Light regulation of photosynthetic membrane structure, organization, and function

Insight into the regulatory networks underlying the high lipid perennial ryegrass growth under different irradiances

Under favourable conditions, perennial ryegrass (Lolium perenne) engineered to accumulated high lipid (HL) carbon sink in their leaves was previously shown to also enhance photosynthesis and growth. The greater aboveground biomass was found to be diminished in a dense canopy compared to spaced pots. Besides, the underlying genetic regulatory network linking between leaf lipid sinks and these physiological changes remains unknown. In this study, we demonstrated that the growth advantage was not displayed in HL Lolium grown in spaced pots under low lights. Under standard lights, analysis of differentiating transcripts in HL Lolium reveals that the plants had elevated transcripts involved in lipid metabolism, light capturing, photosynthesis, and sugar signalling while reduced expression of genes participating in sugar biosynthesis and transportation. The plants also had altered several transcripts involved in mitochondrial oxidative respiration and redox potential. Many of the above upregulated or downregulated transcript levels were found to be complemented by growing the plants under low light. Overall, this study emphasizes the importance of carbon and energy homeostatic regulatory mechanisms to overall productivity of the HL Lolium through photosynthesis, most of which are significantly impacted by low irradiances.

Growth-phase dependent morphological alteration in higher plant thylakoid is accompanied by changes in both photodamage and repair rates

Thylakoid membranes of young leaves consist of grana and stroma lamellae (stroma-grana [SG] structure). The SG thylakoid is gradually converted into isolated grana (IG), almost lacking the stroma lamellae during growth. This morphological alteration was found to cause a reduction in maximum photosynthetic rate and an enhancement of photoinhibition in photosystem II (PSII). In situ microspectrometric measurements of chlorophyll fluorescence in individual chloroplasts suggested an increase of the PSII/PSI ratio in IG thylakoids of mature leaves. Western blot analysis of isolated IG thylakoids showed relative increases in some PSII components, including the core protein (D1) and light-harvesting components CP24 and Lhcb2. Notably, a nonphotochemical quenching-related factor in the PSII supercomplex, PsbS, decreased by 40%. Changes in the high light response of PSII were detected through parameters of pulse-amplitude modulation fluorometry. Chlorophyll fluorescence lifetime indicated an increase of fluorescence quantum yield in IG. A minimal photodamage-repair rate analysis on a lincomycin treatment of the leaves indicated that repair rate constant of IG is slower than that of SG, while photodamage rate of IG is higher than that of SG. These results suggest that IG thylakoids are relatively sensitive to high light, which is not only due to a higher photodamage rate caused by some rearrangements of PS complexes, but also to the retarded PSII repair that may result from the lack of stroma lamellae. The IG thylakoids found among many plant species thus seem to be an adaptive form to low light environments, although their physiological roles still remain unclear.

Pretilachlor toxicity is decided by discrete photo-acclimatizing conditions: Physiological and biochemical evidence from Anabaena sp. and Nostoc muscorum

The current study was undertaken to elucidate the impact of the herbicide pretilachlor (3 µg ml^-1 and 6 µg ml^-1) on cyanobacteria, Nostoc muscorum ATCC 27893 and Anabaena sp. PCC 7120 under three levels of photoacclimatization (suboptimum, 25 µmol photon m^-2 s^-1; optimum, 75 µmol photon m^-2 s^-1; and supra-optimum, 225 µmol photon m^-2 s^-1) by analyzing certain physiological (biomass accumulation, photosynthesis, Chl a fluorescence and respiration) and biochemical parameters (photosynthetic pigments‒ chlorophyll a, carotenoids and phycocyanin; reactive oxygen species‒ O₂^•¯, H₂O₂, lipid peroxidation; antioxidant system‒ superoxide dismutase, peroxidise, catalase and glutathione-S-transferase). The light conditioning played the most prominent role in deciding the extent of herbicide toxicity on both the tested cyanobacteria as the maximum toxicity was observed in suboptimum light acclimatized cyanobacterial cells corroborated by the least growth in the same cells. The impact of pretilachlor treatment on photosystem II photochemistry viz. φP₀, Ѱ₀, φE₀, PI_ABS, ABS/RC, TR₀/RC, ET₀/RC and DI₀/RC was also altered by light acclimatization. The percent rise in oxidative stress markers (SOR and H₂O₂) and consequent lipid peroxidation (MDA equivalents) were also highest in suboptimum light acclimatized cells exposed to pretilachlor which could not be prospered with compatible antioxidant performance. Conversely, supra-optimum light acclimatized cells of both the cyanobacteria was found to accelerate the activities of all the studied enzymes and thus able to counterbalance the pretilachlor toxicity and supported the healthier growth.

Analysis of an Arabidopsis heat-sensitive mutant reveals that chlorophyll synthase is involved in reutilization of chlorophyllide during chlorophyll turnover

Chlorophylls, the most abundant pigments in the photosynthetic apparatus, are constantly turned over as a result of the degradation and replacement of the damage-prone reaction center D1 protein of photosystem II. Results from isotope labeling experiments suggest that chlorophylls are recycled by reutilization of chlorophyllide and phytol, but the underlying mechanism is unclear. In this study, by characterization of a heat-sensitive Arabidopsis mutant we provide evidence of a salvage pathway for chlorophyllide a. A missense mutation in CHLOROPHYLL SYNTHASE (CHLG) was identified and confirmed to be responsible for a light-dependent, heat-induced cotyledon bleaching phenotype. Following heat treatment, mutant (chlg-1) but not wild-type seedlings accumulated a substantial level of chlorophyllide a, which resulted in a surge of phototoxic singlet oxygen. Immunoblot analysis suggested that the mutation destabilized the chlorophyll synthase proteins and caused a conditional blockage of esterification of chlorophyllide a after heat stress. Accumulation of chlorophyllide a after heat treatment occurred during recovery in the dark in the light-grown but not the etiolated seedlings, suggesting that the accumulated chlorophyllides were not derived from de novo biosynthesis but from de-esterification of the existing chlorophylls. Further analysis of the triple mutant harboring the CHLG mutant allele and null mutations of CHLOROPHYLLASE1 (CLH1) and CLH2 indicated that the known chlorophyllases are not responsible for the accumulation of chlorophyllide a in chlg-1. Taken together, our results show that chlorophyll synthase acts in a salvage pathway for chlorophyll biosynthesis by re-esterifying the chlorophyllide a produced during chlorophyll turnover.

Gross morphological changes in thylakoid membrane structure are associated with photosystem I deletion in Synechocystis sp. PCC 6803

Cells of Synechocystis sp. PCC 6803 lacking photosystem I (PSI-less) and containing only photosystem II (PSII) or lacking both photosystems I and II (PSI/PSII-less) were compared to wild type (WT) cells to investigate the role of the photosystems in the architecture, structure, and number of thylakoid membranes. All cells were grown at 0.5μmol photons m(-2)s(-1). The lumen of the thylakoid membranes of the WT cells grown at this low light intensity were inflated compared to cells grown at higher light intensity. Tubular as well as sheet-like thylakoid membranes were found in the PSI-less strain at all stages of development with organized regular arrays of phycobilisomes on the surface of the thylakoid membranes. Tubular structures were also found in the PSI/PSII-less strain, but these were smaller in diameter to those found in the PSI-less strain with what appeared to be a different internal structure and were less common. There were fewer and smaller thylakoid membrane sheets in the double mutant and the phycobilisomes were found on the surface in more disordered arrays. These differences in thylakoid membrane structure most likely reflect the altered composition of photosynthetic particles and distribution of other integral membrane proteins and their interaction with the lipid bilayer. These results suggest an important role for the presence of PSII in the formation of the highly ordered tubular structures.

Light quality regulates expression of chloroplast genes and assembly of photosynthetic membrane complexes

The concentrations of photosystem I (PSI) and photosystem II (PSII) reaction centers and the level of chloroplast reaction center gene transcripts were determined in pea plants grown under different light-quality regimes. In plants grown in light primarily absorbed by PSI ("red" light), the PSII/PSI reaction center ratio was 2-fold greater than that in plants grown in PSII-sensitizing ("yellow") light. In addition, the ratio of a PSII gene (psbB) transcript to a PSI gene (psaA) transcript was 2.6 times greater in red-grown plants relative to yellow-grown plants. Thus, a differential reaction-center concentration in the thylakoid membrane was accompanied by a differential expression of reaction center genes, suggesting that the synthesis of chloroplast membrane complexes and the assembly of photosystems are regulated by light quality at the transcriptional and/or post-transcriptional level.

Effects of glycerol on the fluorescence spectra and chloroplast ultrastructure of Phaeodactylum tricornutum (Bacillariophyta)

Responses of the photosynthetic activity of Phaeodactylum tricornutum (Bacillariophyta) to organic carbon glycerol were investigated. The growth rate, photosynthetic pigments, 77 K fluorescence spectra, and chloroplast ultrastructure of P. tricornutum were examined under photoautotrophic, mixotrophic, and photoheterotrophic conditions. The results showed that the specific growth rate was the fastest under mixotrophic conditions. The cell photosynthetic pigment content and values of Chl a/Chl c were reduced under mixotrophic and photoheterotrophic conditions. The value of carotenoid/Chl a was enhanced under mixotrophic conditions, but was decreased under photoheterotrophic conditions. In comparison with photoautotrophic conditions, the fluorescence emission peaks and fluorescence excitation peaks were not shifted. The relative fluorescence of photosystem (PS) I and PS II and the values of F685/F710 and F685/F738 were decreased. Chloroplast thylakoid pairs were less packed under mixotrophic and photoheterotrophic conditions. There was a strong correlation between degree of chloroplast thylakoid packing and the excitation energy kept in PS II. These results suggested that the PS II activity was reduced by glycerol under mixotrophic conditions, thereby leading to repression of the photosynthetic activity.

Chloroplast division

Chloroplasts are descendants of cyanobacteria and divide by binary fission. Several components of the division apparatus have been identified in the past several years and we are beginning to appreciate the plastid division process at a mechanistic level. In this review, we attempt to summarize the most recent developments in the field and assemble these observations into a working model of plastid division in plants.

Comparative profiling of lipid-soluble antioxidants and transcripts reveals two phases of photo-oxidative stress in a xanthophyll-deficient mutant of Chlamydomonas reinhardtii

Excess light can impose severe oxidative stress on photosynthetic organisms. We have characterized high-light responses in wild-type Chlamydomonas reinhardtii and in the npq1 lor1 double mutant. The npq1 lor1 strain lacks two photoprotective carotenoids, lutein and zeaxanthin, and experiences acute photo-oxidative stress upon exposure to excess light. To examine the ability of npq1 lor1 cells to respond to photo-oxidative stress, we measured changes in lipid-soluble antioxidants following a shift from low light to high light in the wild type and the double mutant. The size of the xanthophyll cycle pool increased in both the wild type and mutant during the first 6 h of exposure to high light levels, but then decreased in the mutant during photo-oxidative bleaching. The level of alpha-tocopherol (vitamin E) was constant in the wild type and mutant during the first 6 h; then it increased by three-fold in the wild type but declined in npq1 lor1 cells. We also used cDNA microarrays and RNA gel-blot analysis to monitor differences in gene expression. Both strains showed an initial light-stress response in the form of a transient increase in expression of (1) GPXH, a glutathione peroxidase gene that has been shown to respond specifically to singlet oxygen and lipid peroxidation; (2) SMT1, a gene for a putative sterol C-methyltransferase; and (3) LI818r, a stress-responsive member of the light-harvesting complex superfamily. These transient changes in gene expression in high light were followed by a second series of changes in npq1 lor1, coincident with declines in lipid-soluble antioxidants but preceding detectable photo-oxidative damage to proteins and lipids. Thus, the response of npq1 lor1 to high light is unexpectedly complex, with initial changes in lipid-soluble antioxidants and RNA levels that are associated with acclimation in the wild type and a second wave of changes that accompanies photo-oxidative bleaching.

Light quality (red:far-red ratio): does it affect photosynthetic activity, net CO2 assimilation, and morphology of young white clover leaves?

We assessed the effects of red:far-red (R:FR) ratio on net CO2 assimilation, photosynthetic activity, and morphology of young white clover leaves developed under a simulated canopy light environment. Isolated cuttings of Trifolium repens L. were grown in controlled conditions under two irradiances in combination with two R:FR ratios. We used far-red light emitting diodes to reduce the R:FR ratio on leaves that were successively developing on the main stolon and were subject to strong competition for light within a sward. A leaf that developed under a low R:FR ratio had a lower chlorophyll content than the controls. However, this change had no consequences on the net CO2 assimilation rate and on the biomass allocated to shoots. The low R:FR ratio modified the leaf morphology by increasing the leaf area. A leaf that developed in lower irradiance had a lower net CO2 assimilation rate and a greater leaf area and petiole length than leaves that developed in higher irradiance. Therefore, irradiance was the main factor limiting the leaf CO2 assimilation under a canopy. Capture of light by clover leaves was improved by FR light only via modifications of leaf morphology, because there were no significant effects of FR light on photosynthesis.

Structural analysis of photosystem II in far-red-light-adapted thylakoid membranes. New crystal forms provide evidence for a dynamic reorganization of light-harvesting antennae subunits

We studied two-dimensional crystals of the major pigment-protein complex, photosystem II, in far-red-light-adapted thylakoid membranes of the viridis-zb63 mutant of barley. Significantly larger grana membranes were produced with an increased synthesis of the entire photosystem II complex. These red-light-adapted membranes also contained two-dimensional crystals with a high frequency. Three different crystal forms of photosystem II were observed, providing the following data which further our understanding of the architecture of the native complex. (a) The oligomeric form of photosystem II in the membrane was monomeric in all crystal forms, but with a clear non-crystallographic pseudo-twofold symmetry. This was more apparent on the lumenal face of the complex. (b) The variability of unit cell contacts in different crystal forms implied that the peripheral light-harvesting antenna complex and the core of the complex were loosely connected. These peripheral subunits were predicted to rearrange so that they can either encircle the core complex or associate in parallel channels separated by lines of core complexes. (c) Grana membranes were found to retain a double-layered inside-out character, with a stromal face-to-stromal face packing. However, the presence of a crystal in one membrane did not necessarily impose crystallinity on its pair.

A dynamin-like protein in Arabidopsis thaliana is involved in biogenesis of thylakoid membranes

Dynamin, a GTP-binding protein found in rat brain, plays a role in endocytosis. Suborganellar fractionation studies of Arabidopsis leaf tissue revealed that a dynamin-like protein, ADL1, is localized in the thylakoid membranes of chloroplasts. This notion was supported further by in vivo targeting experiments using an ADL1-green fluorescent fusion protein and immunogold labeling with the anti-ADL1 antibody. Transgenic plants harboring various deletion mutant genes of ADL1 had a yellow leaf phenotype where the cells had very few chloroplasts. In addition, the remaining chloroplasts appeared morphologically not fully developed. The detailed structure of the chloroplasts revealed by electron microscopy showed a greatly reduced amount of thylakoid membranes. Also, the level of thylakoid membrane proteins such as the light-harvesting complex II and CP29 was greatly reduced in these transgenic plants. When we examined the expression of the ADL1 deletion mutant genes, these genes were highly expressed at the transcriptional level. However, the mutant ADL1s were not detectable at the protein level by Western blot analysis. Moreover, the endogenous ADL1 protein level was greatly reduced in these transgenic plants, probably due to a post-transcriptional silencing effect of the transgenes. We propose, therefore, that ADLl is involved in the biogenesis of thylakoid membranes.

Photosynthetic light gradients and spectral régime within leaves of Medicago sativa

By using a fibre-optic probe, light gradients were measured at 450, 550 and 680 nm in sun leaves, 125 µm thick, of Medicago sativa L. cv. Armor. The space irradiances immediately beneath the leaf surface were 1.5-2.0 times greater than the incident light for these wavelengths, indicating that M. sativa leaves are efficient light traps. Although the palisade was only 60 µm thick, each light gradient declined steeply within this layer. More light appeared to be scattered in forward rather than backward directions and the spectral régime of the light fluxes depended upon their direction of travel within the leaf. Spectra for transmitted light were dependent upon depth within the leaf, whereas back-scattered light consisted of mostly green and far-red light at all depths, PAR (photosynthetically active radiation, 400-700 nm) within both the palisade and spongy mesophyll consisted mostly of green and far-red light, and the spongy mesophyll received only 0.11 of the PAR compared with the midregion of the palisade. Anomalous measurements within the palisade were traced to the epidermis, which was found to act as a mosaic of microlenses that focused light within the palisade layer. In M. sativa leaves, the light microenvironment, leaf anatomy and photosynthesis seem to be strongly interrelated.

Chromatic regulation inChlamydomonas reinhardtii alters photosystem stoichiometry and improves the quantum efficiency of photosynthesis

The work addressed the adjustment of the photosystem ratio in the green algaChlamydomonas reinhardtii. It is shown that green algae, much like cyanophytes and higher plants, adjust and optimize the ratio of the two photosystems in chloroplasts in response to the quality of irradiance during growth. Such adjustments are compensation reactions and helpC. reinhardtii to retain a quantum efficiency of oxygen evolution near the theoretical maximum. Results show variable amounts of PS I and a fairly constant amount of PS II in chloroplasts and suggest that photosystem stoichiometry adjustments, occurring in response to the quality of irradiance during plant growth, are mainly an adjustment in the concentration of PS I. The work delineates chromatic effects on chlorophyll accumulation in the chloroplast ofC. reinhardtii from those pertaining to the regulation of the PS I/PS II ratio. The detection of the operation of a molecular feedback mechanism for the PS I/PS II ratio adjustment in green algae strengthens the notion of the highly conserved nature of this mechanism among probably all oxygen evolving photosynthetic organisms. Findings in this work are expected to serve as the basis of future biochemical and mutagenesis experiments for the elucidation of the photosystem ratio adjustment in oxygenic photosynthesis.

The PSII-S protein of higher plants: a new type of pigment-binding protein

An intrinsic 22 kDa protein of photosystem II has been shown to possess high sequence homology with the CAB gene products, but differs from these proteins by an additional putative fourth transmembrane helix. This protein, designated PSII-S in accordance with the assignment of the name psbS to its gene, has been isolated by nonionic detergents and preparative isoelectric focusing in this study. The isolated PSII-S protein was shown to bind 5 chlorophyll molecules (a and b) per protein unit and also several different kinds of carotenoids. The room temperature absorption spectrum of the Qy transition of the chlorophylls bound to the isolated protein is characterized by a broad band with a maximum at 671 nm. The 77 K fluorescence spectrum exhibits a peak at 672 nm. A single photon counting technique was applied to resolve the room temperature decay kinetics of the first excited singlet states in the chlorophyll ensemble of the PSII-S protein. The data can be satisfactorily described by triexponential kinetics with lifetimes of tau 1 = 1.8 ns, tau 2 = 4.4 ns, and tau 3 = 6.1 ns and normalized amplitudes of 0.09, 0.60, and 0.31, respectively. Circular dichroism spectra suggest that, in contrast to LHCII, virtually no pigment coupling exists in the PSII-S protein. Two copies of the PSII-S protein were found per PSII in spinach thylakoids. It displays an unusually extreme lateral heterogeneity, since the PSII beta centers located in the stroma exposed thylakoid regions contained only residual amounts of the PSII-S protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Adjustments of photosystem stoichiometry in chloroplasts improve the quantum efficiency of photosynthesis

The efficiency of photosynthetic electron transport depends on the coordinated interaction of photosystem II (PSII) and photosystem I (PSI) in the electron-transport chain. Each photosystem contains distinct pigment-protein complexes that harvest light from different regions of the visible spectrum. The light energy is utilized in an endergonic electron-transport reaction at each photosystem. Recent evidence has shown a large variability in the PSII/PSI stoichiometry in plants grown under different environmental irradiance conditions. Results in this work are consistent with the notion of a dynamic, rather than static, thylakoid membrane in which the stoichiometry of the two photosystems is adjusted and optimized in response to different light quality conditions. Direct evidence is provided that photosystem stoichiometry adjustments in chloroplasts are a compensation strategy designed to correct unbalanced absorption of light by the two photosystems. Such adjustments allow the plant to maintain a high quantum efficiency of photosynthesis under diverse light quality conditions and constitute acclimation that confers to plants a significant evolutionary advantage over that of a fixed photosystem stoichiometry in thylakoid membranes.

Quantitation of plastoquinone photoreduction in spinach chloroplasts

The question of plastoquinone (PQ) concentration and its stoichiometry to photosystem I (PSI) and PSII in spinach chloroplasts is addressed here. The results from three different experimental approaches were compared. (a) Quantitation from the light-induced absorbance change at 263 nm (ΔA263) yielded the following ratios (mol:mol); Chl:PQ=70:1, PQ:PSI=9:1 and PQ:PSIIα=7:1. The kinetics of PQ photoreduction were a monophasic but non-exponential function of time. The deviation of the semilogarithmic plots from linearity reflects the cooperativity of several electron transport chains at the PQ pool level. (b) Estimates from the area over the fluorescence induction curve (Afl) tend to exaggerate the PQ pool size because of electron transfer via PSI to molecular oxygen (Mehler reaction) resulting in the apparent increase of the pool of electron acceptors. The reliability of the Afl method is increased substantially upon plastocyanin inhibition by KCN. (c) Quantitation of the number of electrons removed from PQH2 by PSI, either under far-red excitation or after the addition of DCMU to preilluminated chloroplasts, is complicated due to the competitive loss of electrons from PQH2 to molecular oxygen. The latter is biphasic reaction occurring with half-times of about 2 s (30-40% of PQH2) and of about 60 s (60-70% of PQH2).

Phosphorylation of chloroplast membrane proteins partially protects against photoinhibition

Thylakoids isolated from peas (Pisum sativum cv. Kelvedon Wonder) and phosphorylated by incubation with ATP have been compared with non-phosphorylated thylakoids in their sensitivity to photoinhibition by exposure to illumination in vitro. Assays of the kinetics of fluorescence induction at 20° C and the fluorescence emission spectra at-196° C indicate a proportionally larger decrease in fluorescence as a result of photoinhibitory treatment of non-phosphorylated compared with phosphorylated thylakoids. It is concluded that protein phosphorylation can afford partial protection to thylakoids exposed to photoinhibitory conditions.

Effect of light quality on chloroplast-membrane organization and function in pea

The effect of light quality during plant growth of chloroplast membrane organization and function in peas (Pisum sativum L. cv. Alaska) was investigated. In plants grown under photosystem (PS) I-enriched (far-red enriched) illumination both the PSII/PSI stoichiometry and the electrontransport capacity ratios were high, about 1.9. In plants grown under PSII-enriched (far-red depleted) illumination both the PSII/PSI stoichiometry and the electron-transport capacity ratios were significantly lower, about 1.3. In agreement, steady-state electron-transport measurements under synchronous illumination of PSII and PSI demonstrated an excess of PSII in plants grown under far-red-enriched light. Sodium dodecylsulfate polyacrylamide gel electrophoretic analysis of chlorophyll-containing complexes showed greater relative amounts of the PSII reaction center chlorophyll-protein complex in plants grown under farred-enriched light. Additional changes were observed in the ratio of light-harvesting chlorophyll a/b protein to PSII reaction center chlorophyll-protein under the two different light-quality regimes. The results demonstrate the dynamic nature of chloroplast structure and support the notion that light quality is an important factor in the regulation of chloroplast membrane organization and-function.