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。     

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。     

胞间连丝向胞间通道转化的机理

为了适应衰老器官内大量的降解物质在短期内迅速转移到贮藏器官和生长部位,胞间连丝内部结构在酶的作用下瓦解,其周围的细胞壁物质也被降解,胞间连丝结构拓宽,形成胞间通道。     

胞间连丝的次生形成和次生变化

本文介绍了胞间连丝次生形成和次生变化的研究进展。用统计特定细胞壁区段上胞间连丝数量与密度的变化,电镜观察嫁接组合中接穗与砧水间细胞壁上胞间连丝的形成等方法,证明了在植物生长发育过程中,存在着胞间连丝的次生形成。在某些特定部位,某一发育阶段,已形成的胞间连丝常会发生可逆的次生变化,这种变化和植物发育过程中的共质体隔离以及物质运输的调节有关。     

胞间连丝的次生形成和次生变化

本文介绍了胞间连丝次生形成和次生变化的研究进展,用统计特定细胞壁区段上胞间连丝数量与密度的变化,电镜观察嫁接组合中接穗与砧水间细胞壁上胞间连丝的形成等方法,证明了在植物生长发育过程中,存在着胞间连丝的次生形成。在某些特定部位,某一发育阶段,已形成的胞间连丝常会发生可逆的次生变化,这种变化和植物发育过程中的共质体隔离以及物质运输的调节有关。     

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。     

紫竹梅雄蕊毛细胞发育过程中胞间连丝超微结构的变化

紫竹梅（Ｓｅｔｃｒｅａｓｅａ ｐｕｒｐｕｒｅａ）雄蕊毛细胞间的胞间连丝随着细胞的生长、发育、衰老而呈现动态变化的过程．花蕾和开放花的雄蕊毛细胞间的胞间连丝,具备胞间连丝的一般结构,直径约５０ ｎｍ ．衰老花雄蕊毛细胞间的胞间连丝拓宽,内部结构逐步降解、撤离,呈开放式通道,直径约１００ ｎｍ ． 在胞间连丝的动态开放过程中,细胞内的细胞器也发生相应变化． 对胞间连丝形成开放性通道及其机理进行了讨论     

植物的胞间连丝

胞间连丝是植物细胞的重要特征之一。自1879年坦格尔(Tangl.E)发现柿子等胚乳细胞间存在有纤细的原生质连丝以来,随着科学技术的发展和电子显微镜的广泛应用,给胞间连丝结构与功能的研究带来方便。胞间连丝的形成胞间连丝的形成主要有两种方式。其一,胞间连丝的初生形成,即随细胞分裂而出现胞间连丝的过程。在细胞分裂的末期,纺锤丝在靠近两极处消失,中间的纺锤丝保留下来,微管的数量增加,并向四周扩展,形成一个筒状     

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８００～１０００Ｄａ．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。     

Putting the Squeeze on Plasmodesmata: A Role for Reticulons in Primary Plasmodesmata Formation

Primary plasmodesmata (PD) arise at cytokinesis when the new cell plate forms. During this process, fine strands of endoplasmic reticulum (ER) are laid down between enlarging Golgi-derived vesicles to form nascent PD, each pore containing a desmotubule, a membranous rod derived from the cortical ER. Little is known about the forces that model the ER during cell plate formation. Here, we show that members of the reticulon (RTNLB) family of ER-tubulating proteins in Arabidopsis (Arabidopsis thaliana) may play a role in the formation of the desmotubule. RTNLB3 and RTNLB6, two RTNLBs present in the PD proteome, are recruited to the cell plate at late telophase, when primary PD are formed, and remain associated with primary PD in the mature cell wall. Both RTNLBs showed significant colocalization at PD with the viral movement protein of Tobacco mosaic virus, while superresolution imaging (three-dimensional structured illumination microscopy) of primary PD revealed the central desmotubule to be labeled by RTNLB6. Fluorescence recovery after photobleaching studies showed that these RTNLBs are mobile at the edge of the developing cell plate, where new wall materials are being delivered, but significantly less mobile at its center, where PD are forming. A truncated RTNLB3, unable to constrict the ER, was not recruited to the cell plate at cytokinesis. We discuss the potential roles of RTNLBs in desmotubule formation.Plasmodesmata (PD), the small pores that connect higher plant cells, are complex structures of about 50 nm in diameter. Each PD pore is lined by the plasma membrane and contains an axial endoplasmic reticulum (ER)-derived structure known as the desmotubule (Overall and Blackman, 1996; Maule, 2008; Tilsner et al., 2011). The desmotubule is an enigmatic structure whose function has not been fully elucidated. The small spiraling space between the desmotubule and the plasma membrane, known as the cytoplasmic sleeve, is almost certainly a conduit for the movement of small molecules (Oparka et al., 1999). Some reports, however, suggest that the desmotubule may also function in cell-to-cell trafficking, providing an ER-derived pathway between cells along which macromolecules may diffuse (Cantrill et al., 1999). The desmotubule is one of the most tightly constricted membrane structures found in nature (Tilsner et al., 2011), but the forces that generate its intense curvature are not understood. In most PD, the desmotubule is a tightly furled tube of about 15 nm in diameter in which the membranes of the ER are in close contact along its length. The desmotubule may balloon out in the region of the middle lamella into a central cavity, but at the neck regions of the PD pore it is tightly constricted (Robinson-Beers and Evert, 1991; Ding et al., 1992; Glockmann and Kollmann, 1996; Overall and Blackman, 1996; Ehlers and Kollmann, 2001). Studies of PD using GFP targeted to the ER lumen (e.g. GFP-HDEL) have shown that GFP is excluded from the desmotubule due to the constriction of ER membranes in this structure (Oparka et al., 1999; Crawford and Zambryski, 2000; Martens et al., 2006; Guenoune-Gelbart et al., 2008). Therefore, lumenal GFP is unable to move between plant cells unless the membranes of the desmotubule become relaxed in some way. On the other hand, dyes and some proteins inserted into the ER membrane can apparently move through the desmotubule, either along the membrane or through the lumen, at least under some conditions (Grabski et al., 1993; Cantrill et al., 1999; Martens et al., 2006; Guenoune-Gelbart et al., 2008).Recently, a number of proteins have been described in mammalian, yeast, and plant systems that induce extreme membrane curvature. Among these are the RETICULONS (RTNs), integral membrane proteins that induce curvature of the ER to form tubules (Voeltz et al., 2006; Hu et al., 2008; Tolley et al., 2008, 2010; Sparkes et al., 2010). In animals, RTNs have been shown to be involved in a wide array of endomembrane-related processes, including intracellular transport and vesicle formation, and as RTNs can also influence axonal growth, they may have roles in neurodegenerative disorders such as Alzheimer’s disease (Yang and Strittmatter, 2007). Arabidopsis (Arabidopsis thaliana) has 21 RTN homologs, known as RTNLBs (Nziengui et al., 2007; Sparkes et al., 2010), considerably more than in yeast or mammals, but most have not been examined. RTNLBs contain two unusually long hydrophobic helices that form reentrant loops (Voeltz et al., 2006; Hu et al., 2008; Sparkes et al., 2010; Tolley et al., 2010). These are thought to induce membrane curvature by the molecular wedge principle (Hu et al., 2008; Shibata et al., 2009). When RTNLBs are overexpressed transiently in cells expressing GFP-HDEL, the ER becomes tightly constricted and GFP-HDEL is excluded from the lumen of the constricted ER tubules (Tolley et al., 2008, 2010), a situation similar to that which occurs in desmotubules (Oparka et al., 1999; Crawford and Zambryski, 2000; Martens et al., 2006). In vitro studies with isolated membranes have shown that the degree of tubulation is proportional to the number and spacing of RTNLB proteins in the membrane (Hu et al., 2008). For example, to constrict the ER membrane into a structure of 15 nm, the diameter of a desmotubule, would require RTNLBs to be inserted every 2 nm or less along the desmotubule axis (Hu et al., 2008), potentially making the desmotubule an extremely protein-rich structure (Tilney et al., 1991). Interestingly, a number of RTNLB proteins appear in the recently described PD proteome (Fernandez-Calvino et al., 2011), suggesting that RTNLBs are good candidates for proteins that model the cortical ER into desmotubules.Primary PD form at cytokinesis during the assembly of the cell plate (Hawes et al., 1981; Hepler, 1982). Of the numerous studies devoted to the structure of the cell plate, very few have examined the behavior of the ER during cytokinesis. During mitosis, elements of the ER are located in the spindle apparatus, separated from the cytoplasm (Hepler, 1980). Just prior to cytokinesis, there is a relative paucity of ER in the region destined to become the cell plate (Hepler, 1980; Hawes et al., 1981). The studies of Hawes et al. (1981) and Hepler (1982), exploiting heavy-metal impregnation of the ER, showed that during the formation of the new cell plate, strands of cortical ER are inserted across the developing wall, between the Golgi-derived vesicles that deposit wall materials. These ER strands become increasingly thinner during formation of the desmotubule, eventually excluding heavy metal stains from the ER lumen (Hepler, 1982). The center of the desmotubule often appears electron opaque in transmission electron microscopy images and has been referred to as the central rod (Overall and Blackman, 1996). This structure may consist of proteins that extend from the inner ER leaflets or may correspond to head groups of the membrane lipids themselves. In the fully formed primary PD, the desmotubule remains continuous with the cortical ER that runs close to the new cell wall (Hawes et al., 1981; Hepler, 1982; Oparka et al., 1994).Here, we show that two of the RTNLBs present in the PD proteome, RTNLB3 and RTNLB6, become localized to the cell plate during the formation of primary PD. These RTNLBs remain associated with the desmotubule in fully formed PD and are immobile, as evidenced by fluorescence recovery after photobleaching (FRAP) studies. A truncated version of RTNLB3, in which the second hydrophobic region was deleted (Sparkes et al., 2010), was not recruited to the cell plate at cytokinesis. We suggest that RTNLBs play an important role in the formation of primary PD and discuss mechanisms by which these proteins may model the ER into desmotubules.     

Plasmodesmata as a Modulator of Osmotic Water Fluxes in Plants

Solutions to some key problems in the relationships between the structure and functions of plasmodesmata, a component of the plant intercellular communication system, are proposed on the basis of the theory of osmotic flows through porous membranes. The theory accounts for structural characteristics of plasmodesmata, such as their dimension, shape, and length. It considers the steric and adsorption potentials of the solution–cell wall interaction and estimates water and solute (e.g., sucrose) flows under the sustained difference of osmotic pressures at the ends of plasmodesmata. The theory predicts that the water flow through plasmodesmata increases with the widening of the neck constriction and reaches its peak when its size is equal to the diameter of the solute molecule. The water-flow direction was found to depend on the opening of the annulus in neck constrictions at negative adsorption potentials of the plasmodesmata channel walls. Taking into account the presence of sphincters in the neck constrictions, our data suggest the role of plasmodesmata as a modulator of osmotic water fluxes in plants.     

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。     

Plasmodesmata form and function

Intercellular transport via plasmodesmata controls cell fate decisions in plants, and is of fundamental importance in viral movement, disease resistance, and the spread of RNAi signals. Although plasmodesmata appear to be unique to plant cells, they may have structural and functional similarities to the newly discovered tunneling nanotubes that connect animal cells. Recently, proteins that localize to plasmodesmata have been identified, and a microtubule-associated protein was found to negatively regulate the trafficking of viral movement proteins. Other advances have delivered new insights into the function and molecular nature of plasmodesmata and have shown that protein trafficking through plasmodesmata is developmentally regulated, opening up the possibility that the genetic control of plasmodesmal function will soon be understood.