Cholesterol synthesis in rat intestine: effect of fasting and of porphyrogenic chemical allylisopropylacetamide

(a) Administration of allylisopropylacetamide to fasting rats stimulates intestinal sterol synthesis as measured by incorporation of ¹⁴C from [1-¹⁴C]sodium acetate. Stimulatory effect of AIA is confined to the acetate to mevalonate segment of cholesterol biosynthetic pathway.(b) It is also shown that the suppression of sterol synthesis in the ileum of intact rats produced by fasting is of the same order of magnitude as that observed for liver sterol synthesis due to fasting.     

Sterol and sesquiterpenoid biosynthesis during a growth cycle of tobacco cell suspension cultures

The accumulation and biosynthesis of sterols and fungal elicitor-inducible sesquiterpenoids by tobacco (Nicotiana tabacum) cell suspension cultures were examined as a function of a 10 day culture cycle. Sterols accumulated concomitantly with fresh weight gain. The rate of sterol biosynthesis, measured as the incorporation rate of [¹⁴C]acetate and [³H]mevalonate, was maximal when the cultures entered into their rapid phase of growth. Changes in squalene synthetase enzyme activity correlated more closely with thein vivo synthesis rate and accumulation of sterols than 3-hydroxy-3-methylglutaryl CoA reductase (HMGR) enzyme activity. Cell cultures entering into the rapid phase of growth also responded maximally to fungal elicitor as measured by the production of capsidiol, an extracellular sesquiterpenoid. However, the rate of sesquiterpenoid biosynthesis, measured as the incorporation rate of [¹⁴C]acetate and [³H]mevalonate, could not be correlated with elicitor-inducible HMGR or sesquiterpene cyclase enzyme activities, nor elicitor-suppressible squalene synthetase enzyme activity.Abbreviations FPP farnesyl diphosphate - HMGR 3-hydroxy-3-methylglutaryl coenzyme A reductase     

Phenotypic Effects of Isolated pRiA4 TL-DNA rol Genes in the Presence of Intact TR-DNA in Transgenic Plants of Solanum dulcamara L.

The effect of the rol genes, together with the TR-DNA of pRiA4on the phenotype of Solanum dulcamara plants, was analysed.Plants transformed by Agrobacterium strain BN1010: :rolA (rolA^7plus;TR⁺)exhibited severe leaf wrinkling, whereas plants transformedby strain BN1010: :rolC (ro/C⁺TR⁺) had a typical ‘hairyroot’ phenotype. Leaf discs excised from these latterplants produced roots on hormone-free medium. BN1010: :rolABC(rolABC⁺TR⁺) transformed plants had an exaggerated transformedphenotype. Some of the BN1010: :rolABC transformants had positivelygeotropic root growth which correlated with the presence ofmultiple copies of the TR-DNA. S. dulcamara plants, transformedby the TR-DNA region only, exhibited epinasty. Scanning electronmicroscopy of plants containing various regions of agropineRi T-DNA revealed that transformation causes changes in basicplant siructure.     

UDP-Glucose: Sterol Glucosyltransferase and a Steryl Glucoside Acyltransferase Activity in Amyloplast Membranes from Potato Tubers

Envelope membranes were isolated from potato tuber amyloplastby a discontinuous sucrose density gradient and high speed centrifugation.These membranes catalyzed the transfer of [¹⁴C]glucose fromUDP-[¹⁴C]glucose to endogenous sterol acceptors and, in turn,catalyzed the esterification of steryl glucosides with fattyacids from an endogenous acyl donor. The synthesis of sterylglucosides was stimulated in the presence of Triton. X-100,while formation of acyl steryl glucosides was inhibited by thedetergent. However, in the presence of an added sterol acceptorand Triton X-100, the inhibition of acyl steryl glucoside synthesiswas overcome by the addition of phosphatidylethanolamine. Theenzyme involved in steryl glucoside formation was solubilizedby treatment of the envelope membranes with 0.3% Triton X-100.The solubilized enzyme had an almost absolute requirement forsterol acceptors. Key words: Solanum tuberosum, Sterol glucosylation, Steryl glucoside acylation, Amyloplast membrane     

Evidence for negative feedback control of cholesterogenesis from mevalonate in liver: Absence in the intestine of guinea pigs fed A 0,5 % cholesterol diet

The in vitro rate of incorporation of [2-¹⁴C]-acetate and [2-¹⁴C]-mevalonate into cholesterol of liver, ileum and caecum was determined in guinea pigs. In control animals, contrary to the situation observed when acetate was used as precursor, the rate of conversion of mevalonate to cholesterol was higher in liver than in intestine. In this latter tissue, the cholesterogenesis varied depending on the portion tested. The distribution of radiolabel derived from mevalonate between esterified and unesterified cholesterol differed among the various tissues. In cholesterol-fed guinea pigs, the plasma, liver, intestine and aorta cholesterol contents increased significantly. In addition, a negative feedback control existed for hepatic cholesterol synthesis for mevalonate and acetate. This control was absent in intestinal tissues.     

Induction of sesquiterpene cyclase and suppression of squalene synthetase activities in plant cell cultures treated with fungal elicitor

Addition of elicitor, cell wall fragments of the fungus Phytophthora parasitica, to tobacco cell suspension cultures (Nicotiana tabacum) resulted in the rapid synthesis and secretion of large amounts of antibiotic sesquiterpenoids. Pulse-labeling experiments with [¹⁴C]acetate and [³H] mevalonate demonstrated that the induction of sesquiterpenoid biosynthesis, maximal by 6 to 9 hours after elicitor addition to the cell cultures, was paralleled by a rapid and large decline in the incorporation rate of radioactivity into sterols. Consequently, sterol accumulation was also inhibited upon addition of elicitor to the cell cultures. Sesquiterpene cyclase activity was absent from control cell cultures but induced to a maximum within 10 hours of elicitor addition to the cell cultures. The cyclase activity remained elevated for an additional 30 hours before declining. In contrast, squalene synthetase activity was suppressed to less than 15% of that found in control cells within 7 hours of elicitor addition. Our results suggest that the channeling of isoprenoid intermediates, and especially farnesyl diphosphate, into sesquiterpenoids occurred by a coordinated increase in the sesquiterpene cyclase and a decrease in the squalene synthetase enzyme activities. A reexamination of the data pertaining to the transient induction of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity (EC 1.1.1.34) in elicitor-treated cells suggested that, while the reductase activity was necessary for sesquiterpenoid biosynthesis, it functioned more to maintain a sufficient level of intermediates between mevalonate and farnesyl diphosphate rather than as a rate limiting step controlling the synthesis rate of any one class of isoprenoids.     

Use of a Disarmed Ri Plasmid Vector in Analysis of Transformed Root Induction

Nicotiana glauca, N. tabacum, Solanian dulcamara and S. nigrumwere transformed by Agrobacteriun rhizogenes strain BN1010 (TL^–TR⁺).The TR-DNA stimulated agropine-positive root induction and wastransformation competent in the absence of the TL-DNA. An unusualpattern of root induction was seen when stem explants were inoculatedwith this strain; occasionally, agropine-positive roots wereinduced at the inoculation sites, but prolific agropine-negativeroots were formed in profusion down the stems. The utility ofBN1010 as an efficient co-integrating vector was demonstratedby the separate transfer of a fragment containing rol ABC (BN1010::pEM15) and of a chimeric nopaline synthase-kanamycin resistancegene (BN1010:: Neo) into plants. Root cultures of S. dulcamaratransformed with BN1010:: Neo had an unusual, positively geotropicphenotype. Strain BN1010:: pEM15 (rol ABC⁺D^–TR⁺) incitedmore roots down stem explants than strain A4T. This indicatesthat rol D may act to suppress agropine-negative root productionin N. glauca and N. tabacum. Key words: Agrobacterium rhizogenes, TL-DNA, TR-DNA, disarmed Ri vector, transformed roots, Nicotiana glauca, N. tabacun, Solatium dulcamara, S. nigrum     

Glycollate Formation during the Photorespiration of Acetate by Chlorella

WhenChlorella pyrenoidosa photoassimilates ³H-¹⁴C-acetate theglycollic acid formed shows a high ³H/¹⁴C ratio, the only othercompounds showing similar ratios being glycerate and serine.The ³H/¹⁴C ratio of glycollate was unaffected by the TCA cycleinhibitors MFA, diethylmalonate and arsenite showing that ³Hin glycollate does not result from the oxidation of acetatevia the TCA cycle, the resulting NADP³H₂ or NAD³H₂ being usedfor the reduction of the glycollate precursor. Although DCMUdecreased the ³H/¹⁴C ratio, complete inhibition of glycollatelabelling was not observed with 10^–6 M DCMU, at whichconcentration complete inhibition of the Hill reaction is achieved.Although the ³H/¹⁴C ratio was unaltered, total dpm of both ¹⁴Cand ³H in glycollate were increased by INH. The ³H/¹⁴C ratiosof glycerate and serine were decreased by INH, as were the totaldpm of ³H and ¹⁴C incorporated into these compounds. Thus, INHinhibits the further metabolism of glycollate to glycerate andserine. The effect of INH on incorporation of ¹⁴C-I-acetateinto various cell fractions was investigated. The incorporationof ¹⁴C into polysaccharide and lipid was decreased, while theincorporation of ¹⁴C into the water-soluble fraction of cellsand therelease of ¹⁴CO₂ were little affected. Although glycollicacid was an early product of acetate photoassimilation in Chlorellapyrenoidosa, glycollate excretion does not take place undera wide range of environmental conditions shown to favour glycollateexcretion by other algae. However, small amounts of labelledglycollate were detected in the supernatant from the cells duringthe photoassimilation of ³H-¹⁴C-acetate, but this glycollatedid not show the high ³H/¹⁴C ratio of glycollate present withinthe cell. The failure of Chlorella pyrenoidosa to excrete appreciableamounts of glycollate when photoassimilating acetate or carbondioxide was considered to result from the presence of glycollateoxidase (EC 1.1.3.1 [EC] ) which allowed the further metabolism ofglycollate. Besides glycollate oxidase, glyoxylate reductasewas also demonstrated in Chlorella pyrenoidosa so that glycollatecould function in hydrogen transfer during the photoassimilationof acetate.     

Cholesterol biosynthesis by the cornea. Comparison of rates of sterol synthesis with accumulation during early development

The origin of the cholesterol needed by the cornea for growth and cell turnover was addressed by comparing absolute rates of sterol synthesis with rates of sterol accumulation during early development of the rabbit. Linearity of incorporation of 3H2O and [14C]mevalonate into digitonin-precipitable sterols with time of incubation in vitro and a lack of accumulation of 14C in intermediates of sterol biosynthesis indicated that tritiated water can validly be used to measure rates of sterol synthesis by the cornea. The rate of sterol synthesis per unit weight of rabbit cornea was constant between 14 and 60 days of age at an average 1.03 nmol of 3H of 3H2O incorporated/mg dry cornea per 8 h. Essentially all of the synthesized cholesterol and most of the cholesterol mass was present in corneal epithelium. The cumulative sterol synthesized over the 46-day period studied exceeded the observed rate of cholesterol accumulation by sixfold. Cholesterol synthesized in excess of the growth requirement was likely used to support turnover of the epithelium which was estimated at 9 days. Removal of cholesterol from the cornea by excretion into tear fluid and clearance by high density lipoproteins are also considered.     

Role of mevalonate in regulation of cholesterol synthesis and 3-hydroxy-3-methylglutaryl coenzyme A reductase in cultured cells and their cytoplasts

H4-II-E-C3 hepatoma cells in culture respond to lipid-depleted media and to mevinolin with increased sterol synthesis from [14C]acetate and rise of 3-hydroxy-3-methylglutaryl coenzyme A reductase levels. Mevalonate at 4 mM concentration represses sterol synthesis and the reductase, and completely abolishes the effects of mevinolin. Mevalonate has little or no effect on sterol synthesis or reductase in enucleated hepatoma cells (cytoplasts) or on reductase in cytoplasts of cultured Chinese hamster ovary (CHO) cells. The sterol-synthesizing system of hepatoma cell cytoplasts and the reductase in the cytoplasts of CHO cells were completely stable for at least 4 hr. While reductase levels and sterol synthesis from acetate followed parallel courses, the effects on sterol synthesis--both increases and decreases--exceeded those on reductase. In vitro translation of hepatoma cell poly(A)+RNAs under various culture conditions gave an immunoprecipitable polypeptide with a mass of 97,000 daltons. The poly(A)+RNA from cells exposed for 24 hr to lipid-depleted media plus mevinolin (1 microgram/ml) contained 2.8 to 3.6 times more reductase-specific mRNA than that of cells kept in full-growth medium, or cells exposed to lipid-depleted media plus mevinolin plus mevalonate. Northern blot hybridization of H4 cell poly(A)+RNAs with [32P]cDNA to the reductase of CHO cells gave two 32P-labeled bands of 4.6 and 4.2 K-bases of relative intensities 1.0, 0.61-1.1, 2.56, and 1.79 from cells kept, respectively, in full-growth medium, lipid-depleted medium plus mevinolin plus mevalonate, lipid-depleted medium plus mevinolin, and lipid-depleted medium. These values approximate the reductase levels of these cells. We conclude that mevalonate suppresses cholesterol biosynthesis in part by being a source of a product that decreases the level of reductase-specific mRNA.     

Transformation of 5 alpha-cholest-7-en-3 beta-ol to cholesterol and cholestanol in cerebrotendinous xanthomatosis

The metabolism of Delta(7)-cholestenol, cholesterol, and cholestanol was examined in a patient with cerebrotendinous xanthomatosis after intravenous pulse-labeling with a mixture of dl-[2-(14)C]mevalonate and stereospecific 3S,4S,3R,4R-[4-(3)H]mevalonate. Silver nitrate and reversed-phase thin-layer chromatography were used to purify the sterols isolated from the feces, and their identities were confirmed by gas-liquid chromatography-mass spectrometry. The specific activities were determined and plotted as a function of time. Isotope ratio measurements and specific activity decay curves showed that sterol synthesis proceeded in the following sequence: mevalonate, squalene, lanosterol, Delta(7)-cholestenol, cholesterol, cholestanol. Labeled cholesterol precursors might be advantageously used to measure changes in cholesterol synthesis because they appear to equilibrate rapidly and have very short turnover times.     

Transformation and Characterization of Transgenic Plants of Solanum dulcamara L.--Incidence of Transgene Silencing

A transformation system is described for Solanum dulcamara usingthe supervirulentAgrobacterium tumefaciens strain 1065, carryingboth the ß-glucuronidase (gus) and neomycin phosphotransferaseII (npt II) genes adjacent to the right and left T-DNA borders,respectively. Leaf explants were more efficient for the productionof transformed plants compared to stem explants on medium containing50 mg l^-1of kanamycin sulphate. A 1:10 (v:v) dilution of anovernight culture ofAgrobacterium gave optimal transformationin terms of transgenic plant regeneration. From a total of 174kanamycin-resistant plants selected by their antibiotic resistance,16 failed to exhibit GUS activity. Southern analysis revealedthat these GUS-negative transformants originated from threeindependently transformed cell lines. Restriction enzyme analysesshowed that the GUS-negative plants had both the gus and nptII genes integrated into their genome (one plant had a singlecopy of each gene; the other two plants had multiple copies),with major rearrangement of the gus gene occurring in plantswith several copies of the transgene. GUS-negative plants showedleaf malformations, delayed flowering and a reduction in flower,fruit and seed production compared to GUS-positive and non-transformed(control) plants. Although gene silencing of the gus gene occurred,albeit at a low frequency (9.2%), the transformation systemdescribed generates large numbers of phenotypically normal,stably transformed plants. Copyright 2000 Annals of Botany Company Agrobacterium -mediated transformation, gene silencing, Solanum dulcamara L. (Bittersweet, Woody Nightshade), T-DNA truncation, transgene expression     

Effects of peroxisome proliferators gemfibrozil and clofibrate on syntheses of dolichol and cholesterol in rat liver

The effects of two peroxisome proliferators, gemfibrozil and clofibrate, on syntheses of dolichol and cholesterol in rat liver were investigated. Gemfibrozil did not affect the overall content of dolichyl phosphate, but it changed the chain-length distribution of dolichyl phosphate, increasing the levels of species with shorter isoprene units. Gemfibrozil suppressed synthesis of dolichyl phosphate from [(3)H]mevalonate and [(3)H]farnesyl pyrophosphate in rat liver. In contrast, clofibrate increased the content of dolichol (free and acyl ester forms). It remarkably enhanced dolichol synthesis from mevalonate, but did not affect dolichol synthesis from farnesyl pyrophosphate. Gemfibrozil elevated cholesterol synthesis from [(14)C]acetate, but did not affect the synthesis from mevalonate. Clofibrate suppressed cholesterol synthesis from acetate, but did not affect cholesterol synthesis from mevalonate. These results suggest that gemfibrozil suppresses synthesis of dolichyl phosphate by inhibiting, at the least, the pathway from farnesyl pyrophosphate to dolichyl phosphate. As a result, the chain-length pattern of dolichyl phosphate may show an increase in shorter isoprene units. Clofibrate may increase the content of dolichol by enhancing dolichol synthesis from mevalonate. Gemfibrozil may increase cholesterol synthesis by activating the pathway from acetate to mevalonate. Unlike gemfibrozil, clofibrate may decrease cholesterol synthesis by inhibiting the pathway from acetate to mevalonate.     

The activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase and the rate of sterol synthesis diminish in cultures with high cell density

The specific activity of HMG-CoA reductase, the major rate-limiting enzyme in the sterol biosynthetic pathway, declined linearly with increasing cell density in four different lines of mammalian cell cultures. As expected, this caused the rates of sterol synthesis from [14C]acetate to decline in a parallel manner. The decrease in reductase activity in the dense cultures was also correlated with decreased incorporation of [14C]acetate into fatty acids and [3H]thymidine into DNA. In contrast, the activities of two enzymes, NADH dehydrogenase and 5'-nucleotidase, which are not involved in lipid synthesis, were independent of changes in cell density. The simplest explanation for these data is tht HMG-CoA reductase and the synthesis of sterol and fatty acids are regulated in concordance with the rate of cell growth and proliferation.     

Chloroquine inhibits cyclization of squalene oxide to lanosterol in mammalian cells

Chloroquine inhibits the incorporation of [14C]acetate into sterols at a concentration of 10 microM or more in mouse L cells but has no effect on fatty acid synthesis and CO2 production from the same substrate even at a 10-fold higher concentration of the drug. The site of inhibition is distal to the formation of mevalonate since chloroquine also inhibits [14C]mevalonate metabolism to sterols and does not decrease the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34) or the incorporation of [14C]acetate into the total nonsaponifiable lipids. Analyses by thin layer and high pressure liquid chromatography of the nonsaponifiable lipid fraction from cultures incubated with chloroquine show an accumulation of radioactivity in the region of squalene oxide. Identification of the radiolabeled lipid as squalene oxide has been established by: (a) its co-migration with the authentic squalene oxide standard; (b) its conversion into squalene glycol by acid hydrolysis; and (c) its further metabolism to desmosterol when chloroquine is removed from the medium. Addition of chloroquine (12.5-50 microM) to 20,000 X g supernatant fractions of mouse liver homogenates inhibits the incorporation of [14C]mevalonolactone into cholesterol and lanosterol, with corresponding increases of [14C]squalene oxides, in a concentration-dependent manner. It appears, therefore, that chloroquine inhibits the enzymatic step catalyzed by 2,3-oxidosqualene-lanosterol cyclase (EC 5.4.99.7). Incubation of cell cultures with chloroquine (50 microM) arrests cell growth and causes cell death after 1-3 days. However, simultaneous incubation of chloroquine with either cholesterol or lanosterol prevents cell death and permits cell growth. Uptake of chloroquine is not affected by exogenous sterols since intracellular chloroquine concentrations are the same in cells grown with or without added sterols. The cytotoxicity of chloroquine, under our experimental conditions, must, therefore, be due primarily to its inhibition of sterol synthesis. In addition to its well known effect on protein catabolism, chloroquine has been found to inhibit protein synthesis. The significance of these findings concerning the use of chloroquine in studying the regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity is discussed.     

Allocation of Photosynthate to Individual Tubers of Solanum tuberosum L.: I. RELATIONSHIP BETWEEN TUBER GROWTH RATE AND ENZYME ACTIVITIES OF THE STARCH METABOLISM

Potato plants (Solanum tuberosum L.) were grown in water culturein a controlled environment. Cooling (+8°C) of individualtubers decreased their growth rates and increased the growthrates of non-cooled tubers of the same plant. The carbohydrateconcentration in non-cooled and cooled tubers did not differsignificantly, but ¹⁴C-import from labelled photosynthate waslower in cooled than in non-cooled tubers. The markedly lowerconversion rate of ethanol-soluble ¹⁴C to starch in cooled,in comparison to non-cooled tubers, was not associated withsignificant differences in the in vitro activities of starchsynthase, ADPG-pyrophosphorylase and starch phosphorylase understandard assay conditions (+30°C). However, the Q₁₀-valuesof the enzymes differed in vitro in the temperature range between30°C and 8°C, leading to a marked decrease in the activityratio of ADPG-pyrophosphorylase/starch phosphorylase in cooledtubers. In tubers differing in growth rates without manipulation, 14d after tuber initiation significant positive correlations werefound between ¹⁴C-concentration of tuber tissue and the in vitroactivities of starch synthase and ADPG-pyrophosphorylase anda significant negative correlation between ¹⁴C-concentrationand starch phosphorylase. In contrast, in tubers which wereanalysed 5 d after initiation, there were only small differencesbetween tubers in growth rate, ¹⁴C import and the activity ratioADPG-pyrophosphorylase/starch phosphorylase. From various directand indirect evidence it is concluded that the growth rate ofindividual tubers, and thus the sink strength, is at least inpart controlled by the activity of starch synthesizing enzymes. Key words: Potato tuber, cooling, starch synthesizing enzymes     

Enhanced sterol synthesis in concanavalin A-stimulated lymphocytes: correlation with phospholipid synthesis and DNA synthesis.

Incorporation of (14C)choline and (3H)myo-inositol into the total lipid fraction, incorporation of (14C)acetate into the sterol fraction and incorporation of (3H)thymidine into DNA were studied in human lymphocyte cultures. Concanavalin A induced an increase in the incorporation of these labels with the following features: (a) Phospholipid synthesis was increased promptly. The lag time for the increase in sterol synthesis and DNA synthesis were 5 hours and 27 hours respectively; (b) The increase in phospholipid synthesis and sterol synthesis was proportional to ConA concentration initially. Cells treated with a high concentration of ConA showed very low levels of DNA synthesis; (c) The increase in phospholipid synthesis could be abolished immediately by alpha-Methyl-Mannoside. alpha-Methyl-Mannoside blunted but did not abolish the increase in sterol synthesis. alpha-Methyl-Mannoside enhanced DNA synthesis of those cells which had been treated by a high concentration of ConA; and (d) Selective inhibition of sterol synthesis with 25-hydroxycholesterol did not prevent the increase in phospholipid synthesis, but it blocked the increase in DNA synthesis. Supplement of LDL, HDL or total lipoproteins to lymphocyte cultures was effective in preventing the inhibition of DNA synthesis by 25-hydroxy-cholesterol. These results suggest that in lymphocyte activation by ConA phospholipid synthesis, sterol synthesis and DNA synthesis were sequentially increased. The rate of cellular commitment to mitogenesis was proportional to ConA concentrations. High concentrations of ConA arrested the cell growth at a postcommitment point in the G1 phase. Enhanced phospholipid synthesis was a precommitment event. Enhanced sterol synthesis was a postcommitment event and reflected the requirement of an increased cholesterol supply for the passage of cell growth through G1.     

Factors Affecting the Biosynthesis of Abscisic Acid

Incorporation of labelled mevalonate into abscisic acid (ABA)has been demonstrated in the cotyledons of mature avocado seeds,embryos and endosperms of developing wheat seeds, and avocadostems. The increase in ABA concentration on wilting parallelsthe increased incorporation of [2^–14C)mevalonate intoABA in avocado leaves and stems, suggesting that the increasein ABA content occurs by synthesis rather than by release froma stored precursor. Incorporation of [2^–14C]mevalonateby avocado mesocarp segments is unaffected by an 18 per centwater loss. The ABA content of roots was hardly affected bya 30 per cent water loss, indicating that the wilt-activatedmechanism is not fully operative in these tissues. Submerged Ceratophyllum plants and submerged parts of Callitricheshoots show a twofold increase in ABA content on wilting whereasthe aerial rosettes of the latter plant show a sixfold increase.This suggests that the occurrence of the wilt-induced mechanismis affected by previous growth conditions as well as by themorphology of the tissue.     

Effects of Temperature and Photoperiod on Assimilate Partitioning in Potato Plants

The effects of temperature and photoperiod on d. wt partitioningand ¹⁴C translocation were studied in three potato varieties.High temperatures and long days enhanced plant growth in termsof plant height and number of leaves, and also affected d. wtpartitioning between the plant organs. However, no temperatureeffect was noted on total plant d. wt, nor on the export of¹⁴C from the source leaf. Translocation of ¹⁴C to the vegetativeorgans (leaves and stems) was greater at higher temperatures,while translocation to the tubers was less under these conditions.We suggest that, under the temperature regimes studied, themain effect of high temperature is on assimilate partitioningand not on total plant productivity. Differences in responseto high temperatures were observed among varieties, with Norchipshowing the least and Up-to-Date showing the most sensitivity. High temperature, partitioning of assimilates, ¹⁴C-translocation, potato, Solanum tuberosum var. Desirèe, Solanum tuberosum var. Norchip, Solanum tuberosum var. Up-to-Date     

Electroporation Stimulates Plant Regeneration from Protoplasts of the Woody Medicinal Species Solarium dulcamara L.

Exposure of cell suspension protoplasts of the woody medicinalplant Solatium dulcamara L. to voltages of 250 to 1250 V cm^–1for three successive pulses, each of 10–50 us duration,stimulated growth of protoplast-derived tissues. Such tissuesexhibited increased morphogenesis and required a shorter periodin culture to exhibit this effect than tissues from untreatedprotoplasts. Regenerated shoots also rooted more readily anddeveloped more prolific root systems than shoots from untreatedprotoplasts. These observations have important implicationsfor plant genetic manipulation and may have application in therecovery and rooting of shoots from tissues of woody species,normally considered recalcitrant in culture. Key words: Electroporation, protoplasts, shoot regeneration, Solanum dulcamara (woody nightshade, bittersweet)