Contrasting Behavior of Higher Plant Photosystem I and II Antenna Systems during Acclimation

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.

Identification of key residues for pH dependent activation of violaxanthin de-epoxidase from Arabidopsis thaliana

Plants are often exposed to saturating light conditions, which can lead to oxidative stress. The carotenoid zeaxanthin, synthesized from violaxanthin by Violaxanthin De-Epoxidase (VDE) plays a major role in the protection from excess illumination. VDE activation is triggered by a pH reduction in the thylakoids lumen occurring under saturating light. In this work the mechanism of the VDE activation was investigated on a molecular level using multi conformer continuum electrostatic calculations, site directed mutagenesis and molecular dynamics. The pK(a) values of residues of the inactive VDE were determined to identify target residues that could be implicated in the activation. Five such target residues were investigated closer by site directed mutagenesis, whereas variants in four residues (D98, D117, H168 and D206) caused a reduction in enzymatic activity indicating a role in the activation of VDE while D86 mutants did not show any alteration. The analysis of the VDE sequence showed that the four putative activation residues are all conserved in plants but not in diatoms, explaining why VDE in these algae is already activated at higher pH. Molecular dynamics showed that the VDE structure was coherent at pH 7 with a low amount of water penetrating the hydrophobic barrel. Simulations carried out with the candidate residues locked into their protonated state showed instead an increased amount of water penetrating the barrel and the rupture of the H121-Y214 hydrogen bond at the end of the barrel, which is essential for VDE activation. These results suggest that VDE activation relies on a robust and redundant network, in which the four residues identified in this study play a major role.

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.

Acclimation increases freezing stress response of Arabidopsis thaliana at proteome level

This study used 2DE to investigate how Arabidopsis thaliana modulates protein levels in response to freezing stress after sub-lethal exposure at -10°C, both in cold-acclimated and in non-acclimated plants. A map was implemented in which 62 spots, corresponding to 44 proteins, were identified. Twenty-two spots were modulated upon treatments, and the corresponding proteins proved to be related to photosynthesis, energy metabolism, and stress response. Proteins demonstrated differences between control and acclimation conditions. Most of the acclimation-responsive proteins were either not further modulated or they were down-modulated by freezing treatment, indicating that the levels reached during acclimation were sufficient to deal with freezing. Anabolic metabolism appeared to be down-regulated in favor of catabolic metabolism. Acclimated plants and plants submitted to freezing after acclimation showed greater reciprocal similarity in protein profiles than either showed when compared both to control plants and to plants frozen without acclimation. The response of non-acclimated plants was aimed at re-modulating photosynthetic apparatus activity, and at increasing the levels of proteins with antioxidant-, molecular chaperone-, or post-transcriptional regulative functions. These changes, even less effective than the acclimation strategy, might allow the injured plastids to minimize the production of non-useful metabolites and might counteract photosynthetic apparatus injuries.

Assembly of the light-harvesting chlorophyll antenna in the green alga Chlamydomonas reinhardtii requires expression of the TLA2-CpFTSY gene

The truncated light-harvesting antenna2 (tla2) mutant of Chlamydomonas reinhardtii showed a lighter-green phenotype, had a lower chlorophyll (Chl) per-cell content, and higher Chl a/b ratio than corresponding wild-type strains. Physiological analyses revealed a higher intensity for the saturation of photosynthesis and greater P(max) values in the tla2 mutant than in the wild type. Biochemical analyses showed that the tla2 strain was deficient in the Chl a-b light-harvesting complex, and had a Chl antenna size of the photosystems that was only about 65% of that in the wild type. Molecular and genetic analyses showed a single plasmid insertion in the tla2 strain, causing a chromosomal DNA rearrangement and deletion/disruption of five nuclear genes. The TLA2 gene, causing the tla2 phenotype, was cloned by mapping the insertion site and upon complementation with each of the genes that were deleted. Successful complementation was achieved with the C. reinhardtii TLA2-CpFTSY gene, whose occurrence and function in green microalgae has not hitherto been investigated. Functional analysis showed that the nuclear-encoded and chloroplast-localized CrCpFTSY protein specifically operates in the assembly of the peripheral components of the Chl a-b light-harvesting antenna. In higher plants, a cpftsy null mutation inhibits assembly of both the light-harvesting complex and photosystem complexes, thus resulting in a seedling-lethal phenotype. The work shows that cpftsy deletion in green algae, but not in higher plants, can be employed to generate tla mutants. The latter exhibit improved solar energy conversion efficiency and photosynthetic productivity under mass culture and bright sunlight conditions.

Acclimation to intense light implies changes at the level of trimeric subunits involved in the structural organization of the main light-harvesting complex of photosystem II (LHCII) and their isoforms

When plants are grown under stable light conditions their photosynthetic apparatus undergoes a long-term acclimation process. Acclimation to different light intensities involves changes in the organization and/or abundance of protein complexes in the thylakoid membranes. In this study, spinach plants were exposed to differing light intensities, and the structural organization of the major light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) was investigated by analysing their trimeric subunits. Plants were exposed to three different light intensities, 100 μmol quanta m⁻² s⁻¹, 200 μmol quanta m⁻² s⁻¹ and an elevated light intensity, 400 μmol quanta m⁻² s⁻¹, sufficient to provoke a moderate stress response in the form of down regulation of PSII. "MicroRotofor" analysis showed the presence of LHCII with different pIs and revealed a clear decline in their abundance as light intensity increased from 100 to 400 μmol quanta m⁻² s⁻¹. The three subunits (Lhcb1, Lhcb2, Lhcb3) behaved differently from each other as: Lhcb1 decreased more significantly than Lhcb2, whereas Lhcb3 was reduced only at a light window at which Lhcb1 and Lhcb2 abundance has already been depleted under intense irradiation. Interestingly, we also found that isoforms of Lhcb1 subunit (Lhcb1.1; 1.2; 1.3) behaved differently in response to elevated light intensity, suggesting an essential role of these isoforms to light adaption and consequently explaining the presence of this multigenic family, often identified among higher plants.

Acclimation of Chlamydomonas reinhardtii to different growth irradiances

We report on the changes the photosynthetic apparatus of Chlamydomonas reinhardtii undergoes upon acclimation to different light intensity. When grown in high light, cells had a faster growth rate and higher biomass production compared with low and control light conditions. However, cells acclimated to low light intensity are indeed able to produce more biomass per photon available as compared with high light-acclimated cells, which dissipate as heat a large part of light absorbed, thus reducing their photosynthetic efficiency. This dissipative state is strictly dependent on the accumulation of LhcSR3, a protein related to light-harvesting complexes, responsible for nonphotochemical quenching in microalgae. Other changes induced in the composition of the photosynthetic apparatus upon high light acclimation consist of an increase of carotenoid content on a chlorophyll basis, particularly zeaxanthin, and a major down-regulation of light absorption capacity by decreasing the chlorophyll content per cell. Surprisingly, the antenna size of both photosystem I and II is not modulated by acclimation; rather, the regulation affects the PSI/PSII ratio. Major effects of the acclimation to low light consist of increased activity of state 1 and 2 transitions and increased contributions of cyclic electron flow.

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 thylakoid protease Deg2 is involved in stress-related degradation of the photosystem II light-harvesting protein Lhcb6 in Arabidopsis thaliana

• The thylakoid protease Deg2 is a serine-type protease peripherally attached to the stromal side of the thylakoid membrane. Given the lack of knowledge concerning its function, two T-DNA insertion lines devoid of Deg2 were prepared to study the functional importance of this protease in Arabidopsis thaliana. • The phenotypic appearance of deg2 mutants was studied using a combination of stereo and transmission electron microscopy, and short-stress-mediated degradation of apoproteins of minor light-harvesting antennae of photosystem II (PSII) was analysed by immunoblotting in the mutants in comparison with wild-type plants. • Deg2 repression produced a phenotype in which reduced leaf area and modified chloroplast ultrastructure of older leaves were the most prominent features. In contrast to the wild type, the chloroplasts of second-whorl leaves of 4-wk-old deg2 mutants did not display features typical of the early senescence phase, such as undulation of the chloroplast envelope and thylakoids. The ability to degrade the photosystem II light-harvesting protein Lhcb6 apoprotein in response to brief high-salt, wounding, high-temperature and high-irradiance stress was demonstrated to be impaired in deg2 mutants. • Our results suggest that Deg2 is required for normal plant development, including the chloroplast life cycle, and has an important function in the degradation of Lhcb6 in response to short-duration stresses.

A retrieval algorithm to evaluate the Photosystem I and Photosystem II spectral contributions to leaf chlorophyll fluorescence at physiological temperatures

A new computational procedure to resolve the contribution of Photosystem I (PSI) and Photosystem II (PSII) to the leaf chlorophyll fluorescence emission spectra at room temperature has been developed. It is based on the Principal Component Analysis (PCA) of the leaf fluorescence emission spectra measured during the OI photochemical phase of fluorescence induction kinetics. During this phase, we can assume that only two spectral components are present, one of which is constant (PSI) and the other variable in intensity (PSII). Application of the PCA method to the measured fluorescence emission spectra of Ficus benjamina L. evidences that the temporal variation in the spectra can be ascribed to a single spectral component (the first principal component extracted by PCA), which can be considered to be a good approximation of the PSII fluorescence emission spectrum. The PSI fluorescence emission spectrum was deduced by difference between measured spectra and the first principal component. A single-band spectrum for the PSI fluorescence emission, peaked at about 735 nm, and a 2-band spectrum with maxima at 685 and 740 nm for the PSII were obtained. A linear combination of only these two spectral shapes produced a good fit for any measured emission spectrum of the leaf under investigation and can be used to obtain the fluorescence emission contributions of photosystems under different conditions. With the use of our approach, the dynamics of energy distribution between the two photosystems, such as state transition, can be monitored in vivo, directly at physiological temperatures. Separation of the PSI and PSII emission components can improve the understanding of the fluorescence signal changes induced by environmental factors or stress conditions on plants.

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.

A red-shifted antenna protein associated with photosystem II in Physcomitrella patens

Antenna systems of plants and green algae are made up of pigment-protein complexes belonging to the light-harvesting complex (LHC) multigene family. LHCs increase the light-harvesting cross-section of photosystems I and II and catalyze photoprotective reactions that prevent light-induced damage in an oxygenic environment. The genome of the moss Physcomitrella patens contains two genes encoding LHCb9, a new antenna protein that bears an overall sequence similarity to photosystem II antenna proteins but carries a specific motif typical of photosystem I antenna proteins. This consists of the presence of an asparagine residue as a ligand for Chl 603 (A5) chromophore rather than a histidine, the common ligand in all other LHCbs. Asparagine as a Chl 603 (A5) ligand generates red-shifted spectral forms associated with photosystem I rather than with photosystem II, suggesting that in P. patens, the energy landscape of photosystem II might be different with respect to that of most green algae and plants. In this work, we show that the in vitro refolded LHCb9-pigment complexes carry a red-shifted fluorescence emission peak, different from all other known photosystem II antenna proteins. By using a specific antibody, we localized LHCb9 within PSII supercomplexes in the thylakoid membranes. This is the first report of red-shifted spectral forms in a PSII antenna system, suggesting that this biophysical feature might have a special role either in optimization of light use efficiency or in photoprotection in the specific environmental conditions experienced by this moss.

Acclimation- and mutation-induced enhancement of PsbS levels affects the kinetics of non-photochemical quenching in Arabidopsis thaliana

The efficiency of photosystem II antenna complexes (LHCs) in higher plants must be regulated to avoid potentially damaging overexcitation of the reaction centre in excess light. Regulation is achieved via a feedback mechanism known as non-photochemical quenching (NPQ), triggered the proton gradient (ΔpH) causing heat dissipation within the LHC antenna. ΔpH causes protonation of the LHCs, the PsbS protein and triggers the enzymatic de-epoxidation of the xanthophyll, violaxanthin, to zeaxanthin. A key step in understanding the mechanism is to decipher whether PsbS and zeaxanthin cooperate to promote NPQ. To obtain clues about their respective functions we studied the effects of PsbS and zeaxanthin on the rates of NPQ formation and relaxation in wild-type Arabidopsis leaves and those overexpressing PsbS (L17) or lacking zeaxanthin (npq1). Overexpression of PsbS was found to increase the rate of NPQ formation, as previously reported for zeaxanthin. However, PsbS overexpression also increased the rate of NPQ relaxation, unlike zeaxanthin, which is known decrease the rate. The enhancement of PsbS levels in plants lacking zeaxanthin (npq1) by either acclimation to high light or crossing with L17 plants showed that the effect of PsbS was independent of zeaxanthin. PsbS levels also affected the kinetics of the 535 nm absorption change (ΔA535), which monitors the formation of the conformational state of the LHC antenna associated with NPQ, in an identical way. The antagonistic action of PsbS and zeaxanthin with respect to NPQ and ΔA535 relaxation kinetics suggests that the two molecules have distinct regulatory functions.

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.

Acclimation of Nannochloropsis gaditana to different illumination regimes: effects on lipids accumulation

Algae are interesting potential sources of biodiesel, although research is still needed to develop efficient large scale productions. One major factor affecting productivity is light use efficiency. The effect of different light regimes on the seawater alga Nannochloropsis gaditana was accessed monitoring growth rate and photosynthetic performances. N. gaditana showed the capacity of acclimating to different light intensities, optimizing its photosynthetic apparatus to illumination. Thanks to this response, N. gaditana maintained similar growth rates under a wide range of irradiances, suggesting that this organism is a valuable candidate for outdoor productions in variable conditions. In the conditions tested here, without external CO(2) supply, light intensity alone was not found to be a major signal affecting lipids accumulation showing the absence of a direct regulatory link between the light stress and lipids accumulation. Strong illumination can nevertheless indirectly influences lipid accumulation if combined with other stresses or in the presence of excess CO(2).

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.

The light-harvesting complexes of higher-plant Photosystem I: Lhca1/4 and Lhca2/3 form two red-emitting heterodimers

The outer antenna of higher-plant PSI (Photosystem I) is composed of four complexes [Lhc (light-harvesting complex) a1-Lhca4] belonging to the light-harvesting protein family. Difficulties in their purification have so far prevented the determination of their properties and most of the knowledge about Lhcas has been obtained from the study of the in vitro reconstituted antennas. In the present study we were able to purify the native complexes, showing that Lhca2/3 and Lhca1/4 form two functional heterodimers. Both dimers show red-fluorescence emission with maxima around 730 nm, as in the intact PSI complex. This indicates that the dimers are in their native state and that LHCI-680, which was previously assumed to be part of the PSI antenna, does not represent the native state of the system. The data show that the light-harvesting properties of the two dimers are functionally identical, concerning absorption, long-wavelength emission and fluorescence quantum yield, whereas they differ in their high-light response. Implications of the present study for the understanding of the energy transfer process in PSI are discussed. Finally, the comparison of the properties of the native dimers with those of the reconstituted complexes demonstrates that all of the major properties of the Lhcas are reproduced in the in vitro systems.

Analysis of LhcSR3, a protein essential for feedback de-excitation in the green alga Chlamydomonas reinhardtii

In photosynthetic organisms, feedback dissipation of excess absorbed light energy balances harvesting of light with metabolic energy consumption. This mechanism prevents photodamage caused by reactive oxygen species produced by the reaction of chlorophyll (Chl) triplet states with O₂. Plants have been found to perform the heat dissipation in specific proteins, binding Chls and carotenoids (Cars), that belong to the Lhc family, while triggering of the process is performed by the PsbS subunit, needed for lumenal pH detection. PsbS is not found in algae, suggesting important differences in energy-dependent quenching (qE) machinery. Consistent with this suggestion, a different Lhc-like gene product, called LhcSR3 (formerly known as LI818) has been found to be essential for qE in Chlamydomonas reinhardtii. In this work, we report the production of two recombinant LhcSR isoforms from C. reinhardtii and their biochemical and spectroscopic characterization. We found the following: (i) LhcSR isoforms are Chl a/b- and xanthophyll-binding proteins, contrary to higher plant PsbS; (ii) the LhcSR3 isoform, accumulating in high light, is a strong quencher of Chl excited states, exhibiting a very fast fluorescence decay, with lifetimes below 100 ps, capable of dissipating excitation energy from neighbor antenna proteins; (iii) the LhcSR3 isoform is highly active in the transient formation of Car radical cation, a species proposed to act as a quencher in the heat dissipation process. Remarkably, the radical cation signal is detected at wavelengths corresponding to the Car lutein, rather than to zeaxanthin, implying that the latter, predominant in plants, is not essential; (iv) LhcSR3 is responsive to low pH, the trigger of non-photochemical quenching, since it binds the non-photochemical quenching inhibitor dicyclohexylcarbodiimide, and increases its energy dissipation properties upon acidification. This is the first report of an isolated Lhc protein constitutively active in energy dissipation in its purified form, opening the way to detailed molecular analysis. Owing to its protonatable residues and constitutive excitation energy dissipation, this protein appears to merge both pH-sensing and energy-quenching functions, accomplished respectively by PsbS and monomeric Lhcb proteins in plants.

Structural and functional diversification of the light-harvesting complexes in photosynthetic eukaryotes

Eukaryotes acquired photosynthetic metabolism over a billion years ago, and during that time the light-harvesting antennae have undergone significant structural and functional divergence. The antenna systems are generally used to harvest and transfer excitation energy into the reaction centers to drive photosynthesis, but also have the dual role of energy dissipation. Phycobilisomes formed the first antenna system in oxygenic photoautotrophs, and this soluble protein complex continues to be the dominant antenna in extant cyanobacteria, glaucophytes, and red algae. However, phycobilisomes were lost multiple times during eukaryotic evolution in favor of a thylakoid membrane-integral light-harvesting complex (LHC) antenna system found in the majority of eukaryotic taxa. While photosynthesis spread across different eukaryotic kingdoms via endosymbiosis, the antenna systems underwent extensive modification as photosynthetic groups optimized their light-harvesting capacity and ability to acclimate to changing environmental conditions. This review discusses the different classes of LHCs within photosynthetic eukaryotes and examines LHC diversification in different groups in a structural and functional context.

Reorganization of photosystem II is involved in the rapid photosynthetic recovery of desert moss Syntrichia caninervis upon rehydration

The moss Syntrichia caninervis (S. caninervis) is one of the dominant species in biological soil crusts of deserts. It has long been the focus of scientific research because of its ecological value. Moreover, S. caninervis has a special significance in biogenesis research because it is characterized by its fast restoration of photosynthesis upon onset of rehydration of the desiccated organism. In order to study the mechanisms of rapid photosynthetic recovery in mosses upon rewatering, we investigated the kinetics of the recovery process of photosynthetic activity in photosystem (PS) II, with an indirect assessment of the photochemical processes based on chlorophyll (Chl) fluorescence measurements. Our results showed that recovery can be divided into two phases. The fast initial phase, completed within 3 min, was characterized by a quick increase in maximal quantum efficiency of PSII (F(v)/F(m)). Over 50% of the PSII activities, including excitation energy transfer, oxygen evolution, charge separation, and electron transport, recovered within 0.5 min after rehydration. The second, slow phase was dominated by the increase of plastoquinone (PQ) reduction and the equilibrium of the energy transport from the inner antenna to the reaction center (RC) of PSII. Analysis of the recovery process in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) revealed that blocking the electron transport from Q(A) to Q(B) did not hamper Chl synthesis or Chl organization in thylakoid membranes under light conditions. A de novo chloroplast protein synthesis was not necessary for the initial recovery of photochemical activity in PSII. In conclusion, the moss's ability for rapid recovery upon rehydration is related to Chl synthesis, quick structural reorganization of PSII, and fast restoration of PSII activity without de novo chloroplast protein synthesis.

The structure and function of eukaryotic photosystem I

Eukaryotic photosystem I consists of two functional moieties: the photosystem I core, harboring the components for the light-driven charge separation and the subsequent electron transfer, and the peripheral light-harvesting complex (LHCI). While the photosystem I-core remained highly conserved throughout the evolution, with the exception of the oxidizing side of photosystem I, the LHCI complex shows a high degree of variability in size, subunits composition and bound pigments, which is due to the large variety of different habitats photosynthetic organisms dwell in. Besides summarizing the most current knowledge on the photosystem I-core structure, we will discuss the composition and structure of the LHCI complex from different eukaryotic organisms, both from the red and the green clade. Furthermore, mechanistic insights into electron transfer between the donor and acceptor side of photosystem I and its soluble electron transfer carrier proteins will be given. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Physcomitrella patens mutants affected on heat dissipation clarify the evolution of photoprotection mechanisms upon land colonization

Light is the source of energy for photosynthetic organisms; when in excess, however, it also drives the formation of reactive oxygen species and, consequently, photoinhibition. Plants and algae have evolved mechanisms to regulate light harvesting efficiency in response to variable light intensity so as to avoid oxidative damage. Nonphotochemical quenching (NPQ) consists of the rapid dissipation of excess excitation energy as heat. Although widespread among oxygenic photosynthetic organisms, NPQ shows important differences in its machinery. In land plants, such as Arabidopsis thaliana, NPQ depends on the presence of PSBS, whereas in the green alga Chlamydomonas reinhardtii it requires a different protein called LHCSR. In this work, we show that both proteins are present in the moss Physcomitrella patens. By generating KO mutants lacking PSBS and/or LHCSR, we also demonstrate that both gene products are active in NPQ. Plants lacking both proteins are more susceptible to high light stress than WT, implying that they are active in photoprotection. These results suggest that NPQ is a fundamental mechanism for survival in excess light and that upon land colonization, photosynthetic organisms evolved a unique mechanism for excess energy dissipation before losing the ancestral one found in algae.

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.

Regulation of plant light harvesting by thermal dissipation of excess energy

Elucidating the molecular details of qE (energy quenching) induction in higher plants has proven to be a major challenge. Identification of qE mutants has provided initial information on functional elements involved in the qE mechanism; furthermore, investigations on isolated pigment–protein complexes and analysis in vivo and in vitro by sophisticated spectroscopic methods have been used for the elucidation of mechanisms involved. The aim of the present review is to summarize the current knowledge of the phenotype of npq (non-photochemical quenching)-knockout mutants, the role of gene products involved in the qE process and compare the molecular models proposed for this process.

Structure determination and improved model of plant photosystem I

Photosystem I functions as a sunlight energy converter, catalyzing one of the initial steps in driving oxygenic photosynthesis in cyanobacteria, algae, and higher plants. Functionally, Photosystem I captures sunlight and transfers the excitation energy through an intricate and precisely organized antenna system, consisting of a pigment network, to the center of the molecule, where it is used in the transmembrane electron transfer reaction. Our current understanding of the sophisticated mechanisms underlying these processes has profited greatly from elucidation of the crystal structures of the Photosystem I complex. In this report, we describe the developments that ultimately led to enhanced structural information of plant Photosystem I. In addition, we report an improved crystallographic model at 3.3-A resolution, which allows analysis of the structure in more detail. An improved electron density map yielded identification and tracing of subunit PsaK. The location of an additional ten beta-carotenes as well as five chlorophylls and several loop regions, which were previously uninterpretable, are now modeled. This represents the most complete plant Photosystem I structure obtained thus far, revealing the locations of and interactions among 17 protein subunits and 193 non-covalently bound photochemical cofactors. Using the new crystal structure, we examine the network of contacts among the protein subunits from the structural perspective, which provide the basis for elucidating the functional organization of the complex.

An integrated analysis of molecular acclimation to high light in the marine diatom Phaeodactylum tricornutum

Photosynthetic diatoms are exposed to rapid and unpredictable changes in irradiance and spectral quality, and must be able to acclimate their light harvesting systems to varying light conditions. Molecular mechanisms behind light acclimation in diatoms are largely unknown. We set out to investigate the mechanisms of high light acclimation in Phaeodactylum tricornutum using an integrated approach involving global transcriptional profiling, metabolite profiling and variable fluorescence technique. Algae cultures were acclimated to low light (LL), after which the cultures were transferred to high light (HL). Molecular, metabolic and physiological responses were studied at time points 0.5 h, 3 h, 6 h, 12 h, 24 h and 48 h after transfer to HL conditions. The integrated results indicate that the acclimation mechanisms in diatoms can be divided into an initial response phase (0–0.5 h), an intermediate acclimation phase (3–12 h) and a late acclimation phase (12–48 h). The initial phase is recognized by strong and rapid regulation of genes encoding proteins involved in photosynthesis, pigment metabolism and reactive oxygen species (ROS) scavenging systems. A significant increase in light protecting metabolites occur together with the induction of transcriptional processes involved in protection of cellular structures at this early phase. During the following phases, the metabolite profiling display a pronounced decrease in light harvesting pigments, whereas the variable fluorescence measurements show that the photosynthetic capacity increases strongly during the late acclimation phase. We show that P. tricornutum is capable of swift and efficient execution of photoprotective mechanisms, followed by changes in the composition of the photosynthetic machinery that enable the diatoms to utilize the excess energy available in HL. Central molecular players in light protection and acclimation to high irradiance have been identified.

Functional analysis of Photosystem I light-harvesting complexes (Lhca) gene products of Chlamydomonas reinhardtii

The outer antenna system of Chlamydomonas reinhardtii Photosystem I is composed of nine gene products, but due to difficulty in purification their individual properties are not known. In this work, the functional properties of the nine Lhca antennas of Chlamydomonas, have been investigated upon expression of the apoproteins in bacteria and refolding in vitro of the pigment-protein complexes. It is shown that all Lhca complexes have a red-shifted fluorescence emission as compared to the antenna complexes of Photosystem II, similar to Lhca from higher plants, but less red-shifted. Three complexes, namely Lhca2, Lhca4 and Lhca9, exhibit emission maxima above 707 nm and all carry an asparagine as ligand for Chl 603. The comparison of the protein sequences and the biochemical/spectroscopic properties of the refolded Chlamydomonas complexes with those of the well-characterized Arabidopsis thaliana Lhcas shows that all the Chlamydomonas complexes have a chromophore organization similar to that of A. thaliana antennas, particularly to Lhca2, despite low sequence identity. All the major biochemical and spectroscopic properties of the Lhca complexes have been conserved through the evolution, including those involved in "red forms" absorption. It has been proposed that in Chlamydomonas PSI antenna size and polypeptide composition can be modulated in vivo depending on growth conditions, at variance as compared to higher plants. Thus, the different properties of the individual Lhca complexes can be functional to adapt the architecture of the PSI-LHCI supercomplex to different environmental conditions.

The Arabidopsis TSPO-related protein is a stress and abscisic acid-regulated, endoplasmic reticulum-Golgi-localized membrane protein

The Arabidopsis gene At2g47770 encodes a membrane-bound protein designated AtTSPO (Arabidopsis thaliana TSPO-related). AtTSPO is related to the bacterial outer membrane tryptophan-rich sensory protein (TspO) and the mammalian mitochondrial 18-kDa translocator protein (18 kDa TSPO), members of the group of TspO/MBR domain-containing membrane proteins. In this study we show that AtTSPO is mainly detected in dry seeds, but can be induced in vegetative tissues by osmotic or salt stress or abscisic acid (ABA) treatment, corroborating available transcriptome data. Using subcellular fractionation, immunocytochemistry and fluorescent protein tagging approaches we present evidence that AtTSPO is targeted to the secretory pathway in plants. Induced or constitutively expressed AtTSPO can be detected in the endoplasmic reticulum and the Golgi stacks of plant cells. AtTSPO tagged with fluorescent protein in transgenic plants (Arabidopsis and tobacco) was mainly detected in the Golgi stacks of leaf epidermal cells. Constitutive expression of AtTSPO resulted in increased sensitivity to NaCl, but not to osmotic stress, and in reduced greening of cultured Arabidopsis cells under light growing conditions. Transgenic Arabidopsis plants overexpressing AtTSPO were more sensitive to ABA-induced growth inhibition, indicating that constitutive expression of AtTSPO may enhance ABA sensitivity. AtTSPO is rapidly downregulated during seed imbibition, and the ABA-dependent induction in plant is transient. Downregulation of AtTSPO seems to be boosted by treatment with aminolevulinic acid. Taken together, these results suggest that AtTSPO is a highly regulated protein, induced by abiotic stress to modulate, at least in part, transient intracellular ABA-dependent stress perception and/or signalling.

Photosystem II organisation in chloroplasts of Arum italicum leaf depends on tissue location

The growth of plants under stable light quality induces long-term acclimation responses of the photosynthetic apparatus. Light can even cause variations depending on the tissue location, as in Arum italicum leaf, where chloroplasts are developed in the lamina and in the entire thickness of the petiole. We addressed the question whether differences in plastids can be characterised in terms of protein-protein interactions in the thylakoid membranes. Thylakoid assembly was studied in the palisade and spongy tissue of the lamina and in the outer parenchyma and inner aerenchyma of the petiole of the mature winter leaf of Arum italicum. The chlorophyll-protein complexes were analysed by means of blue-native-PAGE and fluorescence emission spectra. The petiole chloroplasts differ from those in the lamina in thylakoid composition: (1) reaction centres are scarce, especially photosystem (PS) I in the inner aerenchyma; (2) light-harvesting complex (LHC) II is abundant, (3) the relative amount of LHCII trimers increases, but this is not accompanied by increased levels of PSII-LHCII supercomplexes. Nevertheless, the intrinsic PSII functionality is comparable in all tissues. In Arum italicum leaf, the gradient in thylakoid organisation, which occurs from the palisade tissue to the inner aerenchyma of the petiole, is typical for photosynthetic acclimation to low-light intensity with a high enrichment of far-red light. The results obtained demonstrate a high plasticity of chloroplasts even in an individual plant. The mutual interaction of thylakoid protein complexes is discussed in relation to the photosynthetic efficiency of the leaf parts and to the ecodevelopmental role of light.

Antenna complexes protect Photosystem I from photoinhibition

BACKGROUND

Photosystems are composed of two moieties, a reaction center and a peripheral antenna system. In photosynthetic eukaryotes the latter system is composed of proteins belonging to Lhc family. An increasing set of evidences demonstrated how these polypeptides play a relevant physiological function in both light harvesting and photoprotection. Despite the sequence similarity between antenna proteins associated with the two Photosystems, present knowledge on their physiological role is mostly limited to complexes associated to Photosystem II.

RESULTS

In this work we analyzed the physiological role of Photosystem I antenna system in Arabidopsis thaliana both in vivo and in vitro. Plants depleted in individual antenna polypeptides showed a reduced capacity for photoprotection and an increased production of reactive oxygen species upon high light exposure. In vitro experiments on isolated complexes confirmed that depletion of antenna proteins reduced the resistance of isolated Photosystem I particles to high light and that the antenna is effective in photoprotection only upon the interaction with the core complex.

CONCLUSION

We show that antenna proteins play a dual role in Arabidopsis thaliana Photosystem I photoprotection: first, a Photosystem I with an intact antenna system is more resistant to high light because of a reduced production of reactive oxygen species and, second, antenna chlorophyll-proteins are the first target of high light damages. When photoprotection mechanisms become insufficient, the antenna chlorophyll proteins act as fuses: LHCI chlorophylls are degraded while the reaction center photochemical activity is maintained. Differences with respect to photoprotection strategy in Photosystem II, where the reaction center is the first target of photoinhibition, are discussed.

Light-induced dissociation of an antenna hetero-oligomer is needed for non-photochemical quenching induction

PsbS plays a major role in activating the photoprotection mechanism known as "non-photochemical quenching," which dissipates chlorophyll excited states exceeding the capacity for photosynthetic electron transport. PsbS activity is known to be triggered by low lumenal pH. However, the molecular mechanism by which this subunit regulates light harvesting efficiency is still unknown. Here we show that PsbS controls the association/dissociation of a five-subunit membrane complex, composed of two monomeric Lhcb proteins (CP29 and CP24) and the trimeric LHCII-M. Dissociation of this supercomplex is indispensable for the onset of non-photochemical fluorescence quenching in high light, strongly suggesting that protein subunits catalyzing the reaction of heat dissipation are buried into the complex and thus not available for interaction with PsbS. Consistently, we showed that knock-out mutants on two subunits participating to the B4C complex were strongly affected in heat dissipation. Direct observation by electron microscopy and image analysis showed that B4C dissociation leads to the redistribution of PSII within grana membranes. We interpreted these results to mean that the dissociation of B4C makes quenching sites, possibly CP29 and CP24, available for the switch to an energy-quenching conformation. These changes are reversible and do not require protein synthesis/degradation, thus allowing for changes in PSII antenna size and adaptation to rapidly changing environmental conditions.

The dynamics of photosynthesis

Despite recent elucidation of the three-dimensional structure of major photosynthetic complexes, our understanding of light energy conversion in plant chloroplasts and microalgae under physiological conditions requires exploring the dynamics of photosynthesis. The photosynthetic apparatus is a flexible molecular machine that can acclimate to metabolic and light fluctuations in a matter of seconds and minutes. On a longer time scale, changes in environmental cues trigger acclimation responses that elicit intracellular signaling between the nucleo-cytosol and chloroplast resulting in modification of the biogenesis of the photosynthetic machinery. Here we attempt to integrate well-established knowledge on the functional flexibility of light-harvesting and electron transfer processes, which has greatly benefited from genetic approaches, with data derived from the wealth of recent transcriptomic and proteomic studies of acclimation responses in photosynthetic eukaroytes.

The occurrence of the psbS gene product in Chlamydomonas reinhardtii and in other photosynthetic organisms and its correlation with energy quenching

To avoid photodamage, photosynthetic organisms have developed mechanisms to evade or dissipate excess energy. Lumen overacidification caused by light-induced electron transport triggers quenching of excited chlorophylls and dissipation of excess energy into heat. In higher plants participation of the PsbS protein as the sensor of low lumenal pH was clearly demonstrated. Although light-dependent energy quenching is a property of all photosynthetic organisms, large differences in amplitude and kinetics can be observed thus raising the question whether a single common mechanism is in action. We performed a detailed study of PsbS expression/accumulation in Chlamydomonas reinhardtii and investigated its accumulation in other algae and plants. We showed that PsbS cannot be detected in Chlamydomonas under a wide range of growth conditions. Overexpression of the endogenous psbs gene showed that the corresponding protein could not be addressed to the thylakoid membranes. Survey of different unicellular green algae showed no accumulation of anti-PsbS reactive proteins differently from multicellular species. Nevertheless, some unicellular species exhibit high energy quenching activity, suggesting that a PsbS-independent mechanism is activated. By correlating growth habitat and PsbS accumulation in different species, we suggest that during the evolution the light environment has been a determinant factor for the conservation/loss of the PsbS function.

Chlorophyll limitation in plants remodels and balances the photosynthetic apparatus by changing the accumulation of photosystems I and II through two different approaches

Arabidopsis plants with a reduced expression of CHL27 (chl27), an enzyme (EC 1.14.13.81) required for the synthesis of Pchlide, are chlorotic and have a Chl a/b ratio two times higher than wild-type (WT). Knockdown plants transformed with a construct constitutively expressing CHL27 recovered regarding Chl level, a/b ratio and 77K fluorescence. A negative correlation was found between total Chl and Chl a/b ratio in the examined plants. The chl27 plants fail to assemble WT amounts of complete PSI and PSII, leading to an elevated PSII/PSI ratio. The PSI remaining in chl27 is fully functional with a quantum yield higher than for WT. Despite a severe reduction of photosystem II antennae protein (LHCII) and an increased proportion of stroma lammella, the chl27 plants are able to perform state transitions. No major differences were found regarding PSII quantum yield, qN and 1 - qp whereas non-photochemical quenching was decreased by a factor two in chl27 plants. The PSII quantum yield for dark-adapted plants and plants given 10 min recovery after high light treatment were similar for both WT and chl27 showing that chl27 plants are not more susceptible to photoinhibition than WT. Taken together the plant manage to acclimate and to balance the two photosystems well even when it is severely limited in Chl. The way to achieve this differs for the two photosystems: regarding PSI a general reduction of core and antenna subunits occurs with no apparent change in the antenna composition; whereas for PSII there is a preferential loss of antenna proteins.

The role of Lhca complexes in the supramolecular organization of higher plant photosystem I

In this work, Photosystem I supercomplexes have been purified from Lhca-deficient lines of Arabidopsis thaliana using a mild detergent treatment that does not induce loss of outer antennas. The complexes have been studied by integrating biochemical analysis with electron microscopy. This allows the direct correlation of changes in protein content with changes in supramolecular structure of Photosystem I to get information about the position of the individual Lhca subunits, the association of the antenna to the core, and the influence of the individual subunits on the stability of the system. Photosystem I complexes with only two or three antenna complexes were purified, showing that the binding of Lhca1/4 and Lhca2/3 dimers to the core is not interdependent, although weak binding of Lhca2/3 to the core is stabilized by the presence of Lhca4. Moreover, Lhca2 and Lhca4 can be associated with the core in the absence of their "dimeric partners." The structure of Photosystem I is very rigid, and the absence of one antenna complex leaves a "hole" in the structure that cannot be filled by other Lhcas, clearly indicating that the docking sites for the individual subunits are highly specific. There is, however, an exception to the rule: Lhca5 can substitute for Lhca4, yielding highly stable PSI supercomplexes with a supramolecular organization identical to the WT.

Loss of chloroplast protease SPPA function alters high light acclimation processes in Arabidopsis thaliana L. (Heynh.)

SPPA1 is a protease in the plastids of plants, located in non-appressed thylakoid regions. In this study, T-DNA insertion mutants of the single-copy SPPA1 gene in Arabidopsis thaliana (At1g73990) were examined. Mutation of SPPA1 had no effect on the growth and development of plants under moderate, non-stressful conditions. It also did not affect the quantum efficiency of photosynthesis as measured by dark-adapted F(v)/F(m) and light-adapted Phi(PSII). Chloroplasts from sppA mutants were indistinguishable from the wild type. Loss of SPPA appears to affect photoprotective mechanisms during high light acclimation: mutant plants maintained a higher level of non-photochemical quenching of Photosystem II chlorophyll (NPQ) than the wild type, while wild-type plants accumulated more anthocyanin than the mutants. The quantum efficiency of Photosystem II was the same in all genotypes grown under low light, but was higher in wild type than mutants during high light acclimation. Further, the mutants retained the stress-related Early Light Inducible Protein (ELIP) longer than wild-type leaves during the early recovery period after acute high light plus cold treatment. These results suggest that SPPA1 may function during high light acclimation in the plastid, but is non-essential for growth and development under non-stress conditions.

Sensing and responding to excess light

Plants and algae often absorb too much light-more than they can actually use in photosynthesis. To prevent photo-oxidative damage and to acclimate to changes in their environment, photosynthetic organisms have evolved direct and indirect mechanisms for sensing and responding to excess light. Photoreceptors such as phototropin, neochrome, and cryptochrome can sense excess light directly and relay signals for chloroplast movement and gene expression responses. Indirect sensing of excess light through biochemical and metabolic signals can be transduced into local responses within chloroplasts, into changes in nuclear gene expression via retrograde signaling pathways, or even into systemic responses, all of which are associated with photoacclimation.

Fast and reversible response of thylakoid-associated polyamines during and after UV-B stress: a comparative study of the wild type and a mutant lacking chlorophyll b of unicellular green alga Scenedesmus obliquus

The functional and biochemical aspects of the photosynthetic apparatus in response to UV-B radiation were examined in unicellular oxygenic algae Scenedesmus obliquus. The wild type (Wt) and a chlorophyll b-less mutant (Wt-lhc) were used as a specific tool for the understanding of antenna role. Photosynthesis was monitored during and after UV-B stress by time resolved fluorescence spectroscopy and polarography. Carotenoids, such as neoxanthin, loroxanthin, lutein, violaxanthin, antheraxanthin, zeaxanthin, alpha- and beta-carotene, cellular and thylakoid-associated putrescine, spermidine, spermine and subcomplexes of light-harvesting complex (LHCII) of photosystem II (PSII) were investigated to assess their possible involvement in response to UV-B. Oxygen evolution depression by UV-B was higher in the Wt-lhc mutant than in the Wt. Photosynthesis recovery occurred in the Wt, but not in the mutant. The dissipation of excess excitation energy during UV-B stress was accompanied by changes in the thylakoid-associated polyamines which were much higher than changes in xanthophylls. We conclude that, at least in the unicellular green alga S. obliquus, mutants lacking chlorophyll b have significant lower capacity for recovery after UV-B stress. In addition, the comparison of xanthophylls and thylakoid-associated polyamines reveals that the latter are more responsive to UV-B stress and in a reversible manner.

In silico and biochemical analysis of Physcomitrella patens photosynthetic antenna: identification of subunits which evolved upon land adaptation

Background

In eukaryotes the photosynthetic antenna system is composed of subunits encoded by the light harvesting complex (Lhc) multigene family. These proteins play a key role in photosynthesis and are involved in both light harvesting and photoprotection. The moss Physcomitrella patens is a member of a lineage that diverged from seed plants early after land colonization and therefore by studying this organism, we may gain insight into adaptations to the aerial environment.

Principal Findings

In this study, we characterized the antenna protein multigene family in Physcomitrella patens, by sequence analysis as well as biochemical and functional investigations. Sequence identification and analysis showed that some antenna polypeptides, such as Lhcb3 and Lhcb6, are present only in land organisms, suggesting they play a role in adaptation to the sub-aerial environment. Our functional analysis which showed that photo-protective mechanisms in Physcomitrella patens are very similar to those in seed plants fits with this hypothesis. In particular, Physcomitrella patens also activates Non Photochemical Quenching upon illumination, consistent with the detection of an ortholog of the PsbS protein. As a further adaptation to terrestrial conditions, the content of Photosystem I low energy absorbing chlorophylls also increased, as demonstrated by differences in Lhca3 and Lhca4 polypeptide sequences, in vitro reconstitution experiments and low temperature fluorescence spectra.

Conclusions

This study highlights the role of Lhc family members in environmental adaptation and allowed proteins associated with mechanisms of stress resistance to be identified within this large family.

Minor antenna proteins CP24 and CP26 affect the interactions between photosystem II subunits and the electron transport rate in grana membranes of Arabidopsis

We investigated the function of chlorophyll a/b binding antenna proteins Chlorophyll Protein 26 (CP26) and CP24 in light harvesting and regulation of photosynthesis by isolating Arabidopsis thaliana knockout lines that completely lacked one or both of these proteins. All three mutant lines had a decreased efficiency of energy transfer from trimeric light-harvesting complex II (LHCII) to the reaction center of photosystem II (PSII) due to the physical disconnection of LHCII from PSII and formation of PSII reaction center depleted domains in grana partitions. Photosynthesis was affected in plants lacking CP24 but not in plants lacking CP26: the former mutant had decreased electron transport rates, a lower DeltapH gradient across the grana membranes, reduced capacity for nonphotochemical quenching, and limited growth. Furthermore, the PSII particles of these plants were organized in unusual two-dimensional arrays in the grana membranes. Surprisingly, overall electron transport, nonphotochemical quenching, and growth of the double mutant were restored to wild type. Fluorescence induction kinetics and electron transport measurements at selected steps of the photosynthetic chain suggested that limitation in electron transport was due to restricted electron transport between Q(A) and Q(B), which retards plastoquinone diffusion. We conclude that CP24 absence alters PSII organization and consequently limits plastoquinone diffusion.

Functional organization of a plant Photosystem I: evolution of a highly efficient photochemical machine

Despite its enormous complexity, a plant Photosystem I (PSI) is arguably the most efficient nano-photochemical machine in Nature. It emerged as a homodimeric structure containing several chlorophyll molecules over 3.5 billion years ago, and has perfected its photoelectric properties ever since. The recently determined structure of plant PSI, which is at the top of the evolutionary tree of this kind of complexes, provided the first relatively high-resolution structural model of the supercomplex containing a reaction center (RC) and a peripheral antenna (LHCI) complexes. The RC is highly homologous to that of the cyanobacterial PSI and maintains the position of most transmembrane helices and chlorophylls during 1.5 years of separate evolution. The LHCI is composed of four nuclear gene products (Lhca1-Lhca4) that are unique among the chlorophyll a/b binding proteins in their pronounced long-wavelength absorbance and their assembly into dimers. In this respect, we describe structural elements, which establish the biological significance of a plant PSI and discuss structural variance from the cyanobacterial version. The present comprehensive structural analysis summarizes our current state of knowledge, providing the first glimpse at the architecture of this highly efficient photochemical machine at the atomic level.

Chlorophyll fluorescence emission spectrum inside a leaf

Chlorophyll a fluorescence can be used as an early stress indicator. Fluorescence is also connected to photosynthesis so it can be proposed for global monitoring of vegetation status from a satellite platform. Nevertheless, the correct interpretation of fluorescence requires accurate physical models. The spectral shape of the leaf fluorescence free of any re-absorption effect plays a key role in the models and is difficult to measure. We present a vegetation fluorescence emission spectrum free of re-absorption based on a combination of measurements and modelling. The suggested spectrum takes into account the photosystem I and II spectra and their relative contribution to fluorescence. This emission spectrum is applicable to describe vegetation fluorescence in biospectroscopy and remote sensing.