic localization of CARs revealed that they are present at functionally and structurally important sites of both the PSI and PSII reaction centers. Characterization of a CAR-less cyanobacterial mutant revealed that while the absence of CARs prevents the formation of PSII complexes, it does not abolish the assembly and function of PSI. CAR molecules assist in the formation of protein subunits of the photosynthetic complexes by gluing together their protein components. In addition to their aforementioned indispensable functions, CARs have a substantial role in the formation and maintenance of proper cellular architecture, and potentially also in the protection of the translational machinery under stress conditions.

Evidence for the absence of enzymatic reactions in the glassy state. A case study of xanthophyll cycle pigments in the desiccation-tolerant moss Syntrichia ruralis

Desiccation-tolerant plants are able to withstand dehydration and resume normal metabolic functions upon rehydration. These plants can be dehydrated until their cytoplasm enters a 'glassy state' in which molecular mobility is severely reduced. In desiccation-tolerant seeds, longevity can be enhanced by drying and lowering storage temperature. In these conditions, they still deteriorate slowly, but it is not known if deteriorative processes include enzyme activity. The storage stability of photosynthetic organisms is less studied, and no reports are available on the glassy state in photosynthetic tissues. Here, the desiccation-tolerant moss Syntrichia ruralis was dehydrated at either 75% or <5% relative humidity, resulting in slow (SD) or rapid desiccation (RD), respectively, and different residual water content of the desiccated tissues. The molecular mobility within dry mosses was assessed through dynamic mechanical thermal analysis, showing that at room temperature only rapidly desiccated samples entered the glassy state, whereas slowly desiccated samples were in a 'rubbery' state. Violaxanthin cycle activity, accumulation of plastoglobules, and reorganization of thylakoids were observed upon SD, but not upon RD. Violaxanthin cycle activity critically depends on the activity of violaxanthin de-epoxidase (VDE). Hence, it is proposed that enzymatic activity occurred in the rubbery state (after SD), and that in the glassy state (after RD) no VDE activity was possible. Furthermore, evidence is provided that zeaxanthin has some role in recovery apparently independent of its role in non-photochemical quenching of chlorophyll fluorescence.

A reaction center-dependent photoprotection mechanism in a highly robust photosystem II from an extremophilic red alga, Cyanidioschyzon merolae

Members of the rhodophytan order Cyanidiales are unique among phototrophs in their ability to live in extremely low pH levels and moderately high temperatures. The photosynthetic apparatus of the red alga Cyanidioschyzon merolae represents an intermediate type between cyanobacteria and higher plants, suggesting that this alga may provide the evolutionary link between prokaryotic and eukaryotic phototrophs. Although we now have a detailed structural model of photosystem II (PSII) from cyanobacteria at an atomic resolution, no corresponding structure of the eukaryotic PSII complex has been published to date. Here we report the isolation and characterization of a highly active and robust dimeric PSII complex from C. merolae. We show that this complex is highly stable across a range of extreme light, temperature, and pH conditions. By measuring fluorescence quenching properties of the isolated C. merolae PSII complex, we provide the first direct evidence of pH-dependent non-photochemical quenching in the red algal PSII reaction center. This type of quenching, together with high zeaxanthin content, appears to underlie photoprotection mechanisms that are efficiently employed by this robust natural water-splitting complex under excess irradiance. In order to provide structural details of this eukaryotic form of PSII, we have employed electron microscopy and single particle analyses to obtain a 17 Å map of the C. merolae PSII dimer in which we locate the position of the protein mass corresponding to the additional extrinsic protein stabilizing the oxygen-evolving complex, PsbQ'. We conclude that this lumenal subunit is present in the vicinity of the CP43 protein, close to the membrane plane.

Potential for green microalgae to produce hydrogen, pharmaceuticals and other high value products in a combined process

Green microalgae for several decades have been produced for commercial exploitation, with applications ranging from health food for human consumption, aquaculture and animal feed, to coloring agents, cosmetics and others. Several products from green algae which are used today consist of secondary metabolites that can be extracted from the algal biomass. The best known examples are the carotenoids astaxanthin and β-carotene, which are used as coloring agents and for health-promoting purposes. Many species of green algae are able to produce valuable metabolites for different uses; examples are antioxidants, several different carotenoids, polyunsaturated fatty acids, vitamins, anticancer and antiviral drugs. In many cases, these substances are secondary metabolites that are produced when the algae are exposed to stress conditions linked to nutrient deprivation, light intensity, temperature, salinity and pH. In other cases, the metabolites have been detected in algae grown under optimal conditions, and little is known about optimization of the production of each product, or the effects of stress conditions on their production. Some green algae have shown the ability to produce significant amounts of hydrogen gas during sulfur deprivation, a process which is currently studied extensively worldwide. At the moment, the majority of research in this field has focused on the model organism, Chlamydomonas reinhardtii, but other species of green algae also have this ability. Currently there is little information available regarding the possibility for producing hydrogen and other valuable metabolites in the same process. This study aims to explore which stress conditions are known to induce the production of different valuable products in comparison to stress reactions leading to hydrogen production. Wild type species of green microalgae with known ability to produce high amounts of certain valuable metabolites are listed and linked to species with ability to produce hydrogen during general anaerobic conditions, and during sulfur deprivation. Species used today for commercial purposes are also described. This information is analyzed in order to form a basis for selection of wild type species for a future multi-step process, where hydrogen production from solar energy is combined with the production of valuable metabolites and other commercial uses of the algal biomass.

Effect of environmental stress factors on ecophysiological traits and susceptibility to pathogens of five Populus clones throughout the growing season

The variability of ecophysiological traits associated with productivity (e.g., water relations, leaf structure, photosynthesis and nitrogen (N) content) and susceptibility to fungal and insect infection were investigated in five poplar clones (Populus deltoides Batr.-Lux clone; Populus nigra L.-58-861 clone and Populus × canadensis Mönch.-Luisa Avanzo, I-214 and Adige clones) during their growing season. The objective of the study was to determine their physiological responses under summer constraints (characteristic of the Mediterranean climate) and to propose clone candidates for environmental restoration activities such as phytoremediation. Relative water content, the radiometric water index and (13)C isotope discrimination (Δ(13)C) results reflected improved water relations in Adige and Lux during summer drought. Leaf structural parameters such as leaf area, leaf mass per area, density (D) and thickness (T) indicated poorer structural adaptations to summer drought in clone 58-861. Nitrogen content and Δ(13)C results pointed to a stomatal component as the main limitant of photosynthesis in all clones. Adige and Lux showed enhanced photoprotection as indicated by the size and the de-epoxidation index of the xanthophyll-cycle pool, and also improved antioxidant defence displayed by higher ascorbate, reduced glutathione, total phenolics and α-tocopherol levels. Photoprotective and antioxidative responses allowed all clones to maintain a high maximum quantum yield of PSII (Fv/Fm) with the exception of Luisa Avanzo and 58-861 which experienced slight photoinhibition in late spring. The study of susceptibility to rust (Melampsora sp.) and lace bug (Monosteira unicostata Muls. and Rey) infections showed Adige and Lux to be the most tolerant. Overall, these two clones presented high adaptability to summer conditions and improved resistance to abiotic and biotic stress, thereby making them highly commendable clones for use in environmental remediation programmes.

The physiological roles and metabolism of ascorbate in chloroplasts

Ascorbate is a multifunctional metabolite in plants. It is essential for growth control, involving cell division and cell wall synthesis and also involved in redox signaling, in the modulation of gene expression and regulation of enzymatic activities. Ascorbate also fulfills crucial roles in scavenging reactive oxygen species, both enzymatically and nonenzymatically, a well-established phenomenon in the chloroplasts stroma. We give an overview on these important physiological functions and would like to give emphasis to less well-known roles of ascorbate, in the thylakoid lumen, where it also plays multiple roles. It is essential for photoprotection as a cofactor for violaxanthin de-epoxidase, a key enzyme in the formation of nonphotochemical quenching. Lumenal ascorbate has recently also been shown to act as an alternative electron donor of photosystem II once the oxygen-evolving complex is inactivated and to protect the photosynthetic machinery by slowing down donor-side induced photoinactivation; it is yet to be established if ascorbate has a similar role in the case of other stress effects, such as high light and UV-B stress. In bundle sheath cells, deficient in oxygen evolution, ascorbate provides electrons to photosystem II, thereby poising cyclic electron transport around photosystem I. It has also been shown that, by supporting linear electron transport through photosystem II in sulfur-deprived Chlamydomonas reinhardtii cells, in which oxygen evolution is largely inhibited, externally added ascorbate enhances hydrogen production. For fulfilling its multiple roles, Asc has to be transported into the thylakoid lumen and efficiently regenerated; however, very little is known yet about these processes.

Photosystem II does not possess a simple excitation energy funnel: time-resolved fluorescence spectroscopy meets theory

The experimentally obtained time-resolved fluorescence spectra of photosystem II (PS II) core complexes, purified from a thermophilic cyanobacterium Thermosynechococcus vulcanus, at 5–180 K are compared with simulations. Dynamic localization effects of excitons are treated implicitly by introducing exciton domains of strongly coupled pigments. Exciton relaxations within a domain and exciton transfers between domains are treated on the basis of Redfield theory and generalized Förster theory, respectively. The excitonic couplings between the pigments are calculated by a quantum chemical/electrostatic method (Poisson-TrEsp). Starting with previously published values, a refined set of site energies of the pigments is obtained through optimization cycles of the fits of stationary optical spectra of PS II. Satisfactorily agreement between the experimental and simulated spectra is obtained for the absorption spectrum including its temperature dependence and the linear dichroism spectrum of PS II core complexes (PS II-CC). Furthermore, the refined site energies well reproduce the temperature dependence of the time-resolved fluorescence spectrum of PS II-CC, which is characterized by the emergence of a 695 nm fluorescence peak upon cooling down to 77 K and the decrease of its relative intensity upon further cooling below 77 K. The blue shift of the fluorescence band upon cooling below 77 K is explained by the existence of two red-shifted chlorophyll pools emitting at around 685 and 695 nm. The former pool is assigned to Chl45 or Chl43 in CP43 (Chl numbering according to the nomenclature of Loll et al. Nature2005, 438, 1040) while the latter is assigned to Chl29 in CP47. The 695 nm emitting chlorophyll is suggested to attract excitations from the peripheral light-harvesting complexes and might also be involved in photoprotection.

Light-induced acclimation of the Arabidopsis chlorina1 mutant to singlet oxygen

Singlet oxygen (¹O₂) is a reactive oxygen species that can function as a stress signal in plant leaves leading to programmed cell death. In microalgae, ¹O₂-induced transcriptomic changes result in acclimation to ¹O₂. Here, using a chlorophyll b-less Arabidopsis thaliana mutant (chlorina1 [ch1]), we show that this phenomenon can also occur in vascular plants. The ch1 mutant is highly photosensitive due to a selective increase in the release of ¹O₂ by photosystem II. Under photooxidative stress conditions, the gene expression profile of ch1 mutant leaves very much resembled the gene responses to ¹O₂ reported in the Arabidopsis mutant flu. Preexposure of ch1 plants to moderately elevated light intensities eliminated photooxidative damage without suppressing ¹O₂ formation, indicating acclimation to ¹O₂. Substantial differences in gene expression were observed between acclimation and high-light stress: A number of transcription factors were selectively induced by acclimation, and contrasting effects were observed for the jasmonate pathway. Jasmonate biosynthesis was strongly induced in ch1 mutant plants under high-light stress and was noticeably repressed under acclimation conditions, suggesting the involvement of this hormone in ¹O₂-induced cell death. This was confirmed by the decreased tolerance to photooxidative damage of jasmonate-treated ch1 plants and by the increased tolerance of the jasmonate-deficient mutant delayed-dehiscence2.

Effects of altered α- and β-branch carotenoid biosynthesis on photoprotection and whole-plant acclimation of Arabidopsis to photo-oxidative stress

Functions of α- and β-branch carotenoids in whole-plant acclimation to photo-oxidative stress were studied in Arabidopsis thaliana wild-type (wt) and carotenoid mutants, lutein deficient (lut2, lut5), non-photochemical quenching1 (npq1) and suppressor of zeaxanthin-less1 (szl1) npq1 double mutant. Photo-oxidative stress was applied by exposing plants to sunflecks. The sunflecks caused reduction of chlorophyll content in all plants, but more severely in those having high α- to β-branch carotenoid composition (α/β-ratio) (lut5, szl1npq1). While this did not alter carotenoid composition in wt or lut2, which accumulates only β-branch carotenoids, increased xanthophyll levels were found in the mutants with high α/β-ratios (lut5, szl1npq1) or without xanthophyll-cycle operation (npq1, szl1npq1). The PsbS protein content increased in all sunfleck plants but lut2. These changes were accompanied by no change (npq1, szl1npq1) or enhanced capacity (wt, lut5) of NPQ. Leaf mass per area increased in lut2, but decreased in wt and lut5 that showed increased NPQ. The sunflecks decelerated primary root growth in wt and npq1 having normal α/β-ratios, but suppressed lateral root formation in lut5 and szl1npq1 having high α/β-ratios. The results highlight the importance of proper regulation of the α- and β-branch carotenoid pathways for whole-plant acclimation, not only leaf photoprotection, under photo-oxidative stress.

Emerging trade-offs - impact of photoprotectants (PsbS, xanthophylls, and vitamin E) on oxylipins as regulators of development and defense

This review summarizes evidence for a mechanistic link between plant photoprotection and the synthesis of oxylipin hormones as regulators of development and defense. Knockout mutants of Arabidopsis, deficient in various key components of the chloroplast photoprotection system, consistently produced greater concentrations of the hormone jasmonic acid or its precursor 12- oxo-phytodienoic acid (OPDA), both members of the oxylipin messenger family. Characterized plants include several mutants deficient in PsbS (an intrinsic chlorophyll-binding protein of photosystem II) or pigments (zeaxanthin and/or lutein) required for photoprotective thermal dissipation of excess excitation energy in the chloroplast and a mutant deficient in reactive oxygen detoxification via the antioxidant vitamin E (tocopherol). Evidence is also presented that certain plant defenses against herbivores or pathogens are elevated for these mutants. This evidence furthermore indicates that wild-type Arabidopsis plants possess less than maximal defenses against herbivores or pathogens, and suggest that plant lines with superior defenses against abiotic stress may have lower biotic defenses. The implications of this apparent trade-off between abiotic and biotic plant defenses for plant ecology as well as for plant breeding/engineering are explored, and the need for research further addressing this important issue is highlighted.

The Arabidopsis nox mutant lacking carotene hydroxylase activity reveals a critical role for xanthophylls in photosystem I biogenesis

Carotenes, and their oxygenated derivatives xanthophylls, are essential components of the photosynthetic apparatus. They contribute to the assembly of photosynthetic complexes and participate in light absorption and chloroplast photoprotection. Here, we studied the role of xanthophylls, as distinct from that of carotenes, by characterizing a no xanthophylls (nox) mutant of Arabidopsis thaliana, which was obtained by combining mutations targeting the four carotenoid hydroxylase genes. nox plants retained α- and β-carotenes but were devoid in xanthophylls. The phenotype included depletion of light-harvesting complex (LHC) subunits and impairment of nonphotochemical quenching, two effects consistent with the location of xanthophylls in photosystem II antenna, but also a decreased efficiency of photosynthetic electron transfer, photosensitivity, and lethality in soil. Biochemical analysis revealed that the nox mutant was specifically depleted in photosystem I function due to a severe deficiency in PsaA/B subunits. While the stationary level of psaA/B transcripts showed no major differences between genotypes, the stability of newly synthesized PsaA/B proteins was decreased and translation of psaA/B mRNA was impaired in nox with respect to wild-type plants. We conclude that xanthophylls, besides their role in photoprotection and LHC assembly, are also needed for photosystem I core translation and stability, thus making these compounds indispensable for autotrophic growth.

Jasmonate: A decision maker between cell death and acclimation in the response of plants to singlet oxygen

Under stress conditions that bring about excessive absorption of light energy in the chloroplasts, the formation of singlet oxygen ( (1)O2) can be strongly enhanced, triggering programmed cell death. However, the (1)O2 signaling pathway can also lead to acclimation to photooxidative stress, when (1)O2 is produced in relatively low amounts. This acclimatory response is associated with a strong downregulation of the jasmonate biosynthesis pathway and the maintenance of low jasmonate levels, even under high light stress conditions that normally induce jasmonate synthesis. These findings suggest a central role for this phytohormone in the orientation of the (1)O2 signaling pathway toward cell death or acclimation. This conclusion is confirmed here in an Arabidopsis double mutant obtained by crossing the (1)O2-overproducing mutant ch1 and the jasmonate-deficient mutant dde2. This double mutant was found to be constitutively resistant to (1)O2 stress and to display a strongly stimulated growth rate compared with the single ch1 mutant. However, the involvement of other phytohormones, such as ethylene, cannot be excluded.

From ecophysiology to phenomics: some implications of photoprotection and shade-sun acclimation in situ for dynamics of thylakoids in vitro

Half a century of research into the physiology and biochemistry of sun-shade acclimation in diverse plants has provided reality checks for contemporary understanding of thylakoid membrane dynamics. This paper reviews recent insights into photosynthetic efficiency and photoprotection from studies of two xanthophyll cycles in old shade leaves from the inner canopy of the tropical trees Inga sapindoides and Persea americana (avocado). It then presents new physiological data from avocado on the time frames of the slow coordinated photosynthetic development of sink leaves in sunlight and on the slow renovation of photosynthetic properties in old leaves during sun to shade and shade to sun acclimation. In so doing, it grapples with issues in vivo that seem relevant to our increasingly sophisticated understanding of ΔpH-dependent, xanthophyll-pigment-stabilized non-photochemical quenching in the antenna of PSII in thylakoid membranes in vitro.

Photosynthetic limitations and volatile and non-volatile isoprenoids in the poikilochlorophyllous resurrection plant Xerophyta humilis during dehydration and rehydration

We investigated the photosynthetic limitations occurring during dehydration and rehydration of Xerophyta humilis, a poikilochlorophyllous resurrection plant, and whether volatile and non-volatile isoprenoids might be involved in desiccation tolerance. Photosynthesis declined rapidly after dehydration below 85% relative water content (RWC). Raising intercellular CO(2) concentrations during desiccation suggest that the main photosynthetic limitation was photochemical, affecting energy-dependent RuBP regeneration. Imaging fluorescence confirmed that both the number of photosystem II (PSII) functional reaction centres and their efficiency were impaired under progressive dehydration, and revealed the occurrence of heterogeneous photosynthesis during desiccation, being the basal leaf area more resistant to the stress. Full recovery in photosynthetic parameters occurred on rehydration, confirming that photosynthetic limitations were fully reversible and that no permanent damage occurred. During desiccation, zeaxanthin and lutein increased only when photosynthesis had ceased, implying that these isoprenoids do not directly scavenge reactive oxygen species, but rather protect photosynthetic membranes from damage and consequent denaturation. X. humilis was found to emit isoprene, a volatile isoprenoid that acts as a membrane strengthener in plants. Isoprene emission was stimulated by drought and peaked at 80% RWC. We surmise that isoprene and non-volatile isoprenoids cooperate in reducing membrane damage in X. humilis, isoprene being effective when desiccation is moderate while non-volatile isoprenoids operate when water deficit is more extreme.

Reactive oxygen species generation and signaling in plants

The introduction of molecular oxygen into the atmosphere was accompanied by the generation of reactive oxygen species (ROS) as side products of many biochemical reactions. ROS are permanently generated in plastids, peroxisomes, mitochiondria, the cytosol and the apoplast. Imbalance between ROS generation and safe detoxification generates oxidative stress and the accumulating ROS are harmful for the plants. On the other hand, specific ROS function as signaling molecules and activate signal transduction processes in response to various stresses. Here, we summarize the generation of ROS in the different cellular compartments and the signaling processes which are induced by ROS.

LCAA, a novel factor required for magnesium protoporphyrin monomethylester cyclase accumulation and feedback control of aminolevulinic acid biosynthesis in tobacco

Low Chlorophyll Accumulation A (LCAA) antisense plants were obtained from a screen for genes whose partial down-regulation results in a strong chlorophyll deficiency in tobacco (Nicotiana tabacum). The LCAA mutants are affected in a plastid-localized protein of unknown function, which is conserved in cyanobacteria and all photosynthetic eukaryotes. They suffer from drastically reduced light-harvesting complex (LHC) contents, while the accumulation of all other photosynthetic complexes per leaf area is less affected. As the disturbed accumulation of LHC proteins could be either attributable to a defect in LHC biogenesis itself or to a bottleneck in chlorophyll biosynthesis, chlorophyll synthesis rates and chlorophyll synthesis intermediates were measured. LCAA antisense plants accumulate magnesium (Mg) protoporphyrin monomethylester and contain reduced protochlorophyllide levels and a reduced content of CHL27, a subunit of the Mg protoporphyrin monomethylester cyclase. Bimolecular fluorescence complementation assays confirm a direct interaction between LCAA and CHL27. 5-Aminolevulinic acid synthesis rates are increased and correlate with an increased content of glutamyl-transfer RNA reductase. We suggest that LCAA encodes an additional subunit of the Mg protoporphyrin monomethylester cyclase, is required for the stability of CHL27, and contributes to feedback-control of 5-aminolevulinic acid biosynthesis, the rate-limiting step of chlorophyll biosynthesis.

Retrograde signaling and photoprotection in a gun4 mutant of Chlamydomonas reinhardtii

GUN4 is a regulatory subunit of Mg-chelatase involved in the control of tetrapyrrole synthesis in plants and cyanobacteria. Here, we report the first characterization of a gun4 insertion mutant of the unicellular green alga Chlamydomonas reinhardtii. The mutant contains 50% of chlorophyll as compared to wild-type and accumulates ProtoIX. In contrast to the increase in LHC transcription, the accumulation of most LHC proteins is drastically diminished, implying posttranscriptional down-regulation in the absence of transcriptional coordination. We found that 803 genes change their expression level in gun4 as compared to wild-type, by RNA-Seq, and this wide-ranging effect on transcription is apparent under physiological conditions. Besides LHCs, we identified transcripts encoding enzymes of the tetrapyrrole pathway and factors involved in signal transduction, transcription, and chromatin remodeling. Moreover, we observe perturbations in electron transport with a strongly decreased PSI-to-PSII ratio. This is accompanied by an enhanced activity of the plastid terminal oxidase (PTOX) that could have a physiological role in decreasing photosystem II excitation pressure.

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.

Retrograde signaling and photoprotection in a gun4 mutant of Chlamydomonas reinhardtii

GUN4 is a regulatory subunit of Mg-chelatase involved in the control of tetrapyrrole synthesis in plants and cyanobacteria. Here, we report the first characterization of a gun4 insertion mutant of the unicellular green alga Chlamydomonas reinhardtii. The mutant contains 50% of chlorophyll as compared to wild-type and accumulates ProtoIX. In contrast to the increase in LHC transcription, the accumulation of most LHC proteins is drastically diminished, implying posttranscriptional down-regulation in the absence of transcriptional coordination. We found that 803 genes change their expression level in gun4 as compared to wild-type, by RNA-Seq, and this wide-ranging effect on transcription is apparent under physiological conditions. Besides LHCs, we identified transcripts encoding enzymes of the tetrapyrrole pathway and factors involved in signal transduction, transcription, and chromatin remodeling. Moreover, we observe perturbations in electron transport with a strongly decreased PSI-to-PSII ratio. This is accompanied by an enhanced activity of the plastid terminal oxidase (PTOX) that could have a physiological role in decreasing photosystem II excitation pressure.

The MORPH algorithm: ranking candidate genes for membership in Arabidopsis and tomato pathways

Closing gaps in our current knowledge about biological pathways is a fundamental challenge. The development of novel computational methods along with high-throughput experimental data carries the promise to help in the challenge. We present an algorithm called MORPH (for module-guided ranking of candidate pathway genes) for revealing unknown genes in biological pathways. The method receives as input a set of known genes from the target pathway, a collection of expression profiles, and interaction and metabolic networks. Using machine learning techniques, MORPH selects the best combination of data and analysis method and outputs a ranking of candidate genes predicted to belong to the target pathway. We tested MORPH on 230 known pathways in Arabidopsis thaliana and 93 known pathways in tomato (Solanum lycopersicum) and obtained high-quality cross-validation results. In the photosynthesis light reactions, homogalacturonan biosynthesis, and chlorophyll biosynthetic pathways of Arabidopsis, genes ranked highly by MORPH were recently verified to be associated with these pathways. MORPH candidates ranked for the carotenoid pathway from Arabidopsis and tomato are derived from pathways that compete for common precursors or from pathways that are coregulated with or regulate the carotenoid biosynthetic pathway.

Zeaxanthin protects plant photosynthesis by modulating chlorophyll triplet yield in specific light-harvesting antenna subunits

Plants are particularly prone to photo-oxidative damage caused by excess light. Photoprotection is essential for photosynthesis to proceed in oxygenic environments either by scavenging harmful reactive intermediates or preventing their accumulation to avoid photoinhibition. Carotenoids play a key role in protecting photosynthesis from the toxic effect of over-excitation; under excess light conditions, plants accumulate a specific carotenoid, zeaxanthin, that was shown to increase photoprotection. In this work we genetically dissected different components of zeaxanthin-dependent photoprotection. By using time-resolved differential spectroscopy in vivo, we identified a zeaxanthin-dependent optical signal characterized by a red shift in the carotenoid peak of the triplet-minus-singlet spectrum of leaves and pigment-binding proteins. By fractionating thylakoids into their component pigment binding complexes, the signal was found to originate from the monomeric Lhcb4-6 antenna components of Photosystem II and the Lhca1-4 subunits of Photosystem I. By analyzing mutants based on their sensitivity to excess light, the red-shifted triplet-minus-singlet signal was tightly correlated with photoprotection in the chloroplasts, suggesting the signal implies an increased efficiency of zeaxanthin in controlling chlorophyll triplet formation. Fluorescence-detected magnetic resonance analysis showed a decrease in the amplitude of signals assigned to chlorophyll triplets belonging to the monomeric antenna complexes of Photosystem II upon zeaxanthin binding; however, the amplitude of carotenoid triplet signal does not increase correspondingly. Results show that the high light-induced binding of zeaxanthin to specific proteins plays a major role in enhancing photoprotection by modulating the yield of potentially dangerous chlorophyll-excited states in vivo and preventing the production of singlet oxygen.

Photosystem II function and dynamics in three widely used Arabidopsis thaliana accessions

Columbia-0 (Col-0), Wassilewskija-4 (Ws-4), and Landsberg erecta-0 (Ler-0) are used as background lines for many public Arabidopsis mutant collections, and for investigation in laboratory conditions of plant processes, including photosynthesis and response to high-intensity light (HL). The photosystem II (PSII) complex is sensitive to HL and requires repair to sustain its function. PSII repair is a multistep process controlled by numerous factors, including protein phosphorylation and thylakoid membrane stacking. Here we have characterized the function and dynamics of PSII complex under growth-light and HL conditions. Ws-4 displayed 30% more thylakoid lipids per chlorophyll and 40% less chlorophyll per carotenoid than Col-0 and Ler-0. There were no large differences in thylakoid stacking, photoprotection and relative levels of photosynthetic complexes among the three accessions. An increased efficiency of PSII closure was found in Ws-4 following illumination with saturation flashes or continuous light. Phosphorylation of the PSII D1/D2 proteins was reduced by 50% in Ws-4 as compared to Col-0 and Ler-0. An increase in abundance of the responsible STN8 kinase in response to HL treatment was found in all three accessions, but Ws-4 displayed 50% lower levels than Col-0 and Ler-0. Despite this, the HL treatment caused in Ws-4 the lagest extent of PSII inactivation, disassembly, D1 protein degradation, and the largest decrease in the size of stacked thylakoids. The dilution of chlorophyll-protein complexes with additional lipids and carotenoids in Ws-4 may represent a mechanism to facilitate lateral protein traffic in the membrane, thus compensating for the lack of a full complement of STN8 kinase. Nevertheless, additional PSII damage occurs in Ws-4, which exceeds the D1 protein synthesis capacity, thus leading to enhanced photoinhibition. Our findings are valuable for selection of appropriate background line for PSII characterization in Arabidopsis mutants, and also provide the first insights into natural variation of PSII protein phosphorylation.

Plastid localization of the key carotenoid enzyme phytoene synthase is altered by isozyme, allelic variation, and activity

Plant carotenoids have unique physiological roles related to specific plastid suborganellar locations. Carotenoid metabolic engineering could enhance plant adaptation to climate change and improve food security and nutritional value. However, lack of fundamental knowledge on carotenoid pathway localization limits targeted engineering. Phytoene synthase (PSY), a major rate-controlling carotenoid enzyme, is represented by multiple isozymes residing at unknown plastid sites. In maize (Zea mays), the three isozymes were transiently expressed and found either in plastoglobuli or in stroma and thylakoid membranes. PSY1, with one to two residue modifications of naturally occurring functional variants, exhibited altered localization, associated with distorted plastid shape and formation of a fibril phenotype. Mutating the active site of the enzyme reversed this phenotype. Discovery of differential PSY locations, linked with activity and isozyme type, advances the engineering potential for modifying carotenoid biosynthesis.

Modulation of photosynthetic energy conversion efficiency in nature: from seconds to seasons

Modulation of the efficiency with which leaves convert absorbed light to photochemical energy [intrinsic efficiency of open photosystem II (PSII) centers, as the ratio of variable to maximal chlorophyll fluorescence] as well as leaf xanthophyll composition (interconversions of the xanthophyll cycle pigments violaxanthin and zeaxanthin) were characterized throughout single days and nights to entire seasons in plants growing naturally in contrasting light and temperature environments. All pronounced decreases of intrinsic PSII efficiency took place in the presence of zeaxanthin. The reversibility of these PSII efficiency changes varied widely, ranging from reversible-within-seconds (in a vine experiencing multiple sunflecks under a eucalypt canopy) to apparently permanently locked-in for entire seasons (throughout the whole winter in a subalpine conifer forest at 3,000 m). While close association between low intrinsic PSII efficiency and zeaxanthin accumulation was ubiquitous, accompanying features (such as trans-thylakoid pH gradient, thylakoid protein composition, and phosphorylation) differed among contrasting conditions. The strongest and longest-lasting depressions in intrinsic PSII efficiency were seen in the most stress-tolerant species. Evergreens, in particular, showed the most pronounced modulation of PSII efficiency and thermal dissipation, and are therefore suggested as model species for the study of photoprotection. Implications of the responses of field-grown plants in nature for mechanistic models are discussed.

Acclimatory responses of Arabidopsis to fluctuating light environment: comparison of different sunfleck regimes and accessions

Acclimation to fluctuating light environment with short (lasting 20 s, at 650 or 1,250 μmol photons m(-2) s(-1), every 6 or 12 min) or long (for 40 min at 650 μmol photons m(-2) s(-1), once a day at midday) sunflecks was studied in Arabidopsis thaliana. The sunfleck treatments were applied in the background daytime light intensity of 50 μmol photons m(-2) s(-1). In order to distinguish the effects of sunflecks from those of increased daily irradiance, constant light treatments at 85 and 120 μmol photons m(-2) s(-1), which gave the same photosynthetically active radiation (PAR) per day as the different sunfleck treatments, were also included in the experiments. The increased daily total PAR in the two higher constant light treatments enhanced photosystem II electron transport and starch accumulation in mature leaves and promoted expansion of young leaves in Columbia-0 plants during the 7-day treatments. Compared to the plants remaining under 50 μmol photons m(-2) s(-1), application of long sunflecks caused upregulation of electron transport without affecting carbon gain in the form of starch accumulation and leaf growth or the capacity of non-photochemical quenching (NPQ). Mature leaves showed marked enhancement of the NPQ capacity under the conditions with short sunflecks, which preceded recovery and upregulation of electron transport, demonstrating the initial priority of photoprotection. The distinct acclimatory responses to constant PAR, long sunflecks, and different combinations of short sunflecks are consistent with acclimatory adjustment of the processes in photoprotection and carbon gain, depending on the duration, frequency, and intensity of light fluctuations. While the responses of leaf expansion to short sunflecks differed among the seven Arabidopsis accessions examined, all plants showed NPQ upregulation, suggesting limited ability of this species to utilize short sunflecks. The increase in the NPQ capacity was accompanied by reduced chlorophyll contents, higher levels of the xanthophyll-cycle pigments, faster light-induced de-epoxidation of violaxanthin to zeaxanthin and antheraxanthin, increased amounts of PsbS protein, as well as enhanced activity of superoxide dismutase. These acclimatory mechanisms, involving reorganization of pigment-protein complexes and upregulation of other photoprotective reactions, are probably essential for Arabidopsis plants to cope with photo-oxidative stress induced by short sunflecks without suffering from severe photoinhibition and lipid peroxidation.

The Arabidopsis szl1 mutant reveals a critical role of β-carotene in photosystem I photoprotection

Carotenes and their oxygenated derivatives, the xanthophylls, are structural determinants in both photosystems (PS) I and II. They bind and stabilize photosynthetic complexes, increase the light-harvesting capacity of chlorophyll-binding proteins, and have a major role in chloroplast photoprotection. Localization of carotenoid species within each PS is highly conserved: Core complexes bind carotenes, whereas peripheral light-harvesting systems bind xanthophylls. The specific functional role of each xanthophyll species has been recently described by genetic dissection, however the in vivo role of carotenes has not been similarly defined. Here, we have analyzed the function of carotenes in photosynthesis and photoprotection, distinct from that of xanthophylls, by characterizing the suppressor of zeaxanthin-less (szl) mutant of Arabidopsis (Arabidopsis thaliana) which, due to the decreased activity of the lycopene-β-cyclase, shows a lower carotene content than wild-type plants. When grown at room temperature, mutant plants showed a lower content in PSI light-harvesting complex I complex than the wild type, and a reduced capacity for chlorophyll fluorescence quenching, the rapidly reversible component of nonphotochemical quenching. When exposed to high light at chilling temperature, szl1 plants showed stronger photoxidation than wild-type plants. Both PSI and PSII from szl1 were similarly depleted in carotenes and yet PSI activity was more sensitive to light stress than PSII as shown by the stronger photoinhibition of PSI and increased rate of singlet oxygen release from isolated PSI light-harvesting complex I complexes of szl1 compared with the wild type. We conclude that carotene depletion in the core complexes impairs photoprotection of both PS under high light at chilling temperature, with PSI being far more affected than PSII.

Drought stress has contrasting effects on antioxidant enzymes activity and phenylpropanoid biosynthesis in Fraxinus ornus leaves: an excess light stress affair?

The experiment was conducted using Fraxinus ornus plants grown outside under full sunlight irradiance, and supplied with 100% (well-watered, WW), 40% (mild drought, MD), or 20% (severe drought, SD) of the daily evapotranspiration demand, with the main objective of exploring the effect of excess light stress on the activity of antioxidant enzymes and phenylpropanoid biosynthesis. Net CO₂ assimilation rate at saturating light and daily assimilated CO₂ were significantly smaller in SD than in WW and MD plants. Xanthophyll-cycle pigments supported nonphotochemical quenching to a significantly greater extent in SD than in MD and WW leaves. As a consequence, the actual efficiency of PSII (Φ(PSII)) was smaller, while the excess excitation-energy in the photosynthetic apparatus was greater in SD than in WW or MD plants. The concentrations of violaxanthin-cycle pigments relative to total chlorophyll (Chl(tot)) exceeded 200 mmol mol⁻¹ Chl(tot) in SD leaves at the end of the experiment. This leads to hypothesize for zeaxanthin a role not only as nonphotochemical quencher, but also as chloroplast antioxidant. Reductions in ascorbate peroxidase and catalase activities, as drought-stress progressed, were paralleled by greater accumulations of esculetin and quercetin 3-O-glycosides, both phenylpropanoids having effective capacity to scavenge H₂O₂. The drought-induced accumulation of esculetin and quercetin 3-O-glycosides in the vacuoles of mesophyll cells is consistent with their putative functions as reducing agents for H₂O₂ in excess light-stressed leaves. Nonetheless, the concentration of H₂O₂ and the lipid peroxidation were significantly greater in SD than in MD and WW leaves. It is speculated that vacuolar phenylpropanoids may constitute a secondary antioxidant system, even on a temporal basis, activated upon the depletion of primary antioxidant defences, and aimed at keeping whole-cell H₂O₂ within a sub-lethal concentration range.

A quadruple mutant of Arabidopsis reveals a β-carotene hydroxylation activity for LUT1/CYP97C1 and a regulatory role of xanthophylls on determination of the PSI/PSII ratio

BACKGROUND

Xanthophylls are oxygenated carotenoids playing an essential role as structural components of the photosynthetic apparatus. Xanthophylls contribute to the assembly and stability of light-harvesting complex, to light absorbance and to photoprotection. The first step in xanthophyll biosynthesis from α- and β-carotene is the hydroxylation of ε- and β-rings, performed by both non-heme iron oxygenases (CHY1, CHY2) and P450 cytochromes (LUT1/CYP97C1, LUT5/CYP97A3). The Arabidopsis triple chy1chy2lut5 mutant is almost completely depleted in β-xanthophylls.

RESULTS

Here we report on the quadruple chy1chy2lut2lut5 mutant, additionally carrying the lut2 mutation (affecting lycopene ε-cyclase). This genotype lacks lutein and yet it shows a compensatory increase in β-xanthophylls with respect to chy1chy2lut5 mutant. Mutant plants show an even stronger photosensitivity than chy1chy2lut5, a complete lack of qE, the rapidly reversible component of non-photochemical quenching, and a peculiar organization of the pigment binding complexes into thylakoids. Biochemical analysis reveals that the chy1chy2lut2lut5 mutant is depleted in Lhcb subunits and is specifically affected in Photosystem I function, showing a deficiency in PSI-LHCI supercomplexes. Moreover, by analyzing a series of single, double, triple and quadruple Arabidopsis mutants in xanthophyll biosynthesis, we show a hitherto undescribed correlation between xanthophyll levels and the PSI-PSII ratio. The decrease in the xanthophyll/carotenoid ratio causes a proportional decrease in the LHCII and PSI core levels with respect to PSII.

CONCLUSIONS

The physiological and biochemical phenotype of the chy1chy2lut2lut5 mutant shows that (i) LUT1/CYP97C1 protein reveals a major β-carotene hydroxylase activity in vivo when depleted in its preferred substrate α-carotene; (ii) xanthophylls are needed for normal level of Photosystem I and LHCII accumulation.

Chemical quenching of singlet oxygen by carotenoids in plants

Carotenoids are considered to be the first line of defense of plants against singlet oxygen ((1)O(2)) toxicity because of their capacity to quench (1)O(2) as well as triplet chlorophylls through a physical mechanism involving transfer of excitation energy followed by thermal deactivation. Here, we show that leaf carotenoids are also able to quench (1)O(2) by a chemical mechanism involving their oxidation. In vitro oxidation of β-carotene, lutein, and zeaxanthin by (1)O(2) generated various aldehydes and endoperoxides. A search for those molecules in Arabidopsis (Arabidopsis thaliana) leaves revealed the presence of (1)O(2)-specific endoperoxides in low-light-grown plants, indicating chronic oxidation of carotenoids by (1)O(2). β-Carotene endoperoxide, but not xanthophyll endoperoxide, rapidly accumulated during high-light stress, and this accumulation was correlated with the extent of photosystem (PS) II photoinhibition and the expression of various (1)O(2) marker genes. The selective accumulation of β-carotene endoperoxide points at the PSII reaction centers, rather than the PSII chlorophyll antennae, as a major site of (1)O(2) accumulation in plants under high-light stress. β-Carotene endoperoxide was found to have a relatively fast turnover, decaying in the dark with a half time of about 6 h. This carotenoid metabolite provides an early index of (1)O(2) production in leaves, the occurrence of which precedes the accumulation of fatty acid oxidation products.

Spatio-temporal heterogeneity in Arabidopsis thaliana leaves under drought stress

Using chlorophyll (chl) fluorescence imaging, we studied the effect of mild (MiDS), moderate (MoDS) and severe (SDS) drought stress on photosystem II (PSII) photochemistry of 4-week-old Arabidopsis thaliana. Spatio-temporal heterogeneity in all chl fluorescence parameters was maintained throughout water stress. After exposure to drought stress, maximum quantum yield of PSII photochemistry (F(v)/F(m)) and quantum efficiency of PSII photochemistry (Φ(PSΙΙ)) decreased less in the proximal (base) than in the distal (tip) leaf. The chl fluorescence parameter F(v) /F(m) decreased less after MoDS than MiDS. Under MoDS, the antioxidant mechanism of A. thaliana leaves seemed to be sufficient in scavenging reactive oxygen species, as evident by the decreased lipid peroxidation, the more excitation energy dissipated by non-photochemical quenching (NPQ) and decreased excitation pressure (1-q(p)). Arabidopsis leaves appear to function normally under MoDS, but do not seem to have particular metabolic tolerance mechanisms under MiDS and SDS, as revealed by the level of lipid peroxidation and decreased quantum yield for dissipation after down-regulation in PSII (Φ(NPQ)), indicating that energy dissipation by down-regulation did not function and electron transport (ETR) was depressed. The simultaneous increased quantum yield of non-regulated energy dissipation (Φ(NO)) indicated that both the photochemical energy conversion and protective regulatory mechanism were insufficient. The non-uniform photosynthetic pattern under drought stress may reflect different zones of leaf anatomy and mesophyll development. The data demonstrate that the effect of different degrees of drought stress on A. thaliana leaves show spatio-temporal heterogeneity, implying that common single time point or single point leaf analyses are inadequate.

Activation of violaxanthin cycle in darkness is a common response to different abiotic stresses: a case study in Pelvetia canaliculata

BACKGROUND

In the violaxanthin (V) cycle, V is de-epoxidized to zeaxanthin (Z) when strong light or light combined with other stressors lead to an overexcitation of photosystems. However, plants can also suffer stress in darkness and recent reports have shown that dehydration triggers V-de-epoxidation in the absence of light. In this study, we used the highly stress-tolerant brown alga Pelvetia canaliculata as a model organism, due to its lack of lutein and its non-photochemical quenching independent of the transthylakoidal-ΔpH, to study the triggering of the V-cycle in darkness induced by abiotic stressors.

RESULTS

We have shown that besides desiccation, other factors such as immersion, anoxia and high temperature also induced V-de-epoxidation in darkness. This process was reversible once the treatments had ceased (with the exception of heat, which caused lethal damage). Irrespective of the stressor applied, the resulting de-epoxidised xanthophylls correlated with a decrease in Fv/Fm, suggesting a common function in the down-regulation of photosynthetical efficiency. The implication of the redox-state of the plastoquinone-pool and of the differential activity of V-cycle enzymes on V-de-epoxidation in darkness was also examined. Current results suggest that both violaxanthin de-epoxidase (VDE) and zeaxanthin-epoxidase (ZE) have a basal constitutive activity even in darkness, being ZE inhibited under stress. This inhibition leads to Z accumulation.

CONCLUSION

This study demonstrates that V-cycle activity is triggered by several abiotic stressors even when they occur in an absolute absence of light, leading to a decrease in Fv/Fm. This finding provides new insights into an understanding of the regulation mechanism of the V-cycle and of its ecophysiological roles.

The oligomeric states of the photosystems and the light-harvesting complexes in the Chl b-less mutant

The reversible associations between the light-harvesting complexes (LHCs) and the core complexes of PSI and PSII are essential for the photoacclimation mechanisms in higher plants. Two types of Chls, Chl a and Chl b, both function in light harvesting and are required for the biogenesis of the photosystems. Chl b-less plants have been studied to determine the function of the LHCs because the Chl b deficiency has severe effects specific to the LHCs. Previous studies have shown that the amounts of the LHCs, especially the LHCII trimer, were decreased in the mutants; however, it is still unclear whether Chl b is required for the assembly of the LHCs and for the association of the LHCs with PSI and PSII. Here, to reveal the function of Chl b in the LHCs, we investigated the oligomeric states of the LHCs, PSI and PSII in the Arabidopsis Chl b-less mutant. A two-dimensional blue native-PAGE/SDS-PAGE demonstrated that the PSI-LHCI supercomplex was fully assembled in the absence of Chl b, whereas the trimeric LHCII and PSII-LHCII supercomplexes were not detected. The PSI-NAD(P)H dehydrogenase (NDH) supercomplexes were also assembled in the mutant. Furthermore, we detected two forms of monomeric LHC proteins. The faster migrating forms, which were detected primarily in the mutant, were probably apo-LHC proteins, whereas the slower migrating forms were probably the LHC proteins that contained Chl a. These findings increase our understanding of the Chl b function in the assembly of LHCs and the association of the LHCs with PSI, PSII and NDH.

The effects of moderately high temperature on zeaxanthin accumulation and decay

Moderately high temperature reduces photosynthetic capacities of leaves with large effects on thylakoid reactions of photosynthesis, including xanthophyll conversion in the lipid phase of the thylakoid membrane. In previous studies, we have found that leaf temperature of 40°C increased zeaxanthin accumulation in dark-adapted, intact tobacco leaves following a brief illumination, but did not change the amount of zeaxanthin in light-adatped leaves. To investigate heat effects on zeaxanthin accumulation and decay, zeaxanthin level was monitored optically in dark-adapted, intact tobacco and Arabidopsis thaliana leaves at either 23 or 40°C under 45-min illumination. Heated leaves had more zeaxanthin following 3-min light but had less or comparable amounts of zeaxanthin by the end of 45 min of illumination. Zeaxanthin accumulated faster at light initiation and decayed faster upon darkening in leaves at 40°C than leaves at 23°C, indicating that heat increased the activities of both violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase (ZE). In addition, our optical measurement demonstrated in vivo that weak light enhances zeaxanthin decay relative to darkness in intact leaves of tobacco and Arabidopsis, confirming previous observations in isolated spinach chloroplasts. However, the maximum rate of decay is similar for weak light and darkness, and we used the maximum rate of decay following darkness as a measure of the rate of ZE during steady-state light. A simulation indicated that high temperature should cause a large shift in the pH dependence of the amount of zeaxanthin in leaves because of differential effects on VDE and ZE. This allows for the reduction in ΔpH caused by heat to be offset by increased VDE activity relative to ZE.

Quenching in Arabidopsis thaliana mutants lacking monomeric antenna proteins of photosystem II

The minor light-harvesting complexes CP24, CP26, and CP29 have been proposed to play a key role in the zeaxanthin (Zx)-dependent high light-induced regulation (NPQ) of excitation energy in higher plants. To characterize the detailed roles of these minor complexes in NPQ and to determine their specific quenching effects we have studied the ultrafast fluorescence kinetics in knockout (ko) mutants koCP26, koCP29, and the double mutant koCP24/CP26. The data provide detailed insight into the quenching processes and the reorganization of the Photosystem (PS) II supercomplex under quenching conditions. All genotypes showed two NPQ quenching sites. Quenching site Q1 is formed by a light-induced functional detachment of parts of the PSII supercomplex and a pronounced quenching of the detached antenna parts. The antenna remaining bound to the PSII core was also quenched substantially in all genotypes under NPQ conditions (quenching site Q2) as compared with the dark-adapted state. The latter quenching was about equally strong in koCP26 and the koCP24/CP26 mutants as in the WT. Q2 quenching was substantially reduced, however, in koCP29 mutants suggesting a key role for CP29 in the total NPQ. The observed quenching effects in the knockout mutants are complicated by the fact that other minor antenna complexes do compensate in part for the lack of the CP24 and/or CP29 complexes. Their lack also causes some LHCII dissociation already in the dark.

Arabidopsis mutants deleted in the light-harvesting protein Lhcb4 have a disrupted photosystem II macrostructure and are defective in photoprotection

The role of the light-harvesting complex Lhcb4 (CP29) in photosynthesis was investigated in Arabidopsis thaliana by characterizing knockout lines for each of the three Lhcb4 isoforms (Lhcb4.1/4.2/4.3). Plants lacking all isoforms (koLhcb4) showed a compensatory increase of Lhcb1 and a slightly reduced photosystem II/I ratio with respect to the wild type. The absence of Lhcb4 did not result in alteration in electron transport rates. However, the kinetic of state transition was faster in the mutant, and nonphotochemical quenching activity was lower in koLhcb4 plants with respect to either wild type or mutants retaining a single Lhcb4 isoform. KoLhcb4 plants were more sensitive to photoinhibition, while this effect was not observed in knockout lines for any other photosystem II antenna subunit. Ultrastructural analysis of thylakoid grana membranes showed a lower density of photosystem II complexes in koLhcb4. Moreover, analysis of isolated supercomplexes showed a different overall shape of the C₂S₂ particles due to a different binding mode of the S-trimer to the core complex. An empty space was observed within the photosystem II supercomplex at the Lhcb4 position, implying that the missing Lhcb4 was not replaced by other Lhc subunits. This suggests that Lhcb4 is unique among photosystem II antenna proteins and determinant for photosystem II macro-organization and photoprotection.

Evolution and functional properties of photosystem II light harvesting complexes in eukaryotes

Photoautotrophic organisms, the major agent of inorganic carbon fixation into biomass, convert light energy into chemical energy. The first step of photosynthesis consists of the absorption of solar energy by pigments binding protein complexes named photosystems. Within photosystems, a family of proteins called Light Harvesting Complexes (LHC), responsible for light harvesting and energy transfer to reaction centers, has evolved along with eukaryotic organisms. Besides light absorption, these proteins catalyze photoprotective reactions which allowed functioning of oxygenic photosynthetic machinery in the increasingly oxidant environment. In this work we review current knowledge of LHC proteins serving Photosystem II. Balance between light harvesting and photoprotection is critical in Photosystem II, due to the lower quantum efficiency as compared to Photosystem I. In particular, we focus on the role of each antenna complex in light harvesting, energy transfer, scavenging of reactive oxygen species, chlorophyll triplet quenching and thermal dissipation of excess energy. This article is part of a Special Issue entitled: Photosystem II.

Role of PSBS and LHCSR in Physcomitrella patens acclimation to high light and low temperature

Photosynthetic organisms respond to strong illumination by activating several photoprotection mechanisms. One of them, non-photochemical quenching (NPQ), consists in the thermal dissipation of energy absorbed in excess. In vascular plants NPQ relies on the activity of PSBS, whereas in the green algae Chlamydomonas reinhardtii it requires a different protein, LHCSR. The moss Physcomitrella patens is the only known organism in which both proteins are present and active in triggering NPQ, making this organism particularly interesting for the characterization of this protection mechanism. We analysed the acclimation of Physcomitrella to high light and low temperature, finding that these conditions induce an increase in NPQ correlated to overexpression of both PSBS and LHCSR. Mutants depleted of PSBS and/or LHCSR showed that modulation of their accumulation indeed determines NPQ amplitude. All mutants with impaired NPQ also showed enhanced photosensitivity when exposed to high light or low temperature, indicating that in this moss the fast-responding NPQ mechanism is also involved in long-term acclimation.

The role of the xanthophyll cycle and of lutein in photoprotection of photosystem II

Photoprotection of photosystem II (PSII) is essential to avoid the light-induced damage of the photosynthetic apparatus due to the formation of reactive oxygen species (=photo-oxidative stress) under excess light. Carotenoids are known to play a crucial role in these processes based on their property to deactivate triplet chlorophyll (³Chl*) and singlet oxygen (¹O₂*). Xanthophylls are further assumed to be involved either directly or indirectly in the non-photochemical quenching (NPQ) of excess light energy in the antenna of PSII. This review gives an overview on recent progress in the understanding of the photoprotective role of the xanthophylls zeaxanthin (which is formed in the light in the so-called xanthophyll cycle) and lutein with emphasis on the NPQ processes associated with PSII of higher plants. The current knowledge supports the view that the photoprotective role of Lut is predominantly restricted to its function in the deactivation of ³Chl*, while zeaxanthin is the major player in the deactivation of excited singlet Chl (¹Chl*) and thus in NPQ (non-photochemical quenching). Additionally, zeaxanthin serves important functions as an antioxidant in the lipid phase of the membrane and is likely to act as a key component in the memory of the chloroplast with respect to preceding photo-oxidative stress. This article is part of a Special Issue entitled: Photosystem II.

Reactive oxygen species and transcript analysis upon excess light treatment in wild-type Arabidopsis thaliana vs a photosensitive mutant lacking zeaxanthin and lutein

BACKGROUND

Reactive oxygen species (ROS) are unavoidable by-products of oxygenic photosynthesis, causing progressive oxidative damage and ultimately cell death. Despite their destructive activity they are also signalling molecules, priming the acclimatory response to stress stimuli.

RESULTS

To investigate this role further, we exposed wild type Arabidopsis thaliana plants and the double mutant npq1lut2 to excess light. The mutant does not produce the xanthophylls lutein and zeaxanthin, whose key roles include ROS scavenging and prevention of ROS synthesis. Biochemical analysis revealed that singlet oxygen (1O2) accumulated to higher levels in the mutant while other ROS were unaffected, allowing to define the transcriptomic signature of the acclimatory response mediated by 1O2 which is enhanced by the lack of these xanthophylls species. The group of genes differentially regulated in npq1lut2 is enriched in sequences encoding chloroplast proteins involved in cell protection against the damaging effect of ROS. Among the early fine-tuned components, are proteins involved in tetrapyrrole biosynthesis, chlorophyll catabolism, protein import, folding and turnover, synthesis and membrane insertion of photosynthetic subunits. Up to now, the flu mutant was the only biological system adopted to define the regulation of gene expression by 1O2. In this work, we propose the use of mutants accumulating 1O2 by mechanisms different from those activated in flu to better identify ROS signalling.

CONCLUSIONS

We propose that the lack of zeaxanthin and lutein leads to 1O2 accumulation and this represents a signalling pathway in the early stages of stress acclimation, beside the response to ADP/ATP ratio and to the redox state of both plastoquinone pool. Chloroplasts respond to 1O2 accumulation by undergoing a significant change in composition and function towards a fast acclimatory response. The physiological implications of this signalling specificity are discussed.

Chlorophyll cycle regulates the construction and destruction of the light-harvesting complexes

Chlorophyll a and chlorophyll b are the major constituents of the photosynthetic apparatus in land plants and green algae. Chlorophyll a is essential in photochemistry, while chlorophyll b is apparently dispensable for their photosynthesis. Instead, chlorophyll b is necessary for stabilizing the major light-harvesting chlorophyll-binding proteins. Chlorophyll b is synthesized from chlorophyll a and is catabolized after it is reconverted to chlorophyll a. This interconversion system between chlorophyll a and chlorophyll b refers to the chlorophyll cycle. The chlorophyll b levels are determined by the activity of the three enzymes participating in the chlorophyll cycle, namely, chlorophyllide a oxygenase, chlorophyll b reductase, and 7-hydroxymethyl-chlorophyll reductase. This article reviews the recent progress on the analysis of the chlorophyll cycle and its enzymes. In particular, we emphasize the impact of genetic modification of chlorophyll cycle enzymes on the construction and destruction of the photosynthetic machinery. These studies reveal that plants regulate the construction and destruction of a specific subset of light-harvesting complexes through the chlorophyll cycle. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Characterization of singlet oxygen-accumulating mutants isolated in a screen for altered oxidative stress response in Chlamydomonas reinhardtii

BACKGROUND

When photosynthetic organisms are exposed to harsh environmental conditions such as high light intensities or cold stress, the production of reactive oxygen species like singlet oxygen is stimulated in the chloroplast. In Chlamydomonas reinhardtii singlet oxygen was shown to act as a specific signal inducing the expression of the nuclear glutathione peroxidase gene GPXH/GPX5 during high light stress, but little is known about the cellular mechanisms involved in this response. To investigate components affecting singlet oxygen signaling in C. reinhardtii, a mutant screen was performed.

RESULTS

Mutants with altered GPXH response were isolated from UV-mutagenized cells containing a GPXH-arylsulfatase reporter gene construct. Out of 5500 clones tested, no mutant deficient in GPXH induction was isolated, whereas several clones showed constitutive high GPXH expression under normal light conditions. Many of these GPXH overexpressor (gox) mutants exhibited higher resistance to oxidative stress conditions whereas others were sensitive to high light intensities. Interestingly, most gox mutants produced increased singlet oxygen levels correlating with high GPXH expression. Furthermore, different patterns of altered photoprotective parameters like non-photochemical quenching, carotenoid contents and α-tocopherol levels were detected in the various gox mutants.

CONCLUSIONS

Screening for mutants with altered GPXH expression resulted in the isolation of many gox mutants with increased singlet oxygen production, showing the relevance of controlling the production of this ROS in photosynthetic organisms. Phenotypic characterization of these gox mutants indicated that the mutations might lead to either stimulated triplet chlorophyll and singlet oxygen formation or reduced detoxification of singlet oxygen in the chloroplast. Furthermore, changes in multiple protection mechanisms might be responsible for high singlet oxygen formation and GPXH expression, which could either result from mutations in multiple loci or in a single gene encoding for a global regulator of cellular photoprotection mechanisms.

Evidence for the existence of one antenna-associated, lipid-dissolved and two protein-bound pools of diadinoxanthin cycle pigments in diatoms

We studied the localization of diadinoxanthin cycle pigments in the diatoms Cyclotella meneghiniana and Phaeodactylum tricornutum. Isolation of pigment protein complexes revealed that the majority of high-light-synthesized diadinoxanthin and diatoxanthin is associated with the fucoxanthin chlorophyll protein (FCP) complexes. The characterization of intact cells, thylakoid membranes, and pigment protein complexes by absorption and low-temperature fluorescence spectroscopy showed that the FCPs contain certain amounts of protein-bound diadinoxanthin cycle pigments, which are not significantly different in high-light and low-light cultures. The largest part of high-light-formed diadinoxanthin cycle pigments, however, is not bound to antenna apoproteins but located in a lipid shield around the FCPs, which is copurified with the complexes. This lipid shield is primarily composed of the thylakoid membrane lipid monogalactosyldiacylglycerol. We also show that the photosystem I (PSI) fraction contains a tightly connected FCP complex that is enriched in protein-bound diadinoxanthin cycle pigments. The peripheral FCP and the FCP associated with PSI are composed of different apoproteins. Tandem mass spectrometry analysis revealed that the peripheral FCP is composed mainly of the light-harvesting complex protein Lhcf and also significant amounts of Lhcr. The PSI fraction, on the other hand, shows an enrichment of Lhcr proteins, which are thus responsible for the diadinoxanthin cycle pigment binding. The existence of lipid-dissolved and protein-bound diadinoxanthin cycle pigments in the peripheral antenna and in PSI is discussed with respect to different specific functions of the xanthophylls.

Identification of the chromophores involved in aggregation-dependent energy quenching of the monomeric photosystem II antenna protein Lhcb5

Non-photochemical quenching (NPQ) of excess absorbed light energy is a fundamental process that regulates photosynthetic light harvesting in higher plants. Among several proposed NPQ mechanisms, aggregation-dependent quenching (ADQ) and charge transfer quenching have received the most attention. In vitro spectroscopic features of both mechanisms correlate with very similar signals detected in more intact systems and in vivo, where full NPQ can be observed. A major difference between the models is the proposed quenching site, which is predominantly the major trimeric light-harvesting complex II in ADQ and exclusively monomeric Lhcb proteins in charge transfer quenching. Here, we studied ADQ in both monomeric and trimeric Lhcb proteins, investigating the activities of each antenna subunit and their dependence on zeaxanthin, a major modulator of NPQ in vivo. We found that monomeric Lhcb proteins undergo stronger quenching than light-harvesting complex II during aggregation and that this is enhanced by binding to zeaxanthin, as occurs during NPQ in vivo. Finally, the analysis of Lhcb5 mutants showed that chlorophyll 612 and 613, in close contact with lutein bound at site L1, are important facilitators of ADQ.

Proton equilibration in the chloroplast modulates multiphasic kinetics of nonphotochemical quenching of fluorescence in plants

In plants, the major route for dissipating excess light is the nonphotochemical quenching of absorbed light (NPQ), which is associated with thylakoid lumen acidification. Our data offer an interpretation for the complex relationship between changes in luminal pH and the NPQ response. Upon steady-state illumination, fast NPQ relaxation in the dark reflects the equilibration between the electrochemical proton gradient established in the light and the cellular ATP/ADP+Pi ratio. This is followed by a slower phase, which reflects the decay of the proton motive force at equilibrium, due to gradual cellular ATP consumption. In transient conditions, a sustained lag appears in both quenching onset and relaxation, which is modulated by the size of the antenna complexes of photosystem II and by cyclic electron flow around photosystem I. We propose that this phenomenon reflects the signature of protonation of specific domains in the antenna and of slow H(+) diffusion in the different domains of the chloroplast.

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.

Mutation analysis of violaxanthin de-epoxidase identifies substrate-binding sites and residues involved in catalysis

Plants are able to deal with variable environmental conditions; when exposed to strong illumination, they safely dissipate excess energy as heat and increase their capacity for scavenging reacting oxygen species. Both these protection mechanisms involve activation of the xanthophyll cycle, in which the carotenoid violaxanthin is converted to zeaxanthin by violaxanthin de-epoxidase, using ascorbate as the source of reducing power. In this work, following determination of the three-dimensional structure of the violaxanthin de-epoxidase catalytic domain, we identified the putative binding sites for violaxanthin and ascorbate by in silico docking. Amino acid residues lying in close contact with the two substrates were analyzed for their involvement in the catalytic mechanism. Experimental results supported the proposed substrate-binding sites and point to two residues, Asp-177 and Tyr-198, which are suggested to participate in the catalytic mechanism, based on complete loss of activity in mutant proteins. The role of other residues and the mechanistic similarity to aspartic proteases and epoxide hydrolases are discussed.