Chloroplast lipid synthesis and lipid trafficking through ER-plastid membrane contact sites

Plant chloroplasts contain an intricate photosynthetic membrane system, the thylakoids, and are surrounded by two envelope membranes at which thylakoid lipids are assembled. The glycoglycerolipids mono- and digalactosyldiacylglycerol, and sulfoquinovosyldiacylglycerol as well as phosphatidylglycerol, are present in thylakoid membranes, giving them a unique composition. Fatty acids are synthesized in the chloroplast and are either directly assembled into thylakoid lipids at the envelope membranes or exported to the ER (endoplasmic reticulum) for extraplastidic lipid assembly. A fraction of lipid precursors is reimported into the chloroplast for the synthesis of thylakoid lipids. Thus polar lipid assembly in plants requires tight co-ordination between the chloroplast and the ER and necessitates inter-organelle lipid trafficking. In the present paper, we discuss the current knowledge of the export of fatty acids from the chloroplast and the import of chloroplast lipid precursors assembled at the ER. Direct membrane contact sites between the ER and the chloroplast outer envelopes are discussed as possible conduits for lipid transfer.     

Galactolipid synthesis in chromoplast internal membranes of the daffodil

Summary Chromoplast internal membranes from Narcissus pseudonarcissus flowers (like chloroplast envelope membranes, as opposed to chloroplast thylakoids) were found to contain high galactolipid synthesizing activities when UDP-galactose plus diglyceride were applied to the purified preparations.Abbreviations MGDG monogalactosyl diglyceride - DGDG digalactosyl diglyceride     

Localization within chloroplasts of protoporphyrinogen oxidase, the target enzyme for diphenylether-like herbicides.

Plant protoporphyrinogen oxidase is of particular interest since it is the last enzyme of the common branch for chlorophyll and heme biosynthetic pathways. In addition, it is the target enzyme for diphenyl ether-type herbicides, such as acifluorfen. Two distinct methods were used to investigate the localization of this enzyme within Percoll-purified spinach chloroplasts. We first assayed the enzymatic activity by spectrofluorimetry and we analyzed the specific binding of the herbicide acifluorfen, using highly purified chloroplast fractions. The results obtained give clear evidence that chloroplast protoporphyrinogen oxidase activity is membrane-bound and is associated with both chloroplast membranes, i.e. envelope and thylakoids. Protoporphyrinogen oxidase specific activity was 7-8 times higher in envelope membranes than in thylakoids, in good agreement with the number of [3H]acifluorfen binding sites in each membrane system: 21 and 3 pmol/mg protein, respectively, in envelope membranes and thylakoids. On a total activity basis, 25% of protoporphyrinogen oxidase activity were associated with envelope membranes. The presence of protoporphyrinogen oxidase in chloroplast envelope membranes provides further evidence for a role of this membrane system in chlorophyll biosynthesis. In contrast, the physiological significance of the enzyme associated with thylakoids is still unknown, but it is possible that thylakoid protoporphyrinogen oxidase could be involved in heme biosynthesis.     

Bizonoplast, a unique chloroplast in the epidermal cells of microphylls in the shade plant Selaginella erythropus (Selaginellaceae)

Study of the unique leaf anatomy and chloroplast structure in shade-adapted plants will aid our understanding of how plants use light efficiently in low light environments. Unusual chloroplasts in terms of size and thylakoid membrane stacking have been described previously in several deep-shade plants. In this study, a single giant cup-shaped chloroplast, termed a bizonoplast, was found in the abaxial epidermal cells of the dorsal microphylls and the adaxial epidermal cells of the ventral microphylls in the deep-shade spike moss Selaginella erythropus. Bizonoplasts are dimorphic in ultrastructure: the upper zone is occupied by numerous layers of 2-4 stacked thylakoid membranes while the lower zone contains both unstacked stromal thylakoids and thylakoid lamellae stacked in normal grana structure oriented in different directions. In contrast, other cell types in the microphylls contain chloroplasts with typical structure. This unique chloroplast has not been reported from any other species. The enlargement of epidermal cells into funnel-shaped, photosynthetic cells coupled with specific localization of a large bizonoplast in the lower part of the cells and differential modification in ultrastructure within the chloroplast may allow the plant to better adapt to low light. Further experiments are required to determine whether this shade-adapted organism derives any evolutionary or ecophysiological fitness from these unique chloroplasts.     

Localization and integration of thylakoid protein translocase subunit cpTatC

Thylakoid membranes have a unique complement of proteins, most of which are nuclear encoded synthesized in the cytosol, imported into the stroma and translocated into thylakoid membranes by specific thylakoid translocases. Known thylakoid translocases contain core multi-spanning, membrane-integrated subunits that are also nuclear-encoded and imported into chloroplasts before being integrated into thylakoid membranes. Thylakoid translocases play a central role in determining the composition of thylakoids, yet the manner by which the core translocase subunits are integrated into the membrane is not known. We used biochemical and genetic approaches to investigate the integration of the core subunit of the chloroplast Tat translocase, cpTatC, into thylakoid membranes. In vitro import assays show that cpTatC correctly localizes to thylakoids if imported into intact chloroplasts, but that it does not integrate into isolated thylakoids. In vitro transit peptide processing and chimeric precursor import experiments suggest that cpTatC possesses a stroma-targeting transit peptide. Import time-course and chase assays confirmed that cpTatC targets to thylakoids via a stromal intermediate, suggesting that it might integrate through one of the known thylakoid translocation pathways. However, chemical inhibitors to the cpSecA-cpSecY and cpTat pathways did not impede cpTatC localization to thylakoids when used in import assays. Analysis of membranes isolated from Arabidopsis thaliana mutants lacking cpSecY or Alb3 showed that neither is necessary for cpTatC membrane integration or assembly into the cpTat receptor complex. These data suggest the existence of another translocase, possibly one dedicated to the integration of chloroplast translocases.     

Chloroplast biogenesis. Cell-free transfer of envelope monogalactosylglycerides to thylakoids.

An ATP- and temperature-dependent transfer of monogalactosylglycerides from the chloroplast envelope to the chloroplast thylakoids was reconstituted in a cell-free system prepared from isolated chloroplasts of garden pea (Pisum sativum) or spinach (Spinacia oleracea). Isolated envelope membranes, in which the label was present exclusively in monogalactosylglycerides, were prepared radiolabeled in vitro with [14C]galactose from UDP-[14C]galactose to label galactolipids as the donor. ATP-dependent transfer of radioactivity from donor to unlabeled acceptor thylakoids, immobilized on nitrocellulose strips, was observed. In some experiments linear transfer for longer than 30 min of incubation was facilitated by the addition of stroma proteins but in other experiments stroma was without effect or inhibitory suggesting no absolute requirements for a soluble protein carrier. Transfer was donor specific. No membrane fraction tested (plasma membrane, tonoplast, endoplasmic reticulum, nuclei, Golgi apparatus, mitochondria or thylakoids) (isolated from tissue radiolabeled in vivo with [14C]acetate) other than chloroplast envelopes demonstrated any significant ability to transfer labeled membrane lipids to immobilized thylakoids. Acceptor specificity, while not absolute, showed a 3-10-fold greater ATP-dependent transfer of labeled galactolipids from chloroplast envelopes to immobilized thylakoids than to other leaf membranes. The results provide independent confirmation of the potential for transfer of galactolipids between chloroplast envelopes and thylakoids suggested previously from ultrastructural studies and of the known location of thylakoid galactolipid biosynthetic activities in the chloroplast envelope.     

Structure and Function of Chloroplast Membrane III. The Effects of Mg++ and K+ Ions Toward the Ultrastructure of Two Kinds of Chloroplast Thylakoid Membranes

Based upon our previous work on the relationship between structure and function of chloroplast of wheat in connection with PSⅡ reaction, we studied the effects of MgCl2 and KC1 toward two kinds of thylakoid membranes. After exposing etiolated wheat seedlings to intermittent light (cycle of 2 min. light, 118 min. dark) for 24 hr, we obtain ed an incompletely developed chleroplast membrane. Completely developed chloroplast membrane was obtained from wheat seedlings grown under normal light-dark regime. Thylakoid membranes of plants grown under intermittent light failed to form grana stacks they remained as single lamellae in the suspension containing Mg++ or K+ of high concentration although simple stackings not more than two thylakoids c.ould be found. However, thylakoids grown under normal light-dark regime showed well developed grand stacks. Isolated chloroplast samples from two kinds of seedlings were suspended in 5 mM MgCl2 and 100 mM KC1 solutions for a definite time, portions of each samples were processed for electron microscopic observations and their photosynthetic activities were measured at the same time (It will be dealt with in another article). When these two kinds of isolated plastids were suspended either in MgCl2 (5 mM) or KC1 (100), the normally developed grana thylakoids stacked closely but the incompletely developed thylakoid- membranes did not stack. The incompletely developed chloroplast thylakoid membranes,, in either Mg++ or K+ ions could not induce stacking of the scattered thylakoid membranes to form grana. Therefore, we presume that light- harvesting chlorophyll a/b protein complex is on internal factor to induce thylakoid- membranes stacking and a definite concentration of caionions is an important factor in maintaining the stacking of thylakoid membranes. These results further prove the close association between structure and function in our previous studies on the mesophyll cell of the winter wheat.     

Growth and development of winter rye at cold-hardening temperatures results in thylakoid membranes with increased sensitivity to low concentrations of osmoticum

Chloroplasts developed at cold-hardening (5°C) and non-hardening temperatures (20°C) were compared with respect to the stability of photosynthetic electron transport activities, the capacity to produce and maintain a H⁺ gradient and the capacity fat photophosphorylation as a function of resuspension in the presence or absence of osmoticum. The results for electron transport indicate that whole chain, photosystem I and pfaotosystem II activities in non-hardened chloroplast thyalkoids were unaffected by resuspension in the presence of high or low osmoticum. In contrast, the same electron transport activities in cold-hardened chloroplast thylakoids exhibited a 3- to 4-fold decrease in activity when resuspended in the presence of low osmoticum. Impairment of electron transport through photosystem II of cold-hardened thylakoids resuspended in the presence of low osmoticum was supported by room temperature fluorescence induction kinetics. Since the presence of Mn²⁺ partially overcame this inhibition, it is concluded that this osmotically-induced inhibition of PSII activity in cold-hardened chloroplast thylakoids may, in part, be due to damage to the H₂O-splitting side of photosystem II. Both the initial rate and the maximum capacity for cyclic photophosphorylation were significantly inhibited in cold-hardened as compared to non-hardened thylakoids upon resuspension in the presence of low concentrations of osmoticum. This was correlated with an inability of the cold-hardened chloroplast thylakoids to maintain a significant transrnembrane H⁺ gradient. The results indicate that cold-hardened thylakoid membranes required an osmotic concentration (0.8 M) twice as high as non-hardened thylakoids (0.4 M) to produce the same initial rate of H⁺ uptake. In addition, the capacity to produce a proton gradient in cold-hardened thylakoids was less stable than that in non-hardened thylakoids regardless of the osmotic concentration tested. It is concluded that development of rye thylakoid membranes at low temperature results in a differential sensitivity to low osmoticum and thus extreme caution should be exercised when comparing the structure and function of isolated thylakoids developed under contrasting thermal regimes.     

甜菊组织培养物中叶绿体的超微结构与脱分代

含有叶绿体的甜菊（Ｓｔｅｖｉａｒｅｂａｕｄｉａｎａ）愈伤组织细胞转移至新鲜培养基后,导致光合片层的逐渐减少或消失,最后叶绿体脱分化形成原质体样的结构。超微结构观察表明,光合片层的减少或消失与降解及叶绿体分裂特别是不均等缢缩分裂而致基质组分和类囊体膜稀释有关。这一过程并不完全同步,一些质体含有少量正常的片展而另一些质体含有退化的片层甚至片展结构完全消失。细胞的一个明显特点是细胞器大多聚集在细胞核附近,细胞质增加并向细胞中央伸出细胞质丝。同时可观察到原质体。培养７ｄ后,许多细胞呈分生状态,细胞质富含细胞器,充满了细胞的大部分空间。此时细胞中的质体大多呈原质体状态。在细胞生长的稳定期,质体内膜组织成基质基粒片层,同时质体核糖体增加。文中讨论了高度液泡化细胞脱分化与细胞中叶绿体脱分化的关系。     

Electron Microscopic Observations on the Symbiosis of Paramecium bursaria and its Intracellular Algae*

SYNOPSIS. Observations were made on the fine structure of Paramecium bursaria and its intracellular Chlorella symbionts. Emphasis was placed on the structure of the algae and structural aspects of the relationship between the organisms. The algae are surrounded by a prominent cell wall and contain a cup-shaped chloroplast which lies just beneath the plasma membrane. Within the cavity formed by the chloroplast are a large nucleus, a mitochondrion, one or more dictyosomes, and numerous ribosomes. The chloroplast itself is made up of a series of lamellar stacks each containing 2–6 or more thylakoids with a granular stroma and starch grains intercalated between the stacks. The thylakoid stacks of mature algae are frequently more compact than those of recently divided algae. A large pyrenoid is located within the base of the chloroplast. It is made up of a granular or fibrillar matrix surrounded by a shell of starch. The matrix is bisected by a stack of 2 thylakoids. Prior to the division of the chloroplast the pyrenoid regresses; pyrenoids subsequently form in the daughter chloroplasts thru condensation of the matrix material and the reappearance of a starch shell. This shell appears to be formed by the hollowing-out of starch grains already present in the chloroplast stroma. Accordingly, in this case, starch moves from the stroma to the pyrenoid. The algae are located thruout the peripheral cytoplasm of the Paramecium. Each alga is located in an individual vacuole except immediately following division of the algae when the daughter cells are temporarily located in the vacuole which harbored the parental cell. Shortly thereafter the vacuole membrane invaginates, thereby isolating the daughter algae into individual vacuoles. Degenerating symbiotic algae are seen; because these are frequently found in vacuoles with bacteria, they are presumed to be undergoing digestion. Due to the conditions of culture these algae could have been either of intracellular or extracellular origin.     

Frost hardening and photosynthetic performance of Scots pine (Pinus sylvestris L.). II. Seasonal changes in the fluidity of thylakoid membranes

The fluidity of chloroplast thylakoid membranes of frost-tolerant and frost-sensitive needles of␣three- to four-year-old Scots pine (Pinus sylvestris L.) trees, of liposomes produced from the lipids of the thylakoids of these needles, and of liposomes containing varying amounts of light-harvesting complex (LHC) II protein was investigated by means of electron paramagnetic resonance (EPR) measurements using spin-labelled fatty acids as probes. Broadening of the EPR-resonance signals of 16-doxyl stearic acid in chloroplast membranes of frost-sensitive needles and changes in the amplitudes of the peaks were observed upon a decrease in temperature from +30 °C to −10 °C, indicating a drastic loss in rotational mobility. The lipid molecules of the thylakoid membranes of frost-tolerant needles exhibited greater mobility. Moderate frost resistance could be induced in Scots pine needles by short-day treatment (Vogg et al., 1997, Planta, this issue), and growth of the trees under short-day illumination (9 h) resulted in a higher mobility of the chloroplast membrane lipids than did growth under long-day conditions (16 h). The EPR spectrum of thylakoids from frost-tolerant needles at −10 °C was typical of a spin label in highly fluid surroundings. However, an additional peak in the low-field range appeared in the subzero temperature range for the chloroplast membranes of frost-sensitive needles, which represents spin-label molecules in a motionally restricted surrounding. The EPR spectra of thylakoids and of liposomes of thylakoid lipids from frost-hardy needles were identical at +30 °C and −10 °C. The corresponding spectra from frost-sensitive plants revealed an additional peak for the thylakoids, but not for the pure liposomes. Hence, the domains with restricted mobility could be attributed to protein-lipid interactions in the membranes. Broadening of the spectrum and the appearance of an additional peak was observed with liposomes of pure distearoyl phosphatidyl glycerol modified to contain increasing amounts of LHC II. These results are discussed with respect to a loss of chlorophyll and chlorophyll-binding proteins in thylakoids of Scots pine needles under winter conditions. Received: 3 March 1997 / Accepted: 16 July 1997     

Release of membrane proteins in relation to heat injury of spinach chloroplasts

Thylakoids isolated from spinach leaves ( Spinacia oleracea L. cvs. Monatol and Montako) were exposed to supraoptimal temperatures that inactivated their photochemical reactions. Membrane injury was accompanied by release of a small amount of membrane proteins. When sucrose was present during high-temperature treatment, thylakoids were partially protected and release of membrane proteins was less pronounced than in the absence of sugar. From thylakoids, which were isolated from heat-damaged spinach leaves, less protein was released when heated again after the isolation procedure, indicating that protein release also takes place during heat inactivation in vivo . Sodium dodecyl sulfate gel electropherograms of thylakoids demonstrated that heat inactivation of the lamellae was not accompanied by significant changes in the pattern of the proteins, which remained in the membranes. The same was found when thylakoids were solubilized with Triton X-100 before and after heat damage. It is suggested that the protein release that occurs during heat treatment is a consequence of irreversible alterations in the membrane structure; these changes may be responsible for thermal damage of chloroplast membranes.     

Ultrastructural changes in major organelles during spermatial differentiation inBangia (Rhodophyta)

Summary The major organelles within the cells of maleBangia atropurpurea (Roth) C. Ag. filaments undergo a series of ultrastructural transformations during the production of spermatia. Initially, thylakoids within the large axial chloroplast develop a reticulate pattern commencing at the central pyrenoid region. Subsequent changes involve loss of lobes and diminution of volume through division; chloroplasts in final stages contain a few dilated, distorted thylakoids and many plastoglobuli. During differentiation the large nucleolus disappears from the nucleus and four masses of chromatin aggregate near the nuclear envelope. Furrows originating from the nuclear envelope form double membranes around each of the chromatin masses and most of the nucleoplasm is eliminated. Several types of fibrillar vesicles are formed during the process and large floridean starch reserves are utilized. Multilamellar bodies and microbody-like structures occur within the cells during certain phases of spermatiogenesis.     

Membrane lipids in heat injury of spinach chloroplasts

Heat treatment of intact leaves and of isolated thylakoid membranes from spinach (Spinacia oleracea L. cvs. Monatol and Montako) caused inactivation of photochemical processes such as electron transport through photosystem II and photophos-phorylation. Membrane lipid analysis demonstrated that heat-induced damage to thylakoids is not caused by chemical alterations in the lipids such as oxidation of unsaturated fatty acids, or release of free fatty acids due to hydrolysis of lipids. Partial extraction of lipids from isolated chloroplast membranes before and after thermal inactivation do not point to drastic changes in the binding relations of the lipids within the membranes. However, it cannot be excluded that during high temperature treatment changes in lipid-lipid interactions and/or delocalization of specific lipids within the thylakoids might be responsible for the disorganization of the functional integrity of the membranes. Since thermostability of chloroplast membranes is decreased when they are exposed to free unsaturated fatty acids, small amounts of membrane lipids which become hydrolyzed during extended heat treatment may partly contribute to primary heat damage.     

Biogenesis and origin of thylakoid membranes.

Thylakoids are photosynthetically active membranes found in Cyanobacteria and chloroplasts. It is likely that they originated in photosynthetic bacteria, probably in close connection to the occurrence of photosystem II and oxygenic photosynthesis. In higher plants, chloroplasts develop from undifferentiated proplastids. These contain very few internal membranes and the whole thylakoid membrane system is built when chloroplast differentiation takes place. During cell and organelle division a constant synthesis of new thylakoid membrane material is required. Also, rapid adaptation to changes in light conditions and long term adaptation to a number of environmental factors are accomplished by changes in the lipid and protein content of the thylakoids. Thus regulation of synthesis and assembly of all these elements is required to ensure optimal function of these membranes.     

Dynamic flexibility in the structure and function of photosystem II in higher plant thylakoid membranes: the grana enigma

Grana are not essential for photosynthesis, yet they are ubiquitous in higher plants and in the recently evolved Charaphyta algae; hence grana role and its need is still an intriguing enigma. This article discusses how the grana provide integrated and multifaceted functional advantages, by facilitating mechanisms that fine-tune the dynamics of the photosynthetic apparatus, with particular implications for photosystem II (PSII). This dynamic flexibility of photosynthetic membranes is advantageous in plants responding to ever-changing environmental conditions, from darkness or limiting light to saturating light and sustained or intermittent high light. The thylakoid dynamics are brought about by structural and organizational changes at the level of the overall height and number of granal stacks per chloroplast, molecular dynamics within the membrane itself, the partition gap between appressed membranes within stacks, the aqueous lumen encased by the continuous thylakoid membrane network, and even the stroma bathing the thylakoids. The structural and organizational changes of grana stacks in turn are driven by physicochemical forces, including entropy, at work in the chloroplast. In response to light, attractive van der Waals interactions and screening of electrostatic repulsion between appressed grana thylakoids across the partition gap and most probably direct protein interactions across the granal lumen (PSII extrinsic proteins OEEp-OEEp, particularly PsbQ-PsbQ) contribute to the integrity of grana stacks. We propose that both the light-induced contraction of the partition gap and the granal lumen elicit maximisation of entropy in the chloroplast stroma, thereby enhancing carbon fixation and chloroplast protein synthesizing capacity. This spatiotemporal dynamic flexibility in the structure and function of active and inactive PSIIs within grana stacks in higher plant chloroplasts is vital for the optimization of photosynthesis under a wide range of environmental and developmental conditions.     

Acclimation of leaves to low light produces large grana: the origin of the predominant attractive force at work

Photosynthetic membrane sacs (thylakoids) of plants form granal stacks interconnected by non-stacked thylakoids, thereby being able to fine-tune (i) photosynthesis, (ii) photoprotection and (iii) acclimation to the environment. Growth in low light leads to the formation of large grana, which sometimes contain as many as 160 thylakoids. The net surface charge of thylakoid membranes is negative, even in low-light-grown plants; so an attractive force is required to overcome the electrostatic repulsion. The theoretical van der Waals attraction is, however, at least 20-fold too small to play the role. We determined the enthalpy change, in the spontaneous stacking of previously unstacked thylakoids in the dark on addition of Mg²⁺, to be zero or marginally positive (endothermic). The Gibbs free-energy change for the spontaneous process is necessarily negative, a requirement that can be met only by an increase in entropy for an endothermic process. We conclude that the dominant attractive force in thylakoid stacking is entropy-driven. Several mechanisms for increasing entropy upon stacking of thylakoid membranes in the dark, particularly in low-light plants, are discussed. In the light, which drives the chloroplast far away from equilibrium, granal stacking accelerates non-cyclic photophosphorylation, possibly enhancing the rate at which entropy is produced.     

BIOGENESIS OF CHLOROPLAST MEMBRANES : V. A Radioautographic Study of Membrane Growth in a Mutant of Chlamydomonas reinhardi y-1

The development of photosynthetic lamellae during greening of dark-grown Chlamydomonas y-1 cells was investigated by radioautography. Acetate-³H was used as a marker for membrane lipids. In short pulse-labeling experiments, about 50–60% of the radioactivity incorporated was found in the lipid fraction and about 25–50% in starch granules present in the chloroplast of these algae. The relative specificity of acetate-³H used as a marker for membranes was artificially increased through quantitative removal of the starch granules from fixed cells by amylase treatment. Analysis of turnover coefficients of different membrane constituents and of the contribution of turnover and net synthesis to the total label incorporated in pulse experiments indicated that the incorporation of acetate into specific lipids was mainly due to net synthesis. The distribution of radioactivity in the different lipid constituents at the end of a short pulse and after 30- and 60-min chases indicated that transacylation is minimal and may be disregarded as a possible cause of randomization of the label. Statistical analysis of radioautographic grain distribution and measurements of different structural parameters indicate that (a) the chloroplast volume and surface remain constant during the process, whereas the growth of the photosynthetic lamellae parallels the increase in chlorophyll; (b) the lamellae do not develop from the chloroplast envelope or from the tubular system of the pyrenoid; (c) all the lamellae grow by incorporation of new material within preexisting structures; (d) different types of lamellae grow at different rates. The pyrenoid tubular system develops faster than the thylakoids, and single thylakoids develop about twice as fast as those which are paired or fused to grana. It is concluded that growth of the membranes occurs by a mechanism of random intussusception of molecular complexes within different types of preexisting membranes.     

A Comparative Ultrastructural Study of Cyanidium caldarium and the Unicellular Red Alga Rhodosorus marinus

The presence of nucleus, chloroplasts, mitochondria, microbodiesand a vacuole are confirmed in Cyanidium caldarium strain CCAP1355/1. Chloroplasts usually contain parallel rows of unstackedthylakoids surrounded by a peripheral thylakoid, although theconcentric arrangement has also been observed. The chloroplastenvelope consists of two closely appressed membranes which canonly be resolved after gluteraldehyde—osmium fixation.The chloroplast of Rhodosorus marinus also contains parallel,unstacked thylakoids surrounded by a peripheral thylakoid but,in addition, a prominent, stalked pyrenoid projects into thecytoplasm and is bounded by both the peripheral thylakoid andthe chloroplast envelope. This structure is covered by a capof starch grains and contains membranous vesicles in the matrix.Phycobilisomes are absent from the chloroplasts of both algae.The recognition of C. caldarium as a rhodophyte is supportedby these observations. Cyanidium caldarium, Rhodosorus marinus, chloroplast ultrastructure     

Biosynthesis of the unique trans-delta 3-hexadecenoic acid component of chloroplast phosphatidylglycerol: evidence concerning its site and mechanism of formation

As in most higher plants, chloroplast membranes of the green alga Dunaliella salina contain phosphatidylglycerol (PG) that is rich in trans-delta 3-hexadecenoic acid (16:1t), a fatty acid found nowhere else in the cell. After labeling D. salina with exogenous [3H]myristic acid [( 3H]14:0), the cis-unsaturated fatty acids of monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol as well as PG had higher specific radioactivities in chloroplast envelopes than in thylakoids. In contrast, 16:1t was very slow to become radioactive, and its specific radioactivity was several times higher in isolated thylakoids than in envelopes after brief (3-20 min) labeling with [3H]14:0. Analysis of individual PG molecular species revealed that the fatty acid paired with 16:1t was also labeled slowly. Thus linoleate (18:2) released from a 16:1t-containing PG had a 350-fold (at 3 min) to 20-fold (at 60 min) lower specific radioactivity than did 18:2 from a palmitate (16:0)-containing PG. The findings suggest that the substrates for trans-desaturation are 16:0-containing PG molecular species which are readily labeled from [3H]14:0 in the envelope but are diluted by the large pool of thylakoid PG before penetrating to the desaturation site. By examining the labeling patterns of individual PG molecular species classes, it was concluded that D. salina 16:1t is formed from 16:0 linked to 18:2/16:0 PG and 18:3/16:0 PG by a trans-desaturase located within the inner recesses of the thylakoid compartment.