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.     

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     

Studies on Plasmodesmata in the Trichomes and Leaf Epidermis of some Asclepiadaceae

The occurrence of plasmodesmata has been studied in the trichomesand leaf epidermis of 12 species of the Asclepiadaceae. Plasmodesmataare observed in the walls of hair cells, epidermal cells, andstomatal apparatus. Plasmodesmata are present between adjacenthair cells from base to the apex; epidermal and basal hair cells;epidermal cells; epidermal and subsidiary cells; subsidiarycells, and subsidiary and guard cells, thus establishing mutualsymplasmatic contacts.     

Plasmodesmal changes are related to different developmental stages of antheridia of Chara species

Summary During the development of the antheridia of Chara species, dynamic changes in the occurrence and ultrastructure of plasmodesmata are observed which are closely correlated to particular developmental phases and presumably regulate the morphogenetic events in the antheridia. The disappearance of plasmodesmata between shield cells and between shied cells and the basal cell leads to a cessation in symplasmic transport around the antheridum and determines its concentric or centrifugal character via centrally situated capitular cells. Unplugged plasmodesmata are present between fully synchronously developing antheridial filament cells and obviously coordinate the development of the cells. In the middle phase of spermiogenesis, rough endoplasmic reticulum in antheridial filaments passes uncompressed through wide plasmodesmata and provides an additional transport pathway for developmental control factors. Plugged plasmodesmata link cells of different types or cells of the same type which are at different phases of cell cycle and guarantee their individual development. The plugging of plasmodesmata is a reversible process that depends on the morphogenetic situation. Plasmodesmata connecting the basal cell and the subbasal cell as well as the basal cell and capitular cells are transformed successively from the simple into the complex type and might be the pathways for an import of gibberellins and nutrients into the strong sink tissues of the developing antheridium. There is a symplasmic connection between the antheridum and the thallus via a basal cell. Prior to the initiation of spermatozoid differentiation (spermiogenesis), plasmodesmata connecting the basal cell with a subbasal cell and the basal cell with capitular cells are spontaneously broken, resulting in symplasmic isolation of the antheridium that is probably a signal which triggers the induction of spermatozoid differentiation. Premature plasmolytically evoked symplasmic isolation of the antheridium leads to the elimination of 1 to 2 cell cycles from the proliferative stage of spermatogenesis. Autoradiographic studies demonstrate that both natural and induced symplasmic isolation drastically decreases the entry of isotopically labeled gibberellic acid into antheridia of Chara species that may be the consequence of the elimination of the hormone's transport through plasmodesmata.Correspondence and reprints: Department of Cytophysiology, University of ód, ulica Pilarskiego 14, 90-231 ód, Poland.Received March 11, 2002; accepted September 19, 2002; published online August 26, 2003     

Parenchyma cells of secondary phloem in Pinus strobus

Summary Parenchyma cells of the secondary phloem in Pinus strobus have all the cellular organelles common in other plant cells. They have mitochondria, endoplasmic reticulum, ribosomes, dictyosomes, and plastids. Parenchyma cells are very conspicuous because of their organic inclusions, starch and lipids. Plasmodesmata in transverse and tangential walls of axial parenchyma cells and in end walls of ray parenchyma cells are regularly distributed and of uniform size, about 500 Å in diameter. In radial walls of axial parenchyma cells and horizontal walls of ray parenchyma cells plasmodesmata are located in primary pit-fields; there they are of variable size and often divided into several branches. The branches are confluent into a median nodule. Perforation of the transverse wall between two axial parenchyma cells and the resultant union of the cellular material of the two connected cells is reported.This research has been supported by NSF Grant GB 3193.     

Subcellular localization determines the availability of non-targeted proteins to plasmodesmatal transport

BACKGROUND: Individual plant cells are encased in a cell wall. To enable cell-to-cell communication, plants have evolved channels, termed plasmodesmata, to span thick walls and interconnect the cytoplasm between adjacent cells. How macromolecules pass through these channels is now beginning to be understood. RESULTS: Using two green fluorescent protein (GFP) reporters and a non-invasive transfection system, we assayed for intercellular macromolecular traffic in leaf epidermal cells. Plasmodesmata were found in different states of dilation. We could distinguish two forms of protein movement across plasmodesmata, non-targeted and targeted. Although leaves have generally been considered closed to non-specific transport of macromolecules, we found that 23% of the cells had plasmodesmatal channels in a dilated state, allowing GFP that was not targeted to plasmodesmata to move into neighboring cells. GFP fusions that were targeted to the cytoskeleton or to the endoplasmic reticulum did not move between cells, whereas those that were localized to the cytoplasm or nucleus diffused to neighboring cells in a size-dependent manner. Superimposed upon this non-specific exchange, proteins that were targeted to the plasmodesmata could transit efficiently between 62% of transfected cells. CONCLUSIONS: A significant population of leaf cells contain plasmodesmata in a dilated state, allowing macromolecular transport between cells. Protein movement potential is regulated by subcellular address and size. These parameters of protein movement illustrate how gradients of signaling macromolecules could be formed and regulated, and suggest that non-cell-autonomous development in plants may be more significant than previously assumed.     

Translocation in the staminal hairs ofSetcreasea purpurea

Summary Investigations into plant intercellular communication were initiated through an examination of plasmodesmata and cell-to-cell passage of molecular probes in the staminal hairs ofSetcreasea purpurea. Plasmodesmata connecting staminal hair cells of small buds are filled with an electron-opaque homogenous material. To examine the permeation selectivity of plasmodesmata, molecular probes made up of fluorescein isothiocyanate (FITC) complexed with amino acids and peptides were injected into the staminal hair cells and the spread of these fluorescent molecules through the symplast, was monitored. Molecules composed of FITC complexed to single amino acids with polar and aliphatic R groups travel rapidly, while those which include peptides travel slowly. Dye molecules composed of an amino acid with an aromatic side group do not pass from cell to cell at all. It is hypothesized that the material occluding the plasmodesmata constitutes the diffusion barrier, by presenting a hydrophilic environment which allows passage of molecules with maximum molecular weights of 700–800 daltons, but which retains those with aromatic side groups.     

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.     

Ultrastructural features of the salt gland of Tamarix aphylla L.

Summary The salt gland in Tamarix is a complex of eight cells composed of two inner, vacuolate, collecting cells and six outer, densely cytoplasmic, secretory cells. The secretory cells are completely enclosed by a cuticular layer except along part of the walls between the collecting cells and the inner secretory cell. This non-cuticularized wall region is termed the transfusion are (Ruhland, 1915) and numerous plasmodesmata connect the inner secretory cells with the collecting cells in this area. Plasmodesmata also connect the collecting cells with the adjacent mesophyll cells.There are numerous mitochondria in the secretory cells and in different glands they show wide variation in form. In some glands wall protuberances extend into the secretory cells forming a labyrinth-like structure; however, in other glands the protuberances are not extensively developed. Numerous small vacuoles are found in some glands and these generally are distributed around the periphery of the secretory cells in association with the wall protuberances. Further, an unusual structure or interfacial apparatus is located along the anticlinal walls of the inner secretory cells. The general structure of the gland including the cuticular encasement, connecting plasmodesmata, interfacial apparatus, and variations in mitochondria, vacuoles, and wall structures are discussed in relation to general glandular function.     

葫芦藓精子发生过程中胞间连丝与胞质桥的超微结构

从超微结构水平上对葫芦藓(Funaria hygrometrica Hedw.)精子发生过程中胞间连接系统的结构及其变化动态进行了研究.结果表明,同一区中的相邻生精细胞由大量胞质桥相连,而不同区的细胞之间则不存在胞质桥.胞间连丝存在于套细胞之间以及套细胞与生精细胞之间,但它在生精细胞间不存在.在精子器发生的后期,当精子细胞壁开始降解时,同一个精子器中所有的精子细胞似乎都由扩大的胞质桥相互连接.胞质桥一直保持到精子分化的后期,最终精子细胞同步分化成精子.胞间连丝与胞质桥具有不同的内部结、分布以及生物发生机制,这表明它们在精子器的发育过程中可能扮演着不同的角色.     

Cell-to-cell communication via plasmodesmata during Arabidopsis embryogenesis

In Arabidopsis embryogenesis, positional information establishes the overall body plan and lineage-dependent cell fate specifies local patterning. Position-dependent gene expression and responses to the plant hormone auxin are also crucial. Recently, another mechanism that delivers positional information has been uncovered. This pathway utilizes cell-to-cell communication via plasmodesmata. Plasmodesmata span the walls between neighboring plant cells. Groups of cells that allow intercellular transport of biotic and abiotic tracers form symplastic domains of shared communication. Initially, cells of the embryo form one symplast. As development proceeds, symplastic sub-domains that correspond to the major morphological regions of the plant (i.e. shoot apex, cotyledons, hypocotyl, and root) are formed. These sub-domains further resolve into tissue-specific domains of communication (such as protodermal and vascular regions). Cell-to-cell communication via plasmodesmata between embryonic and maternal tissues ceases as development proceeds.     

Unusual electron-dense dome associates with compound plasmodesmata in the embryo-suspensor of genus Sedum (Crassulaceae)

Plasmodesmata ensure the continuity of cytoplasm between plant cells and play an important part in the intercellular communication and signal transduction. During the development of the suspensor of both Sedum acre L. and Sedum hispanicum L., changes in the ultrastructure of plasmodesmata and adjoining cytoplasm are observed. Numerous simple plasmodesmata are present in the inner wall of the two-celled embryo separating the basal cell from the apical cell. From the early-globular to the torpedo stage of embryo development, the part of the wall separating the basal cell from the first layer of the chalazal suspensor cells is perforated by unusual, compound plasmodesmata. The role and the sort of transport through these plasmodesmata are discussed.     

Age-related and origin-related control of the numbers of plasmodesmata in cell walls of developing Azolla roots

Plasmodesmata were counted in the longitudinal and transverse walls in developmental sequences of merophytes in roots of Azolla pinnata R.Br. The differences between certain categories of longitudinal wall were traced to factors that govern the surface area of the cell plates, the density of plasmodesmata (number per unit area of cell plate), and the amount by which each type of plate expands. No evidence for secondary augmentation of plasmodesmatal numbers after the cell-plate stage of development was found, but plasmodesmata are lost from the walls of sieve and xylem elements during their differentiation. Losses caused by cell separation occur in other tissues. The relatively high density of plasmodesmata in transverse walls is based not so much on a high density in the cell plates as on the relatively low expansion that these walls undergo. There appears to be a compensatory mechanism that relates initial plasmodesmatal density to the future expansion of the cell plate. The root shows determinate growth, the apical cell dividing about 55 times. Beginning at about the 35th division there is a progressive failure to maintain the plasmodesmatal frequencies that were developed in earlier cell divisions in the apical cell. The divisions that occur within the later-produced merophytes also show progressive diminution of plasmodesmatal numbers. The result is that the apex of the root, and particularly the apical cell, becomes more and more isolated symplastically, a phenomenon which could account for its limited lifespan and the determinate growth pattern of the root.     

PHYSIOLOGICAL IMPLICATIONS OF VICIA FABA AND NICOTIANA TABACUM GUARD-CELL ULTRASTRUCTURE

The guard cells of Vicia faba and Nicotiana tabacum contain numerous mitochondria, elements of endoplasmic reticulum, spherosomes, and peroxisome-like microbodies. A full ribosomal complement appears in young but not in fully mature guard cells. Numerous small lipid droplets external to the plasmalemma were noted in mature Vicia guard cells. Chloroplasts were found in both epidermal and guard cells of both species. Full photosynthetic capacity was indicated by the grana fretwork of guard-cell chloroplasts. A specialized peripheral reticulum was observed in the guard-cell chloroplasts of Vicia. Plasmodesmata were observed in both walls between sister guard cells and between guard and epidermal cells. In the latter case plasmodesmata were found primarily in pit fields of transverse walls. It is postulated that the small volume of guard cells allows them an osmotic advantage over larger neighboring cells in generating turgor.     

SOME ULTRASTRUCTURAL OBSERVATIONS ON ENDODERMAL CELL DEVELOPMENT IN ZEA MAYS ROOTS

The fine structure of primary, secondary, and tertiary stages of Zea endodermal cell development was investigated. The casparian strip formed in situ in the anticlinal walls and remained at a fixed point relative to the endodermis-pericycle boundary. The only protoplasmic structure that had a constant spatial association with the developing strip was the plasmalemma. Plasmodesmata appeared to be more numerous on the tangential walls than on radial walls; only rarely were they located in the casparian strip. The suberized lamella developed on inner and outer tangential walls before it appeared on the radial walls. No cytoplasmic organelles were found to have any particular spatial association with this layer. The suberized lamella was about 0.04 μm thick except near plasmodesmata and along the adaxial margin of the casparian strip, where it was thicker. Occasionally it failed to form along the abaxial margin of the strip. The adherent affinity between plasmalemma and casparian strip was lost after the strip was covered by suberized lamella. The secondary wall became asymmetrically thickened by differential deposition of successive lamellae. A thin layer of secondary wall material extended across the floor of each pit. Pit cavities often contained mitochondria, and plasmodesmata were restricted to the pits. The plasmodesmata were constricted where they entered the thin layer of secondary wall material and where they penetrated the suberized lamella. The various stages of cell development tended to be asynchronous. No passage cells were observed. Endodermal cell development in Zea closely resembles that described for barley.     

Developmental Regulation of Intercellular Protein Trafficking through Plasmodesmata in Tobacco Leaf Epidermis

Plasmodesmata mediate direct cell-to-cell communication in plants. One of their significant features is that primary plasmodesmata formed at the time of cytokinesis often undergo structural modifications, by the de novo addition of cytoplasmic strands across cell walls, to become complex secondary plasmodesmata during plant development. Whether such modifications allow plasmodesmata to gain special transport functions has been an outstanding issue in plant biology. Here we present data showing that the cucumber mosaic virus 3a movement protein (MP):green fluorescent protein (GFP) fusion was not targeted to primary plasmodesmata in the epidermis of young or mature leaves in transgenic tobacco (Nicotiana tabacum) plants constitutively expressing the 3a:GFP fusion gene. Furthermore, the cucumber mosaic virus 3a MP:GFP fusion protein produced in planta by biolistic bombardment of the 3a:GFP fusion gene did not traffic between cells interconnected by primary plasmodesmata in the epidermis of a young leaf. In contrast, the 3a MP:GFP was targeted to complex secondary plasmodesmata and trafficked from cell to cell when a leaf reached a certain developmental stage. These data provide the first experimental evidence, to our knowledge, that primary and complex secondary plasmodesmata have different protein-trafficking functions and suggest that complex secondary plasmodesmata may be formed to traffic specific macromolecules that are important for certain stages of leaf development.     

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。     

A 41 kDa protein isolated from maize mesocotyl cell walls immunolocalizes to plasmodesmata

Summary Plasmodesmata, dynamic pore structures that traverse plant cell walls, function in cytoplasmic transport between contiguous cells. Cell walls containing embedded plasmodesmata were isolated from mesocotyls of etiolated maize seedlings. Proteins associated with the isolated walls were separated by SDS-PAGE and antibodies were generated against a 41 kDa protein, one of several associated with this wall fraction. Immunoblot analysis showed that the 41 kDa polypeptide was also associated with other subcellular fractions obtained following tissue homogenization and differential centrifugation. The wall associated 41 kDa protein is apparently a peripheral membrane protein since it could be extracted by high salt and high pH. Silver-enhanced immunogold light microscopy showed that the 41 kDa protein was associated with the cell walls of cells both in the stele and cortex. The immunolabeling pattern was transwall and punctate. Electron microscopic immuno-gold labeling localized the polypeptide to plasmodesmata and to electron dense cytoplasmic structures that are apparently Golgi membranes. The significance of the presence of this protein in the Golgi is discussed.Abbreviations ER endoplasmic reticulum     

Directional cell-to-cell communication in theArabidopsis root apical meristem III. Plasmodesmata turnover and apoptosis in meristem and root cap cells during four weeks after germination

Summary Plasmodesmata frequency and distribution in root cap cells ofArabidopsis thaliana root tips were characterized during four weeks after germination to understand the symplasmic control of apoptosis. Apoptotic cells in some of the root apical-meristem cells and in root cap cells were identified by the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling reaction and characterized by electron microscopy. Starting at the second week after germination, cells in the outermost layers of the root cap showed typical apoptotic features, including nuclear DNA fragmentation, chromatin condensation, cytoplasmic vacuolation, and organelle destruction. Intercellular connections, indicated by the frequency and number of plasmodesmata per cell length, were significantly reduced in the walls of outer root cap cells. This shows that cells become symplasmically isolated during the apoptosis process. In apoptotic root cap cells, the majority of nonfunctional plasmodesmata were observed to be associated with degenerated endoplasmic reticulum; this state was prior to the detection of any nuclear DNA fragmentation. Other nonfunctional plasmodesmata were sealed by heterogeneous cell wall materials. However, in immature epidermal and cortical cells in 4-week-old arrested roots the endoplasmic reticulum associated with plasmodesmata became disconnected as a result of protoplast condensation and shrinkage. No degenerated endoplasmic reticulum was observed in these cells. These observations suggest that the apoptotic processes in the root body and the root cap are different.     

THE FINE STRUCTURE OF THE TRIGGER HAIR IN VENUS'S FLYTRAP

Trigger hairs of Dionaea muscipula fixed in glutaraldehyde and OsO₄ were prepared for study in the electron microscope. Electron micrographs of the active zone of the trigger hair reveal three regions in which the cells differ in size, shape, and cytoplasmic content. Each region contains large numbers of protein bodies and mitochondria with densely packed tubular cristae. Vacuole-like structures containing protein bodies or an anastomosing system of cisternae, or occasionally both, are also present. Found only in the indentation cells is a complex, whorled endoplasmic reticulum. A concentric lamellar arrangement of the endoplasmic reticulum around the vacuolar structures is often observed. The lateral walls of the indentation cells are disproportionately thick while end walls are thin. The basal walls of these cells contain many plasmodesmata. Plasmodesmata in the anticlinal and podium cells pass through constricted zones in the cell wall and are particularly numerous in the peripheral podium cells. The possible functional significance of these structures is discussed.