胞间连丝研究的进展

胞间连丝为多细胞植物有机体提供了一个直接的细胞间物质运输和信息传递的细胞质通道,把一个个独立的“细胞王国”转变成相互连接的共质体,它是当今细胞生物学中十分活跃的研究领域。日益增多的研究结果揭示,胞间连丝协调基因表达和许多的细胞生理生化过程,对细胞的分裂与分化、形态发生、植物体的生长与发育,以及植物对环境的反应与适应等诸方面都起着十分重要的作用。本文仅就胞间连丝结构的多样性;胞间通道的调节因子;大分子蛋白质和核酸的胞间运输;胞间连丝阻断和共质体分区的形成及其与形态发生、休眠和抗逆性的关系等几个方面的新进展做一个简要的综述,借此例证胞间连丝在植物生命活动中的重要意义。     

胞间连丝与大分子物质的胞间转运机制

胞间连丝是相邻细胞间共质体运输的桥梁。基于对胞间连丝分子组成及超微结构的研究,不同学者提出了不同的胞间连丝结构模型。对其功能的研究表明,胞间连丝在物质运输、信息传递、病毒的周身感染等方面都具有重要作用。文章就胞间连丝的结构、分子组成及病毒介导的大分子胞间转移,以及对内源蛋白质的胞间转运机制诸方面的研究进展作了概述。     

灵武长枣果实维管束韧皮部及周围细胞的超微结构特征

为了探讨灵武长枣果实光合同化物韧皮部卸载和运输的途径,该研究采用透射电镜技术,对不同发育时期灵武长枣果实维管束韧皮部及其周围薄壁细胞的超微结构特征进行了分析。结果表明:筛管/伴胞复合体及其周围韧皮薄壁细胞间在果实膨大前期富含胞间连丝,而韧皮薄壁细胞与周围库细胞以及相邻库细胞间几乎不存在胞间连丝,形成共质体隔离;筛管/伴胞复合体及其与周围薄壁细胞间在果实快速膨大期也存在胞间连丝,但与果实膨大前期相比明显减少;果实着色期,SE/CC复合体及其与周围薄壁细胞间胞间连丝较少,并且出现阻塞现象;果实完熟期,筛管和伴胞之间几乎没有胞间连丝,有的筛管之间有少量胞间连丝,但却出现了阻塞现象,果肉库薄壁细胞与韧皮薄壁细胞间因胞间连丝阻塞现象而形成共质体隔离。综上结果认为,在果实发育的膨大前期阶段,光合同化物以共质体途径经筛分子卸出,卸出后可能以质外体途径进入液泡贮藏与利用;果实快速膨大期,光合同化物的卸出与运输采用共质体和质外体共存的途径;果实着色期和完熟期,光合同化物从筛分子卸出到贮藏薄壁细胞的运输均以质外体途径为主。     

胞间连丝：共质体运输的动态通道

胞间连丝作为一种细胞质结构将相邻的细胞连系起来而形成植物的共质体。胞间连丝通过调控许多离子和分子的共质体运输而广泛地参与植物的生命活动。胞间连丝的主要构成部分是细胞质膜、连丝小管、以及位于二之间的环层细胞质。这三都很容易在电子显微镜下观察到。细胞骨架的成分（肌动蛋白和肌球蛋白）起到稳定胞间连丝的作用。同时,钙结合蛋白可能具有调节间连丝功能的作用。在胞间连丝里,环层细胞质为大多数溶质提供共质体运输的通道,而有些 共质体运输则可能是通过连丝小管的内腔、连丝小管的壳层、甚或是细胞质膜来实现的。共质体可以细分为数个区块,它们各自允许不同大小的分子（从低于1000到高于10000道尔顿）通过。从发生上看,胞间连丝可以是初生的,也可以是次生的。前是伴随着新细胞壁的形成则产生的,而后则是在已有的细胞壁上产生的。胞间连丝的动态性质还表现在它们的频率是处于变化之中,这是由于组织或植物整体的发育和生理状态决定的。虽然共质体运输的基本形式是扩散,但胞间连丝对于某些离子和分子却是选择性的。在病毒感染细胞时,病毒的移动蛋白作用于胞间连丝的受体蛋白,结果,胞间连丝被显地扩张（其机理尚不清楚）。于是,病毒的移动蛋白连同与之结合在一起的病毒基因组进入毗邻的健康细胞。一些植物源性的蛋白质也能够通过胞间连丝来运输;推测其方式类似于病毒的移动蛋白。有些植物蛋白质本身就是信号分子,它们调节分化和其他活动。与此相反,还有一些植物蛋白质的共质体运输并不是通过特异的方式来实现的。     

宁夏枸杞果实韧皮部及其周围细胞超微结构研究

应用透射电镜技术研究了宁夏枸杞果实韧皮部细胞的超微结构变化。结果表明:(1)随着枸杞果实的发育成熟,果实维管组织中的韧皮部筛分子筛域逐渐变宽,筛孔大而多,通过筛孔的物质运输十分活跃;筛分子和伴胞间有胞间连丝联系,伴胞属传递细胞类型,与其相邻韧皮薄壁细胞和果肉薄壁细胞连接处的细胞界面发生质膜内突,整个筛分子/伴胞复合体与韧皮薄壁细胞之间形成共质体隔离,韧皮部糖分的卸载方式主要以质外体途径进行。(2)韧皮薄壁细胞间的胞间连丝较多,而韧皮薄壁细胞与果肉薄壁细胞的胞间连丝相对较少,但果肉薄壁细胞间几乎无胞间连丝;果肉薄壁细胞之间胞间隙较大,细胞壁和质膜内突间形成较大的质外体空间,为质外体的糖分运输创造了条件。(3)筛管、伴胞、韧皮薄壁细胞和果肉薄壁细胞中丰富的囊泡以及活跃的囊泡运输现象,暗示囊泡也参与了果实糖分的运输过程。研究推测,枸杞果实韧皮部同化物的卸载方式以及卸载后的同化物运输主要以质外体途径为主。     

植物体内的短距运输

植物体内水和溶质的运输,可分为短距离运输和长距离运输两种类型。短距离运输是指从细胞到细胞的运输,它包括共质体运输和非原质体(质外体)运输。长距离运输是指通过维管系统进行的运输。在这里,我们只讨论短距离运输。短距离运输,包括共质体运输和非原质体运输。植物细胞内的原生质体,由胞间连丝联系起来,形成一个整体,即共质体。所以,我们把从细胞到细胞通过胞间连丝的运输称为共质     

文冠果果实韧皮部及其周围薄壁细胞的超微结构观察及功能分析

该研究应用透射电镜技术,对生长发育过程中的文冠果果实的韧皮部及其周围薄壁细胞的超微结构进行了观察,以探讨文冠果果实同化物韧皮部卸载的细胞学路径及其机理。结果显示:(1)文冠果果实发育过程中,筛分子细胞胞腔较空,几乎没有细胞器,但有类似于囊泡的丝状不定型物存在;伴胞胞质浓密且细胞器丰富,液泡化程度不一,大多数存在多个小液泡;薄壁细胞具有中央大液泡,发育中期富含线粒体、高尔基体、内质网等细胞器,并存在囊泡运输现象,发育后期细胞器发生降解,说明随着果实生长发育,果实内物质代谢和转运活跃程度逐渐下降。(2)果实发育过程中筛分子和伴胞之间始终有胞间连丝,薄壁细胞之间也一直存在大量的胞间连丝,而筛分子-伴胞复合体与薄壁细胞之间只有在果实发育前期和后期存在一定数量的胞间连丝,发育中期却几乎没有胞间连丝。研究结果表明,文冠果果实发育过程中同化物韧皮部卸载路径可能发生了共质体途径-质外体途径-共质体途径的转变。     

玉米根冠中类外连丝的结构特征及其共质传输功能(简报)

胞间连丝是植物体内连接两个相邻细胞原生质体的共质运输通道,在胞间物质的转运和通讯联络上发挥重要作用。胞间连丝的功能在生理上主要体现在它对胞间转运物质的通透性（permeability）,通透性的变化和调节影响到许多生理过程的进展与协调。在植物的不同组织及其发育的不同阶段,不同的物化因素,不同的逆境胁迫以及病原物的侵染均可导致胞间连丝的通透性呈现相应的变动。胞间连丝存在形式的多样性以及对其不同程度和不同方式的修饰,均可对胞间连丝的生理功能及其通透性有明显的影响。[第一段]     

植物胞间连丝的形成和结构变化

 通过对胞间连丝的起源、形成及其调控物质运输机理的讨论,说明胞间连丝是物质运输和信息传递的重要通道。并且比较了高等植物和低等植物胞间连丝的发生和次生变化。由于酶的作用,胞间连丝向胞间通道转化,形成了约100~1000nm的开放性通道。     

植物胞间连丝的形成和结构变化

通过对胞间连丝的起源,形成及其调控物质运输机理的讨论,说明胞间连丝是物质运输和信息传递的重要通道。并且比较了高等植物和低等植物胞间连线的发生和次生变化。由于酶的作用,胞间连丝向胞间通道转化,形成了约１００￣１０００ｎｍ的开放性通道。     

TIP,a novel host factor linking callose degradation with the cell-to-cell movement of Potato virus X

The cell-to-cell movement of Potato virus X (PVX) requires four virus-encoded proteins, the triple gene block (TGB) proteins (TGB25K, TGB12K, and TGB8K) and the coat protein. TGB12K increases the plasmodesmal size exclusion limit (SEL) and may, therefore, interact directly with components of the cell wall or with plant proteins associated with bringing about this change. A yeast two-hybrid screen using TGB12K as bait identified three TGB12K-interacting proteins (TIP1, TIP2, and TIP3). All three TIPs interacted specifically with TGB12K but not with TGB25K or TGB8K. Similarly, all three TIPs interacted with beta-1,3-glucanase, the enzyme that may regulate plasmodesmal SEL through callose degradation. Sequence analyses revealed that the TIPs encode very similar proteins and that TIP1 corresponds to the tobacco ankyrin repeat-containing protein HBP1. A TIP1::GFP fusion protein localized to the cytoplasm. Coexpression of this fusion protein with TGB12K induced cellular changes manifested as deposits of additional cytoplasm at the cell periphery. This work reports a direct link between a viral movement protein required to increase plasmodesmal SEL and a host factor that has been implicated as a key regulator of plasmodesmal SEL. We propose that the TIPs are susceptibility factors that modulate the plasmodesmal SEL.     

Arabidopsis plasmodesmal proteome

The multicellular nature of plants requires that cells should communicate in order to coordinate essential functions. This is achieved in part by molecular flux through pores in the cell wall, called plasmodesmata. We describe the proteomic analysis of plasmodesmata purified from the walls of Arabidopsis suspension cells. Isolated plasmodesmata were seen as membrane-rich structures largely devoid of immunoreactive markers for the plasma membrane, endoplasmic reticulum and cytoplasmic components. Using nano-liquid chromatography and an Orbitrap ion-trap tandem mass spectrometer, 1341 proteins were identified. We refer to this list as the plasmodesmata- or PD-proteome. Relative to other cell wall proteomes, the PD-proteome is depleted in wall proteins and enriched for membrane proteins, but still has a significant number (35%) of putative cytoplasmic contaminants, probably reflecting the sensitivity of the proteomic detection system. To validate the PD-proteome we searched for known plasmodesmal proteins and used molecular and cell biological techniques to identify novel putative plasmodesmal proteins from a small subset of candidates. The PD-proteome contained known plasmodesmal proteins and some inferred plasmodesmal proteins, based upon sequence or functional homology with examples identified in different plant systems. Many of these had a membrane association reflecting the membranous nature of isolated structures. Exploiting this connection we analysed a sample of the abundant receptor-like class of membrane proteins and a small random selection of other membrane proteins for their ability to target plasmodesmata as fluorescently-tagged fusion proteins. From 15 candidates we identified three receptor-like kinases, a tetraspanin and a protein of unknown function as novel potential plasmodesmal proteins. Together with published work, these data suggest that the membranous elements in plasmodesmata may be rich in receptor-like functions, and they validate the content of the PD-proteome as a valuable resource for the further uncovering of the structure and function of plasmodesmata as key components in cell-to-cell communication in plants.     

Application of microinjection techniques to plant nutrition

To gain a full understanding of the complex processes that underlie plant nutrition requires the elucidation of the genetic, molecular, biochemical, biophysical, physiological and environmental factors that interact, at the cellular, organ and whole plant levels, to allow this sessile organism to optimize the allocation and utilization of available resources. The application of microinjection methods, in conjunction with molecular tools, established a powerful experimental approach to elucidate the processes underlying plant growth and development. Besides providing insight into the molecular nature of many of the membrane transport systems that function in nutrient acquisition and transport, this approach revealed the presence of a unique plasmodesmal macromolecular trafficking system that operates at the cellular/tissue and whole-plant level. This information processing network it discussed in terms of its role in allowing plants to regulate physiological activities at a supracellular level. Future studies aimed at identifying additional genes associated with this plasmodesmal macromolecular trafficking system will advance our understanding of the function and evolution of this novel plant communication system.     

Primary and secondary plasmodesmata: structure, origin, and functioning

Summary In the multicellular organisms of higher plants, plasmodesmata provide pathways for intimate symplasmic communication between neighboring cells. The arguments summarized in the present review demonstrate that plasmodesmata are diverse and highly dynamic structures. Differences in the plasmodesmal origin and modifications of the plasmodesmal structure and functioning at the various cell interfaces are the basic means which give rise to a complicated and flexibile symplasmic network. This complex communication system is discussed to serve a significant role in the coordinated development and in the concerted physiological functioning of the cells within the plant tissues, organs, and organisms.     

Dynamics of plasmodesmal connectivity in successive interfaces of the cambial zone

Frequency, density and branching of plasmodesmata were counted in successive tangential and transverse walls in the cambial zone of tomato stems in order to examine development of the plasmodesmal network in a chronological order. Coincident with progress of cell development, plasmodesmal connectivity increased, both at the xylem- and phloem-side. In transverse walls, the number of secondary plasmodesmata enhanced considerably. The same held for tangential walls, with a superimposed plasmodesmal doubling during the first phase of phloem development. This plasmodesmal doubling was interpreted to result from the deposition of wall material between branched plasmodesmal strands. Structural plasmodesmal development was correlated with production of hydroxyl radicals which control local cell wall alterations. Successive phases of plasmodesmal deployment and modification were distinguished which may coincide with differential functional capacities as documented by intracellular injection of fluorochromes. Diffusion-driven symplasmic transport appeared to be transiently interrupted during cell maturation.     

Opening up the communication channels: recent insights into plasmodesmal function

The past year has seen significant advances in our understanding of the function and regulation of plasmodesmata. Notably, we have learned that plasmodesmata undergo dynamic changes during development and may participate in long-range communication through the transmission of RNA signals. Biochemical studies have enriched our understanding of a putative plasmodesmal receptor and of plant factors involved in viral cell-to-cell movement.     

Peptide antagonists of the plasmodesmal macromolecular trafficking pathway

In plants, cell-to-cell transport of endogenous and viral proteins and ribonucleoprotein complexes (RNPCs) occurs via plasmodesmata. Specificity of this transport pathway appears to involve interaction between such proteins/RNPCs and plasmodesmal chaperones/receptors. Here, KN1 and the cucumber mosaic virus movement protein (CMV-MP) were used, in a modified phage-display screening system, to identify peptides capable of interacting with proteins present in a plasmodesmal-enriched cell wall fraction. Binding/competition assays and microinjection experiments revealed that these phage-displayed peptides and homologous synthetic oligopeptides function as ligand-specific antagonists of macromolecular trafficking through plasmodesmata. A KN1 peptide antagonist had the capacity to interact with a motif involved in the dilation of plasmodesmal microchannels. Although KN1 could still achieve limited movement through plasmodesmata when this SEL motif was blocked, KN1-mediated transport of KN1-sense RNA was fully inhibited. These findings provide direct support for the hypothesis that KN1 requires, minimally, two physically separated signal motifs involved in the dilation of, and protein translocation through, plasmodesmal microchannels, and provide direct proof that plasmodesmal dilation is a prerequisite for the cell-to-cell transport of an RNPC.     

Specific targeting of a plasmodesmal protein affecting cell-to-cell communication

Plasmodesmata provide the cytoplasmic conduits for cell-to-cell communication throughout plant tissues and participate in a diverse set of non–cell-autonomous functions. Despite their central role in growth and development and defence, resolving their modus operandi remains a major challenge in plant biology. Features of protein sequences and/or structure that determine protein targeting to plasmodesmata were previously unknown. We identify here a novel family of plasmodesmata-located proteins (called PDLP1) whose members have the features of type I membrane receptor-like proteins. We focus our studies on the first identified type member (namely At5g43980, or PDLP1a) and show that, following its altered expression, it is effective in modulating cell-to-cell trafficking. PDLP1a is targeted to plasmodesmata via the secretory pathway in a Brefeldin A–sensitive and COPII-dependent manner, and resides at plasmodesmata with its C-terminus in the cytoplasmic domain and its N-terminus in the apoplast. Using a deletion analysis, we show that the single transmembrane domain (TMD) of PDLP1a contains all the information necessary for intracellular targeting of this type I membrane protein to plasmodesmata, such that the TMD can be used to target heterologous proteins to this location. These studies identify a new family of plasmodesmal proteins that affect cell-to-cell communication. They exhibit a mode of intracellular trafficking and targeting novel for plant biology and provide technological opportunities for targeting different proteins to plasmodesmata to aid in plasmodesmal characterisation.     

胞间连丝与大分子物质的胞间转移

胞间连丝是细胞间细胞器,是细胞间通讯的直接途径。一般认为,胞间连丝允许通过物质的分子量上限(SEL)是800～1000 Da．近年来研究的许多证据表明,胞间连丝的SEL随组织种类及其生理状况而异。在某些情况下,它可以允许大分子物质通过,如病毒运动蛋白与胞间连丝相互作用,使病毒通过胞间连丝转移。玉米突变体 kn1基因异常表达的KN1可使包括表皮在内的各层组织结瘤,KN1是细胞间移动的信息物,P-蛋白可由伴胞通过胞间连丝转移到筛管。某些组织中胞间连丝很高的SEL和发育过程胞间连丝SEL的变化可能在植物发育调控中有重要作用。本文对大分子通过胞间连丝转移的机理进行了讨论。