Bacteriological phosphate determination in defined liquid media with Pseudomonas fluorescens var. aureofaciens

Summary A soil organism, identified as Pseudomonas fluorescens var. aureofaciens was used to assay the phosphate content in defined liquid media.The organism showed a linear growth response to K₂HPO₄ from 0.6 mM to 6.10^–6 mM. The organism did not grow at K₂HPO₄ levels below 6.10^–6 mM and the response above 0.6 mM was not correlated with growth.It is suggested that this organism could be used to assay unknown K₂HPO₄ concentrations in defined liquid media between the above values with the aid of a standard growth curve.     

Microautoradiographic studies on host-parasite interactions II. The exchange of 3H-lysine between Uromyces phaseoli and Phaseolus vulgaris

A new species of anaerobic bacterium that degrades the even-numbered carbon fatty acids, butyrate, caproate and caprylate, to acetate and H₂ and the odd-numbered carbon fatty acids, valerate and heptanoate, to acetate, propionate and H₂ was obtained in coculture with either an H₂-utilizing methanogen or H₂-utilizing desulfovibrio. The organism could be grown only in syntrophic association with the H₂-utilizer and no other energy sources or combination of electron donor and acceptors were utilized. It was a Gram-negative helical rod with 2 to 8 flagella, about 20 nm in diameter, inserted in a linear fashion about 130 nm or more apart along the concave side of the cell. It grew with a generation time of 84 h in co-culture with Methanospirillum hungatii and was present in numbers of at least 4.5×10^-6 per g of anaerobic digestor sludge.     

Pyrolobus fumarii, gen. and sp. nov., represents a novel group of archaea, extending the upper temperature limit for life to 113°C

A novel, irregular, coccoid-shaped archaeum was isolated from a hydrothermally heated black smoker wall at the TAG site at the Mid Atlantic Ridge (depth 3650 meters). It grew at between 90°C and 113°C (optimum 106°C) and pH 4.0–6.5 (optimum 5.5) and 1%–4% salt (optimum 1.7%). The organism was a facultatively aerobic obligate chemolithoautotroph gaining energy by H₂-oxidation. Nitrate, S₂O₃ ^2–, and low concentrations of O₂ (up to 0.3% v/v) served as electron acceptors, yielding NH⁺ ₄, H₂S, and H₂O as end products, respectively. Growth was inhibited by acetate, pyruvate, glucose, starch, or sulfur. The new isolate was able to form colonies on plates (at 102°C) and to grow at a pressure of 25000 kPa (250 bar). Exponentially growing cultures survived a one-hour autoclaving at 121°C. The GC content was 53mol%. The core lipids consisted of glycerol–dialkyl glycerol tetraethers and traces of 2,3-di-O-phytanyl-sn-glycerol. The cell wall was composed of a surface layer of tetrameric protein complexes arranged on a p4-lattice (center-to-center distance 18.5nm). By its 16S rRNA sequence, the new isolate belonged to the Pyrodictiaceae. Based on its GC-content, DNA homology, S-layer composition, and metabolism, we describe here a new genus, which we name Pyrolobus (the "fire lobe"). The type species is Pyrolobus fumarii (type strain 1A; DSM). Received: 28 September 1996 / Accepted: 17 October 1996     

Thermodynamics and kinetics of the helix-coil transition of oligomers containing GC base pairs

Absorbance-temperature profiles have been determined for the following self-complementary oligonucleotides or equimolar paris of complementary oligonucleotides containing GC base pairs: A₂GCU₂, A₃GCU₃, A₄GCU₄, A₆CG + CGU₆, A₈CG + CGU₈, A₄G₂ + C₂U₄, A₅G₂ + C₂U₅, A₄G₃ + C₃U₄, and A₅G₃ + C₃U₅. In all cases cooperative melting transitions indicate double-helix formation. As was found previously, the stability of GC containing oligomer helices is much higher than that of AU helices of corresponding length. Moreover, helices with the same length and base composition but different sequences also have quite different stabilites. The melting curves were andlyzed using a zipper model and the thermodynamic parameters for the AU pairs determined previously. The effect of single-strand stacking was considered separately. According to this model, the formation of a GC pair from unstacked single strands is associated with an ethalpy change of ?15 kcal/mole. Due to the high degree of single-strand stacking at room temperature the enthalpy change for the formation of GC pairs from unstacked single strands is only ?5 to ?6 kcal/mole. (The corresponding parameters for AU pairs are ?10.7 kcal/mole and ?5 to ?6 kcal/mole.) The sequence dependence of helix stability seems to be primarily entropic since no differences in ΔH were seen among the sequence isomers. The kinetics of helix formation was investigated for the same molecules using the temperature jump technique. Recombination of strands is second order with rate constants in the range of 10⁵ to 10⁷M^?1 sec^?1 depending on the chain length and the nucleotide sequence. Within a series of oligomers of a given type, the rates of recombination decrease with increasing chain length. Oligomers with the sequence A_nGCU_n recombine six to eight times slower than the other oligomers of corresponding chain length. The experimental enthalpies of activation of 6 to 9 kcal/mole suggest a nucleation length of one or two GC base pairs. The helix dissociation process has rate constants between 0.5 and 500 sec^?1 and enthalpies of activation of 25 to 50 kcal/mole. An increase of chain length within a given nucleotide series leads to decreased rates of dissociation and increased enthalpies of activation. An investigation of the effect of ionic strength on A_nGCU_n helix formation showed that the rates of recombination increase considerably with increased ionic strength.     

Rhodospirillum sodomense,sp. nov., a Dead Sea rhodospirillum species

A new species of halophilic anoxygenic purple bacteria of the genus Rhodospirillum is described. The new organism, isolated from water/sediment of the Dead Sea, was vibrio-shaped and an obligate halophile. Growth was best at 12% NaCl, with only weak growth occurring at 6% or 21% NaCl. Growth occurred at Mg²⁺ concentrations up to 1 M but optimal growth was obtained at 0.05–0.1 M Mg²⁺. Bromide was well tolerated as an alternative anion to chloride. The new organism is an obligate phototroph, growing photoheterotrophically in media containing yeast extract and acetate or a few other organic compounds. Growth of the Dead Sea Rhodospirillum species under optimal culture conditions was slow (minimum t_d 20 h). Cells contained bacteriochlorophyll a and carotenoids of the spirilloxanthin series and mass cultures were pink in color. Absorption spectra revealed the presence of a B875 (light-harvesting I) but no B800/B850 (light-harvesting II) photopigment complex. The new organism shares a number of properties with the previously described halophilic phototrophic bacterium Rhodospirillum salinarum and was shown to be related to this phototroph by 16S rRNA sequencing. However, because of its salinity requirements, photosynthetic properties, and isolation from the Dead Sea, the new phototroph is proposed as a new species of the genus Rhodospirillum, R. sodomense.     

New insights into the energy metabolism and taxonomy of Deferribacteres revealed by the characterization of a new isolate from a hypersaline microbial mat

Strain L21-Ace-BES^T, isolated from a lithifying cyanobacterial mat, could be assigned to a novel species and genus within the class Deferribacteres. It is an important model organism for the study of anaerobic acetate degradation under hypersaline conditions. The metabolism of strain L21-Ace-BES^T was characterized by biochemical studies, comparative genome analyses, and the evaluation of gene expression patterns. The central metabolic pathway is the citric acid cycle, which is mainly controlled by the enzyme succinyl-CoA:acetate-CoA transferase. The potential use of a reversed oxidative citric acid cycle to fix CO₂ has been revealed through genome analysis. However, no autotrophic growth was detected in this strain, whereas sulfide and H₂ can be used mixotrophically. Preferred electron acceptors for the anaerobic oxidation of acetate are nitrate, fumarate and dimethyl sulfoxide, while oxygen can be utilized only under microoxic conditions. Aerotolerant growth by fermentation was observed at higher oxygen concentrations. The redox cycling of sulfur/sulfide enables the generation of reducing power for the assimilation of acetate during growth and could prevent the over-reduction of cells in stationary phase. Extracellular electron transfer appears to be an essential component of the respiratory metabolism in this clade of Deferribacteres and may be involved in the reduction of nitrite to ammonium.     

Intragenome distribution of 5-methylcytosine in DNA of healthy and wilt-infected cotton plants (Gossypium hirsutum L.)

Fractionation of DNA of healthy and wilt-infected cotton plants has been carried out according to the reassociation kinetics and the content of GC and 5-methylcytosine in the resulting fractions has been studied. The genome of cotton plant was found to be methylated quite unevenly. The GC rich (GC=64.7 mole%) fraction of highly reiterated sequences (C ₀ t=0–3.7×10^-2) has a high content of 5-methylcytosine (5.8 mole%), whereas the methylation degree of the fraction of unique sequences (C ₀ t487) is very low (the 5-methylcytosine content is about 0.5 mole%). In plants being infected with wilt, the 5-methylcytosine content in DNA of cotton leaves decreases two-fold; no other changes in the structure and molecular population of DNA has been found. The sharp change in the 5-methylcytosine content in DNA of infected plants takes place at the expense of the decrease in the 5-methylcytosine content in fractions of highly reiterated sequences. The methylation degree of unique sequences (structural genes) remains unchanged.     

Netropsin binding in five duplex-dimer DNA constructs as a function of size and distance between binding sites: circular dichroism and absorption spectroscopy

The optical activity induced on binding the drug netrospin (NET) in the minor groove of DNA is studied in five oligonucleotides (OGNs) as a function of (1) the size of the binding site in (5′-(GC)₂AATT(GC)₂-3′)₂ (OGN 1a) versus (5′-(GC)₂AAATTT(GC)₂-3′)₂ (OGN 1b) and (2) the distance between two AATT binding sites in (5′-(GC)₂AATT(GC) _x AATT(GC)₂-3′)₂, with x = 1, 2, or 3 (OGNs 2a, b, c, respectively). NET binding is monitored via the induced circular dichroism (CD) at ~315 nm, where the nucleic acids are optically inactive. The CD titrations, fit to a tight binding model, yield lower limits for the binding constant, K_a, ≥8 × 10⁷ M⁻¹ for OGN 1a and ≥2 × 10⁸ M⁻¹ for OGNs 2a, b, c in 1 mM buffer. In 100 mM buffer, tight binding occurs in all five OGNs with K_a ≥ 8 × 10⁷ M⁻¹ for OGN 1a and ≥1 × 10⁸ M⁻¹ for OGNs 1b and 2a, b, c. In contrast, the elongated AAATTT binding site of OGN 1b results in weak binding of NET in 1 mM buffer, where competing electrostatic interactions with the solvent environment are lower. In the constructs with two binding sites, the increase in flexibility introduced by intervening GC base pairs does not induce co-operative binding, although differences in the number of binding sites, n (2.05–2.65), indicate that there may be differences in the way NET is bound in OGNs 2a, b, c. In addition, the large shifts in the absorption spectra induced in bound versus free NET, and effects on the CD spectral bands at higher energy, are discussed in terms of electrostatic and excitonic interactions.     

Effects of temperature, salinity and irradiance on the growth of the red tide dinoflagellate Gyrodinium instriatum Freudenthal et Lee

The effects of temperature, salinity and irradiance on the growth of the red tide dinoflagellate Gyrodinium instriatum Freudenthal et Lee were examined in the laboratory. Exposed to 45 different combinations of temperature (10–30 °C) and salinity (0–40) under saturating irradiance, G. instriatum exhibited its maximum growth rate of 0.7 divisions/day at a combination of 25 °C and a salinity of 30. Optimum growth rates (>0.5 divisions/day) were observed at temperatures ranging from 20 to 30 °C and at salinities from 10 to 35. The organism could not grow at ≤10 °C. In addition, G. instriatum burst at a salinity of 0 at all temperatures, but grew at a salinity of 5 at temperatures between 20 and 25 °C. It is noteworthy that G. instriatum is a euryhaline organism that can live under extremely low salinity. Factorial analysis revealed that the contributions of temperature and salinity to its growth of the organism were almost equal. The irradiance at the light compensation point (I₀) was 10.6 μmol/(m² s) and the saturated irradiance for growth (I_s) was 70 μmol/(m² s), which was lower than I_s for several other harmful dinoflagellates (90–110 μmol/(m² s)).     

Carbon dioxide fixation in Pyrobotrys stellata

The green alga Pyrobotrys stellata Korshik., an obligate phototroph, is unable to utilise carbon dioxide for growth, although assimilation of acetate is dependent on the photosynthetic process. The incorporation of ¹⁴CO₂ from ¹⁴C-bicarbonate into the cells of P. stellata is only 3% of that in Chlorella pyrenoidosa Chick. The activity of the key enzyme of the Calvin cycle, ribulose-1-5-diphosphate carboxylase, is very low in P. stellata, being only 7% of that in C. pyrenoidosa. The determination of the products of ¹⁴CO₂ fixation in intact cells confirms that ribulose-1-5-diphosphate activity is very low in P. stellata, since little carbon-14 is found in 1–3 diphosphoglyceric acid, the product of carboxylation of ribulose-1-5-diphosphate. It is concluded that the inability of P. stellata to utilize carbon dioxide for growth in the light is probably the result of the low ribulose-1-5-diphosphate carboxylase activity in the organism.     

Programmed cell death in plants under anaerobic conditions: Effect of Ag+

We investigated epidermal peels from the leaves of pea (Pisum sativum L.) consisting of a monolayer of the cells of two types: stomatal guard cells (GC) with chloroplasts and mitochondria and basic epidermal cells (EC) containing only mitochondria. As inducers of programmed cell death, we used KCN destroying the nuclei in GC and EC and chitosan that destroys nuclei only in EC. AgNO₃ (10 μM) stimulated the destruction of nuclei in GC and EC induced by CN^? and suppressed chitosan-induced destruction of nuclei in EC. The destruction of nuclei in GC induced by CN^? occurred under aerobic conditions and was prevented under anaerobiosis. The destruction of nuclei in GC induced by (CN^? + Ag⁺) occurred both under aerobic and anaerobic conditions and was not suppressed by antioxidants. Among the tested cations of metals (Ag+, Hg²⁺, Fe²⁺, Fe³⁺, Cu²⁺, and Mn²⁺), Ag⁺ turned out to be the most efficient in respect to the stimulation of cyanide-induced destruction of nuclei in GC. Half-maximum concentrations of Ag⁺ and Hg²⁺ were equal to 4–5 μM. In epidermal peels treated with chitosan, GC were permeable to propidium iodide; however, the nuclei in GC (in contrast to EC) were not destructed in the presence of chitosan. It was concluded that Ag⁺, acting as an electron acceptor during photosynthetic electron transfer in the chloroplasts from pea leaves, impeded the O₂ evolution by the chloroplasts treated with ferricyanide or silicomolybdate as electron acceptors, impeded the consumption of O₂ in the course of electron transfer from the (ascorbate + N,N,N′,N′-tetramethyl-p-phenylenediamine) to methylviologen and suppressed the production of reactive oxygen species (ROS) in GC and EC.     

Control of nitrogenase inAzospirillum sp.

Many N₂-fixing organisms can turn off nitrogenase activity in the presence of NH₄ ⁺ and turn it on again when the NH₄ ⁺ is exhausted. One of the most interesting systems for accomplishing this is by covalent modification of one subunit of dinitrogenase reductase by dinitrogenase reductase ADP-ribosyltransferase (DRAT). The system can be reactivated when NH₄ ⁺ is exhausted, by dinitrogenase reductase activating glycohydrolase (DRAG) which removes the inactivating group. It is fascinating that some species of the genusAzospirillum possess the DRAT and DRAG systems (A. lipoferum andA. brasilense), whereasA. amazonense in the same genus lacks DRAT and DRAG.A. amazonense responds to NH₄ ⁺ but does not exhibit modification of dinitrogenase reductase characteristic of the action of DRAT. However, it has been possible to clone DRAT and DRAG and to introduce them intoA. amazonense, whereupon they become functional in this organism. The DRAT and DRAG system does not appear to function inAcetobacter diazotrophicus, an organism isolated from sugar cane, that fixes N₂ at a pH as low as 3.0.A. diazotrophicus does show a rather sluggish response to NH₄ ⁺. A level of about 10 M NH₄ ⁺ is required to switch off the system. The response to NH₄ ⁺ is influenced by the dissolved oxygen concentration (DOC) as has been reported forAzospirillum sp. A DOC in equilibrium with 0.1 to 0.2 kPa O₂ seems optimal for the response inA. diazotrophicus.     

Calcium,protease activation,and cytoskeleton remodeling underlie growth cone formation and neuronal regeneration

The cytoarchitecture, synaptic connectivity, and physiological properties of neurons are determined during their development by the interactions between the intrinsic properties of the neurons and signals provided by the microenvironment through which they grow. Many of these interactions are mediated and translated to specific growth patterns and connectivity by specialized compartments at the tips of the extending neurites: the growth cones (GCs). The mechanisms underlying GC formation at a specific time and location during development, regeneration, and some forms of learning processes, are therefore the subject of intense investigation. Using cultured Aplysia neurons we studied the cellular mechanisms that lead to the transformation of a differentiated axonal segment into a motile GC. We found that localized and transient elevation of the free intracellular calcium concentration ([Ca²⁺] _i ) to 200–300 M induces GC formation in the form of a large lamellipodium that branches up into growing neurites. By using simultaneous on-line imaging of [Ca²⁺] _i and of intraaxonal proteolyticactivity, we found that the elevated [Ca²⁺] _i activate proteases in the region in which a GC is formed. Inhibition of the calcium-activated proteases prior to the local elevation of the [Ca²⁺] _i blocks the formation of GCs. Using retrospective immunofluorescent methods we imaged the proteolysis of the submembrane spectrin network, and the restructuring of the cytoskeleton at the site of GC formation. The restructuring of the actin and microtubule network leads to local accumulation of transported vesicles, which then fuse with the plasma membrane in support of the GC expansion.     

<Emphasis Type="Italic">Arhodomonas recens</Emphasis> sp. nov., a halophilic alkane-utilizing hydrogen-oxidizing bacterium from the brines of flotation enrichment of potassium minerals

A halophilic nonpigmented rod-shaped (0.8–1.0 × 2.0–2.5 μm), gram-negative bacterium with a single polar flagellum (strain RS91) was isolated from acidic brines of flotation enrichment of potassium minerals (Silvinit Co., Solikamsk, Russia). The strain grew in the media with 2 to 25% NaCl (optimum at 10–12%), 20–45°C (optimum at 37°C), and pH 5.5–8.5 (optimum 6.5–7.5). It was an aerobe or facultative anaerobe incapable of fermentation. The strain was characterized by the absence of growth on glucose, fructose, and citrate, extensive aerobic growth on n-hexadecane and in the mineral medium with H₂ + O₂ + CO₂ in the gas phase, anaerobic nitrate reduction with acetate or hydrogen (under H₂ + CO₂ + N₂), and variable fatty acid composition. The DNA G+C content was 68.2 mol %. Phylogenetic analysis based on 16S rRNA gene sequencing revealed that while strain RS91 was most closely related to Arhodomonas aquaeolei HA-1^T (98.3%) and Nitrococcus mobilis (98.1%), it was only remotely related to the halophilic phototroph Halorhodospira halophila (90.6%). Based on the combination of its phenotypic and genotypic characteristics, the organism was classified as a new species of the genus Arhodomonas, family Ectothiorhodospiraceae with the proposed name Arhodomonas recens sp. nov. The type strain is RS91^T (= IEGM 796^T = VKPM B-11280^T).     

Nitric oxide regulates steroid synthesis by bovine antral granulosa cells in a chemically defined medium

Nitric oxide (NO) in bovine ovary has been characterized as one of the controllers of granulosa cells’ (GC) steroidogenesis and apoptosis. One of the pathways used by NO to have these effects is cGMP. The objectives of the present study were to verify the effect of sodium nitroprusside (SNP), a NO donor, on steroidogenesis, cell viability (mitochondrial activity) and GC cell cycle distribution and if this effect occurs by the NO-cGMP signaling pathway with the addition of SNP with or without 1H-[1,2,3] oxadiaziolo[4,3a]quinoxaline-1-one (ODQ), a selective soluble guanylate cyclase inhibitor. The antral GC from 3 to 5 mm diameter cattle follicles was cultured without treatment (control), with ODQ (10⁻⁴ M) and 10⁻⁵, 10⁻³ and 10⁻¹ M SNP with or without ODQ for 24 h. Nitrate/nitrite (NO₃⁻/N0₂⁻) concentrations were evaluated by Griess method, progesterone (P₄) and 17β-estradiol (E₂) concentrations by chemiluminescence, viability and cell cycle stage by MTT method (3-[4,5-dimethylthiazol-2yl]-2,3 dipheniltetrazolium bromide) and flow cytometry, respectively. Nitrate/nitrite concentration in culture medium increased (P < 0.05) in a dose-dependent manner according to SNP concentration added to the culture medium. The GC cultured without treatment, with ODQ and with 10⁻⁵ M SNP in the presence or absence of ODQ developed into cell aggregates and did not vary in cell viability (P > 0.05), while GC cultured with 10⁻³ and 10⁻¹ M SNP with or without ODQ presented disorganized GC aggregates or did not develop into cell aggregates and also had substantially decreased cell viability (mitochondrial activity inhibition) and steroids synthesis (P < 0.05), and effects were not reversed with us of ODQ. Most GC cultured without treatment (control) or with ODQ, 10⁻⁵ and 10⁻³ M SNP with or without ODQ were in the G0/G1 (80–75%) stage and in a lesser proportion (20–25%) in the S + G2/M stage of the cell cycle, while the 10⁻¹ M SNP treatment resulted in GC in G1 phase arrest. The treatment with 10⁻⁵ M SNP increased (P < 0.05) E₂ synthesis and inhibited (P < 0.05) progesterone synthesis. The addition of ODQ reversed (P < 0.05) the stimulatory effect of 10⁻⁵ M SNP treatment on E₂, but not on P₄ synthesis (P > 0.05). These results demonstrated that E₂ synthesis by antral GC from small follicles is modulated by lesser NO concentrations via the cGMP pathway, but not P₄ while steroids inhibition cGMP pathway independent, mitochondrial damage and the interference on cell cycle progression caused by greater NO concentration can lead to cell death.     

Comparison of the effects of cationic porphyrins on DNA properties: influence of GC content of native and synthetic polymers

Interactions of meso-tetra(4-N-methylpyridyl)porphyrin [TMpyP(4)], meso-tetra(2-N-methylpyridyl)porphyrin [TMpyP(2)], and meso-tetra(para-N-trimethylanilinium)porphyrin (TMAP) with several native and synthetic DNAs were studied by a variety of physical techniques: nmr (³¹P and ¹H), absorption spectroscopy, viscosity, and flow dichroism (FD). Of the three porphyrins studied, only the interaction of TMpyP(4) with poly [d(G-C)₂] was fully consistent with intercalation. In particular, a large increase in viscosity, a downfield ³¹P-nmr signal (ca. -1 ppm), and upfield imino proton signals (11 to 12 ppm range) were observed. Comparison of the effects of TMpyP(4) on DNAs of different GC contents revealed larger changes in solution viscosity with increased GC content. However, the characteristic changes in ³¹P- and ¹H-nmr spectra were not observed. The viscosity increases observed in studies with poly[d(A-C)(G-T)] and C. Perf. DNA were much lower than with poly[d(G-C)₂], M. Lys. DNA, and calf thymus DNA. Thus, GC sequence and content are clearly important. The principal change in the ³¹P-nmr signal of native DNA is the appearance of a very broad shoulder centered at ca. -2.0 ppm, which is larger in M. Lys. DNA than in C. Perf. DNA. FD studies indicate highly ordered TMpyP(4) cations arranged perpendicular to the DNA axis of calf thymus DNA. Together, these results suggest the major effects of TMpyP(4) on DNA properties are due to strong GC-binding interactions that influence DNA structure. The data are consistent with combined intercalative and outside binding interactions of TMpyP(4) with GC regions of DNA. In contrast, similar studies with TMAP suggest that it influences AT regions of DNA by an outside binding mode. On the other hand, TMpyP(2) effects on DNA properties are consistent with nonselective outside binding.     

Metabolism of gibberellin A19 is under photoperiodic control in Populus, Salix and Betula, but not in daylength-insensitive Populus overexpressing phytochrome A

Application experiments have suggested that short‐day‐induced cessation of elongation growth in trees is caused by photoperiodic regulation of the conversion of gibberellin GA₁₉ to GA₂₀. In the present study we examined further the photoperiodic control of GA metabolism in trees with focus on the conversion of GA₁₉ in Salix pentandra, hybrid aspen (Populus tremula × tremuloides) and silver birch (Betula pendula) using [17,17‐²H₂]‐GA₁₉ or unlabelled GAs in application studies. GA₂₀ and GA₁ were able to restore growth also in hybrid aspen and silver birch under short days (SD), whereas GA₁₉ had no or only a very small activity. Contrary to hybrid aspen and S. pentandra, the activity of GA₂₀ in silver birch was significantly lower than that of GA₁. Gas chromatography‐mass spectrometry (GC‐MS) analysis revealed a smaller turnover of [²H₂]‐GA₁₉ in SD than in long days (LD) in hybrid aspen. No such difference in turnover of [²H₂]‐GA₁₉ was observed in photoperiod‐insensitive hybrid aspen overexpressing PHYA. Application of unlabelled GAs to seedlings of S. pentandra, hybrid aspen and silver birch under SD followed by quantification of metabolites by GC‐MS analysis, showed that applied GA₁₉ was not readily converted to GA₂₀ and GA₁. Although the sensitivity to GAs is also known to decrease under SD, the present data are in favour of a photoperiodic regulation of the metabolism of GA₁₉in vivo in the woody species S. pentandra, hybrid aspen and silver birch. The data might also suggest that silver birch differs from S. pentandra and hybrid aspen by exhibiting a possible photoperiodic control also of the conversion of GA₂₀ to GA₁.     

Reactive oxygen intermediates regulate cellular response to apoptotic stimuli: An hypothesis

Production of reactive oxygen intermediates (ROI) has been thought for a long time to adversely affect the physiology and survival of a cell. There is now a growing body of evidence to suggest that ROI such as superoxide anion (O^·-₂) and hydrogen peroxide (H₂O₂) can influence the growth, as well as death, of animal cells in vitro. The observation that cells release O^·-₂ or its dismutation product H₂O₂, either constitutively in the case of tumor cells or following cytokine stimulation, has led to the speculation that they might possibly serve as intercellular messengers to stimulate proliferation via mechanisms common to natural growth factors. However, as the balance between cell populations in an organism is tightly controlled by the rate of proliferation and death of constituent cells, an increase in cell numbers could reciprocally be viewed as deregulation of cell death. Hence, it is equally important to decipher how ROI influence the response of cells to signals that activate cell death pathway(s). We propose that ROI not only regulate proliferation but also affect cell sensitivity to triggers which activate the cellular suicide program (apoptosis) versus those that cause accidental (necrotic) cell death.