Membrane phospholipids as a phosphate reserve: the dynamic nature of phospholipid-to-digalactosyl diacylglycerol exchange in higher plants

It is well established that phosphate deficiency induces the replacement of membrane phospholipid with non-phosphorous lipids in extra-plastidial membranes (e.g. plasma membrane, tonoplast, mitochondria). The predominant replacement lipid is digalactosyl diacylglycerol (DGDG). This paper reports that the phospholipid-to-DGDG replacement is reversible, and that when oat seedlings are re-supplied with radio-labelled phosphate, it is initially recovered primarily in phosphatidylcholine (PC). Within 2 d, the shoot contains more than half of the lipid-associated radiolabel, reflecting phosphate translocation. Oat was also cultivated in different concentrations of phosphate and the DGDG/PC ratio in roots and phospholipase activities in isolated plasma membranes was assayed after different times of cultivation. The DGDG/PC ratio in root tissue correlated more closely with plasma membrane-localized phospholipase D, yielding phosphatidic acid (PA), than with plasma membrane-localized PA phosphatase, the activity that results in a decreased proportion of phospolipids. The lipid degradation data did not reflect a significant involvement of phospholipase C, although a putative phospholipase C analogue, non-specific phospholipase C4 (NPC4), was present in oat roots. The correlation between increased phospholipase D activity and DGDG/PC ratio is consistent with a model where phospholipid-to-DGDG replacement involves formation of PA that readily is removed from the plasma membrane for further degradation elsewhere.     

Phosphate-limited oat. The plasma membrane and the tonoplast as major targets for phospholipid-to-glycolipid replacement and stimulation of phospholipases in the plasma membrane

We recently reported that cultivation of oat (Avena sativa L.) without phosphate resulted in plasma membrane phosphoglycerolipids being replaced to a large extent by digalactosyldiacylglycerol (DGDG) (Andersson, M. X., Stridh, M. H., Larsson, K. E., Liljenberg, C., and Sandelius, A. S. (2003) FEBS Lett. 537, 128-132). We report here that DGDG is not the only non-phosphorous-containing lipid that replaces phospholipids but that also the content of glucosylceramides and sterolglycosides increased in plasma membranes as a response to phosphate starvation. In addition, phosphate deficiency induced similar changes in lipid composition in the tonoplast. The phospholipid-to-glycolipid replacement apparently did not occur to any greater extent in endoplasmic reticulum, Golgi apparatus, or mitochondrial inner membranes. In contrast to the marked effects on lipid composition, the polypeptide patterns were largely similar between root plasma membranes from well-fertilized and phosphate-limited oat, although the latter condition induced at least four polypeptides, including a chaperone of the HSP80 or HSP90 family, a phosphate transporter, and a bacterial-type phosphoesterase. The latter polypeptide reacted with an antibody raised against a phosphate deficiency-induced phospholipase C from Arabidopsis thaliana (Nakamura, Y., Awai, K., Masuda, T., Yoshioka, Y., Takamiya, K., and Ohta, H. (2005) J. Biol. Chem. 280, 7469-7476). In plasma membranes from oat, however, a phospholipase D-type activity and a phosphatidic acid phosphatase were the dominant lipase activities induced by phosphate deficiency. Our results reflect a highly developed plasticity in the lipid composition of the plasma membrane and the tonoplast. In addition, phosphate deficiency-induced alterations in plasma membrane lipid composition may involve different sets of lipid-metabolizing enzymes in different plant tissues or species, at different stages of plant development and/or at different stages of stress adjustments.     

The galactolipid digalactosyldiacylglycerol accumulates in the peribacteroid membrane of nitrogen-fixing nodules of soybean and Lotus

The peribacteroid membrane (PBM) surrounding nitrogen fixing rhizobia in the nodules of legumes is crucial for the exchange of ammonium and nutrients between the bacteria and the host cell. Digalactosyldiacylglycerol (DGDG), a galactolipid abundant in chloroplasts, was detected in the PBM of soybean (Glycine max) and Lotus japonicus. Analyses of membrane marker proteins and of fatty acid composition confirmed that DGDG represents an authentic PBM lipid of plant origin and is not derived from the bacteria or from plastid contamination. In Arabidopsis, DGDG is known to accumulate in extraplastidic membranes during phosphate deprivation. However, the presence of DGDG in soybean PBM was not restricted to phosphate limiting conditions. Complementary DNA sequences corresponding to the two DGDG synthases, DGD1 and DGD2 from Arabidopsis, were isolated from soybean and Lotus. The two genes were expressed during later stages of nodule development in infected cells and in cortical tissue. Because nodule development depends on the presence of high amounts of phosphate in the growth medium, the accumulation of the non-phosphorus galactolipid DGDG in the PBM might be important to save phosphate for other essential processes, i.e. nucleic acid synthesis in bacteroids and host cells.     

Long- and short-term phosphate deprivation in bean roots: plasma membrane lipid alterations and transient stimulation of phospholipases

In a long-term experiment bean (Phaseolus vulgaris L.) seedlings were grown for 18 days in hydroponics in either phosphate-sufficient (+P) or phosphate-deficient (-P) nutrient solutions. Phosphate deprivation halved the phosphorous content of roots. In plasma membrane (PM) fractions isolated from -P roots the phospholipid (PL) level was reduced from 35 to 21 mol%, while PL composition and degree of unsaturation were hardly altered. Digalactosyldiacylglycerol (DGDG) accumulated up to 26% of total PM lipids, replacing PL to a large extent. Molecular species and fatty acid compositions of DGDG in root PM were different compared to DGDG present in the -P plastids. In a short-term study, bean seedlings were grown for 18 days in hydroponics with a complete nutrient solution containing phosphate and then incubated in a -P medium for increasing time ranging from 1 up to 96 h. At the end of the starvation period phosphorous content of -P roots was reduced by 30% compared to +P ones. An activation of phospholipase D and phospholipase C was observed after 1 and 2h of phosphate deprivation, respectively. Maximal phosphatidic acid accumulation was detected after 4h of phosphate deprivation, when also DGDG started to accumulate in PM of bean roots. The fatty acid composition of PLD-derived phosphatidylbutanol resembled that of phosphatidylcholine.     

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.     

Can digalactosyldiacylglycerol substitute for phosphatidylcholine upon phosphate deprivation in leaves and roots of Arabidopsis?

To explore the role of digalactosyldiacylglycerol (DGDG) in plants the dgd1 mutant of Arabidopsis thaliana was grown in the presence and absence of inorganic phosphate. Phosphate deficiency in the dgd1 mutant causes a strong decrease in all phospholipids accompanied by an increase in DGDG and sulpholipid. Moreover, a significant DGDG accumulation was found in roots upon phosphate deprivation as well. Our data indicate that DGDG accumulation upon phosphate deprivation is due to the activation of a specific eukaryotic dgd1-independent biosynthetic pathway. We propose that DGDG may substitute for phosphatidylcholine upon phosphate deprivation.     

Adaptation of Synechococcus sp. PCC 7942 to phosphate starvation by glycolipid accumulation and membrane lipid remodeling

Organisms use various adaptive strategies against phosphate stress, including lipid remodeling. Here, the response of major membrane lipids to phosphate stress was analyzed in Synechococcus sp. PCC 7942. Unlike plants and eukaryotic microalgae, no significant increases in neutral lipids were found, whereas glycolipids content increased to as high as 6.13% (of dry cell weight, DCW) and phospholipids decreased to 0.34% (of DCW) after 16 days of cultivation without phosphate. Glycolipids accumulation were mainly attributed to the significant increase of digalactosyldiacylglycerol (DGDG) by 50% and sulfoquinovosyldiaclglycerol (SQDG) by 90%, both of which acted as complementary lipids for phosphatidylglycerol (PG) in the cyanobacterial membrane. Also, a notable increase in content (by 48%) of C18 fatty acids (especially C18:1) was observed in all glycolipids at the expense of C12 and C14 (72%). These changes may contribute to membrane fluidity and photosynthetic activity for basic cell metabolism and phosphate stress adaptation. Lipidomic analyses showed the reduction of PG 18:1/16: 0 (by 52%) with the increase of DGDG 18:1/16:0 (133%) and SQDG 18:1/16:0 (245%), strongly suggesting a direct conversion of PG to DGDG and SQDG. Moreover, the decreasing amount of monogalactosyldiacylglycerol (MGDG) 16:1/16:0 (22%) was consistent with the increase of free fatty acids (125%) on day 2 of phosphate absence, which suggested that MGDG is more likely to provide a pool of fatty acids for de novo synthesis of glycolipids. This study provides valuable insight into cyanobacteria adaptation strategies to phosphate stress by membrane lipid remodeling and unveils the underlying acyl chain fluxes into glycolipids.     

Plasma membrane lipid composition of the halophilic cyanobacterium Aphanothece halophytica

The plasma membrane from Aphanothece halophytica was isolated using both glycerol and sucrose gradient centrifugation. The isolated membrane was characterized for lipid content by TLC and isolated lipids were quantified by chemical analysis. The plasma membrane of A. halophytica was composed of MGDG, DGDG and PG. The sulfur containing lipid SQDG was not detected. The mole percent of each lipid in the plasma membrane varied with the external salinity of the media. MGDG was the most abundant lipid in the plasma membrane of cells grown at one molar external NaCl. At three molar external NaCl, PG was the most abundant lipid. The ratio of uncharged to charged lipids comprising the plasma membrane decreased as the external salinity increased. It is possible that the alteration in lipid composition is of major importance in the adaptation of A. halophytica to changing external salinity.Abbreviations TLC Thin-layer chromatography - MGDG momogalactosyldiacylglycerol - DGDG digaloctosyldiacylglycerol - PG phosphatidylglycerol - SQDG sulphoquinovosyldiacylglycerol     

Cytochemical localization of ATPase activity in oat roots localizes a plasma membrane-associated soluble phosphatase, not the proton pump

Cytochemical techniques employing lead-precipitation of enzymically released inorganic phosphate have been widely used in attempts to localize the plasma membrane proton pump (H⁺-ATPase) in electron micrographs. Using Avena sativa root tissue we have performed a side-by-side comparison of ATPase activity observed in electron micrographs with that observed in in vitro assays using ATPases found in the soluble and plasma membrane fractions of homogenates. Cytochemical analysis of oat roots, which had been fixed in glutaraldehyde in order to preserve subcellular structures, identifies an ATPase located at or near the plasma membrane. However, the substrate specificity and inhibitor sensitivity of the in situ localized ATPase appear identical to those of an in vitro ATPase activity found in the soluble fraction, and are completely unlike those of the plasma membrane proton pump. Further studies demonstrated that the plasma membrane H⁺-ATPase is particularly sensitive to inactivation by the fixatives glutaraldehyde and formaldehyde and by lead. In contrast, the predominant soluble ATPase activity in oat root homogenates is less sensitive to fixation and is completely insensitive to lead. Based on these results, we propose a set of criteria for evaluating whether a cytochemically localized ATPase activity is, in fact, due to the plasma membrane proton pump.     

Drought stress and rehydration affect the balance between MGDG and DGDG synthesis in cowpea leaves

Membranes are main targets of drought, and there is growing evidence for the involvement of membrane lipid in plant adaptation to such an environmental stress. Biosynthesis of the galactosylglycerolipids, monogalactosyl-diacylglycerol (MGDG) and digalactosyl-diacylglycerol (DGDG), which are the main components of chloroplast envelope and thylakoid membranes, could be important for plant tolerance to water deficit and for recovery after rehydration. In this study, galactolipid (GL) biosynthesis in cowpea ( Vigna unguiculata L. Walp) leaves was analysed during drought stress and subsequent rewatering. Comparison of two cowpea cutivars, one drought tolerant and the other drought susceptible submitted to moderate drought stress, revealed patterns associated with water-deficit tolerance: increase in DGDG leaf content, stimulation of DGDG biosynthesis in terms of ¹⁴C-acetate incorporation and messenger accumulation corresponding to four genes coding for GL synthases ( MDG1 , MGD2 , DGD1 and DGD2 ). Similar to phosphate starvation, lack of water enhanced DGDG biosynthesis and it was hypothesized that the drought-induced DGDG accumulated in extrachloroplastic membranes, and thus contributes to plant tolerance to arid environments.     

Critical regulation of galactolipid synthesis controls membrane differentiation and remodeling in distinct plant organs and following environmental changes

The plant galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are the most abundant lipids in chloroplast membranes, and they constitute the majority of total membrane lipids in plants. MGDG is synthesized by two types of MGDG synthase, type-A (MGD1) and type-B (MGD2, MGD3). These MGDG synthases have distinct roles in Arabidopsis. In photosynthetic organs, Type A MGD is responsible for the bulk of MGDG synthesis, whereas Type B MGD is expressed in non-photosynthetic organs such as roots and flowers and mainly contributes to DGDG accumulation under phosphate deficiency. Similar to MGDG synthesis, DGDG is synthesized by two synthases, DGD1 and DGD2; DGD1 is responsible for the majority of DGDG synthesis, whereas DGD2 makes its main contribution under phosphate deficiency. These galactolipid synthases are regulated by light, plant hormones, redox state, phosphatidic acid levels, and various stress conditions such as drought and nutrient limitation. Maintaining the appropriate ratio of these two galactolipids in chloroplasts is important for stabilizing thylakoid membranes and maximizing the efficiency of photosynthesis. Here we review progress made in the last decade towards a better understanding of the pathways regulating plant galactolipid biosynthesis.     

Phosphate deprivation induces transfer of DGDG galactolipid from chloroplast to mitochondria

In many soils plants have to grow in a shortage of phosphate, leading to development of phosphate-saving mechanisms. At the cellular level, these mechanisms include conversion of phospholipids into glycolipids, mainly digalactosyldiacylglycerol (DGDG). The lipid changes are not restricted to plastid membranes where DGDG is synthesized and resides under normal conditions. In plant cells deprived of phosphate, mitochondria contain a high concentration of DGDG, whereas mitochondria have no glycolipids in control cells. Mitochondria do not synthesize this pool of DGDG, which structure is shown to be characteristic of a DGD type enzyme present in plastid envelope. The transfer of DGDG between plastid and mitochondria is investigated and detected between mitochondria-closely associated envelope vesicles and mitochondria. This transfer does not apparently involve the endomembrane system and would rather be dependent upon contacts between plastids and mitochondria. Contacts sites are favored at early stages of phosphate deprivation when DGDG cell content is just starting to respond to phosphate deprivation.     

A Contrast of the Plasma Membrane Lipid Composition of Oat and Rye Leaves in Relation to Freezing Tolerance

The lipid composition of the plasma membrane isolated from leaves of spring oat (Avena sativa L. cv Ogle) was vastly different from that of winter rye (Secale cereale L. cv Puma). The plasma membrane of spring oat contained large proportions of phospholipids (28.8 mol% of the total lipids), cerebrosides (27.2 mol%), and acylated sterylglucosides (27.3 mol%) with lesser proportions of free sterols (8.4 mol%) and sterylglucosides (5.6 mol%). In contrast, the plasma membrane of winter rye contained a greater proportion of phospholipids (36.6 mol%), and there was a lower proportion of cerebrosides (16.4 mol%); free sterols (38.1 mol%) were the predominant sterols, with lesser proportions of sterylglucosides (5.6 mol%) and acylated sterylglucosides (2.9 mol%). Although the relative proportions of individual phospholipids, primarily phosphatidylcholine and phosphatidylethanolamine, and the molecular species of these two phospholipids were similar in oat and rye, the relative proportions of di-unsaturated species of these two phospholipids were substantially lower in oat than in rye. The relative proportions of sterol species in oat were different from those in rye; the molecular species of cerebrosides were similar in oat and rye, with only slight differences in the proportions of the individual species. After 4 weeks of cold acclimation, the proportion of phospholipids increased significantly in both oat (from 28.8 to 36.8 mol%) and rye (from 36.6 to 43.3 mol%) as a result of increases in the proportions of phosphatidylcholine and phosphatidylethanolamine. For both oat and rye, the relative proportions of di-unsaturated species increased after cold acclimation, but the increase was greater in rye than in oat. In both oat and rye, this increase occurred largely during the first week of cold acclimation. During the 4 weeks of cold acclimation, there was a progressive decrease in the proportion of cerebrosides in the plasma membrane of rye (from 16.4 to 10.5 mol%), but there was only a small decrease in oat (from 27.2 to 24.2 mol%). In both oat and rye, there were only small changes in the proportions of free sterols and sterol derivatives during cold acclimation. Consequently, the proportions of both acylated sterylglucosides and cerebrosides remained substantially higher in oat than in rye after cold acclimation. The relationship between these differences in the plasma membrane lipid composition of oat and rye and their freezing tolerance is presented.     

Effects of the sugar headgroup of a glycoglycerolipid on the phase behavior of phospholipid model membranes in the dry state

Glycolipids are important components of almost all biological membranes. They possess unique properties that have only been incompletely characterized so far. The plant glycolipid digalactosyldiacylglycerol (DGDG) strongly influences the physical behavior of phospholipid model membranes in both the dry and hydrated state. It was, however, unclear whether the strong effect of DGDG on the gel to liquid-crystalline phase transition temperature (Tm) in dry phosphatidylcholine (PC) bilayers is mainly due to the high degree of unsaturation of the DGDG fatty acyl chains or to interactions between the DGDG and PC headgroups. Also, no information on the relative effectiveness of membrane bound and free sugars on membrane phase behavior was available. We have used Fourier-transform infrared spectroscopy (FTIR) to investigate the phase properties and H-bonding patterns in dry membranes made from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) containing one saturated and one monounsaturated (16:0/18:1) fatty acid and different fractions of DGDG or 1,2-dilinolenoyl-sn-glycero-3-phosphatidylcholine (DLPC) (18:3/18:3). This was compared to the effects of galactose (Gal) and digalactose (diGal). All additives depressed Tm of the dry membranes, but DGDG was much more effective than DLPC or Gal. diGal had a similar effect as DGDG, pointing to the sugar headgroup as the component with the strongest influence on membrane phase behavior. A combination of DLPC and diGal, which should theoretically be equivalent to DGDG, was much more effective than the galactolipid. H-bonding interactions with the P = O group of PC were also stronger for free diGal than for DGDG, indicating that the free sugar may be structurally more flexible to adopt an optimal conformation for interactions with the PC headgroup.     

Disruption of the two digalactosyldiacylglycerol synthase genes DGD1 and DGD2 in Arabidopsis reveals the existence of an additional enzyme of galactolipid synthesis

Two genes (DGD1 and DGD2) are involved in the synthesis of the chloroplast lipid digalactosyldiacylglycerol (DGDG). The role of DGD2 for galactolipid synthesis was studied by isolating Arabidopsis T-DNA insertional mutant alleles (dgd2-1 and dgd2-2) and generating the double mutant line dgd1 dgd2. Whereas the growth and lipid composition of dgd2 were not affected, only trace amounts of DGDG were found in dgd1 dgd2. The growth and photosynthesis of dgd1 dgd2 were affected more severely compared with those of dgd1, indicating that the residual amount of DGDG in dgd1 is crucial for normal plant development. DGDG synthesis was increased after phosphate deprivation in the wild type, dgd1, and dgd2 but not in dgd1 dgd2. Therefore, DGD1 and DGD2 are involved in DGDG synthesis during phosphate deprivation. DGD2 was localized to the outer side of chloroplast envelope membranes. Like DGD2, heterologously expressed DGD1 uses UDP-galactose for galactosylation. Galactolipid synthesis activity for monogalactosyldiacylglycerol (MGDG), DGDG, and the unusual oligogalactolipids tri- and tetragalactosyldiacylglycerol was detected in isolated chloroplasts of all mutant lines, including dgd1 dgd2. Because dgd1 and dgd2 carry null mutations, an additional, processive galactolipid synthesis activity independent from DGD1 and DGD2 exists in Arabidopsis. This third activity, which is related to the Arabidopsis galactolipid:galactolipid galactosyltransferase, is localized to chloroplast envelope membranes and is capable of synthesizing DGDG from MGDG in the absence of UDP-galactose in vitro, but it does not contribute to net galactolipid synthesis in planta.     

Characterization of a calcium- and lipid-dependent protein kinase associated with the plasma membrane of oat.

A protein kinase that is activated by calcium and lipid has been partially purified from the plasma membrane of oat roots. This protein kinase cross-reacts with four monoclonal antibodies directed against a soluble calcium-dependent protein kinase from soybean described previously [Putman-Evans, C. L., Harmon, A. C., & Cormier, M. J. (1990) Biochemistry 29, 2488-2495; Harper, J. F., Sussman, M. R., Schaller, G. E., Putnam-Evans, C., Charbonneau, H., & Harmon, A. C. (1991) Science 252, 951-954], indicating that the oat enzyme is a member of this calcium-dependent protein kinase family. Immunoblots demonstrate that the membrane-derived protein kinase is slightly larger than that observed in the cytosolic fraction of oat. Limited digestion of the membrane-derived kinase with trypsin generates a smaller water-soluble kinase that is still activated by calcium but is no longer activated by lipid. When posthomogenization proteolysis is minimized, the bulk of the immunoreactive kinase material is localized in the membrane. These results suggest that a calcium-dependent protein kinase observed in the supernatant fraction of oat extracts may originate in situ from a calcium- and lipid-dependent protein kinase which is associated with the oat plasma membrane. They further indicate that, in contrast to animal cells, the predominant calcium- and lipid-dependent protein kinase associated with the plasma membrane of plant cells has biochemical properties and amino acid sequence unlike protein kinase C.     

Effect of Cold Acclimation on the Lipid Composition of the Inner and Outer Membrane of the Chloroplast Envelope Isolated from Rye Leaves

The lipid composition of the inner and outer membranes of the chloroplast envelope isolated from winter rye (Secale cereale L. cv Puma) leaves was characterized before and after cold acclimation. In nonacclimated leaves the inner membrane contained high proportions of monogalactosyldiacylglycerols (MGDG, 47.9 mol% of the total lipids) and digalactosyldiacylglycerols (DGDG, 31.1 mol%) and a low proportion of phosphatidylcholine (PC, 8.1 mol%). The outer membrane contained a similar proportion of DGDG (30.0 mol%); however, the proportion of MGDG was much lower (20.1 mol%) and the proportion of PC was much higher (31.5 mol%). After 4 weeks of cold acclimation, the proportions of these lipid classes were significantly altered in both of the inner and outer membranes. In the inner membrane the proportion of MGDG decreased (from 47.9 to 38.4 mol%) and the proportion of DGDG increased (from 31.1 to 39.3 mol%), with only a slight change in the proportion of PC (from 8.1 to 8.8 mol%). In the outer membrane MGDG decreased from 20.1 to 14.8 mol%, DGDG increased from 30.0 to 39.9 mol%, and PC decreased from 31.5 to 25.4 mol%. Thus, both before and after cold acclimation, the proportion of MGDG was much higher in the inner membrane than in the outer membrane. In contrast, the proportion of PC was higher in the outer membrane than in the inner membrane. The relationship between the lipid composition of the inner and outer membranes of the chloroplast envelope and freeze-induced membrane lesions is discussed.     

A Comparison of Freezing Injury in Oat and Rye: Two Cereals at the Extremes of Freezing Tolerance

A detailed analysis of cold acclimation of a winter rye (Secale cereale L. cv Puma), a winter oat (Avena sativa L. cv Kanota), and a spring oat cultivar (Ogle) revealed that freezing injury of leaves of nonacclimated seedlings occurred at -2[deg]C in both the winter and spring cultivars of oat but did not occur in winter rye leaves until after freezing at -4[deg]C. The maximum freezing tolerance was attained in all cultivars after 4 weeks of cold acclimation, and the temperature at which 50% electrolyte leakage occurred decreased to -8[deg]C for spring oat, -10[deg]C for winter oat, and -21[deg]C for winter rye. In protoplasts isolated from leaves of nonacclimated spring oat, expansion-induced lysis was the predominant form of injury over the range of -2 to -4[deg]C. At temperatures lower than -4[deg]C, loss of osmotic responsiveness, which was associated with the formation of the hexagonal II phase in the plasma membrane and subtending lamellae, was the predominant form of injury. In protoplasts isolated from leaves of cold-acclimated oat, loss of osmotic responsiveness was the predominant form of injury at all injurious temperatures; however, the hexagonal II phase was not observed. Rather, injury was associated with the occurrence of localized deviations of the plasma membrane fracture plane to closely appressed lamellae, which we refer to as the "fracture-jump lesion." Although the freeze-induced lesions in the plasma membrane of protoplasts of spring oat were identical with those reported previously for protoplasts of winter rye, they occurred at significantly higher temperatures that correspond to the lethal freezing temperature.     

光合膜膜脂双半乳糖二酰基甘油的研究进展

本文综述了光合膜膜脂双半乳糖二酰基甘油（DGDG）的生物合成和生理功能的研究进展。DGDG是光合膜中惟一的双半乳糖脂类,在光合膜的不同膜区均有分布。在高等植物叶绿体中,存在着两条不同的DGDG生物合成途径,即原核合成途径和真核合成途径。原核途径只限于在质体内进行,而真核途径还包括一些在内质网内发生的反应。DGDG在维持光系统II捕光色素蛋白复合体的寡聚体结构、调控光系统II和光系统II核心复合物的放氧活性等方面起着重要作用。     

Digalactosyldiacylglycerol is required for better photosynthetic growth of Synechocystis sp. PCC6803 under phosphate limitation

Digalactosyldiacylglycerol (DGDG) is a typical membrane lipid of oxygenic photosynthetic organisms. Although DGDG synthase genes have been isolated from plants, no homologous gene has been annotated in the genomes of cyanobacteria and the unicellular red alga Cyanidioschyzon merolae. Here we used a comparative genomics approach and identified a non-plant-type DGDG synthase gene (designated dgdA) in Synechocystis sp. PCC6803. The enzyme produced DGDG in Escherichia coli when co-expressed with a cucumber monogalactosyldiacylglycerol synthase. A DeltadgdA knock-out mutant showed no obvious phenotype other than loss of DGDG when grown in a BG11 medium, indicating that DGDG is dispensable under optimal conditions. However, the mutant showed reduced growth under phosphate-limited conditions, suggesting that DGDG may be required under phosphate-limited conditions, such as those in natural niches of cyanobacteria.