Limitation in electron transfer in photosystem I donor side mutants of Chlamydomonas reinhardtii. Lethal photo-oxidative damage in high light is overcome in a suppressor strain deficient in the assembly of the light harvesting complex

Strains of Chlamydomonas reinhardtii lacking the PsaF gene or containing the mutation K23Q within the N-terminal part of PsaF are sensitive to high light (>400 microE m(-2) s(-1)) under aerobic conditions. In vitro experiments indicate that the sensitivity to high light of the isolated photosystem I (PSI) complex from wild type and from PsaF mutants is similar. In vivo measurements of photochemical quenching and oxygen evolution show that impairment of the donor side of PSI in the PsaF mutants leads to a diminished linear electron transfer and/or a decrease of photosystem II (PSII) activity in high light. Thermoluminescence measurements indicate that the PSII reaction center is directly affected under photo-oxidative stress when the rate of electron transfer becomes limiting in the PsaF-deficient strain and in the PsaF mutant K23Q. We have isolated a high light-resistant PsaF-deficient suppressor strain that has a high chlorophyll a/b ratio and is affected in the assembly of light-harvesting complex. These results indicate that under high light a functionally intact donor side of PSI is essential for protection of C. reinhardtii against photo-oxidative damage when the photosystems are properly connected to their light-harvesting antennae.     

The luminal helix l of PsaB is essential for recognition of plastocyanin or cytochrome c6 and fast electron transfer to photosystem I in Chlamydomonas reinhardtii.

At the lumenal side of photosystem I (PSI) in cyanobacteria, algae, and vascular plants, proper recognition and binding of the donor proteins plastocyanin (pc) and cytochrome (cyt) c(6) are crucial to allow subsequent efficient electron transfer to the photooxidized primary donor. To characterize the surface regions of PSI needed for the correct binding of both donors, loop j of PsaB of Chlamydomonas reinhardtii was modified using site-directed mutagenesis and chloroplast transformation. Mutant strains D624K, E613K/D624K, E613K/W627F, and D624K/W627F accumulated <20% of PSI as compared with wild type and were only able to grow photoautotrophically at low light intensities. Mutant strains E613N, E613K, and W627F accumulated >50% of PSI as compared with wild type. This was sufficient to isolate the altered PSI and perform a detailed analysis of the electron transfer between the modified PSI and the two algal donors using flash-induced spectroscopy. Such an analysis indicated that residue Glu(613) of PsaB has two functions: (i) it is crucial for an improved unbinding of the two donors from PSI, and (ii) it orientates the positively charged N-terminal domain of PsaF in a way that allows efficient binding of pc or cyt c(6) to PSI. Mutation of Trp(627) to Phe completely abolishes the formation of an intermolecular electron transfer complex between pc and PSI and also drastically diminishes the rate of electron transfer between the donor and PSI. This mutation also hinders binding and electron transfer between the altered PSI and cyt c(6). It causes a 10-fold increase of the half-time of electron transfer within the intermolecular complex of cyt c(6) and PSI. These data strongly suggest that Trp(627) is a key residue of the recognition site formed by the core of PSI for binding and electron transfer between the two soluble electron donors and the photosystem.     

Organization of photosystem I polypeptides. Identification of PsaB domains that may interact with PsaD.

PsaA and PsaB are homologous integral membrane-proteins that form the heterodimeric core of photosystem i (PSI). We used subunit-deficient PSI complexes from the mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 to examine interactions between PsaB and other PSI subunits. Incubation of the wild-type PSI with thermolysin yielded 22-kD C-terminal fragments of PsaB that were resistant to further proteolysis. Modification of the wild-type PSI with N-hydroxysuccinimidobiotin and subsequent cleavage by thermolysin showed that the lysyl residues in the 22-kD C-terminal domain were inaccessible to modification by N-hydroxysuccinimidobiotin. The absence of PsaE, PsaF, PsaI, PsaJ, or PsaL facilitated accumulation of 22-kD C-terminal fragments of PsaB but did not alter their resistance to further proteolysis. When the PsaD-less PSI was treated with thermolysin, the 22-kD C-terminal fragments of PsaB were rapidly cleaved, with concomitant accumulation of a 16-kD fragment and then a 3.4-kD one. We mapped the N termini of these fragments by N-terminal amino acid sequencing and the C termini from their positive reaction with an antibody against the C-terminal peptide of PsaB. The cleavage sites were proposed to be in the extramembrane loops on the cytoplasmic side. Western blot analyses showed resistance of PsaC and PsaI to proteolysis prior to cleavage of the 22-kD fragments. Therefore, we propose that PsaD shields two extramembrane loops of PsaB and protects the C-terminal domain of PsaB from in vitro proteolysis.     

Replacement of the methionine axial ligand to the primary electron acceptor A0 slows the A0 reoxidation dynamics in Photosystem I

The recent crystal structure of photosystem I (PSI) from Thermosynechococcus elongatus shows two nearly symmetric branches of electron transfer cofactors including the primary electron donor, P₇₀₀, and a sequence of electron acceptors, A, A₀ and A₁, bound to the PsaA and PsaB heterodimer. The central magnesium atoms of each of the putative primary electron acceptor chlorophylls, A₀, are unusually coordinated by the sulfur atom of methionine 688 of PsaA and 668 of PsaB, respectively. We [Ramesh et al. (2004a) Biochemistry 43:1369-1375] have shown that the replacement of either methionine with histidine in the PSI of the unicellular green alga Chlamydomonas reinhardtii resulted in accumulation of A₀⁻ (in 300-ps time scale), suggesting that both the PsaA and PsaB branches are active. This is in contrast to cyanobacterial PSI where studies with methionine-to-leucine mutants show that electron transfer occurs predominantly along the PsaA branch. In this contribution we report that the change of methionine to either leucine or serine leads to a similar accumulation of A₀⁻ on both the PsaA and the PsaB branch of PSI from C. reinhardtii, as we reported earlier for histidine mutants. More importantly, we further demonstrate that for all the mutants under study, accumulation of A₀⁻ is transient, and that reoxidation of A₀⁻ occurs within 1-2 ns, two orders of magnitude slower than in wild type PSI, most likely via slow electron transfer to A₁. This illustrates an indispensable role of methionine as an axial ligand to the primary acceptor A₀ in optimizing the rate of charge stabilization in PSI. A simple energetic model for this reaction is proposed. Our findings support the model of equivalent electron transfer along both cofactor branches in Photosystem I.     

Chlorophyll a fluorescence of typical desert plant Alhagi sparsifolia Shap. at two light levels

Alhagi sparsifolia Shap. is exposed to a high-irradiance environment as the main vegetation found in the forelands of the Taklamakan Desert. We investigated chlorophyll a fluorescence emission of A. sparsifolia seedlings grown under ambient (HL) and shade (LL) conditions. Our results indicated that the fluorescence intensity in the leaves was significantly higher for LL-grown plants than that under HL. High values of the maximum quantum yield of PSII for primary photochemistry (φP_o) and the quantum yield that an electron moves further than Q_A - (φE_o) in the plants under LL conditions suggested that the electron flow from Q_A - (primary quinone electron acceptors of PSII) to Q_B (secondary quinone acceptor of PSII) or Q_B - was enhanced at LL compared to natural HL conditions. The efficiency/probability with which an electron from the intersystem electron carriers was transferred to reduce end electron acceptors at the PSI acceptor side and the quantum yield for the reduction of end electron acceptors at the PSI acceptor side were opposite to φP_o, and φE_o. Thus, we concluded that the electron transport on the donor side of PSII was blocked under LL conditions, while acceptor side was inhibited at the HL conditions. The PSII activity of electron transport in the plants grown in shade was enhanced, while the energy transport from PSII to PSI was blocked compared to the plants grown at HL conditions. Furthermore, PSII activity under HL was seriously affected in midday, while the plants grown in shade enhanced their energy transport.     

Effect of light quality on chloroplast-membrane organization and function in pea

The effect of light quality during plant growth of chloroplast membrane organization and function in peas (Pisum sativum L. cv. Alaska) was investigated. In plants grown under photosystem (PS) I-enriched (far-red enriched) illumination both the PSII/PSI stoichiometry and the electrontransport capacity ratios were high, about 1.9. In plants grown under PSII-enriched (far-red depleted) illumination both the PSII/PSI stoichiometry and the electron-transport capacity ratios were significantly lower, about 1.3. In agreement, steady-state electron-transport measurements under synchronous illumination of PSII and PSI demonstrated an excess of PSII in plants grown under far-red-enriched light. Sodium dodecylsulfate polyacrylamide gel electrophoretic analysis of chlorophyll-containing complexes showed greater relative amounts of the PSII reaction center chlorophyll-protein complex in plants grown under farred-enriched light. Additional changes were observed in the ratio of light-harvesting chlorophyll a/b protein to PSII reaction center chlorophyll-protein under the two different light-quality regimes. The results demonstrate the dynamic nature of chloroplast structure and support the notion that light quality is an important factor in the regulation of chloroplast membrane organization and-function.Abbreviations and symbols Chl chlorophyll - CPa PSII reaction center chlorophyll protein complex - CPI PSI chlorophyll protein complex - FR-D light depleted in far-red sensitizing primarily PSII - FR-E light enriched in far-red sensitizing primarily PSI - LHCP PSII light-harvesting chlorophyll a/b protein complex - P ₇₀₀ primary electron donor of PSI - PSI, PSII photosystems I and II, respectively - Q primary electron acceptor of PSII     

Oxidizing side of the cyanobacterial photosystem I. Evidence for interaction between the electron donor proteins and a luminal surface helix of the PsaB subunit.

Photosystem I (PSI) interacts with plastocyanin or cytochrome c6 on the luminal side. To identify sites of interaction between plastocyanin/cytochrome c6 and the PSI core, site-directed mutations were generated in the luminal J loop of the PsaB protein from Synechocystis sp. PCC 6803. The eight mutant strains differed in their photoautotrophic growth. Western blotting with subunit-specific antibodies indicated that the mutations affected the PSI level in the thylakoid membranes. PSI proteins could not be detected in the S600R/G601C/N602I, N609K/S610C/T611I, and M614I/G615C/W616A mutant membranes. The other mutant strains contained different levels of PSI proteins. Among the mutant strains that contained PSI proteins, the H595C/L596I, Q627H/L628C/I629S, and N638C/N639S mutants showed similar levels of PSI-mediated electron transfer activity when either cytochrome c6 or an artificial electron donor was used. In contrast, cytochrome c6 could not function as an electron donor to the W622C/A623R mutant, even though the PSI activity mediated by an artificial electron donor was detected in this mutant. Thus, the W622C/A623R mutation affected the interaction of the PSI complex with cytochrome c6. Biotin-maleimide modification of the mutant PSI complexes indicated that His-595, Trp-622, Leu-628, Tyr-632, and Asn-638 in wild-type PsaB may be exposed on the surface of the PSI complex. The results presented here demonstrate the role of an extramembrane loop of a PSI core protein in the interaction with soluble electron donor proteins.     

Biochemical and molecular characterization of photosystem I deficiency in the NCS6 mitochondrial mutant of maize

Interorganellar signaling interactions are poorly understood. The maize non-chromosomal stripe (NCS) mutants provide models to study the requirement of mitochondrial function for chloroplast biogenesis and photosynthesis. Previous work suggested that the NCS6 mitochondrial mutation, a cytochrome oxidase subunit 2 (cox2) deletion, is associated with a malfunction of Photosystem I (PSI) in defective chloroplasts of mutant leaf sectors (Gu et al., 1993). We have now quantified the reductions of photosynthetic rates and PSI activity in the NCS6 defective stripes. Major reductions of the plastid-coded PsaC and nucleus-coded PsaD and PsaE PSI subunits and of their corresponding mRNAs are seen in mutant sectors; however, although thepsaA/B mRNA is greatly reduced, levels of PsaA and PsaB (the core proteins of PSI) are only slightly decreased. Levels of the PsaL subunit and its mRNA appear to be unchanged. Tested subunits of other thylakoid membrane complexes – PSII, Cyt b₆/f, and ATP synthase, have minor (or no) reductions within mutant sectors. The results suggest that specific signaling pathways sense the dysfunction of the mitochondrial electron transport chain and respond to down-regulate particular PSI mRNAs, leading to decreased PSI accumulation in the chloroplast. The reductions of both nucleus and plastid encoded components indicate that complex interorganellar signaling pathways may be involved.     

Inhibition of respiration and nitrate assimilation enhances photohydrogen evolution under low oxygen concentrations in Synechocystis sp. PCC 6803

In cyanobacterial membranes photosynthetic light reaction and respiration are intertwined. It was shown that the single hydrogenase of Synechocystis sp. PCC 6803 is connected to the light reaction. We conducted measurements of hydrogenase activity, fermentative hydrogen evolution and photohydrogen production of deletion mutants of respiratory electron transport complexes. All single, double and triple mutants of the three terminal respiratory oxidases and the ndhB-mutant without a functional complex I were studied. After activating the hydrogenase by applying anaerobic conditions in the dark hydrogen production was measured at the onset of light. Under these conditions respiratory capacity and amount of photohydrogen produced were found to be inversely correlated. Especially the absence of the quinol oxidase induced an increased hydrogenase activity and an increased production of hydrogen in the light compared to wild type cells. Our results support that the hydrogenase as well as the quinol oxidase function as electron valves under low oxygen concentrations. When the activities of photosystem II and I (PSII and PSI) are not in equilibrium or in case that the light reaction is working at a higher pace than the dark reaction, the hydrogenase is necessary to prevent an acceptor side limitation of PSI, and the quinol oxidase to prevent an overreduction of the plastoquinone pool (acceptor side of PSII). Besides oxygen, nitrate assimilation was found to be an important electron sink. Inhibition of nitrate reductase resulted in an increased fermentative hydrogen production as well as higher amounts of photohydrogen.     

Replacement of the methionine axial ligand to the primary electron acceptor A0 slows the A0- reoxidation dynamics in photosystem I

The recent crystal structure of photosystem I (PSI) from Thermosynechococcus elongatus shows two nearly symmetric branches of electron transfer cofactors including the primary electron donor, P(700), and a sequence of electron acceptors, A, A(0) and A(1), bound to the PsaA and PsaB heterodimer. The central magnesium atoms of each of the putative primary electron acceptor chlorophylls, A(0), are unusually coordinated by the sulfur atom of methionine 688 of PsaA and 668 of PsaB, respectively. We [Ramesh et al. (2004a) Biochemistry 43:1369-1375] have shown that the replacement of either methionine with histidine in the PSI of the unicellular green alga Chlamydomonas reinhardtii resulted in accumulation of A(0)(-) (in 300-ps time scale), suggesting that both the PsaA and PsaB branches are active. This is in contrast to cyanobacterial PSI where studies with methionine-to-leucine mutants show that electron transfer occurs predominantly along the PsaA branch. In this contribution we report that the change of methionine to either leucine or serine leads to a similar accumulation of A(0)(-) on both the PsaA and the PsaB branch of PSI from C. reinhardtii, as we reported earlier for histidine mutants. More importantly, we further demonstrate that for all the mutants under study, accumulation of A(0)(-) is transient, and that reoxidation of A(0)(-) occurs within 1-2 ns, two orders of magnitude slower than in wild type PSI, most likely via slow electron transfer to A(1). This illustrates an indispensable role of methionine as an axial ligand to the primary acceptor A(0) in optimizing the rate of charge stabilization in PSI. A simple energetic model for this reaction is proposed. Our findings support the model of equivalent electron transfer along both cofactor branches in Photosystem I.     

The Accumulation of Protochlorophyllide in Cells of Synechocystis PCC 6714 with a Low PSI/PSII Stoichiometry

Changes in intracellular levels of Chl a precursors were examinedin relation to changes in the PSI/PSII stoichiometry in thecyanophyte Synechocystis PCC 6714. Protochlorophyllide (Pchlide)accumulated markedly in cells with a low PSI/PSII stoichiometrygrown under light that is absorbed by Chl a (PSI light) whereasno accumulation occurred in cells with a high PSI/PSII stoichiometrygrown under light absorbed by phycobilisomes (PSII light). Levelsof Pchlide in cells grown under PSI light decreased rapidlyupon a shift to PSII light. The rapid decrease in Pchlide accompanieda transient increase in chlorophyllide a, indicating that reductionof Pchlide was enhanced by shift to PSII light. The action spectrumindicated that the Pchlide decrease upon the shift to PSII lightdepended on excitation of Pchlide, suggesting that the accumulationof Pchllide was due to limited excitation of Pchlide, so thatPchlide photoreduction, under PSI light. However, comparisonof levels of Pchlide and the photosystem complexes in wild-typePlectonema boryanum with those in a mutant that lacked the darkPchlide reductase (YFC 1004) indicated that dark reduction compensatedfor the limited photoreduction under PSI light. Similar compensationby dark reduction was confirmed with Synechocystis PCC 6714.In cultures of Synechocystis under conditions where Pchlidecould not be photoreduced, accumulation of Pchlide and low PSI/PSIIstoichiometry occurred only when cells were illuminated withlight that preferentially excited PSI. The results indicatethat the low PSI/PSII stoichiometry in cells grown under PSIlight is not a result of inefficient synthesis of Chl a witha reduced rate of Pchlide photoreduction. They suggest furtherthat accumulation of Pchlide under PSI light results from retardationof the Chl a synthesis due to suppression of PSI synthesis. ¹Present address: Tsurukawa 5-15-11, Machida, Tokyo, 195 Japan.     

The hydrophobic recognition site formed by residues PsaA-Trp651 and PsaB-Trp627 of photosystem I in Chlamydomonas reinhardtii confers distinct selectivity for binding of plastocyanin and cytochrome c6

On the lumenal side of photosystem I (PSI), each of the two large core subunits, PsaA and PsaB, expose a conserved tryptophan residue to the surface. PsaB-Trp(627) is part of the hydrophobic recognition site that is essential for tight binding of the two electron donors plastocyanin and cytochrome c(6) to the donor side of PSI (Sommer, F., Drepper, F., and Hippler, M. (2002) J. Biol. Chem. 277, 6573-6581). To examine the function of PsaA-Trp(651) in binding and electron transfer of both donors to PSI, we generated the mutants PsaA-W651F and PsaA-W651S by site-directed mutagenesis and biolistic transformation of Chlamydomonas reinhardtii. The protein-protein interaction and the electron transfer between the donors and PSI isolated from the mutants were analyzed by flash absorption spectroscopy. The mutation PsaA-W651F completely abolished the formation of a first order electron transfer complex between plastocyanin (pc) and the altered PSI and increased the dissociation constant for binding of cytochrome (cyt) c(6) by more than a factor of 10 as compared with wild type. Mutation of PsaA-Trp(651) to Ser had an even larger impact on the dissociation constant. The K(D) value increased another 2-fold when the values obtained for the interaction and electron transfer between cyt c(6) and PSI from PsaA-W651S and PsaA-W651F are compared. In contrast, binding and electron transfer of pc to PSI from PsaA-W651S improved as compared with PSI from PsaA-W651F and admitted the formation of an inter-molecular electron transfer complex, resulting in a K(D) value of about 554 microm that is still five times higher than observed for wild type. These results demonstrate that PsaA-Trp(651) is, such as PsaB-Trp(627), crucial for high affinity binding of pc and cyt c(6) to PSI. Our results also indicate that the highly conserved structural recognition motif that is formed by PsaA-Trp(651) and PsaB-Trp(627) confers a differential selectivity in binding of both donors to PSI.     

Cyclic electron flow plays an important role in photoprotection for the resurrection plant <Emphasis Type="Italic">Paraboea</Emphasis><Emphasis Type="Italic">rufescens</Emphasis> under drought stress

Resurrection plants could survive severe drought stress, but the underlying mechanism for protecting their photosynthetic apparatus against drought stress is unclear. Cyclic electron flow (CEF) has been documented as a crucial mechanism for photoprotection in Arabidopsis and tobacco. We hypothesized that CEF plays an important role in protecting photosystem I (PSI) and photosystem II (PSII) against drought stress for resurrection plants. To address this hypothesis, the effects of mild drought stress on light energy distribution in PSII and P700 redox state were examined in a resurrection plant Paraboea rufescens. Cyclic electron flow was not activated below the photosynthetic photon flux density (PPFD) of 400 μmol m⁻² s⁻¹ in leaves without drought stress. However, CEF was activated under low light in leaves with mild drought stress, and the effective quantum yield of PSII significantly decreased. Meanwhile, non-photochemical quenching (NPQ) was significantly stimulated not only under high light but also under low light. Compared with the control, the fraction of overall P700 that cannot be oxidized in a given state (PSI acceptor side limitation) under high light was maintained at low level of 0.1 in leaves with water deficit, indicating that the over-reduction of the PSI acceptor side was prevented by the significant stimulation of CEF. Furthermore, methyl viologen could significantly increase the PSII photo-inhibition induced by high light compared with chloramphenicol. These results suggested that CEF is an important mechanism for protecting PSI and PSII from drought stress in resurrection plants.     

Absence of PsaC subunit allows assembly of photosystem I core but prevents the binding of PsaD and PsaE in Synechocystis sp. PCC6803

In photosystem I (PSI) of oxygenic photosynthetic organisms the psaC polypeptide, encoded by the psaC gene, provides the ligands for two [4Fe-4S] clusters, F_A and F_B. Unlike other cyanobacteria, two different psaC genes have been reported in the cyanobacterium Synechocystis 6803, one (copy 1) with a deduced amino acid sequence identical to that of tobacco and another (copy 2) with a deduced amino acid sequence similar to those reported for other cyanobacteria. Insertion of a gene encoding kanamycin resistance into copy 2 resulted in a photosynthesis-deficient strain, CDK25, lacking the PsaC, PsaD and PsaE polypeptides in isolated thylakoid membranes, while the PsaA/PsaB and PsaF subunits were found. Growth of the mutant cells was indistinguishable from that of wild-type cells under light-activated heterotrophic growth (LAHG). A reversible P700⁺ signal was detected by EPR spectroscopy in the isolated thylakoids during illumination at low temperature. Under these conditions, the EPR signals attributed to F_A and F_B were absent in the mutant strain, but a reversible F_x signal was present with broad resonances at g=2.079, 1.903, and 1.784. Addition of PsaC and PsaD proteins to the thylakoids gave rise to resonances at g=2.046, 1.936, 1.922, and 1.880; these values are characteristic of an interaction-type spectrum of F_A ^- and F_B ^-. In room-temperature optical spectroscopic analysis, addition of PsaC and PsaD to the thylakoids also restored a 30 ms kinetic transient which is characteristic of the P700⁺ [F_A/F_B]^- backreaction. Expression of copy 1 was not detected in cells grown under LAHG and under mixotrophic conditions. These results demonstrate that copy 2 encodes the PsaC polypeptide in PSI in Synechocystis 6803, while copy 1 is not involved in PSI; that the PsaC polypeptide is necessary for stable assembly of PsaD and PsaE into PSI complex in vivo; and that PsaC, PsaD and PsaE are not needed for assembly of PsaA-PsaB dimer and electron transport from P700 to F_x.     

Production of reactive oxygen species by photosystem II

Photosysthetic cleavage of water molecules to molecular oxygen is a crucial process for all aerobic life on the Earth. Light-driven oxidation of water occurs in photosystem II (PSII) — a pigment-protein complex embedded in the thylakoid membrane of plants, algae and cyanobacteria. Electron transport across the thylakoid membrane terminated by NADPH and ATP formation is inadvertently coupled with the formation of reactive oxygen species (ROS). Reactive oxygen species are mainly produced by photosystem I; however, under certain circumstances, PSII contributes to the overall formation of ROS in the thylakoid membrane. Under limitation of electron transport reaction between both photosystems, photoreduction of molecular oxygen by the reducing side of PSII generates a superoxide anion radical, its dismutation to hydrogen peroxide and the subsequent formation of a hydroxyl radical terminates the overall process of ROS formation on the PSII electron acceptor side. On the PSII electron donor side, partial or complete inhibition of enzymatic activity of the water-splitting manganese complex is coupled with incomplete oxidation of water to hydrogen peroxide. The review points out the mechanistic aspects in the production of ROS on both the electron acceptor and electron donor side of PSII.     

Mitochondrial alternative oxidase pathway protects plants against photoinhibition by alleviating inhibition of the repair of photodamaged PSII through preventing formation of reactive oxygen species in Rumex K-1 leaves

The purpose of this study was to explore how the mitochondrial AOX (alternative oxidase) pathway alleviates photoinhibition in Rumex K-1 leaves. Inhibition of the AOX pathway decreased the initial activity of NADP-malate dehydrogenase (EC 1.1.1.82, NADP-MDH) and the pool size of photosynthetic end electron acceptors, resulting in an over-reduction of the photosystem I (PSI) acceptor side. The over-reduction of the PSI acceptor side further inhibited electron transport from the photosystem II (PSII) reaction centers to the PSII acceptor side as indicated by an increase in V(J) (the relative variable fluorescence at J-step), causing an imbalance between photosynthetic light absorption and energy utilization per active reaction center (RC) under high light, which led to the over-excitation of the PSII reaction centers. The over-reduction of the PSI acceptor side and the over-excitation of the PSII reaction centers enhanced the accumulation of reactive oxygen species (ROS), which inhibited the repair of the photodamaged PSII. However, the inhibition of the AOX pathway did not change the level of photoinhibition under high light in the presence of the chloroplast D1 protein synthesis inhibitor chloramphenicol, indicating that the inhibition of the AOX pathway did not accelerate the photodamage to PSII directly. All these results suggest that the AOX pathway plays an important role in the protection of plants against photoinhibition by minimizing the inhibition of the repair of the photodamaged PSII through preventing the over-production of ROS.     

强光下硫化氢通过促进光系统II的活性来缓解水稻的光抑制

以水稻品种‘II优084’为材料,测定了强光胁迫下,水稻光合速率、叶绿素荧光快速诱导曲线（OJIP）以及O2ˉ·和H2O2在水稻叶片中积累的影响。结果表明强光胁迫下,水稻的净光合速率及气孔导度下降;光系统II（PSII）反应中心关闭的比例以及电子传递链中光系统II受体侧原初醌受体（QA）的还原程度增加;PSII反应中心电子传递的量子产额、能量以及传递到下游电子链的比率下降;光抑制下PSII的过剩能量向PSI的状态装换减少;自由基的产生增加。而施加作为硫化氢（H2S）供体的外源硫氢化钠（NaHS）后,上述影响PSII活性的指标的负变化被缓解,捕光天线复合体LHC通过在两个光系统之间的移动,来调节两个光系统的能量分配。强光下H2S处理能促进LHC离开PSII,与PSI结合,从而减少PSII分配的激发能,增加PSI分配的激发能,缓解了PSII的过度还原。以上结果表明外源H2S通过促进PSII的光合活性来缓解水稻光抑制伤害。     

THE ANTARCTIC PSYCHROPHILE,CHLAMYDOMONAS RAUDENSIS ETTL (UWO241) (CHLOROPHYCEAE,CHLOROPHYTA), EXHIBITS A LIMITED CAPACITY TO PHOTOACCLIMATE TO RED LIGHT1

The psychrophilic Antarctic alga, Chlamydomonas raudensis Ettl (UWO241), grows under an extreme environment of low temperature and low irradiance of a limited spectral quality (blue‐green). We investigated the ability of C. raudensis to acclimate to long‐term imbalances in excitation caused by light quality through adjustments in photosystem stoichiometry. Log‐phase cultures of C. raudensis and C. reinhardtii grown under white light were shifted to either blue or red light for 12 h. Previously, we reported that C. raudensis lacks the ability to redistribute light energy via the short‐term mechanism of state transitions. However, similar to the model of mesophilic alga, C. reinhardtii, the psychrophile retained the capacity for long‐term adjustment in energy distribution between PSI and PSII by modulating the levels of PSI reaction center polypeptides, PsaA/PsaB, with minimal changes in the content of the PSII polypeptide, D1, in response to changes in light quality. The functional consequences of the modulation in PSI/PSII stoichiometry in the psychrophile were distinct from those observed in C. reinhardtii. Exposure of C. raudensis to red light caused 1) an inhibition of growth and photosynthetic rates, 2) an increased reduction state of the intersystem plastoquinone pool with concomitant increases in nonphotochemical quenching, 3) an uncoupling of the major light‐harvesting complex from the PSII core, and 4) differential thylakoid protein phosphorylation profiles compared with C. reinhardtii. We conclude that the characteristic low levels of PSI relative to PSII set the limit in the capacity of C. raudensis to photoacclimate to an environment enriched in red light.     

NaCl胁迫加重强光胁迫下超大甜椒叶片的光系统II和光系统I的光抑制

快速叶绿素荧光动力学可以在无损情况下探知叶片光合机构的损伤程度, 快速叶绿素荧光测定和分析技术(JIP-test)将测量值转化为多种具有生物学意义的参数, 因而被广泛应用于植物光合机构对环境的响应机制研究。该文研究了超大甜椒(Capsicum annuum)幼苗在强光及不同NaCl浓度胁迫下的荧光响应情况。与单纯强光胁迫相比, NaCl胁迫引起了叶绿素荧光诱导曲线的明显改变, 光系统II (PSII)光抑制加重, 同时PSII反应中心和受体侧受到明显影响, 而且高NaCl浓度胁迫下PSII供体侧受伤害明显, 同时PSI反应中心活性(P700⁺)在盐胁迫下明显降低。这些结果表明, NaCl胁迫会增强强光对超大甜椒光系统的光抑制, 并且浓度越高抑制越明显, 但对PSI的抑制作用低于PSII。高NaCl浓度胁迫易对PSII供体侧造成破坏, 且PSI光抑制严重。     

The ferredoxin docking site of photosystem I

The reaction center of photosystem I (PSI) reduces soluble ferredoxin on the stromal side of the photosynthetic membranes of cyanobacteria and chloroplasts. The X-ray structure of PSI from the cyanobacterium Synechococcus elongatus has been recently established at a 2.5 A resolution [Nature 411 (2001) 909]. The kinetics of ferredoxin photoreduction has been studied in recent years in many mutants of the stromal subunits PsaC, PsaD and PsaE of PSI. We discuss the ferredoxin docking site of PSI using the X-ray structure and the effects brought by the PSI mutations to the ferredoxin affinity.