Adaptation versus shock response to polyethylene glycol-induced low water potential in cultured potato cells

We compared long-term adaptation versus short-term or shock response of potato ( Solanum tuberosum ) cells to polyethylene glycol (PEG)-induced low water potential. Potato cells, which were allowed to adapt gradually to a decreasing water potential, were able to grow actively in a medium containing 20% PEG. In contrast, no appreciable gain in dry weight was observed in potato cells shocked by abrupt transfer to the same medium. PEG-adapted cells were also salt-tolerant, as they were able to proliferate in a medium supplemented with 200 m M NaCl. No visible ultrastructural changes of mitochondria or proplastids were observed in adapted cells at values of low water potential (about −2.0 MPa), which caused membrane disruption and appearance of lipid droplets in unadapted cells. ABA cellular content increased 5-fold in PEG-shocked cells but no significant increase was found in PEG-adapted cells. The intracellular content of free proline increased 12.5 times over the basal level in PEG-adapted cells and 6.5 times in PEG-shocked cells. As shown by in vivo protein labeling, shock conditions strongly inhibited protein synthesis, which was completely recovered in PEG-adapted cells. Osmotin, a protein associated with salt adaptation in tobacco, was constitutively expressed at a high level in PEG-adapted cells and accumulated in PEG-shocked cells only three days after the transfer in a medium supplemented with 20% PEG. Proline and osmotin accumulation were coincident with the increase in cellular ABA content in PEG-shocked cells, but not in PEG-adapted cells. These data suggest that this hormone is mainly involved in shock response rather than long-term adaptation.     

Acclimation of leaf growth to low water potentials in sunflower

Abstract Leaf growth is one of the most sensitive of plant processes to water deficits and is frequently inhibited in field crops. Plants were acclimated for 2 weeks under a moderate soil water deficit to determine whether the sensitivity of leaf growth could be altered by sustained exposure to low water potentials. Leaf growth under these conditions was less than in the controls because expansion occurred more slowly and for less of the day than in control leaves. However, acclimated leaves were able to grow at leaf water potentials (Ψ₁) low enough to inhibit growth completely in control plants. This ability was associated with osmotic adjustment and maintenance of turgor in the acclimated leaves. Upon rewatering, the growth of acclimated leaves increased but was less than the growth of controls, despite higher concentrations of cell solute and greater turgor in the acclimated leaves than in controls. Therefore, factors other than turgor and osmotic adjustment limited the growth of acclimated leaves at high ψ₁ Four potentially controlling factors were investigated and the results showed that acclimated leaves were less extensible and required more turgor to initiate growth than control leaves. The slow growth of acclimated leaves was not due to a decrease in the water potential gradient for water uptake, although changes in the apparent hydraulic conductivity for water transport could have occurred. It was concluded that leaf growth acclimated to low ψ₁, by adjusting osmotically, and the concomitant maintenance of turgor permitted growth where none otherwise would occur. However, changes in the extensibility of the tissue and the turgor necessary to initiate growth caused generally slow growth in the acclimated leaves.     

Alteration of the action potential of tissue cultured neuronal cells by growth in the presence of a polyunsaturated fatty acid

The effects of alterations in membrane phospholipid fatty acid composition on the excitability of neuroblastoma × glioma hybrid cells, clone NG108-15, were examined using intracellular recording techniques. Cells were grown in the presence of arachidonate (20:4) added to the culture medium as a complex with bovine serum albumin. Exposure of the cells to 20:4 for 3–21 days produced a 40% decrease in the maximum rate of rise of the action potential (dV/dt) with a small change in its amplitude. The resting membrane potential and passive properties of the cells were unaffected. An effect of 20:4 was not observed until 24 hr after treatment and increased over the next 2 days. The phospholipid content of 20:4 and its metabolite 22:4 increased from 6.9% to 25.3% of total fatty acids during approximately the same time span. It is concluded that the action potential dV/dt can be altered by changes in membrane lipid composition.     

Postharvest changes in endogenous cytokinins and cytokinin efficacy in potato tubers in relation to bud endodormancy

Using an indirect enzyme‐linked immunosorbent assay (ELISA), the effects of postharvest storage duration and temperature on endogenous cytokinins in potato ( Solanum tuberosum L. cv. Russet Burbank) tuber apical bud tissues in relation to endodormancy status were determined. Following fractionation by HPLC, a total of eight cytokinins were detected and these were: zeatin riboside‐5'‐monophosphate (ZRMP), zeatin‐ O ‐glucoside (ZOG), zeatin (Z), zeatin riboside (ZR), isopentenyl adenosine‐5'‐monophosphate (IPMP), isopentenyl adenine‐9‐glucoside (IP‐9‐G), isopentenyl adenine (IP) and isopentenyl adenosine (IPA). Regardless of postharvest storage temperature or endodormancy status, IP‐9‐G was the most abundant cytokinin detected while ZRMP and ZOG were the least abundant ones. In tubers preincubated at a growth‐permissive temperature (20°C) prior to extraction, the loss of endodormancy was preceded by significant increases in the endogenous levels of Z, ZR, IPMP and IP‐9‐G. When stored continuously at a growth‐inhibiting temperature (3°C), significant increases in ZR, IP‐9‐G and IP + IPA were observed. The total content of cytokinins increased by over 7‐fold during postharvest storage and this increase was a result of de novo biosynthesis. Dose‐response studies using IPA and ZR demonstrated a time‐dependent increase in apparent cytokinin sensitivity during postharvest storage. With the exception of IP‐9‐G, injection of any of these endogenous cytokinins resulted in the rapid and complete termination of tuber endodormancy. The significance of these results with respect to endodormancy regulation and the possible mechanisms controlling cytokinin levels in potato tubers are discussed.     

Changes in net photosynthetic rate and chlorophyll fluorescence in potato leaves induced by water stress

Net rates of photosynthesis (PN) saturated by irradiance of >500 mol m-2 s-1 (PAR) significantly decreased in water-stressed potato (Solanum tuberosum L. cv. Kufri Sindhuri) plants. The quantum yield of photochemical energy conversion (Fv/Fm), relative electron transport rate (ETR), and photochemical quenching (Qp) exhibited a parallel decline at high irradiance. A slight decrease in relative water content (RWC) was accompanied by a drastic decline in leaf water potential (w) from -0.2 to -1.0 MPa. Dehydrated leaves showed an increase in the amount of total soluble sugars per unit leaf area which inhibited the photosynthesis in a feedback manner. After rewatering, PN and Fv/Fm were restored to the values of control plants within 24 h, and the restoration was accompanied by a proportionate lowering of content of total soluble sugars in the leaves.     

The presence of a short redox chain in the membrane of intact potato tuber peroxisomes and the association of malate dehydrogenase with the peroxisomal membrane

Peroxisomes and mitochondria were purified from potato tubers (Solanum tuberosum L. cv. Bintje) by differential centrifugation followed by separation on a continuous Percoll gradient containing 0.3 M sucrose in the lower half and 0.3 M mannitol in the upper half. The peroxisomes band at the bottom and the mitochondria in the middle of this type of gradient. Mitochondrial contamination of the peroxisomes was only 2% [as judged by cytochrome c oxidase (EC 1.3.9.1) activity]. Contamination by amyloplasts, plasma membrane and endoplasmic reticulum was also minimal. The peroxisomes were 80% intact as judged by malate dehydrogenase (MDH, NAD^?-dependent; EC 1.1.1.37) latency. The specific activity of NADH-ferricyanide reductase and NADH-Cyt c reductase was 0.22 and 0.051 μmol (mg protein)^?1 min^?1 in freshly isolated peroxisomes, respectively. The active site of the reductase appeared to be on the inner surface of the membrane. The peroxisomes also contained a b-type cytochrome. Frozen peroxisomes were subfractionated by osmotic rupture followed by centrifugation to separate the soluble proteins from the peroxisomal membrane. About half the MDH and 30% of the NADH-ferricyanide reductase activity was associated with the membrane but only 6% of the catalase (EC 1.11.1.6) activity. A further wash removed 75% of the residual catalase with only a small loss of MDH or NADH-ferricyanide reductase activity. MDH appears to be closely associated with the peroxisomal membrane. When the purified peroxisomal membrane was analyzed by SDS-PAGE followed by silver staining, prominent bands at 22, 40, 41, 48, 53 and 74 kDa were observed. After immunoblotting the purified peroxisomal membrane, a band at 53 kDa showed strong cross-reactivity with antibodies raised against NADH-ferricyanide reductase. Since the NADH-ferricyanide reductase activity in the peroxisomal membrane could be shown to be specific for the β-hydrogen of NADH, the activity could not be due to contamination by endoplasmic reticulum where the reductase is α-specific. We conclude that the peroxisomal membrane contains a short redox chain, consisting of a NADH-ferricyanide reductase and a b-type cytochrome, similar to that of e.g. the plasma membrane. The role of this redox chain has yet to be elucidated.     

Very low osmotic water permeability and membrane fluidity in isolated toad bladder granules

Summary Osmotic water permeability of the apical membrane of toad urinary epithelium is increased greatly by vasopressin (VP) and is associated with exocytic addition of granules and aggrephores at the apical surface. To determine the physiological role of granule exocytosis, we measured the osmotic water permeability and membrane fluidity of isolated granules, surface membranes and microsomes prepared from toad bladder in the presence and absence of VP.P _f was measured by stopped-flow light scattering and membrane fluidity was examined by diphenylhexatriene (DPH) fluorescence anisotropy. In response to a 75mm inward sucrose gradient, granule size decreased with a single exponential time constant of 2.3±0.1 sec (sem, seven preparations, 23°C), corresponding to aP _f of 5×10^–4 cm/sec; the activation energy (E _a ) forP _f was 17.6±0.8 kcal/mole. Under the same conditions, the volume of surface membrane vesicles decreased biexponentially with time constants of 0.13 and 1.9 sec; the fast component comprised 70% of the signal. Granule, surface membrane and microsome time constants were unaffected by VP. However, in surface membranes, there was a small decrease (6±2%) in the fraction of surface membranes with fast time constant. DPH anisotropies were 0.253 (granules), 0.224 (surface membrane fluidity is remarkably lower than that of surface and microsomal membranes, and (4) rapid water transport occurs in surface membrane vesicles. The unique physical properties of the granule suggests that apical exocytic addition of granule membrane may be responsible for the low water permeability of the unstimulated apical membrane.     

A moderate change in temperature induces changes in fatty acid composition of storage and membrane lipids in a soil arthropod

A moderate change in ambient temperature can lead to vital physiological and biochemical adjustments in ectotherms, one of which is a change in fatty acid composition. When temperature decreases, the composition of membrane lipids (phospholipid fatty acids) is expected to become more unsaturated to be able to maintain homeoviscosity. Although different in function, storage lipids (triacylglycerol fatty acids) are expected to respond to temperature changes in a similar way. Age-specific differences, however, could influence this temperature response between different life stages. Here, we investigate if fatty acid composition of membrane and storage lipids responds similarly to temperature changes for two different life stages of Orchesella cincta. Juveniles and adults were cold acclimated (15 °C → 5 °C) for 28 days and then re-acclimated (5 °C → 15 °C) for another 28 days. We found adult membranes had a more unsaturated fatty acid composition than juveniles. Membrane lipids became more unsaturated during cold acclimation, and a reversed response occurred during warm acclimation. Membrane lipids, however, showed no warm acclimation, possibly due to the moderate temperature change. The ability to adjust storage lipid composition to moderate changes in ambient temperature may be an underestimated fitness component of temperature adaptation because fluidity of storage lipids permits accessibility of enzymes to energy reserves.     

豚鼠耳蜗螺旋动脉平滑肌细胞和内皮细胞静息膜电位特性

目的:多种内耳疾病和内耳微循环障碍有关,但目前对提供内耳主要血供的耳蜗螺旋动脉平滑肌(SMC)和内皮细胞(EC)的生理学特性还不十分清楚,需要进一步研究。方法:本研究采用双细胞内微电极记录技术和细胞荧光染色技术,研究耳蜗螺旋动脉平滑肌和内皮细胞的膜电位特性和细胞间的通讯联系。结果:研究发现耳蜗螺旋动脉SMC和EC具有高、低两种静息膜电位(RP)状态,两种静息膜电位状态的细胞对乙酰胆碱和高K+的反应完全不同。双微电极可同时记录到EC-ECS、MC-SMC和SMC-EC不同类型的细胞,两个细胞的静息膜电位也可以是双高RP、双低RP和一高一低RP。实验所记录的一高一低RP均是SMC-EC类型,而且EC初始膜电位均为高电位,SMC初始膜电位均为低电位。而双高RP和双低RP可以是SMC-SMC或EC-EC或SMC-EC类型。结论:结果表明耳蜗螺旋动脉的SMC和EC在0.3~0.5 mm的范围内,同类细胞之间有很好的通讯联系,能很好的保持功能的协同和一致,血管壁异类细胞则不同。     

斑蝥素对草地贪夜蛾Sf9细胞膜完整性和膜电位的影响

为明确斑蝥素对昆虫细胞膜的作用及其机理, 本研究利用草地贪夜蛾Spodoptera frugiperda的卵巢细胞系Sf9细胞作为实验材料, 采用透射电子显微技术(transmission electron microscope , TEM)、 激光共聚焦显微镜(laser scanning confocal microscope, LSCM)结合荧光探针FDA/PI及DiBAC4(3)技术研究斑蝥素(cantharidin, CTD)对Sf9细胞膜完整性及膜电位（membrane potential, MP）的影响。结果表明： 32 μmol/L CTD处理6 h和12 h后, 电镜观察均未发现细胞膜结构破损; FDA/PI染色后, 32 μmol/L CTD处理0.5 h后细胞FDA荧光强度比对照显著降低(P<0.05), 碘化丙啶（propidium iodide, PI）染色的细胞比例与对照无显著性差异(P≥0.05)。32 μmol/L CTD处理140 s后即引起MP发生显著性去极化(P<0.05); 64 μmol/L CTD处理瞬时MP发生显著性去极化(P<0.05); 32 μmol/L CTD处理3 h内及64 μmol/L CTD处理2 h 内MP仍保持显著性去极化(P<0.05), 之后去极化程度降低; 32 μmol/L CTD处理6 h及64 μmol/L CTD处理3 h时MP去极化与对照组相比已无显著性差异(P≥0.05)。结果说明, CTD处理短时间内可引起Sf9细胞膜电位去极化并维持一段时间, 同时导致细胞活性发生不可逆下降, 但未对细胞膜结构完整性产生破坏。     

Separation of tonoplast and plasma membrane potential and resistance in cells of oat coleoptiles

Summary Membrane potential and resistance were recorded from parenchymal cells of oat (Avena) coleoptiles, using one and two intracellular electrodes. Membrane potential is largest (–100 mV) in impalements with low input resistance (2–4 M), and is less negative (–50 mV) in penetrations with high input resistance (> 20 m). The interpretation is that the electrode lodges in the vacuole which is positive to the cytoplasm (but still negative to the external solution), and that measurements of net membrane potential are compromised to varying degrees by leakage shunts introduced across the high resistance vacuolar membrane by the electrode. This conclusion is supported by several additional lines of evidence. (1) It is possible to convert large-R/small-V impalements into small-R/large-V penetrations by passing excess current through the electrode or by briefly ringing the capacitance neutralization circuit in the amplifier. The cells usually recover their resistance in a few minutes, with a concomitant decrease in the negativity of the membrane potential. (2) Changes in external [K] affect the measuree potential by an amount that is independent of the input resistance of the impalement. This is consistent with an effect of [K] _o on the potential of the plasma membrane and the occurrence of leakage shunts primarily at the tonoplast. (3) Quantitatively, the effects of a change in [K] _o on resistance indicate that nearly 90 percent of the input resistance of unshunted cells resides in the tonoplast. (4) The effects of metabolic inhibitors (DNP, CN^–) on potential are smaller in large-R than in small-R impalements. This observation suggests there are electrogenic pumps contributing to the membrane potential at both the plasmalemma and tonoplast. Finally, we conclude that with an electrode in the vacuole it is possible to record potentials that are dominated by the contribution of the plasma membrane, provided care is taken to select impalements combining both large, negative potential and low input resistance.     

Conformation and activity of chloroplast coupling factor exposed to low chemical potential of water in cells

(1) Photophosphorylation, fCa²⁺-ATPase and Mg²⁺-ATPase activities of isolated chloroplasts were inhibited 55–65% when the chemical potential of water was decreased by dehydrating leaves to water potentials (ψ_w) of ?25 bars before isolation of the plastids. The inhibition could be reversed in vivo by rehydrating the leaves.(2) These losses in activity were reflected in coupling factor (CF₁) isolated from the leaves, since CF₁ from leaves with low ?_w had less Ca²⁺-ATPase activity than control CF₁ and did not recouple phosphorylation in CF₁-deficient chloroplasts. In contrast, CF₁ from leaves having high ?_w only partially recoupled phosphorylation by CF₁-deficient chloroplasts from leaves having low ?_w. This indicated that low ?_w affected chloroplast membranes as well as CF₁ itself.(3) Coupling factor from leaves having low ψ_w had the same number of subunits, and the same electrophoretic mobility, and could be obtained with the same yields as CF₁ from control leaves. However, direct measurements of fluorescence polarization, ultraviolet absorption, and circular dichroism showed that CF₁ from leaves having low ?_w differed from control CF₁. The CF₁ from leaves having low ?_w also had decreased ability to bind fluorescent nucleotides (?-ATP and ?-ADP).(4) Exposure of isolated CF₁ to low ?_w in vitro by preincubation in sucrose-containing media inhibited the Ca²⁺-ATPase activity of the protein in subsequent assays without sucrose. Inclusion of 5 or 10 mM Mg²⁺ in the preincubation medium markedly inhibited Ca²⁺-ATPase activity.(5) These results show that CF₁ undergoes changes in cells which alter its phosphorylating ability. Since low cell ?_w changed the spectroscopic properties but not other protein properties of CF₁, the changes were most likely caused by altered conformation of the protein. This decreased the binding of nucleotides and, in turn, photophosphorylation. The inhibition of ATPase activity in CF₁ in vitro at low ?_w and high ion concentration mimicked the change in activity seen in vivo.     

Effect of anisotonic media on volume,ion and amino-acid content and membrane potential of kidney cells (MDCK) in culture

Summary Effects of anisotonic media on a monolayer of confluent kidney cells in culture (MDCK) were studied by measuring: cell thickness and cross-section changes, ion and amino-acid content and membrane potential. The volume was also determined with cells in suspension. When cells in a monolayer were incubated in hypotonic media, the lateral and the apical membranes were rapidly stretched. Afterwards the lateral membranes returned to their initial state while the apical membranes remained stretched. This partial regulatory volume decrease (RVD) was verified with cells in suspension. RVD was accompanied by a loss of K⁺, Cl^– and amino acids, but there was no loss of inorganic phosphate. Also a transient hyperpolarization of the membrane potential was observed, suggesting an increase of the K⁺ conductance during RVD. Upon restoring the isotonic medium, a regulatory volume increase (RVI) was observed accompanied by a rapid Na⁺ and Cl^– increase and followed by a slow recovery of the initial K⁺ and Na⁺ content while amino acids remained at their reduced content. A transient depolarization of the membrane potential was measured during this RVI, suggesting that Na⁺ and Cl^– conductance could have increased. In hypertonic media, only a small and slow RVI was observed accompanied by an increase in K⁺ and Cl^– content but without any change of membrane potential. Quinine partly inhibited RVD in hypotonic media with cells in a monolayer while inhibiting RVD completely with cells in suspension. Incubation during four hours in a Ca²⁺ free medium had no effect on RVD. Furosemide and amiloride had no effect on RVD and RVI. Volume regulation, RVD or RVI, was not affected by replacing Cl^– by nitrate. When cells in a monolayer were incubated in a hypotonic K₂SO₄ medium, no RVD was observed. From these results, it seems that MDCK cells in a confluent monolayer regulate their volume by activating specific ion and amino-acid transport pathways. Selective K⁺ and Na⁺ conductances are activated during RVD and RVI, while the activated anion conductance has a low selectivity. The controlling mechanism might not be the free intracellular Ca²⁺ concentration.     

Dynamic changes in plasma membrane properties of semliki forest virus infected cells related to cell fusion

The mechanism of the processes leading to membrane fusion is as yet unknown. In this report we demonstrate that changes in membrane potential and potassium fluxes correlate with Semliki Forest virus induced cell-cell fusion at mildly acidic pH. The changes observed occur only at pH's below 6.2 corresponding to values required to trigger the fusion process. A possible role of these alterations of the plasma membrane related to membrane fusion phenomena is discussed.     

Slow potential changes due to transport number effects in cells with unstirred membrane invaginations or dendrites

Summary Many neurones are extremely invaginated and possess branching processes, axons and dendrites. In general, they are surrounded by a restricted diffusion space. Many of these cells exhibit large, slow potential changes during the passage of current across their membranes. Whenever currents cross membranes separating aqueous solutions, differences in transport numbers of the major permeant ions give rise to local concentration changes of these ions adjacent to the membranes, which will result in various electrical and osmotic effects. These transport number effects are expected to be enhanced by the presence of membrane invaginations. Dendrites are equivalent to reversed invaginations and there should be significant changes in concentrations of permeant ions within them. In general, the effects of such changes on the electrical response of a cell will be greater when the concentration of a major permeant ion is low. The effects have been modelled in terms of two nondimensional parameters: the invagination transport number parameter and the relative area occupied by the invaginations A. If these two parameters are known, the magnitudes and time course of the slow potential changes can immediately be estimated and the time course converted to real time, if the length of the invaginations (l) and ionic diffusion coefficient (D) within them are also known. Both analytical and numerical solutions have been given and predictions compared. It is shown that in the case of large currents and potentials the analytical solution predictions will underestimate the magnitudes and rates of onset of the voltage responses. The relative magnitude of the transport number effect within the invaginations (or dendrites) and other transport number contributions to slow potential changes have also been assessed and order-of-magnitude values of these are estimated for some biological data.     

一株分离自美国龙虾的麦氏弧菌的鉴定及其低温状态下细胞脂肪酸的变化

[目的]对从美国龙虾(Homarus americanus)中分离的菌株F5-1进行种属鉴定并分析其低温状态下脂肪酸组成变化.[方法]通过VITEK 2 Compact全自动微生物鉴定仪分析菌株的生理生化特征和进行药敏试验;16S rRNA基因序列同源性分析确定该菌株的系统发育学地位;通过气相色谱和质谱联用法(GC-MS)分析菌株的全细胞脂肪酸.[结果]菌株F5-1为革兰氏阴性菌,对弧菌抑制剂O/129敏感,对青霉素有耐药性;生理生化特征与麦氏弧菌(Vibrio metschnikovii)的相似性为96％,16S rRNA序列与麦氏弧菌(GenBank No.HQ658055)的相似性为99％;菌株的主要脂肪酸组成为C12∶0、C14∶0、C16∶0和C16∶1(n-7),不饱和脂肪酸(棕榈油酸)相对含量达34％,低温培养状态下,不饱和脂肪酸(棕榈油酸)相对含量增加至40％.[结论]将美国龙虾中分离的菌株F5-1鉴定为麦氏弧菌,该菌对多种药物敏感,菌细胞脂肪酸组成与来源于俄罗斯海参威某饮用水水库中分离的麦氏弧菌有较大差异.     

Dietary low linolenic acid compared with docosahexaenoic acid alter synaptic plasma membrane phospholipid fatty acid composition and sodium-potassium ATPase kinetics in developing rats

The objective of this study was to investigate if maternal dietary 20:4n-6 arachidonic acid (AA) and 22:6n-3 compared with adequate or low levels of 18:3n-3 linolenic acid (LNA) increases synaptic plasma membrane (SPM) cholesterol and phospholipid content, phospholipid 20:4n-6 and 22:6n-3 content, and Na,K-ATPase kinetics in rat pups at two and five weeks of age. At parturition, Sprague-Dawley rats were fed semi-purified diets containing either AA + docosahexaenoic acid (DHA), adequate LNA (control; 18:2n-6 : 18:3n-3 ratio of 7.1 : 1) or low LNA (18:2n-6 : 18:3n-39 ratio of 835 : 1). During the first two weeks of life, the rat pups received only their dams' milk. After weaning, pups received the same diet as their respective dams to five weeks of age. No significant difference was observed among rat pups fed the diet treatments for SPM cholesterol or total and individual phospholipid content at two and five weeks of age. Fatty acid analysis revealed that maternal dietary AA + DHA, compared with feeding the dams the control diet or the low LNA diet, increased 20:4n-6 in phosphatidylserine and 22:6n-3 content of SPM phospholipids. Rats fed dietary AA + DHA or the control diet exhibited a significantly increased Vmax for SPM Na,K-ATPase. Diet treatment did not alter the Km (affinity) of SPM Na,K-ATPase in rat pups at two and five weeks of age. It is concluded that dietary AA + DHA does not alter SPM cholesterol and phospholipid content but increases the 22:6n-3 content of SPM phospholipids modulating activity of Na,K-ATPase.     

Inositol 1,4,5-trisphosphate formation in leaves of winter oilseed rape plants in response to freezing, tissue water potential aed abscisic acid

Time courses of formation of inositol 1,4,5-trisphosphate (IP₃) were followed in the leaves of non-acclimated and cold (2°C)-acclimated winter oilseed rape ( Brassica napus L. var. oleifera ) plants, subjected to different freezing temperatures or to polyethylene glycol 8000 (PEG) and abscisic acid (ABA) treatments. Changes in water potential (Ψ_w) and in ABA level in the frost- and PEG-treated tissues were also determined. Results obtained indicate that temperatures sligthly higher than LT₅₀ induced a transient and substantial increase in IP₃ level, both in non-acclimated and cold-acclimated tissues. At comparable freezing temperature (–5°C) the response of cold-acclimated leaves was lower than that of non-acclimated ones. The PEG-depedent decrease in Ψ_w to –0.9 MPa or ABA (0.1 m M ) treatment gave rise to a transient increase in IP₃ content in non-acclimated tissues only. Collectively, the data indicate that cold acclimation of plants may lead to lower cell responsiveness to the factors studied in terms of induction of IP₃ formation. Changes in the IP₃ content, observed in the present experiments, support our previous suggestion that non-killing freezing temperatures may induce the phosphoinositide pathway, both in non-acclimated and cold-acclimated tissues. Lowering of tissue water potential to some threshold value or a high exogenous ABA supply may mimic the freezing-dependent reaction in the non-acclimated leaves.