Digalactosyl-diacylglycerol deficiency impairs the capacity for photosynthetic intersystem electron transport and state transitions in Arabidopsis thaliana due to photosystem I acceptor-side limitations

Evolutionary implications from lipids in membrane bilayers and photosynthetic complexes in cyanobacteria and chloroplasts

In biomembranes, lipids form bilayer structures that serve as the fluid matrix for membrane proteins and other hydrophobic compounds. Additionally, lipid molecules associate with membrane proteins and impact their structures and functions. In both cyanobacteria and the chloroplasts of plants and algae, the lipid bilayer of the thylakoid membrane consists of four distinct glycerolipid classes: monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol. These lipids are also integral components of photosynthetic complexes such as photosystem II and photosystem I. The lipid-binding sites within the photosystems, as well as the lipid composition in the thylakoid membrane, are highly conserved between cyanobacteria and photosynthetic eukaryotes, and each lipid class has specific roles in oxygenic photosynthesis. This review aims to shed light on the potential evolutionary implications of lipid utilization in membrane lipid bilayers and photosynthetic complexes in oxygenic photosynthetic organisms.

Genetically-determined variations in photosynthesis indicate roles for specific fatty acid species in chilling responses

Using a population of recombinant inbred lines (RILs) cowpea (Vigna unguiculata. L. Walp), we tested for co-linkages between lipid contents and chilling responses of photosynthesis. Under low-temperature conditions (19°C/13°C, day/night), we observed co-linkages between quantitative trait loci intervals for photosynthetic light reactions and specific fatty acids, most strikingly, the thylakoid-specific fatty acid 16:1^Δ3trans found exclusively in phosphatidylglycerol (PG 16:1t). By contrast, we did not observe co-associations with bulk polyunsaturated fatty acids or high-melting-point-PG (sum of PG 16:0, PG 18:0 and PG 16:1t) previously thought to be involved in chilling sensitivity. These results suggest that in cowpea, chilling sensitivity is modulated by specific lipid interactions rather than bulk properties. We were able to recapitulate the predicted impact of PG 16:1t levels on photosynthetic responses at low temperature using mutants and transgenic Arabidopsis lines. Because PG 16:1t synthesis requires the activity of peroxiredoxin-Q, which is activated by H₂ O₂ and known to be involved in redox signalling, we hypothesise that the accumulation of PG 16:1t occurs as a result of upstream effects on photosynthesis that alter redox status and production of reactive oxygen species.

Lipids in photosynthetic protein complexes in the thylakoid membrane of plants, algae, and cyanobacteria

In the thylakoid membrane of cyanobacteria and chloroplasts, many proteins involved in photosynthesis are associated with or integrated into the fluid bilayer matrix formed by four unique glycerolipid classes, monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol. Biochemical and molecular genetic studies have revealed that these glycerolipids play essential roles not only in the formation of thylakoid lipid bilayers but also in the assembly and functions of photosynthetic complexes. Moreover, considerable advances in structural biology have identified a number of lipid molecules within the photosynthetic complexes such as PSI and PSII. These data have provided important insights into the association of lipids with protein subunits in photosynthetic complexes and the distribution of lipids in the thylakoid membrane. Here, we summarize recent high-resolution observations of lipid molecules in the structures of photosynthetic complexes from plants, algae, and cyanobacteria, and evaluate the distribution of lipids among photosynthetic protein complexes and thylakoid lipid bilayers. By integrating the structural information into the findings from biochemical and molecular genetic studies, we highlight the conserved and differentiated roles of lipids in the assembly and functions of photosynthetic complexes among plants, algae, and cyanobacteria.

The decreased PG content of pgp1 inhibits PSI photochemistry and limits reaction center and light-harvesting polypeptide accumulation in response to cold acclimation

Decreased PG constrains PSI activity due to inhibition of transcript and polypeptide abundance of light-harvesting and reaction center polypeptides generating a reversible, yellow phenotype during cold acclimation of pgp1. Cold acclimation of the Arabidopsis pgp1 mutant at 5 °C resulted in a pale-yellow phenotype with abnormal chloroplast ultrastructure compared to its green phenotype upon growth at 20 °C despite a normal cold-acclimation response at the transcript level. In contrast, wild type maintained its normal green phenotype and chloroplast ultrastructure irrespective of growth temperature. In contrast to cold acclimation of WT, growth of pgp1 at 5 °C limited the accumulation of Lhcbs and Lhcas assessed by immunoblotting. However, a novel 43 kD polypeptide of Lhcb1 as well as a 29 kD polypeptide of Lhcb3 accumulated in the soluble fraction which was absent in the thylakoid membrane fraction of cold-acclimated pgp1 which was not observed in WT. Cold acclimation of pgp1 destabilized the Chl-protein complexes associated with PSI and predisposed energy distribution in favor of PSII rather than PSI compared to the WT. Functionally, in vivo PSI versus PSII photochemistry was inhibited in cold-acclimated pgp1 to a greater extent than in WT relative to controls. Greening of the pale-yellow pgp1 was induced when cold-acclimated pgp1 was shifted from 5 to 20 °C which resulted in a marked decrease in excitation pressure to a level comparable to WT. Concomitantly, Lhcbs and Lhcas accumulated with a simultaneous decrease in the novel 43 and 29kD polypeptides. We conclude that the reduced levels of phosphatidyldiacylglycerol in the pgp1 limit the capacity of the mutant to maintain the structure and function of its photosynthetic apparatus during cold acclimation. Thus, maintenance of normal thylakoid phosphatidyldiacylglycerol levels is essential to stabilize the photosynthetic apparatus during cold acclimation.

Galactosyldiacylglycerols: From a Photosynthesis-Associated Apparatus to Structure-Defined In Vitro Assembling

Being ubiquitously present in plants, microalgae, and cyanobacteria and as the major constituents of thylakoid membranes, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) make up approximately 52 and 26%, respectively, of chloroplast lipids. Thylakoid membranes harbor the photosynthetic complexes and numerous essential biochemical pathways where MGDG and DGDG play a central role in facilitating photosynthesis light reaction, maintaining chloroplast morphology, and responding to abiotic stresses. Furthermore, these galactolipids are also bioactive compounds with antitumor, antimicrobial, antiviral, immunosuppressive, and anti-inflammatory activities and important nutritional value. These characteristics are strictly dependent upon their fatty acyl chain length, olefinic nature, and stereoconfiguration. However, their application potentials are practically untapped, largely as a result of the fact that their availability in large quantity and high purity (structured galactolipids) is challenging. In addition to laborious extraction from natural sources, in vitro assembling of these molecules could be a promising alternative. Thus, this review updates the latest advances in elucidating biosynthesis paths of MGDG and DGDG and related enzyme systems, which present invaluable inspiration to design approaches for a retrosynthesis of galactolipids. More critically, this work summarizes recent developments in the biological and enzymatic syntheses of galactolipids, especially the strategic scenarios for the construction of in vitro enzymatic and/or chemoenzymatic synthesis routes. Protein engineering of enzymes involved in the synthesis of MGDG and DGDG to improve their properties is highlighted, and the applications of galactolipids in foods and medicine are also discussed.

Chloroplast Lipids Metabolism and Function. A Redox Perspective

Plant productivity is determined by the conversion of solar energy into biomass through oxygenic photosynthesis, a process performed by protein-cofactor complexes including photosystems (PS) II and I, and ATP synthase. These complexes are embedded in chloroplast thylakoid membrane lipids, which thus function as structural support of the photosynthetic machinery and provide the lipid matrix to avoid free ion diffusion. The lipid and fatty acid composition of thylakoid membranes are unique in chloroplasts and cyanobacteria, which implies that these molecules are specifically required in oxygenic photosynthesis. Indeed, there is extensive evidence supporting a relevant function of glycerolipids in chloroplast biogenesis and photosynthetic efficiency in response to environmental stimuli, such as light and temperature. The rapid acclimation of higher plants to environmental changes is largely based on thiol-based redox regulation and the disulphide reductase activity thioredoxins (Trxs), which are reduced by ferredoxin (Fdx) via an Fdx-dependent Trx reductase. In addition, chloroplasts harbour an NADPH-dependent Trx reductase C, which allows the use of NADPH to maintain the redox homeostasis of the organelle. Here, we summarise the current knowledge of chloroplast lipid metabolism and the function of these molecules as structural basis of the complex membrane network of the organelle. Furthermore, we discuss evidence supporting the relevant role of lipids in chloroplast biogenesis and photosynthetic performance in response to environmental cues in which the redox state of the organelle plays a relevant role.

Protection of PSI and PSII complexes of wheat from toxic effect of anthracene by Bacillus subtilis (NCIM 5594)

Contamination of polycyclic aromatic hydrocarbons (PAHs) in environment indicates a serious problem to the present era. These are carcinogenic and mutagenic compounds and pose a potential risk to photosynthetic organisms. The present study illustrates the protection of Photosystem I and Photosystem II complexes of wheat plant by Bacillus subtilis (NCIM 5594) from toxic effects of anthracene (ANT). Initially, Chl a fluorescence induction curve measurement revealed declined J-I and I-P phase in ANT-treated plants. Efficiency of light absorption, trapping, and electron transport was reduced in ANT-treated plants, while in ANT + Bacillus subtilis (NCIM 5594)-treated plants value of these parameters was restored. Effect of ANT and ANT + Bacillus subtilis (NCIM 5594) on energy conversion of Photosystem I and Photosystem II was measured. Quantum yield of Photosystem I (YI) and Photosystem II (YII) was decreased in the presence of ANT, while these values were recovered in ANT + Bacillus subtilis (NCIM 5594)-treated plants. Reduction in Y(II) was associated with an increase in non-regulated energy dissipation NO. Likewise the reduction of Y(I) was induced due to donor-side and acceptor-side limitation of Photosystem I caused by toxic effect of ANT. Toxic effects of ANT on electron transport rate (ETR_I and ETR_II) were found to be reduced in ANT + Bacillus subtilis (NCIM 5594)-treated plants. Activation of Cyclic electron flow around Photosystem I in ANT-treated plants was recovered by bacteria. It was concluded that toxic effect of ANT on Photosystem I and Photosystem II complexes was recovered by Bacillus subtilis (NCIM 5594) strain, and thus it is useful strain for crop improvement in ANT-polluted soil.

Galactolipid DGDG and Betaine Lipid DGTS Direct De Novo Synthesized Linolenate into Triacylglycerol in a Stress-Induced Starchless Mutant of Chlamydomonas reinhardtii

The increasing demand for triacylglycerol (TAG) enriching polyunsaturated fatty acids (PUFAs) has led to a surge of interest in microalgal TAG metabolism. Polar membrane lipids serve as the desaturation carrier for PUFA, and the functional group of PUFA can be incorporated into TAG. Monogalactoglycerolipid has been found to provide the de novo synthesized oleate acyl group or the nascent polyunsaturated diacylglycerol backbone for TAG biosynthesis in the model green alga, Chlamydomonas reinhardtii. However, whether other membrane lipids take part in the formation of PUFA-attached TAG has not been clearly discovered. A time course study of glycerolipidomics in the starchless mutant of C. reinhardtii, BAFJ5, which hyper-accumulates TAG, revealed that digalactosyldiacylglycerol (DGDG) and diacylglycerol-N,N,N-trimethylhomoserine (DGTS) turned into the main components of membrane lipids, accounting for 62% of the total polar lipids, under nitrogen deprivation combined with high light conditions. In addition, the membrane lipid molecules DGDG 18:3n3/16:0 and DGTS 16:0/18:3n6 were presumed to be involved in the consecutive integration of the de novo synthesized linolenates into TAG. Based on the stoichiometry calculation, DGDG and DGTS were demonstrated to provide a major contribution to the accumulation of linolenate-attached TAG. Our study gives insights into the potential PUFA-attached TAG formation pathway mediated by the turnover of de novo synthesized DGDG and DGTS in the starchless mutant of Chlamydomonas.

Galactolipid deficiency disturbs spatial arrangement of the thylakoid network in Arabidopsis thaliana plants

The chloroplast thylakoid network is a dynamic structure which, through possible rearrangements, plays a crucial role in regulation of photosynthesis. Although the importance of the main components of the thylakoid membrane matrix, galactolipids, in the formation of the network of internal plastid membrane was found before, the structural role of monogalactosyldiacylglycerol (MGDG) and digalactosylidacylglycerol (DGDG) is still largely unknown. We elucidated detailed structural modifications of the thylakoid membrane system in Arabidopsis thaliana MGDG- and DGDG-deficient mutants. An altered MGDG/DGDG ratio was structurally reflected by formation of smaller grana, local changes in grana stacking repeat distance, and significant changes in the spatial organization of the thylakoid network compared with wild-type plants. The decrease of the MGDG level impaired the formation of the typical helical grana structure and resulted in a 'helical-dichotomic' arrangement. DGDG deficiency did not affect spatial grana organization but changed the shape of the thylakoid membrane network in situ from lens like into a flattened shape. Such structural disturbances were accompanied by altered composition of carotenoid and chlorophyll-protein complexes, which eventually led to the decreased photosynthetic efficiency of MGDG- and DGDG-deficient plants.

Photosynthetic response in wheat plants caused by the phototoxicity of fluoranthene

Environmental organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) affect photosynthetic performance in plants. The photooxidation of PAHs in natural sunlight, especially UV radiation, enhances the toxicity of PAHs. However, it is unclear as how these compounds and their photoproducts affect the photosynthetic apparatus. In this study, measurements of PSI and PSII were simultaneously performed in wheat (Triticum aestivum L.) plants treated with fluoranthene (FLT) and photomodified fluoranthene (PFLT). The study aimed to investigate whether the phototoxicity of FLT has a different mechanism of toxicity on the two photosystems. With regard to PSII, FLT and PFLT produced a significant decrease in the quantum yield of PSII and a pronounced increase in the yield of nonregulated energy dissipation. A significant reduction was observed in the yield of nonphotochemical quenching. The toxic effects of the PFLT treatment on PSII's performance were more pronounced. Likewise, we noted severe disruption in the electron transport rate in PSII and a decline in Fm caused by FLT phototoxicity. A decline in the quantum yield of PSI and an increase in donor and acceptor side limitation were observed concomitantly. The impact of PFLT was more evident than that of FLT. The data demonstrated that PSI is more tolerant of FLT but for PFLT, particularly at higher concentrations, a pronounced inhibition was observed in the oxidation-reduction kinetics of P700. All these data suggest that increased cyclic electron flow can confer greater protection from FLT toxicity but not from toxicity induced by higher concentrations of PFLT.

Arbuscular Mycorrhizal fungi (AMF) protects photosynthetic apparatus of wheat under drought stress

Drought stress (DS) is amongst one of the abiotic factors affecting plant growth by limiting productivity of crops by inhibiting photosynthesis. Damage due to DS and its protection by Arbuscular Mycorrhizal fungi (AMF) was studied on photosynthetic apparatus of wheat (Triticum aestivum) plants in pot experiments. DS was maintained by limiting irrigation to the drought stressed (DS) and AMF + DS plants. Relative Water content (RWC) was measured for leaf as well as soil to ensure drought conditions. DS plants had minimum RWC for both leaf and soil. AMF plants showed increased RWC both for leaf and soil indicating that AMF hyphae penetrated deep into the soil and provided moisture to the plants. In Chl a fluorescence induction curve (OJIP), a declined J-I and I-P phase was observed in DS plants. Efficacy of primary photochemistry declined in DS plants as result of DS, while AMF plants showed maximum photochemistry. DS leads to declined quantum efficiency of PSI and PSII in DS plants while it was restored in AMF + DS plants. Electron transport (ETRI and ETRII) decreased while quantum yield of non-photochemical quenching Y(NPQ) increased as a result of drought stress. CEF around PSI increased in DS-stressed plants. Efficient PSI complexes decreased in DS plants while in case of AMF plants PSI complexes were able to perform PSI photochemistry significantly. Thus, it is concluded that drought stress-induced damage to the structure and function of PSII and PSI was alleviated by AMF colonization.

Structural and functional integrity of Sulla carnosa photosynthetic apparatus under iron deficiency conditions

The abundance of calcareous soils makes bicarbonate-induced iron (Fe) deficiency a major problem for plant growth and crop yield. Therefore, Fe-efficient plants may constitute a solution for use on calcareous soils. We investigated the ability of the forage legume Sulla carnosa (Desf.) to maintain integrity of its photosynthetic apparatus under Fe deficiency conditions. Three treatments were applied: control, direct Fe deficiency and bicarbonate-induced Fe deficiency. At harvest, all organs of deficient plants showed severe growth inhibition, the effect being less pronounced under indirect Fe deficiency. Pigment analysis of fully expanded leaves revealed a reduction in concentrations of chlorophyll a, chlorophyll b and carotenoids under Fe deficiency. Electron transport rate, maximum and effective quantum yield of photosystem II (PSII), photochemical quenching (qP), non-photochemical quenching (qN) as well as P700 activity also decreased significantly in plants exposed to direct Fe deficiency, while qN was not affected. The effects of indirect Fe deficiency on the same parameters were less pronounced in bicarbonate-treated plants. The relative abundances of thylakoid proteins related to PSI (PsaA, Lhca1, Lhca2) and PSII (PsbA, Lhcb1) were also more affected under direct than indirect Fe deficiency. We conclude that S. carnosa can maintain the integrity of its photosynthetic apparatus under bicarbonate-induced Fe deficiency, preventing harmful effects to both photosystems under direct Fe deficiency. This suggests a high capacity of this species not only to take up Fe in the presence of bicarbonate (HCO₃^- ) but also to preferentially translocate absorbed Fe towards leaves and prevent its inactivation.

Prediction of Thylakoid Lipid Binding Sites on Photosystem II

The thylakoid membrane has a unique lipid composition, consisting mostly of galactolipids. These thylakoid lipids have important roles in photosynthesis. Here, we investigate to what extent these lipids bind specifically to the Photosystem II complex. To this end, we performed coarse-grain MD simulations of the Photosystem II complex embedded in a thylakoid membrane with realistic composition. Based on >85 μs simulation time, we find that monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol lipids are enriched in the annular shell around the protein, and form distinct binding sites. From the analysis of residue contacts, we conclude that electrostatic interactions play an important role in stabilizing these binding sites. Furthermore, we find that chlorophyll a has a prevalent role in the coordination of the lipids. In addition, we observe lipids to diffuse in and out of the plastoquinone exchange cavities, allowing exchange of cocrystallized lipids with the bulk membrane and suggesting a more open nature of the plastoquinone exchange cavity. Together, our data provide a wealth of information on protein-lipid interactions for a key protein in photosynthesis.

Specific Distribution of Phosphatidylglycerol to Photosystem Complexes in the Thylakoid Membrane

The thylakoid membrane is the site of photochemical and electron transport reactions of oxygenic photosynthesis. The lipid composition of the thylakoid membrane, with two galactolipids, one sulfolipid, and one phospholipid, is highly conserved among oxygenic photosynthetic organisms. Besides providing a lipid bilayer matrix, thylakoid lipids are integrated in photosynthetic complexes particularly in photosystems I and II and play important roles in electron transport processes. Thylakoid lipids are differentially allocated to photosynthetic complexes and the lipid bilayer fraction, but distribution of each lipid in the thylakoid membrane is unclear. In this study, based on published crystallographic and biochemical data, we estimated the proportions of photosynthetic complex-associated and bilayer-associated lipids in thylakoid membranes of cyanobacteria and plants. The data suggest that ∼30 mol% of phosphatidylglycerol (PG), the only major phospholipid in thylakoid membranes, is allocated to photosystem complexes, whereas glycolipids are mostly distributed to the lipid bilayer fraction and constitute the membrane lipid matrix. Because PG is essential for the structure and function of both photosystems, PG buried in these complexes might have been selectively conserved among oxygenic phototrophs. The specific and substantial allocation of PG to the deep sites of photosystems may need a unique mechanism to incorporate PG into the complexes possibly in coordination with the synthesis of photosynthetic proteins and pigments.

PSI becomes more tolerant to fluoranthene through the initiation of cyclic electron flow

Environmental pollution by organic compounds such as polycyclic aromatic hydrocarbons (PAHs) poses a potential ecological risk to photosynthetic organisms. In the present study, the toxic effects of fluoranthene (FLT) on the energy conversion of PSI and PSII in wheat (Triticum aestivum L.) plants were studied. By evaluating the performance of both PSI and PSII, which act as an internal environmental sensor, it was revealed that activity of both photosystems was negatively affected by FLT treatment. However, the quantum yield of PSII, Y(II), was reduced at 5µM FLT, whereas the quantum yield of PSI, Y(I), significantly decreased at 25µM FLT. The decline in Y(II) was accompanied by an increase in nonregulated energy dissipation, Y(NO). The decrease in Y(I) induced by FLT was caused by donor-side, and acceptor side limitation of PSI. Cyclic electron flow (CEF) was activated only at higher concentrations and was associated with the inhibition of linear electron flow (LEF) after exposure to a higher concentration of FLT. The inhibition of LEF and induction of CEF seems to be essential for the tolerance of PSI to FLT toxicity.

Comparative Phosphoproteomics Reveals an Important Role of MKK2 in Banana (Musa spp.) Cold Signal Network

Low temperature is one of the key environmental stresses, which greatly affects global banana production. However, little is known about the global phosphoproteomes in Musa spp. and their regulatory roles in response to cold stress. In this study, we conducted a comparative phosphoproteomic profiling of cold-sensitive Cavendish Banana and relatively cold tolerant Dajiao under cold stress. Phosphopeptide abundances of five phosphoproteins involved in MKK2 interaction network, including MKK2, HY5, CaSR, STN7 and kinesin-like protein, show a remarkable difference between Cavendish Banana and Dajiao in response to cold stress. Western blotting of MKK2 protein and its T31 phosphorylated peptide verified the phosphoproteomic results of increased T31 phosphopeptide abundance with decreased MKK2 abundance in Daojiao for a time course of cold stress. Meanwhile increased expression of MKK2 with no detectable T31 phosphorylation was found in Cavendish Banana. These results suggest that the MKK2 pathway in Dajiao, along with other cold-specific phosphoproteins, appears to be associated with the molecular mechanisms of high tolerance to cold stress in Dajiao. The results also provide new evidence that the signaling pathway of cellular MKK2 phosphorylation plays an important role in abiotic stress tolerance that likely serves as a universal plant cold tolerance mechanism.

Sulla carnosa modulates root invertase activity in response to the inhibition of long-distance sucrose transport under magnesium deficiency

Being the principal product of photosynthesis, sucrose is involved in many metabolic processes in plants. As magnesium (Mg) is phloem mobile, an inverse relationship between Mg shortage and sugar accumulation in leaves is often observed. Mg deficiency effects on carbohydrate contents and invertase activities were determined in Sulla carnosa Desf. Plants were grown hydroponically at different Mg concentrations (0.00, 0.01, 0.05 and 1.50 mM Mg) for one month. Mineral analysis showed that Mg contents were drastically diminished in shoots and roots mainly at 0.01 and 0.00 mM Mg. This decline was adversely associated with a significant increase of sucrose, fructose and mainly glucose in shoots of plants exposed to severe deficiency. By contrast, sugar contents were severely reduced in roots of these plants indicating an alteration of carbohydrate partitioning between shoots and roots of Mg-deficient plants. Cell wall invertase activity was highly enhanced in roots of Mg-deficient plants, while the vacuolar invertase activity was reduced at 0.00 mM Mg. This decrease of vacuolar invertase activity may indicate the sensibility of roots to Mg starvation resulting from sucrose transport inhibition. ¹⁴ CO₂ labeling experiments were in accordance with these findings showing an inhibition of sucrose transport from source leaves to sink tissues (roots) under Mg depletion. The obtained results confirm previous findings about Mg involvement in photosynthate loading into phloem and add new insights into mechanisms evolved by S. carnosa to cope with Mg shortage in particular the increase of the activity of cell wall invertase.

Role of membrane glycerolipids in photosynthesis, thylakoid biogenesis and chloroplast development

The lipid bilayer of the thylakoid membrane in plant chloroplasts and cyanobacterial cells is predominantly composed of four unique lipid classes; monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG). MGDG and DGDG are uncharged galactolipids that constitute the bulk of thylakoid membrane lipids and provide a lipid bilayer matrix for photosynthetic complexes as the main constituents. The glycolipid SQDG and phospholipid PG are anionic lipids with a negative charge on their head groups. SQDG and PG substitute for each other to maintain the amount of total anionic lipids in the thylakoid membrane, with PG having indispensable functions in photosynthesis. In addition to biochemical studies, extensive analyses of mutants deficient in thylakoid lipids have revealed important roles of these lipids in photosynthesis and thylakoid membrane biogenesis. Moreover, recent studies of Arabidopsis thaliana suggest that thylakoid lipid biosynthesis triggers the expression of photosynthesis-associated genes in both the nucleus and plastids and activates the formation of photosynthetic machineries and chloroplast development. Meanwhile, galactolipid biosynthesis is regulated in response to chloroplast functionality and lipid metabolism at transcriptional and post-translational levels. This review summarizes the roles of thylakoid lipids with their biosynthetic pathways in plants and discusses the coordinated regulation of thylakoid lipid biosynthesis with the development of photosynthetic machinery during chloroplast biogenesis.

Global transcriptome analyses provide evidence that chloroplast redox state contributes to intracellular as well as long-distance signalling in response to stress and acclimation in Arabidopsis

Global transcriptome analyses were used to assess the interactive effects of short-term stress versus long-term acclimation to high light (HL), low temperature (LT) and excitation pressure in Arabidopsis. Microarray analyses indicated that exposure to stress resulted in two times as many modulated transcripts in both, high-light-treated and low-temperature-treated plants, compared to plants that were fully acclimated to either one of these conditions. We showed that 10.9 % of all transcripts were regulated in the same way by both stress conditions, and hence, were categorized as excitation pressure regulated, rather than regulated by either high-light or low-temperature stress per se. This group of chloroplast redox-sensitive genes included various photosynthetic genes as well as genes known to be associated with cold acclimation (cbf3, cor15A, cor15B) and gibberellic acid (GA) metabolism and signalling (ga2ox1, gai). Chemical inhibition of the photosynthetic electron transport by either DCMU or DBMIB indicated that although the plastoquinone pool contributes significantly to redox regulation of the transcriptome (8.6 %), it appears that PSI represents the major source of redox signals (89 %), whereas PSII appears to contribute only 3.1 %. A comparison of the gene expression profiles between stress and acclimated plants indicated that 10 % of the genes induced by a short, 1-h stress were also associated with long-term acclimation to high excitation pressure. This included the APETALA2/ETHYLENE-RESPONSIVE-BINDING PROTEIN family, the MYB domain- and MYB-related transcription factor family as well as the GRAS transcription factor family important in GA signalling confirming that acclimation to stress is a time-nested phenomenon. We suggest that acclimation to photosynthetic redox imbalance extends beyond the chloroplast and the leaf cell to systemic ROS signalling. This is discussed in terms of the control of plant phenotype through regulation of the nuclear encoded cbf regulon and GA metabolism.

Monogalactosyldiacylglycerol and digalactosyldiacylglycerol role, physical states, applications and biomimetic monolayer films

The relevance of biomimetic membranes using galactolipids has not been expressed in any extensive experimental study of these lipids. Thus, on the one hand, we present an in-depth article about the presence and role of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) in thylakoid membranes, their physical states and their applications. On the other hand, we use the Langmuir and Langmuir-Blodgett (LB) techniques to prepare biomimetic monolayers of saturated galactolipids MGDG, DGDG and MGDG:DGDG 2:1 mixture (MD)--biological ratio--. These monolayers are studied using surface pressure-area isotherms and their data are processed to enlighten their physical states and mixing behaviour. These monolayers, once transferred to a solid substrate at several surface pressures are topographically studied on mica using atomic force microscopy (AFM) and using cyclic voltammetry for studying the electrochemical behaviour of the monolayers once transferred to indium-tin oxide (ITO), which has good optical and electrical properties. Moreover, MD presents other differences in comparison with its pure components that are explained by the presence of different kinds of galactosyl headgroups that restrict the optimal orientation of the MGDG headgroups.

Two Hymenophyllaceae species from contrasting natural environments exhibit a homoiochlorophyllous strategy in response to desiccation stress

Hymenophyllaceae is a desiccation tolerant family of Pteridophytes which are poikilohydric epiphytes. Their fronds are composed by a single layer of cells and lack true mesophyll cells and stomata. Although they are associated with humid and shady environments, their vertical distribution varies along the trunk of the host plant with some species inhabiting the drier sides with a higher irradiance. The aim of this work was to compare the structure and function of the photosynthetic apparatus during desiccation and rehydration in two species, Hymenophyllum dentatum and Hymenoglossum cruentum, isolated from a contrasting vertical distribution along the trunk of their hosts. Both species were subjected to desiccation and rehydration kinetics to analyze frond phenotypic plasticity, as well as the structure, composition and function of the photosynthetic apparatus. Minimal differences in photosynthetic pigments were observed upon dehydration. Measurements of ϕPSII (effective quantum yield of PSII), ϕNPQ (quantum yield of the regulated energy dissipation of PSII), ϕNO (quantum yield of non-regulated energy dissipation of PSII), and TL (thermoluminescence) indicate that both species convert a functional photochemical apparatus into a structure which exhibits maximum quenching capacity in the dehydrated state with minimal changes in photosynthetic pigments and polypeptide compositions. This dehydration-induced conversion in the photosynthetic apparatus is completely reversible upon rehydration. We conclude that H. dentatum and H. cruentum are homoiochlorophyllous with respect to desiccation stress and exhibited no correlation between inherent desiccation tolerance and the vertical distribution along the host tree trunk.

Roles of Lipids in Photosynthesis

Thylakoid membranes in cyanobacterial cells and chloroplasts of algae and higher plants are the sites of oxygenic photosynthesis. The lipid composition of the thylakoid membrane is unique and highly conserved among oxygenic photosynthetic organisms. Major lipids in thylakoid membranes are glycolipids, monogalactosyldiacylglycerol, digalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol, and the phospholipid, phosphatidylglycerol. The identification of almost all genes involved in the biosynthesis of each lipid class over the past decade has allowed the generation and isolation of mutants of various photosynthetic organisms incapable of synthesizing specific lipids. Numerous studies using such mutants have revealed that these lipids play important roles not only in the formation of the lipid bilayers of thylakoid membranes but also in the folding and assembly of the protein subunits in photosynthetic complexes. In addition to the studies with the mutants, recent X-ray crystallography studies of photosynthetic complexes in thylakoid membranes have also provided critical information on the association of lipids with photosynthetic complexes and their activities. In this chapter, we summarize our current understanding about the structural and functional involvement of thylakoid lipids in oxygenic photosynthesis.

DGDG and Glycolipids in Plants and Algae

Photosynthetic organelles in plants and algae are characterized by the high abundance of glycolipids, including the galactolipids mono- and digalactosyldiacylglycerol (MGDG, DGDG) and the sulfolipid sulfoquinovosyldiacylglycerol (SQDG). Glycolipids are crucial to maintain an optimal efficiency of photosynthesis. During phosphate limitation, the amounts of DGDG and SQDG increase in the plastids of plants, and DGDG is exported to extraplastidial membranes to replace phospholipids. Algae often use betaine lipids as surrogate for phospholipids. Glucuronosyldiacylglycerol (GlcADG) is a further glycolipid that accumulates under phosphate deprived conditions. In contrast to plants, a number of eukaryotic algae contain very long chain polyunsaturated fatty acids of 20 or more carbon atoms in their glycolipids. The pathways and genes for galactolipid and sulfolipid synthesis are largely conserved between plants, Chlorophyta, Rhodophyta and algae with complex plastids derived from secondary or tertiary endosymbiosis. However, the relative contribution of the endoplasmic reticulum- and plastid-derived lipid pathways for glycolipid synthesis varies between plants and algae. The genes for glycolipid synthesis encode precursor proteins imported into the photosynthetic organelles. While most eukaryotic algae contain the plant-like galactolipid (MGD1, DGD1) and sulfolipid (SQD1, SQD2) synthases, the red alga Cyanidioschyzon harbors a cyanobacterium-type DGDG synthase (DgdA), and the amoeba Paulinella, derived from a more recent endosymbiosis event, contains cyanobacterium-type enzymes for MGDG and DGDG synthesis (MgdA, MgdE, DgdA).

Photoinhibition of photosystem I in a pea mutant with altered LHCII organization

Comparative analysis of in vivo chlorophyll fluorescence imaging revealed that photosystem II (PSII) photochemical efficiency (Fv/Fm) of leaves of the Costata 2/133 pea mutant with altered pigment composition and decreased level of oligomerization of the light harvesting chlorophyll a/b-protein complexes (LHCII) of PSII (Dobrikova et al., 2000; Ivanov et al., 2005) did not differ from that of WT. In contrast, photosystem I (PSI) activity of the Costata 2/133 mutant measured by the far-red (FR) light inducible P700 (P700(+)) signal exhibited 39% lower steady state level of P700(+), a 2.2-fold higher intersystem electron pool size (e(-)/P700) and higher rate of P700(+) re-reduction, which indicate an increased capacity for PSI cyclic electron transfer (CET) in the Costata 2/133 mutant than WT. The mutant also exhibited a limited capacity for state transitions. The lower level of oxidizable P700 (P700(+)) is consistent with a lower amount of PSI related chlorophyll protein complexes and lower abundance of the PsaA/PsaB heterodimer, PsaD and Lhca1 polypeptides in Costata 2/133 mutant. Exposure of WT and the Costata 2/133 mutant to high light stress resulted in a comparable photoinhibition of PSII measured in vivo, although the decrease of Fv/Fm was modestly higher in the mutant plants. However, under the same photoinhibitory conditions PSI photochemistry (P700(+)) measured as ΔA820-860 was inhibited to a greater extent (50%) in the Costata 2/133 mutant than in the WT (22%). This was accompanied by a 50% faster re-reduction rate of P700(+) in the dark indicating a higher capacity for CET around PSI in high light treated mutant leaves. The role of chloroplast thylakoid organization on the stability of the PSI complex and its susceptibility to high light stress is discussed.

Investigating changes in the redox state of Photosystem I at low pH

Changes in the redox state of Photosystem I (PSI) were studied in spinach leaf discs suspended in buffers of different pH (pH 7.5, 6.5, 5.5 and 4.5). By measuring absorbance changes at 820 nm, it was observed that under normal conditions, the electrons were supplied by Photosystem II (PSII) for the photo-oxidation of P700 while in the presence of DCMU when electrons coming from PSII are blocked, cyclic electron flow (CEF) around PSI was the major source for the absorbance changes observed at 820 nm. This was supported by complete inhibition in the reduction of both single turnover (ST) area and multiple turnover (MT) area, in the presence of DCMU, which is generally filled up by the electrons coming from PSII. In the absence of DCMU, the intersystem electron pool or plastoquinone (PQ) pool was increased at low pH which was probably due to enhanced cyclic electron flow around PSI. Our results also suggest that at low pH, in the absence of DCMU, the major contribution for faster dark re-reduction of P700(+) is attributed mainly by PSII and CEF PSI while in the presence of DCMU, the significant contribution is provided by CEF PSI and other stromal components.

Preferential damaging effects of limited magnesium bioavailability on photosystem I in Sulla carnosa plants

Magnesium deficiency preferentially inhibits photosystem I rather than photosystem II in Sulla carnosa plants. The effects of magnesium (Mg(2+)) deficiency on growth, photosynthetic performance, pigment and polypeptide composition of chloroplast membranes were studied in the halophyte Sulla carnosa (Desf.), an annual legume endemic to Tunisia and Algeria. The results demonstrate a gradual decrease in biomass production with decreasing Mg(2+) availability in the growth medium. The increase of Mg(2+) deficiency was also associated with a decline of the net CO2 assimilation (Pn) in fully expanded leaves, a decrease in the amount of photosynthetic pigments, and an increase in the lipid peroxidation in plants exposed to decreased Mg(2+) concentrations. Interestingly, while CO2 assimilation already was affected at Mg(2+) concentrations below 1.5 mM, the photochemical efficiency of photosystem II (PSII) declined only in the absence of Mg(2+). In contrast, plants of S. carnosa grown in Mg(2+)-deficient conditions exhibited a significant decrease in photosystem I (PSI) photochemistry in vivo at much higher Mg(2+) levels compared to PSII photochemical activity. The inhibitory effect of Mg(2+) deficiency on PSI photochemistry strongly correlated with significantly lower relative abundance of PSI-related chlorophyll-protein complexes and lower amounts of PSI-associated polypeptides, PsaA, PsaB, and Lhca proteins within the same range of Mg(2+) concentrations. These observations were associated with a higher intersystem electron pool size, restricted linear electron transport and a lower rate of reduction of P700(+) in the dark indicating restricted capacity for PSI cyclic electron transfer in plants exposed to Mg(2+)-deficient conditions compared to controls. These results clearly indicate that PSI, rather than PSII is preferentially targeted and damaged under Mg(2+)-deficiency conditions.

Biomimetic monolayer films of digalactosyldiacylglycerol incorporating plastoquinone

The photosynthesis is the process used by plants and bacteria cells to convert inorganic matter in organic thanks to the light energy. This process consist on several steps, being one of them the electronic transport from the photosystem II to the cytochrome thanks to plastoquinone-9 (PQ). Here we prepare membranes that mimic the characteristics and composition of natural photosynthetic cell membranes and we characterize them in order to obtain the PQ molecules position in the membrane and their electrochemical behaviour. The selected galactolipid is digalactosyldiacylglycerol (DGDG) that represents the 30% of the thylakoid membrane lipid content. The results obtained are worthful for several science fields due to the relevance of galactolipids as anti-algal, anti-viral, anti-tumor and anti-inflammatory agents and the antioxidant and free radical scavenger properties of prenylquinones. Both pure components (DGDG and PQ) and the DGDG:PQ mixtures have been studied using surface pressure-area isotherms. These isotherms give information about the film stability and indicate the thermodynamic behaviour of the mixture and their physical state. The Langmuir-Blodgett (LB) film has been transferred forming a monolayer that mimics the bottom layer of the biological membranes. This monolayer on mica has been topographically characterized using AFM and both the height and the physical state that they present have been obtained. Moreover, these monolayers have been transferred onto ITO that is a hydrophilic substrate with good optical and electrical features, so that, it is suitable for studying the electrochemical behaviour of these systems and it is a good candidate for energy producing devices.

Effects of cadmium on lipids of almond seedlings (Prunus dulcis)

BACKGROUND

Cadmium uptake and distribution, as well as its effects on lipid composition was investigated in almond seedlings (Prunus dulcis) grown in culture solution supplied with two concentrations of Cd (50 and 150 μM).

RESULTS

The accumulation of Cd increased with external metal concentrations, and was considerably higher in roots than in leaves. Fourteen days after Cd treatment, the membrane lipids were extracted and separated on silica-gel thin layer chromatography (TLC). Fatty acid methyl esters were analyzed by FID-GC on a capillary column. Our results showed that Cd stress decreased the quantities of all lipids classes (phospholipids, galactolipids and neutral lipids). Galactolipid, phospholipid and neutral lipid concentrations decreased more in roots than in leaves by Cd-treatment. In almost all lipid classes the proportion of palmitic acid (16:0), linoleic (18: 2) and that of linolenic (18: 3) acid decreased, suggesting that heavy metal treatment induced an alteration in the fatty acid synthesis processes.

CONCLUSIONS

In conclusion, our results show that the changes found in total fatty acids, in the quantities of all lipids classes, and in the in the profiles of individual polar lipids suggest that membrane structure and function might be altered by Cd stress.

Global analysis of transcriptome responses and gene expression profiles to cold stress of Jatropha curcas L

BACKGROUND

Jatropha curcas L., also called the Physic nut, is an oil-rich shrub with multiple uses, including biodiesel production, and is currently exploited as a renewable energy resource in many countries. Nevertheless, because of its origin from the tropical MidAmerican zone, J. curcas confers an inherent but undesirable characteristic (low cold resistance) that may seriously restrict its large-scale popularization. This adaptive flaw can be genetically improved by elucidating the mechanisms underlying plant tolerance to cold temperatures. The newly developed Illumina Hiseq™ 2000 RNA-seq and Digital Gene Expression (DGE) are deep high-throughput approaches for gene expression analysis at the transcriptome level, using which we carefully investigated the gene expression profiles in response to cold stress to gain insight into the molecular mechanisms of cold response in J. curcas.

RESULTS

In total, 45,251 unigenes were obtained by assembly of clean data generated by RNA-seq analysis of the J. curcas transcriptome. A total of 33,363 and 912 complete or partial coding sequences (CDSs) were determined by protein database alignments and ESTScan prediction, respectively. Among these unigenes, more than 41.52% were involved in approximately 128 known metabolic or signaling pathways, and 4,185 were possibly associated with cold resistance. DGE analysis was used to assess the changes in gene expression when exposed to cold condition (12°C) for 12, 24, and 48 h. The results showed that 3,178 genes were significantly upregulated and 1,244 were downregulated under cold stress. These genes were then functionally annotated based on the transcriptome data from RNA-seq analysis.

CONCLUSIONS

This study provides a global view of transcriptome response and gene expression profiling of J. curcas in response to cold stress. The results can help improve our current understanding of the mechanisms underlying plant cold resistance and favor the screening of crucial genes for genetically enhancing cold resistance in J. curcas.

Expression of the minor isoform pea ferredoxin in tobacco alters photosynthetic electron partitioning and enhances cyclic electron flow

Ferredoxins (Fds) are ferrosulfoproteins that function as low-potential electron carriers in plants. The Fd family is composed of several isoforms that share high sequence homology but differ in functional characteristics. In leaves, at least two isoforms conduct linear and cyclic photosynthetic electron transport around photosystem I, and mounting evidence suggests the existence of at least partial division of duties between these isoforms. To evaluate the contribution of different kinds of Fds to the control of electron fluxes along the photosynthetic electron transport chain, we overexpressed a minor pea (Pisum sativum) Fd isoform (PsFd1) in tobacco (Nicotiana tabacum) plants. The transplastomic OeFd1 plants exhibited variegated leaves and retarded growth and developmental rates. Photosynthetic studies of these plants indicated a reduction in carbon dioxide assimilation rates, photosystem II photochemistry, and linear electron flow. However, the plants showed an increase in nonphotochemical quenching, better control of excitation pressure at photosystem II, and no evidence of photoinhibition, implying a better dynamic regulation to remove excess energy from the photosynthetic electron transport chain. Finally, analysis of P700 redox status during illumination confirmed that the minor pea Fd isoform promotes enhanced cyclic flow around photosystem I. The two novel features of this work are: (1) that Fd levels achieved in transplastomic plants promote an alternative electron partitioning even under greenhouse light growth conditions, a situation that is exacerbated at higher light intensity measurements; and (2) that an alternative, minor Fd isoform has been overexpressed in plants, giving new evidence of labor division among Fd isoforms.

Excess manganese differentially inhibits photosystem I versus II in Arabidopsis thaliana

The effects of exposure to increasing manganese concentrations (50-1500 µM) from the start of the experiment on the functional performance of photosystem II (PSII) and photosystem I (PSI) and photosynthetic apparatus composition of Arabidopsis thaliana were compared. In agreement with earlier studies, excess Mn caused minimal changes in the PSII photochemical efficiency measured as F(v)/F(m), although the characteristic peak temperature of the S(2/3)Q(B) (-) charge recombinations was shifted to lower temperatures at the highest Mn concentration. SDS-PAGE and immunoblot analyses also did not exhibit any significant change in the relative abundance of PSII-associated polypeptides: PSII reaction centre protein D1, Lhcb1 (major light-harvesting protein of LHCII complex), and PsbO (OEC33, a 33 kDa protein of the oxygen-evolving complex). In addition, the abundance of Rubisco also did not change with Mn treatments. However, plants grown under excess Mn exhibited increased susceptibility to PSII photoinhibition. In contrast, in vivo measurements of the redox transients of PSI reaction centre (P700) showed a considerable gradual decrease in the extent of P700 photooxidation (P700(+)) under increased Mn concentrations compared to control. This was accompanied by a slower rate of P700(+) re-reduction indicating a downregulation of the PSI-dependent cyclic electron flow. The abundance of PSI reaction centre polypeptides (PsaA and PsaB) in plants under the highest Mn concentration was also significantly lower compared to the control. The results demonstrate for the first time that PSI is the major target of Mn toxicity within the photosynthetic apparatus of Arabidopsis plants. The possible involvement mechanisms of Mn toxicity targeting specifically PSI are discussed.

Implications of alternative electron sinks in increased resistance of PSII and PSI photochemistry to high light stress in cold-acclimated Arabidopsis thaliana

Exposure of control (non-hardened) Arabidopsis leaves to high light stress at 5 °C resulted in a decrease of both photosystem II (PSII) (45 %) and Photosystem I (PSI) (35 %) photochemical efficiencies compared to non-treated plants. In contrast, cold-acclimated (CA) leaves exhibited only 35 and 22 % decrease of PSII and PSI photochemistry, respectively, under the same conditions. This was accompanied by an accelerated rate of P700(+) re-reduction, indicating an up-regulation of PSI-dependent cyclic electron transport (CET). Interestingly, the expression of the NDH-H gene and the relative abundance of the Ndh-H polypeptide, representing the NDH-complex, decreased as a result of exposure to low temperatures. This indicates that the NDH-dependent CET pathway cannot be involved and the overall stimulation of CET in CA plants is due to up-regulation of the ferredoxin-plastoquinone reductase, antimycin A-sensitive CET pathway. The lower abundance of NDH complex also implies lower activity of the chlororespiratory pathway in CA plants, although the expression level and overall abundance of the other well-characterized component involved in chlororespiration, the plastid terminal oxidase (PTOX), was up-regulated at low temperatures. This suggests increased PTOX-mediated alternative electron flow to oxygen in plants exposed to low temperatures. Indeed, the estimated proportion of O(2)-dependent linear electron transport not utilized in carbon assimilation and not directed to photorespiration was twofold higher in CA Arabidopsis. The possible involvement of alternative electron transport pathways in inducing greater resistance of both PSII and PSI to high light stress in CA plants is discussed.

Characterization of photosynthetic performance during senescence in stay-green and quick-leaf-senescence Zea mays L. inbred lines

The net photosynthetic rate, chlorophyll content, chlorophyll fluorescence and 820 nm transmission were investigated to explore the behavior of the photosynthetic apparatus, including light absorption, energy transformation and the photoactivities of photosystem II (PSII) and photosystem I (PSI) during senescence in the stay-green inbred line of maize (Zea mays) Q319 and the quick-leaf-senescence inbred line of maize HZ4. The relationship between the photosynthetic performance and the decrease in chlorophyll content in the two inbred lines was also studied. Both the field and laboratory data indicated that the chlorophyll content, net photosynthetic rate, and the photoactivities of PSII and PSI decreased later and slower in Q319 than in HZ4, indicating that Q319 is a functional stay-green inbred line. In order to avoid the influence of different development stages and environmental factors on senescence, age-matched detached leaf segments from the two inbred lines were treated with ethephon under controlled conditions to induce senescence. The net photosynthetic rate, light absorption, energy transformation, the activities of PSII acceptor side and donor side and the PSI activities decreased much slower in Q319 than in HZ4 during the ethephon-induced senescence. These results suggest that the retention of light absorption, energy transformation and activity of electron transfer contribute to the extended duration of active photosynthesis in Q319. Although the chlorophyll content decreased faster in HZ4, with decrease of chlorophyll content induced by ethephon, photosynthetic performance of Q319 deteriorated much more severely than that of HZ4, indicating that, compared with Q319, HZ4 has an advantage at maintaining higher photosynthetic activity with decrease of chlorophyll although HZ4 is a quick-leaf-senescence inbred line. We conclude that attention should be paid to two favorable characteristics in breeding long duration of active photosynthesis hybrids: 1) maintaining more chlorophyll content during senescence and 2) maintaining higher photosynthetic activity during the loss of chlorophyll.

Genetic decrease in fatty acid unsaturation of phosphatidylglycerol increased photoinhibition of photosystem I at low temperature in tobacco leaves

Leaves of transgenic tobacco plants with decreased levels of fatty acid unsaturation in phosphatidylglycerol (PG) exhibited a slightly lower level of the steady state oxidation of the photosystem I (PSI) reaction center P700 (P700(+)) than wild-type plants. The PSI photochemistry of wild-type plants was only marginally affected by high light treatments. Surprisingly, all plants of transgenic lines exhibited much higher susceptibility to photoinhibition of PSI than wild-type plants. This was accompanied by a 2.5-fold faster re-reduction rate of P700(+) in the dark, indicating a higher capacity for cyclic electron flow around PSI in high light treated transgenic leaves. This was associated with a much higher intersystem electron pool size suggesting over-reduction of the PQ pool in tobacco transgenic lines with altered PG unsaturation compared to wild-type plants. The physiological role of PG unsaturation in PSI down-regulation and modulation of the capacity of PSI-dependent cyclic electron flows and distribution of excitation light energy in tobacco plants under photoinhibitory conditions at low temperatures is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Early changes in the fatty acid composition of photosynthetic membrane lipids from Populus nigra grown on a metallurgical landfill

We compared the fatty acid composition of leaves taken from poplars on a metal-contaminated landfill, and on the uncontaminated roadside bordering this site. For the first time, it is shown that the percentage of linolenic acid, which is mainly associated with thylakoid lipids, was significantly lower in tree species within the landfill than within the control area. A correlation study was carried out to investigate relationships between the C18:3/(C18:0 + C18:1 + C18:2) fatty acid ratios and the metal contents in soils and leaves. Lead and chromium leaf contents were significantly negatively correlated to this fatty acid ratio. The impact of each of these metals remains difficult to evaluate, but chromium in leaf likely plays a major role in toxicity. In addition, the decrease in the C18:3/(C18:0 + C18:1 + C18:2) fatty acid ratio occurred at low leaf metal content, and therefore it is shown that this ratio can be used as an early indicator of the effect of metals.

Ginsenosidases and the pathogenicity of Pythium irregulare

American ginseng (Panax quinquefolius L.) produces triterpenoid saponins, ginsenosides, that possess mild fungitoxic activity toward some common ginseng leaf pathogens. However, numerous oomycete root pathogens of ginseng, most notably Pythium irregulare Buisman, are able to partially deglycosylate 20 (S)-protopanaxadiol ginsenosides Rb1, Rd and gypenoside XVII via extracellular glycosidases, leading to a common product, ginsenoside F2. Conversion of the common 20 (S)-protopanaxadiols into F2 requires both β (1→6) and β (1→2) glucosidase activity. In the present study, the ability of nine distinct isolates of P. irregulare, as well as a P. ultimum Trow isolate and two isolates of Trichoderma hamatum (Bonord.) Bainier, to deglycosylate 20 (S)-protopanaxadiols, in vitro was examined. The pathogenicity of each isolate was also examined by scoring the severity of disease symptoms caused by each in separate inoculations of one- and two-year old ginseng seedlings. Disease severity was scored using a disease severity index, as well as by taking F(v)/F(m) measurements of leaves during a 14-day infection period. Based on these measurements, it was concluded that (1) the use of direct F(v)/F(m) measurements correlates strongly with observations of disease severity (R(2)=0.79), and that (2) the pathogenicity of P. irregulare isolates correlates with their ability to deglycosylate ginsenosides (R(2)=0.57). These results further support the hypothesis that the pathogenicity of P. irregulare on ginseng roots is dependent, in part, on the ability of this organism to deglycosylate ginsenosides.

Correlation between spatial (3D) structure of pea and bean thylakoid membranes and arrangement of chlorophyll-protein complexes

BACKGROUND

The thylakoid system in plant chloroplasts is organized into two distinct domains: grana arranged in stacks of appressed membranes and non-appressed membranes consisting of stroma thylakoids and margins of granal stacks. It is argued that the reason for the development of appressed membranes in plants is that their photosynthetic apparatus need to cope with and survive ever-changing environmental conditions. It is not known however, why different plant species have different arrangements of grana within their chloroplasts. It is important to elucidate whether a different arrangement and distribution of appressed and non-appressed thylakoids in chloroplasts are linked with different qualitative and/or quantitative organization of chlorophyll-protein (CP) complexes in the thylakoid membranes and whether this arrangement influences the photosynthetic efficiency.

RESULTS

Our results from TEM and in situ CLSM strongly indicate the existence of different arrangements of pea and bean thylakoid membranes. In pea, larger appressed thylakoids are regularly arranged within chloroplasts as uniformly distributed red fluorescent bodies, while irregular appressed thylakoid membranes within bean chloroplasts correspond to smaller and less distinguished fluorescent areas in CLSM images. 3D models of pea chloroplasts show a distinct spatial separation of stacked thylakoids from stromal spaces whereas spatial division of stroma and thylakoid areas in bean chloroplasts are more complex. Structural differences influenced the PSII photochemistry, however without significant changes in photosynthetic efficiency. Qualitative and quantitative analysis of chlorophyll-protein complexes as well as spectroscopic investigations indicated a similar proportion between PSI and PSII core complexes in pea and bean thylakoids, but higher abundance of LHCII antenna in pea ones. Furthermore, distinct differences in size and arrangements of LHCII-PSII and LHCI-PSI supercomplexes between species are suggested.

CONCLUSIONS

Based on proteomic and spectroscopic investigations we postulate that the differences in the chloroplast structure between the analyzed species are a consequence of quantitative proportions between the individual CP complexes and its arrangement inside membranes. Such a structure of membranes induced the formation of large stacked domains in pea, or smaller heterogeneous regions in bean thylakoids. Presented 3D models of chloroplasts showed that stacked areas are noticeably irregular with variable thickness, merging with each other and not always parallel to each other.

Characterization of photosynthesis in Arabidopsis ER-to-plastid lipid trafficking mutants

Vascular plants use two pathways to synthesize galactolipids, the predominant lipid species in chloroplasts-a prokaryotic pathway that resides entirely in the chloroplast, and a eukaryotic pathway that involves assembly in the endoplasmic reticulum. Mutants deficient in the endoplasmic reticulum pathway, trigalactosyldiacylglycerol (tgd1-1 and tgd2-1) mutants, had been previously identified with reduced contents of monogalactosyldiacylglycerol and digalactosyldiacylglycerol, and altered lipid molecular species composition. Here, we report that the altered lipid composition affected photosynthesis in lipid trafficking mutants. It was found that proton motive force as measured by electrochromic shift was reduced by ~40% in both tgd mutants. This effect was accompanied by an increase in thylakoid conductance attributable to ATPase activity and so the rate of ATP synthesis was nearly unchanged. Thylakoid conductance to ions also increased in tgd mutants. However, gross carbon assimilation in tgd mutants as measured by gas exchange was only marginally affected. Rubisco activity, electron transport rate, and photosystem I and II oxidation status were not altered. Despite the large differences in proton motive force, responses to heat and high light stress were similar between tgd mutants and the wild type.

Identification of plant-like galactolipids in Chromera velia, a photosynthetic relative of malaria parasites

Apicomplexa are protist parasites that include Plasmodium spp., the causative agents of malaria, and Toxoplasma gondii, responsible for toxoplasmosis. Most Apicomplexa possess a relict plastid, the apicoplast, which was acquired by secondary endosymbiosis of a red alga. Despite being nonphotosynthetic, the apicoplast is otherwise metabolically similar to algal and plant plastids and is essential for parasite survival. Previous studies of Toxoplasma gondii identified membrane lipids with some structural features of plastid galactolipids, the major plastid lipid class. However, direct evidence for the plant-like enzymes responsible for galactolipid synthesis in Apicomplexan parasites has not been obtained. Chromera velia is an Apicomplexan relative recently discovered in Australian corals. C. velia retains a photosynthetic plastid, providing a unique model to study the evolution of the apicoplast. Here, we report the unambiguous presence of plant-like monogalactosyldiacylglycerol and digalactosyldiacylglycerol in C. velia and localize digalactosyldiacylglycerol to the plastid. We also provide evidence for a plant-like biosynthesis pathway and identify candidate galactosyltranferases responsible for galactolipid synthesis. Our study provides new insights in the evolution of these important enzymes in plastid-containing eukaryotes and will help reconstruct the evolution of glycerolipid metabolism in important parasites such as Plasmodium and Toxoplasma.

Regulation of energy partitioning and alternative electron transport pathways during cold acclimation of lodgepole pine is oxygen dependent

Second year needles of Lodgepole pine (Pinus contorta L.) were exposed for 6 weeks to either simulated control summer ['summer'; 25 °C/250 photon flux denisty (PFD)], autumn ('autumn'; 15°C/250 PFD) or winter conditions ('winter'; 5 °C/250 PFD). We report that the proportion of linear electron transport utilized in carbon assimilation (ETR(CO2)) was 40% lower in both 'autumn' and 'winter' pine when compared with the 'summer' pine. In contrast, the proportion of excess photosynthetic linear electron transport (ETR(excess)) not used for carbon assimilation within the total ETR(Jf) increased by 30% in both 'autumn' and 'winter' pine. In 'autumn' pine acclimated to 15°C, the increased amounts of 'excess' electrons were directed equally to 21  kPa O2-dependent and 2  kPa O2-dependent alternative electron transport pathways and the fractions of excitation light energy utilized by PSII photochemistry (Φ(PSII)), thermally dissipated through Φ(NPQ) and dissipated by additional quenching mechanism(s) (Φ(f,D)) were similar to those in 'summer' pine. In contrast, in 'winter' needles acclimated to 5 °C, 60% of photosynthetically generated 'excess' electrons were utilized through the 2  kPa O2-dependent electron sink and only 15% by the photorespiratory (21  kPa O2) electron pathway. Needles exposed to 'winter' conditions led to a 3-fold lower Φ(PSII), only a marginal increase in Φ(NPQ) and a 2-fold higher Φ(f,D), which was O2 dependent compared with the 'summer' and 'autumn' pine. Our results demonstrate that the employment of a variety of alternative pathways for utilization of photosynthetically generated electrons by Lodgepole pine depends on the acclimation temperature. Furthermore, dissipation of excess light energy through constitutive non-photochemical quenching mechanisms is O2 dependent.

Digalactosyl-diacylglycerol-deficiency lowers the thermal stability of thylakoid membranes

We investigated the effects of digalactosyl-diacylglycerol (DGDG) on the organization and thermal stability of thylakoid membranes, using wild-type Arabidopsis thaliana and the DGDG-deficient mutant, dgd1. Circular-dichroism measurements reveal that DGDG-deficiency hampers the formation of the chirally organized macrodomains containing the main chlorophyll a/b light-harvesting complexes. The mutation also brings about changes in the overall chlorophyll fluorescence lifetimes, measured in whole leaves as well as in isolated thylakoids. As shown by time-resolved measurements, using the lipophylic fluorescence probe Merocyanine 540 (MC540), the altered lipid composition affects the packing of lipids in the thylakoid membranes but, as revealed by flash-induced electrochromic absorbance changes, the membranes retain their ability for energization. Thermal stability measurements revealed more significant differences. The disassembly of the chiral macrodomains around 55°C, the thermal destabilization of photosystem I complex at 61°C as detected by green gel electrophoresis, as well as the sharp drop in the overall chlorophyll fluorescence lifetime above 45°C (values for the wild type-WT) occur at 4-7°C lower temperatures in dgd1. Similar differences are revealed in the temperature dependence of the lipid packing and the membrane permeability: at elevated temperatures MC540 appears to be extruded from the dgd1 membrane bilayer around 35°C, whereas in WT, it remains lipid-bound up to 45°C and dgd1 and WT membranes become leaky around 35 and 45°C, respectively. It is concluded that DGDG plays important roles in the overall organization of thylakoid membranes especially at elevated temperatures.

De novo biosynthesis of fatty acids plays critical roles in the response of the photosynthetic machinery to low temperature in Arabidopsis

The Arabidopsis thaliana kas3 mutant was isolated based on the hypersensitivity of PSII to low temperature using a Chl fluorescence imaging technique. Chl content was lower in kas3 seedlings cultured at 23 degrees C than in the wild type, but PSII activity was only mildly affected. However, after the chilling treatment at 4 degrees C for 7 d, PSII activity was severely impaired in kas3. PSII was more sensitive to light at 4 degrees C in the presence of lincomycin, suggesting that the kas3 mutation accelerates at least the PSII photodamage. The kas3 mutation causes an amino acid alteration in 3-ketoacyl-ACP synthase III (KasIII), leading to the partial loss of the de novo synthesis pathway for fatty acids in plastids. Consequently, the total fatty acid level was reduced to 75% of the wild-type level in kas3 at 23 degrees C and was further reduced to 60% at 4 degrees C. The composition of fatty acids was also slightly affected in kas3 at both 4 and 23 degrees C. Consistent with the results of the electron transport analysis, the chilling treatment also destabilized PsaA and cytochrome (Cyt) f and D1 in kas3. An analysis of double mutants with pgr1 conditionally defective in Cyt b(6)f activity and with var2 defective in FtsH protease suggested that the kas3 mutation has pleiotropic effects on chloroplast function, probably impacting both the Cyt b(6)f activity and translation in chloroplasts at 23 degrees C. The full activity of KasIII is required for the biogenesis of the intact electron transport machinery in thylakoid membranes and is especially important for the process of responding to low temperature.

Type A and type B monogalactosyldiacylglycerol synthases are spatially and functionally separated in the plastids of higher plants

Mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively) constitute the bulk of membrane lipids in plant chloroplasts. The final step in MGDG biosynthesis occurs in the plastid envelope and is catalyzed by MGDG synthase. In Arabidopsis, the three MGDG synthases are classified into type A (atMGD1) and type B MGD isoforms (atMGD2 and atMGD3). atMGD1 is an inner envelope membrane-associated protein of chloroplasts and is responsible for the bulk of galactolipid biosynthesis in green tissues. MGD1 function is indispensable for thylakoid membrane biogenesis and embryogenesis. By contrast, type B atMGD2 and atMGD3 are localized in the outer envelopes and have no important role in chloroplast biogenesis or plant development under nutrient-sufficient conditions. These type B MGD genes are, however, strongly induced by phosphate (Pi) starvation and are essential for alternative galactolipid biosynthesis during Pi starvation. MGD1 gene expression is up-regulated by light and cytokinins. By contrast, Pi starvation-dependent expression of atMGD2/3 is suppressed by cytokinins but induced through auxin signaling pathways. These growth factors may control the functional sharing of the inner envelope pathway by atMGD1 and the outer envelope pathway by atMGD2/3 according to the growth environment.

Galactolipid synthesis in chloroplast inner envelope is essential for proper thylakoid biogenesis, photosynthesis, and embryogenesis

The biogenesis of thylakoid membranes, an indispensable event for the photoautotrophic growth of plants, requires a significant increase in the level of the unique thylakoid membrane lipid monogalactosyldiacylglycerol (MGDG), which constitutes the bulk of membrane lipids in chloroplasts. The final step in MGDG biosynthesis occurs in the plastid envelope and is catalyzed by MGDG synthase. Here we report the identification and characterization of an Arabidopsis mutant showing a complete defect in MGDG synthase 1. The mutant seeds germinated as small albinos only in the presence of sucrose. The seedlings lacked galactolipids and had disrupted photosynthetic membranes, leading to the complete impairment of photosynthetic ability and photoautotrophic growth. Moreover, invagination of the inner envelope, which is not seen in mature WT chloroplasts, was observed in the mutant, supporting an old hypothesis that envelope invagination is a major event in early chloroplast biogenesis. In addition to the defective seedling phenotype, embryo development was arrested in the mutant, although seeds with impaired embryos could germinate heterotrophically. These results demonstrate the importance of galactolipids not only in photosynthetic growth but also in embryogenesis.

Structure and function of glycoglycerolipids in plants and bacteria

Phosphoglycerolipids are abundant membrane constituents in prokaryotic and eukaryotic cells. However, glycoglycerolipids are the predominant lipids in chloroplasts of plants and eukaryotic algae and in cyanobacteria. Membrane composition in chloroplasts and cyanobacteria is highly conserved, with monogalactosyldiacylglycerol (MGD) and digalactosyldiacylglycerol (DGD) representing the most abundant lipids. The genes encoding enzymes of galactolipid biosynthesis have been isolated from Arabidopsis. Galactolipids are crucial for growth under normal and phosphate limiting conditions. Furthermore, they are indispensable for maximal efficiency of photosynthesis. A wide variety of glycoglycerolipids is found in different bacteria. These lipids contain glucose or galactose, in some cases also mannose or other sugars with different glycosidic linkages in their head group. Some bacterial species produce unusual glycoglycerolipids, such as glycophospholipids or glycoglycerolipids carrying sugar head groups esterified with acyl residues. A number of genes coding for bacterial glycoglycerolipid synthases have been cloned and the enzymes characterized. In contrast to the breadth of information available on their structural diversity, much less is known about functional aspects of bacterial glycoglycerolipids. In some bacteria, glycoglycerolipids are required for membrane bilayer stability, they serve as precursors for the formation of complex membrane components, or they are crucial to support anoxygenic photosynthesis or growth during phosphate deficiency.

Cold acclimation of the Arabidopsisdgd1mutant results in recovery from photosystem I-limited photosynthesis

We compared the thylakoid membrane composition and photosynthetic properties of non- and cold-acclimated leaves from the dgd1 mutant (lacking >90% of digalactosyl-diacylglycerol; DGDG) and wild type (WT) Arabidopsis thaliana. In contrast to warm grown plants, cold-acclimated dgd1 leaves recovered pigment-protein pools and photosynthetic function equivalent to WT. Surprisingly, this recovery was not correlated with an increase in DGDG. When returned to warm temperatures the severe dgd1 mutant phenotype reappeared. We conclude that the relative recovery of photosynthetic activity at 5 degrees C resulted from a temperature/lipid interaction enabling the stable assembly of PSI complexes in the thylakoid.