Phenylpropanoids as a Protectant of Aluminum Toxicity in Cultured Tobacco Cells

Aluminum (Al) enhances ferrous ion [Fe(II)]-mediated peroxidationof lipids, which is lethal to normal tobacco cells, but notto phosphate (P_i)-starved cells ( –P cells). We foundthat tobacco cells accumulated phenylpropanoid compounds includingchlorogenic acid (CGA) and caffeic acid (CA) during P_i starvation.The accumulation was inhibited by 2-aminoindan-2-phosphonicacid (AIP), a specific inhibitor of L-phenylalanine ammonialyase (PAL). CGA, CA and also an extract containing the phenylpropanoidcompounds from –P cells protected normal cells ( +P cells)efficiently from both lipid peroxidation and the loss of viabilitycaused by the combined application of Al and Fe(II), indicatingthat the phenylpropanoids acted as antioxidant molecules. –Pcells exhibited approximately 25-fold higher specific activityof PAL than +P cells. The content of the phenylpropanoids andthe activity of PAL were not affected by the combined treatmentwith Al and Fe(II) in either +P cells or –P cells. Theseresults suggest that an increase in PAL activity during P_i starvationenhances the accumulation of phenylpropanoids, and that thephenylpropanoids protect tobacco cells from cytotoxic lipidperoxidation caused by the combination of Al and Fe(II) ⁴CREST, Japan Science and Technology Corporation (JST).     

Induction of Ferric Reductase Activity and of Iron Uptake Capacity in Chlorococcum littorale Cells under Extremely High-CO2 and Iron-Deficient Conditions

The marine green alga, Chlorococcum littorale, accumulated ironin its cells and showed high activity of plasma membrane ferricreductase under high-CO₂ and iron-deficient conditions. Theseactivities disappeared upon exposure to ordinary air and byadding excess FeSO₄. The iron uptake had high affinity for theFe(II) form (K_m of 0.13 µM). Carbonylcyanide m-chlorophenylhydrazoneand N,N-dicyclohexylcarbodiimide significantly suppressed theiron uptake, suggesting that the Fe(II) uptake was driven byATPase. These results indicate that high CO₂ and iron deficiencycooperatively induce the Fe(II) uptake and cell-surface ferricreductase activity. (Received October 20, 1997; Accepted January 29, 1998)     

Protective effect of glutathione on the cytotoxicity caused by a combination of aluminum and iron in suspension-cultured tobacco cells

The role of endogenous glutathione (GSH) in the protection of suspension-cultured tobacco cells from aluminum (Al) toxicity was examined. Cells at the logarithmic phase of growth were treated with or without Al in nutrient medium prepared without P_i and EDTA. In the absence of Al, total GSH content (including oxidized glutathione [GSSG]) increased gradually. In the presence of Al, the increase of GSH was repressed. This effect was observed before the loss of plasma membrane integrity and the loss of cell viability. In contrast, GSSG content in cells increased in the presence of Al. GSH-deprived cells were prepared by culturing cells with buthionine sulfoximine (an inhibitor of Γ -glutamylcysteine synthetase) for 24 h. Total GSH content in GSH-deprived cells was 6% of that in normal cells. The GSH-deprived cells exhibited a higher degree of lipid peroxidation, increased accumulation of Al, and greater loss of viability than normal cells. These results suggest that GSH protects cells from the oxidative membrane damage caused by a combination of Al and Fe(II) possibly by both direct consumption of GSH and oxidation of GSH.     

Aluminum Tolerance Acquired during Phosphate Starvation in Cultured Tobacco Cells

Al toxicity in cultured tobacco cells (Nicotiana tabacum L. cv Samsun; nonchlorophyllic cell line SL) has been investigated in nutrient medium. In this system, Al and Fe(II) (ferrous ion) in the medium synergistically result in the accumulation of both Al and Fe, the peroxidation of lipids, and eventually death in cells at the logarithmic phase of growth (+P cells). A lipophilic antioxidant, N,N[prime]-diphenyl-p-phenylenediamine, protected +P cells from the peroxidation of lipids and from cell death, suggesting that a relationship exists between the two. Compared with +P cells, cells that had been starved of Pi (-P cells) were more tolerant to Al, accumulated 30 to 40% less Al and 70 to 90% less Fe, and did not show any evidence of the peroxidation of lipids during Al treatment. These results suggest that -P cells exhibit Al tolerance because their plasma membranes are protected from the peroxidation of lipids caused by the combination of Al and Fe(II). It seems likely that the exclusion of Fe from -P cells might suppress directly Fe-mediated peroxidation of lipids. Furthermore, since -P cells accumulated [beta]-carotene, it is proposed that this carotenoid pigment might function as a radical-trapping antioxidant in the plasma membrane of cells starved of Pi.     

Characteristics of Sulfite Transport by Chlorella vulgaris

The rate of short-term accumulation of [³⁵S]sulfite in Chlorellavulgaris cells was found to be strongly dependent on the pHof the medium. The rate increased with decreased pH, and theincrease in rate closely paralleled the increase in the concentrationof the un-ionized form of sulfite. When the pH of the mediumwas increased, fast accumulation ceased immediately. The rateof accumulation showed a strong temperature dependence, withan apparent temperature coefficient of 1.93 per 10°C rise,between 10 and 25°C. Because pK_a values of sulfite shiftwith temperature, the rates were corrected by dividing by theconcentration of the un-ionized form of sulfite present at therespective temperatures. The temperature coefficient was thenfound to decrease to 1.45. When cells which had been allowedto accumulate [³⁵S]sulfite for 20 min were transferred to amedium containing no sulfite, more than 50% of the accumulated[³⁵S] was released into the medium in 20 min. Our results arecompatible with a simple diffusion model of SO₂ transport intoChlorella cells. (Received September 26, 1996; Accepted January 20, 1997)     

Ferric chelate reduction by sunflower (Helianthus annuus L.) leaves: influence of light, oxygen, iron-deficiency and leaf age

The presence of ferric chelate reducing activity in sunflower[Helianthus annuus L.) leaves has been studied by submergingleaf discs in a solution with Fe(III)-ethylenediaminetetra-acetate(FeEDTA), batho-phenanthroline disulphonate (BPDS) and vacuuminfiltration. The effect of different factors on the Fe(III)reduction rate was studied. Ferric reduction rate was about10-fold higher in the light than in darkness. The light effectwas greatly inhibited by 3-(3,4-dichloro-phenyl)-1,1-dimethylurea(DCMU), a photosystem II inhibitor. Several inhibitors of redoxsystems [cis-platinum (II) diamine dichloride (cis-platin),p-nitro-phenylacetate (p-NPA) and p-hydroxymercuribenzoic acid(pHMB)] decreased the FeEDTA reduction rate. The greatest inhibitionwas produced by the - SH group reagent pHMB (17% of control,in light). The FeEDTA reduction rate was much higher in theabsence of O₂ than with air or 100% O₂. Superoxide dismutase(SOD) decreased FeEDTA reduction with air in the light. Youngleaves reduced Fe(III)-chelate at a higher rate than did olderleaves. In iron-deficient plants, leaves did not exhibit enhancedferric chelate-reducing activity as was observed in roots. Itis suggested that at least two different redox systems or twostates of the same redox system work in the light and in darkness. Key words: Iron, leaves, plasma membrane-redox, light, oxygen level     

Degradation of Endogenous Organic Acids Induced by Pi Uptake in Catharanthus roseus Cells: Involvement of the Biochemical pH-Stat

We have previously shown that inorganic orthophosphate (P_i)uptake by Catharanthus roseus cells proceeds by a proton/P_icotransport mechanism [Sakano (1990) Plant Physiol. 93: 479]that acidifies the cytoplasm [Sakano et al. (1992) Plant Physiol.99: 672]. In the present study, we analyzed changes in the contentof endogenous organic acids, carbon dioxide evolution, and oxygenconsumption upon P_i application. The results are consistentwith the operation of the biochemical pH-stat mechanism [Davies(1986) Physiol. Plant. 67: 702] during and after P_i uptake. (Received November 13, 1997; Accepted March 30, 1998)     

DYNAMIC RESPONSE OF ROOT BORDER CELLS AND THEIR ASSOCIATED MUCILAGE EXUDATION IN SOYBEAN TO AI STRESS AND RECOVERY

 以耐铝性明显差异的两个大豆(Glycine max)基因型‘浙秋2号’(耐性)和‘浙春3号’(敏感)为材料, 研究根尖边缘细胞比活度、粘液分泌和根长对铝胁迫和解除胁迫的反应, 明确边缘细胞的粘液分泌对策在铝毒环境中的生态学意义。结果表明, ‘浙秋2号’在100~400 µmol&;#8226;L^–1 Al³⁺处理的3~12 h, 边缘细胞比活率呈递减趋势, 12 h后比活率又略有上升。‘浙春3号’在300和400 µmol&;#8226;L^–1 Al³⁺处理的变化与前者一致。两个大豆基因型的粘液层随着Al³⁺浓度增加和时间延长而增厚, 并于400 µmol&;#8226;L^–1 Al³⁺处理24 h时达到最大(>17 µm)。‘浙秋2号’在低浓度Al3+ (100和200 µmol&;#8226;L^–1)处理3~6 h后就会分泌大量粘液, ‘浙春3号’则在300 µmol&;#8226;L^–1 Al³⁺处理12 h后才有类似的变化。‘浙秋2号’在400 µmol&;#8226;L^–1 Al³⁺处理下的根相对伸长率均高于100~300 µmol&;#8226;L^–1 Al³⁺处理, ‘浙春3号’则表现为Al³⁺浓度越高, 根伸长受抑越明显。Al³⁺胁迫解除后, ‘浙秋2号’的粘液分泌速度和分泌量急剧下降, ‘浙春3号’在胁迫解除后的24 h, 仍会持续、大量地分泌粘液(>19 µm)。可见, 耐性大豆通过在铝胁迫初期快速、大量地分泌粘液以维持较高的边缘细胞活性和解除胁迫后迅速降低粘液的分泌速度及分泌量来适应铝毒害环境。     

Aluminate-Induced Changes in Morphology and Ultrastructure of Thinopyrum Roots

The effects of aluminate [Al(OH)_4$$$] on the morphology andultrastructure of root cells were studied in the salt-tolerantgrasses Thinopyrum bessarabicum (Savul. and Rayss) A. Lve (2) and Thinopyrum junceum (L.) A. Lve (6) by light and transmissionelectron microscopy. Seedlings were grown in nutrient solutioncontaining 1 mol m^–3 [Al] and 5 mol m^–3 Na₂CO₃ atpH 100. Light microscopy revealed that root tips of [Al]-treated plantsdisplayed bending. Many cells of the cortex in the elongatingregion contained a fibrillar/granular material which renderedthem densely staining. Radial (anticlinal) walls of the epidermalcells were either cleft apart of unusually thickened. Amyloplastsof the central root cap cells contained fewer starch grains,while their distribution was disturbed. Electron microscopy showed that the most serious effects of[Al] toxicity occurred at the cell walls of the epidermal androot cap cells, as they lost their fibrillar fine structureand contained an amorphous electron-dense material distributedall over the wall section. Electron-opaque droplets were encounteredat the plasma membrane region of epidermal cells, while theelectron-dense material observed in the vacuoles of cortex cellscould be aluminate which had accumulated there. Thus, despitethe presence of a barrier to aluminate uptake, some [Al] doesenter the symplast. However, the cytoplasm of many epidermalcells displayed a normal fine structure and contained the usualsubcellular components. Dictyosomes, in particular, were abundantand surrounded by many vesicles denoting an active state. Theseobservations stress the role of cell walls as the major [Al]pool and of the plasma membrane as the ultimate barrier thatprotects the cytoplasm. Results are further discussed in relation to the findings inother plant species and it is concluded that, although aluminateis less toxic than Al³⁺, it causes morphological, structuraland, presumably, functional damage-to the roots of the speciesinvestigated. Key words: Thinopyrum, aluminate toxicity, cell walls, root bending     

Active oxygen participation in chlorophyll destruction and lipid peroxidation in SO2-fumigated leaves of spinach

Chlorophyll a and carotenoids of spinach began to be destroyed2 to 3 hr after fumigation with 2 ppm SO₂ under light, whereaschlorophyll b was undamaged during 8 hr of exposure to SO₂.Pheophytin a was not affected by the fumigation. When disks excised from leaves fumigated with SO₂ at 2 ppm for2 hr were illuminated, chlorophyll a and carotenoids were brokendown, while they were not destroyed in darkness. The destructionof these pigments was suppressed under nitrogen. Chlorophylla destruction was inhibited by l,2-dihydroxybenzene-3,5-disulfonate(tiron), hydro-quinone and ascorbate, but not by l,4-diazabicyclo-[2,2,2]-octane(DABCO), methio-nine, histidine, benzoate and formate. Chlorophylla destruction was inhibited by phenazine methosulfate but stimulatedby methyl viologen. Addition of superoxide dismutase (SOD) tothe homogenate of SO₂-fumigated leaves inhibited the chlorophylla destruction. The activity of endogenous SOD was reduced to40% by 2-hr fumigation before the loss of chlorophyll was observed.These results suggest that chlorophyll a destruction by SO₂was due to superoxide radicals (O₂^–). Moreover, malondialdehyde (MDA), a product of lipid peroxidation,was formed in SO₂-fumigated leaves. MDA formation was inhibitedby tiron, hydroquinone and DABCO but not by benzoate and formate.MDA formation was increased by D₂O. These results suggest thatlipid peroxidation in SO₂-fumigated leaves was due to singletoxygen ¹O₂ produced from O_2^–. (Received May 15, 1980; )     

Active oxygen participation in chlorophyll destruction and lipid peroxidation in SO2-fumigated leaves of spinach

Chlorophyll a and carotenoids of spinach began to be destroyed2 to 3 hr after fumigation with 2 ppm SO₂ under light, whereaschlorophyll b was undamaged during 8 hr of exposure to SO₂.Pheophytin a was not affected by the fumigation. When disks excised from leaves fumigated with SO₂ at 2 ppm for2 hr were illuminated, chlorophyll a and carotenoids were brokendown, while they were not destroyed in darkness. The destructionof these pigments was suppressed under nitrogen. Chlorophylla destruction was inhibited by l,2-dihydroxybenzene-3,5-disulfonate(tiron), hydro-quinone and ascorbate, but not by l,4-diazabicyclo-[2,2,2]-octane(DABCO), methio-nine, histidine, benzoate and formate. Chlorophylla destruction was inhibited by phenazine methosulfate but stimulatedby methyl viologen. Addition of superoxide dismutase (SOD) tothe homogenate of SO₂-fumigated leaves inhibited the chlorophylla destruction. The activity of endogenous SOD was reduced to40% by 2-hr fumigation before the loss of chlorophyll was observed.These results suggest that chlorophyll a destruction by SO₂was due to superoxide radicals (O₂^–). Moreover, malondialdehyde (MDA), a product of lipid peroxidation,was formed in SO₂-fumigated leaves. MDA formation was inhibitedby tiron, hydroquinone and DABCO but not by benzoate and formate.MDA formation was increased by D₂O. These results suggest thatlipid peroxidation in SO₂-fumigated leaves was due to singletoxygen ¹O₂ produced from O_2^–. (Received May 15, 1980; )     

Regulation of KCa current by store-operated Ca2+ influx depends on internal Ca2+ release in HSG cells

This study examines theCa²⁺ influx-dependent regulationof the Ca²⁺-activatedK⁺ channel(K_Ca) in human submandibulargland (HSG) cells. Carbachol (CCh) induced sustained increases in theK_Ca current and cytosolic Ca²⁺ concentration([Ca²⁺]_i),which were prevented by loading cells with1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Removal of extracellularCa²⁺ and addition ofLa³⁺ orGd³⁺, but notZn²⁺, inhibited the increases inK_Ca current and[Ca²⁺]_i.Ca²⁺ influx during refill (i.e.,addition of Ca²⁺ to cells treatedwith CCh and then atropine inCa²⁺-free medium) failed to evokeincreases in the K_Ca current but achieved internal Ca²⁺ storerefill. When refill was prevented by thapsigargin,Ca²⁺ readdition induced rapidactivation of K_Ca. These dataprovide further evidence that intracellularCa²⁺ accumulation provides tightbuffering of[Ca²⁺]_iat the site of Ca²⁺ influx (H. Mogami, K. Nakano, A. V. Tepikin, and O. H. Petersen. Cell 88: 49-55, 1997). We suggestthat the Ca²⁺ influx-dependentregulation of the sustained K_Cacurrent in CCh-stimulated HSG cells is mediated by the uptake ofCa²⁺ into the internalCa²⁺ store and release via theinositol 1,4,5-trisphosphate-sensitive channel.

     

Physiology and Growth of Wheat Across a Subambient Carbon Dioxide Gradient

Two cultivars of wheat (Triticum aestivum L.), 'Yaqui 54' and'Seri M82', were grown along a gradient of daytime carbon dioxideconcentrations ([CO₂]) from near 350-200 µmol CO₂ mol^-1air in a 38 m long controlled environment chamber. Carbon dioxidefluxes and evapotranspiration were measured for stands (plantsand soil) in five consecutive 7·6-m lengths of the chamberto determined potential effects of the glacial/interglacialincrease in atmospheric [CO₂] on C₃ plants. Growth rates andleaf areas of individual plants and net assimilation per unitleaf area and daily (24-h) net CO₂ accumulation of wheat standsrose with increasing [CO₂]. Daytime net assimilation (P_D, mmolCO₂ m^-2 soil surface area) and water use efficiency of wheatstands increased and the daily total of photosynthetic photonflux density required by stands for positive CO₂ accumulation(light compensation point) declined at higher [CO₂]. Nighttimerespiration (R_N, mmol CO₂ m^-2 soil surface) of wheat, measuredat 369-397 µmol mol^-1 CO₂, apparently was not alteredby growth at different daytime [CO₂], but R_N /P_D of stands declinedlinearly as daytime [CO₂] and P_D increased. The responses ofwheat to [CO₂], if representative of other C₃ species, suggestthat the 75-100% increase in [CO₂] since glaciation and the30% increase since 1800 reduced the minimum light and waterrequirements for growth and increased the productivity of C₃plants.Copyright 1993, 1999 Academic Press Atmospheric carbon dioxide, carbon accumulation, evapotranspiration, light compensation point, net assimilation, respiration, Triticum aestivum, water use efficiency, wheat     

Acidification and Alkalinization of Culture Medium by Catharanthus roseus cellsIs Anoxic Production of Lactate a Cause of Cytoplasmic Acidification?

Catharanthus roseus(L.) G. Don cells acidified Mura-shige-Skoogmedium rapidly. Upon transfer to fresh medium, the medium pH(initially5.3) dropped below 4 within 2 d. This acidificationwas reversed under hypoxic conditions. The cells induced a similaracidification in a simple medium consisting of CaCl₂, KCl, andglucose: medium pH dropped below 4 within 6 h. The acidificationwas accompanied by an influx of K⁺ at a H⁺(efflux)/K⁺ ratioof ca 0.6 as well as by an expansion of endogenous organic acidpool, in which malic and citric acids were the major components.Anoxia reversed all these processes: the direction of both K⁺and H⁺ fluxes reversed with a H⁺/K⁺ ratio of 1.70. Anoxia induceda cytoplasmic acidification from pH 7.6 (aerobic) to 7.4 asmeasured by 31P-NMR, accompanied by a rapid, long-lasting lactateaccumulation at expense of malic and citric acids. Evidencesuggested that accumulation of lactic acid was not a cause ofcytoplasmic acidification under anoxia, but a result of pH regulationby the biochemical pH-stat [Davies (1973) Symp. Soc. Exp. Biol.27: 513]. The anoxic acidification of the cytoplasm was ascribedto the influx of H⁺ from the medium. (Received April 18, 1997; Accepted July 8, 1997)     

Chromatic Regulation of Cytochrome Composition and Electron Transfer from Cytochrome b6-f Complex to Photosystem I in Anacystis nidulans (Tx 20)

Cytochrome composition of the cyanobacterial photosyntheticsystem was studied with Anacystis nidulans (Tx 20) in relationto the chromatic regulation of photosystem composition. Comparisonof cytochrome compositions in cells with a high PS I/II ratio(3.0, grown under weak orange light) and with a low ratio (1.6,grown under weak red light) indicated that cytochrome compositionwas also changed in the chromatic regulation of photosystemcomposition. Two types of cytochrome change were observed: 1)contents of cytochromes C₅₅₃ and c₅₄₈ were changed in parallelwith the changes in PS I content, and 2) cytochrome b₅₅₃ andcytochrome b₆-f complex were held at a constant molar ratioto PS II. The molar ratio, PS II : cytochrome b₅₅₉ : cytochromeb₆-f complex : cytochrome c₅₅₃ : PS I : cytochrome C₅₄₈, inthe red-grown cells was 1 : 2.5 : 1.3 : 0.17 : 1.6 : 0.67, andthe ratio in the orange-grown cells, 1:2.4:0.9:0.32:3.0:1.2.In both types of cells, almost all cytochrome f in the cytochromeb₆-f complex was rapidly oxidized after multiple flash activation,indicating that all cytochrome b₆-f complexes in cells of bothtypes are functionally connected to PS I, even when the molarratio to PS I is largely changed. The content of cytochromeC₅₅₃ was at most 0.14 of PS I, suggesting that the cytochrometurns over several times per one turnover of PS I. ¹Present address: Department of Biology, Faculty of Science,Tokyo Metropolitan University, Fukazawa 2-1-1, Setagaya, Tokyo158, Japan. (Received January 20, 1986; Accepted March 17, 1986)     

When does the lung die? Kfc, cell viability, and adenine nucleotide changes in the circulation-arrested rat lung

Jones, David R., Randy M. Becker, Steve C. Hoffmann, John J. Lemasters, and Thomas M. Egan. When does the lungdie? K_fc, cellviability, and adenine nucleotide changes in the circulation-arrested rat lung. J. Appl. Physiol. 83(1):247-252, 1997.Lungs harvested from cadavericcirculation-arrested donors may increase the donor pool for lungtransplantation. To determine the degree and time course ofischemia-reperfusion injury, we evaluated the effect ofO₂ ventilation on capillarypermeability [capillary filtration coefficient(K_fc)],cell viability, and total adenine nucleotide (TAN) levels in in situcirculation-arrested rat lungs.K_fc increased with increasing postmortem ischemic time(r = 0.88). Lungs ventilated withO₂ 1 h postmortem had similarK_fc andwet-to-dry ratios as controls. Nonventilated lungs had threefold(P < 0.05) and sevenfold (P < 0.0001) increases inK_fc at 30 and 60 min postmortem compared with controls. Cell viability decreased inall groups except for 30-min postmortemO₂-ventilated lungs. TAN levelsdecreased with increasing ischemic time, particularly in nonventilatedlungs. Loss of adenine nucleotides correlated with increasingK_fc values (r = 0.76). This study indicates thatlungs retrieved 1 h postmortem may have normalK_fc withpreharvest O₂ ventilation. Therelationship betweenK_fc and TANsuggests that vascular permeability may be related to lung TAN levels.

     

Influence of Drought on Mitochondrial Activity, Photosynthesis, Nocturnal Acid Accumulation and Water Relations in the CAM Plants Prenia sladeniana(ME-type) and Crassula lycopodioides(PEPCK-type)

The activity of photosynthesis and mitochondrial respiration,nocturnal organic acid accumulation and water relations wereinvestigated in Prenia sladeniana L. Bol. [malic enzyme (ME)-type]andCrassula lycopodioides Lam. [phosphoenolpyruvate carboxykinase(PEPCK)-type] to compare the physiological responses to waterdeficit in crassulacean acid metabolism (CAM) plants differingin their decarboxylating enzyme systems. Withholding water inhibiteddaytime gas exchange within 2 d while night time CO₂gain andmalic acid accumulation remained relatively unchanged in bothspecies. In P. sladeniana, maximum photochemical efficiency(F_v/F_m) and photosynthetic electron transport declined to nearlythe same degree as CO₂supply was restricted during drought.Despite limited CO₂availability, photosynthetic activity waslargely unaffected in C. lycopodioides, as were mitochondrialproperties. There is no indication of a drought-induced increasein the capability to totally oxidize malate, yielding 4 CO₂, in either species. Nevertheless, the enhanced ratio of malateto glycine oxidation may have increased the in vivo capabilityfor malate oxidation in P. sladeniana. Although pressure potentialwas maintained throughout the experiment in both species, activeosmotic adaptation occurred only inP. sladeniana. The observeddecrease in photosynthetic and mitochondrial activity may haveresulted from the large increase in osmotic concentration inthis species. Copyright 2000 Annals of Botany Company Chlorophyll fluorescence analysis, Crassula lycopodioides Lam., crassulacean acid metabolism, citric acid, gas exchange, malic acid, mitochondria, photosynthetic electron transport, Prenia sladeniana L. Bol., water relations     

Effects of CaCl2-washing on EPR Signals of Cytochrome b559 in Photosystem II Preparation from Spinach Chloroplasts

Washing of PS II preparation by 1 M CaCl₂ inactivates oxygenevolution without loss of bound manganese [Ono and Inoue (1983)FEBS Lett. 164: 255]. Most of the high-potential Cyt b₅₅₀, whichamounts to about a half of the total Cyt b₅₅₉ in untreated preparation,was converted to its low-potential form by CaCl₂-washing. Theeffect was similar to that of Tris-washing. The peak positionof the g_s band of the EPR spectrum of the CaCl₂-washed preparation(g=2.95) was the same as that of the low potential form of untreatedpreparation but was slightly different from that of the Tris-washedor heat-treated preparation (g=2.98). ¹ Present address: Department of Biology, Faculty of Science,Tokyo Metropolitan University, Fukazawa 2-1-1, Setagaya, Tokyo158, Japan. (Received November 14, 1984; Accepted January 30, 1985)     

Induction of Glucose 6-Phosphate Transport Activity in Chloroplasts of Mesembryanthemum crystallinum by the C3-CAM Transition

To study possible changes in the transport metabolites betweenchloroplasts and cytoplasm during CAM induction of Mesembryanthemumcrystallinum, we compared substrate specificity of P1¹ translocator(s)in isolated chloroplasts from the C₃ and CAM-induced plants.The [¹⁴C]glu-cose 6-phosphate (G6P) transport activity was significantonly in the chloroplasts of CAM-mode plants and not detectablein those of C₃-mode, while a similar high rate of [³²P]P_i uptakewas observed with both types of chloroplasts. Kinetic analysisof G6P uptake in the CAM chloroplasts showed a high V_max [10.6µmol (mg Chl)^–1 h^–1] and a comparatively lowK_m value (0.41 mM); the latter was similar to K_i values of P_i,3-phosphoglycerate and phospho-enolpyruvate, 0.30, 0.34 and0.47 mM, respectively. On the other hand, [32P]Pi uptake inthe CAM chloroplasts was inhibited competitively by G6P witha K_i value (8.4 mM) 20-fold higher than the K_m value for G6Puptake, while that in C₃ chloroplasts was not inhibited at all.These results suggest that a new G6P/P_i, counterexchange mechanismis induced in the chloroplast envelope of CAM-induced M. crystallinumin addition to the ordinary type of P, translocator, that cannottransport G6P, already present in the C₃-type chloroplasts. (Received March 17, 1997; Accepted May 10, 1997)     

Effects of osmotic swelling on voltage-gated calcium channel currents in rat anterior pituitary cells

Decrease in extracellular osmolarity ([Os]_e) results in stimulation of hormone secretion from pituitary cells. Different mechanisms can account for this stimulation of hormone secretion. In this study we examined the possibility that hyposmolarity directly modulates voltage-gated calcium influx in pituitary cells. The effects of hyposmolarity on L-type (I_L) and T-type (I_T) calcium currents in pituitary cells were investigated by using two hyposmotic stimuli, moderate (18-22% decrease in [Os]_e) and strong (31-32% decrease in [Os]_e). Exposure to moderate hyposmotic stimuli resulted in three response types in I_L (a decrease, a biphasic effect, and an increase in I_L) and in increase in I_T. Exposure to strong hyposmotic stimuli resulted only in increases in both I_L and I_T. Similarly, in intact pituitary cells (perforated patch method), exposure to either moderate or strong hyposmotic stimuli resulted only in increases in both I_L and I_T. Thus it appears that the main effect of decrease in [Os]_e is increase in calcium channel currents. This increase was differential (I_L were more sensitive than I_T) and voltage independent. In addition, we show that these hyposmotic effects cannot be explained by activation of an anionic conductance or by an increase in cell membrane surface area. In conclusion, this study shows that hyposmotic swelling of pituitary cells can directly modulate voltage-gated calcium influx. This hyposmotic modulation of I_L and I_T may contribute to the previously reported hyposmotic stimulation of hormone secretion. The mechanisms underlying these hyposmotic effects and their possible physiological relevance are discussed. L-type channels; mechanosensitivity; somatotrophs; lactotrophs