Function Of Thylakoid Membranes Research Articles

Relationship Between Glycerolipids and Photosynthetic Components During Recovery of Thylakoid Membranes From Nitrogen Starvation-Induced Attenuation in Synechocystis sp. PCC 6803.

Thylakoid membranes, the site of photochemical and electron transport reactions of oxygenic photosynthesis, are composed of a myriad of proteins, cofactors including pigments, and glycerolipids. In the non-diazotrophic cyanobacterium Synechocystis sp. PCC 6803, the size and function of thylakoid membranes are reduced under nitrogen (N) starvation but are quickly recovered after N addition to the starved cells. To understand how the functionality of thylakoid membranes is adjusted in response to N status in Synechocystis sp. PCC 6803, we examined changes in thylakoid components and the photosynthetic activity during the N starvation and recovery processes. In N-starved cells, phycobilisome content, photosystem II protein levels and the photosynthetic activity substantially decreased as compared with those in N-sufficient cells. Although the content of chlorophyll (Chl) a, total protein and total glycerolipid also decreased under the N-starved condition based on OD730 reflecting cell density, when based on culture volume, the Chl a and total protein content remained almost constant and total glycerolipid content even increased during N starvation, suggesting that cellular levels of these components decrease under the N-starved condition mainly through dilution due to cell growth. With N addition, the photosynthetic activity quickly recovered, followed by full restoration of photosynthetic pigment and protein levels. The content of phosphatidylglycerol (PG), an essential lipid constituent of both photosystems, increased faster than that of Chl a, whereas the content of glycolipids, the main constituents of the thylakoid lipid bilayer, gradually recovered after N addition. The data indicate differential regulation of PG and glycolipids during the construction of the photosynthetic machinery and regeneration of thylakoid membranes. Of note, addition of PG to the growth medium slightly accelerated the Chl a accumulation in wild-type cells during the recovery process. Because PG is required for the biosynthesis of Chl a and the formation of functional photosystem complexes, rapid PG biosynthesis in response to N acquisition may be required for the rapid formation of the photosynthetic machinery during thylakoid regeneration.

Overaccumulation of glycine betaine makes the function of the thylakoid membrane better in wheat under salt stress

Wheat (Triticum aestivum L.) lines T1, T4, and T6 were genetically modified to increase glycine betaine (GB) synthesis by introduction of the BADH (betaine aldehyde dehydrogenase, BADH) gene from mountain spinach (Atriplex hortensis L.). These transgenic lines and WT of wheat (T. aestivum L.) were used to study the effect of increased GB synthesis on wheat tolerance to salt stress. Salt stress due to 200mmolL−1 NaCl impaired the photosynthesis of the four wheat lines, as indicated by declines in net photosynthetic rate (Pn), stomatal conductance (Gs), maximum photochemical efficiency of PSII (Fv/Fm), and actual photochemical efficiency of PSII (ФPSII) and an increase in intercellular CO2 concentration (Ci). In comparison with WT, the effect of salinity on the three transgenic lines was mild. Salt stress caused disadvantageous changes in lipids and their fatty acid compositions in the thylakoid membrane of the transgenic lines and WT. Under salt stress, the three transgenic lines showed slightly higher chlorophyll and carotenoid contents and higher Hill reaction activities and Ca2+-ATPase activity than WT. All the results suggest that overaccumulation of GB resulting from introduction of the BADH gene can enhance the salt tolerance of transgenic plants, especially in the protection of the components and function of thylakoid membranes, thereby making photosynthesis better. Changes in lipids and fatty acid compositions in the thylakoid membrane may be involved in the increased salt stress tolerance of the transgenic lines.

Extraplastidial cytidinediphosphate diacylglycerol synthase activity is required for vegetative development in Arabidopsis thaliana

Cytidinediphosphate diacylglycerol synthase (CDS) catalyzes the activation of phosphatidic acid to cytidinediphosphate (CDP)-diacylglycerol, a central intermediate in glycerolipid biosynthesis in prokaryotic and eukaryotic organisms. Cytidinediphosphate-diacylglycerol is the precursor to phosphatidylinositol, phosphatidylglycerol (PG) and cardiolipin of eukaryotic phospholipids that are essential for various cellular functions. Isoforms of CDS are located in plastids, mitochondria and the endomembrane system of plants and are encoded by five genes in Arabidopsis. Two genes have previously been shown to code for the plastidial isoforms which are indispensable for the biosynthesis of plastidial PG, and thus biogenesis and function of thylakoid membranes. Here we have focused on the extraplastidial CDS isoforms, encoded by CDS1 and CDS2 which are constitutively expressed contrary to CDS3. We provide evidence that these closely related CDS genes code for membrane proteins located in the endoplasmic reticulum and possess very similar enzymatic properties. Development and analysis of Arabidopsis mutants lacking either one or both CDS1 and CDS2 genes clearly shows that these two genes have redundant functions. As reflected in the seedling lethal phenotype of the cds1cds2 double mutant, plant cells require at least one catalytically active microsomal CDS isoform for cell division and expansion. According to the altered glycerolipid composition of the double mutant in comparison with wild-type seedlings, it is likely that the drastic decrease in the level of phosphatidylinositol and the increase in phosphatidic acid cause defects in cell division and expansion.

THE ROLE OF PHOSPHOLIPASE C IN SIGNAL TRANSDUCTION DURING DROUGHT AND COLD STRESSES IN WHEAT (Triticum aestivum L.)

Environmental stresses are one of the major challenges to crop production. Molecular biologists have investigated these problems through the study of the genes whose expression could be changed in response to stress with the hypothesis that these genes may contribute to tolerance to these stresses. Zhu (2002) stated that salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification response pathways, and growth regulation pathways. Some phospholipids systems are activated by osmotic stress, generating a diverse array of messenger molecules, some of them function upstream of the osmotic stress-activated protein kinases. Also, ABA biosynthesis is regulated by osmotic stress. Both ABA dependent and independent osmotic stress signaling start with the modification of the expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream, stress tolerance effect or genes. Many cold responsive genes have been identified and characterized since the 1980’s. More recently DNA microarrays analysis using a 24 K GeneChips representing about 24000 Arabidopsis genes have been used to characterize changes in gene expression in response to cold treatment (Zhu et al., 2007). One study identified 655 genes cold upregulated and 284 as cold downregulated genes after chilling treatment (Lee et al., 2005). Many of the early cold responsive genes encode transcription factors that probably activate the genes that are induced after longer periods of exposure to the cold. Only one transcription factor was down regulated during early cold stress, suggesting that cold responses in plants are started by transcriptional activation and not by repression of genes. To investigate the interaction of proteins that may have signalling or regulatory function, protein-protein interaction was studied with yeast 2 hybrid analysis (Tardif et al., 2007). Phospholipases are sets of enzymes which have the ability to hydrolize phospholipids. They have different forms in plants, including phosphlipases C, D and A. Phospho-lipase C and D have important roles in signal transduction. The general structure of phospholipids consists of two fatty acyl chains esterified to a glycerol backbone at the sn-1 and sn-2 positions, and a phosphate at sn-3 position to which a variable head group (R) is attached. Phos-phatidylglycerol plays a specific role in photosynthesis (Sakurai et al., 2007) by helping in the organization and function of thylakoid membranes in plant cells. The first cell change during stress injury is an alteration in the structure and function of cell membranes. In many plants, alterations in lipids, particularly in phospholipids and sterols were very clear due to water stress (Liljenberg, 1992) or salt treatment (Kuiper, 1984). These alterations in the membrane seem to control the fluidity and the surrounding proteins, which influences membrane functions such as bilayer permeability carrier-mediated transport and the activity of membrane bound enzymes including ATPase activity (Surjus and Durand, 1996). In plants, G-proteins are involved in the regulation of ion channel and abscisic acid signalling, modulation of cell proliferation, and in many other processes such as seed germination, shoot and root growth, and stomatal regulation. Phospholipase C (PLC) has been shown to function as an intercellular effector molecule in plants for the α-subunit of pea heterotrimeric G-proteins whose activity is regulated by salinity stress (Misra et al., 2007). Gα protein interacts with pea phos-pholipase-C (PLC) at the calcium-binding domain (C2) leading to increasing GTPase activity, thus the signal transduction can be reduced by PLC association with the Gα-protein (Tuteja, 2007). The main objective of this study was to characterize the expression profiles of three full length cDNA clones from two phosphogyceride specific phospholipase C’s wheat genes under drought and cold stresses.

Two Closely Related Genes of Arabidopsis Encode Plastidial Cytidinediphosphate Diacylglycerol Synthases Essential for Photoautotrophic Growth

Cytidinediphosphate diacylglycerol synthase (CDS) catalyzes the formation of cytidinediphosphate diacylglycerol, an essential precursor of anionic phosphoglycerolipids like phosphatidylglycerol or -inositol. In plant cells, CDS isozymes are located in plastids, mitochondria, and microsomes. Here, we show that these isozymes are encoded by five genes in Arabidopsis (Arabidopsis thaliana). Alternative translation initiation or alternative splicing of CDS2 and CDS4 transcripts can result in up to 10 isoforms. Most of the cDNAs encoding the various plant isoforms were functionally expressed in yeast and rescued the nonviable phenotype of the mutant strain lacking CDS activity. The closely related genes CDS4 and CDS5 were found to encode plastidial isozymes with similar catalytic properties. Inactivation of both genes was required to obtain Arabidopsis mutant lines with a visible phenotype, suggesting that the genes have redundant functions. Analysis of these Arabidopsis mutants provided further independent evidence for the importance of plastidial phosphatidylglycerol for structure and function of thylakoid membranes and, hence, for photoautotrophic growth.

Glycine betaine improves thylakoid membrane function of tobacco leaves under low-temperature stress

Glycine betaine (GB) is an effective compatible solute that improves the tolerance in plants to various stresses. We investigated the effects of 2 mM GB applied to the roots of a tobacco (Nicotiana tabacum L.) cultivar on enhancing photosynthesis under low-temperature (LT) stress (5/5 °C, 12/12 h, 300 µmol m-2 s-1) and in the subsequent recovery (25/18 °C) from the stress. The net photosynthetic rate, intrinsic efficiency measured as the ratio of variable to maximum fluorescence, and actual efficiency of the photochemistry of photosystem 2 as well as the ATPase activity in the thylakoid membrane decreased, and a distinct K step in the fluorescence transient O-J-I-P appeared under cold stress. Exogenous GB alleviated the decrease in all these parameters. The LT-stress induced the accumulation of 33-66 kDa polypeptides and decreased the proportion of unsaturated fatty acids in the thylakoid membrane. In plants subjected to LT-stress, GB protected these polypeptides from damage and enhanced the proportion of unsaturated fatty acids. An increase in non-radiative energy dissipation (NPQ) may be involved in the improvement of the function of the thylakoid membrane by GB since exogenous GB protected violaxanthin de-epoxidase and enhanced NPQ.

Effect of glycinebetaine on function of thylakoid membranes in wheat flag leaves under drought stress

Two wheat cultivars, HF9703 (drought tolerant) and SN215953 (drought sensitive) were used to examine the effects of glycinebetaine (GB, 100 mM) on lipid composition and function of thylakoid membranes under drought stress. GB application mitigated negative effect of drought on Ca2+-ATPase and Hill reaction activities, chlorophyll content, gas exchange and photosynthesis. These positive effects of GB application maybe, in part, correlated with improving the lipid composition of the thylakoid membranes.

Ethylene control of seed coat development in low and high sterile semidwarf indica rice cultivars

Phosphatidylglycerol (PG) is of crucial importance for the organization and function of thylakoid membranes. The reason for a decrease of PG concentration in plants under phosphate deprivation stress still remains unclear. By comparing PG concentration and PG hydrolase activity of wheat leaves at different developmental stages, we show that when the first leaves are fully developed, the PG concentration and PG hydrolase activity in phosphate-deficient plants were similar to those of the controls. With age, there was a significant decrease in PG concentration combined with an increase in PG hydrolase activity for phosphate-deficient plants. The close correlation between the decrease in PG concentration and the increase in PG hydrolases activities suggests that PG hydrolysis was the most important factor responsible for the decrease in its concentration. The main hydrolysis products of PG are phosphatidic acid (PA), diacylglycerol (DAG) and free fatty acid (FFA). The application of an inhibitor, n-butanol, which blocks the formation of PA via the PLD pathway, to the in vitro enzyme reaction mixture may restrict PA and DAG production. Neomycin sulfate, a PLC inhibitor, was shown to exhibit significant inhibition in DAG generation. These results suggest that both PLD and PLC were responsible for PG degradation in the leaves of phosphate-deficient wheat. The possible role of PLA activity for PG degradation is also discussed.

Cyclodextrins: A potential tool for studying the role of glycerolipids in photosynthetic membranes

A novel cyclodextrin derivative, i.e., permethylated-alpha-cyclodextrin (PM-alpha-CD), was used for removing glycerolipids from spinach thylakoid membranes and investigating their role in photosynthetic activities. A three-step selective removal of each lipid class was observed in treated membranes. Up to a concentration of 4 mM, PM-alpha-CD (in the presence of 75 microg chlorophyll a+b/mL), PM-alpha-CD displayed a marked selectivity for anionic lipids [sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG)] in comparison with galactolipids. At this concentration, half of PG and SQDG were removed. Within this range of concentration, the volume response of treated thylakoids to variation of osmolarity, an indirect mean of verifying the structural integrity of the membrane, was not altered. Similarly, neither photosystem II (PSII) nor photosystem I (PSI) activity was affected. In contrast, the low-temperature fluorescence ratio F695/F740 drastically diminished from 1.45 to about 0.7, essentially due to the decrease of PSII fluorescence. The results derived from the fast fluorescence rise expressed in the form of a spider suggest that the fraction of inactivated (non-Q(A) reducing) reaction centers (RC) increases while the active (Q(A) reducing) RC remained intact. Raising the concentration of PM-alpha-CD from 4 to 7 mM resulted in a progressive but greater diminution of the galactolipid level than that of SQDG and PG. Within this concentration range, the integrity of the membrane was not altered, nor was either PSII or PSI activity, whereas the F695/F740 ratio decreased to about 0.45 as well as the fraction of inactivated RC. At concentrations above 7 mM of PM-alpha-CD, the integrity of the membrane was impaired, resulting in a decrease of both electron transport activities. At all concentrations, PM-alpha-CD did not show any selectivity toward either the acyl chains of the lipid molecules or the molecular species of PG. The results are discussed in terms of the role of glycerolipids in thylakoid membrane function and the relationship of the chemical structure of PM-alpha-CD and its lipid removal capacity.

Structure-function correlation during the etioplast-chloroplast transition in cucumber cotyledonary leaves

We studied the development of chloroplasts from etioplasts in the cotyledonary leaves of 4-d-old dark-grown cucumber (Cucumis sativus) seedlings after irradiation (20 μmol m-2 s-1). Upon irradiation, the triggering of chlorophyll (Chl) synthesis and accumulation showed a relatively short lag phase. The irradiation of etiolated seedlings initiated the synthesis of apoproteins of pigment-protein complexes. While Chl-protein 2 (CP2) was detected at 6 h after irradiation, CP1 only after 29 h. The appearance and accumulation of some of the apoproteins were monitored by Western-blotting. LHC2 apoprotein was detected after a 6 h-irradiation. The amounts of D1 protein of photosystem (PS) 2 and PsaA/B protein of PS1 were quantitated by ELISA. Further, the thylakoid membrane function during this time period in terms of PS1- and PS2-mediated electron transfer activity and intersystem electron pool size were analyzed. While PS1 activity was detected after 4 h, PS2-mediated O2 evolution was detected only after a 17 h-irradiation. Fv/Fm value of Chl a fluorescence measurements indicated that the photochemical efficiency of these leaves reached its maximum after 29 h of irradiation. The intersystem pool size of cotyledonary leaves was equivalent to that of the control cotyledonary leaves grown for 25 h under continuous irradiation. Thus etioplasts develop into fully functional chloroplasts after approximately 25 h when 4 d-dark grown cucumber seedlings are continuously moderately irradiated. The development of photosynthetic electron transport chain seems to be limited in time at the level of PS2, possibly at the donor side.

The contribution of drought‐related decreases in foliar nitrogen concentration to decreases in photosynthetic capacity during and after drought in prairie grasses

While stomatal closure usually limits photosynthesis during drought, our previous results suggest that drought‐related decreases in foliar nitrogen concentration (NL) limit photosynthesis during recovery from drought in prairie grasses. Here we estimate the importance of decreases in NL to decreased photosynthetic capacity (PScap) during drought and a subsequent recovery period in three perennial C4 prairie grasses. PScap (O2 evolution at light and CO2 saturation) decreased 69 to 78% during drought in these grasses, and full recovery of PScap required 8 to 12 days, until younger leaves were expanded or older leaves were repaired, depending on species. Decreases in NL explained 38 to 51% of the loss of PScap during drought and accounted for 51 to 69% of the total loss of PScap integrated over the post‐drought recovery period. N‐related loss of PScap appeared to result more from decreases in ribulose‐1,5‐bisphosphate carboxylase/oxygenase (EC 4.1.1.39), phosphoenolpyruvate carboxylase (4.1.1.31), and other soluble photosynthetic enzymes, than from decreases in thylakoid N‐containing compounds. Decreases in quantum yield of O2 evolution and Fv/Fm (variable‐to‐maximum fluorescence of dark‐adapted leaves) during drought were small, so we assumed that little damage to photsystem II (PSII) and thylakoid membrane function occurred. Further, F0 (minimum F) decreased or remained unchanged, dark F0 was greater than light F0, and decreases in photochemical quenching (the fraction of oxidized PSII) were reversed within 1–3 days after drought. Therefore, prolonged increases in non‐photochemical quenching (qn; thermal dissipation of excess light energy) during and after drought were indicative of protective downregulation and were likely associated with disproportionate loss of soluble photosynthetic proteins during drought. In support of this, post‐drought recovery of qn paralleled recovery of NL and PScap. Thus, in C4 prairie grasses, loss of PScap during drought is largely the result of decreases in shoot NL and of associated protective downregulation, decreasing carbon assimilation for 1–2 weeks after drought.

Dynamics of Photosystem Stoichiometry Adjustment by Light Quality in Chloroplasts.

Long-term imbalance in light absorption and electron transport by photosystem I (PSI) and photosystem II (PSII) in chloroplasts brings about changes in the composition, structure, and function of thylakoid membranes. The response entails adjustment in the photosystem ratio, which is optimized to help the plant retain a high quantum efficiency of photosynthesis (W.S. Chow, A. Melis, J.M. Anderson [1990] Proc Nat Acad Sci USA 87: 7502-7506). The dynamics of photosystem ratio adjustment were investigated upon the transfer of pea {Pisum sativum} plants from a predominantly PSI-light to a predominantly PSII-light environment and vice versa. The concentration of functional components (primary electron accepting plastoquinone of PSII [QA], P700) and that of constituent proteins were monitored during acclimation by A difference spectrophotometry and immunoblot analysis, respectively. Fully reversible changes in photosystem ratio occurred with a half-time of about 20 h. They involved closely coordinated changes in the concentration of the QA, reaction center protein D1, D2, and the 9-kD apoprotein of the cytochrome b559 for PSII. Similarly, closely coordinated changes in the relative concentration of P700 and reaction center proteins of PSI were observed. The level of chlorophyll b and that of the light-harvesting complex II changed in accordance with the concentration of PSII in the acclimating thylakoids. Overall, adjustments in the photosystem ratio in response to PSI- or PSII-light conditions appeared to be a well-coordinated reaction in the chloroplast. The response was absent in the chlorophyll b-less chlorina f2 mutant of barley (Hordeum vulgare) and in a phycobilisomeless mutant of Agmenellum quadruplicatum, suggesting that photosystem accessory pigments act as the light-quality perception molecules and that PSI and PSII themselves play a role in the signal transduction pathway.

Response of Soybean Photosynthesis and Chloroplast Membrane Function to Canopy Development and Mutual Shading

The effect of natural shading on photosynthetic capacity and chloroplast thylakoid membrane function was examined in soybean (Glycine max. cv Young) under field conditions using a randomized complete block design. Seedlings were thinned to 15 plants per square meter at 20 days after planting. Leaves destined to function in the shaded regions of the canopy were tagged during early expansion at 40 days after planting. To investigate the response of shaded leaves to an increase in available light, plants were removed from certain plots at 29 or 37 days after tagging to reduce the population from 15 to three plants per square meter and alter the irradiance and spectral quality of light. During the transition from a sun to a shade environment, maximum photosynthesis and chloroplast electron transport of control leaves decreased by two- to threefold over a period of 40 days followed by rapid senescence and abscission. Senescence and abscission of tagged leaves were delayed by more than 4 weeks in plots where plant populations were reduced to three plants per square meter. Maximum photosynthesis and chloroplast electron transport activity were stabilized or elevated in response to increased light when plant populations were reduced from 15 to three plants per square meter. Several chloroplast thylakoid membrane components were affected by light environment. Cytochrome f and coupling factor protein decreased by 40% and 80%, respectively, as control leaves became shaded and then increased when shaded leaves acclimated to high light. The concentrations of photosystem I (PSI) and photosystem II (PSII) reaction centers were not affected by light environment or leaf age in field grown plants, resulting in a constant PSII/PSI ratio of 1.6 +/- 0.3. Analysis of the chlorophyll-protein composition revealed a shift in chlorophyll from PSI to PSII as leaves became shaded and a reversal of this process when shaded leaves were provided with increased light. These results were in contrast to those of soybeans grown in a growth chamber where the PSII/PSI ratio as well as cytochrome f and coupling factor protein levels were dependent on growth irradiance. To summarize, light environment regulated both the photosynthetic characteristics and the timing of senescence in soybean leaves grown under field conditions.

The composition and function of thylakoid membranes from pea plants grown under white or green light with or without far‐red light

Pea plants (Pisum sativum L. ev. Greenfeast) were grown for 2 to 3 weeks in while (˜ 50 μmol photons m−2 s−1; 400–700 nm) or green (˜ 30 μmol photons m−2 s −1 400–700 nm) light (16 h day/8 h night), with or without far‐red light. Supplementary far‐red light decreased leaf area and increased internodal length in both white and green light, demonstrating that phytochrome influenced leaf size and plant growth. However, there was no effect of far‐red light on chlorophyll a/chlorophyll b ratios, chlorophyll‐protein composition, the stoichiometry of electron transport complexes or photosynthetic function of isolated thylakoids. These results suggest that phytochrome is ineffective in modulating the composition and function of thylakoids in pea plants grown at low irradiance. One possible explanation of the ineffectiveness of phytochrome on thylakoids is discussed in terms of the drastic attenuation of red relative to far‐red light in green tissue.

High-temperature damage and acclimation of the photosynthetic apparatus

The influence of mono- (K(+)) and divalent (Mg(2+)) cations and protons (pH) on the temperature sensitivity of thylakoid membranes was investigated in three groups of young bean plants (control, heat-acclimated and non-acclimated). Thylakoid-membrane function was monitored by second and millisecond delayed fluorescence and 9-aminoacridine fluorescence quenching. It was established that metal ions at investigated concentrations decreased the thermostability of the photosynthetic parameters - an increase of MgSO4 concentration from 0.1 to 20 mM decreased the temperature of their half-inactivation (T50) by 13°C. At the same time the pH dependence of the thermal stability of these parameters showed a maximum at pH 5.5-6.5. The half-inactivation temperatures of those photosynthetic parameters connected with the ability of the thylakoid membrane to form light-induced proton gradients increased by 6-7°C in the heat-acclimated plants compared with the control. It was assumed that the temperature inactivation of photosynthetic electron transfer and the energization of the thylakoid membrane was determined both by the thermoinduced dissociation of the light-harvesting chlorophyll a/b protein complex from PSII, leading to destruction of the excitation energy transfer to the reaction centres, and by the thermal denaturation of the membrane-protein components. The rate of these processes was probably controlled by the size of the negative surface charge and the viscosity of the thylakoid membrane.

Changes in leaf phosphate status have only small effects on the photochemical apparatus of sugar beet leaves

The effects of orthophosphate (P i) on the structure and function of thylakoid membranes was investigated by monitoring changes in thylakoid polypeptides, pigment-proteins, cytochromes (Cyt), atrazine-binding sites, leaf fluorescence characteristics, photosynthesis quantum yield and electron transport (including Cyt f dark reduction) in leaves of sugar beet ( Beta vulgaris L. cv. F58-554H1) plants cultured hydroponically with different levels of P i supply. Low-P treatment (i.e. low external P i supply) decreased soluble leaf phosphorus (P) by 78% and total plant dry matter by 77%, and decreased light-saturated photosynthetic CO 2 fixation by 32%. The results show that low-P treatment had only minor effects on thylakoid membrane composition and function. There was some evidence that low-P increased the amounts of photosystem (PS) I and light harvesting chlorophyll-protein complexes and of the Cyt f and b 563 , relative to the amounts of PS II reaction center complex, the number of atrazine binding sites and Cyt b 559 . Low-P treatment had very little effect, if any, on photosynthetic quantum yield and small effects on chlorophyll fluorescence.

Light-quality and irradiance adaptation of the composition and function of pea-thylakoid membranes

The effect of light quality and light quantity during growth on the composition and function of pea thylakoids was investigated. Pea plants were grown at seven irradiance levels in four light-quality regimes with different red to far-red ratios (control, far-red-enriched, far-red-deficient and red). Both the chlorophyll a chlorophyll b ( Chl a Chl b ) ratios and the coupling-factor activities increased with increasing levels of irradiance in a curvilinear response in each light regime. This results in a linear relationship between Chl a Chl b ratios, and hence modulations of chlorophyll-proteins and coupling-factor activities in each case, regardless of the red/far-red ratio of the four light-quality sources. Under conditions of long-term irradiance in peas, the modulations of the amounts of core Photosystem II (PS II) complex, core Photosystem I complex, cytochrome b f complex and ATP synthetase on the one hand vs. the antenna chlorophyll a b- proteins of PS II and PS I on the other do not appear to be influenced by the phytochrome photoequilibrium. We suggest that the levels of NADPH and ATP formed during photosynthesis are the crucial factor in promoting gene expression and regulating the composition, structure and function of thylakoid membranes.

Changes in topography and function of thylakoid membranes following membrane protein phosphorylation

Changes in topography and function of pea (Pisum sativum L.) thylakoid membrane fractions following membrane protein phosphorylation have been studied. After protein phosphorylation the stromal membrane fraction had a higher chlorophyll a/b ratio, an increased content of light-harvesting chlorophyll protein and a higher ratio of chlorophyll to cytochrome f. This indicates that a pool of light-harvesting chlorophyll protein migrates from the photosystem II-enriched grana regions to the photosystem I-enriched stroma lamellae, in agreement with Kyle et al. (1984, Biochim. Biophys. Acta 765, 89-96) and Larsson et al. (1983, Eur. J. Biochem. 136, 25-29). Phosphorylation caused a stimulation in the rate of light-limited photosystem-I electron transfer in the unappressed membrane fraction, indicating that the translocated LHC-II becomes functionally associated with photosystem I.

Effect of Light Quality on the Composition, Function, and Structure of Photosynthetic Thylakoid Membranes of Asplenium australasicum (Sm.) Hook

The effect of light quality on the composition, function and structure of the thylakoid membranes, as well as on the photosynthetic rates of intact fronds from Asplenium australasicum, a shade plant, grown in blue, white, or red light of equal intensity (50 microeinsteins per square meter per second) was investigated. When compared with those isolated from plants grown in white and blue light, thylakoids from plants grown in red light have higher chlorophyll a/chlorophyll b ratios and lower amounts of light-harvesting chlorophyll a/b-protein complexes than those grown in blue light. On a chlorophyll basis, there were higher levels of PSII reaction centers, cytochrome f and coupling factor activity in thylakoids from red light-grown ferns, but lower levels of PSI reaction centers and plastoquinone. The red light-grown ferns had a higher PSII/PSI reaction center ratio of 4.1 compared to 2.1 in blue light-grown ferns, and a larger apparent PSI unit size and a lower PSII unit size. The CO(2) assimilation rates in fronds from red light-grown ferns were lower on a unit area or fresh weight basis, but higher on a chlorophyll basis, reflecting the higher levels of electron carriers and electron transport in the thylakoids.The structure of thylakoids isolated from plants grown under the three light treatments was similar, with no significant differences in the number of thylakoids per granal stack or the ratio of appressed membrane length/nonappressed membrane length. The large freeze-fracture particles had the same size in the red-, blue-, and white-grown ferns, but there were some differences in their density. Light quality is an important factor in the regulation of the composition and function of thylakoid membranes, but the effects depend upon the plant species.

A Mutant of Arabidopsis Lacking a Chloroplast-Specific Lipid

In order to investigate the functional significance of membrane lipid unsaturation, we have isolated a series of mutants of Arabidopsis thaliana which are deficient in particular membrane fatty acids. The first of these mutants completely lacks Delta3-trans-hexadecenoate, an acyl group that until now has been thought to play an important role in the structure or function of thylakoid membranes in photosynthetic eukaryotes. The apparent absence of any marked physiological effect of the mutation illustrates the potential of this approach to the analysis of membrane structure and function.