PsbY is required for prevention of photodamage to photosystem II in a PsbM-lacking mutant of <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803

The PsbM (3.9 kDa) and PsbY (4.2 kDa) proteins are membrane-spanning, single-helix, subunits associated with the chlorophyll-binding CP47 pre-complex of photosystem II (PSII). Removal of PsbM resulted in accumulation of PSII pre-assembly complexes and impaired electron transfer between the primary (Q_A) and secondary (Q_B) plastoquinone electron acceptors of PSII indicating that the Q_B-binding site and bicarbonate binding to the non-heme iron were altered in this strain. Removal of PsbY alone had only a minor impact on PSII activity but deleting PsbY in the ΔPsbM background led to additional modification of the acceptor side resulting in ΔPsbM:ΔPsbY cells being susceptible to photodamage and this required protein synthesis for recovery. Addition of bicarbonate was able to compensate for the light-induced damage in ΔPsbM:ΔPsbY cells potentially re-occupying the modified bicarbonate-binding site in the ΔPsbM:ΔPsbY strain and complementation of ΔPsbM:ΔPsbY cells with the psbY gene restored the ΔPsbM phenotype.     

Quantification of bound bicarbonate in photosystem II

In this study, we presented a new approach for quantification of bicarbonate (HCO₃^?) molecules bound to PSII. Our method, which is based on a combination of membrane-inlet mass spectrometry (MIMS) and ¹⁸O-labelling, excludes the possibility of “non-accounted” HCO₃^? by avoiding (1) the employment of formate for removal of HCO₃^? from PSII, and (2) the extremely low concentrations of HCO₃^?/CO₂ during online MIMS measurements. By equilibration of PSII sample to ambient CO₂ concentration of dissolved CO₂/HCO₃^?, the method ensures that all physiological binding sites are saturated before analysis. With this approach, we determined that in spinach PSII membrane fragments 1.1 ± 0.1 HCO₃^? are bound per PSII reaction center, while none was bound to isolated PsbO protein. Our present results confirmed that PSII binds one HCO₃^? molecule as ligand to the non-heme iron of PSII, while unbound HCO₃^? optimizes the water-splitting reactions by acting as a mobile proton shuttle.     

‘Birth defects’ of photosystem II make it highly susceptible to photodamage during chloroplast biogenesis

High solar flux is known to diminish photosynthetic growth rates, reducing biomass productivity and lowering disease tolerance. Photosystem II (PSII) of plants is susceptible to photodamage (also known as photoinactivation) in strong light, resulting in severe loss of water oxidation capacity and destruction of the water‐oxidizing complex (WOC). The repair of damaged PSIIs comes at a high energy cost and requires de novo biosynthesis of damaged PSII subunits, reassembly of the WOC inorganic cofactors and membrane remodeling. Employing membrane‐inlet mass spectrometry and O₂‐polarography under flashing light conditions, we demonstrate that newly synthesized PSII complexes are far more susceptible to photodamage than are mature PSII complexes. We examined these ‘PSII birth defects’ in barley seedlings and plastids (etiochloroplasts and chloroplasts) isolated at various times during de‐etiolation as chloroplast development begins and matures in synchronization with thylakoid membrane biogenesis and grana membrane formation. We show that the degree of PSII photodamage decreases simultaneously with biogenesis of the PSII turnover efficiency measured by O₂‐polarography, and with grana membrane stacking, as determined by electron microscopy. Our data from fluorescence, Q_B‐inhibitor binding, and thermoluminescence studies indicate that the decline of the high‐light susceptibility of PSII to photodamage is coincident with appearance of electron transfer capability Q_A^? → Q_B during de‐etiolation. This rate depends in turn on the downstream clearing of electrons upon buildup of the complete linear electron transfer chain and the formation of stacked grana membranes capable of longer‐range energy transfer.     

Acceptor side effects on the electron transfer at cryogenic temperatures in intact photosystem II

In intact PSII, both the secondary electron donor (Tyr_Z) and side-path electron donors (Car/Chl_Z/Cyt_b559) can be oxidized by P₆₈₀⁺ at cryogenic temperatures. In this paper, the effects of acceptor side, especially the redox state of the non-heme iron, on the donor side electron transfer induced by visible light at cryogenic temperatures were studied by EPR spectroscopy. We found that the formation and decay of the S₁Tyr_Z EPR signal were independent of the treatment of K₃Fe(CN)₆, whereas formation and decay of the Car⁺/Chl_Z⁺ EPR signal correlated with the reduction and recovery of the Fe³⁺ EPR signal of the non-heme iron in K₃Fe(CN)₆ pre-treated PSII, respectively. Based on the observed correlation between Car/Chl_Z oxidation and Fe³⁺ reduction, the oxidation of non-heme iron by K₃Fe(CN)₆ at 0 °C was quantified, which showed that around 50-60% fractions of the reaction centers gave rise to the Fe³⁺ EPR signal. In addition, we found that the presence of phenyl-p-benzoquinone significantly enhanced the yield of Tyr_Z oxidation. These results indicate that the electron transfer at the donor side can be significantly modified by changes at the acceptor side, and indicate that two types of reaction centers are present in intact PSII, namely, one contains unoxidizable non-heme iron and another one contains oxidizable non-heme iron. Tyr_Z oxidation and side-path reaction occur separately in these two types of reaction centers, instead of competition with each other in the same reaction centers. In addition, our results show that the non-heme iron has different properties in active and inactive PSII. The oxidation of non-heme iron by K₃Fe(CN)₆ takes place only in inactive PSII, which implies that the Fe³⁺ state is probably not the intermediate species for the turnover of quinone reduction.     

The nonheme iron in photosystem II

Photosystem II (PSII), the light-driven water:plastoquinone (PQ) oxidoreductase of oxygenic photosynthesis, contains a nonheme iron (NHI) at its electron acceptor side. The NHI is situated between the two PQs Q_A and Q_B that serve as one-electron transmitter and substrate of the reductase part of PSII, respectively. Among the ligands of the NHI is a (bi)carbonate originating from CO₂, the substrate of the dark reactions of oxygenic photosynthesis. Based on recent advances in the crystallography of PSII, we review the structure of the NHI in PSII and discuss ideas concerning its function and the role of bicarbonate along with a comparison to the reaction center of purple bacteria and other enzymes containing a mononuclear NHI site.     

Fluorescence Induction Kinetics in Membrane Preparations of Photosystem II with Heterogeneous Metal Clusters (Mn/Fe) in the Oxygen-Evolving Complex

Transport of electrons in spinach photosystem II (PSII) whose oxygen-evolving complex (OEC) contains heterogeneous metal clusters 2Mn2Fe and 3Mn1Fe was studied by measuring the fluorescence induction kinetics (FIK). PSII(2Mn,2Fe) and PSII(3Mn,1Fe) preparations were produced using Cadepleted PSII membranes (PSII(–Ca)). It was found that FIK in PSII(2Mn,2Fe) membranes is similar in form to FIK in PSII(–Ca) samples, but the fluorescence yield is lower in PSII(2Mn,2Fe). The results demonstrate that, just as in PSII(–Ca) preparations, there is electron transfer from the metal cluster in the OEC to the primary plastoquinone electron acceptor Q_A. They also show that partial substitution of Mn cations with Fe has no effect on the electron transport on the acceptor side of PSII. Thus, these data demonstrate the possibility of water oxidation either by the heterogeneous metal cluster or just by the manganese dimer. We established that FIK in PSII(3Mn,1Fe) preparations are similar in form to FIK in PSII(2Mn,2Fe) membranes but PSII(3Mn,1Fe) is characterized by a slightly higher maximal fluorescence yield, F_max. The electron transfer rate in PSII(3Mn,1Fe) preparations significantly (by a factor of two) increases in the presence of Ca²⁺, whereas Ca²⁺ has hardly any effect on the electron transport in PSII(2Mn,2Fe) membranes. In Mndepleted PSII membranes, FIK reaches its maximum (the so-called peak K), after which the fluorescence yield starts to decrease as the result of two factors: the oxidation of reduced primary plastoquinone Q _A ^? and the absence of electron influx from the donor side of PSII. The replacement of Mn cations by Fe in PSII(?Mn) preparations leads to fluorescence saturation and disappearance of the K peak. This is possibly due to the deceleration of the charge recombination process that takes place between reduced primary electron acceptor Q _A ^? and oxidized tyrosine Y _Z ^+. which is an electron carrier between the OEC and the primary electron donor P680.     

PsbP-induced protein conformational changes around Cl<Superscript>−</Superscript> ions in the water oxidizing center of photosystem II

PsbP is an extrinsic protein of PSII having a function of Ca²⁺ and Cl^? retention in the water-oxidizing center (WOC). In order to understand the mechanism how PsbP regulates the Cl^? binding in WOC, we examined the effect of PsbP depletion on the protein structures around the Cl^? sites using Fourier transform infrared (FTIR) spectroscopy. Light-induced FTIR difference spectra upon the S¹→S₂ transition were obtained using Cl^?-bound and NO₃^?-substituted PSII membranes in the presence and absence of PsbP. A clear difference in the amide I band changes by PsbP depletion was observed between Cl^?-bound and NO₃^?-substituted PSII samples, indicating that PsbP binding perturbed the protein conformations around the Cl^?ion(s) in WOC. It is suggested that PsbP stabilizes the Cl^? binding by regulating the dissociation constant of Cl^? and/or an energy barrier of Cl^? dissociation through protein conformational changes around the Cl^? ion(s).     

Aluminum resistance in wheat involves maintenance of leaf Ca<Superscript>2+</Superscript> and Mg<Superscript>2+</Superscript> content,decreased lipid peroxidation and Al accumulation,and low photosystem II excitation pressure

The phytotoxic aluminum species (Al³⁺) is considered as the primary factor limiting crop productivity in over 40 % of world’s arable land that is acidic. We evaluated the responses of two wheat cultivars (Triticum aestivum L.) with differential Al resistance, cv. Yecora E (Al-resistant) and cv. Dio (Al-sensitive), exposed to 0, 37, 74 and 148 μM Al for 14 days in hydroponic culture at pH 4.5. With increasing Al concentration, leaf Ca²⁺ and Mg²⁺ content decreased, as well as the effective quantum yield of photosystem II (PSII) photochemistry (Φ _PSII ), while a gradual increase in leaf membrane lipid peroxidation, Al accumulation, photoinhibition (estimated as F _v /F _m ), and PSII excitation pressure (1 ? q _p ) occurred. However, the Al-resistant cultivar with lower Al accumulation, retained larger concentrations of Ca²⁺ and Mg²⁺ in the leaves and kept a larger fraction of the PSII reaction centres (RCs) in an open configuration, i.e. a higher ratio of oxidized to reduced quinone A (Q_A), than plants of the Al-sensitive cultivar. Four times higher Al concentration in the nutrient solution was required for Al-resistant plants (148 μM Al) than for Al-sensitive (37 μM Al), in order to establish the same closed RCs. Yet, the decline in photosynthetic efficiency in the cultivar Dio was not only due to closure of PSII RCs but also to a decrease in the quantum yield of the open RCs. We suggest that Al³⁺ toxicity may be mediated by nutrient deficiency and oxidative stress, and that Al-resistance of the wheat cultivar Yecora E, may be due at least partially, from the decreased Al accumulation that resulted to decreased reactive oxygen species (ROS) formation. However, under equal internal Al accumulation (exposure Al concentration: Dio 74 μM, Yecora E 148 μM) that resulted to the same oxidative stress, the reduced PSII excitation pressure and the better PSII functioning of the Al-resistant cultivar was probably due to the larger concentrations of Ca²⁺ and Mg²⁺ in the leaves. We propose that the different sensitivities of wheat cultivars to Al³⁺ toxicity can be correlated to differences in the redox state of Q_A. Thus, chlorophyll fluorescence measurements can be a promising tool for rapid screening of Al resistance in wheat cultivars.     

Assessment of the preventive effect of vermicompost on salinity resistance in tomato (<Emphasis Type="Italic">Solanum lycopersicum</Emphasis> cv. Ailsa Craig)

To determine the effects of vermicompost leachate (VCL) on resistance to salt stress in plants, young tomato seedlings (Solanum lycopersicum, cv. Ailsa Craig) were exposed to salinity (150 mM NaCl addition to nutrient solution) for 7 days after or during 6 mL L^??1 VCL application. Salt stress significantly decreased leaf fresh and dry weights, reduced leaf water content, significantly increased root and leaf Na⁺ concentrations, and decreased K⁺ concentrations. Salt stress decreased stomatal conductance (g_s), net photosynthesis (A), instantaneous transpiration (E), maximal efficiency of PSII photochemistry in the dark-adapted state (F_v/F_m), photochemical quenching (qP), and actual PSII photochemical efficiency (ΦPSII). VCL applied during salt stress increased leaf fresh weight and g_s, but did not reduce leaf osmotic potential, despite increased proline content in salt-treated plants. VCL reduced Na⁺ concentrations in leaves (by 21.4%), but increased them in roots (by 16.9%). VCL pre-treatment followed by salt stress was more efficient than VCL concomitant to salt stress, since VCL pre-treatment provided the greatest osmotic adjustment recorded, with maintenance of net photosynthesis and K⁺/Na⁺ ratios following salt stress. VCL pre-treatment also led to the highest proline content in leaves (50 µmol g^??1 FW) and the highest sugar content in roots (9.2 µmol g^??1 FW). Fluorescence-related parameters confirmed that VCL pre-treatment of salt-stressed plants showed higher PSII stability and efficiency compared to plants under concomitant VCL and salt stress. Therefore, VCL represents an efficient protective agent for improvement of salt-stress resistance in tomato.     

Dynamic changes in energy metabolism and electron transport of photosystem II in <Emphasis Type="Italic">Nicotiana tabacum</Emphasis> infested by nymphs of <Emphasis Type="Italic">Bemisia tabaci</Emphasis> (Middle East-Asia Minor 1)

Bemisia tabaci Middle East-Asia Minor 1 (MEAM1) infestation adversely affected photosynthesis of host plants. In the current study, chlorophyll a fluorescence was measured to determine the effects of MEAM1 nymph infestation of tobacco local and systemic leaves on energy metabolism and electron transport of photosystemII(PSII). The results showed that the density of PSII reaction centres per excited cross section (CS) (RC/CS) of infested and systemic leaves was reduced at 14 and 20 days. In systemic leaves, the number of PSII closed reaction centres (1-qP) increased significantly at 14 and 20 days. Absorption flux per Q_A^? reducing PSII reaction centre (RC) (ABS/RC), trapped energy flux per RC (TRo/RC), and electron transport per RC (ETo/RC) of infested and systemic leaves increased with MEAM1 nymph infestation. The most obvious increase in absorption flux per CS (ABS/CSo) and trapped energy flux per CS (TRo/CSo) of infested and systemic leaves occurred at 14 days. MEAM1 nymph infestation significantly reduced the energy required for PSII Q_A to be completely reduced (Sm) in tobacco leaves. These results suggested that MEAM1 nymph infestation caused changes in light-harvesting antenna system and deactivation of the reaction centre, resulting in the reduction of photons absorbed by reaction centres per unit area. MEAM1 nymph infestation, particularly the 3rd instar nymphs, decreased light utilization ability and increased excess excitation energy in tobacco leaves. With MEAM1 nymph infestation, the relative electron transport capacity of the entire electron transport chain decreased, and more light energy was used to reduce Q_A.     

The Chlorophyll <Emphasis Type="Italic">a</Emphasis> Fluorescence Characteristic in Different Types of Leaf Senescence in <Emphasis Type="Italic">Alhagi sparsifolia</Emphasis>

Senescence is both a highly controlled and a strictly regulated process that is gene dependent. To study the PSII reaction in different types of leaf senescence processes, stem girdling was performed on Alhagi sparsifolia to investigate the leaf status in the control, natural senescence, and girdling-induced senescence leaves. The results showed that during senescence, leaf soluble sugar content, starch content, and the energy absorbed by the unit reaction center (ABS/RC) increased; whereas leaf photosynthetic rate, photosynthetic pigment content, maximum photochemical efficiency (φ _Po), and energy used by the acceptor site in electron transfer (ET_o/RC) decreased. The result of the present research implied that stem girdling significantly accelerated leaf senescence, which was due to the accumulation of carbohydrate. Natural senescence is a highly controlled process, which is an ordered process played by genes, whereas girdling-induced senescence is a disordered one. In addition, natural senescence slightly inhibits the acceptor site of PSII but did not damage the donor site of PSII. Conversely, girdling-induced senescence not only damaged the donor site of PSII (for example, oxygen-evolving complex), but also significantly inhibited the acceptor site of PSII. Moreover, both types of senescence led to an increase in the energy absorbed by the unit reaction center (ABS/RC), which subsequently resulted in an increasing excitation pressure in the reaction center (DI_o/RC), as well as additional saved carotenoid for absorbing residual light energy and quenching reactive oxygen species during senescence.     

Changes in photosynthesis of alpine plant <Emphasis Type="Italic">Saussurea superba</Emphasis> during leaf expansion

The native alpine plant Saussurea superba is widely distributed in Qinghai–Tibetan Plateau regions. The leaves of S. superba grow in whorled rosettes, and are horizontally oriented to maximize sunlight exposure. Experiments were conducted in an alpine Kobresia humilis meadow near Haibei Alpine Meadow Ecosystem Research Station (37°29′–37°45′N, 101°12′–101°33′E; alt. 3200 m). Leaf growth, photosynthetic pigments and chlorophyll fluorescence parameters were measured in expanding leaves of S. superba. The results indicate that leaf area increased progressively from inner younger leaves to outside fully expanded ones, and then slightly decreased in nearly senescent leaves, due to early unfavorable environmental conditions, deviating from the ordinary growth pattern. The specific leaf area decreased before leaves were fully expanded, and the leaf thickness was largest in mature leaves. There were no significant changes in the content of chlorophylls (Chl) and carotenoids (Car), but the ratios of Chl a/b and Car/Chl declined after full expansion of the leaves. The variation of Chl a/b coincided well with changes in photochemical quenching (q _P) and the fraction of open PSII reaction centers (q _L). The maximum quantum efficiency of PSII photochemistry after 5 min dark relaxation (F _(v)/F _(m)) continuously increased from younger leaves to fully mature leaves, suggesting that mature leaves could recover more quickly from photoinhibition than younger leaves. The light-harvesting capacity was relatively steady during leaf expansion, as indicated by the maximum quantum efficiency of open PSII centers (\(F_{\text{v}}^{{\prime }}\)/\(F_{\text{m}}^{{\prime }}\)). UV-absorbing compounds could effectively screen harmful solar radiation, and are a main protection way on the photosynthetic apparatus. The decline of q _P and q _L during maturation, together with limitation of quantum efficiency of PSII reaction centers (L _(PFD)), shows a decrease of oxidation state of Q_A in PSII reaction centers under natural sunlight. Furthermore, light-induced (Φ _NPQ) and non-light-induced quenching (Φ _NO) were consistent with variation of L _(PFD). It is concluded that the leaves of S. superba could be classified into four functional groups: young, fully expanded, mature, and senescent. Quick recovery from photoinhibition was correlated with protection by screening pigments, and high level of light energy trapping was correlated with preservation of photosynthetic pigments. Increasing of Φ _NPQ and Φ _NO during leaves maturation indicates that both thermal dissipation of excessive excitation energy in safety and potential threat to photosynthetic apparatus were strengthened due to the declination of q _P and q _L, and enhancement of L _(PFD).     

Photosynthetic physiological performance and proteomic profiling of the oleaginous algae <Emphasis Type="Italic">Scenedesmus acuminatus</Emphasis> reveal the mechanism of lipid accumulation under low and high nitrogen supplies

In this study, we presented cellular morphological changes, time-resolved biochemical composition, photosynthetic performance and proteomic profiling to capture the photosynthetic physiological response of Scenedesmus acuminatus under low nitrogen (3.6 mM NaNO₃, N^?) and high nitrogen supplies (18.0 mM NaNO₃, N⁺). S. acuminatus cells showed extensive lipid accumulation (53.7% of dry weight) and were enriched in long-chain fatty acids (C16 & C18) under low nitrogen supply. The activity of PSII and photosynthetic rate decreases, whereas non-photochemical quenching and dark respiration rates were increased in the N^? group. In addition, the results indicated a redistribution of light excitation energy between PSII and PSI in S. acuminatus exists before lipid accumulation. The iTRAQ results showed that, under high nitrogen supply, protein abundance of the chlorophyll biosynthesis, the Calvin cycle and ribosomal proteins decreased in S. acuminatus. In contrast, proteins associated with the photosynthetic machinery, except for F-type ATPase, were increased in the N⁺ group (N⁺, 3 vs. 9 days and 3 days, N⁺ vs. N^?). Under low nitrogen supply, proteins involved in central carbon metabolism, fatty acid synthesis and branched-chain amino acid metabolism were increased, whereas the abundance of proteins of the photosynthetic machinery had decreased, with exception of PSI (N^?, 3 vs. 9 days and 9 days, N⁺ vs. N^?). Collectively, the current study has provided a basis for the metabolic engineering of S. acuminatus for biofuel production.     

Carboxylate shifts steer interquinone electron transfer in photosynthesis

Understanding the mechanisms of electron transfer (ET) in photosynthetic reaction centers (RCs) may inspire novel catalysts for sunlight-driven fuel production. The electron exit pathway of type II RCs comprises two quinone molecules working in series and in between a non-heme iron atom with a carboxyl ligand (bicarbonate in photosystem II (PSII), glutamate in bacterial RCs). For decades, the functional role of the iron has remained enigmatic. We tracked the iron site using microsecond-resolution x-ray absorption spectroscopy after laser-flash excitation of PSII. After formation of the reduced primary quinone, Q_A⁻, the x-ray spectral changes revealed a transition (t_½ ≈ 150 μs) from a bidentate to a monodentate coordination of the bicarbonate at the Fe(II) (carboxylate shift), which reverted concomitantly with the slower ET to the secondary quinone Q_B. A redox change of the iron during the ET was excluded. Density-functional theory calculations corroborated the carboxylate shift both in PSII and bacterial RCs and disclosed underlying changes in electronic configuration. We propose that the iron-carboxyl complex facilitates the first interquinone ET by optimizing charge distribution and hydrogen bonding within the Q_AFeQ_B triad for high yield Q_B reduction. Formation of a specific priming intermediate by nuclear rearrangements, setting the stage for subsequent ET, may be a common motif in reactions of biological redox cofactors.     

Growth and ecophysiological response in juvenile clones of <Emphasis Type="Italic">Guadua</Emphasis> (Guaduinae: Bambusoideae) cultivated in an altered lowland tropical region

Guadua amplexifolia and Guadua angustifolia are the most promising timber substitutes amongst American bamboos due to their outstanding dimensions and structural properties. Despite the commercial potential of these species, there are few studies on the survival and adaptability of juveniles in plantations. The present study dealt with survival, growth, and ecophysiological response of juvenile clonal plants of these species, cultivated in abandoned pastures in Mérida, Venezuela. Survivorship, growth (height and culm diameter), and ecophysiological parameters were monitored the first year during wet and dry seasons. Survival rates were high in both species (95% in G. amplexifolia and 89% in G. angustifolia). Midday leaf water potentials decreased in both species during dry months (–1.28 to–2.72 MPa in G. amplexifolia and–1.67 to–2.37 MPa in G. angustifolia, respectively). Net photosynthetic rates measured during wet [16.57 ± 1.40 and 13.68 ± 2.40 μmol(CO₂) m^–2 s^–1, respectively] and dry seasons [12.19 ± 2.82 and 8.12 ± 1.81 μmol(CO₂) m^–2 s^–1, respectively], demonstrated that G. amplexifolia maintained consistently higher photosynthetic rates compared to G. angustifolia, which could explain the higher growth rates of the former. Similar trends were observed for stomatal conductance, transpiration, water-use efficiency, electron transport rate, and photochemical quenching of PSII. G. angustifolia maintained higher nonphotochemical quenching as well as a higher consumption of electrons per molecule of CO₂ fixed, indicating a lower photosynthetic efficiency. The maximal photochemical efficiency of PSII (0.73–0.76) suggested that neither of these species suffered from photoinhibition, despite persistently high radiation and air temperatures at the study site.     

Amino acid composition of leaf,grain and bracts of japonica rice (<Emphasis Type="Italic">Oryza Sativa</Emphasis> ssp. <Emphasis Type="Italic">japonica</Emphasis>) and its response to nitrogen fertilization

Heat stress is one of the main abiotic stresses that limit plant growth. The effects of high temperature on oxidative damage, PSII activity and D1 protein turnover were studied in three wheat varieties with different heat susceptibility (CS, YN949 and AK58). The results showed that heat stress induced lower lipid peroxidation in AK58 and YN949 than CS, which was related to different changes of SOD, CAT, POD and H₂O₂. Similarly, AK58 and YN949 performed better PSII photochemical efficiency (F_v/F_m, ΦPSII and ETR) under high temperature, which was attributed to rapid synthesis and degradation of D1 protein. Moreover, higher expression of D1 protein turnover-related genes (PsbA, STN8, PBCP, Deg1, Deg2, Deg5, Deg8, FtsH1/5 and FtsH2/8) and SOD activity in AK58 and YN949 under normal conditions also established a basis for acclimatizing high temperatures, thereby alleviating PSII photoinhibition and reducing oxidative damage when exposed to heat stress.     

Control of the maximal chlorophyll fluorescence yield by the Q<Subscript>B</Subscript> binding site

Differences in maximal yields of chlorophyll variable fluorescence (F_m) induced by single turnover (ST) and multiple turnover (MT) excitation are as great as 40%. Using mutants of Chlamydomonas reinhardtii we investigated potential mechanisms controlling F_m above and beyond the Q_A redox level. F_m was low when the Q_B binding site was occupied by PQ and high when the Q_B binding site was empty or occupied by a PSII herbicide. Furthermore, in mutants with impaired rates of plastoquinol reoxidation, F_m was reached rapidly during MT excitation. In PSII particles with no mobile PQ pool, F_m was virtually identical to that obtained in the presence of PSII herbicides. We have developed a model to account for the variations in maximal fluorescence yields based on the occupancy of the Q_B binding site. The model predicts that the variations in maximal fluorescence yields are caused by the capacity of secondary electron acceptors to reoxidize Q_A^–.     

Exogenous proline induces soluble sugar accumulation and alleviates drought stress effects on photosystem II functioning of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> leaves

The effects of exogenous applied proline (Pro), on photosystem II (PSII) photochemistry of drought stressed (DS) 4-week old Arabidopsis thaliana plants, was studied by using chlorophyll (chl) fluorescence imaging. The maximum quantum yield of PSII photochemistry (F _v /F _m) in DS plants decreased significantly to 77% of that of the control value, suggesting that DS plants could not maintain PSII function, possibly due to accelerated photoinhibition of PSII. Free Pro and total soluble sugars (SS) increased, in response to DS. Exogenous foliar application of Pro by spraying, led to a remarkable increase in the accumulation of Pro and surprisingly also of SS. Both of them served to scavenge reactive oxygen species (ROS), as it was evident by the decreased lipid peroxidation level measured as malondialdehyde (MDA). DS plants sprayed with Pro showed a tolerance to photoinhibition, this indicated by F _v/F _m being close to values typical of healthy leaves by maintaining more than 98% of PSII function. Also the higher quantum efficiency of PSII photochemistry (Φ _PSΙΙ ) and the decreased excitation pressure (1 ? q _p ) recorded for stressed leaves with Pro, lead us to conclude that Pro appears to be involved in the protection of chloroplast structures by quenching ROS. The enhanced dissipation of excess light energy of PSII, in part accounts for the observed increased resistance to DS in A. thaliana leaves with Pro. Our data pointed out that Pro signalling interacts with SS signaling pathway and provided a new insight in Pro metabolism.     

Mutagenesis of CP43-arginine-357 to serine reveals new evidence for (bi)carbonate functioning in the water oxidizing complex of Photosystem II.

The chlorophyll-binding protein CP43 is an inner subunit of the Photosystem II (PSII) reaction center core complex of all oxygenic photoautotrophs. X-Ray structural evidence places the guanidinium cation of the conserved arginine 357 residue of CP43 within a few Angstroms to the Mn(4)Ca cluster of the water-oxidizing complex (WOC) and has been implicated as a possible carbonate binding site. To test the hypothesis, the serine mutant, CP43-R357S, from Synechocystis PCC 6803 was investigated by PSII variable fluorescence (F(v)/F(m)) and simultaneous flash O(2) yield measurements in cells and thylakoid membranes. The R357S mutant assembles PSII-WOC centers, but is unable to grow photoautotrophically. Reconstitution of O(2) evolution by photoactivation and the occurrence of period-four oscillations of F(v)/F(m) establishes that the R357S mutant contains an assembled Mn(4)Ca cluster, but turnover is impaired as seen by an 11-fold larger Kok double miss parameter and faster decay of upper S states. Using pulsed light to avoid photoinactivation, wild-type cells and thylakoid membranes exhibit a 2-4-fold loss in O(2) evolution rate upon partial bicarbonate depletion under multiple turnover conditions, while the R357S mutant is unaffected by bicarbonate. Arginine R357 appears to function in binding a (bi)carbonate ion essential to normal catalytic turnover of the WOC. The quantum yield of electron donation from the WOC into PSII increases with decreasing turnover rate in R357S mutant cells and involves an aborted two-flash pathway that is distinct from the classical four-flash pattern. We speculate that an altered photochemical mechanism for O(2) production occurs via formation of hydrogen peroxide, by analogy to other treatments that retard the kinetics of proton release into the lumen.     

ISC-like [2Fe–2S] ferredoxin (FdxB) dimer from <Emphasis Type="Italic">Pseudomonas putida</Emphasis> JCM 20004: structural and electron–nuclear double resonance characterization

The crystal structure of the ISC-like [2Fe–2S] ferredoxin (FdxB), probably involved in the de novo iron-sulfur cluster biosynthesis (ISC) system of Pseudomonas putida JCM 20004, was determined at 1.90-Å resolution and displayed a novel tail-to-tail dimeric form. P. putida FdxB lacks the consensus free cysteine usually present near the cluster of ISC-like ferredoxins, indicating its primarily electron transfer role in the iron-sulfur cluster. Orientation-selective electron–nuclear double resonance spectroscopic analysis of reduced FdxB in conjunction with the crystal structure has identified the innermost Fe2 site with a high positive spin population as the nonreducible iron retaining the Fe³⁺ valence and the outermost Fe1 site as the reduced iron with a low negative spin density. The average g _max direction is skewed, forming an angle of about 27.3° (±4°) with the normal of the [2Fe–2S] plane, whereas the g _int and g _min directions are distributed in the cluster plane, presumably tilted by the same angle with respect to this plane. These results are related to those for other [2Fe–2S] proteins in different electron transport chains (e.g. adrenodoxin) and suggest a significant distortion of the electronic structure of the reduced [2Fe–2S] cluster under the influence of the protein environment around each iron site in general.