Reduction Of P700 Research Articles

Effects of drought stress on the oxidation of the reaction center chlorophyll of photosystem I and grain yield in paddy-field grown rice plants

ABSTRACT Drought is a serious abiotic stress that leads to decreased yield. In pot experiments, the reaction center chlorophyll of photosystem I (PSI) (P700) has been reported to be oxidized in response to drought stress. In the present study, we examined whether drought stress that decreases grain yield was detected by P700 oxidation in paddy-field-grown rice (Oryza sativa L.) plants. The P700 reduction and SPAD values were also evaluated. Drought stress was imposed by the prolonged mid-season drainage in 2019 and 2020. P700 oxidation was measured at saturated CO2 levels before heading. P700 oxidation was not stimulated in the 2019-drought plots. In contrast, P700 oxidation was stimulated in the 2020-drought plot. However, this phenomenon in 2020 was primarily accounted for by decreases in total leaf-N levels, as previously observed in the rice plants grown under low N availability. Rough rice yields and ratios of filled spikelets tended to decrease in the drought plots. Therefore, drought stress responsible for a decrease in grain yields could not be directly detected by P700 oxidation in paddy-field-grown rice plants, whereas the P700 oxidation responded to changes in total leaf-N levels due to the drought stress treatment. The same conclusion was drawn for the P700 reduction and SPAD value.

Response of Amaranthus tricolor to cesium stress in hydroponic system: Growth, photosynthesis and cesium accumulation

Remediation of the cesium-contaminated environment is of paramount importance, and phytoremediation is a cost-effective and green technique. In this paper, the response of Amaranthus tricolor to cesium ions in hydroponic solution was investigated at various cesium concentration (0, 0.05, 0.2, 0.4 and 0.6 mM), in terms of the growth weight, height and photosynthesis. The maximal Cs content in stems and leaves of A. tricolor was 13.05 mg/g dry wt under spiked Cs level of 0.4 mM in solution. The maximal transfer factor (TF) and bioconcentration factor (BCF) were 1.87 and 181.25 respectively, when the corresponding Cs content in roots and shoots was 7.04 mg/g and 13.05 mg/g dry wt respectively. TFs are higher than 1 in the conditions of normal plant growth. The growth of A. tricolor was enhanced after the treatment of Cs at low concentrations (0.05 and 0.2 mM), while it was inhibited at 0.4 and 0.6 mM. The leaf number and dry weight of stem, leaf parts and root parts were maximum at the spiked cesium level of 0.2 mM, which significantly increased by 19.19%, 47.56% and 94.56% respectively, compared with the control samples. Under 0.6 mM cesium stress, curl and withering of the leaves occurred, and the plant growth and cesium accumulation dropped to the minimum. Cs at the spiked level of 0.6 mM in solution inhibited the performance of PSII, especially in terms of blockage in electron transfer process beyond QA and restraint of P700 reduction. On contrast, the performance of PSII was enhanced by the spiked Cs at level of 0.2 mM, leading to the growing density of reaction centers per excited cross-section and increasing electron transfer process beyond QA. In summary, A. tricolor has potential for remediating the Cs-contaminated environment.

Biphasic effects of typical chlorinated organophosphorus flame retardants on Microcystis aeruginosa

The potential accumulation of chlorinated organophosphorus flame retardants (Cl-OPFRs) in aquatic environments sparked interest in studying the effects of Cl-OPFRs on cyanobacterial blooms. In this work, two common Cl-OPFRs, tris(1,3-dichloro-2-propyl) phosphate (TDCPP) and tris(2-chloroethyl) phosphate (TCEP), induced dose-dependent biphasic effect on bloom-forming M. aeruginosa. The hormetic response to low-dose Cl-OPFRs was associated with the upregulation of the type I NADH dehydrogenase (NDH-1) complex and its mediated cyclic electron transfer (CET) pathway, as reflected by a transient post-illumination increase in chlorophyll fluorescence, the dark reduction of P700+ and the change of NDH-1-related gene expression. The increased CET activity and carotenoid content jointly reduced the intracellular ROS production, facilitating cyanobacterial growth. Conversely, a higher concentration of both Cl-OPFRs induced severe inhibition of growth and photosynthetic oxygen-evolving activity through an imbalance between PSII and PSI. Toxic-dose Cl-OPFRs inhibited state transition and fixed cells into the State I with a higher PSII/PSI ratio, as indicated by chlorophyll fluorescence induction, 77 K fluorescence emission spectra and photosystem stoichiometry. The elevated PSII/PSI ratio created an imbalance between the two photosystems and eventually lead to ROS overproduction, which generate adverse effects on cell growth. This work provides important insights into the hormetic mechanism of Cl-OPFRs on Microcystis aeruginosa and their potential roles in harmful cyanobacteria blooms.

Oxidation of the reaction center chlorophyll of photosystem I is induced via close cooperation of photosystems II and I with progress of drought stress in soybean seedlings

ABSTRACT Drought is a serious abiotic stress that impairs plant productivity. We previously suggested that in rice (Oryza sativa L.) plants, the reaction center chlorophyll of photosystem I (PSI) (P700) is oxidized in response to drought stress via close cooperation of photosystem II (PSII) and PSI to protect PSI, whose fitness is crucial for photosynthesis and plant growth. It has rarely been reported whether such a defense system functions in other plant species. To collect another example, this subject was examined using soybean [Glycine max (L.)]. Irrigation of soil-cultured soybean seedlings was restricted to gradually impose drought stress. A decrease in the quantum efficiency of PSII [Y(II)], induction of non-photochemical quenching (NPQ) of excess light energy at PSII, reduction of the primary quinone electron acceptor of PSII (QA), and P700 oxidation proceeded before severe drought stress led to severe leaf wilting and photoinhibition of PSII and PSI. P700 oxidation, Y(II), and reduction of QA were strongly correlated in this period. These results are in accordance with those observed in rice. In contrast, such correlations disappeared when severely drought-stressed seedlings were included, as P700 oxidation was hampered while Y(II) further decreased in such seedlings. These results suggest that close cooperation of PSII and PSI leading to P700 oxidation functions to protect PSI against drought stress in soybean, and that similar defense system functions in soybean and rice unless drought stress are severe. The drought-stress responses of PSII and PSI are suggested to be regulated by acidification of thylakoid lumen in soybean, according to the changing patterns in the PSII and PSI parameters. Based on the similarity of the photochemistry of soybean and rice in response to drought stress, the effectiveness of P700 oxidation as an index for drought stress diagnosis is discussed. P700 oxidation is affected by factors other than drought stress such as N nutrition levels. However, effects of N nutrition levels are expected to be canceled by using another parameter obtained in P700 measurement as reference, suggesting the suitability of P700 oxidation for drought stress diagnosis.

In vivo electron donation from plastocyanin and cytochrome c6 to PSI in Synechocystis sp. PCC6803

Many cyanobacteria species can use both plastocyanin and cytochrome c6 as lumenal electron carriers to shuttle electrons from the cytochrome b6f to either photosystem I or the respiratory cytochrome c oxidase. In Synechocystis sp. PCC6803 placed in darkness, about 60% of the active PSI centres are bound to a reduced electron donor which is responsible for the fast re-reduction of P700in vivo after a single charge separation. Here, we show that both cytochrome c6 and plastocyanin can bind to PSI in the dark and participate to the fast phase of P700 reduction, but the fraction of pre-bound PSI is smaller in the case of cytochrome c6 than with plastocyanin. Because of the inter-connection of respiration and photosynthesis in cyanobacteria, the inhibition of the cytochrome c oxidase results in the over-reduction of the photosynthetic electron transfer chain in the dark that translates into a lag in the kinetics of P700 oxidation at the onset of light. We show that this is true both with plastocyanin and cytochrome c6, indicating that the partitioning of electron transport between respiration and photosynthesis is regulated in the same way independently of which of the two lumenal electron carriers is present, although the mechanisms of such regulation are yet to be understood.

Co-suppression in Pyropia yezoensis (Rhodophyta) Reveals the Role of PyLHCI in Light Harvesting and Generation Switch.

The red macroalga Pyropia yezoensis is an economically important seaweed widely cultured in Asian countries and is a model organism for molecular biological and commercial research. This species is unique in that it utilizes both phycobilisomes and transmembrane light-harvesting proteins as its antenna system. Here, one of the genes of P. yezoensis (PyLHCI) was selected for introduction into its genome to overexpress PyLHCI. However, the co-suppression phenomenon occurred. This is the first documentation of co-suppression in algae, in which it exhibits a different mechanism from that in higher plants. The transformant (T1) was demonstrated to have higher phycobilisomes and lower LHC binding pigments, resulting in a redder color, higher sensitivity to salt stress, smaller in size, and slower growth rate than the wildtype (WT). The photosynthetic performances of T1 and WT showed similar characteristics; however, P700 reduction was slower in T1. Most importantly, T1 could release a high percentage of carpospores in young blades to switch generation during its life cycle, which was rarely seen in WT. The co-suppression of PyLHCI revealed its key roles in light harvesting, stress resistance, and generation alternation (generation switch from gametophytes to sporophytes, and reproduction from asexual to sexual).

A Commonly Used Photosynthetic Inhibitor Fails to Block Electron Flow to Photosystem I in Intact Systems.

In plant science, 2,4-dinitrophenylether of iodonitrothymol (DNP-INT) is frequently used as an alternative to 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone (DBMIB) to examine the capacity of plastoquinol and semiquinone to reduce O2. DNP-INT is considered to be an effective inhibitor of the photosynthetic electron transfer chain (PETC) through its binding at the Q0 site of Cyt-b6f. The binding and action of DNP-INT has been previously characterized spectroscopically in purified Cyt-b6f complex reconstituted with Plastocyanin, PSII membranes and plastoquinone, as well as in isolated thylakoids based on its property to block MV-mediated O2 consumption. Contrary to the conclusions obtained from these experiments, we observed clear reduction of P700+ in samples incubated with DNP-INT during our recent investigation into the sites of oxygen consumption in isolated thylakoids. Therefore, we carried out an extensive investigation of DNP-INT’s chemical efficacy in isolated thylakoids and intact leaves. This included examination of its capacity to block the PETC before PSI, and therefore its inhibition of CO2 fixation. P700 redox kinetics were measured using Dual-PAM whilst Membrane Inlet Mass Spectrometry (MIMS) was used for simultaneous determination of the rates of O2 evolution and O2 consumption in isolated thylakoids and CO2 fixation in intact leaves, using two stable isotopes of oxygen (16O2, 18O2) and CO2 (12C, 13C), respectively. Based on these investigations we confirmed that DNP-INT is unable to completely block the PETC and CO2 fixation, therefore its use may produce artifacts if applied to isolated thylakoids or intact cells, especially when determining the locations of reactive oxygen species formation in the photosynthetic apparatus.

Stimulation of cyclic electron flow around photosystem I upon a sudden transition from low to high light in two angiosperms Arabidopsis thaliana and Bletilla striata

In angiosperms, cyclic electron flow (CEF) around photosystem I (PSI) is more important for photoprotection under fluctuating light than under constant light. However, the underlying mechanism is not well known. In the present study, we measured the CEF activity, P700 redox state and electrochromic shift signal upon a sudden transition from low to high light in wild-type plants of Arabidopsis thaliana and Bletilla striata (Orchidaceae). Within the first 20 s after transition from low to high light, P700 was highly reduced in both species, which was accompanied with a sufficient proton gradient (ΔpH) across the thylakoid membranes. Meanwhile, the level of CEF activation was elevated. After transition from low to high light for 60 s, the plants generated an optimal ΔpH. Under such condition, PSI was highly oxidized and the level of CEF activation decreased to the steady state. Furthermore, the CEF activation was positively correlated to the P700 reduction ratio. These results indicated that upon a sudden transition from low to high light, the insufficient ΔpH led to the over-reduction of PSI electron carriers, which in turn stimulated the CEF around PSI. This transient stimulation of CEF not only favored the rapid ΔpH formation but also accepted electrons from PSI, thus protecting PSI at donor and acceptor sides. These findings provide new insights into the important role of CEF in regulation of photosynthesis under fluctuating light.

Changes in the photosynthesis properties and photoprotection capacity in rice (Oryza sativa) grown under red, blue, or white light.

Non-photochemical quenching (NPQ) of the excited state of chlorophyll a is a major photoprotective mechanism plants utilize to survive under high light. Here, we report the impact of long-term light quality treatment on photosynthetic properties, especially NPQ in rice. We used three LED-based light regimes, i.e., red (648-672nm), blue (438-460nm), and "warm" white light (529-624nm), with the incident photon flux density of 300µmolphotonsm-2s-1, the difference in the absorbed photon flux densities by leaves grown under different light quality being less than 7%. Our results show that blue light, as compared to white light, induced a significant decrease in Fv/Fm, a decreased rate of reduction of P700+ after P700 was completely oxidized; furthermore, blue light also induced higher NPQ with an increased initial speed of NPQ induction, which corresponds to the qE component of NPQ, and a lower maximum quantum yield of PSII, i.e., Y(II). In contrast, rice grown under long-term red light showed decreased Y(II) and increased NPQ, but with no change in Fv/Fm. Furthermore, we found that rice grown under either blue or red light showed decreased transcript abundance of both catalase and ascorbate peroxidase, together with an increased H2O2 content, as compared to rice grown under white light. All these data suggest that even under a moderate incident light level, rice grown under blue or red light led to compromised antioxidant system, which contributed to decreased quantum yield of photosystem II and increased NPQ.

Photoinduction of electron transport on the acceptor side of PSI in Synechocystis PCC 6803 mutant deficient in flavodiiron proteins Flv1 and Flv3

After transferring the dark-acclimated cyanobacteria to light, flavodiiron proteins Flv1/Flv3 serve as a main electron acceptor for PSI within the first seconds because Calvin cycle enzymes are inactive in the dark. Synechocystis PCC 6803 mutant Δflv1/Δflv3 devoid of Flv1 and Flv3 retained the PSI chlorophyll P700 in the reduced state over 10 s (Helman et al., 2003; Allahverdiyeva et al., 2013). Study of P700 oxidoreduction transients in dark-acclimated Δflv1/Δflv3 mutant under the action of successive white light pulses separated by dark intervals of various durations indicated that the delayed oxidation of P700 was determined by light activation of electron transport on the acceptor side of PSI. We show that the light-induced redox transients of chlorophyll P700 in dark-acclimated Δflv1/Δflv3 proceed within 2 min, as opposed to 1–3 s in the wild type, and comprise a series of kinetic stages. The release of rate-limiting steps was eliminated by iodoacetamide, an inhibitor of Calvin cycle enzymes. Conversely, the creation with methyl viologen of a bypass electron flow to O2 accelerated P700 oxidation and made its extent comparable to that in the wild-type cells. The lack of major sinks for linear electron flow in iodoacetamide-treated Δflv1/Δflv3 mutant, in which O2- and CO2-dependent electron flows were impaired, facilitated cyclic electron flow, which was evident from the decreased steady-state oxidation of P700 and from rapid dark reduction of P700 during and after illumination with far-red light. The results show that the photosynthetic induction in wild-type Synechocystis PCC 6803 is largely hidden due to the flavodiiron proteins whose operation circumvents the rate-limiting electron transport steps controlled by Calvin cycle reactions.

Redox changes of ferredoxin, P700, and plastocyanin measured simultaneously in intact leaves

Properties and performance of the recently introduced Dual/KLAS-NIR spectrophotometer for simultaneous measurements of ferredoxin (Fd), P700, and plastocyanin (PC) redox changes, together with whole leaf chlorophyll a (Chl) fluorescence (emission >760, 540 nm excitation) are outlined. Spectral information on in vivo Fd, P700, and PC in the near-infrared region (NIR, 780–1000 nm) is presented, on which the new approach is based. Examples of application focus on dark–light and light–dark transitions, where maximal redox changes of Fd occur. After dark-adaptation, Fd reduction induced by moderate light parallels the Kautsky effect of Chl fluorescence induction. Both signals are affected analogously by removal of O2. A rapid type of Fd reoxidation, observed after a short pulse of light before light activation of linear electron transport (LET), is more pronounced in C4 compared to C3 leaves and interpreted to reflect cyclic PS I (CET). Light activation of LET, as assessed via the rate of Fd reoxidation after short light pulses, occurs at very low intensities and is slowly reversed (half-time ca. 20 min). Illumination with strong far-red light (FR, 740 nm) reveals two fractions of PS I, PS I (LET), and PS I (CET), differing in the rates of Fd reoxidation upon FR-off and the apparent equilibrium constants between P700 and PC. Parallel information on oxidation of Fd and reduction of P700 plus PC proves essential for identification of CET. Comparison of maize (C4) with sunflower and ivy (C3) responses leads to the conclusion that segregation of two types of PS I may not only exist in C4 (mesophyll and bundle sheath cells), but also in C3 photosynthesis (grana margins plus end membranes and stroma lamellae).

Effect of chromate ions on marine microalgae Phaeodactylum tricornutum

Effect of chromate ions on the culture of a marine diatom Phaeodactylum tricornutum was studied using an M-PEA-2 fluorimeter, which carries out simultaneous measurement of fluorescence induction and redox transformations of the P700 pigment within a millisecond range. Chromate ions were shown to inhibit electron transport in PS II and decrease the rate of QА reduction. This results in decreased values of the quantum yield of electron transport in PS II (ϕEo) and performance index (PI ABS), lower rates of P700 reduction, and increased energy (DI0/RC) and ΔpH-dependent nonphotochemical quenching (q E ). Emergence of the slow component of P700 reduction was observed, indicating the activation of cyclic transport in the presence of chromate. Performance index (PI ABS), which was the most sensitive parameter, may be recommended for detection of chromate ions at early stages of their toxic action. The fluorescence parameter F O is promising application in biotesting to assess the algal growth rates.

Lateral heterogeneity of the proton potential along the thylakoid membranes of chloroplasts

This work is devoted to critical analysis and reexamination of the problem of lateral heterogeneity of the trans-thylakoid pH difference (ΔpH=pHout−pHin) in thylakoid membranes of chloroplasts. Correct measurements of ΔpH may be complicated by nonuniform partitioning of the protons pumped into the lumen of granal (stacked) and stroma-exposed thylakoids. We have compared results of ΔpH estimations in isolated bean chloroplasts by two different methods. One of the methods is based on the use of pH-sensitive spin-probes. Another approach relies on the analysis of pH-dependent post-illumination reduction of P700+ - oxidized reaction center of photosystem I (PSI). Both methods lead to virtually the same values of ΔpH, as measured in the state of photosynthetic control when the ATP synthase complexes are inactive (ΔpH~2.3–2.5 at pHout~8). Under the photophosphorylation conditions (metabolic state 3), ΔpH decreases due to the proton drain from the lumen to stroma via active ATP synthases (ΔpH~1–2 at pHout~8). In this state, ΔpH values derived from kinetic data are smaller than ΔpH measured with the pH-probing amines. Such a discrepancy can be explained by the coexistence of thylakoids with different pHin established in granal and stromal thylakoids. The kinetic method is equivalent to the use of a “local pH-meter”, which is sensitive to less significant decrease in pHin inside the stroma-exposed thylakoids. Otherwise, pH-indicating probes give information on pHin values averaged out at both granal and stromal thylakoids. Experimental results have been analyzed within the framework of our mathematical model developed for simulation of electron and proton transport processes in laterally heterogeneous thylakoids. The model provides a reasonable description of experimental data, supporting the notion that the long-range diffusion of protons within the lumen and obstructed diffusion of mobile electron carriers (PQH2 and Pc) influence the lateral profiles of pH along the thylakoid membranes. The model predicts significant alkalization of the inter-thylakoid gap and the establishment of nonuniform lateral profiles of ΔpH under the photophosphorylation conditions. These results are discussed in the context of the problem of energy coupling in laterally heterogeneous lamellar system of chloroplasts.

Kinetics of plastoquinol oxidation by the Q-cycle in leaves

Electrochromic shift measurements confirmed that the Q-cycle operated in sunflower leaves. The slow temporarily increasing post-pulse phase was recorded, when ATP synthase was inactivated in the dark and plastoquinol (PQH2) oxidation was initiated by a short pulse of far-red light (FRL). During illumination by red light, the Q-cycle-supported proton arrival at the lumen and departure via ATP synthase were simultaneous, precluding extreme build-up of the membrane potential. To investigate the kinetics of the Q-cycle, less than one PQH2 per cytochrome b6f (Cyt b6f) were reduced by illuminating the leaf with strong light pulses or single-turnover Xe flashes. The post-pulse rate of oxidation of these PQH2 molecules was recorded via the rate of reduction of plastocyanin (PC+) and P700+, monitored at 810 and 950nm. The PSII-reduced PQH2 molecules were oxidized with multi-phase overall kinetics, τd=1, τp=5.6 and τs=16 ms (22 °C). We conclude that τd characterizes PSII processes and diffusion, τp is the bifurcated oxidation of the primary quinol and τs is the Q-cycle—involving reduction of the secondary quinol at the n-site, its transport to the p-site, and bifurcated oxidation there. The extraordinary slow kinetics of the Q-cycle may be related to the still unsolved mechanism of the “photosynthetic control.”

Temperature dependence of the oxidation kinetics of TyrZ and TyrD in oxygen-evolving photosystem II complexes throughout the range from 320 K to 5 K

The photo-induced oxidation of TyrZ and TyrD by P680•+, that involves both electron and proton transfer (PCET), has been studied in oxygen-evolving photosystem II from Thermosynechococcus elongatus. We used time-resolved absorption spectroscopy to measure the kinetics of P680•+ reduction by tyrosine after the first flash given to dark-adapted PS II as a function of temperature and pH. The half-life of TyrZ oxidation by P680•+ increases from 20ns at 300K to about 4μs at 150K. Analyzing the temperature dependence of the rate, one obtains a reorganization energy of about 770meV. Between 260K and 150K, the reduction of P680•+ by TyrZ is increasingly replaced by charge recombination between P680•+ and QA•−. We propose that the driving force for TyrZ oxidation by P680•+ decreases upon lowering the temperature. TyrZ oxidation cannot be excluded in a minority of PS II complexes at cryogenic temperatures.TyrD oxidation by P680•+ with a half-life of about 30ns was observed at high pH. The pH dependence of the yield of TyrD oxidation can be described by a single protonable group with a pK of approximately 8.4. The rate of TyrD oxidation by P680•+ is virtually identical upon substitution of solvent exchangeable protons with deuterons indicating that the rate is limited by electron transfer. The rate is independent of temperature between 5K and 250K. It is concluded that TyrD donates the electron to P680•+ via PD2.

Preferential damaging effects of limited magnesium bioavailability on photosystem I in Sulla carnosa plants.

Magnesium deficiency preferentially inhibits photosystem I rather than photosystem II in Sulla carnosa plants. The effects of magnesium (Mg(2+)) deficiency on growth, photosynthetic performance, pigment and polypeptide composition of chloroplast membranes were studied in the halophyte Sulla carnosa (Desf.), an annual legume endemic to Tunisia and Algeria. The results demonstrate a gradual decrease in biomass production with decreasing Mg(2+) availability in the growth medium. The increase of Mg(2+) deficiency was also associated with a decline of the net CO2 assimilation (Pn) in fully expanded leaves, a decrease in the amount of photosynthetic pigments, and an increase in the lipid peroxidation in plants exposed to decreased Mg(2+) concentrations. Interestingly, while CO2 assimilation already was affected at Mg(2+) concentrations below 1.5 mM, the photochemical efficiency of photosystem II (PSII) declined only in the absence of Mg(2+). In contrast, plants of S. carnosa grown in Mg(2+)-deficient conditions exhibited a significant decrease in photosystem I (PSI) photochemistry in vivo at much higher Mg(2+) levels compared to PSII photochemical activity. The inhibitory effect of Mg(2+) deficiency on PSI photochemistry strongly correlated with significantly lower relative abundance of PSI-related chlorophyll-protein complexes and lower amounts of PSI-associated polypeptides, PsaA, PsaB, and Lhca proteins within the same range of Mg(2+) concentrations. These observations were associated with a higher intersystem electron pool size, restricted linear electron transport and a lower rate of reduction of P700(+) in the dark indicating restricted capacity for PSI cyclic electron transfer in plants exposed to Mg(2+)-deficient conditions compared to controls. These results clearly indicate that PSI, rather than PSII is preferentially targeted and damaged under Mg(2+)-deficiency conditions.

Photosystem activity and state transitions of the photosynthetic apparatus in cyanobacterium Synechocystis PCC 6803 mutants with different redox state of the plastoquinone pool.

To better understand how photosystem (PS) activity is regulated during state transitions in cyanobacteria, we studied photosynthetic parameters of photosystem II (PSII) and photosystem I (PSI) in Synechocystis PCC 6803 wild type (WT) and its mutants deficient in oxidases (Ox(-)) or succinate dehydrogenase (SDH(-)). Dark-adapted Ox(-) mutant, lacking the oxidation agents, is expected to have a reduced PQ pool, while in SDH(-) mutant the PQ pool after dark adaptation will be more oxidized due to partial inhibition of the respiratory chain electron carriers. In this work, we tested the hypothesis that control of balance between linear and cyclic electron transport by the redox state of the PQ pool will affect PSII photosynthetic activity during state transition. We found that the PQ pool was reduced in Ox(-) mutant, but oxidized in SDH(-) mutant after prolonged dark adaptation, indicating different states of the photosynthetic apparatus in these mutants. Analysis of variable fluorescence and 77K fluorescence spectra revealed that the WT and SDH(-) mutant were in State 1 after dark adaptation, while the Ox(-) mutant was in State 2. State 2 was characterized by ~1.5 time lower photochemical activity of PSII, as well as high rate of P700 reduction and the low level of P700 oxidation, indicating high activity of cyclic electron transfer around PSI. Illumination with continuous light 1 (440nm) along with flashes of light 2 (620nm) allowed oxidation of the PQ pool in the Ox(-) mutant, thus promoting it to State 1, but it did not affect PSII activity in dark adapted WT and SDH(-) mutant. State 1 in the Ox(-) mutant was characterized by high variable fluorescence and P700(+) levels typical for WT and the SDH(-) mutant, indicating acceleration of linear electron transport. Thus, we show that PSII of cyanobacteria has a higher photosynthetic activity in State 1, while it is partially inactivated in State 2. This process is controlled by the redox state of PQ in cyanobacteria through enhancement/inhibition of electron transport on the acceptor side of PSII.

Regulation of pepc gene expression in Anabaena sp. PCC 7120 and its effects on cyclic electron flow around photosystem I and tolerances to environmental stresses.

Since pepc gene encoding phosphoenolpyruvate carboxylase (PEPCase) has been cloned from Anabaena sp. PCC 7120 and other cyanobacteria, the effects of pepc gene expression on photosynthesis have not been reported yet. In this study, we constructed mutants containing either upregulated (forward) or downregulated (reverse) pepc gene in Anabaena sp. PCC 7120. Results from real-time quantitative polymerase chain reaction (RT-qPCR), Western blot and enzymatic analysis showed that PEPCase activity was significantly reduced in the reverse mutant compared with the wild type, and that of the forward mutant was obviously increased. Interestingly, the net photosynthesis in both the reverse mutant and the forward mutant were higher than that of the wild type, but dark respiration was decreased only in the reverse mutant. The absorbance changes of P700 upon saturation pulse showed the photosystem I (PSI) activity was inhibited, as reflected by Y(I), and Y(NA) was elevated, and dark reduction of P700(+) was stimulated, indicating enhanced cyclic electron flow (CEF) around PSI in the reverse mutant. Additionally, the reverse mutant photosynthesis was higher than that of the wild type in low temperature, low and high pH, and high salinity, and this implies increased tolerance in the reverse mutant through downregulated pepc gene.

Functional characterization of the small regulatory subunit PetP from the cytochrome b6f complex in Thermosynechococcus elongatus.

The cyanobacterial cytochrome b(6)f complex is central for the coordination of photosynthetic and respiratory electron transport and also for the balance between linear and cyclic electron transport. The development of a purification strategy for a highly active dimeric b(6)f complex from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 enabled characterization of the structural and functional role of the small subunit PetP in this complex. Moreover, the efficient transformability of this strain allowed the generation of a ΔpetP mutant. Analysis on the whole-cell level by growth curves, photosystem II light saturation curves, and P700(+) reduction kinetics indicate a strong decrease in the linear electron transport in the mutant strain versus the wild type, while the cyclic electron transport via photosystem I and cytochrome b(6)f is largely unaffected. This reduction in linear electron transport is accompanied by a strongly decreased stability and activity of the isolated ΔpetP complex in comparison with the dimeric wild-type complex, which binds two PetP subunits. The distinct behavior of linear and cyclic electron transport may suggest the presence of two distinguishable pools of cytochrome b(6)f complexes with different functions that might be correlated with supercomplex formation.

Physiological role of AOX1a in photosynthesis and maintenance of cellular redox homeostasis under high light in Arabidopsis thaliana

As plants are sessile, they often face high light (HL) stress that causes damage of the photosynthetic machinery leading to decreased photosynthesis. The importance of alternative oxidase (AOX) in optimizing photosynthesis is well documented. In the present study, the role of AOX in sustaining photosynthesis under HL was studied using AOX1a knockout mutants (aox1a) of Arabidopsis thaliana. Under growth light (GL; 50 μmol photons m−2 s−1) conditions, aox1a plants did not show any changes in photosynthetic parameters, NAD(P)/H redox ratios, or respiratory O2 uptake when compared to wild-type (WT). Upon exposure to HL (700 μmol photons m−2 s−1), respiratory rates did not vary between WT and aox1a. But, photosynthetic parameters related to photosystem II (PSII) and NaHCO3 dependent O2 evolution decreased, while the P700 reduction state increased in aox1a compared to WT. Further, under HL, the redox state of cellular NAD(P)/H pools increased with concomitant rise in reactive oxygen species (ROS) and malondialdehyde (MDA) content in aox1a compared to WT. In presence of HL, the transcript levels of several genes related to antioxidant, malate-oxaloacetate (malate-OAA) shuttle, photorespiratory and respiratory enzymes was higher in aox1a compared to WT. Taken together, these results demonstrate that under HL, in spite of significant increase in transcript levels of several genes mentioned above to maintain cellular redox homeostasis and minimize ROS production, Arabidopsis plants deficient in AOX1a were unable to sustain photosynthesis as is the case in WT plants.