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。     

The ultrastructure of plasmodesmata in the filamentous green alga,Bulbochaete hiloensis (Nordst.) tiffany

Summary The ultrastructure of the plasmodesmata found in the green alga Bulbochaete hiloensis has been examined by electron microscopy of ultra-thin sections. Unlike most other plasmodesmata that have been described recently, there are no internal components such as a desmotubule or a derivative of the endoplasmic reticulum. Each plasmodesma consists of a cylindrical connection between the plasma membranes of adjacent cells. The cylinder is constricted at each end to orifices which may be less than 100 Å in diameter. Within the cylinder the cytoplasmic face of the plasma membrane is lined with material probably consisting of helically arranged particles. The lumen here is 400–450 Å in diameter.The observations are discussed in relation to possible functions in intercellular transport.     

Plasmodesmal widening accompanies the short-term increase in symplasmic phloem unloading in pea root tips under osmotic stress

Summary Root tips ofPisum sativum seedlings were exposed to 350 mM mannitol, which was shown to effect a transient but dramatic increase in phloem unloading, and investigated by electron microscopy. After chemical fixation and embedding, extremely thin sections of the root extension zone were examined. Outer, inner, and desmotubule diameters of 830 primary plasmodesmata in transverse walls of cortical cells were measured. Statistical analysis indicated that the majority of plasmodesmata had no neck constriction during osmoregulation. Compared to controls, a highly significant increase in mean plasmodesmata diameter was found, but the desmotubule diameter remained unchanged. Both loss of neck constriction and widening of the cytoplasmic sleeve indicate an increase in effective passage area of plasmodesmata. Spokes between plasma membrane and desmotubule were preserved. Continued exposure of the root tips to mannitol led to a return to control values for plasmodesmal diameters. In contrast to these responses, plasmolysis of cortical cells by 1,000 mM sucrose, diminishing phloem unloading, was accompanied by a reduction in those plasmodesmata classified as open. This is the first report showing a correlation between the ultrastructure of plasmodesmata and the rate of symplasmic transport. The role of the different plasmodesmal components in controlling the passage area of symplasmic transport is discussed.     

Substructure of freeze-substituted plasmodesmata

Summary The substructure of plasmodesmata in freeze-substituted tissues of developing leaves of the tobacco plant (Nicotiana tabacum L. var. Maryland Mammoth) was studied by high resolution electron microscopy and computer image enhancement techniques. Both the desmotubule wall and the inner leaflet of the plasmodesmatal plasma membrane are composed of regularly spaced electron-dense particles approximately 3 nm in diameter, presumably proteinaceous and embedded in lipid. The central rod of the desmotubule is also particulate. In plasmodesmata with central cavities, spoke-like extensions are present between the desmotubule and the plasma membrane in the central cavity region. The space between the desmotubule and the plasma membrane appears to be the major pathway for intercellular transport through plasmodesmata. This pathway may be tortuous and its dimensions could be regulated by interactions between desmotubule and plasma membrane particles.Abbreviations ER endoplasmic reticulum - PJF propane jet freezing - HPF high pressure freezing - CRT cathode ray tube - IP₃ inositoltrisphosphate     

On the physics of CNS memory

The effects of geometry, type of fluid and properties of the desmotubule membrane on the fluid transport in plasmodesma are discussed from a hydrodynamics viewpoint. It is shown that the “necking” of the ends of plasmodesma has a profound effect on the volume flow rates reducing them by several orders of magnitude. Most of the pressure drop occurs across the “neck” regions. There appears to be little significant difference in the volume flow rates if we consider a Newtonian or powerlaw fluid or if we allow the desmotubule membrane to be permeable or slightly flexible, at least in comparison to the dominating feature of plasmodesma geometry.     

A new interpretation of plasmodesmatal ultrastructure

Summary It is shown that simple, unbranched, plasmodesmata between young xylem ray cells of willow have no direct intercellular continuity apart from the plasmalemma which limits the cytoplasm and lines the plasmodesmatal canal. Each plasmodesma is traversed by a 200 Å diameter tubule (the desmotubule) which has a wall with probably 11 subunits arranged around a central cavity through which runs a 40 Å diameter rod. This rod is connected to the inside of the tubule wall, by fine filaments. At the ends of each plasmodesma the plasmalemma and cell wall are closely appressed to the tubule, thus precluding direct continuity between the cytoplasm of adjacent cells. Through the central part of the plasmodesmata the tubule is separated from the plasmalemma by a 90–100 Å wide gap. Cytoplasmic microtubules in the same tissue have a diameter of approximately 250 Å and a wall probably composed of 13 subunits: both desmotubules and cytoplasmic microtubules therefore have a centre-to-centre subunit spacing of about 47 Å. It is suggested that the desmotubules are not microtubules but may be nuclear spindle fibres which become trapped in the wall during cell plate formation. The endoplasmic reticulum, while closely approaching the plasmodesmata, is not continuous across them. It is thought most unlikely that the endoplasmic reticulum traverses plasmodesmata, as the dimensions of the central tubule — found here as well as by other workers — are smaller than those which would be expected to allow a stable molecular configuration in a unit membrane. The plasmalemma, where it lines the plasmodesmatal canal, appears to have particulate subunits in the outer opaque layers and the presence of these subunits may be attributable to the need for stability in membranes arranged about so small a radius.     

Distribution and structure of the plasmodesmata in mesophyll and bundle-sheath cells of Zea mays L.

In leaf blades of Zea mays L. plasmodesmata between mesophyll cells are aggregated in numerous thickened portions of the walls. The plasmodesmata are unbranched and all are characterized by the presence of electron-dense structures, called sphincters by us, near both ends of the plasmodesmatal canal. The sphincters surround the desmotubule and occlude the cytoplasmic annulus where they occur. Plasmodesmata between mesophyll and bundle-sheath cells are aggregated in primary pit-fields and are constricted by a wide suberin lamella on the sheath-cell side of the wall. Each plasmodesma contains a sphincter on the mesophyll-cell side of the wall. The outer tangential and radial walls of the sheath cells exhibit a continuous suberin lamella. However, on the inner tangential wall only the sites of plasmodesmatal aggregates are consistently suberized. Apparently the movement of photosynthetic intermediates between mesophyll and sheath cells is restricted largely or entirely to the plasmodesmata (symplastic pathway) and transpirational water movement to the cell walls (apoplastic pathway).Abbreviation ER endoplasmic reticulum     

Absence of plasma membrane H+-ATPase in plasmodesmata located in pit-fields of the young reactive pulvinus ofMimosa pudica L.

Summary Immunocytochemical techniques were employed to study the spatial distribution of the plasma membrane H⁺-ATPase within various cell types of the young reactive primary pulvinus ofMimosa pudica L. These cells were interconnected by large numbers of plasmodesmata, being concentrated within pit-fields. Although we could routinely detect evidence of the H⁺-ATPase along the plasma membrane, immunolabelling was rarely, if ever, observed along the plasma membranes of the plasmodesmata. This finding is discussed with respect to the likely specialized supramolecular structure of the plasmodesma.Abbreviations SEL size exclusion limit of plasmodesmata     

Filament Disruption in Funaria Protonemata: Occlusion of Plasmodesmata

Plasmodesmata are occluded when Funaria chloronemata are fragmented by the development of tmema cells (TCs). The TC deposits a new wall layer along the cross wall toward the neighbouring non-sister cell (NC). This wall layer cuts off the plasmodesmata and its connection with the cross wall is soon lost. The plasmodesmata become isolated when the NC forms a new wall layer along the former cross wall. At the end of TC development, before its disintegration, the sister cell (SC) also deposits a new wall layer along the cross wall toward the TC, cutting off the plasmodesmata. For some time the plasmalemma of the plasmodesmata remains connected to the NC or the TC, whereas the desmotubule soon disappears. Relicts of the plasmalemma remain even after the isolation of the plasmodesmata and the disintegration of the TC. During the decay of the plasmodesmata, a cylinder of electron-dense material is frequently formed along the border of the plasmodesmatal channel. This may extend over the surface of the cell wall. Eventually, the plasmodesmatal channel is filled with wall material. Callose is only observed around functional plasmodesmata and does not seem to play a role in their occlusion.     

Fine structure of plasmodesmata in mature leaves of sugarcane

The fine structure of plasmodesmata in vascular bundles and contiguous tissues of mature leaf blades of sugarcane (Saccharum interspecific hybrid L62–96) was studied with the transmission electron microscope. Tissues were fixed in glutaraldehyde, with and without the addition of tannic acid, and postfixed in OsO₄. The results indicate that the fine structure of plasmodesmata in sugarcane differs among various cell combinations in a cell-specific manner, but that three basic structural variations can be recognized among plasmodesmata in the mature leaf: 1) Plasmodesmata between mesophyll cells. These plasmodesmata possess amorphous, electron-opaque structures, termed sphincters, that extend from plasma membrane to desmotubule near the orifices of the plasmodesmata. The cytoplasmic sleeve is filled by the sphincters where they occur; elsewhere it is open and entirely free of particulate or spokelike components. The desmotubule is tightly constricted and has no lumen within the sphincters, but between the sphincters it is a convoluted tubule with an open lumen. 2) Plasmodesmata that traverse the walls of chlorenchymatous bundle-sheath cells and mestome-sheath cells. In addition to the presence of sphincters, these plasmodesmata are modified by the presence of suberin lamellae in the walls. Although the plasmodesmata are quite narrow and the lumens of the desmotubules are constricted where they traverse the suberin lamellae, the cytoplasmic sleeves are still discernible and appear to contain substructural components there. 3) Plasmodesmata between parenchymatous cells of the vascular bundles. These plasmodesmata strongly resemble those found in the roots of Azolla, in that their desmotubules are closed for their entire length and their cytoplasmic sleeves appear to contain substructural components for their entire length. The structural variations exhibited by the plasmodesmata of the sugarcane leaf are compared with those proposed for a widely-adopted model of plasmodesmatal structure.Abbreviation ER endoplasmic reticulum This study was supported by National Science Foundation grants DCB 87-01116 and DCB 90-01759 to R.F.E. and a University of Wisconsin-Madison Dean's Fellowship to K. R.-B. We also thank Claudia Lipke and Kandis Elliot for photographic and artistic assistance, respectively.     

Intercellular communication inAzolla roots: I. Ultrastructure of plasmodesmata

Summary A model is proposed for the structure of the plasmodesmata ofAzolla root primordia, based on micrographs obtained by a combination of fixation in glutaraldehyde/p-formaldehyde/tannic acid/ferric chloride, digestion of cell walls and the use of stereo pairs. Unlike the model for plasmodesmatal structure proposed byRobards (1971), the desmotubule is depicted as a virtually closed cylindrical bilayer providing little or no open pathway for transport. In this respect it is similar to the model ofLópez-Sáez et al. (1966). An analysis of the molecular packing of types of lipids found in endoplasmic reticulum (of which the desmotubule is an extension) indicates that the model is geometrically feasible. Details cannot be discerned with accuracy, but material, possibly particulate, occupies much of the space between desmotubule and plasma membrane, the cytoplasmic lumen being reduced to inter-particle spaces of cross-sectional area comparable to that of the bore in a gap junction connexon. Implications for intercellular transport are discussed.     

Cell-to-cell transport via the lumen of the endoplasmic reticulum

Plasmodesmata are plasma membrane‐lined channels through which cytoplasmic molecules move from cell‐to‐cell in plants. Most plasmodesmata contain a desmotubule, a central tube of endoplasmic reticulum (ER), that connects the ER of adjacent cells. Here we demonstrate that molecules of up to 10.4 kDa in size can move between the ER lumen of neighbouring leaf trichome or epidermal cells via the desmotubule lumen. Fluorescent molecules of up to 10 kDa, microinjected into the ER of Nicotiana trichome cells, consistently moved into the ER and nuclei of neighbouring trichome cells. This movement occurred more rapidly than movement via the cytoplasmic pathway. A fluorescent 3‐kDa dextran microinjected into the ER of a basal trichome cell moved into the ER and nuclei of epidermal cells across a barrier to cytoplasmic movement. We constructed a 10.4‐kDa recombinant ER‐lumenal reporter protein (LRP) from a fragment of the endogenous ER‐lumenal binding protein AtBIP1. Following transient expression of the LRP in the ER of Tradescantia leaf epidermal cells, it often moved into the nuclear envelopes of neighbouring cells. However, green fluorescent protein targeted to the ER lumen (ER‐GFP) did not move from cell to cell. We propose that the ER lumen of plant cells is continuous with that of their neighbours, and allows movement of small ER‐lumenal molecules between cells.     

Ultrastructural study of plasmodesmata in the brown alga Dictyota dichotoma (Dictyotales, Phaeophyceae)

Plasmodesmata are intercellular bridges that directly connect the cytoplasm of neighboring cells and play a crucial role in cell-to-cell communication and cell development in multicellular plants. Although brown algae (Phaeophyceae, Heterokontophyta) are phylogenetically distant to land plants, they nevertheless possess a complex multicellular organization that includes plasmodesmata. In this study, the ultrastructure and formation of plasmodesmata in the brown alga Dictyota dichotoma were studied using transmission electron microscopy and electron tomography with rapid freezing and freeze substitution. D. dichotoma possesses plasma membrane-lined, simple plasmodesmata without internal endoplasmic reticulum (desmotubule). This structure differs from those in land plants. Plasmodesmata were clustered in regions with thin cell walls and formed pit fields. Fine proteinaceous "internal bridges" were observed in the cavity. Ultrastructural observations of cytokinesis in D. dichotoma showed that plasmodesmata formation began at an early stage of cell division with the formation of tubular pre-plasmodesmata within membranous sacs of the cytokinetic diaphragm. Clusters of pre-plasmodesmata formed the future pit field. As cytokinesis proceeded, electron-dense material extended from the outer surface of the mid region of the pre-plasmodesmata and finally formed the nascent cell wall. From these results, we suggest that pre-plasmodesmata are associated with cell wall development during cytokinesis in D. dichotoma.     

The ER-Membrane Transport System Is Critical for Intercellular Trafficking of the NSm Movement Protein and Tomato Spotted Wilt Tospovirus

Plant viruses move through plasmodesmata to infect new cells. The plant endoplasmic reticulum (ER) is interconnected among cells via the ER desmotubule in the plasmodesma across the cell wall, forming a continuous ER network throughout the entire plant. This ER continuity is unique to plants and has been postulated to serve as a platform for the intercellular trafficking of macromolecules. In the present study, the contribution of the plant ER membrane transport system to the intercellular trafficking of the NSm movement protein and Tomato spotted wilt tospovirus (TSWV) is investigated. We showed that TSWV NSm is physically associated with the ER membrane in Nicotiana benthamiana plants. An NSm-GFP fusion protein transiently expressed in single leaf cells was trafficked into neighboring cells. Mutations in NSm that impaired its association with the ER or caused its mis-localization to other subcellular sites inhibited cell-to-cell trafficking. Pharmacological disruption of the ER network severely inhibited NSm-GFP trafficking but not GFP diffusion. In the Arabidopsis thaliana mutant rhd3 with an impaired ER network, NSm-GFP trafficking was significantly reduced, whereas GFP diffusion was not affected. We also showed that the ER-to-Golgi secretion pathway and the cytoskeleton transport systems were not involved in the intercellular trafficking of TSWV NSm. Importantly, TSWV cell-to-cell spread was delayed in the ER-defective rhd3 mutant, and this reduced viral infection was not due to reduced replication. On the basis of robust biochemical, cellular and genetic analysis, we established that the ER membrane transport system serves as an important direct route for intercellular trafficking of NSm and TSWV.     

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     

Ultrastructural Changes of Plasmodesmata in Staminal Hairs of Setcreasea purpurea During Development

Structural changes of plasmodesmata occurred in along with growth, development and senescence of staminal hair cells of Setcreasea purpurea. The plasmodesmata in the staminal hair cells of buds and open flowers were normal having a diameter of 50 mm. Those of senescent flowers became enlarged and underwent modification, such as the appressed endoplasmic reticulum disintegrated and the cell wall around the plasmodesmata degraded, so that it formed a channel with such as a diameter of 100 nm, twice or threefold as that of normal plasmodesmata. In the process of plasmodesma enlargement and modification, a series of changes occurred in the organelles.     

The Ultrastructure and Computer-enhanced Digital Image Analysis of Plasmodesmata at the Kranz Mesophyll-Bundle Sheath Interface of Themeda triandra var. imberbis (Retz) A. Camus in Conventionally-fixed Blades

The ultrastructure of the plasmodesmata at the Kranz mesophyll-bundlesheath (KMS-BS) interface in Themeda triandra, and the substructureswithin the plasmodesmata were investigated, using conventionallyfixed leaf-blade material, enhanced by the addition of 0·1%tannic acid to the primary fixative. Examination of high-resolution electronmicrographs, and computer-enhanceddigital images suggests that these plasmodesmata are complexstructures, comprised of helically-arranged particulate material.The electron-dense particles are between 2·5 and 3·0nm in diameter. These particles are specifically associatedwith the inner face of the inner plasmalemma membrane leaflet,and the outer region of the desmotubule wall. The electron-denseparticles are presumably proteinaceous and embedded in a lipidmatrix. In the constricted median portion of the KMS-BS plasmodesmata,the space between the desmotubule and the inner plasmalemmamembrane leaflet and areas surrounding the proteinaceous particlesthereof (the cytoplasmic sleeve) is about 3 nm in cross-section,and constitutes what we believe to be the space through whichintercellular transport takes place.Copyright 1993, 1999 AcademicPress Themeda triandra, Poaceae, cytoplasmic sleeve, image analysis, plasmodesmatal structure, desmotubule, Kranz mesophyll     

Ultrastructural analysis of leaf trichome plasmodesmata reveals major differences from mesophyll plasmodesmata

Functional studies on molecular transport through plasmodesmata in leaf mesophyll and trichome cells revealed significant differences in their basal size-exclusion limits and their response to microinjected tobacco mosaic virus movement protein (E. Waigmann et al., 1994, Proc. Natl. Acad. Sci. USA 91: 1433–1437; E. Waigmann and P. Zambryski, 1995, Plant Cell 7: 2069–2079). To address the basis for these functional differences, Nicotiana clevelandii trichome and mesophyll plasmodesmata were compared ultrastructurally. Trichome plasmodesmata increase in ultrastructural complexity from the tip to the base cell. Their neck regions, thought to control molecular traffic through plasmodesmata, are clearly distinct from necks of mesophyll plasmodesmata. In contrast to the electron-dense desmotubular area in mesophyll plasmodesmata, trichome plasmodesmata contain an electron-translucent circle in their center, surrounded by an electron-dense ring. This latter ring is connected to the inner leaflet of the plasma membrane by multiple spokes or filaments. Two monoclonal antibodies raised against a maize plasmodesmal protein preparation (A. Turner et al., 1994, J Cell Sci. 107: 3351–3361) interact with both trichome and mesophyll N. clevelandii plasmodesmata. Based on the localization pattern and the high degree of cross-reactivity, both antibodies likely recognize a conserved structural component of plasmodesmata, and may be useful to mark plasmodesma in a variety of plants and tissues. Received: 24 January 1997 / Accepted: 3 March 1997     

Plasmodesmata: composition,structure and trafficking

Plasmodesmata are highly specialized gatable trans-wall channels that interconnect contiguous cells and function in direct cytoplasm-to-cytoplasm intercellular transport. Computer-enhanced digital imaging analysis of electron micrographs of plasmodesmata has provided new information on plasmodesmatal fine structure. It is now becoming clear that plasmodesmata are dynamic quasi-organelles whose conductivity can be regulated by environmental and developmental signals. New findings suggest that signalling mechanisms exist which allow the plasmodesmatal pore to dilate to allow macromolecular transport. Plant viruses spread from cell to cell via plasmodesmata. Two distinct movement mechanisms have been elucidated. One movement mechanism involves the movement of the complete virus particle along virus-induced tubular structures within a modified plasmodesma. Apparently two virus-coded movement proteins are involved. A second movement mechanism involves the movement of a non-virion form through existing plasmodesmata. In this mechanism, the viral movement protein causes a rapid dilation of existing plasmodesmata to facilitate protein and nucleic acid movement. Techniques for the isolation of plasmodesmata have been developed and information on plasmodesma-associated proteins is now becoming available. New evidence is reviewed which suggests that plasmodesmatal composition and regulation may differ in different cells and tissues.     

Cell-to-cell communication via plant endomembranes

Cell-to-cell communication was investigated in epidermal cells cut from stem internodal tissue of Nicotiana tabacum and Torenia fournieri. Fluorescently labelled peptides and dextrans were microinjected using iontophoresis into the cytoplasm andcortical endomembrane network of these cells. The microinjected endomembrane network was similar in location and structure to the endoplasmic reticulum (ER) as revealed by staining with 3, 3'-dihexyloxacarbocyanine iodide (DiOC(6)). No cell-to-cell movement of dextrans was observed following cytoplasmic injections but injection of dextrans into the endomembrane network resulted in rapid diffusion of the probes to neighbouring cells. It is proposed that the ER acts as a pathway for intercellular communication via the desmotubule through plasmodesmata.