The Effect of Temperature on the Level and Biosynthesis of Unsaturated Fatty Acids in Diacylglycerols of Brassica napus Leaves

Experiments on the effects of temperature on the levels of unsaturated fatty acids and their rates of desaturation in Brassica napus leaf lipids have shown that significant differences occur in the composition of all diacylglycerols in the leaf between plants grown at high and low temperatures. In the major thylakoid diacylglycerols, monogalactosyl-diacylglycerol and digalactosyldiacylglycerol, not only is there an increase in the level of unsaturation at low temperatures, but there is a change in the balance between molecular species of chloroplastic origin (16/18C) and cytosolic origin (18/18C). Radioactivity tracer data indicate that at low temperatures there are two distinct phases of desaturation in the fatty acids of the major diacylglycerols of these leaves. A rapid phase, which appears in plants grown at low temperatures and results in the desaturation of palmitic acid to hexadecadienoic acid and oleic acid to linoleic acid may explain the high levels of unsaturated fatty acids found in the leaf diacylglycerols from plants grown at low temperatures. The appearance of this rapid phase is controlled by the temperature at which the plant is grown and is not subject to rapid variations in environmental temperature.     

Fatty acid desaturation in monogalactosyldiacylglycerol of Brassica napus leaves during low temperature acclimation

Brassica napus plants grown at 5°C have the potential to desaturate fatty acids in the major membrane diacylglycerols of leaves at rates much higher than those of plants grown at 30°C. This low temperature-induced desaturation (LTD) is rapidly deactivated if plants grown at 5°C are transferred to 30°C for several hours. By exposure to ¹⁴CO₂ and a computer simulation program, we estimated the rate of desaturation of monogalactosyldiacylglycerol by (ω9-, ω6- and ω3-desaturases of plants grown at 5, 10, 20 and 30°C. Data show that LTD can be induced in mature leaves of plants grown at 20 and 30°C after transfer to 5°C. Full activation of LTD takes several weeks and occurs more rapidly in plants grown at 20°C than 30°C. This activation is largely due to the increased activity of ω9- and ω6-desaturases on C16 fatty acids and ω6-desaturase on C18 fatty acids.     

The effects of altered levels of phosphatidylcholine and phosphatidylethanolamine on fatty acid desaturase activity and sterol metabolism during temperature acclimation in a choline auxotroph of Neurospora crassa

Experiments were conducted to determine the effects of choline deprivation on levels of phospholipid fatty acids in a choline auxotroph (chol-1; chol-2) of Neurospora crassa with respect to high (37 degrees C) and low (15 degrees C) growth temperatures and during acclimation following a shift from high to low temperature conditions. Although grossly altered levels of phosphatidylethanolamine and phosphatidylcholine were observed at both temperatures, phospholipid fatty acid levels remained virtually identical to those found in a phenotypically wild-type and maximally supplemented chol-1; chol-2 strains grown under the same conditions. Deprivation of choline from supplemented cultures of the mutant followed by a shift from high to low growth temperatures did not significantly affect the level of fatty acid desaturation with respect to control cultures. Free sterols did not significantly affect the level of fatty acid desaturation with respect to control cultures. Free sterols were reduced, however, and sterol ester levels were elevated in choline-deprived cultures, suggesting that sterol interconversions may be closely tied to aspects of phospholipid biosynthesis. These experiments suggest that although major modifications in membrane fluidity may be brought about by thermally induced changes in fatty acid desaturase activity, it seems probable that additional cellular mechanisms may be involved if fluidity is under precise control.     

Effects of the temperature range and the lack of beta-ketoacyl acyl-carrier protein synthase II on fatty acid synthesis in Escherichia coli K12 after shifts in temperature

Escherichia coli K12 cells grown at higher temperatures and then subjected to lower temperatures produce fatty acids with higher unsaturated/saturated ratios than cells completely adapted to the lower temperatures (Okuyama et al. (1982) J. Biol. Chem. 257, 4812-4817). This hyper-response was not an artefact of chloramphenicol treatment and was observed when the shift-down was more than 20 degrees C in the cells grown at either 40 degrees C or 35 degrees C. In contrast, cells grown at either 25 degrees C or 30 degrees C showed no appreciable hyper-response in terms of unsaturated/saturated ratio on temperature shifts to as low as 10 degrees C. By combining shift-down and shift-up experiments, we could show the presence of different types of temperature dependency in the fatty acid-synthesizing systems of cells grown at various temperatures. Contrary to wild-type cells which synthesized mainly cis-vaccenate on down-shift to 10 degrees C, a mutant strain lacking beta-ketoacyl acyl-carrier protein synthase II synthesized more palmitoleate (16:1) and less palmitate at 10 degrees C than at 40 degrees C. The average chain lengths of saturated and unsaturated fatty acids also changed, but differently, between the mutant and wild-type cells on shifts of temperature. Thus, the mutant strain has a temperature-dependent fatty acid-synthesizing system qualitatively different from that seen in a wild-type strain.     

Metabolic control of the membrane fluidity in Bacillus subtilis during cold adaptation

Membrane fluidity adaptation to the low growth temperature in Bacillus subtilis involves two distinct mechanisms: (1) long-term adaptation accomplished by increasing the ratio of anteiso- to iso-branched fatty acids and (2) rapid desaturation of fatty acid chains in existing phospholipids by induction of fatty acid desaturase after cold shock. In this work we studied the effect of medium composition on cold adaptation of membrane fluidity. Bacillus subtilis was cultivated at optimum (40 degrees C) and low (20 degrees C) temperatures in complex medium with glucose or in mineral medium with either glucose or glycerol. Cold adaptation was characterized by fatty acid analysis and by measuring the midpoint of phospholipid phase transition T(m) (differential scanning calorimetry) and membrane fluidity (DPH fluorescence polarization). Cells cultured and measured at 40 degrees C displayed the same membrane fluidity in all three media despite a markedly different fatty acid composition. The T(m) was surprisingly the highest in the case of a culture grown in complex medium. On the contrary, cultivation at 20 degrees C in the complex medium gave rise to the highest membrane fluidity with concomitant decrease of T(m) by 10.5 degrees C. In mineral media at 20 degrees C the corresponding changes of T(m) were almost negligible. After a temperature shift from 40 to 20 degrees C, the cultures from all three media displayed the same adaptive induction of fatty acid desaturase despite their different membrane fluidity values immediately after cold shock.     

Temperature effect on a high stearic acid sunflower mutant.

Vegetable oil with elevated saturated fatty acid content may be useful for producing solid fat without hydrogenation or transesterification. Under the nutritional point of view stearic acid is preferred to other saturated fatty acids because of its neutral effect on serum cholesterol lipoproteins. Selection of a very high stearic acid sunflower (Helianthus annuus L.) line (CAS-14), with up to a 37.3% of stearic acid in the seed oil, and the relationship between the expression of this character and the growth temperature are presented. The mutant was selected from the M(2) progeny of 3000 mutagenized seeds (4 mM sodium azide mutagenesis treatment) by analysing the fatty acid composition of half-seed by gas liquid chromatography. In order to genetically fix the mutant character, plants were grown at high day/night temperatures during seed formation. We found that temperatures higher than 30/20 degrees C are required for good expression of the phenotype, the maximum stearic acid content being obtained at 39/24 degrees C. This behaviour is totally opposed to that observed in normal and previously isolated high-stearic acid sunflower lines that contain more stearic acid at low temperature. Thus, a new type of temperature regulation on the stearate desaturation must occur. This line is the sunflower mutant with the highest stearic acid content reported so far.     

Trienoic fatty acids are required to maintain chloroplast function at low temperatures

The chloroplast membranes of all higher plants contain very high proportions of trienoic fatty acids. To investigate how these lipid structures are important in photosynthesis, we have generated a triple mutant line of Arabidopsis that contains negligible levels of trienoic fatty acids. For mutant plants grown at 22 degrees C, photosynthetic fluorescence parameters were indistinguishable from wild type at 25 degrees C. Lowering the measurement temperature led to a small decrease in photosynthetic quantum yield, Phi(II), in the mutant relative to wild-type controls. These and other results indicate that low temperature has only a small effect on photosynthesis in the short term. However, long-term growth of plants at 4 degrees C resulted in decreases in fluorescence parameters, chlorophyll content, and thylakoid membrane content in triple-mutant plants relative to wild type. Comparisons among different mutant lines indicated that these detrimental effects of growth at 4 degrees C are strongly correlated with trienoic fatty acid content with levels of 16:3 + 18:3, approximately one-third of wild type being sufficient to sustain normal photosynthetic function. In total, our results indicate that trienoic fatty acids are important to ensure the correct biogenesis and maintenance of chloroplasts during growth of plants at low temperatures.     

Lizards, lipids, and dietary links to animal function

Our experiments were designed to test the hypotheses that dietary lipids can affect whole-animal physiological processes in a manner concordant with changes in the fluidity of cell membranes. We measured (1) the lipid composition of five tissues, (2) body temperatures selected in a thermal gradient (T(sel)), (3) the body temperature at which the righting reflex was lost (critical thermal minimal [CTMin]), and (4) resting metabolic rate (RMR) at three body temperatures in desert iguanas (Dipsosaurus dorsalis) fed diets enriched with either saturated or unsaturated fatty acids. The composition of lipids in tissues of the lizards generally reflected the lipids in their diets, but the particular classes and ratios of fatty acids varied among sampled organs, indicating the conservative nature of some tissues (e.g., brain) relative to others (e.g., depot fat). Lizards fed the diet enriched with saturated fatty acids selected warmer nighttime body temperatures than did lizards fed a diet enriched with unsaturated fatty acids. This difference is concordant with the hypothesis that the composition of dietary fats influences membrane fluidity and that ectotherms may compensate for such changes in fluidity by selecting different body temperatures. The CTMin of the two treatment groups was indistinguishable. This may reflect the conservatism of some tissues (e.g., brain) irrespective of diet treatment. The RMR of the saturated treatment group nearly doubled between 30 degrees and 40 degrees C. Here, some discrete membrane domains in the lizards fed the saturated diet may have been in a more-ordered phase at 30 degrees C and then transformed to a less-ordered phase at 40 degrees C. In contrast, the RMR of the unsaturated treatment group exhibited temperature independence in metabolic rate from 30 degrees to 40 degrees C. Perhaps the unsaturated diet resulted in membranes that developed a higher degree of disorder (i.e., a certain phase) at a lower temperature than were membranes of lizards fed the saturated diet. Our study demonstrates links between dietary fats and whole-animal physiology; however, the mechanistic basis of these links, and the general knowledge of lipid metabolism in squamate reptiles, remain poorly understood and warrant further study.     

Possible involvement of acetyl coenzyme A carboxylase as well as fatty acid synthetase in the temperature-controlled synthesis of fatty acids in Saccharomyces cerevisiae

Fatty acid synthetase (FAS) preparations from Saccharomyces cerevisiae cells grown at either 35 or 10 degrees C produced the same products at different temperatures and showed quite similar temperature-dependencies in Arrhenius plots, with break points at 25 degrees C. This break point does not appear to reflect a phase transition of phospholipids present in the purified FAS preparations but rather is associated with protein conformational changes. S. cerevisiae cells grown at 35 degrees C and then shifted to 10 degrees C produced fatty acids with a shorter average chain length than those fatty acids synthesized at 10 degrees C by cells already adapted to 10 degrees C (hyper response). Acetyl-CoA carboxylase activity was relatively higher in the cells grown at 35 degrees C than in the cells grown at 10 degrees C; moreover, fatty acids with longer average chain lengths were synthesized in vitro at higher malonyl-CoA concentrations, which was consistent with the difference in the average chain lengths of newly synthesized fatty acids in cells grown at 35 and 10 degrees C. However, the activity levels of acetyl-CoA carboxylase and fatty acid synthetase alone did not account for the hyper response phenomena.     

Effect of growth temperature on the lipids, outer membrane proteins, and lipopolysaccharides of Pseudomonas aeruginosa PAO.

Growth of Pseudomonas aeruginosa PAO1 at 15 to 45 degrees C in tryptic soy broth resulted in changes in the lipids, lipopolysaccharides (LPSs), and outer membrane proteins of the cells. Cells grown at 15 degrees C contained, relative to those cultivated at 45 degrees C, increased levels of the phospholipid fatty acids hexadecenoate and octadecenoate and reduced levels of the corresponding saturated fatty acids. Furthermore, the lipid A fatty acids also showed thermoadaptation with decreases in dodecanoic and hexadecanoic acids and increases in the level of 3-hydroxydecanoate and 2-hydroxdodecanoate as the growth temperature decreased. In addition, LPS extracted from cells cultivated at the lower temperatures contained a higher content of long-chain S-form molecules than that isolated from cells grown at higher temperatures. On the other hand, the percentage of LPS cores substituted with side-chain material decreased from 37.6 mol% at 45 degrees C to 19.3 mol% at 15 degrees C. The outer membrane protein profiles indicated that at low growth temperatures there was an increase in a polypeptide with an apparent molecular weight of 43,000 and decreases in the content of 21,000 (protein H1)- and 27,500-molecular-weight proteins.     

Low-temperature-induced desaturation of fatty acids and expression of desaturase genes in the cyanobacterium Synechococcus sp. PCC 7002

Changes in response to temperature of lipid classes, fatty acid composition and mRNA levels for acyl-lipid desaturase genes were studied in the marine unicellular cyanobacterium, Synechococcus sp. PCC 7002. The degree of unsaturation of C₁₈ fatty acids increased in cells grown at lower temperature for all lipid classes, and ω3 desaturation occurred specifically in cells grown at low temperature. While the level of 18:1(9) fatty acids declined, desaturation at the ω3 position of C₁₈ fatty acids increased gradually during a 12-h period after a temperature shift-down to 22°C. However, the mRNA levels of the desA (Δ12 desaturase), desB (ω3 desaturase) and desC (Δ9 desaturase) genes increased within 15 min after a temperature shift-down to 22°C; the desaturase gene mRNA levels also rapidly declined within 15 min after a temperature shift-up to 38°C. Therefore, the elevation of mRNA levels for the desaturase genes is not the rate-limiting event for the increased desaturation of membrane lipids after a temperature shift-down. The rapid, low-temperature-induced changes in mRNA levels occurred even when cells were grown under light-limiting conditions for which the growth rates at 22°C and 38°C were identical. These studies indicate that the ambient growth temperature, and not some other growth rate-related process, regulates the expression of acyl lipid desaturation in this cyanobacterium.     

Critical role of anteiso-C15:0 fatty acid in the growth of Listeria monocytogenes at low temperatures.

Listeria monocytogenes is a food-borne pathogen capable of growth at refrigeration temperatures. Membrane lipid fatty acids are major determinants of a sufficiently fluid membrane state to allow growth at low temperatures. L. monocytogenes was characterized by a fatty acid profile dominated to an unusual extent (> 95%) by branched-chain fatty acids, with the major fatty acids being anteiso-C15:0, anteiso-C17:0, and iso-C15:0 in cultures grown in complex or defined media at 37 degrees C. Determination of the fatty acid composition of L. monocytogenes 10403S and SLCC 53 grown over the temperature range 45 to 5 degrees C revealed two modes of adaptation of fatty acid composition to lower growth temperatures: (i) shortening of fatty acid chain length and (ii) alteration of branching from iso to anteiso. Two transposon Tn917-induced cold-sensitive mutants incapable of growth at low temperatures had dramatically altered fatty acid compositions with low levels of i-C15:0, a-C15:0, and a-C17:0 and high levels of i-C14:0, C14:0, i-C16:0, and C16:0. The levels of a-C15:0 and a-C17:0 and the ability to grow at low temperatures were restored by supplementing media with 2-methylbutyric acid, presumably because it acted as a precursor of methylbutyryl coenzyme A, the primer for synthesis of anteiso odd-numbered fatty acids. When mid-exponential-phase 10403S cells grown at 37 degrees C were temperature down-shocked to 5 degrees C they were able, for the most part, to reinitiate growth before the membrane fatty acid composition had reset to a composition more typical for low-temperature growth. No obvious evidence was found for a role for fatty acid unsaturation in adaptation of L. monocytogenes to cold temperature. The switch to a fatty acid profile dominated by a-C15:0 at low temperatures and the association of cold sensitivity with deficiency of a-C15:0 focus attention on the critical role of this fatty acid in growth of L. monocytogenes in the cold, presumably through its physical properties and their effects, in maintaining a fluid, liquid-crystalline state of the membrane lipids.     

Temperature-independent and -dependent expression of desaturase genes in filamentous cyanobacterium Spirulina platensis strain C1 (Arthrospira sp. PCC 9438)

The alteration of the degree of unsaturated fatty acids in membrane lipids has been shown to be a key mechanism in the tolerance to temperature stress of living organisms. The step that most influences the physiology of membranes has been proposed to be the amount of di-unsaturated fatty acids in membrane lipids. In this study, we found that the desaturation of fatty acid to yield the di-unsaturated fatty acid 18:2(9,12), in Spirulina platensis strain C1, was not regulated by temperature. As shown by the fatty acid composition and gene expression patterns, the levels of 18:1(9) and 18:2(9,12) remained almost constant either when the cells were grown at 35 degrees C (normal growth temperature) or 22 and 40 degrees C. The expression of desC (Delta9) and desA (Delta12) genes, which are responsible for the introduction of first and second double bonds into fatty acids, respectively, was not affected by the temperature shift from 35 to 22 degrees C or to 40 degrees C. Only the expression and mRNA stability of the desD gene (Delta6) that is responsible for the introduction of a third double bond into fatty acids were enhanced by a temperature shift from 35 to 22 degrees C, but not the shift from 35 to 40 degrees C. The increase in the level of desD mRNA elevated the desaturation of fatty acid from 18:2(9,12) to 18:3(6,9,12) at 22 degrees C. However, the increased level of 18:3(6,9,12) was observed after 36 h of incubation at 22 degrees C, indicating a slow response to temperature of fatty acid desaturation in this cyanobacterium. These findings suggest that the desaturation of fatty acids might not be a key mechanism in the response to the temperature change of S. platensis strain C1.     

Different cellular fatty acid pattern behaviours of two strains of Listeria monocytogenes Scott A and CNL 895807 under different temperature and salinity conditions

Cells of two strains of Listeria monocytogenes CNL 895807 and Scott A were grown to late exponential phase at different growth temperatures (37, 20 and 4 degrees C) with or without NaCl (7%), and their fatty acid compositions were analysed. The results showed that low thermal adaptation response of L. monocytogenes CNL was different than that of the Scott A strain, and it was based on both an increase of anteiso-branched-chain fatty acids and a significant decrease of straight-chain fatty acids. However, the main modifications observed in the Scott A strain when grown at a low temperature were a decrease of the proportion of ai17:0 and an increase of ai15:0. In hyperosmotic medium and over the entire temperature range (4 degrees C, 20 degrees C and 37 degrees C) the two L. monocytogenes strains showed a cellular fatty acid profile dominated by ai15:0. In addition, a decrease of the two major straight-chain fatty acids (14:0 and 16:0) was observed in the CNL strain. These results demonstrated that the CNL strain showed different behaviours of low thermal and salt adaptation to maintain membrane fluidity, which are based both on an increase of anteiso-branched-chain fatty acids, and a significant decrease of straight-chain fatty acids.     

Induction of polyunsaturated fatty-acid synthesis enhances tolerance of a cyanobacterium, Cylindrospermopsis raciborskii, to low-temperature photoinhibition

Acyl-lipid desaturation introduces double bonds (unsaturated bonds) at specifically defined positions of fatty acids that are esterified to the glycerol backbone of membrane glycerolipids. Desaturation pattern of the glycerolipids of Cylindrospermopsis raciborskii (C. raciborskii), a filamentous cyanobacterial strain, was determined in cells grown at 35 degrees C and 25 degrees C. The lowering of the growth temperature from 35 degrees C to 25 degrees C resulted in a considerable accumulation of polyunsaturated octadecanoic fatty acids in all lipid classes. Lipid unsaturation of C. raciborskii was also compared to Synechocystis PCC6803. In C. raciborskii cells, a shift in growth temperature induced a much more pronounced alteration in the desaturation pattern of all lipid classes than in Synechocystis PCC6803. The tolerance to low-temperature photoinhibition of the C. raciborskii cells grown at 25 degrees C and 35 degrees C was also compared to the tolerance of Synechocystis cells grown at the same temperatures. Lower growth temperature increased the tolerance of C. raciborskii cells but not that of Synechocystis cells. These results strengthen the importance of polyunsaturated glycerolipids in the tolerance to environmental stresses and may give a physiological explanation for the determinative role of C. raciborskii strain in algal blooming in the Lake Balaton (Hungary).     

The tolerance of cyanobacterium Cylindrospermopsis raciborskii to low-temperature photo-inhibition affected by the induction of polyunsaturated fatty acid synthesis

Acyl-lipid desaturation introduces double bonds (unsaturated bonds) at specifically defined positions of fatty acids that are esterified to the glycerol backbone of membrane glycerolipids. Desaturation patterns of the glycerolipids of Cylindrospermopsis raciborskii, a filamentous cyanobacterium, were determined in cells grown at 35 degrees C and 25 degrees C. The lowering of the growth temperature from 35 degrees C to 25 degrees C resulted in a considerable accumulation of polyunsaturated octadecanoic fatty acids in all lipid classes. The tolerance to low-temperature photo-inhibition of the C. raciborskii cells grown at 25 degrees C and 35 degrees C was also compared. The lower growth temperature increased the tolerance of C. raciborskii cells. These results strengthen the importance of polyunsaturated glycerolipids in the tolerance to environmental stresses and may give a physiological explanation for the determinative role of C. raciborskii in algal blooming in Lake Balaton (Hungary).     

Perkinsus marinus, a protozoan parasite of the Eastern oyster (Crassostrea virginica): effects of temperature on the uptake and metabolism of fluorescent lipid analogs and lipase activities

The effects of temperature on the uptake and metabolism of fluorescent labeled palmitic acid (FLC16) and phosphatidylcholine (FLPC) and lipase activities in the oyster protozoan parasite, Perkinsus marinus, meront stage were tested at 10, 18, and 28 degrees C. Temperature significantly affected not only the uptake, assimilation, and metabolism of both FLC16 and FLPC in P. marinus, but also its triacylglycerol (TAG) lipase activities. The incorporation of both FLC16 and FLPC increased with temperature and paralleled the increase in the amount of total fatty acids in P. marinus meront cultures. The incorporation of FLC16 was higher than FLPC at all temperatures. The percentage of FLC16 metabolized to TAG was significantly higher at higher temperatures. Trace amounts of incorporated FLC16 were detected in monoacylglycerol (MAG) and PC at 18 and 28 degrees C. P. marinus meronts metabolized FLPC to TAG, diacylglycerol (DAG), monoacylglycerol (MAG), free fatty acids (FFA), phosphatidylethanolamine (PE), and cardiolipin (CL). The conversion of FLPC to TAG and PE was highest at 28 degrees C. The relative proportions of individual fatty acids and total saturated, monounsaturated and polyunsaturated fatty acids changed with temperatures. While total saturated fatty acids (SAFAs) increased with temperature, total monounsaturated fatty acids (MUFAs) decreased with temperature. Total polyunsaturated fatty acids (PUFAs) increased from 28 to 18 degrees C. The findings of increase of total SAFAs and decrease of total MUFAs with the increase of temperatures and upward shift of total PUFAs from 28 to 18 degrees C suggest that, as in other organisms, P. marinus is capable of adapting to changes in environmental temperatures by modifying its lipid metabolism. Generally, higher lipase activities were noted at higher cultivation temperatures. Both TAG lipase and phospholipase activities were detected in P. marinus cells and their extra cellular products (ECP), but phospholipase activities in both the cell pellets and ECP were very low. Also, lipase activities were much lower in ECP than in the cells. The observations of low metabolism, bioconversion of incorporated fluorescent lipid analogs and lipase activities at low temperatures are consistent with the low in vitro growth rate and low infectivity of P. marinus at low temperatures.     

Low temperature-induced changes in the distribution of H2O2 and antioxidants between the bundle sheath and mesophyll cells of maize leaves

The distribution of antioxidants between bundle sheath and mesophyll cells of maize leaves was analysed in plants grown at 20 degrees C, 18 degrees C and 15 degrees C. The purity of the isolated bundle sheath and mesophyll fractions was determined using compartment-specific marker enzymes. In plants grown at 15 degrees C, ascorbate peroxidase, CuZn-superoxide dismutase (CuZn-SOD) and monodehydroascorbate reductase activities were increased in the bundle sheath cells, and glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase activities were enhanced in the mesophyll cells. SOD was absent from the mesophyll of plants grown at 20 degrees C but an Fe-SOD activity was found in the mesophyll of plants grown at 15 degrees C. Foliar Mn-SOD activities were decreased at 15 degrees C compared to 20 degrees C. Catalase was undetectable in the mesophyll extracts of plants grown at 15 degrees C. Ascorbate and glutathione contents were considerably higher in the mesophyll than the bundle sheath fractions of plants grown at 20 degrees C. The ratios of reduced to oxidized forms of these antioxidants were significantly decreased in the bundle sheath, but increased in the mesophyll of leaves grown at 15 degrees C. Foliar H2O2 accumulated at 15 degrees C compared to 20 degrees C. Most of the foliar H2O2 was localized in the mesophyll tissues at all growth temperatures. The differential distribution of antioxidants between leaf bundle sheath and mesophyll tissues, observed at 20 degrees C, is even more pronounced when plants are grown at 15 degrees C and may contribute to the extreme sensitivity of maize to low temperatures.     

The effects of temperature on the composition and physical properties of the lipids of Pseudomonas fluorescens

1. Pseudomonas fluorescens was grown at various temperatures between 5 degrees C and 33 degrees C. The extractable lipids from organisms at various stages of growth and grown at different temperatures were examined. 2. The extractable lipids contained phosphatidylethanolamine, diphosphatidylglycerol, phosphatidylglycerol, phosphatidylcholine, and an ornithine-containing lipid. The relative amounts of these lipids did not vary significantly during growth or with the changes in growth temperature. 3. The major fatty acids were hexadecanoic, hexadecenoic and octadecenoic acids and the cyclopropane acids methylene-hexadecanoic and methylene-octadecanoic acids. The relative amount of unsaturated acids (including cyclopropane acids) did not change significantly during growth, but increased with decreasing temperature. 4. Phosphatidylethanolamines with different degrees of unsaturation and containing different amounts of cyclopropane acids were isolated from organisms grown at 5 degrees C and 22 degrees C and their surface and phase behaviour in water was investigated. Thermodynamic parameters for fusion and monolayer results for cyclopropane and other fatty acids were examined. 5. The surface pressure-area isotherms of phosphatidylethanolamines containing different amounts of unsaturated fatty acids show small differences but the individual isotherms remain essentially unchanged over the temperature range 5-22 degrees C. X-ray-diffraction methods show that the structures (lamellar+hexagonal) formed in water by phosphatidylethanolamine, isolated from organisms grown at 5 degrees C and 22 degrees C, are identical when compared at the respective growth temperatures. This points to a control mechanism of the physical state of the lipids that is sensitive to the operating temperature of the organism. 6. The molecular packing of cyclopropane acids is intermediate between that of the corresponding cis- and trans-monoenoic acids. However, substitution of a cyclopropane acid for a cis-unsaturated acid has insignificant effects on the molecular packing of phospholipids containing these acids.     

The effect of growth temperature on the thermotropic behavior of the membranes of a thermophilic Bacillus. Composition-structure-function relationships

The following study was carried out with the aim of widening our understanding of the thermoadaptive mechanisms of the membrane of thermophiles, using Bacillus stearothermophilus var. nondiastaticus as test-organism. The phospholipids and their acyl chain composition of this Bacillus studied in relation to the physical properties of its membrane from bacteria grown at various temperatures. Phospholipids account for 68-75 weight% of the total lipid in cells grown at 45, 55 or 65 degrees C. Phosphatidylglycerol and diphosphatidylglycerol constitute up to 90% of the total phospholipids; no amino phospholipids were found. Increasing the growth temperatures from 45 degrees to 65 degrees C caused an approximately 4-fold decrease in the proportion of the branched-chain fatty acids and a 2-fold increase in the amount of the saturated acyl chains. The reduced proportion of the branched fatty acids was mainly due to a decrease in their anteiso forms. Unsaturated fatty acids were not produced by cells grown at 65 degrees C. In accordance with the fatty acid composition, the molecular packing of phospholipids in monolayers was more expanded with phospholipids from 45 degrees C grown cells as compared with cultures grown at 55 degrees C. The thermotropic gel to liquid-crystalline phase transition of the membrane lipids was monitored by differential scanning calorimetry and fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene. With increase of the growth temperature the phase transition was progressively shifted to higher but narrower range of temperatures. Completion of the lipid melting occurred always at temperatures below those employed for growth. A constructed phase diagram enabled to relate the growth temperature, the fatty acid composition and the lipid apparent microviscosity at temperatures not used in the present study for growth of the thermophile. The minimum temperature for growth and the upper boundary temperature of the least saturated lipid crystallization were extrapolated in this manner; they correspond to the experimentally determined minimal growth temperature. The apparent microviscosity, a measure of membrane order, decreased gradually and conspicuously as the growth temperature was elevated. The delimiting apparent microviscosity values, at the maximal (65 degrees C) and minimal (41 degrees C) growth temperatures were 0.8 and 1.8 poise, respectively. This lack of rigorous homeostatic control of the bulk lipid viscosity prompted reevaluation of the physiological significance of 'homeoviscous adaptation' in Bacillus stearothermophilus.