植物光破坏防御机制的研究进展

植物在长期进化过程中,形成了一系列保护光合机构免受强光破坏的光破坏防御机制,本文仅就近年来光破坏防御机制中3Chl途径、能量耗散和叶绿体呼吸作了简要综述。     

植物光破坏防御机制研究进展

光破坏防御机制是植物为应对复杂多变的自然环境而产生的保护措施,这些措施从形态、生理和生化等方面反映了植物对环境的适应能力。本文根据光抑制的机理,对近年来植物的光破坏防御机制以及高等植物叶黄素循环机制的研究现状进行综述,认为叶黄素循环防御机制是植物光保护作用的重要措施之一。     

突变体在植物光破坏防御研究中的应用

用突变体研究植物的光破坏防御机制是一种有效的方法,与传统方法相比,具有专一性强、副效应少、直接在基因水平上起作用、可信度高等优点。本文简要介绍了突变体的获得、筛选、鉴定,并总结了几类与光破坏防御有关的突变体。     

植物叶片花青素的光破坏防御机制研究进展

植物花青素广泛分布在植物的根、茎、叶、花和果实等器官中,是植物形态建成过程中或响应逆境而产生的一种次生代谢物质.植物叶片中的花青素具有特殊的化学结构和光谱特性,在光破坏防御机制方面发挥了重要的作用,已经成为植物光合生理生态的研究热点.本文综述了近年来植物叶片花青素与光合作用的研究进展,从叶片花青素的分布、光谱特性及其与光合色素的关系等方面说明花青素对植物光合作用的影响,重点介绍了叶片花青素通过光吸收、抗氧化剂和渗透调节等在植物光破坏防御机制方面的作用,展望了今后的主要研究方向     

突变体在植物光破坏防御研究中的应用

用突变体研究植物的光破坏防御机制是一种有效的方法，与传统方法相比，具有专一性强、副效应少、直接在基因水平上起作用、可信度高等优点。本文简要介绍了突变体的获得、筛选、鉴定，并总结了几类与光破坏防御有关的突变体。     

植物对增强UV-B辐射的防御机制研究进展

由于大气同温层的臭氧层逐渐被破坏变薄,增加了太阳UV-B辐射抵达地球表面的强度,对植物产生不同程度的影响.本文综述了近年来有关植物对增强UV-B辐射的防御机制,包括植物在生长、繁育与次生代谢过程中存在的防御机制、植物体内生物大分子对于UV-B辐射增强的防御和不同植物对于UV-B辐射增强的防御能力的差异,以及如何有效地利用该机制;同时提出了今后研究的方向和重点.     

AOX途径在苹果离体叶片失水过程中的光破坏防御作用

为探讨线粒体交替氧化酶呼吸途径（AOX途径）对水分胁迫下苹果叶片光破坏的防御作用,以苹果砧木平邑甜茶离体叶片为试材,通过AOX抑制剂水杨基羟肟酸(SHAM)处理,同时测定苹果叶片叶绿素荧光诱导动力学曲线和820 nm光的吸收曲线,结合JIP test分析,探讨了失水过程中AOX途径的光保护作用。结果表明：水分胁迫条件下,平邑甜茶叶片的AOX活性显著增加, SHAM抑制AOX途径后,叶片发生更严重的光抑制;在失水胁迫条件下,平邑甜茶叶片PSⅡ原初光化学反应的量子产额（TRo/ABS）、PSⅡ捕获的电子从Q_A传递到Q_B的概率（ETo/TRo）下降,PSⅡ单位反应中心吸收的光能（ABS/RC）上升,而PSⅠ的最大氧化还原活性（ΔI/I_o）未受影响;SHAM抑制AOX途径后,TRo/ABS和ETo/TRo进一步下降,ABS/RC进一步上升,同时引起了ΔI/I_o的下降。研究认为,水分胁迫条件下,平邑甜茶叶片PSⅡ发生了光抑制,而SHAM处理在加重PSⅡ光抑制的同时,引起了PSⅠ的光抑制;叶片失水过程中,AOX呼吸上调是平邑甜茶叶片的重要光破坏防御机制,特别是对PSⅠ具有重要的保护作用。     

植物光胁迫研究中的几个问题

结合作者的研究工作简要讨论了植物光胁迫研究中关于光合作用的光抑制、光合机构的光破坏和光破坏防御等几个问题.     

高等植物光系统Ⅱ对高温的响应

光系统Ⅱ(PSⅡ)是位于类囊体膜上的高度组织的色素蛋白复合体,主要由反应中心复合体、捕光天线复合体等亚单位组成。主要介绍PSⅡ的结构和功能、高温对PSⅡ的影响以及PSⅡ对高温胁迫的防御机制。     

叶黄素循环和D1蛋白周转在毛竹叶片光保护中的作用

利用叶绿素荧光技术,对强光胁迫下以及叶黄素循环抑制剂-二硫苏糖醇(DTT)和D1蛋白合成抑制剂-硫酸链霉素(SM)处理后毛竹(Phyllostachys edulis (Carr.) Lehaie)的光抑制特征进行研究。结果显示:在夏季中午强光或人为强光胁迫下,毛竹叶片最大光化学效率Fv/Fm均显著降低;在下午光强减弱或黑暗、弱光条件下,Fv/Fm可有效恢复。DTT和SM均可抑制毛竹叶片非光化学淬灭(NPQ),且DTT效果明显优于SM。另外,在强光下,DTT和SM处理均能使毛竹叶片Fv/Fm、实际光化学效率Y(Ⅱ)和光化学淬灭qP等荧光参数下降幅度增大。研究结果表明毛竹叶片具有完善的光破坏防御机制,NPQ与叶黄素循环和D1蛋白周转紧密关联,在叶片光保护机制中具有重要作用。     

Changes in the In Vivo Absorption and Fluorescence Excitation Spectra with Growth Irradiance in Three Species of Phytoplankton

The fluorescence excitation spectrum is sometimes used as aproxy for the action spectrum of photosynthesis in phytoplankton.The main assumption behind this approximation is that the shapesof absorption and fluorescence excitation spectra are similarexcept for the absorption by photoprotective pigments, whichdo not contribute to the fluorescence spectrum. In this study,we compare the shapes of the absorption and fluorescence spectrain three species of phytoplankton grown at differentirradiances:two diatoms (Thalassiosira weissflogii and Chaetoceros sp.)and a cyanophyte (Synechococcus sp.). The contribution to absorptionby photoprotective pigments was estimated for each experiment.Results showed that the differences between the shapes of absorptionand fluorescence spectra were similar to the estimated absorptionby photoprotective pigments only in the case ofT. weissflogii.In Synechococcus sp., and to a lesser degree in Chaetocerossp., the differences between the two types of spectra were largerthan the absorption by photoprotective pigments. In the caseof Synechococcus sp., the difference between these spectra wasapparently due mainly to the extreme imbalance of chlorophylla distribution between the two photosystems. Chaetoceros sp.seemed to be an intermediate case: a small part of the chlorophylla of the cell appeared to be exclusively associated with photosystemI and therefore did not contribute to fluorescence. Fluorescenceand absorption values were normalized to their values at 545nm, and the ratio of normalized absorption to normalized fluorescencewas computed for the blue (439 nm) and red (676 nm) peaks inthe spectra. The results showed that these peak ratios can beused to distinguish between the effects of photoprotective pigmentsand the arrangement of the photosynthetic apparatus on differencesbetweenfluorescence and absorption spectra.     

Photoprotective compounds from marine organisms

The substantial loss in the stratospheric ozone layer and consequent increase in solar ultraviolet radiation on the earth’s surface have augmented the interest in searching for natural photoprotective compounds in organisms of marine as well as freshwater ecosystems. A number of photoprotective compounds such as mycosporine-like amino acids (MAAs), scytonemin, carotenoids and several other UV-absorbing substances of unknown chemical structure have been identified from different organisms. MAAs form the most common class of UV-absorbing compounds known to occur widely in various marine organisms; however, several compounds having UV-screening properties still need to be identified. The synthesis of scytonemin, a predominant UV-A-photoprotective pigment, is exclusively reported in cyanobacteria. Carotenoids are important components of the photosynthetic apparatus that serve both light-harvesting and photoprotective functions, either by direct quenching of the singlet oxygen or other toxic reactive oxygen species or by dissipating the excess energy in the photosynthetic apparatus. The production of photoprotective compounds is affected by several environmental factors such as different wavelengths of UVR, desiccation, nutrients, salt concentration, light as well as dark period, and still there is controversy about the biosynthesis of various photoprotective compounds. Recent studies have focused on marine organisms as a source of natural bioactive molecules having a photoprotective role, their biosynthesis and commercial application. However, there is a need for extensive work to explore the photoprotective role of various UV-absorbing compounds from marine habitats so that a range of biotechnological and pharmaceutical applications can be found.     

Growth form defines physiological photoprotective capacity in intertidal benthic diatoms

In intertidal marine sediments, characterized by rapidly fluctuating and often extreme light conditions, primary production is frequently dominated by diatoms. We performed a comparative analysis of photophysiological traits in 15 marine benthic diatom species belonging to the four major morphological growth forms (epipelon (EPL), motile epipsammon (EPM-M) and non-motile epipsammon (EPM-NM) and tychoplankton (TYCHO)) found in these sediments. Our analyses revealed a clear relationship between growth form and photoprotective capacity, and identified fast regulatory physiological photoprotective traits (that is, non-photochemical quenching (NPQ) and the xanthophyll cycle (XC)) as key traits defining the functional light response of these diatoms. EPM-NM and motile EPL showed the highest and lowest NPQ, respectively, with EPM-M showing intermediate values. Like EPL, TYCHO had low NPQ, irrespective of whether they were grown in benthic or planktonic conditions, reflecting an adaptation to a low light environment. Our results thus provide the first experimental evidence for the existence of a trade-off between behavioural (motility) and physiological photoprotective mechanisms (NPQ and the XC) in the four major intertidal benthic diatoms growth forms using unialgal cultures. Remarkably, although motility is restricted to the raphid pennate diatom clade, raphid pennate species, which have adopted a non-motile epipsammic or a tychoplanktonic life style, display the physiological photoprotective response typical of these growth forms. This observation underscores the importance of growth form and not phylogenetic relatedness as the prime determinant shaping the physiological photoprotective capacity of benthic diatoms.     

Structural Determinants Underlying Photoprotection in the Photoactive Orange Carotenoid Protein of Cyanobacteria

The photoprotective processes of photosynthetic organisms involve the dissipation of excess absorbed light energy as heat. Photoprotection in cyanobacteria is mechanistically distinct from that in plants; it involves the orange carotenoid protein (OCP), a water-soluble protein containing a single carotenoid. The OCP is a new member of the family of blue light-photoactive proteins; blue-green light triggers the OCP-mediated photoprotective response. Here we report structural and functional characterization of the wild type and two mutant forms of the OCP, from the model organism Synechocystis PCC6803. The structural analysis provides high resolution detail of the carotenoid-protein interactions that underlie the optical properties of the OCP, unique among carotenoid-proteins in binding a single pigment per polypeptide chain. Collectively, these data implicate several key amino acids in the function of the OCP and reveal that the photoconversion and photoprotective responses of the OCP to blue-green light can be decoupled.     

ΔpH-Dependent Fluorescence Quenching and Its Photoprotective Role in the Unicellular Red Alga Rhodella Violacea

Plants have developed various photoprotective mechanisms to resist irradiation stress. One of the photoprotective mechanisms described in the literature for LHC2-containing organisms involves a down-regulation of photosystem (PS) 2 occurring simultaneously with the build-up of a proton gradient across the thylakoid membrane (ΔpH). It is often correlated with deepoxidation of xanthophylls located in LHC2. In Rhodophyta instead of LHC2, the peripheral antenna of PS2 consists of a large extramembrane complex, the phycobilisome (PBS), which transfers its excitation to the core antennae of PS2 composed of the CP43 and CP47 protein-chlorophyll complexes and there is no xanthophyll cycle. In the red alga Rhodella violacea a ΔpH-dependent chlorophyll (Chl) a fluorescence quenching can be formed. We characterised this quenching, studied the effects of various irradiances and inhibitors. Under photoinhibitory conditions, the ΔpH-dependent Chl fluorescence quenching exerts a photoprotective role and delays the kinetics of photoinhibition. It is the first time that such a photoprotective mechanism is described in PBS-containing organisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Response of Phytoplankton Photophysiology to Varying Environmental Conditions in the Sub-Antarctic and Polar Frontal Zone

Climate-driven changes are expected to alter the hydrography of the Sub-Antarctic Zone (SAZ) and Polar Frontal Zone (PFZ) south of Australia, in which distinct regional environments are believed to be responsible for the differences in phytoplankton biomass in these regions. Here, we report how the dynamic influences of light, iron and temperature, which are responsible for the photophysiological differences between phytoplankton in the SAZ and PFZ, contribute to the biomass differences in these regions. High effective photochemical efficiency of photosystem II (/ 0.4), maximum photosynthesis rate (), light-saturation intensity (), maximum rate of photosynthetic electron transport (1/), and low photoprotective pigment concentrations observed in the SAZ correspond to high chlorophyll and iron concentrations. In contrast, phytoplankton in the PFZ exhibits low / ( 0.2) and high concentrations of photoprotective pigments under low light environment. Strong negative relationships between iron, temperature, and photoprotective pigments demonstrate that cells were producing more photoprotective pigments under low temperature and iron conditions, and are responsible for the low biomass and low productivity measured in the PFZ. As warming and enhanced iron input is expected in this region, this could probably increase phytoplankton photosynthesis in this region. However, complex interactions between the biogeochemical processes (e.g. stratification caused by warming could prevent mixing of nutrients), which control phytoplankton biomass and productivity, remain uncertain.     

Solar radiation as a factor in the evolution of scleromorphic leaf anatomy in Proteaceae

Species of the major Southern Hemisphere family, Proteaceae, have many scleromorphic anatomical structures in their leaves. Many of these structures (very thick cuticles and five anatomically distinct structures beneath the epidermis) are associated with the leaf surface exposed to direct light. These structures increase the path through which solar radiation must pass before reaching the mesophyll. In this study, such structures are proposed to protect the mesophyll from excess solar radiation, including photosynthetically active, ultraviolet, and possibly infrared radiation. Scleromorphic structures of the upper leaf surface and nonscleromorphic photoprotective structures (dense trichomes and papillae of the upper surface) occur almost exclusively in open vegetation. Open vegetation species of Proteaceae occur in oligotrophic and/or cold and/or dry places, where protection from light in excess of photosynthetic capacity and damage from ultraviolet light should be most important. Data from 123 species and a supertree constructed from available molecular phylogenies are used to show that the proposed photoprotective structures evolved many times within Proteaceae. In tests of correlated evolution, the proposed photoprotective structures are significantly associated with open vegetation, but not with dry habitats.     

How carotenoids protect bacterial photosynthesis

The essential function of carotenoids in photosynthesis is to act as photoprotective agents, preventing chlorophylls and bacteriochlorophylls from sensitizing harmful photodestructive reactions in the presence of oxygen. Based upon recent structural studies on reaction centres and antenna complexes from purple photosynthetic bacteria, the detailed organization of the carotenoids is described. Then with specific reference to bacterial antenna complexes the details of the photoprotective role, triplet triplet energy transfer, are presented.     

UV-A/B radiation rapidly activates photoprotective mechanisms in Chlamydomonas reinhardtii

Conversion of light energy into chemical energy through photosynthesis in the chloroplasts of photosynthetic organisms is essential for photoautotrophic growth, and non-photochemical quenching (NPQ) of excess light energy prevents the generation of reactive oxygen species and maintains efficient photosynthesis under high light. In the unicellular green alga Chlamydomonas reinhardtii, NPQ is activated as a photoprotective mechanism through wavelength-specific light signaling pathways mediated by the phototropin (blue light) and ultra-violet (UV) light photoreceptors, but the biological significance of photoprotection activation by light with different qualities remains poorly understood. Here, we demonstrate that NPQ-dependent photoprotection is activated more rapidly by UV than by visible light. We found that induction of gene expression and protein accumulation related to photoprotection was significantly faster and greater in magnitude under UV treatment compared with that under blue- or red-light treatment. Furthermore, the action spectrum of UV-dependent induction of photoprotective factors implied that C. reinhardtii senses relatively long-wavelength UV (including UV-A/B), whereas the model dicot plant Arabidopsis (Arabidopsis thaliana) preferentially senses relatively short-wavelength UV (mainly UV-B/C) for induction of photoprotective responses. Therefore, we hypothesize that C. reinhardtii developed a UV response distinct from that of land plants.

In contrast to land plants, which sense short-wave UV light, the unicellular green alga Chlamydomonas reinhardtii senses long-wavelength UV light for photoprotective responses.