What you get out of high-time resolution electron paramagnetic resonance: example from photosynthetic bacteria

Photosynthesis Research - The primary energy conversion steps of natural photosynthesis proceed via light-induced radical ion pairs as short-lived intermediates. Time-resolved electron paramagnetic...     

The hidden function of photosynthesis: a sensing system for environmental conditions that regulates plant acclimation responses

Plants convert light energy from the sun into chemical energy by photosynthesis. Since they are sessile, they have to deal with a wide range of conditions in their immediate environment. Many abiotic and biotic parameters exhibit considerable fluctuations which can have detrimental effects especially on the efficiency of photosynthetic light harvesting. During evolution, plants, therefore, evolved a number of acclimation processes which help them to adapt photosynthesis to such environmental changes. This includes protective mechanisms such as excess energy dissipation and processes supporting energy redistribution, e.g. state transitions or photosystem stoichiometry adjustment. Intriguingly, all these responses are triggered by photosynthesis itself via the interplay of its light reaction and the Calvin–Benson cycle with the residing environmental condition. Thus, besides its primary function in harnessing and converting light energy, photosynthesis acts as a sensing system for environmental changes that controls molecular acclimation responses which adapt the photosynthetic function to the environmental change. Important signalling parameters directly or indirectly affected by the environment are the pH gradient across the thylakoid membrane and the redox states of components of the photosynthetic electron transport chain and/or electron end acceptors coupled to it. Recent advances demonstrate that these signals control post-translational modifications of the photosynthetic protein complexes and also affect plastid and nuclear gene expression machineries as well as metabolic pathways providing a regulatory framework for an integrated response of the plant to the environment at all cellular levels.     

The hidden function of photosynthesis: a sensing system for environmental conditions that regulates plant acclimation responses

Plants convert light energy from the sun into chemical energy by photosynthesis. Since they are sessile, they have to deal with a wide range of conditions in their immediate environment. Many abiotic and biotic parameters exhibit considerable fluctuations which can have detrimental effects especially on the efficiency of photosynthetic light harvesting. During evolution, plants, therefore, evolved a number of acclimation processes which help them to adapt photosynthesis to such environmental changes. This includes protective mechanisms such as excess energy dissipation and processes supporting energy redistribution, e.g. state transitions or photosystem stoichiometry adjustment. Intriguingly, all these responses are triggered by photosynthesis itself via the interplay of its light reaction and the Calvin-Benson cycle with the residing environmental condition. Thus, besides its primary function in harnessing and converting light energy, photosynthesis acts as a sensing system for environmental changes that controls molecular acclimation responses which adapt the photosynthetic function to the environmental change. Important signalling parameters directly or indirectly affected by the environment are the pH gradient across the thylakoid membrane and the redox states of components of the photosynthetic electron transport chain and/or electron end acceptors coupled to it. Recent advances demonstrate that these signals control post-translational modifications of the photosynthetic protein complexes and also affect plastid and nuclear gene expression machineries as well as metabolic pathways providing a regulatory framework for an integrated response of the plant to the environment at all cellular levels.     

Structure-function investigations of bacterial photosynthetic reaction centers

During photosynthesis light energy is converted into energy of chemical bonds through a series of electron and proton transfer reactions. Over the first ultrafast steps of photosynthesis that take place in the reaction center (RC) the quantum efficiency of the light energy transduction is nearly 100%. Compared to the plant and cyanobacterial photosystems, bacterial RCs are well studied and have relatively simple structure. Therefore they represent a useful model system both for manipulating of the electron transfer parameters to study detailed mechanisms of its separate steps as well as to investigate the common principles of the photosynthetic RC structure, function, and evolution. This review is focused on the research papers devoted to chemical and genetic modifications of the RCs of purple bacteria in order to study principles and mechanisms of their functioning. Investigations of the last two decades show that the maximal rates of the electron transfer reactions in the RC depend on a number of parameters. Chemical structure of the cofactors, distances between them, their relative orientation, and interactions to each other are of great importance for this process. By means of genetic and spectral methods, it was demonstrated that RC protein is also an essential factor affecting the efficiency of the photochemical charge separation. Finally, some of conservative water molecules found in RC not only contribute to stability of the protein structure, but are directly involved in the functioning of the complex.     

Efficiency of electron transport processes

Efficiency of electron transport along the linear chain of molecules was investigated from a dynamic viewpoint. It was proposed that two kinds of efficiency are important for electron transport; one is energy efficiency, the other quantum efficiency. In this paper, these two efficiencies are defined for a linear chain system and the correlation between these quantities and the arrangement of various electron transfers is investigated. The optimization of energy and quantum efficiency is found to set different conditions on the arrangement of the rate constants of electron transfer, and there is strong correlation between neighboring electron transfers. In order to maximize both efficiencies, the rate constants of forward and backward transfers of electrons should be bounded by one another in a limited range. In particular, when there are some bypass reactions on the linear chain, as is the case for photosynthesis and respiration, the rate of the backward transfer should be the same order of magnitude as that of the next forward transfer. The present results are applied to some biological processes. In the early stage of photosynthetic electron transfer it seems that quantum efficiency is more important than energy efficiency. The quantum efficiency is close to unity, whereas a considerable part of the free energy is wasted as heat during the primary electron transfers. On the other hand, in the slower electron transfer processes in photosynthesis and respiration, which take place mostly near equilibrium, the energy efficiency seems to be more important than the quantum efficiency. The relation of these properties to biological function is discussed.     

光合作用对光和CO2响应模型的研究进展

光合作用对光和CO₂响应模型是研究植物生理和植物生态学的重要工具, 可为植物光合特性对主要环境因子的响应提供科学依据。该文综述了当前光合作用对光和CO₂响应模型的研究进展和存在的问题, 并在此基础上探讨了这些模型的可能发展趋势。光合作用涉及光能的吸收、能量转换、电子传递、ATP合成、CO₂固定等一系列复杂的物理和化学反应过程。光合作用由原初反应、同化力形成和碳同化3个基本过程构成, 任一个过程均可对光合作用速率产生直接的影响。光合作用对光响应模型只涉及光能的转换, 而光合作用的生化模型包含了同化力形成和碳同化这两个基本过程。把光合作用的原初反应, 即把参与光能吸收、传递和转换的捕光色素分子的物理参数(如捕光色素分子数、捕光色素分子光能吸收截面、捕光色素分子处于激发态的平均寿命等)结合到生化模型中, 可能是今后光合作用对光响应机理模型的发展方向。     

外源钙对干旱胁迫下烤烟幼苗光系统Ⅱ功能的影响

以叶绿素快相荧光动力学曲线(OJIP)为探针,研究了外源钙对干旱胁迫下烤烟幼苗光系统Ⅱ(PSⅡ)功能的影响.结果表明:干旱胁迫降低了烤烟幼苗PSⅡ原初光能转换效率(Fv/Fm)和电子传递速率(ETR),抑制了光合作用的原初过程,烤烟幼苗叶片发生了明显的光抑制.叶面喷施10.0 mmol·L-1CaCl2溶液后烤烟叶片的光合电子传递能量比例(ФEo)在干旱胁迫下的降低幅度明显小于对照(喷施清水),电子转运效率(ET0/RC)在干旱胁迫下明显高于对照.叶面喷施CaC12溶液增加了PSⅡ捕获光能用于光合电子传递的比例、剩余有活性反应中心的效率和电子传递链中的能量传递,使烤烟叶片的光系统Ⅱ在干旱胁迫下保持相对较高的活性,从而提高了烤烟幼苗的抗旱能力.     

IMPACT OF IRON LIMITATION ON THE PHOTOSYNTHETIC APPARATUS OF THE DIATOM CHAETOCEROS MUELLERI (BACILLARIOPHYCEAE)

Iron starvation induced marked increases in flavodoxin abundance and decreases in light-saturated and light-limited photosynthesis rates in the diatom Chaetoceros muelleri. Consistent with the substitution of flavodoxin for ferredoxin as an early response to iron starvation, increases of flavodoxin abundance were observed before declines of cell division rate or chl a specific photosynthesis rates. Changes in the abundance of flavodoxin after the addition of iron to iron-starved cells indicated that flavodoxin was not actively degraded under iron-replete conditions. Greater declines in light-saturated oxygen evolution rates than dark oxygen consumption rates indicated that the mitochondrial electron transfer chain was not affected as greatly by iron starvation as the photosynthetic electron transfer chain. The carbon:nitrogen ratio was unaffected by iron starvation, suggesting that photosynthetic electron transfer was a primary target of iron starvation and that reductions in nitrate assimilation were due to energy limitation (the C:N ratio would be expected to rise under nitrogen-limited but energy-replete conditions). Parallel changes were observed in the maximum light-saturated photosynthesis rate and the light-limited initial slope of the photosynthesis-light curve during iron starvation and recovery. The lowest photosynthesis rates were observed in iron-starved cells and the highest values in iron-replete cells. The light saturation parameter, I_k, was not affected by iron starvation, nor was the chl-to-C ratio markedly reduced. These observations were consistent with iron starvation having a similar or greater effect on photochemical charge separation in PSII than on downstream electron transfer steps. Declines of the ratio of variable to maximum fluorescence in iron-starved cells were consistent with PSII being a primary target of iron starvation. The functional cross-section of PSII was affected only marginally (<20%) by iron starvation, with the largest values observed in iron-starved cells. The rate constant for electron transfer calculated from fast repetition rate fluorescence was found to covary with the light-saturated photosynthesis rate; it was lowest in the most severely starved cells.     

Low potential manganese ions as efficient electron donors in native anoxygenic bacteria

Systematic control over molecular driving forces is essential for understanding the natural electron transfer processes as well as for improving the efficiency of the artificial mimics of energy converting enzymes. Oxygen producing photosynthesis uniquely employs manganese ions as rapid electron donors. Introducing this attribute to anoxygenic photosynthesis may identify evolutionary intermediates and provide insights to the energetics of biological water oxidation. This work presents effective environmental methods that substantially and simultaneously tune the redox potentials of manganese ions and the cofactors of a photosynthetic enzyme from native anoxygenic bacteria without the necessity of genetic modification or synthesis. A spontaneous coordination with bis-tris propane lowered the redox potential of the manganese (II) to manganese (III) transition to an unusually low value (~400?mV) at pH?9.4 and allowed its binding to the bacterial reaction center. Binding to a novel buried binding site elevated the redox potential of the primary electron donor, a dimer of bacteriochlorophylls, by up to 92?mV also at pH?9.4 and facilitated the electron transfer that is able to compete with the wasteful charge recombination. These events impaired the function of the natural electron donor and made BTP-coordinated manganese a viable model for an evolutionary alternative.     

Application of electrochemical kinetics to photosynthesis and oxidative phosphorylation: The redox element hypothesis and the principle of parametric energy coupling

It is suggested that the transfer of electrons within the biological electron transfer chain is subject to the laws of electrochemical kinetics, when membrane-bound electron carriers are involved. Consequently, small tightly bound molecular complexes of two or more electron transfer proteins of different redox potential within an energy transducing membrane, which accept electrons from a donor at one membrane surface and donate it to an acceptor at the other, may be regarded as real and functioning molecular redox elements, which convert the free energy of electrons into electrochemical energy. Especially, the transfer of an electron from excited chlorophyll to an electron acceptor can be looked upon as an electrochemical oxidation of excited chlorophyll at such a complex. In this reaction the electron acceptor complex behaves like a polarized electrode, in which the electrochemical potential gradient is provided by a gradient of redox potential of its constituents.Calculations and qualitative considerations show that this concept leads to a consistent understanding of both primary and secondary reactions in photosynthesis (electron capture, delayed light emission, ion transfer, energy conversion) and can also be applied to oxidative phosphorylation. Within the proposed concept, ion transfer and the development of ion gradients have to be considered as results of electrochemical activity—not as intermediates for energy conversion. For energetic reasons, a non steady state, periodic energy coupling mechanism is postulated which functions by periodic changes of the capacity of the (electrochemically) charged energy transducing membrane, during which capacitive surplus energy is released as chemical energy. Energy transducing membranes may thus be considered as electrochemical parametric energy transformers. This concept explains active periodic conformation changes and mechanochemical processes of energy transducing membranes as energetically essential events, which trigger energy conversion according to the principle of variable parameter energy transformers.The electrochemical approach presented here has been suggested and is supported by the observation, that with respect to electron capture and conversion of excitation energy into electrochemical energy, the behaviour of excited chlorophyll at suitable solid state (semiconductor) electrodes is very similar to that of chlorophyll in photosynthetic reaction centers.     

On the energetics of the photosyntheses in green sulfur bacteria

The quantum efficiency of photosynthesis by the green sulfur bacterium, Chlorobium thiosulfatophilum, has been determined in systems in which thiosulfate, tetrathionate, and molecular hydrogen served as electron donors. It was found that about 10 ± 1 quanta are used for the assimilation of 1 molecule of CO₂, and that the quantum number is independent of the nature of the electron donor. These results are considered as support for the view that also in the bacterial photosyntheses the primary photochemical reaction consists in the photolysis of H₂O, and that the chemical energy released during the oxidation of the electron donor is not utilized for CO₂ assimilation. Hence the photosynthetic processes of the green sulfur bacteria are thermodynamically less efficient than is green plant photosynthesis.     

On the role of plant mitochondrial metabolism and its impact on photosynthesis in both optimal and sub-optimal growth conditions

Given that the pathways of photosynthesis and respiration catalyze partially opposing processes, it follows that their relative activities must be carefully regulated within plant cells. Recent evidence has shown that the components of the mitochondrial electron transport chain are essential for the proper maintenance of intracellular redox gradients, to allow considerable rates of photorespiration and in turn efficient photosynthesis. Thus considerable advances have been made in understanding the interaction between respiration and photosynthesis during the last decades and the potential mechanisms linking mitochondrial function and photosynthetic efficiency will be reviewed. Despite the fact that manipulation of various steps of mitochondrial metabolism has been demonstrated to alter photosynthesis under optimal growth conditions, it is likely that these changes will, by and large, not be maintained under sub-optimal situations. Therefore producing plants to meet this aim remains a critical challenge. It is clear, however, that although there have been a range of studies analysing changes in respiratory and photosynthetic rates in response to light, temperature and CO₂, our knowledge of the environmental impact on these processes and its linkage still remains fragmented. We will also discuss the metabolic changes associated to plant respiration and photosynthesis as important components of the survival strategy as they considerably extend the period that a plant can withstand to a stress situation.     

Factors influencing carbon fixation and water use by mediterranean sclerophyll shrubs during summer drought

Summary Mediterranean sclerophyll shrubs respond to seasonal drought by adjusting the amount of leaf area exposed and by reducing gas exchange via stomatal closure mechanisms. The degree to which each of these modifications can influence plant carbon and water balances under typical mediterranean-type climate conditions is examined. Leaf area changes are assessed in the context of a canopy structure and light microclimate model. Shifts in physiological response are examined with a mechanistically-based model of C₃ leaf gas exchange that simulates progressive reduction of maximum photosynthesis and transpiration rates and increasingly strong midday stomatal closure over the course of drought. The results demonstrate that midday stomatal closure may effectively contribute to drought avoidance, increase water use efficiency, and strongly alter physiological efficiency in the conversion of intercepted light energy to photoproducts. Physiological adjustments lead to larger reductions in water use than occur when comparing leaf area index 3.5 to 1.5, extremes found for natural stands of sclerophyll shrubs in the California chaparral. Reductions in leaf area have the strongest effect on resource capture and use during non-water-stressed periods and the least effect under extreme drought conditions, while shifts in physiological response lead to large savings of water and efficient water use under extreme stress. An important model parameter termed GFAC (proportionality factor expressing the relation of conductance [g] to net photosynthesis rate) is utilized, which changes in response to the integrated water stress experimence of shrubs and alters the degree to which stomata may open for a given rate of carbon fixation. We attempt to interpret this parameter in terms of physiological mechanisms known to modify control of leaf gas exchange during drought. The analysis helps visualize means by which canopy gas exchange behavior may be coupled to physiological changes occurring in the root environment during soil drying.     

不同CO2浓度下大豆叶片的光合生理生态特性

CO₂是光合作用的原料和底物,影响着光合作用的进程和光合产物的数量.利用Li-6400-40B同时测量大豆叶片在不同CO₂浓度(300、400、500和600 μmol·mol^-1)下的光合电子传递速率和光合作用对光的响应曲线,并用构建的光合作用对光响应机理模型拟合这些光响应曲线,获得大豆叶片一系列的光合参数、生理生态参数和捕光色素分子的物理参数.结果表明: 电子利用效率、最大电子传递速率和最大净光合速率随CO₂浓度的升高而增加;光补偿点和暗呼吸速率随CO₂浓度的升高而下降;光能利用效率和内禀(瞬时)水分利用效率随CO₂浓度的升高而增加,不同CO₂浓度下的最大光能利用效率和最大内禀(瞬时)水分利用效率之间存在显著差异,但不同CO₂浓度下的最大羧化效率的差异不显著.CO₂浓度的大小对光合作用中原初光反应存在一定程度的影响,即高CO₂浓度有利于减小捕光色素分子处于最低激发态的最小平均寿命,以提高光能传递的速度及增加大豆光合电子流的利用效率.     

Chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence transient during freezing and recovery in winter wheat

Chlorophyll (Chl) a fluorescence measurements as evaluators of plant freezing tolerance are frequently insufficiently sensitive to detect the early metabolic changes that are initiated following exposure to freezing temperatures. Using cold-acclimated winter wheat, I analysed the polyphasic transience (from 50 μs to 1 s) of Chl a fluorescence. This enabled detailed studies of the progressive energy flows and efficiencies within the photosystem 2 (PS2) complex that ensue following initial exposure to freezing temperatures right through to the plant recovery stage. The initial consequences of mild frosts that may cause primary damage involve a disturbance to the energy transfer subsequent to Q_A (the primary quinone electron acceptor of PS2). Lower freezing temperatures, on the other hand, may deter energy flow between the PS2 reaction centre (RC), Chl, and Q_A. All primary damage could only be repaired partially. Further freezing-triggered dysfunction of the electron transfer between the PS2 RCs and Q_A was connected with secondary damage that could lead to PS2 deactivation. Both primary and secondary freezing damages were reflected in decreased PI_ABS, the Performance Index based on equal absorption that characterizes all energy bifurcations in PS2. PI_ABS also differentiated cultivars with contrasting freezing-tolerance either subsequent to the onset of freezing or during the recovery stage. In contrast, the potential quantum yield of PS2 (F_v/F_m), which characterizes efficiency of energy trapping in the PS2 RCs, was only different in cultivars with contrasting freezing-tolerance during the recovery stage.     

Fundamental limits on wavelength, efficiency and yield of the charge separation triad

In an attempt to optimize a high yield, high efficiency artificial photosynthetic protein we have discovered unique energy and spatial architecture limits which apply to all light-activated photosynthetic systems. We have generated an analytical solution for the time behavior of the core three cofactor charge separation element in photosynthesis, the photosynthetic cofactor triad, and explored the functional consequences of its makeup including its architecture, the reduction potentials of its components, and the absorption energy of the light absorbing primary-donor cofactor. Our primary findings are two: First, that a high efficiency, high yield triad will have an absorption frequency more than twice the reorganization energy of the first electron transfer, and second, that the relative distance of the acceptor and the donor from the primary-donor plays an important role in determining the yields, with the highest efficiency, highest yield architecture having the light absorbing cofactor closest to the acceptor. Surprisingly, despite the increased complexity found in natural solar energy conversion proteins, we find that the construction of this central triad in natural systems matches these predictions. Our analysis thus not only suggests explanations for some aspects of the makeup of natural photosynthetic systems, it also provides specific design criteria necessary to create high efficiency, high yield artificial protein-based triads.     

不同CO2浓度下大豆叶片的光合生理生态特性

CO₂是光合作用的原料和底物,影响着光合作用的进程和光合产物的数量.利用Li-6400-40B同时测量大豆叶片在不同CO₂浓度(300、400、500和600 μmol·mol^-1)下的光合电子传递速率和光合作用对光的响应曲线,并用构建的光合作用对光响应机理模型拟合这些光响应曲线,获得大豆叶片一系列的光合参数、生理生态参数和捕光色素分子的物理参数.结果表明: 电子利用效率、最大电子传递速率和最大净光合速率随CO₂浓度的升高而增加;光补偿点和暗呼吸速率随CO₂浓度的升高而下降;光能利用效率和内禀(瞬时)水分利用效率随CO₂浓度的升高而增加,不同CO₂浓度下的最大光能利用效率和最大内禀(瞬时)水分利用效率之间存在显著差异,但不同CO₂浓度下的最大羧化效率的差异不显著.CO₂浓度的大小对光合作用中原初光反应存在一定程度的影响,即高CO₂浓度有利于减小捕光色素分子处于最低激发态的最小平均寿命,以提高光能传递的速度及增加大豆光合电子流的利用效率.