The xanthophyll cycle of higher plants: influence of antenna size and membrane organization

Leaky mutations in the zeaxanthin epoxidase in Capsicum annuum result in bright-red fruit containing a high amount of zeaxanthin

Fruit color is one of the most important traits in peppers due to its esthetic value and nutritional benefits and is determined by carotenoid composition, resulting from diverse mutations of carotenoid biosynthetic genes. The EMS204 line, derived from an EMS mutant population, presents bright-red color, compared with the wild type Yuwolcho cultivar. HPLC analysis indicates that EMS204 fruit contains more zeaxanthin and less capsanthin and capsorubin than Yuwolcho. MutMap was used to reveal the color variation of EMS204 using an F3 population derived from a cross of EMS204 and Yuwolcho, and the locus was mapped to a 2.5-Mbp region on chromosome 2. Among the genes in the region, a missense mutation was found in ZEP (zeaxanthin epoxidase) that results in an amino acid sequence alteration (V291 → I). A color complementation experiment with Escherichia coli and ZEP in vitro assay using thylakoid membranes revealed decreased enzymatic activity of EMS204 ZEP. Analysis of endogenous plant hormones revealed a significant reduction in abscisic acid content in EMS204. Germination assays and salinity stress experiments corroborated the lower ABA levels in the seeds. Virus-induced gene silencing showed that ZEP silencing also results in bright-red fruit containing less capsanthin but more zeaxanthin than control. A germplasm survey of red color accessions revealed no similar carotenoid profiles to EMS204. However, a breeding line containing a ZEP mutation showed a very similar carotenoid profile to EMS204. Our results provide a novel breeding strategy to develop red pepper cultivars containing high zeaxanthin contents using ZEP mutations.

The Amount of Zeaxanthin Epoxidase But Not the Amount of Violaxanthin De-Epoxidase Is a Critical Determinant of Zeaxanthin Accumulation in Arabidopsis thaliana and Nicotiana tabacum

The generation of violaxanthin (Vx) de-epoxidase (VDE), photosystem II subunit S (PsbS) and zeaxanthin (Zx) epoxidase (ZEP) (VPZ) lines, which simultaneously overexpress VDE, PsbS and ZEP, has been successfully used to accelerate the kinetics of the induction and relaxation of non-photochemical quenching (NPQ). Here, we studied the impact of the overexpression of VDE and ZEP on the conversion of the xanthophyll cycle pigments in VPZ lines of Arabidopsis thaliana and Nicotiana tabacum. The protein amount of both VDE and ZEP was determined to be increased to about 3- to 5-fold levels of wild-type (WT) plants for both species. Compared to WT plants, the conversion of Vx to Zx, and hence VDE activity, was only marginally accelerated in VPZ lines, whereas the conversion of Zx to Vx, and thus ZEP activity, was strongly increased in VPZ lines. This indicates that the amount of ZEP but not the amount of VDE is a critical determinant of the equilibrium of the de-epoxidation state of xanthophyll cycle pigments under saturating light conditions. Comparing the two steps of epoxidation, particularly the second step (antheraxanthin to Vx) was found to be accelerated in VPZ lines, implying that the intermediate Ax is released into the membrane during epoxidation by ZEP.

Zeaxanthin Epoxidase Activity Is Downregulated by Hydrogen Peroxide

The xanthophyll zeaxanthin (Zx) serves important photoprotective functions in chloroplasts and is particularly involved in the dissipation of excess light energy as heat in the antenna of photosystem II (PSII). Zx accumulates under high-light (HL) conditions in thylakoid membranes and is reconverted to violaxanthin by Zx epoxidase (ZEP) in low light or darkness. ZEP activity is completely inhibited under long-lasting HL stress, and the ZEP protein becomes degraded along with the PSII subunit D1 during photoinhibition of PSII. This ZEP inactivation ensures that high levels of Zx are maintained under harsh HL stress. The mechanism of ZEP inactivation is unknown. Here, we investigated ZEP inactivation by reactive oxygen species (ROS) under in vitro conditions. Our results show that ZEP activity is completely inhibited by hydrogen peroxide (H2O2), whereas inhibition by singlet oxygen or superoxide seems rather unlikely. Due to the limited information about the amount of singlet oxygen and superoxide accumulating under the applied experimental conditions, however, a possible inhibition of ZEP activity by these two ROS cannot be generally excluded. Despite this limitation, our data support the hypothesis that the accumulation of ROS, in particular H2O2, might be responsible for HL-induced inactivation of ZEP under in vivo conditions.

Xanthophyll cycles in the juniper haircap moss (Polytrichum juniperinum) and Antarctic hair grass (Deschampsia antarctica) on Livingston Island (South Shetland Islands, Maritime Antarctica)

The summer climate in Maritime Antarctica is characterised by high humidity and cloudiness with slightly above zero temperatures. Under such conditions, photosynthetic activity is temperature-limited and plant communities are formed by a few species. These conditions could prevent the operation of the photoprotective xanthophyll (VAZ) cycle as low irradiance reduces the excess of energy and low temperatures limit enzyme activity. The VAZ cycle regulates the dissipation of the excess of absorbed light as heat, which is the main mechanism of photoprotection in plants. To test whether this mechanism operates dynamically in Antarctic plant communities, we characterised pigment dynamics under natural field conditions in two representative species: the moss Polytrichum juniperinum and the grass Deschampsia antarctica. Pigment analyses revealed that the total VAZ pool was in the upper range of the values reported for most plant species, suggesting that they are exposed to a high degree of environmental stress. Despite cloudiness, there was a strong conversion of violaxanthin (V) to zeaxanthin (Z) during daytime. Conversely, the dark-induced enzymatic epoxidation back to V was not limited by nocturnal temperatures. In contrast with plants from other cold ecosystems, we did not find any evidence of overnight retention of Z or sustained reductions in photochemical efficiency. These results are of interest for modelling, remote sensing and upscaling of the responses of Antarctic vegetation to environmental challenges.

Protein dynamics and lipid affinity of monomeric, zeaxanthin-binding LHCII in thylakoid membranes

The xanthophyll cycle in the antenna of photosynthetic organisms under light stress is one of the most well-known processes in photosynthesis, but its role is not well understood. In the xanthophyll cycle, violaxanthin (Vio) is reversibly transformed to zeaxanthin (Zea) that occupies Vio binding sites of light-harvesting antenna proteins. Higher monomer/trimer ratios of the most abundant light-harvesting protein, the light-harvesting complex II (LHCII), usually occur in Zea accumulating membranes and have been observed in plants after prolonged illumination and during high-light acclimation. We present a combined NMR and coarse-grained simulation study on monomeric LHCII from the npq2 mutant that constitutively binds Zea in the Vio binding pocket. LHCII was isolated from ¹³C-enriched npq2 Chlamydomonas reinhardtii (Cr) cells and reconstituted in thylakoid lipid membranes. NMR results reveal selective changes in the fold and dynamics of npq2 LHCII compared with the trimeric, wild-type and show that npq2 LHCII contains multiple mono- or digalactosyl diacylglycerol lipids (MGDG and DGDG) that are strongly protein bound. Coarse-grained simulations on npq2 LHCII embedded in a thylakoid lipid membrane agree with these observations. The simulations show that LHCII monomers have more extensive lipid contacts than LHCII trimers and that protein-lipid contacts are influenced by Zea. We propose that both monomerization and Zea binding could have a functional role in modulating membrane fluidity and influence the aggregation and conformational dynamics of LHCII with a likely impact on photoprotection ability.

Acclimation of photosynthetic apparatus in the mesophilic red alga Dixoniella giordanoi

Eukaryotic algae are photosynthetic organisms capable of exploiting sunlight to fix carbon dioxide into biomass with highly variable genetic and metabolic features. Information on algae metabolism from different species is inhomogeneous and, while green algae are, in general, more characterized, information on red algae is relatively scarce despite their relevant position in eukaryotic algae diversity. Within red algae, the best-known species are extremophiles or multicellular, while information on mesophilic unicellular organisms is still lacunose. Here, we investigate the photosynthetic properties of a recently isolated seawater unicellular mesophilic red alga, Dixoniella giordanoi. Upon exposure to different illuminations, D. giordanoi shows the ability to acclimate, modulate chlorophyll content, and re-organize thylakoid membranes. Phycobilisome content is also largely regulated, leading to almost complete disassembly of this antenna system in cells grown under intense illumination. Despite the absence of a light-induced xanthophyll cycle, cells accumulate zeaxanthin upon prolonged exposure to strong light, likely contributing to photoprotection. D. giordanoi cells show the ability to perform cyclic electron transport that is enhanced under strong illumination, likely contributing to the protection of Photosystem I from over-reduction and enabling cells to survive PSII photoinhibition without negative impact on growth.

Accumulation of geranylgeranylated chlorophylls in the pigment-protein complexes of Arabidopsis thaliana acclimated to green light: effects on the organization of light-harvesting complex II and photosystem II functions

Light quality significantly influences plant metabolism, growth and development. Recently, we have demonstrated that leaves of barley and other plant species grown under monochromatic green light (500-590 nm) accumulated a large pool of chlorophyll a (Chl a) intermediates with incomplete hydrogenation of their phytyl chains. In this work, we studied accumulation of these geranylgeranylated Chls a and b in pigment-protein complexes (PPCs) of Arabidopsis plants acclimated to green light and their structural-functional consequences on the photosynthetic apparatus. We found that geranylgeranylated Chls are present in all major PPCs, although their presence was more pronounced in light-harvesting complex II (LHCII) and less prominent in supercomplexes of photosystem II (PSII). Accumulation of geranylgeranylated Chls hampered the formation of PSII and PSI super- and megacomplexes in the thylakoid membranes as well as their assembly into chiral macrodomains; it also lowered the temperature stability of the PPCs, especially that of LHCII trimers, which led to their monomerization and an anomaly in the photoprotective mechanism of non-photochemical quenching. Role of geranylgeranylated Chls in adverse effects on photosynthetic apparatus of plants acclimated to green light is discussed.

Single-Walled Carbon Nanotubes Modify Leaf Micromorphology, Chloroplast Ultrastructure and Photosynthetic Activity of Pea Plants

Single-walled carbon nanotubes (SWCNTs) emerge as promising novel carbon-based nanoparticles for use in biomedicine, pharmacology and precision agriculture. They were shown to penetrate cell walls and membranes and to physically interact and exchange electrons with photosynthetic complexes in vitro. Here, for the first time, we studied the concentration-dependent effect of foliar application of copolymer-grafted SWCNTs on the structural and functional characteristics of intact pea plants. The lowest used concentration of 10 mg L-1 did not cause any harmful effects on the studied leaf characteristics, while abundant epicuticular wax generation on both leaf surfaces was observed after 300 mg L-1 treatment. Swelling of both the granal and the stromal regions of thylakoid membranes was detected after application of 100 mg L-1 and was most pronounced after 300 mg L-1. Higher SWCNT doses lead to impaired photosynthesis in terms of lower proton motive force generation, slower generation of non-photochemical quenching and reduced zeaxanthin content; however, the photosystem II function was largely preserved. Our results clearly indicate that SWCNTs affect the photosynthetic apparatus in a concentration-dependent manner. Low doses (10 mg L-1) of SWCNTs appear to be a safe suitable object for future development of nanocarriers for substances that are beneficial for plant growth.

Shedding light on the dark side of xanthophyll cycles

Xanthophyll cycles are broadly important in photoprotection, and the reversible de-epoxidation of xanthophylls typically occurs in excess light conditions. However, as presented in this review, compiling evidence in a wide range of photosynthetic eukaryotes shows that xanthophyll de-epoxidation also occurs under diverse abiotic stress conditions in darkness. Light-driven photochemistry usually leads to the pH changes that activate de-epoxidases (e.g. violaxanthin de-epoxidase), but in darkness alternative electron transport pathways and luminal domains enriched in monogalactosyl diacyl glycerol (which enhance de-epoxidase activity) likely enable de-epoxidation. Another 'dark side' to sustaining xanthophyll de-epoxidation is inactivation and/or degradation of epoxidases (e.g. zeaxanthin epoxidase). There are obvious benefits of such activity regarding stress tolerance, and indeed this phenomenon has only been reported in stressful conditions. However, more research is required to unravel the mechanisms and understand the physiological roles of dark-induced formation of zeaxanthin. Notably, the de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin in darkness is still a frequently ignored process, perhaps because it questions a previous paradigm. With that in mind, this review seeks to shed some light on the dark side of xanthophyll de-epoxidation, and point out areas for future work.

Exogenous application of cytokinin during dark senescence eliminates the acceleration of photosystem II impairment caused by chlorophyll b deficiency in barley

Recent studies have shown that chlorophyll (Chl) b has an important role in the regulation of leaf senescence. However, there is only limited information about senescence of plants lacking Chl b and senescence-induced decrease in photosystem II (PSII) and photosystem I (PSI) function has not even been investigated in such plants. We have studied senescence-induced changes in photosynthetic pigment content and PSII and PSI activities in detached leaves of Chl b-deficient barley mutant, chlorina f2^f2 (clo). After 4 days in the dark, the senescence-induced decrease in PSI activity was smaller in clo compared to WT leaves. On the contrary, the senescence-induced impairment in PSII function (estimated from Chl fluorescence parameters) was much more pronounced in clo leaves, even though the relative decrease in Chl content was similar to wild type (WT) leaves (Hordeum vulgare L., cv. Bonus). The stronger impairment of PSII function seems to be related to more pronounced damage of reaction centers of PSII. Interestingly, exogenously applied plant hormone cytokinin 6-benzylaminopurine (BA) was able to maintain PSII function in the dark senescing clo leaves to a similar extent as in WT. Thus, considering the fact that without BA the senescence-induced decrease in PSII photochemistry in clo was more pronounced than in WT, the relative protective effect of BA was higher in Chl b-deficient mutant than in WT.

Monochromatic green light induces an aberrant accumulation of geranylgeranyled chlorophylls in plants

Light quality is an important environmental factor affecting the biosynthesis of photosynthetic pigments whose production seems to be affected not only quantitatively but also qualitatively. In this work, we set out to identify unusual pigment detected in leaves of barley (Hordeum vulgare L.) and explain its presence in plants grown under monochromatic green light (GL; 500-590 nm). The chromatographic analysis (HPLC-DAD) revealed that a peak belonging to this unknown pigment is eluted between chlorophyll (Chl) a and b. This pigment exhibited the same absorption spectrum and fluorescence excitation and emission spectra as Chl a. It was negligible in control plants cultivated under white light of the same irradiance (photosynthetic photon flux density of 240 μmol m^-2 s^-1). Mass spectrometry analysis of this pigment (ions m/z = 889 [M-H]^-; m/z = 949 [M+acetic acid-H]^-) indicates that it is Chl a with a tetrahydrogengeranylgeraniol side chain (containing two double bonds in a phytyl side chain; Chl a_THGG), which is an intermediate in Chl a synthesis. In plants grown under GL, the proportion of Chl a_THGG to total Chl content rose to approximately 8% and 16% after 7 and 14 days of cultivation, respectively. Surprisingly, plants cultivated under GL exhibited drastically increased concentration of the enzyme geranylgeranyl reductase, which is responsible for the reduction of phytyl chain double bonds in the Chl synthesis pathway. This indicates impaired activity of this enzyme in GL-grown plants. A similar effect of GL on Chl synthesis was observed for distinct higher plant species.

Conservation of core complex subunits shaped the structure and function of photosystem I in the secondary endosymbiont alga Nannochloropsis gaditana

Photosystem I (PSI) is a pigment protein complex catalyzing the light-driven electron transport from plastocyanin to ferredoxin in oxygenic photosynthetic organisms. Several PSI subunits are highly conserved in cyanobacteria, algae and plants, whereas others are distributed differentially in the various organisms. Here we characterized the structural and functional properties of PSI purified from the heterokont alga Nannochloropsis gaditana, showing that it is organized as a supercomplex including a core complex and an outer antenna, as in plants and other eukaryotic algae. Differently from all known organisms, the N. gaditana PSI supercomplex contains five peripheral antenna proteins, identified by proteome analysis as type-R light-harvesting complexes (LHCr4-8). Two antenna subunits are bound in a conserved position, as in PSI in plants, whereas three additional antennae are associated with the core on the other side. This peculiar antenna association correlates with the presence of PsaF/J and the absence of PsaH, G and K in the N. gaditana genome and proteome. Excitation energy transfer in the supercomplex is highly efficient, leading to a very high trapping efficiency as observed in all other PSI eukaryotes, showing that although the supramolecular organization of PSI changed during evolution, fundamental functional properties such as trapping efficiency were maintained.

Photoacclimation of photosynthesis in the Eustigmatophycean Nannochloropsis gaditana

Nannochloropsis is an eukaryotic alga of the phylum Heterokonta, originating from a secondary endosymbiotic event. In this work, we investigated how the photosynthetic apparatus responds to growth in different light regimes in Nannochloropsis gaditana. We found that intense illumination induces the decrease of both photosystem I and II contents and their respective antenna sizes. Cells grown in high light showed a larger capacity for electron transport, with enhanced cyclic electron transport around photosystem I, contributing to photoprotection from excess illumination. Even when exposed to excess light intensities for several days, N. gaditana cells did not activate constitutive responses such as nonphotochemical quenching and the xanthophyll cycle. These photoprotection mechanisms in N. gaditana thus play a role in acclimation to fast changes in illumination within a time range of minutes, while regulation of the electron flow capacity represents a long-term response to prolonged exposure to excess light.

Photosynthesis in extreme environments: responses to different light regimes in the Antarctic alga Koliella antarctica

Antarctic algae play a fundamental role in polar ecosystem thanks to their ability to grow in an extreme environment characterized by low temperatures and variable illumination. Here, for prolonged periods, irradiation is extremely low and algae must be able to harvest light as efficiently as possible. On the other side, at low temperatures even dim irradiances can saturate photosynthesis and drive to the formation of reactive oxygen species. Colonization of this extreme environment necessarily required the optimization of photosynthesis regulation mechanisms by algal organisms. In order to investigate these adaptations we analyzed the time course of physiological and morphological responses to different irradiances in Koliella antarctica, a green microalga isolated from Ross Sea (Antarctica). Koliella antarctica not only modulates cell morphology and composition of its photosynthetic apparatus on a long-term acclimation, but also shows the ability of a very fast response to light fluctuations. Koliella antarctica controls the activity of two xanthophyll cycles. The first, involving lutein epoxide and lutein, may be important for the growth under very low irradiances. The second, involving conversion of violaxanthin to antheraxanthin and zeaxanthin, is relevant to induce a fast and particularly strong non-photochemical quenching, when the alga is exposed to higher light intensities. Globally K. antarctica thus shows the ability to activate a palette of responses of the photosynthetic apparatus optimized for survival in its natural extreme environment.

Tissue-specific accumulation and regulation of zeaxanthin epoxidase in Arabidopsis reflect the multiple functions of the enzyme in plastids

The enzyme zeaxanthin epoxidase (ZEP) catalyzes the conversion of zeaxanthin to violaxanthin, a key reaction for ABA biosynthesis and the xanthophyll cycle. Both processes are important for acclimation to environmental stress conditions, in particular drought (ABA biosynthesis) and light (xanthophyll cycle) stress. Hence, both ZEP functions may require differential regulation to optimize plant fitness. The key to understanding the function of ZEP in both stress responses might lie in its spatial and temporal distribution in plant tissues. Therefore, we analyzed the distribution of ZEP in plant tissues and plastids under drought and light stress by use of a ZEP-specific antibody. In addition, we determined the pigment composition of the plant tissues and chloroplast membrane subcompartments in response to these stresses. The ZEP protein was detected in all plant tissues (except flowers) concomitant with xanthophylls. The highest levels of ZEP were present in leaf chloroplasts and root plastids. Within chloroplasts, ZEP was localized predominantly in the thylakoid membrane and stroma, while only a small fraction was bound by the envelope membrane. Light stress affected neither the accumulation nor the relative distribution of ZEP in chloroplasts, while drought stress led to an increase of ZEP in roots and to a degradation of ZEP in leaves. However, drought stress-induced increases in ABA were similar in both tissues. These data support a tissue- and stress-specific accumulation of the ZEP protein in accordance with its different functions in ABA biosynthesis and the xanthophyll cycle.

Response of green reflectance continuum removal index to the xanthophyll de-epoxidation cycle in Norway spruce needles

A dedicated field experiment was conducted to investigate the response of a green reflectance continuum removal-based optical index, called area under the curve normalized to maximal band depth between 511 nm and 557 nm (ANMB511-557), to light-induced transformations in xanthophyll cycle pigments of Norway spruce [Picea abies (L.) Karst] needles. The performance of ANMB511-557 was compared with the photochemical reflectance index (PRI) computed from the same leaf reflectance measurements. Needles of four crown whorls (fifth, eighth, 10th, and 15th counted from the top) were sampled from a 27-year-old spruce tree throughout a cloudy and a sunny day. Needle optical properties were measured together with the composition of the photosynthetic pigments to investigate their influence on both optical indices. Analyses of pigments showed that the needles of the examined whorls varied significantly in chlorophyll content and also in related pigment characteristics, such as the chlorophyll/carotenoid ratio. The investigation of the ANMB511-557 diurnal behaviour revealed that the index is able to follow the dynamic changes in the xanthophyll cycle independently of the actual content of foliar pigments. Nevertheless, ANMB511-557 lost the ability to predict the xanthophyll cycle behaviour during noon on the sunny day, when the needles were exposed to irradiance exceeding 1000 µmol m(-2) s(-1). Despite this, ANMB511-557 rendered a better performance for tracking xanthophyll cycle reactions than PRI. Although declining PRI values generally responded to excessive solar irradiance, they were not able to predict the actual de-epoxidation state in the needles examined.

The response of Nannochloropsis gaditana to nitrogen starvation includes de novo biosynthesis of triacylglycerols, a decrease of chloroplast galactolipids, and reorganization of the photosynthetic apparatus

Microalgae of the genus Nannochloropsis are capable of accumulating triacylglycerols (TAGs) when exposed to nutrient limitation (in particular, nitrogen [N]) and are therefore considered promising organisms for biodiesel production. Here, after nitrogen removal from the medium, Nannochloropsis gaditana cells showed extensive triacylglycerol accumulation (38% TAG on a dry weight basis). Triacylglycerols accumulated during N deprivation harbored signatures, indicating that they mainly stemmed from freshly synthesized fatty acids, with a small proportion originating from a recycling of membrane glycerolipids. The amount of chloroplast galactoglycerolipids, which are essential for the integrity of thylakoids, decreased, while their fatty acid composition appeared to be unaltered. In starved cells, galactolipids were kept at a level sufficient to maintain chloroplast integrity, as confirmed by electron microscopy. Consistently, N-starved Nannochloropsis cells contained less photosynthetic membranes but were still efficiently performing photosynthesis. N starvation led to a modification of the photosynthetic apparatus with a change in pigment composition and a decrease in the content of all the major electron flow complexes, including photosystem II, photosystem I, and the cytochrome b(6)f complex. The photosystem II content was particularly affected, leading to the inhibition of linear electron flow from water to CO(2). Such a reduction, however, was partially compensated for by activation of alternative electron pathways, such as cyclic electron transport. Overall, these changes allowed cells to modify their energetic metabolism in order to maintain photosynthetic growth.

Reflectance continuum removal spectral index tracking the xanthophyll cycle photoprotective reactions in Norway spruce needles

This laboratory experiment tested the ability of the spectral index called 'area under curve normalised to maximal band depth' (ANMB) to track dynamic changes in the xanthophyll cycle of Norway spruce (Picea abies (L.) Karsten) needles. Four-year-old spruce seedlings were gradually acclimated to different photosynthetic photon flux densities (PPFDs) and air temperature regimes. The measurements were conducted at the end of each acclimation period lasting for 11 days. A significant decline in the chlorophylls to carotenoids ratio and the increase of the amount of xanthophyll cycle pigments indicated a higher need for carotenoid-mediated photoprotection in spruce leaves acclimated to high PPFD conditions. Similarly, the photochemical reflectance index (PRI) changed from positive to negative values after changing light conditions from low to high intensity as a consequence of the increase in carotenoid content. Systematic responses of PRI to the de-epoxidation state of xanthophyll cycle pigments (DEPS) were, however, observed only during high temperature treatments and after the exposition of needles to high irradiance. The ANMB index computed from needle reflectance between 507 and 556nm was able to track dynamic changes in DEPS without any influence induced by changing the content of leaf photosynthetic pigments (chlorophylls, carotenoids).

Non-photochemical fluorescence quenching in Chromera velia is enabled by fast violaxanthin de-epoxidation

Non-photochemical quenching (NPQ) is a mechanism protecting photosynthetic organisms against excessive irradiation. Here, we analyze a unique NPQ mechanism in the alga Chromera velia, a recently discovered close relative of apicomplexan parasites. NPQ in C. velia is enabled by an operative and fast violaxanthin de-epoxidation to zeaxanthin without accumulation of antheraxanthin. In C. velia violaxanthin also serves as a main light-harvesting pigment. Therefore, in C. velia violaxanthin acts as a key factor in both light harvesting and photoprotection. This is in contrast to a similar alga, Nannochloropsis limnetica, where violaxanthin has only light-harvesting function.

Photosystem II fluorescence lifetime imaging in avocado leaves: contributions of the lutein-epoxide and violaxanthin cycles to fluorescence quenching

Lifetime-resolved imaging measurements of chlorophyll a fluorescence were made on leaves of avocado plants to study whether rapidly reversible ΔpH-dependent (transthylakoid H(+) concentration gradient) thermal energy dissipation (qE) and slowly reversible ΔpH-independent fluorescence quenching (qI) are modulated by lutein-epoxide and violaxanthin cycles operating in parallel. Under normal conditions (without inhibitors), analysis of the chlorophyll a fluorescence lifetime data revealed two major lifetime pools (1.5 and 0.5 ns) for photosystem II during the ΔpH build-up under illumination. Formation of the 0.5-ns pool upon illumination was correlated with dark-retention of antheraxanthin and photo-converted lutein in leaves. Interconversion between the 1.5- and 0.5-ns lifetime pools took place during the slow part of the chlorophyll a fluorescence transient: first from 1.5 ns to 0.5 ns in the P-to-S phase, then back from 0.5 ns to 1.5 ns in the S-to-M phase. When linear electron transport and the resulting ΔpH build-up were inhibited by treatment with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), the major fluorescence intensity was due to a 2.2-ns lifetime pool with a minor faster contribution of approximately 0.7 ns. In the presence of DCMU, neither the intensity nor the lifetimes of fluorescence were affected by antheraxanthin and photo-converted lutein. Thus, we conclude that both antheraxanthin and photo-converted lutein are able to enhance ΔpH-dependent qE processes that are associated with the 0.5-ns lifetime pool. However, unlike zeaxanthin, retention of antheraxanthin and photo-converted lutein may not by itself stabilize quenching or cause qI.

Acute exposure to UV-B sensitizes cucumber, tomato, and Arabidopsis plants to photooxidative stress by inhibiting thermal energy dissipation and antioxidant defense

To characterize a change in NPQ upon exposure to ultraviolet-B (UV-B), the xanthophyll cycle-dependent and -independent NPQs were compared in Cucumis sativus, Lycopersicum esculentum, and Arabidopsis thaliana leaves. The xanthophyll cycle-dependent NPQ was dramatically but reversibly suppressed by UV-B radiation. This suppression was correlated more strongly with a marked decrease in photosynthetic electron transport rather than changes in xanthophyll cycle enzymes such as violaxanthin de-epoxidase and zeaxanthin epoxidase. Accordingly, the UV-B-induced suppression of NPQ cannot be attributed to changes in expressions of VDE and ZEP. However, suppression of the xanthophyll cycle-dependent NPQ could only account for the 77 K fluorescence emission spectra of thylakoid membranes and the increased level of (1)O(2) production, but not for the decreased levels of •O(2)(-) production and H(2)O(2) scavenging. These results suggest that a gradual reduction of H(2)O(2) scavenging activity as well as a transient and reversible suppression of thermal energy dissipation may contribute differentially to increased photooxidative damages in cucumber, tomato, and Arabidopsis plants after acute exposure to UV-B radiation.

Carotenoid biosynthesis in diatoms

Diatoms are ubiquitous and constitute an important group of the phytoplankton community having a major contribution to the total marine primary production. These microalgae exhibit a characteristic golden-brown colour due to a high amount of the xanthophyll fucoxanthin that plays a major role in the light-harvesting complex of photosystems. In the water column, diatoms are exposed to light intensities that vary quickly from lower to higher values. Xanthophyll cycles prevent photodestruction of the cells in excessive light intensities. In diatoms, the diadinoxanthin-diatoxanthin cycle is the most important short-term photoprotective mechanism. If the biosynthetic pathways of chloroplast pigments have been extensively studied in higher plants and green algae, the research on carotenoid biosynthesis in diatoms is still in its infancy. In this study, the data on the biosynthetic pathway of diatom carotenoids are reviewed. The early steps occur through the 2-C-methyl-D: -erythritol 4-phosphate (MEP) pathway. Then a hypothetical pathway is suggested from dimethylallyl diphosphate (DMAPP) and isopentenyl pyrophosphate (IPP). Most of the enzymes of the pathway have not been so far isolated from diatoms, but candidate genes for each of them were identified using protein similarity searches of genomic data.

Acclimation of Norway spruce photosynthetic apparatus to the combined effect of high irradiance and temperature

Diurnal courses of photosynthetic gas exchange parameters, chlorophyll a fluorescence characteristics and the de-epoxidation state of the xanthophyll cycle pigments (DEPS) were measured during the gradual acclimation of 4-year-old Norway spruce seedlings to different photosynthetic photon flux density (PPFD) and air temperature (T(air)) regimes, simulating cloudy days with moderate T(air) (LI, maximum PPFD 300 micromol m(-2)s(-1), T(air) range 15-25 degrees C), sunny days with moderate T(air) (HI, maximum PPFD 1000 micromol m(-2)s(-1), T(air) range 15-25 degrees C) and hot sunny days (HI-HT, maximum PPFD 1000 micromol m(-2)s(-1), T(air) range 20-35 degrees C). The plants were acclimated inside a growth chamber and each acclimation regime lasted for 13d. Acclimation to HI conditions led to a strong depression of the net CO(2) assimilation rates (A(N)), particularly during noon and afternoon periods. Exposure to the HI-HT regime led to a further decrease of A(N) even during the morning period. Insufficient stomatal conductance was found to be the main reason for depressed A(N) under HI and HI-HT conditions. Only slight changes of the maximum photosystem II (PSII) photochemical efficiency (F(V)/F(M)), in the range of 0.78-0.82, supported the resistance of the Norway spruce photosynthetic apparatus against PSII photoinhibition during acclimation to both HI and HI-HT conditions. The HI plants showed increased content of xanthophyll cycle pigments (VAZ) and enhanced efficiency of thermal energy dissipation within PSII (D) that closely correlated with the increased DEPS. In contrast, acclimation to the HI-HT regime resulted in a slight reduction of VAZ content and significantly diminished D and DEPS values during the entire day in comparison with HI plants. These results indicate a minor role of the xanthophyll cycle-mediated thermal dissipation in PSII photoprotection under elevated temperatures. The different contributions of the thermal dissipation and non-assimilatory electron transport pathways in PSII photoprotection during acclimation of the Norway spruce photosynthetic apparatus to excess irradiance and heat stresses are discussed.

Dynamics of the xanthophyll cycle and non-radiative dissipation of absorbed light energy during exposure of Norway spruce to high irradiance

The response of Norway spruce saplings (Picea abies [L.] Karst.) was monitored continuously during short-term exposure (10 days) to high irradiance (HI; 1000micromol m(-2)s(-1)). Compared with plants acclimated to low irradiance (100micromol m(-2)s(-1)), plants after HI exposure were characterized by a significantly reduced CO(2) assimilation rate throughout the light response curve. Pigment contents varied only slightly during HI exposure, but a rapid and strong response was observed in xanthophyll cycle activity, particularly within the first 3 days of the HI treatment. Both violaxanthin convertibility under HI and the amount of zeaxanthin pool sustained in darkness increased markedly under HI conditions. These changes were accompanied by an enhanced non-radiative dissipation of absorbed light energy (NRD) and the acceleration of induction of both NRD and de-epoxidation of the xanthophyll cycle pigments. We found a strong negative linear correlation between the amount of sustained de-epoxidized xanthophylls and the photosystem II (PSII) photochemical efficiency (F(V)/F(M)), indicating photoprotective down-regulation of the PSII function. Recovery of F(V)/F(M) at the end of the HI treatment revealed that Norway spruce was able to cope with a 10-fold elevated irradiance due particularly to an efficient NRD within the PSII antenna that was associated with enhanced violaxanthin convertibility and a light-induced accumulation of zeaxanthin that persisted in darkness.

Short-term down-regulation of zeaxanthin epoxidation in Arabidopsis thaliana in response to photo-oxidative stress conditions

The epoxidation of zeaxanthin (Zx) to violaxanthin after exposure to different light stress conditions has been studied in Arabidopsis (Arabidopsis thaliana). Formation of Zx was induced by illumination of intact leaves for up to 8 h at different light intensities and temperatures. The kinetics of epoxidation was found to be gradually retarded with increasing light stress during pre-illumination, indicating a gradual down-regulation of the Zx epoxidase activity. Retardation of the epoxidation rates by a factor of up to 10 was inducible either by increasing the light intensity or by extending the illumination time or by decreasing the temperature during pre-illumination. The retardation of the epoxidation kinetics was correlated with a decrease of the PSII quantum efficiency after the pre-illumination treatment. Experiments with the stn7/stn8 mutant of Arabidopsis indicated that the thylakoid protein kinases STN7 and STN8, which are required for the phosphorylation of PSII proteins, are not involved in the short-term down-regulation of Zx epoxidation. However, the retardation of Zx epoxidation was maintained in thylakoids isolated from pre-illuminated leaves, indicating that a direct modification of the Zx epoxidase is most likely involved in the light-induced down-regulation.

The giant kelp Macrocystis pyrifera presents a different nonphotochemical quenching control than higher plants

Here the mechanisms involved in excitation energy dissipation of Macrocystis pyrifera were characterized to explain the high nonphotochemical quenching of chlorophyll a (Chla) fluorescence (NPQ) capacity of this alga. We performed a comparative analysis of NPQ and xanthophyll cycle (XC) activity in blades collected at different depths. The responses of the blades to dithiothreitol (DTT) and to the uncoupler NH4Cl were also assayed. The degree of NPQ induction was related to the amount of zeaxanthin synthesized in high light. The inhibition of zeaxanthin synthesis with DTT blocked NPQ induction. A slow NPQ relaxation upon the addition of NH4Cl, which disrupts the transthylakoid proton gradient, was detected. The slow NPQ relaxation took place only in the presence of de-epoxidated XC pigments and was related to the epoxidation of zeaxanthin. These results indicate that in M. pyrifera, in contrast to higher plants, the transthylakoid proton gradient alone does not induce NPQ. The role of this gradient seems to be related only to the activation of the violaxanthin de-epoxidase enzyme.

Characterization of a nonphotochemical quenching-deficient Arabidopsis mutant possessing an intact PsbS protein, xanthophyll cycle and lumen acidification

Arabidopsis thaliana plants grown from ethyl methane sulfonate-treated seeds were screened for so-called que mutants, which are affected in non-photochemical energy quenching. Based on video imaging of chlorophyll fluorescence an energy dissipation mutant, que1, was identified, isolated and characterized. Similar to the npq mutants, the que1 mutant showed a drastically reduced capacity for pH-dependent energy dissipation, qE, but without affecting the Delta pH-dependent conformational changes at 535 nm (DeltaA (535)), which have been supposed to be obligatorily correlated with qE and to reflect pH-regulated binding of zeaxanthin to the PsbS protein. Western blot and DNA sequence analysis revealed that neither a reduced expression of the PsbS protein nor a mutation in the PsbS gene was responsible for the missing qE in que1. Measurements of 9-aminoacridine fluorescence quenching showed that the acidification of the thylakoid lumen was also not affected in the mutant. Furthermore, que1 was able to convert violaxanthin to zeaxanthin. However, unusual characteristics of zeaxanthin formation in the mutant pointed at an altered availability of violaxanthin for de-epoxidation. This was further accompanied by a decrease of the photochemical quenching of chlorophyll fluorescence (qP), an increase of the portion of oxidized P700 and a reduction of the electron transport rate. These characteristics indicate changes in the organization of the thylakoid membrane that affect linear electron transport (but not lumen acidification) and the formation of energy dissipation in photosystem II. Preliminary genetic analysis revealed that the phenotype of que1 is related to two different mutations, mapped to the lower arms of chromosomes 1 and 4.

In diatoms, a transthylakoid proton gradient alone is not sufficient to induce a non-photochemical fluorescence quenching

Non-photochemical fluorescence quenching (NPQ) in diatoms is associated with a xanthophyll cycle involving diadinoxanthin (DD) and its de-epoxidized form, diatoxanthin (DT). In higher plants, an obligatory role of de-epoxidized xanthophylls in NPQ remains controversial and the presence of a transthylakoid proton gradient (DeltapH) alone may induce NPQ. We used inhibitors to alter the amplitude of DeltapH and/or DD de-epoxidation, and coupled NPQ. No DeltapH-dependent quenching was detected in the absence of DT. In diatoms, both DeltapH and DT are required for NPQ. The binding of DT to protonated antenna sites could be obligatory for energy dissipation.

Xanthophyll pigments in light-harvesting complex II in monomolecular layers: localisation, energy transfer and orientation

Monomolecular layers of the largest light-harvesting pigment-protein complex of Photosystem II (LHCII) were formed at the argon-water interface. The molecular area of the LHCII monomer in monomolecular layers determined from the isotherms of compression is found to be close to 14 nm2, which corresponds well to the molecular dimensions of the protein evaluated on the basis of crystallographic studies. Monolayers of LHCII were deposited on a glass support by means of the Langmuir-Blodgett technique and subjected to spectroscopic studies: electronic absorption spectrophotometry and spectrofluorometry. The fluorescence excitation spectra of chlorophyll a in monolayers of LHCII were analysed using gaussian deconvolution. Comparison of the absorption and fluorescence excitation spectra enabled calculation of the rate of excitation energy transfer in the system. Excitation energy was found to be transferred to chlorophyll a from chlorophyll b with 97% efficiency, from neoxanthin with 85%, from lutein with 62% and from violaxanthin with at least 54% efficiency. The analysis of the position of the 0-0 absorption band of the xanthophylls revealed that neoxanthin is located in the same protein environment as lutein but in a different environment than violaxanthin. The analysis of fluorescence excitation spectra of chlorophyll a in LHCII, recorded with the excitation light beam polarised in two orthogonal directions, enabled the determination of the mean orientation angle of the accessory xanthophyll pigments with respect to the plane of the sample. The mean orientation of lutein found in this study (approx. 51 degrees ) corresponds well to the crystallographic data. Neoxanthin was found to adopt a similar orientation to lutein. The transition dipole moment of violaxanthin was found to form a mean angle of 71 degrees with the axis spanning two polar regions of the protein, perpendicular to the plane of the monolayer, suggesting planar orientation of this pigment with respect to the plane of the thylakoid membrane. These experimentally determined xanthophyll orientations are discussed in terms of importance of peripheral xanthophyll pigments in supramolecular organisation of LHCII and the operation of the xanthophyll cycle within the thylakoid membrane.

Greening under high light or cold temperature affects the level of xanthophyll-cycle pigments, early light-inducible proteins, and light-harvesting polypeptides in wild-type barley and the chlorina f2 mutant

Etiolated seedlings of wild type and the chlorina f2 mutant of barley (Hordeum vulgare) were exposed to greening at either 5 degrees C or 20 degrees C and continuous illumination varying from 50 to 800 &mgr;mol m-2 s-1. Exposure to either moderate temperature and high light or low temperature and moderate light inhibited chlorophyll a and b accumulation in the wild type and in the f2 mutant. Continuous illumination under these greening conditions resulted in transient accumulations of zeaxanthin, concomitant transient decreases in violaxanthin, and fluctuations in the epoxidation state of the xanthophyll pool. Photoinhibition-induced xanthophyll-cycle activity was detectable after only 3 h of greening at 20 degrees C and 250 &mgr;mol m-2 s-1. Immunoblot analyses of the accumulation of the 14-kD early light-inducible protein but not the major (Lhcb2) or minor (Lhcb5) light-harvesting polypeptides demonstrated transient kinetics similar to those observed for zeaxanthin accumulation during greening at either 5 degrees C or 20 degrees C for both the wild type and the f2 mutant. Furthermore, greening of the f2 mutant at either 5 degrees C or 20 degrees C indicated that Lhcb2 is not essential for the regulation of the xanthophyll cycle in barley. These results are consistent with the thesis that early light-inducible proteins may bind zeaxanthin as well as other xanthophylls and dissipate excess light energy to protect the developing photosynthetic apparatus from excess excitation. We discuss the role of energy balance and photosystem II excitation pressure in the regulation of the xanthophyll cycle during chloroplast biogenesis in wild-type barley and the f2 mutant.

The kinetics of zeaxanthin formation is retarded by dicyclohexylcarbodiimide

The de-epoxidation of violaxanthin to antheraxanthin (Anth) and zeaxanthin (Zeax) in the xanthophyll cycle of higher plants and the generation of nonphotochemical fluorescence quenching in the antenna of photosystem II (PSII) are induced by acidification of the thylakoid lumen. Dicyclohexylcarbodiimide (DCCD) has been shown (a) to bind to lumen-exposed carboxy groups of antenna proteins and (b) to inhibit the pH-dependent fluorescence quenching. The possible influence of DCCD on the de-epoxidation reactions has been investigated in isolated pea (Pisum sativum L.) thylakoids. The Zeax formation was found to be slowed down in the presence of DCCD. The second step (Anth --> Zeax) of the reaction sequence seemed to be more affected than the violaxanthin --> Anth conversion. Comparative studies with antenna-depleted thylakoids from plants grown under intermittent light and with unstacked thylakoids were in agreement with the assumption that binding of DCCD to antenna proteins is probably responsible for the retarded kinetics. Analyses of the DCCD-induced alterations in different antenna subcomplexes showed that Zeax formation in the PSII antenna proteins was predominantly influenced by DCCD, whereas Zeax formation in photosystem I was nearly unaffected. Our data support the suggestion that DCCD binding to PSII antenna proteins is responsible for the observed alterations in xanthophyll conversion.