Dichlorophenyl ) -1,1-dimethylurea Research Articles

Transport Systems of Ventricaria ventricosa: Asymmetry of the Hyper- and Hypotonic Regulation Mechanisms

Hyper- and hypotonic stresses elicit apparently symmetrical responses in the alga Ventricaria. With hypertonic stress, membrane potential difference (PD) between the vacuole and the external medium becomes more positive, conductance at positive PDs (Gmpos) increases and KCl is actively taken up to increase turgor. With hypotonic stress, the membrane PD becomes more negative, conductance at negative PDs (Gmneg) increases and KCl is lost to decrease turgor. We used inhibitors that affect active transport to determine whether agents that inhibit the K(+) pump and hypertonic regulation also inhibit hypotonic regulatory responses. Cells whose turgor pressure was held low by the pressure probe (turgor-clamped) exhibited the same response as cells challenged by hyperosmotic medium, although the response was maintained longer than in osmotically challenged cells, which regulate turgor. The role of active K(+) transport was confirmed by the effects of decreased light, dichlorophenyldimethyl urea and diethylstilbestrol, which induced a uniformly low conductance (quiet state). Cells clamped to high turgor exhibited the same response as cells challenged by hypo-osmotic medium, but the response was similarly transient, making effects of inhibitors hard to determine. Unlike clamped cells, cells challenged by hypo-osmotic medium responded to inhibitors with rapid, transient, negative-going PDs, with decreased Gmneg and increased Gmpos (linearized I-V), achieving the quiet state as PD recovered. These changes are different from those exerted on the pump state, indicating that different transport systems are responsible for turgor regulation in the two cases.

Photosynthetic electron flow affects H2O2 signaling by inactivation of catalase in Chlamydomonas reinhardtii

A specific signaling role for H2O2 in Chlamydomonas reinhardtii was demonstrated by the definition of a promoter that specifically responded to this ROS. Expression of a nuclear-encoded reporter gene driven by this promoter was shown to depend not only on the level of exogenously added H2O2 but also on light. In the dark, the induction of the reporter gene by H2O2 was much lower than in the light. This lower induction was correlated with an accelerated disappearance of H2O2 from the culture medium in the dark. Due to a light-induced reduction in catalase activity, H2O2 levels in the light remained higher. Photosynthetic electron transport mediated the light-controlled down-regulation of the catalase activity since it was prevented by 3-(3′4′-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II. In the presence of light and DCMU, expression of the reporter gene was low while the addition of aminotriazole, a catalase inhibitor, led to a higher induction of the reporter gene by H2O2 in the dark. The role of photosynthetic electron transport and thioredoxin in this regulation was investigated by using mutants deficient in photosynthetic electron flow and by studying the correlation between NADP-malate dehydrogenase and catalase activities. It is proposed that, contrary to expectations, a controlled down-regulation of catalase activity occurs upon a shift of cells from dark to light. This down-regulation apparently is necessary to maintain a certain level of H2O2 required to activate H2O2-dependent signaling pathways.

Changes in the photosynthetic reaction centre II in the diatom Phaeodactylum tricornutum result in non‐photochemical fluorescence quenching

Diatoms are an important group of primary producers in the aquatic environment. They are able to acclimate to fast changes in the light intensity by various mechanisms including a rise in non-photochemical fluorescence quenching (NPQ). The latter has been attributed to the xanthophyll cycle (XC) following activation of diadinoxanthin de-epoxidase by the acidification of the thylakoid lumen. To examine whether fluorescence quenching in the diatom Phaeodactylum tricornutum depends on the DeltapH generated by the photosynthetic electron transport, we arrested the latter by 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU). This treatment hardly affected the NPQ or XC, even when methylviologen was present. Dissipation of the DeltapH by 2,4-dinitrophenol inhibited the XC but did not alter NPQ. Similar results, i.e. inhibition of the XC but normal fluorescence quenching, were observed when the experiments were performed at 3 degrees C. Measurements of thermoluminescence showed that excess light treatment caused a marked decline in the signals obtained as a result of recombination of Q(B) (-) with the S(3) state of the Mn cluster; this was also observed in cells treated with DCMU (recombination of Q(A) (-) with S(2)). Light treatment also diminished the Q(A) (-) re-oxidation signals. The data suggest that changes in PSII core centre itself due to exposure to excess light conditions play an important part in the acclimation of P. tricornutum to the changing light conditions.

Hydrogen peroxide is involved in high blue light-induced chloroplast avoidance movements in Arabidopsis

One of the most important functions of blue light (BL) is to induce chloroplast movements in order to reduce the damage to the photosynthetic machinery under excess light. Hydrogen peroxide (H(2)O(2)), which is commonly generated under various environmental stimuli, can act as a signalling molecule that regulates a number of developmental processes and stress responses. To investigate whether H(2)O(2) is involved in high-fluence BL-induced chloroplast avoidance movements, a laser scanning confocal microscope and a luminescence spectrometer were used to observe H(2)O(2) generation in situ with the assistance of the fluorescence probe dichlorofluorescein diacetate (H(2)DCF-DA). After treatment with high-fluence BL, an enhanced accumulation of H(2)O(2), indicated by the fluorescence intensity of DCF, can be observed in leaf cells of Arabidopsis thaliana. Exogenously applied H(2)O(2) promotes the high-fluence BL-induced chloroplast movements in a concentration-dependent manner within the range of 0-10(-4) M, not only increasing the degree of movements but also accelerating the start of migrations. Moreover, the high-fluence BL-induced H(2)O(2) generation and the subsequent chloroplast movements can be largely abolished by the administration of the H(2)O(2)-specific scavenger catalase and other antioxidants. In addition, in-depth subcellular experiments indicated that high-fluence BL-induced H(2)O(2) generation can be partly abolished by the addition of diphenyleneiodonium (DPI), which is an NADPH oxidase inhibitor, and the blocker of electron transport chain dichlorophenyl dimethylurea (DCMU), respectively. The results presented here suggest that high-fluence BL can induce H(2)O(2) generation at both the plasma membrane and the chloroplast, and that the production of H(2)O(2) is involved in high-fluence BL-induced chloroplast avoidance movements.

Fusaproliferin effects on the photosystem in the cells of maize seedling leaves

The possible role of the fusariotoxin, fusaproliferin in plant pathology was investigated with respect to cell membrane potential. Electron microscopy was used to study both the early effect of fusaproliferin on the host’s plasma membrane and ultrastructure responses in the cells of maize leaves. The seedlings of resistant (Lucia) and susceptible (Pavla) to the fusaproliferin maize cultivars were grown in the presence of fusaproliferin at different concentrations, namely 5 and 35 μg ml−1, respectively, and electrophysiological measurements were compared with those obtained using two different toxic compounds, namely fusicoccin and 3-3(3,4 dichlorophenyl)-1,1-dimethylurea (DCMU). It was observed that only the higher concentration of fusaproliferin induced the onset of visible symptoms on the leaves. Comparing the effect of fusaproliferin to that of fusicoccin and DCMU at the higher toxin concentration, it was observed that functional differences in membrane potential induced severe damage to the mesophyll and outer chloroplast membrane; the extent of changes in electrophysiology and ultrastructure disturbances depended on the toxin concentration and was greater in the susceptible cv. Pavla. Results indicated that fusaproliferin could be involved in Fusarium pathogenesis either as a virulence factor or by enhancing the activity of other toxins that might be concomitantly present in infected plants.

Expression of psbA Genes is Regulated at Multiple Levels in the Cyanobacterium Synechococcus sp. PCC 7942

In cyanobacterium Synechococcus sp. PCC 7942 the photosystem II reaction-center protein D1 is encoded by three psbA genes. The psbAI gene encodes D1:1 protein, the form used for acclimated growth, and psbAII and psbAIII genes encode the stress-induced form, D1:2 protein. Strong light and low temperature have been shown to induce the expression of psbAII/III genes and down-regulate the expression of psbAI gene. Recently, we reported the involvement of reduced thiols in the up-regulation of psbAII/III genes. In this study, we have analyzed the regulation of psbA gene expression in Synechococcus further, at both the transcriptional and post-transcriptional levels. We show that the inhibitors of the photosynthetic electron-transfer chain, which have different effects on the redox state of the plastoquinone (PQ) pool, have similar effect on the transcription of psbA genes. The inhibitors 3-(3,4 dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) do not cause any changes in psbA gene expression when added under low-light conditions, but dramatically reduce the high-light induction of psbAII/III genes when added upon a high-light shift. Moreover, when the thiol reductant, dithiothreitol, is added to Synechococcus cells together with DCMU concomitant with the high-light shift, no inhibition of psbAII/III gene up-regulation takes place, indicating that the thiol redox state rather than the redox state of the PQ pool regulates psbA gene transcription. We also provide evidence for post-transcriptional regulation of psbA gene expression, particularly, inhibition of translation of psbAI transcripts at high light, and demonstrate that the D1 protein synthesis and degradation processes are coregulated in Synechococcus.

Chromium(VI)-reducing Chlorella spp. isolated from disposal sites of paper-pulp and electroplating industry

We isolated four cultures of chromate resistant, unicellular, non-motile green algae from disposal sites of the paper-pulp and electroplating industries. These algae were maintained in Tris-acetate-glycerophosphate medium containing 30 μM K2Cr2O7. The morphological features as well as analysis of the 500-bp fragment of 18S rDNA (NS 12 region) showed that these isolates belong to Chlorella spp. These isolates showed EC50 values for chromate ranging from 60 to 125 μM. Uptake studies with radioactive 51Cr(VI) showed that 10–19% of total radioactivity was intracellular, and 1–2% was bound to the cell wall. The rest of the activity remained in the medium, suggesting that resistance was not related to accumulation of Cr(VI) in the cells. Interestingly, when these isolates were grown in the presence of 30 μM of K2Cr2O7, a decrease in the Cr(VI) concentration in the medium was observed. Only live cells could deplete Cr(VI) from the supernatant, suggesting the presence of chromium reduction activity in these Chlorella isolates. Cr(VI) reduction activity of the cells of Chlorella was stimulated by light as well as by acetate and glycerophosphate. Treatment of Chlorella cells with 3-(3,4 dichlorophenyl),1,1dimethyl urea (DCMU) did not affect the Cr(VI) reduction. However, if the cells were treated with sodium azide, Cr(VI) reduction was severely affected. Though chromate resistance has been well documented in algae, the information on chromate reduction by algae is scant. This paper discusses the Cr(VI) reduction by Cr(VI) resistant Chlorella, which may find a use in the effective bioremediation of Cr(VI).

Death of stoma guard cells in leaf epidermis under disturbance of energy provision.

Cyanide is an apoptosis inducer in stoma guard cells from pea leaf epidermis. Unlike CN-, the uncoupler of oxidative and photosynthetic phosphorylation carbonyl cyanide m-chlorophenylhydrazone (CCCP), the combination of CCCP, 3-(3 ,4 -dichlorophenyl)-1,1-dimethylurea (DCMU), benzylhydroxamate (BH), myxothiazol, antimycin A, and a glycolysis inhibitor 2-deoxyglucose (DG) did not induce destruction of guard cell nuclei for 20 h of incubation of epidermal peels in the light. DCMU prevented the effect of CN- as a programmed cell death (PCD) inducer. CCCP, the combination of DCMU and CCCP, or the combination of DCMU, CCCP, BH, myxothiazol, antimycin A, and DG supplemented by CN- caused destruction of cell nuclei; the number of the cells lacking nuclei in this case was higher than with CN- alone. DG and CCCP caused cell destruction after longer incubation of the isolated epidermis - after 2 days and to a greater degree after 4 days. The effect of DG and CCCP was reduced by illumination. Cell destruction during long-term incubation was prevented by the combination of DG and CCCP. From data of electron microscopy, DCMU and dinitrophenyl ester of iodonitrothymol (DNP-INT) prevented apoptotic changes of the nuclear ultrastructure induced by CN-. The suppression of the destruction of the guard cell nuclei under combined action of DG and CCCP was apparently caused by switching of cell death from PCD to necrosis. Thus, the type of cell death - via apoptosis or necrosis - is controlled by the level of energy provision.

Imaging the production of singlet oxygen in vivo using a new fluorescent sensor, Singlet Oxygen Sensor Green(R)

Singlet oxygen is known to be produced by cells in response to photo-oxidative stresses and wounding. Due to singlet oxygen being highly reactive, it is thought to have a very short half-life in biological systems and, consequently, it is difficult to detect. A new commercially available reagent (singlet oxygen sensor green, SOSG), which is highly selective for singlet oxygen, was applied to a range of biological systems that are known to generate singlet oxygen. Induction of singlet oxygen production by the addition of myoglobin to liposome preparations demonstrated that the singlet oxygen-induced increases in SOSG fluorescence closely followed the increase in the concentration of conjugated dienes, which is stoichiometrically related to singlet oxygen production. Applications of photo-oxidative stresses to diatom species and leaves, which are known to result in the production of singlet oxygen, produced large increases in SOSG fluorescence, as did the addition of 3-(3',4'-dichlorophenyl)1,1-dimethylurea (DCMU) to these systems, which inhibits electron transport in photosystem II and stimulates singlet oxygen production. The conditional fluorescent (flu) mutant of Arabidopsis produces singlet oxygen when exposed to light after a dark period, and this coincided with a large increase in SOSG fluorescence. Wounding of leaves was followed by an increase in SOSG fluorescence, even in the dark. It is concluded that SOSG is a useful in vivo probe for the detection of singlet oxygen.

Regulation of nitrate reductase in Chlamydomonas reinhardtii by the redox state of the plastoquinone pool

In the chlorophyte alga Chlamydomonas reinhardtii, expression of the nuclear gene NIA1, encoding nitrate reductase, is regulated by light, but the signal transduction mechanism is poorly understood. Using inhibitors, mutants, and physiological manipulation, we searched for signals in the photosynthetic electron transport chain that potentially regulate NIA1 expression. In the NIA1 + wild-type clone CC-1692, nitrate reductase activity is strongly down-regulated when the reduction of plastoquinone is blocked by 3-(3′4′-dichlorophenyl)-1,1′-dimethyl urea (DCMU), but unaffected or stimulated when the oxidation of plastoquinol is inhibited by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). Simultaneously, although DBMIB reduced NIA1 expression by ∼30% over a 6-h period relative to the control, DCMU inhibited expression of the gene by over 80%. A cross between CC-1692 and a site-directed mutant, CC-3388 A251I, in which amino acid 251 in the PSII core protein, D1, was altered from alanine to isoleucine, thereby decreasing the binding affinity for QB, produced a cell with markedly reduced expression of NIA1. Our results indicate that expression of nitrate reductase is coupled to photosynthesis via a sensor related to the redox poise of the plastoquinone pool. When the pool is oxidized, carbon fixation is low and nitrate reductase is down-regulated; conversely, when the pool is reduced, carbon fixation is high and the gene and enzyme activity are up-regulated. These experimental observations suggest a model for the coupled light regulation of photosynthesis and nitrate assimilation.

Coral calcification: Use of radioactive isotopes and metabolic inhibitors to study the interactions with photosynthesis and respiration

In order to characterize the process of calcification in scleractinian corals, a series of laboratory experiments were conducted using radioactive isotopes. Labelled calcium, bicarbonate and glucose were used and the fates of the labelled tracers were followed in the skeleton and the tissue fractions of the coral Galaxea fascicularis. In addition, a variety of metabolic inhibitors were used to test the effects of various enzymes and processes on the incorporation rates. The incorporation rate of 45Ca into the coral skeleton decreased to about one-fifth upon inhibition of metabolic respiration by the specific inhibitor NaCN suggesting a major role of metabolic respiration in coral calcification, and decreased to one-half upon inhibition of carbonic anhydrase by the specific inhibitor acetazolamide indicating a role of the enzyme in the process. The results obtained have also shown that corals are able to incorporate carbon from seawater bicarbonate and added glucose in both skeleton and tissue fractions. The process of incorporation was influenced by light conditions, carbonic anhydrase, respiration and photosynthesis. The incorporation rate of 14C-HCO was reduced to about one-tenth in the skeleton, and one-fifth in the tissue, upon inhibition of carbonic anhydrase suggesting a major influence of the enzyme in the incorporation process. The inhibition of photosynthesis had more influence on the tissue incorporation rate of the tracer than the skeleton suggesting that photosynthesis is the main process responsible for tissue use of seawater bicarbonate in the coral. 14C-glucose incorporation into the skeleton was mainly affected by NaCN addition and to a lesser extent by dichlorophenyldimethylurea (DCMU) addition, while the tissue fraction was mainly affected by NaCN addition. It was concluded that respiration and photosynthesis, in addition to the enzyme carbonic anhydrase, are limiting factors for the process of calcification in the coral, and that various forms and sources of carbon can be used in coral calcification and tissue growth.

High-Temperature Induced Chlorophyll Fluorescence Rise in Plants at 40–50 °C: Experimental and Theoretical Approach

We studied the temperature dependence of chlorophyll fluorescence intensity in barley leaves under weak and actinic light excitation during linear heating from room temperature to 50 degrees C. The heat-induced fluorescence rise usually appearing at around 40-50 degrees C under weak light excitation was also found in leaves treated with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) or hydroxylamine (NH(2)OH). However, simultaneous treatment with both these compounds caused a disappearance of the fluorescence rise. We have suggested that the mechanism of the heat-induced fluorescence rise in DCMU-treated leaves is different than that in untreated or NH(2)OH-treated leaves. In DCMU-treated leaves, the heat-induced fluorescence rise reflects an accumulation of Q(A) (-) even under weak light excitation due to the thermal inhibition of the S(2)Q(A) (-) recombination as was further documented by a decrease in the intensity of the thermoluminescence Q band. Mathematical model simulating this experimental data also supports our interpretation. In the case of DCMU-untreated leaves, our model simulations suggest that the heat-induced fluorescence rise is caused by both the light-induced reduction of Q(A) and enhanced back electron transfer from Q(B) to Q(A). The simulations also revealed the importance of other processes occurring during the heat-induced fluorescence rise, which are discussed with respect to experimental data.

GLYCEROL'S OFF THE MENU FOR ANEMONES

It's quite easy working out what we consume in a day – measuring what goes in is straightforward. But it's a different matter when your favourite food source is packed away inside you. Keeping track of the metabolic trade between symbionts seems virtually impossible. Lynne Whitehead explains that the nutritional traffic in an intact symbioses had only been successfully traced in one symbiotic partnership, and that most metabolite identification was done on symbionts that had been separated. But Whitehead realised that she might be able to untangle a symbioses' complex food web if she could make the donor symbiont dormant, and then feed the host a simulated cocktail of the symbiont's products to see if it satisfied the recipient's metabolic requirements (p.3149). Unsure whether her unconventional approach would reveal anything about the symbioses'nutritional traffic, she decided to `just try, things might work'.First Whitehead needed a symbiotic tenant that she could shut down as simply as throwing a switch; a photosynthetic tenant. Whitehead could control the symbiont's donations to its host simply by switching off the lights. Knowing that the small green sea anemone, Anemonia viridis, harboured the photosynthetic dinoflagellate, Symbiodinium, Whitehead left her lab in York, and travelled across the UK to a cold beach in North Wales. Up to her knees in the icy water, Whitehead collected the small green anemones from their tide pool homes, ready to see whether the animals could thrive on a nutrient cocktail when it's dinoflagellate lodger was dormant.Back in the lab, Whithead provided the intact symbioses with radiolabelled bicarbonate and allowed the dinoflagellates to photosynthesise for a few minutes before separating the symbioses' metabolic products with ion exchange resin and thin layer chromatography. But the amount of each radioactive metabolite produced by the anemone was vanishingly small; Whitehead had a tantalising 6-week wait while she exposed the chromatography plate to film before she could see the metabolite pattern produced by the micro-organism and its host.But would the symbioses produce the same distribution of metabolites when the dinoflagellates were on standby? Instead of dimming the lights, Whitehead used dichlorophenyl dimethyl urea to inhibit the dinflagellates' photoreactive cascade, then she exposed the anemones to the radiolabeled sugars and organic acids that she suspected were released by the micro-organisms. After another nailbiting wait, Whitehead compared the metabolite patterns produced by the inhibited symbioses with the photosynthesising symbioses. They were essentially identical! The anemones that had been supplied with glucose, and fumarate or succinate, had produced a realistic metabolite pattern.Knowing that several microbial symbionts seemed to produce high levels of glycerol when separated from their host, Whitehead also tried supplementing the half active symbioses with glycerol. But instead of producing a natural pattern of metabolites, she realised that these anemones didn't consume the sugar. She suspects that glycerol release from the microbes could turn out to be an artefact of separation, rather than an essential nutrient for some symbioses. Amazed that the long-shot had worked, Whitehead is optimistic that it might be possible to begin identifying who's supplying who, and with what,by shutting off other microbe's energy supplies in intact symbioses. But for the time being, she's sticking with the little green anemone, and its tidal tastes.

Light Induced Enhancement in Proline Levels Under Stress is Regulated by Non-Photosynthetic Events

Vigna radiata (L.) seedlings (5-d-old) were exposed to different concentrations of NaCl in light and in dark. The content of proline in the shoots increased with an increase in NaCl concentration, in light as well as in dark. But, irrespective of the concentration of NaCl, proline accumulation in the shoots was higher in light than in dark. Pretreatment of seedlings with dichlorophenyl dimethyl urea (DCMU) did not make any significant difference in light promoted stress induced proline accumulation. As DCMU is a potent inhibitor of photosynthetic electron transport, the light reaction of photosynthesis was not responsible for the observed light promotion of stress induced proline accumulation. In another set of experiments, 5-d-old green as well as etiolated seedlings were exposed to NaCl stress in the presence of different concentrations of sucrose. Irrespective of the concentration of sucrose used, proline content in shoots of stressed seedlings was higher in light than in dark. Although, sucrose enhanced NaCl stress induced increase in proline content in dark by about 32 %, this enhancement was not comparable to the 286 % increase in proline content brought about by light. These results showed that certain factors other than photosynthesis play a role in light promotion of stress induced proline accumulation.

Polyribosome loading of spinach mRNAs for photosystem I subunits is controlled by photosynthetic electron transport.

In light-, but not in dark-grown spinach seedlings, the mRNAs for the nuclear-encoded photosystem I subunits D, F and L are associated with polyribosomes and this association is prevented by the application of 3-(3',4'-dichlorophenyl)-1,1'-dimethyl urea (DCMU), an inhibitor of the photosynthetic electron transport. To identify the cis-elements which are responsible for this regulation, we generated a series of chimeric PsaD constructs and tested them in transgenic tobacco. The spinach PsaD 5'-untranslated region is sufficient to confer light- and photosynthesis-dependent polyribosome association onto the uidA reporter gene, while the tobacco PsaD 5'-untranslated region directs constitutive polyribosome association. These results are discussed with regard to signals from photosynthetic electron flow which control processes in the cytoplasm.

Photosynthetic control of the plasma membrane H+-ATPase in Vallisneria leaves. I. Regulation of activity during light-induced membrane hyperpolarization.

In mesophyll cells of the aquatic angiosperm Vallisneria gigantea Graebner, red, blue, or blue plus far-red light induced a typical membrane hyperpolarization, whereas far-red light alone had little effect. Both N,N'-dicyclohexylcarbodiimide, a potent inhibitor of H+-ATPase, and carbonylcyanide m-chlorophenylhydrazone, an uncoupler, produced a considerable membrane depolarization in the dark-adapted cells and a complete suppression of the light-induced hyperpolarization. Although 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosynthetic electron transport, did not affect the membrane potential in darkness, it completely inhibited the light-induced membrane hyperpolarization. In vivo illumination of the leaves with red light caused a substantial decrease in the Km for ATP, not only of the vanadate-sensitive ATP-hydrolyzing activity in leaf homogenate, but also of the ATP-dependent H+-transporting activity in plasma membrane (PM) vesicles isolated from the leaves by aqueous polymer two-phase partitioning methods. The effects of red light were negated by the presence of DCMU during illumination. In vivo illumination with far-red light had no effect on the Km for ATP of H+-transporting activity. These results strongly suggest that an electrogenic component in the membrane potential of the mesophyll cell is generated by the PM H+-ATPase, and that photosynthesis-dependent modulation of the enzymatic activity of the PM H+-ATPase is involved in the light-induced membrane hyperpolarization.

Potassium drives daily reversible thallus enlargement in the marine red alga Porphyra leucosticta (Rhodophyta).

Growth rate in terms of area expansion per 30 min was measured in the marine red algae Porphyra leucosticta under light/dark cycles of 6:6 h. Thalli grown in artificial seawater (ASW), under controlled ionic concentrations, showed a rapid thallus expansion just after light-on (morning-peak). Dark phase began with a significant thallus contraction (dark-peak) but no growth was observed until the next light phase. The removal of K+ from the medium inhibited the reversible growth peak that this species shows after light-on. On the other hand, the removal of Na+ did not have an apparent effect on growth pattern. Addition of Rb+ to K+-free ASW restored the morning-peak to 60% of its value in ASW, but addition of Li+ failed to restore the morning-peak. Intracellular ion analyses revealed that after light-on, the internal K+ content of the cells of this species increased 4-fold in 90 min, reaching an intracellular concentration of up to 300 micromol K+ per gram fresh weight, and that this value remained fairly constant over the light phase. Addition of 100 mM of tetraethylammonium, a specific K+-channel blocker, inhibited the morning-peak by 40%. The Na+ and Cl- contents in the cells increased rapidly in the first 45 min of the light phase, but then the internal concentrations of both ions decreased to their minimum values in the light phase. The carbonic anhydrase inhibitors acetazolamide and 6-ethoxyzolamide did not affect thallus expansion during the light phase. In contrast, thallus expansion in the light phase was completely inhibited by diethylstilbestrol, an inhibitor of the putative primary pump, and by 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosynthesis, but not by sodium vanadate, a specific inhibitor of the plasma-membrane H+-ATPase. We suggest that the periodic oscillation of the short-term growth rate of P. leucosticta occurs in response to potassium fluxes, which control the osmotic pressure and eventually the relative cell volume. The possible effects of the loosening of the cell wall and the internal K+ concentration on the growth of this species are also discussed.

Enhanced photochemical light utilization and decreased chilling-induced photoinhibition of photosystem II in cotton overexpressing genes encoding chloroplast-targeted antioxidant enzymes.

The aim of this study was to determine whether increases in stromal superoxide dismutase (SOD; EC 1.15.1.1), ascorbate peroxidase (APX; EC 1.11.1.11) and glutathione reductase (GR; EC 1.6.4.2) via transformation could reduce photosystem (PS) II photoinhibition at low temperature for cotton (Gossypium hirsutum L.) plants and to determine by what mechanism this protection may be realized. During 3-h exposures of lincomycin-treated leaf discs to 10 degrees C and a photon flux density of 500 &mgr;mol m-2 s-1, all transgenic plants exhibited significantly greater PSII activity and O2 evolution than did wild-type plants. Also, the rate constant of PSII photoinactivation was significantly lower for all transgenic plants than for wild-type plants. No significant differences existed between genotypes in non-photochemical quenching of chlorophyll a fluorescence and the regulated component of the thermal dissipation of excitation energy. The relationship between changes in variable to maximum chlorophyll fluorescence (Fv/Fm) and the time-dependent averaged excessive light exposure was similar for all genotypes. This observation excluded the possibility that differences in PSII photodamage were due to improvements in the direct protection of PSII from active oxygen by antioxidant enzyme overproduction. Similar decreases in Fv/Fm during the stress treatment for all genotypes when leaves were pre-treated with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) suggested that the effect of overproduction involved events downstream of PSII in the electron transfer pathway. Since all transgenic plants exhibited a significantly higher photochemical quenching of chlorophyll fluorescence during the chilling treatment, we concluded that, under the conditions used in this study, the enhancement of the protection of PSII from photodamage by increasing the stromal antioxidant enzyme activity in cotton leaves was due to the maintenance of a higher rate of electron transport and, consequently, a lower reduction state of QA.

Photosynthesis and calcification in the calcifying algae Halimeda discoidea studied with microsensors

AbstractWith microsensors, we measured the steady‐state microprofiles of O2, pH and Ca2+ on the topside of young segments of Halimeda discoidea, as well as the surface dynamics upon light–dark shifts. The effect of several inhibitors was studied. The steady‐state measurements showed that under high light intensity, calcium and protons were taken up, while O2 was produced. In the dark, O2 was consumed, the pH decreased to below seawater level and Ca2+ uptake was reduced to 50%. At low light intensity (12 mmol photons m‐2 s‐1), Ca2+ efflux was observed. Upon light–dark shifts, a complicated pattern of both the pH and calcium surface dynamics was observed. Illumination caused an initial pH decrease, followed by a gradual pH increase: this indicated that the surface pH of H. discoidea is determined by more than one light‐induced process. When photosynthesis was inhibited by dichlorophenyl dimethyl urea (DCMU), a strong acidification was observed upon illumination. The nature and physiological function of this putative pump is not known. The calcium dynamics followed all pH dynamics closely, both in the presence and absence of DCMU. The Ca‐channel blockers verapamil and nifedipine had no effect on the Ca2+ dynamics and steady‐state profiles. Thus, in H. discoidea, calcification is not regulated by the alga, but is a consequence of pH increase during photosynthesis. Acetazolamide had no effect on photosynthesis, whereas ethoxyzolamide inhibited photosynthesis at higher light intensities. Therefore, all carbonic anhydrase activity is intracellular. Carbonic anhydrase is required to alleviate the CO2 limitation. Calcification cannot supply sufficient protons and CO2 to sustain photosynthesis.

Research note: On the intermediate steps between the O and P steps in chlorophyll a fluorescence rise measured at different intensities of exciting light

It is known that the characteristic O–I–P transient of the fast chlorophyll a fluorescence rise (FR) detected under low intensity of exciting light, changes to the O–J–I–P (O–I 1 –I 2 –P) transient under higher intensity of exciting light. In this work, we extend an application of the mathematical model of FR [Lazár et al. (1997)l Pesticide Biochemistry and Physiology 57, 200–210], involving photosystem II (PSII) heterogeneity and simulating the O–J–I–P transient, for modeling of FR under different intensities of exciting light. Our simulations qualitatively agree well with experimental FR curves obtained with wheat leaves. The simulations demonstrate that the first step after the O fluorescence level (the I step and the J step under lower and higher intensities of exciting light, respectively) is caused mainly by the accumulation of reduced QA electron acceptors without reduction of subsequent electron acceptors. The first step appears at shorter times with increasing intensity of exciting light as reported previously [Strasser et al. (1995) Photochemistry and Photobiology 61, 32–42]. Our simulations also demonstrate, in accordance with previous publications, that the O–I phase in FR detected under low intensity of exciting light, reflects mainly the accumulation of Q A – in the Q B-non-reducing PSII centers and partly also the accumulation of Q A – in the Q B -reducing PSII centers. In our model, the accumulation of Q A – in the Q B -non-reducing PSII centers gradually shifts with increasing intensity of exciting light to shorter times and participates in the formation of the O–J phase; this is also supported by measurements of FR with wheat leaves treated with 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU). Under high intensity of exciting light, the O–J phase in FR also markedly reflects the accumulation of Q A – in the Q B -reducing PSII centers. Our simulations also support the previously reported suggestion of Strasser’s group that the I step in FR obtained under high intensity of exciting light (in the position of the I step detected under lower intensity of exciting light) appears mainly owing to the accumulation of the Q A –Q B 2– and Q A–Q BH 2f orms.