The effect on photosynthetic electron transport of temperature-dependent changes in the fluidity of the thylakoid membrane in a thermophilic blue-green alga

Various electron transport reactions in cell or isolated thylakoid membranes of the thermophilic blue-green alga, Synechococcus sp. were measured at different temperatures between 72 and 3 degrees C. They are classified into two groups with respect to their temperature dependency. The first group involves cytochrome 553 photooxidation, methyl viologen photoreduction with reduced 2,6-dichlorophenolindophenol as electron donor and 3-(3',4'-dichlorophenyl)-1,1-dimethylurea-resistant ferricyanide photoreduction determined in the presence or absence of silicomolybdate. The Arrhenius plot of these reactions showed a single straight line with the activation energy of about 10 kcal/mol throughout wide temperature ranges studied. Methyl viologen photoreduction with water as electron donor, reduction of flash-oxidized cytochrome 553, ferricyanide photoreduction and photosynthetic O2 evolution form the second group. Their arrhenius plots are characterized by discontinuities or breaks at about 30 and 10 degrees C, which respectively correspond to the upper and lower boundaries of the lateral phase separation of the membrane lipids. The first group reactions represent short spans of electron transport which are mediated either by Photosystem I or Photosystem II alone and not related to plastoquinone, whereas all the reactions of the second group involve plastoquinone. It is concluded therefore that the membrane fluidity affect electron transport specifically at the region of plastoquinone. It is proposed that the reaction center chlorophyll-protein complexes of both Photosystems I and II are closely associated with related electron carrier proteins to form functional supramolecular assemblies so that electron transfer within such a cluster of proteins proceeds independently of the phase changes in the membrane lipids. On the other hand, the role of plastoquinone as a mobile electron carrier mediating electron transfer from the protein assembly of Photosystem II to that of Photosystem I through the fluid hydrophobic matrix of the membranes is highly sensitive to the physical state of the membrane lipids.     

Superoxide formation in pea chloroplasts by a dioxathiadiaza-2,5-pentalene derivative,a new lipophilic: Photosystem I acceptor

The dioxathiadiaza-2,5-pentalene derivative, HEP II, has herbicidal effects similar to those of methyl viologen. HEP II promotes superoxide formation when added to illuminated pea chloroplasts. Superoxide dismutase, but not catalase, diminished formation of the Superoxide whereas cyanide and azide enhanced its formation, presumably by inhibiting the endogenous superoxide dismutase activity. DCMU, which inhibits photosynthetic electron transport, inhibited Superoxide formation. Rates of superoxide formation and oxygen uptake were very similar when equal concentrations of methyl viologen or HEP II were added. At subsaturating concentrations of electron acceptor, Mg²⁺ decreased the rate of oxygen uptake with methyl viologen but not with HEP II, probably reflecting differences in their interactions with the Photosystem I electron donation site. It is likely that HEP II, by analogy with methyl viologen, is reduced by chloroplast Photosystem I and reoxidised by molecular oxygen, generating superoxide.     

H2O2 intensifies CN−-induced apoptosis in pea leaves

H2O2 intensifies CN(-)-induced apoptosis in stoma guard cells and to lesser degree in basic epidermal cells in peels of the lower epidermis isolated from pea leaves. The maximum effect of H2O2 on guard cells was observed at 10(-4) M. By switching on non-cyclic electron transfer in chloroplasts menadione and methyl viologen intensified H2O2 generation in the light, but prevented the CN--induced apoptosis in guard cells. The light stimulation of CN- effect on guard cell apoptosis cannot be caused by disturbance of the ribulose-1,5-bisphosphate carboxylase function and associated OH* generation in chloroplasts with participation of free transition metals in the Fenton or Haber-Weiss type reactions as well as with participation of the FeS clusters of the electron acceptor side of Photosystem I. Menadione and methyl viologen did not suppress the CN(-)-induced apoptosis in epidermal cells that, unlike guard cells, contain mitochondria only, but not chloroplasts. Quinacrine and diphenylene iodonium, inhibitors of NAD(P)H oxidase of cell plasma membrane, had no effect on the respiration and photosynthetic O2 evolution by leaf slices, but prevented the CN(-)-induced guard cell death. The data suggest that NAD(P)H oxidase of guard cell plasma membrane is a source of reactive oxygen species (ROS) needed for execution of CN(-)-induced programmed cell death. Chloroplasts and mitochondria were inefficient as ROS sources in the programmed death of guard cells. When ROS generation is insufficient, exogenous H2O2 exhibits a stimulating effect on programmed cell death. H2O2 decreased the inhibitory effects of DCMU and DNP-INT on the CN(-)-induced apoptosis of guard cells. Quinacrine, DCMU, and DNP-INT had no effect on CN(-)-induced death of epidermal cells.     

The energy source for CO<Subscript>2</Subscript> transport in the marine microalga <Emphasis Type="Italic">Nannochloris atomus</Emphasis>

CO2 fluxes in the marine microalga Nannochloris atomus were studied by mass spectrometry using inhibitors and artificial acceptors of photosynthetic electron transport to investigate the energy source for CO2 uptake. This algal species is capable of taking up CO2 from the external medium by active transport but lacks active HCO(3)(-) transport and extracellular carbonic anhydrase. The capacity of cells to take up CO2 was a function of photosynthetic photon flux density. Dark respiration rates were also dependent upon the light intensity during the preceding illumination period, indicating the presence of light-enhanced dark respiration. Addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea to illuminated cell suspensions that had been allowed to concentrate inorganic carbon internally during photosynthesis caused a rapid burst of CO2, demonstrating that active CO2 transport had been abolished. A similar response was obtained when cell suspensions were treated with 2,5-dibromo-6-isopropyl-3methyl-1,4-benzoqinone or hydroxylamine. When methyl viologen was used to drain electrons from ferredoxin, cells were still able to take up CO2 from the external medium, although C-fixation decreased with time. These results demonstrate that active CO2 transport in N. atomus is supported by photosynthetic linear electron transport.     

NADH and NADPH as electron donors to respiratory and photosynthethic electron transport in the blue-green alga,Aphanocapsa

In the blue-green alga, Aphanocapsa, light inhibits respiration. This can be observed with spheroplasts when O₂ uptake is measured with NADH or NADPH as electron donor. However, NAD(P)H oxidation is unaffected by illumination. Furthermore, it was possible to demonstrate electron transfer from NAD(P)H to Photosystem I. Thus, the inhibition of respiratory oxygen uptake by light is explained by a competition of cytochrome oxidase and Photosystem I for reduction equivalents. Based on studies with inhibitors, electron transfer from NAD(P)H to Photosystem I involves the chloroplast cytochrome b₆-f complex.     

H2O2-Induced Inhibition of Photosynthetic O2 Evolution by Anabaena variabilis Cells

Hydrogen peroxide inhibits photosynthetic O2 evolution. It has been shown that H2O2 destroys the function of the oxygen-evolving complex (OEC) in some chloroplast and Photosystem (PS) II preparations causing release of manganese from the OEC. In other preparations, H2O2 did not cause or caused only insignificant release of manganese. In this work, we tested the effect of H2O2 on the photosynthetic electron transfer and the state of OEC manganese in a native system (intact cells of the cyanobacterium Anabaena variabilis). According to EPR spectroscopy data, H2O2 caused an increase in the level of photooxidation of P700, the reaction centers of PS I, and decreased the rate of their subsequent reduction in the dark by a factor larger than four. Combined effect of H2O2, CN-, and EDTA caused more than eight- to ninefold suppression of the dark reduction of P700+. EPR spectroscopy revealed that the content of free (or loosely bound) Mn2+ in washed cyanobacterial cells was ~20% of the total manganese pool. This content remained unchanged upon the addition of CN- and increased to 25-30% after addition of H2O2. The content of the total manganese decreased to 35% after the treatment of the cells with EDTA. The level of the H2O2-induced release of manganese increased after the treatment of the cells with EDTA. Incubation of cells with H2O2 for 2 h had no effect on the absorption spectra of the photosynthetic pigments. More prolonged incubation with H2O2 (20 h) brought about degradation of phycobilins and chlorophyll a and lysis of cells. Thus, H2O2 causes extraction of manganese from cyanobacterial cells, inhibits the OEC activity and photosynthetic electron transfer, and leads to the destruction of the photosynthetic apparatus. H2O2 is unable to serve as a physiological electron donor in photosynthesis.     

Effect of oxidative stress inductors on the photosynthetic apparatus in cyanobacterium Synechocystis sp. PCC 6803 Prq20 mutant resistant to methyl viologen

The damaging effect of oxidative stress inductors: methyl viologen, benzyl viologen, cumene hydroperoxide, H2O2, menadion, and high irradiance on the photosynthetic apparatus of cyanobacterium Synechocystis sp. PCC 6803 in cells of the wild type strain and the methyl viologen-resistant Prq20 mutant with the disrupted function of the regulatory gene prqR has been investigated by measuring the delayed fluorescence of chlorophyll a and the rate of CO2dependent -O2 gas exchange. It has been shown that the damage to the photosynthetic apparatus in the Prq20 mutant as compared with the wild type was less in the presence of methyl viologen and benzyl viologen. Reasons for the enhanced resistance of the photosynthetic apparatus in the mutant Prq20 to methyl viologen and benzyl viologen are discussed.     

Temperature Dependence of Photosynthetic Activities in Wheat Seedlings Grown in the Presence of BASF 13.338 (4-Chloro-5-Dimethylamino-2-Phenyl-3(2H)Pyridazinone)

When Triticum vulgare cv HD 2189 seedlings were grown in the presence of 125 micromolar BASF 13.338 (4-chloro-5-dimethylamino-2-phenyl-3(2H)pyridazinone), the rate of electron transport (H₂O → methyl viologen) in chloroplast thylakoids isolated from the treated seedlings was higher (by 50%) as compared to the control at assay temperatures above 30°C. Below 30°C, however, the rate with the treated seedlings was lower than the control rate. The temperature dependence of the rate of photosystem I electron transport (2-6-dichlorophenol indophenol-reduced → methyl viologen) in the treated system was similar to that in the control. At high temperatures (>30°C), with diphenyl carabazide as electron donor, the rates of electron transfer (diphenyl carbazide → methyl viologen) were similar in the treated and in the control thylakoids. Direct addition of BASF 13.338 to the assay mixture for the measurement of rate of electron transport (H₂O → methyl viologen) in the thylakoids isolated from the control plants did not cause any change in the temperature dependence of photosynthetic electron transport. These results suggested that the donor side of photosystem II became tolerant to heat in the treated plants. Chlorophyll a fluorescence emission was monitored continuously in the leaves of control and BASF 13.338 treated wheat seedlings during continuous increase in temperature (1°C per minute). The fluorescence-temperature profile showed a decrease in the fluorescence yield above 55°C; this decrease was biphasic in the control and monophasic in the treated plants.     

Determination of H+/e- ratios in chloroplasts with flashing light.

Using a rapid pH electrode, measurements were made of the flash-induced proton transport in isolated spinach chloroplasts. To calibrate the system, we assumed that in the presence of ferricyanide and in steady-state flashing light, each flash liberates from water one proton per reaction chain. We concluded that with both ferricyanide and methylviologen as acceptors two protons per electron are translocated by the electron transport chain connecting Photosystem II and I. With methyl viologen but not with ferricyanide as an acceptor, two additional protons per electron are taken up due to Photosystem I activity. One of these latter protons is translocated to the inside of the thylakoid while the other is taken up in H2O2 formation. Assuming that the proton released during water splitting remains inside the thylakoid, we compute H+/e- ratios of 3 and 4 for ferricyanide and methylviologen, respectively. In continuous light of low intensity, we obtained the same H+/e- ratios. However, with higher intensities where electron transport becomes rate limited by the internal pH, the H+/e- ratio approached 2 as a limit for both acceptors. A working model is presented which includes two sites of proton translocation, one between the photoacts, the other connected to Photosystem I, each of which translocates two protons per electron. Each site presents a approximately 30 ms diffusion barrier to proton passage which can be lowered by uncouplers to 6-10 ms.     

远紫外辐射对紫杉幼苗针叶膜脂过氧化及内源保护系统的影响

实验室条件下用远紫外线(UV-BC)光源照射紫杉幼苗,随照射时间延长,针叶的离子渗出率、膜脂过氧化水平、组织自动氧化速率及H₂O₂含量显著增加,可溶性蛋白、抗坏血酸、类胡萝卜素和叶绿素含量下降,叶绿体光系统II电子传递活性显著下降,外源活性氧清除剂苯甲酸钠和抗坏血酸对针叶膜脂过氧化有抑制作用;甲基紫精和DDC对针叶膜脂过氧化有促进效果,远紫外线引起的紫杉伤害可能和针叶树的越冬光氧化伤害有类似之处.紫杉苗对紫外辐射的抗性远高于一般农作物.     

Two membrane-associated NiFeS-carbon monoxide dehydrogenases from the anaerobic carbon-monoxide-utilizing eubacterium Carboxydothermus hydrogenoformans

Two monofunctional NiFeS carbon monoxide (CO) dehydrogenases, designated CODH I and CODH II, were purified to homogeneity from the anaerobic CO-utilizing eubacterium Carboxydothermus hydrogenoformans. Both enzymes differ in their subunit molecular masses, N-terminal sequences, peptide maps, and immunological reactivities. Immunogold labeling of ultrathin sections revealed both CODHs in association with the inner aspect of the cytoplasmic membrane. Both enzymes catalyze the reaction CO + H(2)O --> CO(2) + 2 e(-) + 2 H(+). Oxidized viologen dyes are effective electron acceptors. The specific enzyme activities were 15,756 (CODH I) and 13,828 (CODH II) micromol of CO oxidized min(-1) mg(-1) of protein (methyl viologen, pH 8.0, 70 degrees C). The two enzymes oxidize CO very efficiently, as indicated by k(cat)/K(m) values at 70 degrees C of 1.3. 10(9) M(-1) CO s(-1) (CODH I) and 1.7. 10(9) M(-1) CO s(-1) (CODH II). The apparent K(m) values at pH 8.0 and 70 degrees C are 30 and 18 microM CO for CODH I and CODH II, respectively. Acetyl coenzyme A synthase activity is not associated with the enzymes. CODH I (125 kDa, 62.5-kDa subunit) and CODH II (129 kDa, 64.5-kDa subunit) are homodimers containing 1.3 to 1.4 and 1.7 atoms of Ni, 20 to 22 and 20 to 24 atoms of Fe, and 22 and 19 atoms of acid-labile sulfur, respectively. Electron paramagnetic resonance (EPR) spectroscopy revealed signals indicative of [4Fe-4S] clusters. Ni was EPR silent under any conditions tested. It is proposed that CODH I is involved in energy generation and that CODH II serves in anabolic functions.     

Electron pathways involved in H(2)-metabolism in the green alga Scenedesmus obliquus

The green alga Scenedesmus obliquus is capable of both uptake and production of H(2) after anaerobic adaptation (photoreduction of CO(2) or photohydrogen production). The essential enzyme for H(2)-metabolism is a NiFe-hydrogenase with a [2Fe-2S]-ferredoxin as its natural redox partner. Western blot analysis showed that the hydrogenase is constitutively expressed. The K(m) values were 79.5 microM and 12.5 microM, determined with ferredoxin and H(2), respectively, as electron donor for the hydrogenase. In vitro, NADP(+) was reduced by H(2) in the presence of the hydrogenase, the ferredoxin and a ferredoxin-NADP reductase. From these results and considerations on the stoichiometry we propose that this light-independent electron transfer is part of the photoreduction of CO(2) in vivo. For ATP synthesis, necessary for the photoreduction of CO(2), light-dependent cyclic electron transfer around Photosystem (PS) I accompanies this 'dark reaction'. PS II fluorescence data suggest that (a) in S. obliquus H(2)-reduction might function as the anaerobic counterpart of the O(2)-dependent Mehler reaction, and (b) the presence of either a ferredoxin quinone-reductase or NAD(P)-dehydrogenase (complex I) in S. obliquus chloroplasts.     

Changes in Photosystem II Account for the Circadian Rhythm in Photosynthesis in Gonyaulax polyedra

Cell-free extracts that show activity in photosynthetic electron flow have been prepared from the unicellular dinoflagellate, Gonyaulax polyedra. Electron flow, as O₂ uptake, was measured through both photo-system I and II from water to methyl viologen, through photosystem I alone from reduced 2,6-dichlorophenol indophenol to methyl viologen which does not include the plastoquinone pool or from duroquinol to methyl viologen which includes the plastoquinone pool. Electron flow principally through photosystem II was measured from water to diaminodurene and ferricyanide, as O₂ evolution. Cultures of Gonyaulax were grown on a 12-hour light:12 hour dark cycle to late log phase, then transferred to constant light at the beginning of a light period. After 3 days, measurements of electron flow were made at the maximum and minimum of the photosynthetic rhythm, as determined from measurements of the rhythm of bioluminescence. Photosynthesis was also measured in whole cells, either as ¹⁴C fixation or O₂ evolution. Electron flow through both photosystems and through photosystem II alone were clearly rhythmic, while electron flow through photosystem I, including or excluding the plastoquinone pool, was constant with time in the circadian cycle. Thus, only changes in photosystem II account for the photosynthesis rhythm in Gonyaulax.     

Electron Fluxes through Photosystem I in Cucumber Leaf Discs Probed by far-red Light

Cucumber leaf discs were illuminated at room-temperature with far-red light to photo-oxidise P700, the chlorophyll dimer in Photosystem (PS) I. The post-illumination kinetics of P700(+) re-reduction were studied in the presence of inhibitors or cofactors of photosynthetic electron transport. The re-reduction kinetics of P700(+) were well fitted as the sum of three exponentials, each with its amplitude and rate coefficient, and an initial flux (at the instant of turning off far-red light) given as the product of the two. Each initial flux is assumed equal to a steady state flux during far-red illumination. The fast phase of re-reduction, with rate coefficient k (1) approximately 10 s(-1), was completely abolished by a saturating concentration of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU); it is attributed to electron flow to P700(+) from PS II, which was stimulated to some extent by far-red light. The intermediate phase, with rate coefficient k (1) approximately 1 s(-1), was only partly diminished by methyl viologen (MV) which diverts electron flow to oxygen. The intermediate phase is attributed to electron donation from reduced ferredoxin to the intersystem pool; reduced ferredoxin could be formed: (1) directly by electron donation on the acceptor of PS I; and/or (2) indirectly by stromal reductants, in line with only a partial inhibition of the intermediate phase by MV. Duroquinol enhanced the intermediate phase in the presence of DCMU, presumably through its interaction with thylakoid membrane components leading to the partial reduction of plastoquinone. The slow phase of P700(+) re-reduction, with rate coefficient k (1) approximately 0.1 s(-1), was unaffected by DCMU and only slightly affected by MV; it could be associated with electron donation to either: (1) the intersystem chain by stromal reductants catalysed by NAD(P)H dehydrogenase slowly; or (2) plastocyanin/P700(+) by ascorbate diffusing across the thylakoid membrane to the lumen. It is concluded that a post-illumination analysis of the fluxes to P700(+) can be used to probe the pathways of electron flow to PS I in steady state illumination.     

Temperature-dependent changes in Photosystem II heterogeneity support a cycle of Photosystem II during photoinhibition

High light treatments were given to attached leaves of pumpkin (Cucurbita pepo L.) at room temperature and at 1°C where the diffusion- and enzyme-dependent repair processes of Photosystem II are at a minimum. After treatments, electron transfer activities and fluorescence induction were measured from thylakoids isolated from the treated leaves. When the photoinhibition treatment was given at 1°C, the Photosystem II electron transfer assays that were designed to require electron transfer to the plastoquinone pool showed greater inhibition than electron transfer from H₂O to paraphenyl-benzoquinone, which measures all PS II centers. When the light treatment was given at room temperature, electron transfer from H₂O to paraphenyl-benzoquinone was inhibited more than whole-chain electron transfer. Variable fluorescence measured in the presence of ferricyanide decreased only during room-temperature treatments. These results suggest that reaction centers of one pool of Photosystem II, non-Q_B-PS II, replace photoinhibited reaction centers at room temperature, while no replacement occurs at 1°C. A simulation of photoinhibition at 1°C supports this conclusion.Abbreviations BSA bovine serum albumin - Chl chlorophyll - DCMU 3-(3,4,-dichlorophenyl)-1,1,-dimethylurea - DCPIP dichlorophenol-indophenol (2,6-dichloro-4((4-hydroxyphenyl)imino)-2,5-cyclohexadien-1-one) - DPC diphenyl carbazide (2,2-diphenylcarbonic dihydrazide) - FeCN ferricyanide (hexacyanoferrate(III)) - _app apparent quantum yield of photosynthetic oxygen evolution - MV methyl viologen (1,1-dimethyl-4,4-bipyridinium dichloride) - PPBQ phenyl-p-benzoquinone - PPFD photosynthetic photon flux density - PQ pool plastoquinone - Q_B secondary quinone acceptor of PS II - RT room temperature - WC whole chain electron transfer     

Stabilization by glutaraldehyde of high-rate electron transport in isolated chloroplasts

Treatment of isolated chloroplasts with glutaraldehyde affects their ability to photoreduce artificial electron acceptors. The remaining rate of O₂ evolution approaches zero with methyl viologen, is low with ferricyanide, but nearly normal with lipophilic Photosystem II acceptors, like oxidized p-phenylenediamine and oxidized diaminodurene. Since Photosystem I donor reactions are also affected, a specific site of inhibition of electron transport to Photosystem I is indicated. At the same time, glutaraldehyde prolongs the longevity of the chloroplasts stored in dark. In control samples the half-life of Photosystem II activity varied between 5 days at 4 °C and 1 day at 25 °C. Glutaraldehyde treatment increased these half times approx. 3-fold. The glutaraldehyde doses required to induce inhibition and stabilization were very similar.     

Functional and structural organization of chlorophyll in the developing photosynthetic membranes of Euglena gracilis Z. II. Formation of system II photosynthetic units during greening under optimal light intensity.

The relationships between light-harvesting chlorophyll and reaction centers in Photosystem II were analyzed during the chloroplast development of dark-grown, non-dividing Euglena gracilis Z. Comparative measurements included light saturation of photosynthesis, oxygen evolution under flashing-light and fluorescence induction. The results obtained can be summarized as follows: (1) Photosystem II photocenters are formed in parallel with chlorophyll synthesis, but after a long lag phase. (2) As a consequence, the chlorophyll reaction center ratio (Emerson's type photosynthetic unit) decreases during greening. (3) This decrease is accompanied by considerable changes in the energy transfer and trapping properties of Photosystem II. Most of the initially synthesized chlorophylls are inactive in the transfer of excitations to active photochemical centers and are shared among newly formed Photosystem II photocenters; as a consequence, the number of chlorophylls functionally connected to each Photosystem II photocenter decreases and cooperatively between these centers appears. Results are discussed in terms of chlorophyll organization in developing photosynthetic membranes with reference to the lake or puddle models of photosynthetic unit organization.     

Photosynthetic utilization of bicarbonate in Zostera marina is reduced by inhibitors of mitochondrial ATPase and electron transport

When Zostera marina was irradiated after a period of darkness, initiation of photosynthetic O2 evolution occurred in two phases. During a lag phase, lasting 4 to 5 min, photosynthesis was supported by a diffusive entry of CO2. Photosynthesis then rapidly increased to its full rate. Tris buffer, at a concentration of 50 mm, completely inhibited this increase without affecting CO2-supported photosynthesis during the lag phase. These results verify that the increase in photosynthesis after the lag phase depended on an activation of bicarbonate (HCO3-) utilization through acid zones generated by proton pumps located to the outer cell membrane. In similar experiments, 6.25 microm of the mitochondrial ATPase blocker oligomycin inhibited photosynthetic HCO3(-) utilization by more than 60%. Antimycin A, a selective blocker of mitochondrial electron transport, caused a similar inhibition of HCO3(-) utilization. Measurements at elevated CO2 concentrations verified that neither oligomycin nor antimycin interfered with linear photosynthetic electron transport or with CO2 fixation. Thus, a major part of the ATP used for the generation of acid zones involved in HCO3(-) utilization in Z. marina was derived from mitochondrial respiration.     

Electron transport and photophosphorylation by Photosystem I in vivo in plants and cyanobacteria

Recently, a number of techniques, some of them relatively new and many often used in combination, have given a clearer picture of the dynamic role of electron transport in Photosystem I of photosynthesis and of coupled cyclic photophosphorylation. For example, the photoacoustic technique has detected cyclic electron transport in vivo in all the major algal groups and in leaves of higher plants. Spectroscopic measurements of the Photosystem I reaction center and of the changes in light scattering associated with thylakoid membrane energization also indicate that cyclic photophosphorylation occurs in living plants and cyanobacteria, particularly under stressful conditions.In cyanobacteria, the path of cyclic electron transport has recently been proposed to include an NAD(P)H dehydrogenase, a complex that may also participate in respiratory electron transport. Photosynthesis and respiration may share common electron carriers in eukaryotes also. Chlororespiration, the uptake of O₂ in the dark by chloroplasts, is inhibited by excitation of Photosystem I, which diverts electrons away from the chlororespiratory chain into the photosynthetic electron transport chain. Chlororespiration in N-starved Chlamydomonas increases ten fold over that of the control, perhaps because carbohydrates and NAD(P)H are oxidized and ATP produced by this process.The regulation of energy distribution to the photosystems and of cyclic and non-cyclic phosphorylation via state 1 to state 2 transitions may involve the cytochrome b ₆-f complex. An increased demand for ATP lowers the transthylakoid pH gradient, activates the b ₆-f complex, stimulates phosphorylation of the light-harvesting chlorophyll-protein complex of Photosystem II and decreases energy input to Photosystem II upon induction of state 2. The resulting increase in the absorption by Photosystem I favors cyclic electron flow and ATP production over linear electron flow to NADP and poises the system by slowing down the flow of electrons originating in Photosystem II.Cyclic electron transport may function to prevent photoinhibition to the photosynthetic apparatus as well as to provide ATP. Thus, under high light intensities where CO₂ can limit photosynthesis, especially when stomates are closed as a result of water stress, the proton gradient established by coupled cyclic electron transport can prevent over-reduction of the electron transport system by increasing thermal de-excitation in Photosystem II (Weis and Berry 1987). Increased cyclic photophosphorylation may also serve to drive ion uptake in nutrient-deprived cells or ion export in salt-stressed cells.There is evidence in some plants for a specialization of Photosystem I. For example, in the red alga Porphyra about one third of the total Photosystem I units are engaged in linear electron transfer from Photosystem II and the remaining two thirds of the Photosystem I units are specialized for cyclic electron flow. Other organisms show evidence of similar specialization.Improved understanding of the biological role of cyclic photophosphorylation will depend on experiments made on living cells and measurements of cyclic photophosphorylation in vivo.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - cyt cytochrome - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCCD dicyclohexylcarbodiimide - DCHC dicyclohexyl-18-crown-6 - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - FCCP carbonylcyanide 4-(trifluoromethoxy) phenylhydrazone - LHC light harvesting chlorophyll - LHCP II light harvesting chlorophyll protein of Photosystem II - PQ plastoquinone - PS I, II Photosystem I, II - SHAM salicyl hydroxamic acid - TBT Tri-n-butyltin CIW/DPB Publication No. 1146     

Shikonin isovalerate: An inhibitor of photosystem II in Chlorogloeopsis fritschi

Shikonin isovalerate, extracted from the roots of the desert plant Arnebia decumbens, was tested for its effect on photosynthetic electron transport system of Chlorogloeopsis fritschii. The ferricyanide-Hill reaction with water and DPC as electron donors was inhibited completely with 10^-5 M shikonin isovalerate. The photoreduction of DCPIP through photosystem II was only slightly inhibited. Photosystem I from durohydroquinone to methyl viologen was not affected using 10^-6 M shikonin isovalerate. The same concentration caused 49% inhibition of cyclic photophosphorylation. These results suggest that shikonin isovalerate inhibits photosynthetic electron flow at the plastoquinone pool.Abbreviations DCMU 3-(3,4-dichlorophenyl)-N,N-dimethyl urea - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-P-benzoquinone - DCPIP 2–6-dichlorophenolindophenol - DPC Diphenylcarbazide - Tricine N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine