植物光合机构的状态转换

植物光合机构的状态转换是一种通过光系统Ⅱ的捕光天线色素蛋白复合体（LHCⅡ）的可逆磷酸化调节激发能在两个光系统间的分配来适应环境中光质等短期变化的机制．一般植物光合机构的LHCⅡ磷酸化主要受电子递体质醌和细胞色素b6f复合体氧化还原状态的调节,从而影响其在两种光系统间的移动。植物光合机构的状态转换也可以通过两种光系统相互接近导致激发能满溢来平衡两个光系统的激发能分配。外界离子浓度骤变可以引起盐藻LHCⅡ磷酸化,其调节过程与电子递体的氧化还原状态无关。绿藻的状态转换可以调节细胞内的ATP供求关系。     

植物光合机构对光环境的适应机制:状态转换

在自然条件下,植物接受的照光量经常变化,而植物在进化过程中已形成了相应的适应机制,用以维持光环境变化过程中2个光反应之间光能转换的能量平衡.植物的调控系统不但能通过调控叶片和叶绿体的运动以及光合色素的积累调节光的吸收,还可以通过光系统的状态转换灵活地调节捕光色素蛋白复合体吸收的能量分配.特别是在低光强下,植物通过可对电子传递链的氧化还原状态做出响应的激酶和磷酸酶调控光系统Ⅱ捕光色素蛋白复合体(LHCⅡ)的可逆磷酸化,从而调节激发能在PSⅠ与PSⅡ之间的分配.植物的状态转换机制是植物适应光质等光环境变化的重要机制.本文综述了植物状态转换机制的研究进展,阐述了LHCⅡ的磷酸化及其在PSⅠ与PSⅡ两个光系统间的移动及其状态转换在植物适应光环境变化中的生理意义,并展望了今后的主要研究方向.     

植物光合系统对高温胁迫的响应机制

温度变化是影响植物生长和发育的一个非常重要的因素,而光合作用是植物对温度变化最为敏感的生理过程.高温胁迫给植物光合器官造成了严重的危害,但在高温胁迫下,植物并不是消极被动的,并且能够在生理生化及分子水平上发生各种变化来渡过逆境.本文结合当今国内外研究进展,从光合系统热量耗散与光合修复的相关因素,如类囊体膜上相关蛋白,热激蛋白,水杨酸,抗过氧化物酶及抗坏血酸等几个方面展开分析,阐述了植物光合系统对高温胁迫的防御机制,并对今后的研究方向进行了探讨和展望.     

青藏高原高山嵩草光合功能对模拟夜间低温的响应

昼夜温差大是青藏高原的典型气候特征,夜间低温作为植物生长季内非常频繁的非生物胁迫因子,对典型高山植物日间光合生理功能的影响如何,尚缺乏研究。该研究以采自青海大学-清华大学三江源高寒草地生态系统野外观测站的高山嵩草(Kobresiapygmaea)为材料,应用叶绿素荧光图像分析手段,研究了模拟夜间低温对叶片光系统Ⅱ(PSⅡ)非光化学猝灭中光诱导和非光诱导的量子产量,及慢弛豫相和快弛豫相组分的影响。结果表明:0℃夜间低温对日间PSⅡ相对电子传递速率、PSⅡ反应中心开放比率(qL)和PSⅡ非光化学猝灭系数(qNP)的快速光响应曲线影响较小;400和1 500μmol·m–2·s–1稳态作用光强下的比较证实,夜间低温并没有影响到光合机构活性及非光化学能量耗散过程。PSⅡ反应中心激发能分配的量子通量分析表明, PSⅡ实际光化学量子效率、PSⅡ非光化学猝灭中非调节性和调节性能量耗散量子产量的相对比率在第3天高光强下,对照组和夜间低温组分别为:36:19:45和38:19:43;较低光强下为66:22:12和66:23:11。非光化学猝灭(NPQ)中快弛豫相(NPQf)为主要组分,而慢弛豫相(NPQ...     

民勤沙生植物园4种云杉属植物光化学特性的趋同适应

采用叶绿素荧光图像分析手段,结合叶绿素含量和主枝生长量测定,研究了沙地云杉、青海云杉、蓝云杉、白扦PSⅡ光化学效率和非光化学能量耗散的光响应特性及对稳态光强的适应性。结果表明:在相同生境和管理条件下,15a苗龄的4种云杉属植物生长势态良好,均能适应民勤荒漠气候环境;蓝云杉针叶的叶绿素含量较高,而青海云杉的叶绿素a、b比值(Chl a/b)较低;4种植物PSⅡ光化学效率的光响应曲线相似,但蓝云杉PSⅡ非光化学猝灭系数(NPQ)的光响应明显有别于其余3种;150μmol m~(-2)s~(-1)低光强下4种植物间NPQ的差异与PSⅡ最大光化学量子效率(Fv/Fm)一致,是内禀光合特性的反映;1500μmol m~(-2)s~(-1)高光强下的NPQ和PSⅡ最大效率(Fv'/Fm')在4云杉属植物间没有差异,呈现光合生理的趋同适应。综合比较分析可知,蓝云杉和白扦在低光强具有略低的PSⅡ非光化学猝灭能力,在高光强具有相对高的PSⅡ运行效率(Fq'/Fm'),光驯化适应能力较大;沙地云杉和青海云杉具有几乎一致的PSⅡ光化学和非光化学猝灭特性,其耐荫性和喜光性相近;4种云杉属植物光合机构对干旱荒漠生境的驯化适应具有趋同性,可作为我国北方防护林建设和城市绿化的重要树种。     

脱水和复水过程中金发藓(Polytrichum commune)与湿地匐灯藓(Plagiomnium acutum)叶绿素荧光特性变化的比较研究

以来自不同水分生境的金发藓(Polytrichum commune)和湿地匐灯藓(Plagiomnium acutum)为材料,利用叶绿素荧光成像技术比较了脱水和复水过程中两种藓类的荧光光响应曲线、光系统Ⅱ光能转化效率(ratio ofchlorophyll variation fluorescence,Fv/Fm)、光系统Ⅱ光量子产量(quantum yielding of PSⅡ,Y(Ⅱ))、光化学猝灭(photochemical quenching,qP)和非光化学猝灭(none-photochemical quenching,NPQ)的变化.结果显示,在脱水过程中,金发藓的抑制光强可维持在800μmol/(m2.s)以上,而湿地匐灯藓可低至400μmol/(m2.s)左右;金发藓ETR(electron transportation rate)值始终可维持在20附近,而湿地匐灯藓可降至0;两种藓类的Fv/Fm、Y(Ⅱ)、qP均下降,但金发藓较湿地匐灯藓高;NPQ先升后降,金发藓的峰值早于湿地匐灯藓,而幅度低于湿地匐灯藓.在复水过程中,两种藓类抑制光强和ETR均迅速恢复后略有下降,金发藓的恢复较湿地匐灯藓慢但波动小;两种藓类Fv/Fm和Y(Ⅱ)均能恢复到正常水平,金发藓均高于湿地匐灯藓;两种藓类qP略有上升,NPQ则略有下降.说明藓类植物对脱水伤害的耐受能力主要体现在复水的修复能力上,而脱水持续和程度会对不同生境的藓类产生不同的胁迫效应.从光保护能力的角度来看,生活于易产生水分亏缺条件下的金发藓比生活在水分充沛条件下的湿地匐灯藓具有更强的脱水耐受能力.     

小麦叶片的状态转换涉及PSⅡ向PSⅠ激发能满溢的变化

叶片照远红光后,其叶绿素荧先参数Fm／Fo和两个光系统低温荧光产量比值F685／F735升高,照红先后,其Fm／Fo和F685／F735降低;在照远红光或红先过程中,与F685／F735的变化相比,Fm／Fo的变化幅度在较短的时间内达到最大;NaF预处理的叶片经远红光照射时,其Fm／Fo和F685／F735不增加;DCMU预处理的叶片经红光照射时,其Fm／Fo和F685／F735降低的幅度比对照小。这些结果表明,小麦叶片状态转换过程中两个先系统间能量分配的变化至少部分地与激发能满溢变化有关。这种满溢的变化与捕光色素蛋白复合体LHCⅡ的磷酸化相联,并且,与光吸收截面变化相比,满溢的变化是对两个光系统不平衡光吸收的较快响应。     

植物光合速率对光强从饱和到有限转变响应方式的物种依赖性

在考察过的57种植物中,有32种的光合速率对光强转换的响应曲线为V型,25种的响应曲线为L型。这种响应类型对物种的依赖性即物种差异,与植物科属分类无关,而与碳同化途径的不同有关,可能还与物种起源时的光环境有关。虽然在饱和光下2类植物体内电子传递速率与羧化速率的比值差异不大,但是在有限光下响应曲线为V型的植物该比值远高于响应曲线为L型的植物。这个差别可以部分地用前者具有较大的捕光天线并且在饱和光下部分天线从反应中心复合体可逆脱离来解释。     

光强转换对不同生长环境下桑树叶片光化学效率的影响

以桑树品种‘蒙古桑’为试验材料,利用叶绿素荧光技术研究了光强转换对生长在不同光强下的桑树叶片实际光化学效率(ΦPSⅡ)、电子传递速率(ETR)和非光化学淬灭(NPQ)的影响,分析了非光化学淬灭(NPQ)3个组分的变化.结果表明:当光强从黑暗或弱光转换到自然光条件下,自然光桑树叶片的光量子转化效率高于弱光叶片,ΦPSⅡ、ETR诱导平衡较快,NPQ诱导呈先升后降趋势.自然光叶片在强光下状态转换淬灭组分(qT)占NPQ的18%,而弱光叶片qT仅占NPQ的7%.与弱光桑树叶片相比,自然光桑树叶片可以通过较高的光量子转化效率和较强的调节激发能在PSⅠ和PSⅡ之间的分配能力来适应光强的变化.     

超氧阴离子诱导的叶绿素荧光猝灭

分别通过黄嘌呤(X)与黄嘌呤氧化酶(XO)反应和甲基紫金(MV)的作用,观察了O·-2诱导莴苣叶绿体的叶绿素荧光猝灭过程.结果表明,O-·2的产生明显使光化学猝灭(qP)和非光化学猝灭(qN)增加.叶绿体内SOD被DDC抑制后,X+XO诱导的叶绿素荧光猝灭过程中,qP下降,qN上升;MV诱导的叶绿素荧光猝灭过程中,qP上升幅度不大,qN增加不明显.当碳代谢被碘乙酰胺(JAA)抑制后, qP下降,qN上升.解偶联剂NH4Cl增加质子跨类囊体膜的通透性,导致qP增加和qN降低,加入MV后qP和qN增加不明显.分析认为,-·2的产生和及时被清除对保持光合电子传递和增加跨膜ΔpH有很重要的作用,有利于叶绿体吸收的光能得到转化和耗散,在一定程度上减轻过量光能引起的光抑制损伤.     

Regulation of the photosynthetic apparatus under fluctuating growth light

Safe and efficient conversion of solar energy to metabolic energy by plants is based on tightly inter-regulated transfer of excitation energy, electrons and protons in the photosynthetic machinery according to the availability of light energy, as well as the needs and restrictions of metabolism itself. Plants have mechanisms to enhance the capture of energy when light is limited for growth and development. Also, when energy is in excess, the photosynthetic machinery slows down the electron transfer reactions in order to prevent the production of reactive oxygen species and the consequent damage of the photosynthetic machinery. In this opinion paper, we present a partially hypothetical scheme describing how the photosynthetic machinery controls the flow of energy and electrons in order to enable the maintenance of photosynthetic activity in nature under continual fluctuations in white light intensity. We discuss the roles of light-harvesting II protein phosphorylation, thermal dissipation of excess energy and the control of electron transfer by cytochrome b₆f, and the role of dynamically regulated turnover of photosystem II in the maintenance of the photosynthetic machinery. We present a new hypothesis suggesting that most of the regulation in the thylakoid membrane occurs in order to prevent oxidative damage of photosystem I.     

Adjustment of photosynthetic activity to drought and fluctuating light in wheat

Drought is a major cause of losses in crop yield. Under field conditions, plants exposed to drought are usually also experiencing rapid changes in light intensity. Accordingly, plants need to acclimate to both, drought and light stress. Two crucial mechanisms in plant acclimation to changes in light conditions comprise thylakoid protein phosphorylation and dissipation of light energy as heat by non-photochemical quenching (NPQ). Here, we analyzed the acclimation efficacy of two different wheat varieties, by applying fluctuating light for analysis of plants, which had been subjected to a slowly developing drought stress as it usually occurs in the field. This novel approach allowed us to distinguish four drought phases, which are critical for grain yield, and to discover acclimatory responses which are independent of photodamage. In short-term, under fluctuating light, the slowdown of NPQ relaxation adjusts the photosynthetic activity to the reduced metabolic capacity. In long-term, the photosynthetic machinery acquires a drought-specific configuration by changing the PSII-LHCII phosphorylation pattern together with protein stoichiometry. Therefore, the fine-tuning of NPQ relaxation and PSII-LHCII phosphorylation pattern represent promising traits for future crop breeding strategies.     

Protein tyrosine phosphorylation in the transition to light state 2 of chloroplast thylakoids

Redox dependent protein phosphorylation in chloroplast thylakoids regulates distribution of excitation energy between the two photosystems of photosynthesis, PS I and PS II. Several thylakoid phosphoproteins are known to be phosphorylated on N-terminal threonine residues exposed to the chloroplast stroma. Phosphorylation of light harvesting complex II (LHC II) on Thr-6 is thought to account for redistribution of light energy from PS II to PS I during the transition to light state 2. Here, we present evidence that a protein tyrosine kinase activity is required for the transition to light state 2. With an immunological approach using antibodies directed specifically towards either phospho-tyrosine or phospho-threonine, we observed that LHC II became phosphorylated on both tyrosine and threonine residues. The specific protein tyrosine kinase inhibitor genistein, at concentrations causing no direct effect on threonine kinase activity, was found to prevent tyrosine phosphorylation of LHC II, the transition to light state 2, and associated threonine phosphorylation of LHC II. Possible reasons for an involvement of tyrosine phosphorylation in light state transitions are proposed and discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

光照或黑暗条件下野火病菌侵染对烟草光合机构的影响

烟草野火病菌(Pst)是一种兼性营养型的细菌致病菌,它可以引起烟草发生褐色病斑,名为野火病.近年来Pst受到很多关注,然而大多数对Pst的研究主要集中在寄主和非寄主植物对Pst侵染的防御机制和产自于野火病菌的野火毒素上,Pst侵染对烟草叶片光合性能的影响及其机理尚未见报道.研究Pst侵染后对光系统Ⅱ(PSⅡ)的影响不仅可以帮助阐明烟草-Pst 相互作用的机制,还可以从生理角度加深对细菌致病菌病害的了解.本研究采用叶绿素荧光快速诱导动力学曲线分析、类囊体膜蛋白Western分析、活性氧(ROS)和叶绿素含量测定等方法,探讨光照(200 μmol·m^-2·s^-1)或黑暗条件下Pst侵染对烟草光系统Ⅱ的影响.结果表明: 与未处理相比,Pst侵染3 d后在光照和黑暗条件下叶片侵染区域叶绿素含量均显著下降,出现萎黄病变,注射区域呈现出明显的野火病特征.光照和黑暗条件下,侵染3 d后烟草叶片过氧化氢含量明显升高,光照条件下要比黑暗条件下升高比例更大.Pst侵染3 d后,光照和黑暗条件下烟草叶片注射区域叶绿素荧光动力学曲线中K点和J点的相对可变荧光W_K和V_J逐渐增大,叶片最大光化学效率(F_v/F_m)和单位面积有活性反应中心的数目(RC/CSm)均显著下降.此外,相对于光照条件,Pst侵染后在黑暗条件下W_K和V_J的升高程度更大,说明对K点和J点的抑制程度更严重.Pst侵染3 d后,在光照和黑暗条件下放氧复合体(OEC)的核心组分PsaO、光系统Ⅱ反应中心核心蛋白D1蛋白均发生明显的降解,且在黑暗条件下降解更为严重.表明Pst侵染后,在光照和黑暗条件下均会使光合电子传递链Q_A到Q_B的电子传递受到限制,放氧复合体受到伤害,烟草叶片光系统Ⅱ供体侧、受体侧、反应中心的数目和活性均受到伤害,光系统Ⅱ发生光抑制或类似光抑制的伤害,且在黑暗条件下对光系统Ⅱ的伤害程度比光照条件下更为严重.     

Modifications to the photosynthetic apparatus of higher plants in response to changes in the light environment

A brief review of the photosynthetic apparatus of higher plants is given, followed by a consideration of the modifications induced in this apparatus by changes in light intensity and light quality. Possible strategies by which plants may optimize photosynthetic activity by both long- and short-term modifications of their photosynthetic apparatus in response to changing light regimes are discussed.     

Mechanism of the light state transition in photosynthesis. I. Analysis of the kinetics of cytochrome f oxidation in State 1 and State 2 in the red alga, Porphyridium cruentum

The kinetics of photooxidation and reduction of cytochrome f were examined spectrophotometrically in the red alga Porphyridium cruentum in light State 1 and light State 2. Experiments were performed on intact cells that had been chemically fixed and stabilized in the light states. The cytochrome f turnover was measured during conditions of linear electron transport driven by both photosystems and during several cyclic reactions mediated by the long-wavelength Photosystem (PS) I. The data show that the rate of photooxidation of cytochrome f increased in State 2 when the cells were activated by subsaturating intensities of green light absorbed primarily by the phycobilisome. No differences in kinetics were found between algae in State 1 or State 2 when they were activated by light absorbed primarily by the chlorophyll of PS I. The results confirm that changes in energy distribution between the two photosystems occur as a result of the light state transition and verify that the redistribution of excitation results in the predicted changes in electron transport.     

Isolation of state transition mutants of Chlamydomonas reinhardtii by fluorescence video imaging

Oxygenic photosynthetic organisms adapt to varying light conditions by changing the distribution of light energy between Photosystem II (PS II) and photosystem I (PS I) during so-called state transitions. To identify the genes involved in this process, we have exploited a simple chlorophyll fluorescence video-imaging technique to screen a library of nuclear mutants of Chlamydomonas reinhardtii for colonies grown on agar plates that are disturbed in their ability to regulate light energy distribution between PS I and PS II. Subsequent modulated fluorescence measurements at room temperature and 77 K fluorescence emission spectra confirmed that 5 mutants (0.025% of total number screened) were defective in state transitions. [³²P]orthophosphate phosphorylation experiments in vivo revealed that in one of these mutants, designated stm1, the level of LHC II polypeptide phosphorylation was drastically reduced compared with wild type. Despite WT levels of PS I and PS II, stm1 grew photoautotrophically at reduced rates, compared with WT especially under low light conditions, which is consistent with an important physiological role for state transitions. Our results highlight the feasibility of video imaging in tandem with mutagenesis as a means of identifying the genes involved in controlling state transitions in eukaryotic photosynthetic organisms.     

Protein kinases and phosphatases involved in the acclimation of the photosynthetic apparatus to a changing light environment

Photosynthetic organisms are subjected to frequent changes in light quality and quantity and need to respond accordingly. These acclimatory processes are mediated to a large extent through thylakoid protein phosphorylation. Recently, two major thylakoid protein kinases have been identified and characterized. The Stt7/STN7 kinase is mainly involved in the phosphorylation of the LHCII antenna proteins and is required for state transitions. It is firmly associated with the cytochrome b₆f complex, and its activity is regulated by the redox state of the plastoquinone pool. The other kinase, Stl1/STN8, is responsible for the phosphorylation of the PSII core proteins. Using a reverse genetics approach, we have recently identified the chloroplast PPH1/TAP38 and PBPC protein phosphatases, which counteract the activity of STN7 and STN8 kinases, respectively. They belong to the PP2C-type phosphatase family and are conserved in land plants and algae. The picture that emerges from these studies is that of a complex regulatory network of chloroplast protein kinases and phosphatases that is involved in light acclimation, in maintenance of the plastoquinone redox poise under fluctuating light and in the adjustment to metabolic needs.     

High light intensity protects photosynthetic apparatus of pea plants against exposure to lead.

The electron transport rates and coupling factor activity in the chloroplasts; adenylate contents, rates of photosynthesis and respiration in the leaves as well as activity of isolated mitochondria were investigated in Pisum sativum L. leaves of plants grown under low or high light intensity and exposed after detachment to 5 mM Pb(NO(3))(2). The presence of Pb(2+) reduced rate of photosynthesis in the leaves from plants grown under the high light (HL) and low light (LL) conditions, whereas the respiration was enhanced in the leaves from HL plants. Mitochondria from Pb(2+) treated HL-leaves oxidized glycine at a higher rate than those isolated from LL leaves. ATP content in the Pb-treated leaves increased to a greater extend in the HL than LL grown plants. Similarly ATP synthase activity increased markedly when chloroplasts isolated from control and Pb-treated leaves of HL and LL grown plants were subjected to high intensity light. The presence of Pb ions was found inhibit ATP synthase activity only in chloroplasts from LL grown plants or those illuminated with low intensity light. Low light intensity during growth also lowered PSI electron transport rates and the Pb(2+) induced changes in photochemical activity of this photosystem were visible only in the chloroplasts isolated from LL grown plants. The activity of PSII was influenced by Pb ions on similar manner in both light conditions. This study demonstrates that leaves from plants grown under HL conditions were more resistant to lead toxicity than those obtained from the LL grown plants. The data indicate that light conditions during growth might play a role in regulation of photosynthetic and respiratory energy conservation in heavy metal stressed plants by increasing the flexibility of the stoichiometry of ATP to ADP production.