Relationship between Tracheary Element Differentiation and DNA Synthesis in Single Cells Isolated from the Mesophyll of Zinnia elegans--Analysis by Inhibitors of DNA Synthesis

Effects of inhibitors of DNA synthesis on tracheary element(TE) differentiation were investigated in a culture of singlecells isolated from the mesophyll of Zinnia elegans L. cv. Canarybird. In this system, neither mitosis nor replication of thewhole genome during the S phase in the cell cycle is a prerequisitefor TE differentiation [Fukuda and Komamine (1980) Plant Physiol.65: 61, unpublished data]. Fluorouracil (FU), fluorodeoxyuridine(FUdR), mitomycin G (MC), arabinosyl cytosine (ara-C) and aphidicolin,inhibitors of DNA synthesis, prevented the incorporation of[³H]-thymidine into nucleic acid, cell division and cytodifferentiationto TE. However, neither FUdR nor aphidicolin prevented the incorporationof [14C]-leucine into protein. Thymidine reversed the inhibitoryeffect of FUdR when given simultaneously with FUdR. These resultsshow that the inhibitors of DNA synthesis prevent TE differentiationvia blockage of the synthesis of some DNA, although replicationof the whole genome during the S phase is not a prerequisitefor cytodifferentiation. The role of DNA synthesis in TE differentiationis discussed. (Received October 13, 1980; Accepted November 17, 1980)     

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.     

Effect of Inorganic Phosphate on the Biosynthesis of Purine and Pyrimidine Nucleotides in Suspension-Cultured Cells of Catharanthus roseus

The levels of purine and pyrimidine nucleotides in suspensioncultures of Catharanthus roseus were determined 24 h after stationary-phasecells were transferred to fresh complete (‘+Pi’)or phosphate-deficient (‘–Pi’) Murashige-Skoogmedium. The levels of ATP, GTP, UTP and CTP were from approx.3 to 5-fold greater in the cells grown in ‘+Pi’medium than in the cells grown in ‘–Pi’ medium.The levels of almost all other nucleotides were slightly higherin the cells in ‘+Pi’ medium. The rates of de novoand salvage biosynthesis of purine and pyrimidine nucleotideswere estimated from the rates of incorporation of radioactivityfrom [¹⁴C]formate, [2–¹⁴C]glycine, NaH¹⁴CO₃, [6–¹⁴C]orotate,[8–¹⁴C]adenine, [8–¹⁴C]adenosine, [2–¹⁴C]uraciland [2–¹⁴C]uridine. The results indicated that the activityof both the de novo and the salvage pathway was higher in thecells in ‘+Pi’ medium than in the cells in ‘–Pi’medium. The rate of degradation estimated from the rate of releaseof ¹⁴CO₂ from labelled purines and pyrimidines indicated thatdegradation of uridine was significantly reduced in the cellsin ‘+Pi’ medium, but no significant difference wasfound in the degradation of adenine, adenosine and uracil. Thepossible role of Pi in the control of the biosynthesis of nucleotidesand in the degradation of uridine is discussed. Catharanthus roseus, Madagascar periwinkle, suspension culture, inorganic phosphate, nucleotides, purines, pyrimidines, biosynthesis, degradation     

Inhibition of Cell Division and DNA Synthesis by Gibberellin in Isolated Zinnia Mesophyll Cells

A culture system of isolated mesophyll cells of Zinnia eleganswas used to examine the action of gibberellic acid (GA) on celldivision. Isolated Zinnia mesophyll cells cultured in a mediumcontaining auxin and cytokinin reinitiated cell division ina partly synchronized manner. When mesophyll cells isolatedfrom 21-day-old seedlings were used, GA added to the culturemedium at concentrations of 1 x 10^–6 M or higher suppressedthe initial rise in the number of divided cells. Tracer experimentswith [³H]-dThd revealed that GA treatment inhibited the incorporationof [³H]-dThd into DNA in the nucleus without inhibiting theuptake of [³H]-dThd into the cells, indicating that GA inhibitedDNA synthesis. GA applied at 48 h inhibited the incorporationof [³H]-dThd into DNA during the following 24 h, but GA appliedat 72 h did not inhibit the incorporation during the subsequent24 h. This suggests that GA affects the process of reinitiationof DNA synthesis, but does not affect DNA synthesis once cellshave become proliferative. (Received January 14, 1986; Accepted March 31, 1986)     

Biosynthesis of Xyloglucan in Suspension-Cultured Soybean Cells. Synthesis of Xyloglucan from UDP-Glucose and UDP-Xylose in the Cell-Free System

When UDP-[¹⁴C]glucose or UDP-[¹⁴C]xylose was incubated witha particulate fraction from soybean cells, radioactive polymerswere synthesized. On digestion with Aspergillus oryzae enzymes,these polymers gave ¹⁴C-monosaccharides and a ¹⁴C-disaccharidewith chromatographic and electrophoretic mobilities indistinguishablefrom those of authentic isoprimeverose (6-O--D-xylopyranosyl-D-glucopyranose).The disaccharide consisted of xylose and glucose, and the latterwas located at the reducing end. Evidence that the disaccharideis isoprimeverose was provided by methylation analysis. Hydrolysisof the methylated disaccharide yielded 2,3,4-tri-O-methyl-D-xyloseand 2,3,4-tri-O-methyl-D-glucose. Thus, incorporation of radioactivityinto isoprimeverose, the smallest structural unit of xyloglucan,suggests that xyloglucan is synthesized in vitro from UDP-glucoseand UDP-xylose. (Received November 20, 1980; Accepted February 14, 1981)     

Formation of L-(+)-Tartaric Acid in Leaves of the Bean, Phaseolus vulgaris L.: Radioisotopic Studies with Putative Precursors

The biosynthetic pathway from D-glucose to L-(+)-tartaric acid(TA) in detached leaves of the bean, Phaseolus vulgaris L.,was studied in three cultivars, two of which were known to containTA and one of which lacked TA, with the aid of several putativeradiolabeled intermediates, namely D-[l-¹⁴C]glucose, D-[6-¹⁴C]glucose,D-[U-¹⁴C]glucose, D-[U-¹⁴C]gluconate, L-[U-¹⁴C]-ascorbic acid,L-[l-^l4C]idonate, D-xylo-5-[U-¹⁴C]hexulosonate, D-xylo-5-[l-¹⁴C]hexulosonate,D-xylo-5-[6-^l4C]hexulosonate and L-[U-^l4C]threonate. D-[U-¹⁴C]Glucoseand D-[U-^l4C]gluconate were converted to TA with low isotopicyield but this yield was further reduced when leaf tissues weresupplied with unlabeled D-gluconate or D-xylo-5-hexulosonate.D-xylo-5-[U-¹⁴C]Hexulosonate and D-xylo-5-[l-¹⁴C]hexulosonatewere good precursors of TA. D-xylo-5-[6-¹⁴C]Hexulosonate didnot furnish ¹⁴C to TA. Addition of a metabolic product of D-xylo-5-hexulosonate(which was labeled by D-xylo-5-[l-¹⁴C]hexulosonate but not byD-xylo-5-[6-¹⁴C]hexulosonate) to leaves labeled with D-xylo-5-[l-¹⁴C]hexulosonatedoubled the incorporation of ¹⁴C into TA. L-[U-¹⁴C]Ascorbicacid, L-[l-¹⁴C]idonate and L-[U-¹⁴C]threonate failed to producelabeled TA. A metabolic scheme to accommodate these observationsis presented. (Received October 21, 1988; Accepted March 29, 1989)     

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.     

Biosynthesis of Resin Terpenes in Leaves and Glandular Hairs of Newcastelia viscida

Label from ¹⁴CO₂ and [2-¹⁴C]mevalonic acid was incorporatedinto resin components of Newcastelia viscida. About 25% of thelabel from [2-¹⁴C]MVA was incorporated into resin componentsafter 4 h. Approximately 80% of the label in the resin was ina TLC band coinciding with the major terpenoid components, with60% associated with the major tricyclic diterpene, a pimaradienewhich contributes 15% to the leaf resin dry weight. Experimentswith detached trichomes showed about 0?01% incorporation oflabel from [2-¹⁴C]MVA into pimaradiene, with most of the [abelbeing associated with ß-sitosterol. It is proposedthat the glandular hairs are the sites of terpene resin synthesis,as well as secretion, in this species.     

Adenine and Guanine Salvage in Suspension Cultured Cells of Catharanthus roseus

Changes in the activity of adenine and guanine salvage in nucleotideand nucleic acid synthesis during the growth of Catharanthusroseus were investigated. Incorporation of [8-¹⁴C]adenine intoATP and ADP and that of [8-¹⁴C]guanine into GTP and GDP increasedmarkedly in the lag phase of cell growth and then sharply decreased.The incorporation into RNA from both precursors showed a similarpattern. The role of rapid purine salvage observed in the lagphase of cell growth is discussed. Catharanthus roseus, Madagascar periwinkle, suspension culture cells, purine salvage, adenine, guanine     

Post-transcriptional Control of Nitrate Reductase Formation in Green Algae

Cycloheximide (2·0 µg ml^–1) inhibits theincorporation of [¹⁴C]phenylalanine and [¹⁴C]adenine into insolublecompounds in Ankistrodesmus braunii. 6-Methylpurine (1·0mM) inhibits only the incorporation of [14C]adenine and it isconcluded that it inhibits RNA synthesis. When ammonium-growncells of Ankistrodesmus or Chlorella are nitrogen-starved orwhen ammonium-grown cells of Dunalitlla are resuspended in nitratemedium, the appearance of nitrate reductase in these organismsis not inhibited by 6-methylpurine. The appearance of nitratereductase activity in Ankistrodesmus or Chlorella is inhibitedby 6-methylpurine when ammonium-grown organisms are preincubatedwith this substance for 1-2 h before nitrogen starvation. Itis concluded that cells growing with ammonium and lacking nitratereductase activity nevertheless contain preformed mRNA for nitratereductase synthesis.     

Effects of ABA on Synthesis of Cell-Wall Polysaccharides in Segments of Etiolated Squash Hypocotyl I. Changes in Incorporation of Glucose and myo-Inositol into Cell-Wall Components

Externally supplied [³H]myo-inositol and [¹⁴C]glucose were incorporatedin cell-wall fractions of segments of etiolated squash hypocotyl.The extent of incorporation of [¹⁴C]glucose into cell-wall fractionswas very much greater than that of [³H]myo-inositol. Radioactivityfrom [¹⁴C]-glucose was effectively incorporated into hemicelluloseB and cellulose fractions and was incorporated uniformly intohexose, pentose and uronic acid residues, but radioactivityfrom [³H]myo-inositol was incorporated predominantly into uronicacid and pentose residues in the pectin and hemicellulose Bfractions. Exogenously applied ABA significantly suppressed the elongationof segments of squash hypocotyl and the incorporation of radioactivityfrom [^l4C]glucose and [³H]myo-inositol into the segments. Furthermore,ABA significantly inhibited the distribution of incorporatedradioactivity from [¹⁴C]glucose into the cellulose fraction,but did not affect distribution into the pectic fraction. Bycontrast, ABA only slightly inhibited the distribution of theincorporated radioactivity from [³H]myo-inositol into the pecticfraction. These results suggest that most of the cell-wall polysaccharidesin segments of squash hypocotyl are synthesized via the UDP-sugarpathway, and that ABA significantly inhibits the synthesis ofcellulose but not the synthesis of pectic polysaccharides whenABA suppresses the elongation of the segments. (Received March 25, 1988; Accepted November 15, 1988)     

Metabolism of Pyrimidines in Protoplasts from Cultured Catharanthus roseus Cells

The metabolism of [2-¹⁴C]thymine, [2-¹⁴C]thymidine, [2-¹⁴C]uraciland [¹⁴C]uridine was investigated in protoplasts obtained fromsuspension cultures of Catharanthus roseus. Most of the exogenouslysupplied thymine, thymidine and uracil was degraded, and salvageof these pyrimidines accounted for 5–36 per cent of thetotal amount of ¹⁴C-labelled precursors which was metabolized.However, more than 80 per cent of the labelled uridine was utilizedfor the biosynthesis of nucleotides and nucleic acids, and therest was degraded. In contrast to the results from protoplastsof sugar cane cells in suspension culture, the data indicatethat protoplasts possess a pathway for the degradation of pyrimidines,and that the overall metabolism of these pyrimidines in protoplastsis very similar to the metabolism in the intact cells. Catharanthus roseus, madagascar periwinkle, protoplasts, pyrimidine metabolism     

Alterations in Protein Synthesis in vivo in Chilling Sensitive Mung Bean Hypocotyls Caused by Chilling Stress

The effects of chilling on protein synthesis in vivo in etiolatedhypocotyls of Vigna radiata L. were investigated. After exposureof the tissues to 0?C for various periods of time, proteinswere labeled with [³⁵S]-methionine at 26?C. The total amountof ³⁵S incorporated into soluble and membrane proteins was reversiblyreduced by chilling for 24 h, during which time the tissuessuffered no injury. Further prolonged chilling produced an irreversibledecline both in the incorporation of radioactivity and in cellviability as assessed by the extent of leakage of electrolyte.The ³⁵S-labeled proteins in the soluble and the total membranefractions were analyzed quantitatively. Chilling of etiolatedhypocotyls for one or two days induced the syntheses of twosoluble proteins (82 and 74 kDa) and one membrane protein (80kDa). Moreover, three heat-shock proteins (HSPs) that were inducedby heat stress (41 ?C, 4h) had the same electrophoretic mobilitiesas those of the proteins induced directly or indirectly by thechilling treatment. ¹Contribution No. 3170 from the Institute of Low TemperatureScience. (Received April 22, 1988; Accepted September 30, 1988)     

Biosynthesis of Caffeine in Flower Buds of Camellia sinensis

The biosynthesis of purine alkaloids in flower buds of tea plantswas investigated. More than 25% of total radioactivity of [8-¹⁴C]adeninetaken up by stamens isolated from tea flower buds was foundto have been incorporated into purine alkaloids, namely, theobromineand caffeine, 24 h after administration of the labelled compound.Pulse-chase experiments indicated that [8-¹⁴C]adenine takenup by the stamens was converted to adenine nucleotides and subsequentlyincorporated into theobromine and caffeine. Since 5 µMcoformycin, an inhibitor of AMP deaminase, inhibited the incorporationof radioactivity into the purine alkaloids, synthesis of caffeinefrom adenine nucleotides seems to be initiated by the reactionof AMP deaminase. Although most of the radioactivity from [8-¹⁴C]inosinewas recovered as CO₂ and ureides, considerable amounts of radioactivitywere recovered as purine alkaloids. The incorporation of radioactivityfrom [8-¹⁴C]inosine into the purine alkaloids was not affectedby coformycin. The five enzymes involved in synthesis of 5-phosphoribosyl-1-pyrophosphatefrom glucose were present in the stamens and petals of tea flowerbuds. From present and previous results, the pathway for thebiosynthesis of caffeine from adenine nucleotides in flowerbuds of tea is discussed.Copyright 1993, 1999 Academic Press Camellia sinensis, tea, stamen, flower, biosynthesis, purine alkaloids, caffeine, theobromine, adenine nucleotides, nucleotide biosynthesis     

Cell Wall Metabolism in Developing Strawberry Fruits

Cell wall metabolism was studied in strawberry receptacles (Fragariaananassa, Duchesne) of known age in relation to petal fall (PF).Polysaccharide and protein composition, incorporation of [¹⁴C]glucoseand [¹⁴C]proline by excised tissue, and the fate of ¹⁴CO₂ fixedby young, attached fruits were followed in relation to celldivision, cell expansion, fine structure, and ethylene synthesis. Cell division continued for about 7 d after PF although vacuolationof cells was already beginning at PF and the subsequent cellexpansion was logarithmic. There was an associated logarithmicincrease in sugar content per cell and a decreasing rate ofethylene production per unit fresh weight. During cell expansion radioactivity from [¹⁴C]glucose was incorporatedinto fractions identified as starch and soluble polyuronideand into glucose and galactose residues in the cell wall. Radioactivityfrom [¹⁴C]proline was also incorporated into the cell wall,but only 10 per cent of this activity was found in hydroxyproline.Correspondingly wall protein contained a low proportion of hydroxyprolineresidues. The proportion of radioactivity from ¹⁴CO₂ fixed byfruitlets remained constant in most sugar residues in the cellwall. The proportion of radioactivity in galactose fell, indicatingturnover of these residues. Between 21 and 28 d after PF receptacles became red and softenedbut there was no change in the rate of ethylene production.Cell expansion continued for at least 28 d. Tubular proliferationof the tonoplast and hydration of middle lamella and wall matrixmaterial had begun 7–14 d after PF but became extremeduring ripening. Associated with the hydration of the wall,over 70 per cent of the polyuronide in the wall became freelysoluble, and arabinose and galactose residues lost from thewall appeared in soluble fractions. There was no increase intotal polysaccharide during ripening and incorporation of [¹⁴C]glucoseinto polysaccharides ceased, although protein increased andincorporation of [¹⁴C]proline into wall protein continued.     

Effects of different vertebrate growth factors on primary cultures of hemocytes from the gastropod mollusc,Haliotis tuberculata

Summary— A useful experimental system from primary cultures of hemocytes from Haliotis tuberculata has been established. Six days after initiation of the culture, the viability of hemocytes remained constant as measured by the MTT assay. In addition, hemocytes showed physiological responses as judged by protein and DNA syntheses in response to treatment with vertebrate growth factors. Porcine insulin and human epidermal growth factor (EGF) stimulated [³H]-leucine and [³H]-thymidine incorporation in hemocytes in a dose-dependent manner. No additive effect of insulin and EGF is observed either for [³H]-leucine or for [³H]-thymidine incorporation. The response of primary cultures of abalone hemocytes to vertebrate growth factors confirms their growth potential in vitro and provides a suitable model for further studies on regulation of the control of cellular processes such as cell growth, differentiation and migration in invertebrate cells.     

Metabolism and Biosynthesis of Cytokinins in Tobacco Crown Gall Cells

The metabolism of ribosylzeatin (RZ) was studied using tobaccocrown gall cells which produce RZ as one of the major endogenouscytokinins. When [8-¹⁴C]RZ was fed to the cells, it was convertedinto its phosphate (which was rigorously determined to be the5'-monophosphate), RZ-O-glucoside, inosine (or its phosphate),adenosine and adenosine-O-glucoside. When [8-¹⁴C]N⁶-(²-isopentenyl)adenosine(i⁶Ado), a probable precursor of RZ, was fed to the cells, itwas converted into (i⁶Ado)-O-glucoside, inosine (or its phosphate),adenosine, adenosine-O-glucoside and adenosine phosphate, butno incorporation of radioactivity into RZ was observed. Thepresent study led to the following conclusions: i) i⁶Ado isnot a precursor of RZ in the cells, ii) both deaminase and cytokininoxidase are involved in the catabolism of cytokinin, and iii)the metabolism of RZ is quite different from that of i⁶Ado. (Received December 24, 1985; Accepted April 1, 1986)     

Suppression by 5-Bromo-2'-deoxyuridine of Transdifferentiation into Tracheary Elements of Isolated Mesophyll Cells of Zinnia elegans

5-Bromo-2'-deoxyuridine (BrdU), a thymidine analog, suppressedthe transdifferentiation into tracheary elements (TE) of isolatedmesophyll cells of Zinnia elegans without affecting cell division.Tracer experiments with [³H]BrdU indicated that 76% and 24%of the incorporated radioactivity was located in the DNA andthe RNA, respectively. Both thymidine and uridine counteractedthe inhibitory effect of BrdU on transdifferentiation but thymidinewas much more effective than uridine. These results suggestthat BrdU might interfere with transdifferentiation via itsincorporation into DNA. The timing of effective suppressionby BrdU was examined by monitoring the effects of the sequentialaddition of BrdU and thymidine with an interval of 12 h at varioustimes in culture. Transdifferentiation was most sensitive toBrdU between the 24th and the 36th hour of culture. This resultsuggests that this window of time is critical for DNA synthesisduring the transdifferentiation of isolated mesophyll cellsof Zinnia elegans into TE. ³Present address: Department of Chemical and Biological Sciences,Faculty of Science, Japan Women's University, Mejiro, Tokyo,112 Japan     

Changes in Some Enzyme Activities and Respiration in the Early Stage of Callus Formation in a Carrot-root Tissue Culture

The respiratory rate increased in two phases during early stages of callus formation in carrot (Daucus carota)-root phloem slices cultured in vitro, showing separate peaks at about 24 and 96 h of culture. In the first phase (within 24 h of culture), the activities of phosphoglucose isomerase, phosphofructokinase and succinate dehydrogenase did not increase, but active syntheses of RNA and protein, indicated by experiments with incorporation of [¹⁴C]-uracil and -leucine, resulted in an active turnover of ATP, to which the first increase in the respiratory rate may be attributable. On the other hand, the activity of phosphoribosylpyrophosphate synthetase, an enzyme related to nucleic acid synthesis, increased in the first 24 h of culture. In the second phase (24–96 h of culture), the activities of the respiratory enzymes investigated increased. This increase was repressed by cycloheximide, indicating de novo syntheses of the respiratory enzymes during this time, which may result in an enlargement of the respiratory capacity, to which the second increase in respiration may be mainly attributable. In the first phase, exogenous supplied 2,4-dichlorophenoxyacetic acid had little or no effect on the respiratory rate and the activities of the respiratory enzymes, but it enhanced synthesis of RNA and the activity of phosphoribosylpyrophosphate synthetase. In the second phase, it increased all the activities of enzymes investigated as well as the respiratory rate and RNA synthesis.     

Radiolabelling Studies of Lipids in the Marine Cryptomonad Chroomonas salina in Relation to Fatty Acid Desaturation

Cells of Chroomonas salina were exposed to [¹⁴C]acetate, [^l4C]16:0,[¹⁴C]18:0, [¹⁴C]18:1(n-9), [¹⁴C]18:2(n–6) or [¹⁴C]18:3(n–3)for 1 h and then incubated for 24 h in non-radioactive medium.At the end of the pulse period, non-glycolipid polar lipidscontained the highest proportions of radioactivity incorporatedfrom [¹⁴C]acetate and [¹⁴C]18:3(n–3) whereas with [¹⁴C]16:0,[¹⁴C]18:1 and [¹⁴C]18:2(n–6), triacylglycerols were mosthighly labelled. ¹⁴C-18:0 was recovered mainly as non-esterifiedfatty acid. Monogalactosyldiacylglycerol initially contained17% of label incorporated from [¹⁴C]acetate but less than 3%of that from [¹⁴C]fatty acids. With all substrates, excluding[¹⁴C]18:0, a gradual transfer of label from polar lipids totriacylglycerols was observed during the chase period. Saturatesand monoenes synthesised from [¹⁴C]acetate were mostly transferedfrom phospholipids and glycolipids to neutral lipid withoutfurther desaturation. Most of the incorporated ¹⁴C-fatty acidsremained unchanged and only with [¹⁴C]18:3(n–3) was substantialamounts of label recovered in penta- and hexaenoic fatty acids.The results indicate that, under the conditions of the study,lipid synthesis in the algae was heavily dominated by triacylglycerolformation and that the mechanisms of fatty acid desaturationin this species may differ from those in higher plants. (Received December 10, 1991; Accepted March 6, 1992)