Eva Selstam

PHASE-EQUILIBRIA OF MIXTURES OF PLANT GALACTOLIPIDS - THE FORMATION OF A BICONTINUOUS CUBIC PHASE

Abstract The chloroplast galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were isolated from wheat leaves. The phase equilibria of galactolipid-water systems with MGDG / DGDG molar ratios equal to 0:1, 1:2, 1.2:1, 2:1 and 1:0 were investigated, using nuclear magnetic resonance (NMR) methods. MGDG and DGDG form reversed hexagonal and lamellar phases, respectively, at temperatures between 10 and 40°C at all water contents studied (up to about 14 mol 2H2O per mol lipid). The galactolipid mixtures show a complex phase forming reversed hexagonal, lamellar and reversed cubic phases, depending on water content and temperature. It was found that the water hydration is similar for the lamellar and hexagonal phases formed by DGDG and MGDG, respectively. The non-lamellar phase areas increase with increasing content of MGDG. Small-angle X-ray measurements show that the cubic phase belongs to the Ia3d space group. From translational diffusion studies by NMR it is concluded that the structure of this cubic phase is bicontinuous.

Iron Deficiency in Cyanobacteria Causes Monomerization of Photosystem I Trimers and Reduces the Capacity for State Transitions and the Effective Absorption Cross Section of Photosystem I in Vivo

The induction of the isiA (CP43′) protein in iron-stressed cyanobacteria is accompanied by the formation of a ring of 18 CP43′ proteins around the photosystem I (PSI) trimer and is thought to increase the absorption cross section of PSI within the CP43′-PSI supercomplex. In contrast to these in vitro studies, our in vivo measurements failed to demonstrate any increase of the PSI absorption cross section in two strains (Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803) of iron-stressed cells. We report that iron-stressed cells exhibited a reduced capacity for state transitions and limited dark reduction of the plastoquinone pool, which accounts for the increase in PSII-related 685 nm chlorophyll fluorescence under iron deficiency. This was accompanied by lower abundance of the NADP-dehydrogenase complex and the PSI-associated subunit PsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturating polyacrylamide gel electrophoresis separation of the chlorophyll-protein complexes indicated that the monomeric form of PSI is favored over the trimeric form of PSI under iron stress. Thus, we demonstrate that the induction of CP43′ does not increase the PSI functional absorption cross section of whole cells in vivo, but rather, induces monomerization of PSI trimers and reduces the capacity for state transitions. We discuss the role of CP43′ as an effective energy quencher to photoprotect PSII and PSI under unfavorable environmental conditions in cyanobacteria in vivo.

Membrane lipid composition of the unusual cyanobacterium Gloeobacter violaceus sp. PCC 7421, which lacks sulfoquinovosyl diacylglycerol

Gloeobacter violaceus sp. PCC 7421 is an unusual cyanobacterium with only one cellular membrane, which lacks the thylakoid membranes found in other oxygenic photosynthetic organisms. The cell membrane lipids in G. violaceus sp. PCC 7421 are monogalactosyl diacylglycerol, digalactosyl diacylglycerol, phosphatidyl glycerol and phosphatidic acid in the molar proportion of 51, 24, 18 and 4% respectively. This lipid composition resembles that of the cell membrane from other cyanobacteria, but completely lacks sulfoquinovosyl diacylglycerol. This lack of sulfoquinovosyl diacylglycerol is exceptional for a photosynthetic membrane. The membrane lipids are esterified to 14:0, 16:0, 16:1, 18:0, 18:1, 18:2 and α18:3 fatty acids.

Calcium modifies Cd effect on runner bean plants

The effect of different Ca concentrations in the growth medium on the toxicity of 25 mu M CdSO4 was studied in runner bean plants (var. Piekny Jas) at two different growth stages of primary leaves. ...

Activities of glutathione reductase and superoxide dismutase in relation to changes of lipids and pigments due to ozone in seedlings of Pinus sylvestris (L.)

Scots pine seedlings (Pinus sylvestris L.) were exposed to ozone at a high-level, 300 ppb 8 h/day for 5 days, and at a low-level, 70-80 ppb 8 h/day for 10 days. In the high-level and low-level experiment the chlorophyll a content decreased 11% and 8%, respectively. The high-level ozone experiment affected the xanthophyll cycle by a 19% higher level of violaxanthin in the exposed seedlings compared to controls at day 5. This was also accompanied by a 42% lower concentration of zeaxanthin. No significant changes of the carotenoids neoxanthin, lutein, alpha-carotene or beta-carotene were found. The molar ratio monogalactosyl diacylglycerol/digalactosyl diacylglycerol decreased by 11% in the high-level ozone exposure. This suggests that the galactolipid:galactolipid galactosyl transferase, located in the outer envelope membrane of the chloroplast, was stimulated by ozone. No significant changes in the total membrane lipid or the acyl group composition was detected. In the high-level ozone experiment the concentration of soluble protein increased in the exposed seedlings during the last 3 days of the experimental period without any change in total protein. No visible damages were observed. The results showed that the activity of the defence enzymes superoxide dismutase (EC 1.15.1.1) and glutathione reductase (EC 1.6.4.2) was not altered after ozone exposure.

X‐ray diffraction studies of the structural organisation of prolamellar bodies isolated from Zea mays

Transmission electron microscopy (TEM) indicates that maize prolamellar bodies (PLBs) are built up of tetrapodal units based on a highly convoluted but continuous lipid bilayer exhibiting diamond cubic (Fd3m) symmetry. Such lattices are often described in terms of infinite periodic minimal surfaces (IMPS) exhibiting zero net curvature and dividing the system into two identical subvolumes. If so, X‐ray diffraction measurements would be expected to index on a double‐diamond (Pn3m) lattice with a unit cell length half that of the TEM lattice. Our measurements index on a Fd3m lattice with a similar repeat distance to the TEM images. The PLB membrane is thus inherently asymmetric, probably as the result of the distribution of membrane protein.

Development of Thylakoid Membranes with Respect to Lipids

Chloroplast membranes contain four highly unsaturated lipids: two galactolipids, one sulfolipid and one phospholipid. These are also present in proplastid and etioplast membranes. During chloroplast differentiation thylakoid membranes are formed by vesicles which bud off from the inner envelope membrane and plastids multiply by division. Monogalactosyldiacylglycerol, which has a reversed hexagonal phase structure, facilitates this fusion process. Monogalactosyldiacylglycerol, together with protochlorophyllide oxidoreductase, also mediates the formation of the prolamellar body, a membrane with a cubic phase structure. The chloroplast membranes also contain negatively charged lipids and the fraction of the charged lipids is very constant. The maintenance of a stable fraction of negatively charged lipids is probably due to the role that these lipids play in regulating the synthesis of monogalactosyldiacylglycerol. Current data from studies with mutants and transgenic plants suggests that the high level of thylakoid lipid unsaturation probably is more important for plastid differentiation than for photosynthetic function at low temperature. Furthermore, the repair of Photosystem II following photoinhibition at low temperature has been shown to be sensitive to the level of unsaturation suggesting a function for lipid unsaturation in membrane protein assembly and processing. Specific functions for the different chloroplast lipids have been difficult to assign. However, there is increasing evidence that phosphatidyl glycerol is tightly bound to the LHC polypeptide and facilitates trimer formation of the LHC complex.

Effects of frost hardening on the composition of galactolipids and phospholipids occurring during isolation of chloroplast thylakoids from needles of scots pine

EFFECTS OF FROST HARDENING ON THE COMPOSITION OF GALACTOLIPIDS AND PHOSPHOLIPIDS OCCURRING DURING ISOLATION OF CHLOROPLAST THYLAKOIDS FROM NEEDLES OF SCOTS PINE

Cold acclimation of the Arabidopsis dgd1 mutant 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 °C resulted from a temperature/lipid interaction enabling the stable assembly of PSI complexes in the thylakoid.

Acyl-chain-dependent incorporation of chlorophyll and cholesterol in membranes of Acholeplasma laidlawii

Abstract Lipid compositions in thylakoid membranes from photosynthetic organisms and plasma membranes of the cell-wall-less bacterium Acholeplasma laidlawii are very similar. The ability of chlorophyll a to affect lipid packing was investigated by incorporation into acyl-chain modified A. laidlawii membranes during growth. Cholesterol was used as a reference. A. laidlawii grows well with mono- and diunsaturated, but not at all with triunsaturated fatty acids. However, both chlorophyll and cholesterol allowed growth with linolenic acid. Hence, in linolenoyl media, A. laidlawii is dependent on these membrane additives for survival. Incorporated amounts of chlorophyll, and to a lesser extent cholesterol, rose dramatically upon increased acyl-chain unsaturation. The absorbance maximum of chlorophyll in A. laidlawii membranes was similar to that in lipids from photosynthetic membranes. The major membrane lipids, monoglucosyldiacylglycerol and diglucosyldiacylglycerol, form reversed hexagonal and lamellar liquid-crystalline phases in water, respectively. From the regulation of these lipids as a response to the presence of chlorophyll or cholesterol, it is concluded that chlorophyll stabilizes and cholesterol destabilizes the lamellar phases in unsaturated membrane lipids.