Photosynthetic energy conversion under extreme conditions--II: the significance of lipids under light limited growth in Antarctic sea ice diatoms

Low photosynthetic active radiation is a strong determinant in the development and growth of sea ice algae. The algae appear to have universal mechanisms to overcome light limitation. One important process, which is induced under light limitation, is the desaturation of chloroplast membrane lipids. In order to discover whether this process is universally valid in sea ice diatoms, we investigated three species coexisting in chemostats illuminated with 15 and 2 micromol photons m(-2) s(-1) at -1 degrees C. Growth under 2 micromol photons m(-2) s(-1) caused a 50% increase in monogalactosyldiacylglycerols (MGDG) thylakoid membrane related 20:5 n-3 fatty acids. This fatty acid supports the fluidity of the thylakoid membrane and therefore the velocity of electron flow, which is indicated by increasing rate constants for the electron transport between Q(A) (first stable electron acceptor) and bound Q(B) (second stable electron acceptor) (11.16 +/- 1.34 to 23.24 +/- 1.35 relative units). Two micromol photons m(-2) s(-1) furthermore resulted in higher amounts of non-lipid bilayer forming MGDG in relation to other bilayer forming lipids, especially digalactosydiacylglycerol (DGDG). The ratio of MGDG:DGDG increased from 3.4 +/- 0.3 to 5.7 +/- 0.3. The existence of bilayer thylakoid membranes with high proportions of non. bilayer forming lipids is only possible when sufficient thylakoid pigment-protein complexes are present. If more thylakoid pigment-protein complexes are present in membranes, as found under extreme light limitation, less bilayer forming lipids such as DGDG are required to stabilize the bilayer structure. Differences in protein contents between both light intensities were not found. Consequently pigment contents which nearly doubled under 2 micromol photons m(-2) s(-1) must be responsible in balancing the potential stability loss resulting from an increase in MGDG:DGDG ratio.     

Aluminum mediates compositional alterations of polar lipid classes in maize seedlings

Changes in lipid composition were investigated on maize roots and shoots under aluminum stress. After 4d exposure to 100 microM Al, root growth was inhibited while shoot growth was not affected. In roots, the decrease of the DBI (double bond index) of total fatty acids may signal a decrease in membrane fluidity. The total lipids (TL) decreased by 49%, but phospholipids (PL), phosphatidylcholine (PC) and phosphatidylinositol (PI) increased to approximately 3-fold. The MGDG increased to 2-fold but no significant change was found in the DGDG. The steryl lipids (SL) increased by 69%. The SL/PL ratio decreased from 2.64 to 1.52 and the MGDG/DGDG ratio increased from 0.45 to 1.06 in roots of Al-stressed plants. Al leads to oxidative stress in roots of treated plants as indicated by the increase of malondialdehyde (MDA) concentrations. In shoots, changes in fatty acid composition were associated with an increase of the DBI in all lipid classes except that of the DGDG decreased. The PG was the lipid class which shows the large variation of fatty acid composition. No significant changes were found either for TL, PL, SL or MDA concentrations in shoots of Al-treated plants. While PE levels did not show significant change, PI and PG increased and PC decreased. However, the Al caused 87% decrease in the GL levels. The MGDG and DGDG decreased to 19- and 8-fold, respectively. The deleterious effects of Al on polar lipids could be caused by a direct intervention of Al on plasma membrane and/or alteration of cell metabolism.     

Effect of indole-3-acetic acid on surface properties of the wheat plastid lipids

Surface parameters of polar lipids extracted from winter wheat plastids were investigated by the Langmuir and X-ray differentiation scattering methods. Highly purified plastids were isolated from non-embryogenic (NE) and embryogenic (E) calli initiated from inflorescences. NE plastids contained more monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) and less phospholipids (PL) fraction than E plastids. Moreover, in E calli, unsaturated fatty acids were detected in a higher proportion than in NE for both MGDG and DGDG. No significant differences in fatty acids saturation of PL between NE and E objects were detected. Aqueous surface monolayers were prepared from separate lipids and from mixtures of glycolipids and PL. In the case of MGDG, isotherms showed specific shoulders, contrary to continuous isotherms obtained for other investigated lipids. On the base of pi-A isotherms, the surface parameters: limiting area (A(lim)) and collapse pressure (pi(coll)) were calculated. Indole-3-acetic acid (IAA) increased the A(lim) of all separated lipids about 4-10 angstrom2/mol. However, for NE lipid mixture, the effect of IAA was much smaller (about 2 angstroms2/mol) than for other objects (usually about 5 angstroms2/mol). X-ray experiments for liposomes, obtained from mixtures of glycolipids and PL of NE and E plastids, showed continuous scattering curves with maxima characteristic for lipid bilayer membranes. Calculations of distance distribution functions indicated that bilayer thickness was 41 and 38 angstroms for NE and E, respectively. IAA influence on membrane structures was detected especially in E liposomes and increased the distance between head groups by about 2 angstroms. It is suggested that changes occur during embryogenesis in specific structure of plastid membranes determined also the formation of domains, similar to that suggested for plasmalemma (Plant Sci. 165 (2003) 265). IAA treatment influenced the membrane structure, especially E plastids increasing distances between polar groups.     

Functional roles of the major chloroplast lipids in the violaxanthin cycle

Monogalactosyldiacylglyceride (MGDG) and digalactosyldiacylglyceride (DGDG) are the major membrane lipids of chloroplasts. The question of the specialized functions of these unique lipids has received limited attention. One function is to support violaxanthin de-epoxidase (VDE) activity, an enzyme of the violaxanthin cycle. To understand better the properties of this system, the effects of galactolipids and phosphatidylcholines on VDE activity were examined by two independent methods. The results show that the micelle-forming lipid (MGDG) and bilayer forming lipids (DGDG and phosphatidylcholines) support VDE activity differently. MGDG supported rapid and complete de-epoxidation starting at a threshold lipid concentration (10 μM) coincident with complete solubilization of violaxanthin. In contrast, DGDG supported slow but nevertheless complete to nearly complete de-epoxidation at a lower lipid concentration (6.7 μM) that did not completely solubilize violaxanthin. Phosphotidylcholines showed similar effects as DGDG except that de-epoxidation was incomplete. Since VDE requires solubilized violaxanthin, aggregated violaxanthin in DGDG at low concentration must become solubilized as de-epoxidation proceeds. High lipid concentrations had lower activity possibly due to formation of multilayered structures (liposomes) that restrict accessibility of violaxanthin to VDE. MGDG micelles do not present such restrictions. The results indicate VDE operates throughout the lipid phase of the single bilayer thylakoid membrane and is not limited to putative MGDG micelle domains. Additionally, the results also explain the differential partitioning of violaxanthin between the envelope and thylakoid as due to the relative solubilities of violaxanthin and zeaxanthin in MGDG, DGDG and phospholipids. The violaxanthin cycle is hypothesized to be a linked system of the thylakoid and envelope for signal transduction of light stress.     

The effect of variations in growth temperature, fatty acid composition and cholesterol content on the lipid polar head-group composition of Acholeplasma laidlawii B membranes

We have systematically investigated the effect of variations in growth temperature, fatty acid composition and cholesterol content on the membrane lipid polar headgroup composition of Acholeplasma laidlawii B. Two important lipid compositional parameters have been determined from such an analysis. The first parameter studied was the ratio of the two major neutral glycolipids of this organism, monoglucosyldiacylglycerol (MGDG) and diglucosyldiacylglycerol (DGDG). As the former lipid prefers to exist in a reversed hexagonal phase at higher temperatures, with unsaturated fatty acyl chains or in the presence of cholesterol, the ratio of these two lipids reflects the phase state preference of the total A. laidlawii membrane lipids. Although we find that the MGDG/DGDG ratio is reduced in response to an increase in fatty acid unsaturation, increases in growth temperature or cholesterol content reduce this ratio only in cells enriched in a saturated but not an unsaturated fatty acid. The second parameter studied was the ratio of these neutral glycolipids to the only phosphatide in the A. laidlawii membrane, phosphatidylglycerol (PG); this parameter reflects the relative balance of uncharged and charged lipids in the membrane of this organism. We find that the MGDG + DGDG/PG ratio is lowest in cells enriched in the saturated fatty acid even though these cells already have the highest lipid bilayer surface charge density. Moreover, this ratio is not consistently related to growth temperature or changes in cholesterol levels, as expected. We therefore conclude that A. laidlawii strain B, apparently unlike strain A, does not possess coherent regulatory mechanisms for maintaining either the phase preference or the surface charge density of its membrane lipid constant in response to variations in growth temperature, fatty acid composition or cholesterol content.     

Mono‐ and digalactosyldiacylglycerol composition of the marennine‐producing diatom,Haslea ostrearia: Comparison to a selection of pennate and centric diatoms

Diatoms are one of the largest groups of primary producers in the oceans, yet despite their environmental importance little is known about their plastidial lipid biochemistry. It has been previously reported that Skeletonema species contain primarily C₁₆/C₁₆ and C₂₀/C₁₆ forms of mono‐ and digalactosyldiacylglycerol (MGDG and DGDG, respectively). Likewise, it was also reported that Phaeodactylum tricornutum contains primarily C₁₆/C₁₆ and C₂₀/C₂₀ forms of MGDG and DGDG. We seek to relate their studies to other diatoms, both in the centrics and pennates, with particular focus on the marennine‐producing pennate diatom, Haslea ostrearia. To this end, the composition and positional distribution of fatty acids of MGDG and DGDG were examined using positive‐ion electrospray ionization/mass spectrometry (ESI/MS). Two centric diatoms, Skeletonema marinoi and Thalassiosira weissflogii, and the pennate diatom, P. tricornutum, contained primarily C₂₀/C₁₆ (sn‐1/sn‐2) and C₁₈/C₁₆ forms of MGDG and DGDG. The other pennate diatoms, H. ostrearia and Navicula perminuta, contained primarily C₁₈/C₁₆ or C₁₈/C₁₈ forms of MGDG and DGDG, indicating a previously unrecognized fatty acid diversity in diatom MGDG and DGDG.     

Biosynthesis and desaturation of prokaryotic galactolipids in leaves and isolated chloroplasts from spinach

Mono- and digalactosyldiacylglycerol (MGDG and DGDG) were isolated from the leaves of sixteen 16:3 plants. In all of these plant species, the sn-2 position of MGDG was more enriched in C₁₆ fatty acids than sn-2 of DGDG. The molar ratios of prokaryotic MGDG to prokaryotic DGDG ranged from 4 to 10. This suggests that 16:3 plants synthesize more prokaryotic MGDG than prokaryotic DGDG. In the 16:3 plant Spinacia oleracea L. (spinach), the formation of prokaryotic galactolipids was studied both in vivo and in vitro. In intact spinach leaves as well as in chloroplasts isolated from these leaves, radioactivity from [1-¹⁴C]acetate accumulated 10 times faster in MGDG than in DGDG. After 2 hours of incorporation, most labeled galactolipids from leaves and all labeled galactolipids from isolated chloroplasts were in the prokaryotic configuration. Both in vivo and in vitro, the desaturation of labeled palmitate and oleate to trienoic fatty acids was higher in MGDG than in DGDG. In leaves, palmitate at the sn-2 position was desaturated in MGDG but not in DGDG. In isolated chloroplasts, palmitate at sn-2 similarly was desaturated only in MGDG, but palmitate and oleate at the sn-1 position were desaturated in MGDG as well as in DGDG. Apparently, palmitate desaturase reacts with sn-1 palmitate in either galactolipid, but does not react with the sn-2 fatty acid of DGDG. These results demonstrate that isolated spinach chloroplasts can synthesize and desaturate prokaryotic MGDG and DGDG. The finally accumulating molecular species, MGDG(18:3/16:3) and DGDG(18:3/16:0), are made by the chloroplasts in proportions similar to those found in leaves.     

Effect of Cold-Hardening on Thylakoid Membrane Lipids and Proteins of Spring Wheat and Winter Wheat

By comparison of thylakoid membrane lipids and their fatty acid composition, the supermolecular structure of light harvesting chlorophyll a/b-protein complex of Photosystem Ⅱ (LHC Ⅱ ) and the spectroscopic characteristics of thylakoids in winter wheat (Yanda 1817) with those in spring wheat (8901) before and after cold-hardening, it was found that after cold-hardening: (1)The trans-3-hexadeeenoic acid content of phosphatidyl alycerol (PG) in both cultivars decreased significantly, the ratio of monogalactosyl diglyceride (MGDG)/digalactosyl diglyceride (DGDG) in the thylakoid of Yanda 1817 decreased, but had no distinct change in 8901. (2)The lipid/chlorophyll ratio in thylakoids of Yanda 1817 increased significantly, but had no distinct change in 8901. (3) The LHC Ⅱ oligomer content decreased in thylakoids of both cultivars. (4) The A683/A652 ratio of the 4th derivative absorption spectra increased in both cultivars. (5)The F685/F738 ratio of low temperature (77K) fluorescence spectra of thylakoids in 8901 increased but was not affected in Yanda 1817. It was concluded that one of the major strategies of wheat to adapt low temperature was the increase of thylakoid membrane fluidity, and that the decrease of MGDG content may play an important role in stabilizing the bilayer structure of the thylakoid membrane at low temperature.     

Chlorophyll-sensitized Photooxidation Products of Spinach Galactolipids

Spinach monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were oxidized with singlet molecular oxygen by the use of chlorophyll a as the photosensitizer. The oxidation products were separated from the unoxidized MGDG and DGDG by reverse-phase high performance liquid chromatography (HPLC). The products separated by HPLC were identified to be mono- and di-hydroperoxides formed by ¹O₂ oxidation of the 16:3 or 18:3 component of MGDG and DGDG. Each unsaturated fatty acid moiety in the MGDG and DGDG produced isomeric hydroperoxides in a manner similar to the corresponding fatty acid methyl ester.     

Lipid dependence of diadinoxanthin solubilization and de-epoxidation in artificial membrane systems resembling the lipid composition of the natural thylakoid membrane

In the present study, the solubility and enzymatic de-epoxidation of diadinoxanthin (Ddx) was investigated in three different artificial membrane systems: (1) Unilamellar liposomes composed of different concentrations of the bilayer forming lipid phosphatidylcholine (PC) and the inverted hexagonal phase (H(II) phase) forming lipid monogalactosyldiacylglycerol (MGDG), (2) liposomes composed of PC and the H(II) phase forming lipid phosphatidylethanolamine (PE), and (3) an artificial membrane system composed of digalactosyldiacylglycerol (DGDG) and MGDG, which resembles the lipid composition of the natural thylakoid membrane. Our results show that Ddx de-epoxidation strongly depends on the concentration of the inverted hexagonal phase forming lipids MGDG or PE in the liposomes composed of PC or DGDG, thus indicating that the presence of inverted hexagonal structures is essential for Ddx de-epoxidation. The difference observed for the solubilization of Ddx in H(II) phase forming lipids compared with bilayer forming lipids indicates that Ddx is not equally distributed in the liposomes composed of different concentrations of bilayer versus non-bilayer lipids. In artificial membranes with a high percentage of bilayer lipids, a large part of Ddx is located in the membrane bilayer. In membranes composed of equal proportions of bilayer and H(II) phase forming lipids, the majority of the Ddx molecules is located in the inverted hexagonal structures. The significance of the pigment distribution and the three-dimensional structure of the H(II) phase for the de-epoxidation reaction is discussed, and a possible scenario for the lipid dependence of Ddx (and violaxanthin) de-epoxidation in the native thylakoid membrane is proposed.     

Mono- and digalactosyldiacylglycerol composition of dinoflagellates. VIII. Temperature effects and a perspective on the curious case of Karenia mikimotoi as a producer of the unusual, ‘green algal’ fatty acid hexadecatetraenoic acid [16:4(n-3)]

Previous studies have shown that dinoflagellates with different plastid ancestries have distinct differences in the fatty acid compositions and regiochemistries of their chloroplast-associated galactolipids, mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively), thus reflecting plastid origin as a major factor in plastid membrane composition. Specifically, dinoflagellates with aberrant plastids (e.g. Karenia brevis, Kryptoperidinium foliaceum and Lepidodinium chlorophorum) possess certain MGDG- and DGDG-associated fatty acids which are not found in peridinin-containing dinoflagellates (the largest group of photosynthetic dinoflagellates with a red algal plastid ancestry which is thought to be an evolutionary precursor to aberrant plastids), but which are common to other algal groups. For example, hexadecatetraenoic acid (16:4(n-3)) is common to green algae and is found in the MGDG and DGDG of L. chlorophorum, which agrees with its green algal plastid ancestry, while hexadecatrienoic acid (16:3) and hexadecadienoic acid (16:2) are found in the MGDG and DGDG of K. foliaceum, which agrees with its diatom plastid ancestry. Notably, 16:4 has been found by others in the total fatty acids and galactolipids of Karenia mikimotoi, but in no other examined members of the Kareniaceae (all of which have plastids of haptophyte origin). However, these findings lack information as to the regiochemistry of 16:4. We have utilized positive-ion electrospray ionization/mass spectrometry (ESI/MS) and ESI/MS/MS to demonstrate that 16:4, which aside from L. chlorophorum is not found conclusively in the MGDG and DGDG of any other dinoflagellates examined to date irrespective of plastid ancestry, is found in K. mikimotoi as 18:5/16:4 (sn-1/sn-2 regiochemistry) MGDG and DGDG, and that its presence is not modulated (i.e. does not become more saturated) with an increase in growth temperature. Considering an aberrant pigment composition as described by others, we present a perspective where galactolipid-associated 16:4 in K. mikimotoi indicates a plastid ancestry more convoluted than for other members of the Kareniaceae.     

The lipid dependence of diadinoxanthin de-epoxidation presents new evidence for a macrodomain organization of the diatom thylakoid membrane

The present study shows that thylakoid membranes of the diatom Cyclotella meneghiniana contain much higher amounts of negatively charged lipids than higher plant or green algal thylakoids. Based on these findings, we examined the influence of SQDG on the de-epoxidation reaction of the diadinoxanthin cycle and compared it with results from the second negatively charged thylakoid lipid PG. SQDG and PG exhibited a lower capacity for the solubilization of the hydrophobic xanthophyll cycle pigment diadinoxanthin than the main membrane lipid MGDG. Although complete pigment solubilization took place at higher concentrations of the negatively charged lipids, SQDG and PG strongly suppressed the de-epoxidation of diadinoxanthin in artificial membrane systems. In in vitro assays employing the isolated diadinoxanthin cycle enzyme diadinoxanthin de-epoxidase, no or only a very weak de-epoxidation reaction was observed in the presence of SQDG or PG, respectively. In binary mixtures of the inverted hexagonal phase forming lipid MGDG with the negatively charged bilayer lipids, comparable suppression took place. This is in contrast to binary mixtures of MGDG with the neutral bilayer lipids DGDG and PC, where rapid and efficient de-epoxidation was observed. In complex lipid mixtures resembling the lipid composition of the native diatom thylakoid membrane, we again found strong suppression of diadinoxanthin de-epoxidation due to the presence of SQDG or PG. We conclude that, in the native thylakoids of diatoms, a strict separation of the MGDG and SQDG domains must occur; otherwise, the rapid diadinoxanthin de-epoxidation observed in intact cells upon illumination would not be possible.     

Fatty Acid Composition of Polar Lipids in Ceratodon purpureus and Pleurozium schreberi

The total amount of fatty acids in the mono- (MGDG) and diglycosyl diglyceride (DGDG) and more polar lipid fractions of frozen Ceratodon purpureus shoots was 4.6, 3.4 and 4.0 mg/g dry weight, respectively. The respective values for the tops of frozen Pleurozium schreberi were 2.6, 3.3 and 3.8 mg/g dry weight. The molar ratios MGDG/DGDG and MGDG + DGDG/chlorophyll were 1.3 and 3.7, respectively, for C. purpureus and 0.8 and 3.5 for P. schreberi. In C. purpureus the main fatty acids in the MGDG fraction were C 18:3ω3 (44% of the total fatty acids) and C 16:3ω3 (26%); in the DGDG fraction C 18:3ω3 (70%); and in the more polar lipid fraction C 18: 3ω3 (26%) and C 16:0 (25%). The proportion of C 20 polyunsaturated fatty acids was 15, 12 and 19% of the total fatty acids found in the MGDG, DGDG and more polar lipid fractions, respectively. In P. schreberi the proportion of C 20 polyunsaturated fatty acids was high in all polar lipid fractions (47, 42 and 25% in MGDG, DGDG and more polar lipid fractions, respectively). In addition, MGDG and DGDG fractions contained abundantly C 18:3ω3 (32 and 45%, respectively), and the more polar lipid fraction both C 18: 3ω3 (24%) and C 16:0 (27%).     

Lipid dependence of diadinoxanthin solubilization and de-epoxidation in artificial membrane systems resembling the lipid composition of the natural thylakoid membrane

In the present study, the solubility and enzymatic de-epoxidation of diadinoxanthin (Ddx) was investigated in three different artificial membrane systems: (1) Unilamellar liposomes composed of different concentrations of the bilayer forming lipid phosphatidylcholine (PC) and the inverted hexagonal phase (H_II phase) forming lipid monogalactosyldiacylglycerol (MGDG), (2) liposomes composed of PC and the H_II phase forming lipid phosphatidylethanolamine (PE), and (3) an artificial membrane system composed of digalactosyldiacylglycerol (DGDG) and MGDG, which resembles the lipid composition of the natural thylakoid membrane. Our results show that Ddx de-epoxidation strongly depends on the concentration of the inverted hexagonal phase forming lipids MGDG or PE in the liposomes composed of PC or DGDG, thus indicating that the presence of inverted hexagonal structures is essential for Ddx de-epoxidation. The difference observed for the solubilization of Ddx in H_II phase forming lipids compared with bilayer forming lipids indicates that Ddx is not equally distributed in the liposomes composed of different concentrations of bilayer versus non-bilayer lipids. In artificial membranes with a high percentage of bilayer lipids, a large part of Ddx is located in the membrane bilayer. In membranes composed of equal proportions of bilayer and H_II phase forming lipids, the majority of the Ddx molecules is located in the inverted hexagonal structures. The significance of the pigment distribution and the three-dimensional structure of the H_II phase for the de-epoxidation reaction is discussed, and a possible scenario for the lipid dependence of Ddx (and violaxanthin) de-epoxidation in the native thylakoid membrane is proposed.     

Development of Oat Prothylakoids into Thylakoids during Greening Does Not Change Transmembrane Galactolipid Asymmetry but Preserves the Thylakoid Bilayer

The lipase from Rhizopus arrhizus and the lipolytic acyl hydrolase from potato tubers have been used to determine the transmembrane distribution of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) in prothylakoids and thylakoids from oat (Avena sativa). Both galactolipids were found to be asymmetrically distributed. The molar outside/inside distribution was 70 ± 8/30 ± 8 for MGDG and 10 ± 4/90 ± 4 for DGDG in the prothylakoid membrane. Mature thylakoids presented a similar distribution, i.e. 63 ± 4/37 ± 4 for MGDG and 12 ± 3/88 ± 3 for DGDG. This distribution has been assessed under a variety of different conditions, namely (a) in media favoring thylakoid stacking or unstacking and inducing various membrane surface potentials, (b) in the presence of defatted bovine serum albumin which removed free fatty acids and partially lyso-galactolipids, (c) under various temperature conditions which resulted in different hydrolysis rates and degrees of fluidity of the membrane, and (d) in the presence of different enzyme concentrations which influenced the hydrolysis rate. The above distribution was found to be independent of the type of conditions used. Nonbilayer forming/bilayer forming lipid ratios suggest that both monolayers of the prothylakoid and the inner monolayer of oat thylakoid membranes should display lamellar structures (e.g. ratios <2.5). In contrast the outer monolayer of the thylakoid membrane should display non-lamellar configurations (e.g. ratio >2.5). Thus, it is concluded that the incorporation of chlorophyll-protein complexes into the nascent thylakoid membrane modifies neither the galactolipid nor the phospholipid transmembrane distribution. However, these complexes appear to be crucial to preserve a bilayer configuration to the greening membrane which, otherwise, would adopt nonlamellar structures. The possible origin of galactolipid transversal asymmetry which appears very early during the biogenesis of oat thylakoid membranes is discussed.     

Differential changes in galactolipid and phospholipid species in soybean leaves and roots under nitrogen deficiency and after nodulation

The availability of nitrogen (N) to plants has a profound impact on carbohydrate and protein metabolism, but little is known about its effect on membrane lipid species. This study examines the changes in galactolipid and phospholipid species in soybean as affected by the availability of N, either supplied to soil or obtained through Bradyrhizobium japonicum nodulation. When N was limited in soil, the content of galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacyglycerol (DGDG), decreased drastically in leaves, while a smaller decrease of DGDG was observed in roots. In both leaves and roots, the overall content of different phospholipid classes was largely unchanged by N limitation, although some individual phospholipid molecular species did display significant changes. Nodulation with Bradyrhizobium of soybean grown in N-deficient soil resulted in a large increase in levels of plastidic lipid classes, MGDG, DGDG, and phosphatidylglycerol, along with smaller increases in non-plastidic phospholipids in leaves. Nodulation also led to higher levels of phospholipids in roots without changes in root levels of MGDG and DGDG. Overall, N availability alters lipid content more in leaves than roots and more in galactolipids than phospholipids. Increased N availability leads to increased galactolipid accumulation in leaves, regardless of whether N is supplied from the soil or symbiotic fixation.     

Effect of monogalactosyldiacylglycerol on the interaction between photosystem II core complex and its antenna complexes in liposomes of thylakoid lipids

The non-bilayer lipid monogalactosyldiacylglycerol (MGDG) is the most abundant type of lipid in the thylakoid membrane and plays an important role in regulating the structure and function of photosynthetic membrane proteins. In this study, we have reconstituted the isolated major light-harvesting complexes of photosystem II (PSII) (LHCIIb) and a preparation consisting of PSII core complexes and minor LHCII of PSII (PSIICC) into liposomes that consisted of digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG), with or without MGDG. Transmission electron microscopy and freeze-fracture studies showed unilamellar proteoliposomes, and demonstrated that most of the MGDG is incorporated into bilayer structures. The impact of MGDG on the functional interaction between LHCIIb and PSIICC was investigated by low temperature (77 K) fluorescence emission spectra and the photochemical activity of PSII. The additional incorporation of LHCIIb into liposomes containing PSIICC markedly increased oxygen evolution of PSIICC. Excitation at 480 nm of chlorophyll (Chl) b in LHCIIb stimulated a characteristic fluorescence emission of the Chl a in PSII (684.2 nm), rather than that of the Chl a in LHCIIb (680 nm) in the LHCIIb–PSIICC proteoliposomes, which indicated that the energy was transferred from LHCIIb to PSIICC in liposome membranes. Increasing the percentage of MGDG in the PSIICC–LHCIIb proteoliposomes enhanced the photochemical activity of PSII, due to a more efficient energy transfer from LHCIIb to PSIICC and, thus, an enlarged antenna cross section of PSII.     

Growth temperature effects on thylakoid membrane lipid and protein content of pea chloroplasts

The lipid composition and level of unsaturation of fatty acids has been determined for chloroplast thylakoid membranes isolated from Pisum sativum grown under cold (4°/7°C) or warm (14°/17°C) conditions. Both the relative amounts of lipid classes and degree of saturation were not greatly changed for the two growth conditions. In cold-grown plants, there was a slightly higher linolenic and lower linoleic acid content for the glycolipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and sulfoquinovosyldiacylglycerol. In contrast to thylakoid membranes, a non-thylakoid leaf membrane fraction including the chloroplast envelope, had a higher overall level of fatty acid unsaturation in cold-grown plants due mainly to an increase in the linolenic acid content of MGDG, DGDG, phosphatidylglycerol, and phosphatidylcholine. The most clear cut change in the thylakoid membrane composition was the lipid to protein ratio which was higher in the cold-grown plants.     

Glycolipids of turions and leaves of Utricularia vulgaris at different stages of development

Turions of Utricularia vulgaris L. were germinated in long-day conditions at 15°C for 1,3 and 6 days and their glycolipid composition was compared with that of resting but vernalized turions. Digalactosyldiacylglycerides (DGDG), monogalactosyldiacylglycerides (MGDG) and cerebrosides were present at all stages of development. No great changes were found in the glycolipid classes during sprouting but some differences were noted in the proportions of fatty acids. The most common fatty acids in all three glycolipid classes studied were 16:0, 18:0 and 18:2. MGDG and DGDG also contained relatively much 18:3 and its proportion increased during germination. Young turions and full-grown leaves collected from nature contained the same glycolipid classes as the sprouting turions. The developmental stage of the organs studied is reflected in the fatty acid composition of DGDG and MGDG but is not so evident in the cerebrosides. The 18:2 fatty acid is rather typical of the resting turions, especially in DGDG.     

The Effect of Light Intensity, Day Length, and Temperature on Fatty Acid Synthesis and Desaturation in Vicia faba L.

Some effects of light intensity, day length, and temperatureon the fatty acid composition of the major glycerolipids ofleaves of Vicia faba L. (cv. Giant Windsor) were observed. Increasinglight intensity caused an increase in the relative concentrationsof 16 : 1 in PG and 18 : 3 in MGDG and DGDG. Increasing daylength during growth (and continuous illumination of leaf tissue)had no effect on 16 : 1 in PG but caused a decrease in the 18: 3 content of PG, PC, MGDG, and DGDG. Since the quantitiesof these lipids increased under these conditions, the decreasewas not due to photodestruction but to the differences in therelative rates of biosynthesis and desaturation of fatty acids.Incubation of leaf tissue in the dark for 4 d had little effecton the fatty acid composition of MGDG, DGDG, and PG. Temperaturealso controls fatty acid synthesis and desaturation. Above theoptimum growth temperature (20 °C), the 18 : 3 content ofMGDG, DGDG, PG, and PC decreased. In mature leaf tissue, thedegree of unsaturation of MGDG may be modified upward in responseto temperature changes. When plants were grown at 30 °Cand transferred to 20 °C the level of 18 : 3 in MGDG ofthe leaf tissue increased to levels found in plants grown onlyat 20 °C. The level of 18 : 3 in MGDG does not decreaseas rapidly when plants grown at 20 °C were transferred to30 °C. This suggests that the lower temperature induceddesaturation of 18 : 2 to 18 : 3.