Metabolism of Glycerolipids in Plastidial and Extraplastidial Membranes of Etiolated Wheat Leaves: In Vivo and In Vitro Studies

Etiolated wheat leaves were fed with [l-¹⁴C]-acetate and chaseexperiments were performed in the dark or under light. In bothconditions, in extraplastidial membranes, phosphatidylglycerol(PG) and phosphatidylcholine (PC) were fatty acid donors, PGand PC providing palmitate and oleate respectively. The labelof linoleate increased only in phosphatidylethanolamine (PE).In etioplasts, PG was also a palmitate donor but PC, ratherpoor in labelled oleate, was an oleate acceptor, contrary towhat was observed in chloroplasts. The galactolipids and sulfoquinovosyldiacylglycerol(SL) remained poorly labelled. When isolated etioplasts were labelled in vitro, during thefirst two hours they incorporated the same amount of [l-¹⁴C]-acetatein their phospholipids, whether they were in the presence orin the absence of extraplastidial membranes. Afterwards, theaddition of a mitochondrial fraction enhanced the label of PG,mainly in palmitate, then in oleate, and to some extent, andonly under light in palmitoleate and linoleate. The mitochondrialfraction might be regarded here as a supplier of labelled precursorto he etioplasts which rapidly accumulated radioactivity inpalmitoyl-PG. In PC of isolated etioplasts, only palmitate wasfairly labelled. The deficiency in labelled oleoyl-PC in plastidsof dark-grown plants and of linoleoyl-PC in extraplastidialmembranes might be the reason for the delay in the labellingof unsaturated galactolipids. ¹ (Received March 9, 1987; Accepted August 21, 1987)     

Radiolabelling Studies on the Lipid Metabolism in the Marine Brown Alga Dictyopteris membranacea

The lipid metabolism of the marine brown alga D. membranaceawas investigated using [2–¹⁴C]acetate, [1–¹⁴C]myristate,[l–^I4C]oleate and [l–¹⁴C]arachidonate as precursors.On incubation with [2–¹⁴C]acetate, 18:1 and 16:0 werethe main products formed by de novo synthesis and incorporatedinto polar lipids. With all the exogenous substrates used, DGTAwas strongly labelled and the subsequent rapid turnover of radioactivitysuggested a key role for this lipid in the redistribution ofacyl chains and most likely also in the biosynthesis of theeukaryotic galacto-lipids produced in the absence of PC. Inthe glycolipids a continuous accumulation of radioactivity wasobserved with all the substrates used. The labelling kineticsof molecular species of MGDG suggested the desaturation of 18:1to 18:4 and of 20:4 (n-6) to 20:5 (n–3) acids on thislipid. Both PG and PE were primary acceptors of de novo synthesizedfatty acids and exogenous [l–¹⁴C]oleate, but no evidenceexists for a further processing of acyl chains on these lipids.TAG, although strongly labelled with all exogenous [l–¹⁴CJacids,was not labelled when [2–¹⁴C]acetate was used as a precursorindicating the flux of endogenous fatty acids to be differentof that of exogenously supplied fatty acids. (Received November 4, 1997; Accepted February 23, 1998)     

Effects of Light on the Metabolism of Glycerolipids in Plastidial and Cytoplasmic Membranes of Wheat Leaves: Studies in Vivo and in Vitro

Wheat leaves were labelled with [l-₁₄C]-glycerol or [l-₁₄C]-acetateand chase experiments performed in the dark or under light.In plastids, both in the dark and under light, the results indicatea transfer of [l-₁₄C]-glycerol from phospholipids to galactolipidsand of [l-₁₄C]-acetate from phosphatidylcholine (PC) to monogalactosyldiacylglycerol (MGDG). They also argue for a transfer of [l-₁₄C]-glyceroland [1-₁₄C]-acetate from phosphatidylcholine (PC) to phosphatidylethanolamine(PE) in extraplastidial membranes. During chase experimentsin the dark, the chloroplasts accumulated higher amounts ofradioactive precursor in saturated fatty acids. In the darkor under light, oleoyl-PC labelling equally decreased in plastids,but decreased much more under light in extraplastidial membranes.Light enhanced polyunsaturated fatty acid synthesis, mainlyin MGDG, PC, PE and plastidial phosphatidylglycerol (PG). In the dark or under light, all glycerolipids were labelledwhen purified plastids were incubated with [l-₁₄C]-acetate.Light stimulated the incorporation of the label in palmitoyl-MGDG,PG and sulfoquinovosyldiacylglycerol (SL) and also the transferof oleate from PC to MGDG. Only under light and when extraplastidialmembranes were added to isolated plastids, linoleoyl-MGDG, PGand PC were slightly labelled. These results argue for a stimulating effect of light on glycerolipidsynthesis in wheat leaf chloroplasts, on the transfer of oleatefrom PC to MGDG and on the desaturase activity. (Received March 8, 1986; Accepted September 26, 1986)     

Phospholipid metabolism of stimulated lymphocytes: Preferential incorporation of polyunsaturated fatty acids into plasma membrane phospholipid upon stimulation with concanavalin A

Rabbit thymocytes were isolated and incubated for various lengths of time with concanavalin A. The cultures were pulsed for the last 12.5 min of incubation with equimolar mixtures of radioactively labelled fatty acids, either [³H]arachidonate and [¹⁴C]oleate or [³H]arachidonate and [¹⁴C]palmitate, and the uptake of each fatty acid into phospholipid of plasma membrane was determined. Upon binding of the mitogen, the fatty acids were incorporated at an increased rate with a new steady state being reached between 12.5 and 42.5 min after stimulation. Initially after 12.5 min, when the two fatty acids were added together, no preferential incorporation of the polyunsaturated fatty acid arachidonate was seen compared to the saturated or monounsaturated ones, palmitate or oleate. However shortly thereafter arachidonate, when compared to palmitate or oleate, started to be preferentially incorporated into plasma membrane phospholipid so that by 4 h after activation, only arachidonate was incorporated at an increased rate: the uptake of palmitate and oleate had reverted to that of unstimulated cells. In contrast, when palmitate or oleate were added alone, after 4 h of activation incorporation was increased similar to that of arachidonate, suggesting that all long chain fatty acids compete for the same activated enzyme(s). A detailed analysis of incorporation into phospholipid species showed that all fatty acids were taken up with the highest rate into phosphatidylcholine. After activation, fatty acid incorporation was increased by approx. 50% for phosphatidylcholine: the highest stimulation rates were observed with phosphatidylinositol (3–7-fold) and phosphatidylethanolamine (2–3-fold). The data suggest that shortly after stimulation with mitogens, the membrane phospholipids start to change by replacing saturated and monounsaturated fatty acids by polyunsaturated ones, thus creating a new membrane.     

Polyunsaturated Fatty Acid Synthesis by a Mixture of Chloroplasts and Microsomes from Spinach Leaves: Evidence for Two Distinct Pathways of the Biosynthesis of Trienoic Acids

In a mixture of chloroplasts and microsomes from spinach leaves,all the leaf lipids were synthesized from (1-¹⁴C)-acetate. Inthis system, all the lipids contained labelled oleate, linoleateand linolenate but labelled linolenate was mainly concentratedinto diacylgalactosylglycerol (MGDG). A small but significantlabelling was found in the linolenate of the diacyldigalactosylglycerol(DGDG). On the other hand, labelled hexadecamonoenoic acid (C^16:1),hexadecadienoic acid (C^16:2) and hexadecatrienoic acid (C^16:3)were only found into MGDG. In such a reconstituted system, atthe end of the incubation period, labelled MGDG was almost exclusivelyrecovered into the chloroplast while the labelled phosphatidylcholine(PC) was found highly concentrated in the microsomes In the MGDG of the chloroplast, C^16:1, C^16:2 and C^16:3 werefound at the C2 position of the glycerol while oleic acid (C^18:1),linoleic acid (C^18:2) and a-linolenic acid (^18:3) esterifiedspecifically the position 1 of the glycerol. No C¹⁸ acids werefound in position 2. In the PC of the microsomes, C^18:1, C^18:2and C^18:3 were found at the Cl and C2 positions of the glycerolwhile palmitic acid esterified exclusively the Cl of the glycerol. The biosynthetic pathway of trienoic fatty acids in leaves ofhigher plants is discussed. (Received July 19, 1982; Accepted October 18, 1982)     

Acyl Lipid Metabolism in Germinating Hazel Seeds (Corylus avellana L.)

A decreasing percentage of radioactivity from [U-¹⁴C]-oleateand [U-¹⁴C]-palmitate was recovered in the lipid fractions ofgerminating hazel cotyledons. The pattern of incorporation ofthe acids into the cotyledon glycerides was consistent withtheir degree of saturation. The relative incorporation intothe cotyledon phospholipids changed during germination. Radioactivityfrom both acids was recovered in increasingly unsaturated fattyacids in the cotyledon hpids. Diversion of both [U-¹⁴C]-fatty acids into acyl lipid synthesisoccurred in the germinating embryonic axes. Both were increasinglyrecovered in mixed glyceride groups which contained diglyceridesand highly unsaturated triglycerides. The two acids gave differentpatterns of incorporation in the axis phospholipids. Desaturationof the acids occurred to a lesser extent than in the germinatingcotyledons.     

Evidence that a Proliferation of the Endoplasmic Reticulum Precedes the Formation of Glyoxysomes and Mitochondria in Germinating Castor Bean Endosperm

Excised castor bean endosperm halves incubated with CDP-[Me-¹⁴C]cholineactively incorporated this compound into membrane phosphatidylcholine.The capacity of the tissue to synthesize phosphatidyl-[¹⁴C]cholineincreased during the first 3 d of germination and subsequentlydeclined. At the onset of germination phosphatidyl-[^l4C]cholinewas exclusively recovered in the ER membrane fraction. The rateof incorporation into the ER membranes increased strikinglyduring the first 24 h of germination while that into mitochondriaand glyoxysomes remained low. At later developmental stagesan increasing proportion of the newly synthesized phosphatidyl-[¹⁴C]cholinewas present in mitochondria and glyoxysomes; the rate of incorporationinto the membranes of these organelles increased while thatinto the ER membrane began to level off. The kinetics of CDP-[¹⁴C]cholineincorporation into membrane phosphatidylcholine of the majororganelle fractions of 3-d-old endosperm tissue showed thatthe ER was immediately labelled, whereas a lag period precededthe labelling of mitochondria and glyoxysomes. Assuming that the incorporation of CDP-[¹⁴C]choline into phosphatidylcholineserves as a reliable indicator of membrane synthesis, the resultsobtained suggest that a proliferation of ER membranes precedesthe formation of glyoxysomes and mitochondria in germinatingcastor bean endosperm. A comparison of developmental changesin (a) total ER and glyoxysomal phospholipid content and (b)ER and mitochondrial NADH cytochrome c reductase activity providedadditional evidence supporting this conclusion.     

Lipids in Cryptomonas CR-1. II. Biosynthesis of Betaine Lipids and Galactolipids

The biosynthesis of lipids in Cryptomonas strain CR-1 was studiedusing radioactive tracers. For studies of general aspects ofthe biosynthesis of lipids, the cells were labelled with [¹⁴C]NaHCO₃or with [l,3-¹⁴]glycerol. In both cases, monogalactosyl diacylglycerol(MGDG) was the most heavily labelled lipid. Phosphatidylcholineand the alanine lipid DGTA were not labelled to specific activitiescomparable to those of MGDG and DGDG. It is improbable thatthe so-called "eukaryotic pathway", which has been suggestedas the pathway for the synthesis of " eukaryotic" molecularspecies of MGDG from PC in higher plants, is operative in Cryptomonascells which contain typical "eukaryotic" MGDG. The homoserinelipid DGTS was labelled to a significant level only in its polargroup. The C-3 and C-4 atoms of methionine, as well as the methylcarbon of methionine, were incorporated into both DGTS and DGTA,whereas the C-l carbon of methionine was incorporated uniquelyinto DGTS. Results of pulse-chase experiments with [3,4-¹⁴C]methionineand [methyl.-^l4C]methionine suggest the conversion of DGTS toDGTA. (Received April 22, 1991; Accepted June 12, 1991)     

The utilisation of fatty-acid substrates in triacylglycerol biosynthesis by tissue-slices of developing safflower (Carthamus tinctorius L.) and sunflower (Helianthus annuus L.) cotyledons

Developing cotyledons of safflower (Carthamus tinctorius L.) and sunflower (Helianthus annuus L.) readily utilised exogenously supplied ¹⁴C-labelled fatty-acid substrates for the synthesis of triacylglycerols. The other major radioactive lipids were phosphatidylcholine and diacylglycerol. In safflower cotyledons, [¹⁴C]oleate was rapidly transferred to position 2 of sn-phosphatidylcholine and concomitant with this was the appearance of radioactive linoleate. The linoleate was further utilised in the synthesis of diacyl- and triacyl-glycerol via the reactions of the so-called Kennedy pathway. Supplying [¹⁴C]linoleate, however, resulted in a more rapid labelling of the diacylglycerols than from [¹⁴C]oleate. In contrast, sunflower cotyledons readily utilised both labelled acyl substrates for rapid diacylglycerol formation as well as incorporation into position 2 of sn-phosphatidylcholine. In both species, however, [¹⁴C]palmitate largely entered sn-phosphatidylcholine at position 1 during triacylglycerol synthesis. The results support our previous in-vitro observations with isolated microsomal membrane preparations that (i) the entry of oleate into position 2 of sn-phosphatidylcholine, via acyl exchange, for desaturation to linoleate is of major importance in regulating the level of polyunsaturated fatty acids available for triacylglycerol formation and (ii) Palmitate is largely excluded from position 2 of sn-phosphatidylcholine and enters this phospholipid at position 1 probably via the equilibration with diacylglycerol. Specie differences appear to exist between safflower and sunflower in relation to the relative importance of acyl exchange and the interconversion of diacylglycerol with phosphatidylcholine as mechanisms for the entry of oleate into the phospholipid for desaturation.Abbreviations FW fresh weight - TLC thin-layer chromatography     

The effect of hypolipidemic drugs WY14643 and DH990, and lysophospholipids on the metabolism of oleate in plants

The effects of the addition of hypolipidemic drugs and 1-acylglycerolipids on the metabolism of oleate in plants have been studied in vivo and in vitro. Using aged potato slices with [¹⁴C]oleate as a precursor, it was found that these drugs markedly inhibited both the incorporation into complex lipids and the desaturation of oleate to linoleate. Moreover, in vitro experiments, carried out with microsomes prepared from developing safflower seeds and [¹⁴C]oleate or [¹⁴C]oleoyl-CoA as precursors, confirmed the inhibitory effect of the drugs on oleate desaturation, and showed that while WY14643 mainly affected oleoyl thiokinase activity, DH990 exerted its strongest effect on the formation of PL, indicating that the mode of action of these two drugs in safflower microsomes is essentially different. Addition of LPC or LPE stimulated the incorporation of radiolabeled precursor into PC and PE, respectively, as well as the desaturation of oleate to linoleate when [¹⁴C]oleoyl-CoA was the precursor. The evidence obtained suggests that oleoyl-PE, as well as oleoyl-PC, should be considered as a possible substrate for oleate desaturation in plants.     

Lipid Metabolism in the Brown Marine Algae Fucus vesiculosus and Ascophyllum nodosum

Lipid metabolism and environmental effects on this process havebeen studied in the marine brown algae Fucus vesiculosus andAscophyllum nodosum. These algae showed very similar patternsof lipid metabolism during 24 h incubations. Labelling from[1-¹⁴C]acetate showed the major labelled lipids to be the ß-alanineether lipid and the neutral lipid fraction in both algae. Ofthe glycolipids, only sulphoquinovosyldiacylglycerol was welllabelled and the phosphoglycerides were all poorly labelled.The major labelled fatty acids were palmitate and oleate, againin both algae, although Fucus vesiculosus also showed significantlabelling of stearate and behenate. Although the amount of fattyacid labelling increased with time, the proportion of labelin palmitate and oleate remained approximately constant. Verylong chain fatty acids (arachidic, behenic) were increasinglylabelled with time. Lowered incubation temperatures decreased labelling of the saturatedfatty acids. Cu²⁺ increased the proportion of oleate labelledin both algae, and of linoleate in Fucus vesiculosus. This cationdecreased the percentage labelling of stearate and myristatein Ascophyllum nodosum. Lipid metabolism in Ascophyllum nodosumwas more sensitive to raised Cu²⁺ levels than in Fucus vesiculosus Key words: Acyl lipid metabolism, Fucus vesiculosus, temperature effects, Ascophyllum nodosum, copper pollution     

The Partitioning of Photosynthetically Assimilated 14C into Amino Acids in Wheat 1. Partitioning During Transport to the Grain

When ¹⁴CO₂ was fed to flag leaf laminae at 20 d post-anthesis,the transport organs between the leaf and the grains containedappreciable ¹⁴C in glutamine, glutamate, serine, alanine, threonineand glycine. Smaller amounts of ¹⁴C were present in gamma-aminobutyricacid (GABA), aspartate and cysteine. Other amino acids whichwere labelled in the source leaf were not labelled in the transportorgans. The export of labelled glutamine, serine, glycine andthreonine from the source leaf was favoured in comparison tothe other amino acids mentioned. Threonine accumulated, andwas subsequently metabolised, in the rachis. [¹⁴C]GABA alsoaccumulated in the rachis. In the grains, the relative amountof soluble [¹⁴C]alanine increased with chase time. This wasprobably due to de novo synthesis and reflected the specialrole of alanine in grain nitrogen metabolism. Wheat, Triticum aestivum, ¹⁴CO₂, amino acids, transport, carbon metabolism     

Betaine Lipids in Lower Plants. Biosynthesis of DGTS and DGTA in Ochromonas danica (Chrysophyceae) and the Possible Role of DGTS in Lipid Metabolism

Membrane lipids and fatty acids of Ochromonas danica were analyzed.Of the two betaine lipids, the homoserine lipid DGTS mainlycontains 14:0 and 18:2 fatty acids, while the alanine lipidDGTA is enriched in 18:0, 18:2 and 22:5 fatty acids. Of thepolar moiety of DGTA, improved NMR data are presented. On incubationof cells with [3,4-¹⁴C]methionine, DGTS as well as DGTA werelabelled. With [1-¹⁴C]methionine as a substrate, the label appearedin DGTS only. If double labelled [³H](glycerol)/[¹⁴C](polarpart)DGTS was used as a precursor, radioactivity was incorporatedspecifically into DGTA in which the isotope ratio was unchangedcompared to the precursor. Thus, the glyceryltrimethylhomoserinepart of DGTS acts as the precursor of the polar group of DGTA.Labelling of cells with [1-¹⁴C]oleate in a pulse-chase mannerand subsequent analysis of the label in the fatty acids andmolecular species of different lipids including DGTS and DGTA,suggested a clearly different role of the two betaine lipids:DGTS acts as a i) primary acceptor for exogenous C₁₈ monoeneacid, ii) substrate for the desaturation of 18:1 to 18:2 acid,and iii) donor of mainly 18:2 fatty acid to be distributed amongPE and other membrane lipids. Into DGTA, in contrast, fattyacids are introduced only after elongation and desaturation.As a result, the biosynthesis of DGTA from DGTS involves a decarboxylationand recarboxylation of the polar part and a simultaneous deacylationand reacylation of the glycerol moiety. (Received January 28, 1992; Accepted March 11, 1992)     

Effect of hypoglycemia on the brain free fatty acid level and the uptake of fatty acids by phospholipids

The effect of hypoglycemia on the uptake of [1-¹⁴C]arachidonate and [1-¹⁴C]oleate into a synaptosomal and microsomal glycerophospholipids was investigated. In the presence of ATP, Mg²⁺ and CoA, rat brain synaptosomes and micorsomes catalyze the transfer of arachidonate and oleatc into glycerophospholipids. Arachidonate was mainly incorporated into phosphatidylinositol (PI) and phosphatidylcholine (PC), whereas oleate was incorporated into phosphatidylcholine and phosphatidylethanolamine (PE).Hypoglycemia was produced by intraperitoneal injection of 10 or 100 units of crystalline insulin per kg body weight. Two hours after injection the blood glucose level decreased to 10–20 mg%. The content of brain phospholipids was slightly decreased but the change was not statistically significant. The level of free fatty acids (FFA) was increased. More pronounced and reproducible changes were found when hypoglycemia was produced by injection of 100 units of insulin per/kg body weight. Changes in brain cortex were similar to those observed in microsomes and synaptosomes. Hypoglycemia affected the incorporation of arachidonic acid into glycerophospholipids of brain membranes. Uptake of [1-¹⁴C]arachidonate was decreased selectively by 50% (into phosphatidic acid /PA/) when hypogiycemia was produced by injection of 10 units of insulin per kg body weight. The Higher dose of insulin 100 units per kg body weight produced a 20% inhibition of arachidonate incorporation into synaptosomal PI and a 13% decrease of incorporation into microsomal phosphatidylcholine. Incorporation of [1-¹⁴C]oleate into membrane phospholipids was not changed by hypoglycemic insult. It is proposed that the disturbances in fatty acid level, particularly arachidonate, and decreased uptake of arachidonic acid by synaptosomal glycerophospholipids may be responsible for alteration of membrane function and changes of synaptic processes.     

Metabolism of Exogenous Malonate by Coffee Leaf Tissue

When [l-¹⁴C]-malonate was supplied to discs cut from matureleaves of Coffea arabica, ¹⁴CO₂ was released (approximately12% of the total CO₂ respired) and organic acids of the Krebscycle, uronic acids, sugars and amino acids became radioactive.There was no incorporation of MC into either lipids or phenoliccompounds. The formation of glucose from malonate has not beenobserved in other studies with plant tissues. The synthesisof labelled glucose together with an active pentose phosphatepathway that is stimulated by malonate explains the accumulationof radioactive phosphogluconate in the leaf discs. Tentativeproposals are made for pathways to account for the results obtained. Key words: Coffee leaves, Malonate metabolism, Pentose phosphate pathway     

Fatty acid biosynthesis in the leaves of barley, wheat and pea.

1. The incorporation of radioactivity from [1-14C]acetate into the leaf lipids of barley, pea and wheat has been studied in pulse-labelling experiments. 2. There was little increase in the total labelling of lipids after the leaves were transferred to non-radioactive medium. However, there was an increase in the relative labelling of unsaturated fatty acids. In addition, there was an increase in the relative labelling of diacylgalactosylglycerol. 3. The principal radioactively labelled acyl lipids were diacylgalactosylglycerol and phosphatidylcholine. Phosphatidylcholine showed a decreasing proportion of [14C]oleate and an increasing amount of [14C]linoleate with time. Diacylgalactosylglycerol also had decreasing amounts of [14C]oleate but, in addition, had an increasing proportion of [14C]linolenate with time. 4. The absence of significant amounts of [14C]linolenate in phosphatidylcholine appeared to exclude a role for this phospholipid in linoleate desaturation. 5. The specific radioactivities of oleate and linoleate in phosphatidylcholine, diacylgalactosylglycerol and diacylgalabiosylglycerol were very similar in any single experiment. It was concluded that these fatty acids can rapidly exchange between the three intact lipids.     

The Long Term Metabolism of ({+/-})-(2-14C)Abscisic Acid by Apple Seeds

Apple seeds soaked in a solution of [±]-[2-¹⁴C]abscisicacid for up to 70 d formed phaseate and the epimeric dihydrophaseatesand all four acids were converted into alkali-hydrolysable conjugates.This metabolism occurred in the husks and in the embryos. Bothparts of untreated control seeds contained the four acids andtheir conjugates. In addition to these characterized metabolites,a number of other, labelled, acidic products were isolated fromthe seeds. ¹⁴CO₂ was evolved from unsterilized seeds but notfrom surface-sterilized ones. Bacterial cultures isolated fromsoil and rotting fruit metabolized [¹⁴C]ABA to a range of compounds,but not to phaseic or the dihydro-phaseic acids. A new, acidicconjugate of ABA is described.     

Glucose and Pyruvate Metabolism in Daucus carota Cells: The Effect of the Ammonium Ion

In Daucus carota cells cultivated in vitro, the ammonium ionstimulates the incorporation of radioactivity from labelledglucose and labelled pyruvate into CO₂ and into the residueinsoluble in 60 per cent (v/v) ethanol. There is a higher ¹⁴CO₂production from [6-¹⁴C₂] glucose than from [6-¹⁴C] glucose.These results suggest a possible stimulation of glycolysis bythe ammonium ion.     

Rhamnogalacturonan-II--a biologically active fragment

Rhamnogalacturonan-II inhibited the uptake of [¹⁴C]leucine and,consequently, the incorporation of [¹⁴C]leucine into acid-precipitableproteins by suspension-cultured tomato cells. Fractionationof rhamnogalacturonan-II showed that the lower molecular componentswere the most effective. KDO and apiose, both constituents ofrhamnogalacturonan-II, also inhibited [¹⁴C]leucine incorporationweakly, suggesting that these sugar residues may be an integralrequirement for the biological activity of rhamnogalacturonan-II.The incorporation of [¹⁴C]glutamate and [¹⁴C]histidine, andto a lesser extent [¹⁴C]proline and [¹⁴C]arginine, was alsoinhibited by rhamnogalacturonan-II; the incorporation of [¹⁴C]tyrosineand [¹⁴C]phenylalanine was little affected. This suggests thatrhamnogalacturonan-II exerts its effect by acting on certainmembrane transport systems. Key words: Rhamnogalacturonan-II, inhibition, protein synthesis, amino acid incorporation     

The utilization and desaturation of oleate and linoleate during glycerolipid biosynthesis in olive (Olea europaea L.) callus cultures

Callus cultures from olive (Olea europaea L.) were used to study characteristics of desaturation in this oil-rich tissue. The incorporation of [1-(14)C]oleate and [1-(14)C]linoleate into complex lipids and their further desaturation was followed in incubations of up to 48 h. Both radiolabelled fatty acids were rapidly incorporated into lipids, especially phosphatidylcholine and triacylglycerol. Radiolabelling of these two lipids peaked after 1-4 h, after which it fell. In contrast, other phosphoglycerides and the galactosylglycerides were labelled in a more sustained manner. [1-(14)C]Linoleate was almost exclusively found in the galactolipids. With [1-(14)C]linoleate as a precursor, the only significant desaturation to linolenate was in the galactolipids. Monogalactosyldiacylglycerol was the first lipid in which [1-(14)C]linoleate and [1-(14)C]linolenate appeared after incubation of the calli with [1-(14)C]oleate and [1-(14)C]linoleate, respectively. The presence of radioactivity in the plastidial lipids shows that both [1-(14)C]oleate and [1-(14)C]linoleate can freely enter the chloroplast. Two important environmental effects were also examined. Raised incubation temperatures (30-35 degrees C) reduced oleate desaturation and this was also reflected in the endogenous fatty acid composition. Low light also caused less oleate desaturation. The data indicate that lysophosphatidylcholine acyltransferase is important for the entry of oleate and linoleate into olive callus lipid metabolism and phospholipid:diacylglycerol acyltransferase may be involved in triacylglycerol biosynthesis. In addition, it is shown that plastid desaturases are mainly responsible for the production of polyunsaturated fatty acids. Individual fatty acid desaturases were differently susceptible to environmental stresses with FAD2 being reduced by both high temperature and low light, whereas FAD7 was only affected by high temperature.