叶绿体移动现象

叶绿体是植物细胞中最重要的光能转化细胞器,叶绿体在细胞中具有一定分布区域,当植物受外界环境刺激时叶绿体会发生位移,包括回避反应、聚集反应等运动方式.近年来,以模式植物拟南芥为材料,利用正向遗传学和反向遗传学等方法,发现一些重要基因编码的蛋白控制叶绿体移动行为,其中向光素蓝光受体PHOT1和PHOT2以及肌动蛋白结合蛋白CHIP1(chloroplast unusual positioning 1)与叶绿体移动有密切关系.简要介绍目前对叶绿体移动机制的研究进展.     

拟南芥NPH3/RPT2-Like (NRL)家族蛋白在向光素信号转导通路中的作用研究进展

向光素PHOT1和PHOT2感受蓝光刺激后发生自磷酸化激活, 调节植物气孔开放、叶绿体运动、叶片伸展和定位以及向光性(包括根的负向光性和下胚轴的向光性)等多种适应性反应。拟南芥(Arabidopsis thaliana) NRL (NPH3/RPT2-Like)家族成员在向光素介导的信号途径中发挥重要作用, 其中NPH3特异调控下胚轴的向光性以及叶片的伸展与定位, RPT2参与调节植物向光性、叶片的伸展与定位以及叶绿体聚光反应等。NCH1是新发现的NRL家族成员, 与RPT2以功能冗余的方式调节叶绿体的聚光反应, 但不调节避光反应。该文主要综述了NRL蛋白家族成员在向光素介导蓝光信号通路中的作用, 并展望了未来的研究方向, 旨在为全面揭示NRL家族成员的功能提供线索。     

拟南芥NPH3/RPT2-Like(NRL)家族蛋白在向光素信号转导通路中的作用研究进展

向光素PHOT1和PHOT2感受蓝光刺激后发生自磷酸化激活,调节植物气孔开放、叶绿体运动、叶片伸展和定位以及向光性(包括根的负向光性和下胚轴的向光性)等多种适应性反应。拟南芥(Arabidopsis thaliana) NRL (NPH3/RPT2-Like)家族成员在向光素介导的信号途径中发挥重要作用,其中NPH3特异调控下胚轴的向光性以及叶片的伸展与定位,RPT2参与调节植物向光性、叶片的伸展与定位以及叶绿体聚光反应等。NCH1是新发现的NRL家族成员,与RPT2以功能冗余的方式调节叶绿体的聚光反应,但不调节避光反应。该文主要综述了NRL蛋白家族成员在向光素介导蓝光信号通路中的作用,并展望了未来的研究方向,旨在为全面揭示NRL家族成员的功能提供线索。     

向光素调节植物向光性及其与光敏色素/隐花色素的相互关系

蓝光受体向光素(PHOT1/PHOT2)调节蓝光诱导的植物运动反应, 包括植物向光性、叶绿体运动、气孔运动和叶片伸展等。其中, 向光素介导的植物向光性能够促使植物弯向光源, 确保其以最佳取向捕获光源, 优化光合作用。光敏色素和隐花色素作为光受体也参与植物的向光性调节。该文综述了向光素介导的拟南芥(Arabidopsis thaliana)下胚轴向光弯曲信号转导及其与光敏色素、隐花色素协同作用的分子机制, 以期为改造植物光捕获能力及提高光利用效率提供理论基础。     

植物向光素受体与信号转导机制研究进展

向光素(phototropin,PHOT)是继光敏色素、隐花色素之后分离的植物蓝光受体。PHOT介导蓝光诱导的向光反应,叶绿体运动,气孔开放、叶片伸展及叶片定位等生理反应。近年来关于PHOT受体介导这些生理反应的分子机制探讨愈来愈受研究者的广泛关注。主要从拟南芥PHOT结构及信号转导方面的研究进展进行综述。     

低等植物蓝光受体信号传导研究

拟南芥含有5个已分离的蓝光受体和至少1个未鉴定的蓝光/紫外光-A受体.隐花色素(CRY1、CRY2和CRY3) 调节植物的形态建成、开花和生物节律性,而向光素 (PHOT1和PHOT2) 调节植物的向光性、叶绿体运动和气孔开放.黄素可以吸收蓝光和紫外光-A,是CRY和PHOT蓝光受体的生色团.对这些光受体的结构和作用模式已了解很多.苔藓植物小立碗藓中含有2个已分离的隐花色素(CRY1a和CRY1b),负责调节侧枝形成和调控生长素反应;有4个向光素(PHOTA1,PHOTA2,PHOTB1,PHOTB2) 调节叶绿体的运动.苔藓细胞内蓝光/紫外光-A引发的信号转导有Ca2+参与.     

植物向光素

向光素是继光敏色素、隐花色素之后发现的一种蓝光受体,分子量120 kD,能够结合黄素单核苷酸(FMN)进行自动磷酸化作用,它介导植物向光性运动、叶绿体移动与气孔开放等反应,在蓝光信号传导反应中它启动生长素载体的运动和诱导Ca2 的流动,从而调节植物细胞相关的反应.文章就这一领域的研究作介绍.     

植物的蓝光受体

文章介绍植物隐花色素、向光素和其他蓝光受体的研究进展。     

PHOT2介导拟南芥下胚轴向光弯曲调节子的筛选与鉴定

向光素(PHOT1和PHOT2)功能冗余调节单侧强蓝光诱导的拟南芥(Arabidopsis thaliana)黄化苗下胚轴向光弯曲表现功能冗余,限制了人们对PHOT2信号转导机制的深入研究。通过化学诱变剂甲基磺酸乙酯(EMS)诱变拟南芥phot1突变体,避开PHOT1基因的干扰,寻找PHOT2下游信号分子。研究结果表明,已成功筛选到1株遗传稳定的下胚轴向蓝光不弯曲突变体。遗传分析结果显示,该突变体可能是PHOT2下游信号分子突变,将其命名为p2sa1(phototropin2 signaling associated1)。用100μmol·m–2·s–1强蓝光单侧照射,phot1p2sa1下胚轴向光弯曲缺失,呈现phot1phot2双突变的表型,然而phot1p2sa1在强蓝光下叶绿体避光正常,明显不同于phot1phot2。实验证实P2SA1可能位于PHOT2的下游,参与调节PHOT2介导的拟南芥下胚轴向光弯曲反应。     

植物的蓝光受体及其信号转导

近年来,对拟南芥及其它植物的分子遗传研究,在隐花色素和向光素的分子、基因和蓝光信号转导方面取得了显著进展。本文就这两种蓝光受体的基本结构及蓝光信号转导进行介绍。     

How and why do plant nuclei move in response to light?

We recently found that nuclei take different intracellular positions depending upon dark and light conditions in Arabidopsis thaliana leaf cells. Under dark conditions, nuclei in both epidermal and mesophyll cells are distributed baso-centrally within the cell (dark position). Under light conditions, in contrast, nuclei are distributed along the anticlinal walls (light position). Nuclear repositioning from the dark to light positions is induced specifically by blue light at >50 µmol m⁻² s⁻¹ in a reversible manner. Using analysis of mutant plants, it was demonstrated that the response is mediated by the blue-light photoreceptor phototropin2. Intriguingly, phototropin2 also seems to play an important role in the proper positioning of nuclei and chloroplasts under dark conditions. Light-dependent nuclear positioning is one of the organelle movements regulated by phototropin2. However, the mechanisms of organelle motility, physiological significance, and generality of the phenomenon are poorly understood. In this addendum, we discussed how and why nuclei move depending on light, together with future perspectives.Key words: actin, Arabidopsis, blue light, cytoskeleton, nuclear positioning, nucleus, phototropin     

Subcellular motility: A correlated light and electron microscopic study using cultured cells

To assess the possible role of filaments in subcellular motility, particular cultured cells were studied by light and electron microscopy. Motile cell margins always contained meshworks of ~50 Å diam. filaments. Organelles moved within cytoplasm occupied by a meshwork of 50–100 Å filaments and microtubules. When cells were treated with cytochalasin B, movements of cell margins stopped, but organelle movements continued; electron microscopically, while subplasmalemmal filaments had disappeared, subcortical filaments and microtubules remained. When cells were treated with hypertonic medium, organelle movements ceased but marginal movements continued; electron microscopically, although cell margins contained normal filament arrays, few subcortical filaments remained. It is concluded that while cell margins are moved by a meshwork of filaments, organelle movement is mediated by a subcortical meshwork of filaments and microtubules.     

Light perception and the role of the xanthophyll cycle in blue-light-dependent chloroplast movements in lemna trisulca L

In most higher plants, chloroplasts move towards the periclinal cell walls in weak blue light (WBL) to increase light harvesting for photosynthesis, and towards the anticlinal walls as an escape reaction, thus avoiding photo-damage in strong blue light (SBL). The photo- receptor(s) triggering these responses have not yet been identified. In this study, the role of zeaxanthin as a blue-light photoreceptor in chloroplast movements was investigated. Time-lapse 3D confocal imaging in Lemna trisulca showed that individual chloroplasts responded to local illumination when one half of the cell was treated with light of different intensity or spectral quality to that received by the other half, or was maintained in darkness. Thus the complete signal perception, transduction and effector system has a high degree of spatial resolution and is consistent with localization of part of the transduction chain in the chloroplasts. Turnover of xanthophylls was determined using HPLC, and a parallel increase was observed between zeaxanthin and chloroplast movements in SBL. Ascorbate stimulated both a transient increase in zeaxanthin levels and chloroplast movement to profile in physiological darkness. Conversely, dithiothreitol blocked zeaxanthin production and responses to SBL and, to a lesser extent, WBL. Norflurazon preferentially inhibited SBL-dependent chloroplast movements. Increases in zeaxanthin were also observed in strong red light (SRL) when no directional chloroplast movements occurred. Thus it appears that a combination of zeaxanthin and blue light is required to trigger responses. Blue light can cause cis-trans isomerization of xanthophylls, thus photo-isomerization may be a critical link in the signal transduction pathway.     

Actin and cortical fiber reticulation in the siphonaceous alga Vaucheria sessilis

Since light-induced organellae aggregation in the siphonaceous alga Vaucheria sessilis (Vauch.) D.C. is accompanied by the formation of a cortical fiber reticulum in the light, we proposed that this process of reticulation might be causally related to aggregation (Blatt and Briggs, 1980). In this paper we report the tentative identification of actin filaments and filament bundles in the cortical cytoplasm of V. sessilis, and present additional evidence, obtained using the inhibitors cytochalasin B and phalloidin and indicating that aggregation in response to low-intensity point irradiation with blue light is dependent upon the formation of a cortical fiber reticulum. Phalloidin stabilized the cortical fibers, preventing both reticulum formation and organelle aggregation in blue light. Cytochalasin B partially destabilized the cortical fibers to the extent of permitting light-induced reticulum formation and organelle aggregation in the light in the presence of phalloidin.C.I.W.-D.P.B. Publication No. 643     

Actin-based mechanisms for light-dependent intracellular positioning of nuclei and chloroplasts in Arabidopsis

The plant organelles, chloroplast and nucleus, change their position in response to light. In Arabidopsis thaliana leaf cells, chloroplasts and nuclei are distributed along the inner periclinal wall in darkness. In strong blue light, they become positioned along the anticlinal wall, while in weak blue light, only chloroplasts are accumulated along the inner and outer periclinal walls. Blue-light dependent positioning of both organelles is mediated by the blue-light receptor phototropin and controlled by the actin cytoskeleton. Interestingly, however, it seems that chloroplast movement requires short, fine actin filaments organized at the chloroplast edge, whereas nuclear movement does cytoplasmic, thick actin bundles intimately associated with the nucleus. Although there are many similarities between photo-relocation movements of chloroplasts and nuclei, plant cells appear to have evolved distinct mechanisms to regulate actin organization required for driving the movements of these organelles.Key words: actin, Arabidopsis, blue light, chloroplast positioning, phototropin, nuclear positioning     

Effects of Light on the Membrane Potential of Protoplasts from Samanea saman Pulvini : Involvement of K Channels and the H -ATPase

Rhythmic light-sensitive movements of the leaflets of Samanea saman depend upon ion fluxes across the plasma membrane of extensor and flexor cells in opposing regions of the leaf-movement organ (pulvinus). We have isolated protoplasts from the extensor and flexor regions of S. saman pulvini and have examined the effects of brief 30-second exposures to white, blue, or red light on the relative membrane potential using the fluorescent dye, 3,3′-dipropylthiadicarbocyanine iodide. White and blue light induced transient membrane hyperpolarization of both extensor and flexor protoplasts; red light had no effect. Following white or blue light-induced hyperpolarization, the addition of 200 millimolar K⁺ resulted in a rapid depolarization of extensor, but not of flexor protoplasts. In contrast, addition of K⁺ following red light or in darkness resulted in a rapid depolarization of flexor, but not of extensor protoplasts. In both flexor and extensor protoplasts, depolarization was completely inhibited by tetraethylammonium, implicating channel-mediated movement of K⁺ ions. These results suggest that K⁺ channels are closed in extensor plasma membranes and open in flexor plasma membranes in darkness and that white and blue light, but not red light, close the channels in flexor plasma membranes and open them in extensor plasma membranes. Vanadate treatment inhibited hyperpolarization in response to blue or white light, but did not affect K⁺ -induced depolarization. This suggests that white or blue light-induced hyperpolarization results from activation of the H⁺ -ATPase, but this hyperpolarization is not the sole factor controlling the opening of K⁺ channels.     

The phototropin family as photoreceptors for blue light-induced chloroplast relocation

Blue light-induced chloroplast accumulation and avoidance relocation movements are controlled by the blue light photoreceptor phototropin. The Arabidopsis thaliana genome has two phototropin genes encoding phot1 and phot2. Each of these photoreceptors contains two LOV (light oxygen and voltage) domains and a kinase domain. The LOV domains absorb blue light though an associated flavin mononucleotide chromophore, while the kinase domain is thought to be associated with signal transduction. The phototropins control not only chloroplast relocation movement, but also blue light-induced phototropic responses, leaf expansion and stomatal opening. Here I review the role of phototropin as a photoreceptor for chloroplast photorelocation movement. Electronic Publication     

The structure of cytoplasm in directly frozen cultured cells. II. Cytoplasmic domains associated with organelle movements

The relationship between organelle movement and cytoplasmic structure in cultured fibroblasts or epithelial cells was studied using video-enhanced differential interference contrast microscopy and electron microscopy of directly frozen whole mounts. Two functional cytoplasmic domains are characterized by these techniques. A central domain rich in microtubules is associated with directed as well as Brownian movements of organelles, while a surrounding domain rich in f-actin supports directed but often intermittent organelle movements more distally along small but distinct individual microtubule tracks. Differences in the organization of the cytoplasm near microtubules may explain why organelle movements are typically continuous in central regions but usually intermittent along the small tracks through the periphery. The central type of cytoplasm has a looser cytoskeletal meshwork than the peripheral cytoplasm which might, therefore, interfere less frequently with organelles moving along microtubules there.     

Light-induced chloroplast movements in Lemna trisulca. Identification of the motile system

In mesophyll cells of the water plant Lemna trisulca L. chloroplasts redistribute in response to blue light. In the present study it is shown that an actin depolymerizing agent cytochalasin D, a crosslinker of actin subunits in F-actin m-maleimidobenzoic acid N-hydroxysuccinimide ester (MBS) as well as N-ethylmaleimide (NEM)—a sulfhydryl group reagent, are potent inhibitors of these blue light-induced chloroplast movements in Lemna. Extraction with cold, buffered glycerol solution preserves light-induced chloroplast arrangements within cells producing permeabilized cell models. ‘Reactivation’ of these cell models by Mg-ATP results in remarkable movements which can be inhibited by treatment with NEM and cytochalasin D. Immunofluorescence microscopy demonstrates that a component which is associated with isolated Lemna chloroplasts cross-reacts with antibodies directed against bovine myosin. These results indicate that a contractile actomyosin system is involved in blue light-induced chloroplast movements in Lemna and a putative motor protein, similar to myosin, is associated with the surface of Lemna chloroplasts.     

Probing and tracking organelles in living plant cells

Intracellular organelle movements and positioning play pivotal roles in enabling plants to proliferate life efficiently and to survive diverse environmental stresses. The elaborate dissection of organelle dynamics and their underlying mechanisms (e.g., the role of the cytoskeleton in organelle movements) largely depends on the advancement and efficiency of organelle tracking systems. Here, we provide an overview of some recently developed tools for labeling and tracking organelle dynamics in living plant cells.