Characterization of Sulfate Transport in the Green Alga Chlorella ellipsoidea

A rapid induction of sulfate transport was observed in the greenalga Chlorella ellipsoidea during sulfur-limited growth. Bothaffinity and V_max increased about five-fold within 6 h of transferringcells from Bold's basal medium with 350 µM MgSO₄ to sulfur-deficientBold's medium. High affinity sulfate transport was induced within15 min and reached maximum rate within 3 h of transferring cellsto sulfur-deficient condition, indicating that a new, high-affinity-sulfatetransport system is induced by sulfur starvation in C. ellipsoidea.Eadie-Hofstee plots of initial rates of sulfate uptake indicatedthat the K of sulfur-starved cells was about 17 µM. Bothsulfur-starved and unstarved cells grown in air had a V_max of1.5 times higher than that of high-CO₂ grown cells. Sulfatetransport was completely inhibited by 30 µM CCCP or 800µMKCN both in the light and the dark but transport in the lightwas not inhibited by 20 µM DCMU. Treatment with 50 µMor 500 µM vanadate caused 50% inhibition of uptake. Therate of sulfate uptake in the dark was twice that in the lightand was stimulated by low pH. These results suggest that thesulfate transport system in C. ellipsoidea is operated by protonsymport across the plasmamembrane which is partially mediatedby P-type ATPase and that these systems depend exclusively onenergy derived from oxidative phosphorylation in the mitochondria. (Received June 28, 1995; Accepted August 8, 1995)     

High pH Causes Disintegration of the Root Surface in Lupinus angustifolius L.

The effect of high pH on the morphology and anatomy of the rootsof lupin (Lupinus angustifolius L. cv. Yandee) and pea (Pisumsativum L. cv. Dundale) was examined in buffered solution. Themorphology and anatomy of lupin roots were markedly altered,and root growth was reduced by increasing solution pH from 5·2to 7·5, whereas pea roots were unaffected. In lupin roots,pH 7·5 caused disintegration of the root surface andimpaired root hair formation. Lupin roots grown at pH 7·5also had decreased cell lengths but increased cell diameterin both the epidermis and the cortex in comparison to rootsgrown at pH 5·2. High pH reduced cell volume greatlyin the epidermis, to a lesser extent in the outer cortex andnot at all in the inner cortex. It appears that in lupins, theprimary detrimental effects of growth at pH 7·5 is reducedlongitudinal growth of cells near the root surface with a consequentreduction in elongation of the cells in inner cortex.Copyright1993, 1999 Academic Press Lupinus angustifolius L., Pisum sativum L., high pH, root morphology, root anatomy     

Short-term Measurements of the Cytoplasmic pH of Chara corallina Derived from the Intracellular Equilibration of 5,5-Dimethyloxazolidine-2,4-Dione (DMO)

Smith, F. A. 1986. Short-term measurements of the cytoplasmicpH of Chara corallina derived from the intracellular equilibrationof 5,5-dimethyloxazolidine-2,4-dione (DMO).—J. exp. Bot.37: 1733–1745. Measurements of the time-course of influx of ¹⁴C-labelled 5,5-dimethyloxazolidine-2,4-dione(DMO) into the cytoplasm and vacuole of internodal cells ofChara corallina, and of efflux of DMO into non-radioactive solutions,have shown that exchange of DMO across the tonoplast is veryrapid compared with exchange across the plasma membrane. Thishas made possible calculations of cytoplasmic pH from distributionof DMO between cytoplasm and vacuole over short periods (5 or10 min) even when intracellular DMO is not at flux equilibriumwith external DMO. Using this new method, estimates have beenmade of the rates and magnitude of: (i) acidification of thecytoplasm caused by acidic growth regulators (IAA and NAA) andby metabolic inhibitors (azide, DNP, CCCP and DCMU), and (ii)alkalinization caused by uptake of ammonium and methylammoniumions. The potential application of the method to future studiesof membrane transport in charophyte cells is assessed. Key words: Charophyles, cytoplasmic pH.     

L-Leucine Transport in Isolated Protoplasts of Vinca Suspension Culture: Characterization of Uptake

The uptake of L-leucine into Vinca protoplasts was studied undervarious conditions. The uptake was highly pH-dependent, withthe optimal pH between 3.0 and 4.0. The uptake was also energydependent, since azide, 2,4-dinitrophenol (DNP), carbonyl cyanidem-chlorophenyl hydrazone (CCCP), and iodoacetate inhibited theuptake. Oligomycin, N,N'-dicycIohexyI carbodiimide (DCCD) andvanadate, but not ouabain, inhibited the uptake, suggestingthat ATPase for H⁺ electrogenic extrusion was necessary to theuptake of L-leucine. The uptake showed stereospecificity, butwas partially inhibited by other L-amino acids. A kinetic studyof the uptake showed that the uptake was multiphasic with threesaturable phases and one unsaturable phase which occurred atconcentrations of L-leucine over 1 mM. The K_m values of thethree affinity sites were 1.4 x 10^–3 M, 1.3 x 10^–4M, 4.3 x 10^–5 M; the maximum velocity values were 3.3x 10^–8, 4.5 x 10^–9, 1.8 x 10_–9 mol/10 min/4x 10⁶ cells. (Received April 18, 1981; Accepted August 25, 1981)     

Light-induced changes in membrane potential in Spirogyra

Spirogyra cells exhibited changes in membrane potential whenthey were exposed to light. Cells made chloroplast-free didnot show any light-induced potential change (LPC) upon illuminationwith white light and also monochromatic red (680 nm) and farred (720 nm) light. LPC was observed when the cell containedonly a small fragment of chloroplast, whether the cell had anucleus or not. The magnitude of LPC depended on the amountof chloroplast in the cell. DCMU at 10^–5 M, CCCP at 10^–5 M and DNP at 10^–4M at pH 5.5 suppressed LPC, while CCCP at 1–5 ? 10^–6M, NH₄Cl at 5 ? 10^–2 M and DNP at 10^–4 M at pH 7.0stimulated LPC. PMS at 10^–4 M stimulated LPC and couldinduce LPC which was completely inhibited by DCMU. These factssuggest that LPC is related to noncyclic and cyclic electronflows. The influences of light and dark conditions and various metabolicinhibitors (DCMU, DNP, CCCP, NH₄Cl) on ATP level have been investigated.No significant difference in the ATP level was observed betweencells in the light and dark. DNP at 10^–4 M (pH 5.5) andCCCP at 5 ? 10^–6 M decreased the ATP level significantly,while DCMU and NH₄Cl only slightly. Good correlation was notfound between the total ATP level and LPC in Spirogyra. LPC occurred even when the external medium contained only asingle salt such as KCl, NaCl or CaSO₄. LPC was also recorded in chloroplasts in situ and in vitro.The mode of LPC of chloroplasts was quite different from thatof the cell. On illumination, the chloroplast potential changedvery rapidly and transiently in the positive direction thenrecovered spontaneously to almost the original potential level. Possible causes of LPC are discussed in relation to the electrogenicion pump. ¹ Present address: Department of Botany, Faculty of Science,University of Tokyo, Hongo, Bunkyo, Tokyo 113, Japan. (Received November 9, 1977; )     

Purification and Properties of NADH:Nitrate Reductase from the Red Alga Porphyra yezoensis

Assimilatory nitrate reductase (NADH) (EC 1.6.6.1 [EC] ) from thered alga Porphyra yezoensis was purified 5,700-fold by a combinationof polyethylene glycol (PEG) treatment, ammonium sulfate fractionation,chromatography on columns of butyl-Toyopearl 650-M, Blue SepharoseCL-6B, DEAE-cellulose (DE 52), and hydroxyapatite, gel filtrationon Sephacryl S-400. The purest preparation of the enzyme hada specific activity of 12.5 units mg^–1 protein. A singleband of protein was detected after polyacrylamide gel electrophoresisunder nondenaturing conditions. This band corresponded to aband that stained positive for reduced methyl viologen-nitratereductase activity. The molecular weight of the native enzymewas estimated to be 220,000. A single band of a protein witha molecular weight of 100,000 was detected after sodium dodecylsulfate-polyacrylamide gel electrophoresis. These results indicatethat the native nitrate reductase is composed of two identicalsubunits. The homogeneous preparation of nitrate reductase hada UV/visible spectrum typical of a b-type cytochrome. The K_mvalues for NADH and KNO₃ were 23 µM and 64 µM, respectively.The pH optimum for the reaction catalyzed by the nitrate reductasewas 8.3, while pH values that supported maximum partial activitiesranged from 7.0 to 8.5. Sulfhydryl reagents, such as p-HMB andNEM, inhibited full and NADH-utilizing partial activities, whilecyanide and azide were effective inhibitors of full and nitrate-reducingpartial activities. (Received March 3, 1993; Accepted September 6, 1993)     

Beryllium uptake by excised barley roots

The uptake of beryllium by excised barley roots from a solutioncontaining 111 µM beryllium and 500 µM calcium wasstudied. The exchange-adsorbed portion was 15% of the totalBe uptake after 30 min of exposure. The total Be content couldbe reduced to 55% of the control in 1 hr with demineralizedwater. At pH levels above 6, Be uptake was increased dramatically.A Q₁₀ of 1.2 was found to exist between 1 and 23°C. Mg (111µM) did not influence Be absorption while Zn and Al atthe same concentration reduced it, possibly competitively, 27and 62% of the control, respectively. Dinitro-phenol (500 µM),cyanide (500µM), azide (500 µM) and m-chlorocarbonylcyanidephenylhydrazone (100 µM) at least doubled it indicatingthe existence of a possible metabolic barrier to beryllium entrywhich was disrupted by these compounds. Absorption kineticsover an external beryllium concentration range of 11 to 444µM suggested a number of binding sites. Above 444 µMa scattering effect was noted due possibly to its toxic effect. ¹Pesticide Programs, Environmental Protection Agency (P.S. -769),Washington, D.C. 20460, U.S.A. (Received August 13, 1979; )     

Effect of Oxygen on Photosynthetic 14CO2 Fixation in Chroomonas sp. (Cryptophyta) II. Effects of Inhibitors, Uncouplers and an Artificial Electron Mediator on the Inhibition of 14CO2 Fixation by Anaerobiosis

In "air-grown" Chroomonas sp. cells, low concentrations of DCMU(less than 0.1 µM) could prevent the inhibition of ¹⁴CO₂fixation by anaerobiosis under light-saturating conditions (morethan 40 W.m^–2), with phenazine methosulfate showing asimilar effect. Antimycin A, carbonyl cyanide m-chlorophenylhydrazone(CCCP), and N,N'-dicyclohexylcarbodiimide strongly inhibitedanaerobic photosynthesis at concentrations which did not significantlyinhibit the rate under 2% O₂ at high light intensity (200 W.m^–2),although 0.2 µM CCCP stimulated the rate under 2% O₂ tosome extent. On the other hand, KCN inhibited the rate muchmore strongly under 2% O₂ than N₂, although it inhibited therate very strongly at concentrations above 5 µM both underN₂ and 2% O₂. These results suggest that the inhibition of photosynthetic¹⁴CO₂ fixation by anaerobiosis in this alga result from ATPdeficiency caused by over-reduction of electron carriers ofthe cyclic electron flow and that oxygen can prevent the over-reduction.Cyclic electron flow seems to be necessary to provide additionalATP for CO₂ reduction under anaerobic conditions, although itseems to be less necessary under aerobic conditions. (Received July 21, 1983; Accepted January 23, 1984)     

Effect of Metabolic Inhibitors on Pinocytosis of Uranyl lons by Radish Root Cells: Probable Mechanisms of Pinocytosis

Pinocytotic uptake of uranyl ions by root cells of the radishRaphanus sativus L. cv. Red Giant and effects of monoiodacetate(MIA), 2,4-dinitrophenol (DNP), 2-mercaptoethanol (ME) and cytochalasinB (CB) on this process were studied. The number of invaginationsand pinocytotic vesicles in cells incubated with 0.15 mM uranylacctate (UA) is 15–20 times greater than in control cells,as estimated by counting the structures (plasmalemma derivatives)in serial sections. Monoiodacetate (0.05 mM) slightly stimulatedand ME (1.5 mM) completely inhibited pinocytosis. DNP (0.1 mM)inhibits UA pinocytosis by 45 per cent: DNP combined with MIAdiminishes pinocytotic activity by nearly 70 per cent. In thepresence of CB (4 µM) and UA quite large invaginations(exceeding 1 µm) have been observed. Cells treated withUA and UA $ MIA contain considerably more secretory vesicles.The results provide evidence for two types of pinocytosis inradish root cells; one independent of metabolic energy (I) andthe other essentially dependent on energy from respiration (II).Experiments with liposomes which show a pinocytosis-like behaviourin the presence of the pinocytosis inductors (including UA)indicate that type I is due solely to the effect exerted bythe inductor on the membrane. Type II is probably directly dependenton metabolism and may be regarded as a combination of uptakeand of sui generis removal of any excess plasmalemma increasedby exocytotic secretion. Raphanus sativus, pinocytosis, exocytosis, metabolic inhibitors, liposomes, electrostatic interaction, lipid peroxidation     

Subcellular Distribution of Inorganic Phosphate, and Levels of Nucleoside Triphosphate, in Mature Maize Roots at Low External Phosphate Concentrations: Measurements with 31P-NMR

Maize plants were grown in nutrient solution without phosphate,or in which inorganic phosphate (Pi) was maintained at nearlyconstant concentrations of 1 µM, 10µM or 0·5mM. In vivo ³¹P-NMR measurements showed that there was no discernibledifference in the cytoplasmic Pi content (µmol cm^–3root volume) of the mature roots of plants exposed to 1 µM,10µM or 0·5 mM external phosphate for up to 12d. However, the vacuolar Pi content of the mature roots variedabout 10-fold between these three groups. The cytoplasmic Pi content of roots receiving no external phosphatedecreased significantly after about 7 d total growth, and atabout this time the vacuolar pool of Pi became too small foraccurate measurement. The presence of 1 µM Pi in the nutrientsolution completely prevented this decline in cytoplasmic Pi,and there was some evidence that it also raised the Pi contentof the root vacuoles above the almost undetectable level foundin the totally P-starved roots. During the first 7–9 d of growth, the nucleoside triphosphatecontent of the mature roots was unaffected by the concentrationof phosphate in the nutrient solution. The results highlight the close control of cytoplasmic concentrationsof certain important phosphorus metabolites in roots growingin soil of normal agricultural fertility. Key words: Vacuole, cytoplasm, intracellular compartmentation, NTP, P-nutrition     

Calcification in the Green Alga Halimeda: IV. THE ACTION OF METABOLIC INHIBITORS ON PHOTOSYNTHESIS AND CALCIFICATION

The effects of a number of metabolic inhibitors on calcificationand photosynthesis in Halimeda tuna, H. discoidea, and H. macrolobaare described. The inhibitors used are CCCP, DNP, DCMU, azide,cyanide, chloramphenicol, cycloheximide, and Diamox. The effectsof these inhibitors, although complex, are consistent with ourmodel of calcification in Halimeda. Inhibition of photosyntheticCO₂ uptake inhibits calcification as does stimulation of respiratoryCO₂ evolution (i.e. uncoupling). There is also indirect evidencefor the presence of a possible light stimulated H⁺ efflux whichinhibits calcification. The observed calcification rate is thereforethe result of a number of factors which affect the concentrationof CO^–and the pH in the intercellular space of the Halimedathallus. The results obtained with the carbonic anhydrase inhibitor Diamoxprovide further evidence for the effective separation of theintercellular space from the external medium by the appressedperipheral utricles.     

Morphology and Microtubule Organization in Arabidopsis Roots Exposed to Oryzalin or Taxol

In roots of Arabidopsis thaliana, we examined the effects oflow concentrations of microtubule inhibitors on the polarityof growth and on the organization of microtubule arrays. Intact6 d old seedlings were transplanted onto plates containing inhibitors,and sampled 12 h, 24 h and 48 h later. Oryzalin, a compoundthat causes microtubule depolymerization, stimulates the radialexpansion of roots. The amount of radial swelling is linearlyproportional to the logarithm of the oryzalin concentration,from the response threshold, 170 nM, to 1 µM. Cells inthe zone of division were slightly more sensitive to oryzalinthan were cells in the zone of pure elongation. Radial swellingis also stimulated by taxol, a compound that causes microtubulepolymerization. Taxol at 1 µM causes little swelling,but at 10µM causes extensive radial swelling of cellsin the elongation zone, and does not affect cells in the divisionzone. To examine the microtubules in these roots, we used methacrylatesections with immunofluorescence microscopy. At all concentrationsof oryzalin, cortical arrays are disorganized and depleted ofmicrotubules, and the microtubules themselves often appear fragmented.These effects increase in severity with concentration, but areunmistakable at 170 nM. In taxol, cortical arrays appear tobe more intensely stained than those of controls. At 10 µM,many cells in growing regions of the stele have longitudinalmicrotubules, whereas many cells in the cortex appear to havetransversely aligned microtubules. Taxol affects microtubulesin cells of division and elongation zones to the same extent,despite the observed difference in growth. We conclude thatthe precise, spatial pattern of cortical microtubules may notbe primarily responsible for controlling growth anisotropy;and that control over growth anisotropy may differ between dividingand non-dividing cells. (Received December 6, 1993; Accepted June 7, 1994)     

High-frequency Organogenesis from Direct Seed Culture in Lathyrus

Culture conditions were developed for inducing a high frequencyof direct shoot morphogenesis and whole plant regeneration incultures of intact seedlings of Lathyrus cicera L., L. ochrus(L.) DC., L. sativus L., and L. tingitanus L. The procedureof shoot regeneration involved culturing of whole, mature seedson MS medium containing cytokinins, or thidiazuron (TDZ), asubstituted phenylurea with cytokinin-like activity. Differentiationof shoots occurred without an intervening callus phase fromthe cotyledonary node and surrounding tissues of the intactseedlings developed from seeds germinated on media containingkinetin, BAP or TDZ. An average of 19·0, 15·8,28·8 and 43·0 shoots were regenerated at optimalconcentrations of 50·0, 50·0 and 80·0 µMBAP in L. cicera, L. tingitanus, L. ochrus and L. sativus, respectively.TDZ enhanced the shoot formation at the concentration of 10µM to an average of 33·1 and 33·7 shootsper seedling in L. cicera and L. tingitanus and at 50 µM,to 57·4 shoots per seedling in L. sativus. Regeneratedshoots developed roots on a modified MS medium containing 2·5µM NAA and the surviving plantlets were transferred tosoil. Histological studies revealed de novo formation of shoot budsfrom the epidermal or subepidermal cells of the basal and nodalregion of multiple epicotyls. Several meristematic centres consistingof actively-dividing cells developed in the subepidermal celllayer of the nodal tissue and adjacent areas within 7 d of seedculture on a cytokinin- or TDZ-supplemented medium. The patternof the development of these meristematic centres and shoot developmentwas similar in all four species.Copyright 1993, 1999 AcademicPress Seed culture, direct shoot morphogenesis, Lathyrus, thidiazuron, grass pea, vetch ochrus     

Role of pulvinar chloroplasts in light-driven leaf movements of the trifoliate leaf of bean (Phaseolus vulgaris L.)

Laminar pulvini of bean (Phaseolus vulgaris L.) contain numerouschloroplasts in cells of their motor tissue. The quantitativerelationships of the chloroplast pigments, chlorophyll a andb, ß-carotene, lutein, neoxanthin as well as the xanthophyllcycle carotenoids (violaxanthin, antheraxanthin and zeaxanthin)were similar to those of mesophyll chloroplasts from leafletlaminae. Exposure of pulvinules to light caused deepoxidationof violaxanthin to zeaxanthin, showing that the xanthophyllcycle is functioning. Chlorophyll fluorescence analysis of pulvinulesconfirmed that their chloroplasts are capable of both photosyntheticelectron transport and non-photochemical fluorescence quenching,showing that they build up a considerable transthylakoid protongradient in the light. Application of DCMU to excised pulvinulesand laminar discs, as well as to pulvinules of intact, attachedterminal leaflets blocked electron transport and fluorescencequenching. Application of the uncoupler CCCP to intact pulvinulesalso prevented non-photochemical fluorescence quenching. Therate of movement of the low-light-adapted terminal leaflet inresponse to exposure of its pulvinule to overhead red light(500 µmol m^–2 s^–1) was reduced when the pulvinulewas pretreated with DCMU. The pulvinar response to overheadblue light (50 µmol ^–2 s^–1), which is morepronounced than to red light, was not affected by similar pretreatmentwith DCMU. Pretreatment with CCCP caused a short lag in theresponse to red light, but did not affect its subsequent rate.The results suggest that the pulvinar response to red, but notto blue light, requires non-cyclic electron transport and theresulting generation of ATP Key words: Leaf movements, light, non-cyclic electron transport, Phaseolus, pulvinar chloroplasts     

Effects of Inhibitors on the Light-Induced Changes in Electric Potential in Pulvinar Motor Cells of Phaseolus vulgaris L.

To analyze the mechanism of the light-induced changes in electricpotential in motor cells of the pulvinus of Phaseolus vulgarisL., inhibitors were applied to the pulvinus by the xylem perfusionmethod. The membrane potential was –60 to –80 mV,which indicated that the polarization was less than that ofcells of a pulvinus in air. A pulse (30 s) of blue light (BL)induced transient depolarization of the membrane in the motorcells. Red light (RL) caused hyperpolarization of the membrane.The magnitude of BL pulse-induced transient depolarization wasgreater under the hyperpolarized state caused by the RL. The membrane was depolarized to –30 to –40 mV onperfusion with the respiratory inhibitor NaN₃ (1 mM) and a pulseof BL or irradiation with RL did not cause any change in thepotential in the depolarized state. Hyperpolarization of themembrane by RL was inhibited by perfusion with DCMU (50 µM),an inhibitor of electron transport in photosynthesis. However,the magnitude of the depolarization induced by the pulse ofBL was not affected. Perfusion with a proton ionophore CCCP(100µM) depolarized the membrane and no change in thepotential was induced by a pulse of BL or by RL in the depolarizedstate. The extent of the BL pulse-induced depolarization of the membranewas proportional to the magnitude of the membrane potentialat the time of which the pulse of BL was applied. It is suggestedthat the active component of the membrane potential was inhibitedby the pulse of BL. The experimental results further supportthe hypothesis that BL inhibits the activity of the proton ATPaseand, thus, causes loss of the electrogenic component of themembrane potential of the pulvinar motor cells. (Received June 22, 1992; Accepted August 24, 1992)     

Relationship between light-induced potential change and internal ATP concentration in tonoplast-free Chara cells

The effect of the intracellular concentration of ATP ([ATP]₁)on the light-induced potential change (LPC) in tonoplast-freeChara cells was studied. The LPC was hardly affected by loweringthe [ATP]₁ by about 1/10 or by raising it to about 10 timesthe normal cytoplasmic concentration (0.5–1.3 mM). Theinsensitivity of LPC to [ATP]₁ excludes the possibility thatan increase in [ATP]₁ due to photosynthesis may induce the LPC.However, extreme lowering of the [ATP]₁ to about 1–2 µMcompletely inhibited LPC, although photosynthetic O₂ evolutionwas not significantly inhibited. This fact supports the hypothesisthat light stimulates the putative H⁺pump fueled by ATP. Theuncoupling agents DNP and CCCP greatly depolarized the membrane,and inhibited LPC strongly, but they did not decrease [ATP]₁.Photosynthetic O₂ evolution was inhibited to some extent by2 µM CCCP and strongly inhibited by 0.1 mM DNP. Sincethe membrane resistance increased significantly, these chemicalsare believed to act on the membrane as an inhibitor of the electrogenicH⁺ pump not as an H⁺conductor. Introduction of 1 mM ATP intocells treated with uncouplers, to a large extent restored theirability to produce LPC although the membrane potential in darknesswas maintained at a low level. ¹Present address: Niigata College of Pharmacy, 5829 Kamishinei-cho,Niigata 950-21, Japan. ²Present address: Department of Agricultural Chemistry, Collegeof Agriculture, Kyoto University, Kyoto 606, Japan. ³Present address: Department of Botany, Faculty of Science,University of Tokyo, Hongo, Tokyo 113, Japan. (Received March 9, 1979; )     

Acyl-CoA Synthetase in Maturing Safflower Seeds

Acyl-CoA synthetase in maturing seeds of safflower (Carthamustinctorius) was membranebound, and the highest specific activitywas associated with microsomes. Activity absolutely dependedon the concentrations of fatty acid, CoA, ATP and Mg²⁺. Theapparent K_m values were 4.2 µM for oleate, 24 µMfor CoA, and 250 µM for ATP. The optimum pH of the reactionwas 7.5. Triacsin C, a potent inhibitor of the animal and bacterialacyl-CoA synthetase, was ineffective for the safflower enzyme.The enzyme utilized C₁₆ and C₁₈ long-chain fatty acids preferentially,while medium-chain and very-long-chain fatty acids were poorsubstrates. The order of specificity for native fatty acidswas linoleate > oleate=palmitate > stearate. Althoughactivity per seed varied during seed maturation, it was enoughto account for the rate of triacylglycerol synthesis in vivo. (Received February 2, 1993; Accepted March 3, 1993)     

Effects of lanthanum and cerium on the growth and mineral nutrition of corn and mungbean

Background and Aims: Plant growth responses to the rare earth elements lanthanum(La) and cerium (Ce) have been reported, but little is knownabout the effects of these two elements on plant mineral nutrition. Methods: Corn (Zea mays ‘Hycorn 82’) and mungbean (Vignaradiata ‘Berken’) were grown in continuous flowingnutrient solutions containing 0, 0·2, 1·0 and5·0 µM La or Ce. At harvest plants were dividedinto roots and shoots, dried, weighed and analysed for macro-and micronutrients, as well as for La and Ce. Key Results: La and Ce did not increase the growth of corn or mungbean. Thedry weight of corn shoots was decreased by 32 % in the presenceof 5·0 µM Ce; the other La and Ce concentrationshad no effect. La and Ce concentrations of 0·9 and 5·0µM decreased the shoot dry weight of mungbean by 75 or95 %, the two elements having closely similar effects. Decreasesin the uptake of Ca, Na, Zn and Mn by corn were observed withincreases in solution La and Ce. For mungbean, the uptake ratesof all measured elements decreased with increases in solutionLa and Ce. The concentrations of La and Ce in the roots of bothspecies were higher than in the shoots and increased stronglywith increasing concentrations of La or Ce in solution. TheLa and Ce concentrations in mungbean shoots were always higherthan in corn shoots. Conclusions: La and Ce did not enhance the growth of corn or mungbean, butdecreased the growth, root function and consequently the nutritionalstatus of mungbean at concentrations >0·2 µMin solution. It is concluded that if La or Ce have positiveeffects on corn and mungbean growth, they can only occur atsolution concentrations below 0·2 µM.     

Changes of Adenine Nucleotide Levels in Chora Internodes during Metabolic Inhibition

The internal ATP concentration of Chara internodes was measuredduring metabolic poisoning. When the Chara was kept in the dark,the ATP level did not show any marked change at least for thefirst hour. Later it decreased slowly to less than half in about4 days. The decrease in the ATP level was accelerated when theChara was treated with 2 µM triphenyltin chloride (TPC),0.2 mM 2,4-dinitrophenol (DNP), 1 mM monoiodoacetic acid (MIA)or 50 µM DCCD. The final decreased level of ATP in TPCor DNP was about 50% in the light, but less than 40% in thedark. The speed of the decrease in the ATP level depended onthe concentration of these poisons. Additional application of1 mM MIA lowered the final level to almost 10%. Simultaneousmeasurements of ADP and AMP concentrations showed a temporaryincrease of ADP and a later slow increase of AMP during poisoning. (Received May 26, 1983; Accepted August 26, 1983)     

A role for ERK1/2 in EGF- and ATP-dependent regulation of amiloride-sensitive sodium absorption

Receptor-mediated inhibition of amiloride-sensitive sodium absorption was observed in primary and immortalized murine renal collecting duct cell (mCT12) monolayers. The addition of epidermal growth factor (EGF) to the basolateral bathing solution of polarized monolayers reduced amiloride-sensitive short-circuit current (I_sc) by 15–25%, whereas the addition of ATP to the apical bathing solution decreased I_sc by 40–60%. Direct activation of PKC with phorbol 12-myristate 13-acetate (PMA) and mobilization of intracellular calcium with 2,5-di-tert-butyl-hydroquinone (DBHQ) reduced amiloride-sensitive I_sc in mCT12 monolayers by 46 ± 4% (n = 8) and 22 ± 2% (n = 8), respectively. Exposure of mCT12 cells to EGF, ATP, PMA, and DBHQ caused an increase in phosphorylation of p42/p44 (extracellular signal-regulated kinase; ERK1/2). Pretreatment of mCT12 monolayers with an ERK kinase inhibitor (PD-98059; 30 µM) prevented phosphorylation of p42/p44 and significantly reduced EGF, ATP, and PMA-induced inhibition of amiloride-sensitive I_sc. In contrast, pretreatment of monolayers with a PKC inhibitor (bisindolylmaleimide I; GF109203x; 1 µM) almost completely blocked the PMA-induced decrease in I_sc, but did not alter the EGF- or ATP-induced inhibition of I_sc. The DBHQ-mediated decrease in I_sc was due to inhibition of basolateral Na⁺-K⁺-ATPase, but EGF-, ATP-, and PMA-induced inhibition was most likely due to reduced apical sodium entry (epithelial Na⁺ channel activity). The results of these studies demonstrate that acute inhibition of amiloride-sensitive sodium transport by extracelluar ATP and EGF involves ERK1/2 activation and suggests a role for MAP kinase signaling as a negative regulator of electrogenic sodium absorption in epithelia. mitogen-activated protein kinase; epithelial ion transport; epithelial sodium channel