Simple Plasmodesmata Research Articles

Trentepohlia brevicellulis comb. et stat. nov. (Trentepohliaceae, Ulvophyceae) found in Japan

SUMMARYThe family Trentepohliaceae contains many common aerial algae, most widely distributed, particularly in tropical and subtropical regions. Eight species of the genus Trentepohlia have been reported in Japan. We report another taxon, Trentepohlia brevicellulis (Cribb) S. Handa and H. Tsubota comb. et stat. nov., growing on bark in western Honshu, Japan. This alga was previously described as Trentepohlia bosseae var. brevicellulis Cribb. However, the relationship between the bosseae and brevicellulis varieties of T. bosseae could not be ascertained owing to the lack of genetic information for brevicellulis. Therefore, we aimed to examine the detailed morphological characteristics of brevicellulis in wild samples and cultured strains and to review the taxonomy, including systematic information obtained by genetic analysis. Its phylogenetic position was resolved based on 18S rDNA sequence data. It differs from T. bosseae and its variety samoensis in exhibiting a warty zoosporangium surface, an unthickened septum with no central pit field, and a non‐rusty‐brown cell wall. The four key characteristics of T. brevicellulis include a warty layer on the inner surface of the non‐bent side of the suffultory cell wall, an ostiole located near the base of the zoosporangium, curved tapering filaments with thickened cell walls, and globular vegetative spores in the basal portion of the filament. Transmission electron microscopy revealed a pit field wherein simple plasmodesmata were present in the septum. Phylogenetic analysis suggested that this alga and T. bosseae form distinct clades and that T. brevicellulis is separate from T. bosseae. Thus, our study helped redefine the accurate taxonomic position of the species under study as Trentepohlia brevicellulis (Cribb) S. Handa and H. Tsubota comb. et stat. nov., a clade distinct from T. bosseae and reported for the first time in Japan.

Development of Embryo Suspensors for Five Genera of Crassulaceae with Special Emphasis on Plasmodesmata Distribution and Ultrastructure.

The suspensor in the majority of angiosperms is an evolutionally conserved embryonic structure functioning as a conduit that connects ovule tissues with the embryo proper for nutrients and growth factors flux. This is the first study serving the purpose of investigating the correlation between suspensor types and plasmodesmata (PD), by the ultrastructure of this organ in respect of its full development. The special attention is paid to PD in representatives of Crassulaceae genera: Sedum, Aeonium, Monanthes, Aichryson and Echeveria. The contribution of the suspensor in transporting nutrients to the embryo was confirmed by the basal cell structure of the suspensor which produced, on the micropylar side of all genera investigated, a branched haustorium protruding into the surrounding ovular tissue and with wall ingrowths typically associated with cell transfer. The cytoplasm of the basal cell was rich in endoplasmic reticulum, mitochondria, dictyosomes, specialized plastids, microtubules, microbodies and lipid droplets. The basal cell sustained a symplasmic connection with endosperm and neighboring suspensor cells. Our results indicated the dependence of PD ultrastructure on the type of suspensor development: (i) simple PD are assigned to an uniseriate filamentous suspensor and (ii) PD with an electron-dense material are formed in a multiseriate suspensor. The occurrence of only one or both types of PD seems to be specific for the species but not for the genus. Indeed, in the two tested species of Sedum (with the distinct uniseriate/multiseriate suspensors), a diversity in the structure of PD depends on the developmental pattern of the suspensor. In all other genera (with the multiseriate type of development of the suspensor), the one type of electron-dense PD was observed.

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.

Plasmodesmata during development: re-examination of the importance of primary, secondary, and branched plasmodesmata structure versus function.

Plasmodesmata (PD) structure and function vary temporally and spatially during all stages of plant development. PD that originate during, or post, cell division are designated as primary or secondary according to classical terminology. PD structure may be simple, twinned, or branched. Studies of PD during leaf, root, and embryo development have lead to the generalization that cells in less mature tissues contain predominantly simple PD. New quantitative analyses reveal that twinned and branched PD also occur in immature tissues. New data also highlight the versatility of viral movement proteins as tags for labeling PD in immature tissues as well as PD in mature tissues. A summary of the formation and function of primary, secondary, and branched PD during leaf, trichome, embryo, apical meristem, vascular cambium, and root development underscores the remarkable and indispensible plant-specific intercellular communication system that is mediated by PD.

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.

Peeking into Pit Fields: A Multiple Twinning Model of Secondary Plasmodesmata Formation in Tobacco

In higher plants, plasmodesmata (PD) are major conduits for cell-cell communication. Primary PD are laid down at cytokinesis, while secondary PD arise during wall extension. During leaf development, the basal cell walls of trichomes extend radially without division, providing a convenient system for studying the origin of secondary PD. We devised a simple freeze-fracture protocol for examining large numbers of PD in surface view. In the postcytokinetic wall, simple PD were distributed randomly. As the wall extended, PD became twinned at the cell periphery. Additional secondary pores were inserted at right angles to these, giving rise to pit fields composed of several paired PD. During wall extension, the number of PD increased fivefold due to the insertion of secondary PD. Our data are consistent with a model in which a subset of the original primary PD pores function as templates for the insertion of new secondary PD, spatially fixing the position of future pit fields. Many of the new PD shared the same wall collar as the original PD pore, suggesting that new PD pores may arise by fissions of existing PD progenitors. Different models of secondary PD formation are discussed. Our data are supported by a computational model, Plasmodesmap, which accurately simulates the formation of radial pit fields during cell wall extension based on the occurrence of multiple PD twinning events in the cell wall. The model predicts PD distributions with striking resemblance to those seen on fractured wall faces.

INCREASED SIZE EXCLUSION LIMIT 2 encodes a putative DEVH box RNA helicase involved in plasmodesmata function during Arabidopsis embryogenesis.

Here, we characterize the Arabidopsis thaliana embryo-defective mutant increased size exclusion limit2 (ise2). In contrast with wild-type embryos, ise2 mutants continue to traffic 10-kD fluorescent dextran in the mid-torpedo stage of development. ise2 embryos contain branched as well as simple plasmodesmata (PD) compared with wild-type embryos, which only contain simple PD. Positional cloning reveals that the ISE2 gene encodes a putative DEVH box RNA helicase that shares sequence homology with RNA helicases involved in RNA degradation pathways in other organisms. ISE2 localizes to granule-like structures in the cytoplasm. These granules increase in number when plant cells are stressed. These features are characteristic of stress granules (SGs) in mammalian cells, suggesting that ISE2 granules represent plant-specific SGs. Genetic data demonstrate that the ISE2 helicase is involved in posttranscriptional gene silencing and the determination of cell fate. These data together suggest that ISE2 function affects PD structure and function through the regulation of RNA metabolism and consequent gene expression.

Intracellular Trafficking of Potato Leafroll VirusMovement Protein in Transgenic Arabidopsis

Intracellular trafficking of viral movement proteins (MPs) in plants has mainly been studied using Tobacco mosaic virus MP30 (TMV MP30) as a model system. Because of the limitations of TMV MP30 expression in Arabidopsis thaliana, these studies have mostly been restricted to tobacco plants. Here we present data on the analysis of transgenic Arabidopsis plants expressing Potato leafroll virus 17-kDa movement protein (MP17) fused to green fluorescent protein. MP17 localizes to secondary branched plasmodesmata (PD) in source but not to simple PD in sink tissues, where MP17 is believed to be degraded by proteolysis. To unravel the intracellular transport path of MP17, we analyzed the relevance of the cytoskeleton and of the secretory pathway on MP17 targeting. To this end, a new incubation system for in vivo analysis of immediate and long-term responses of whole Arabidopsis plants to inhibitor treatments was established. Microscopic and histochemical analysis showed that MP17 is targeted to PD in an actin- and endoplasmic reticulum-Golgi-dependent manner. In contrast, degradation of MP17 in sink tissues required intact microtubules and occurred at 26S proteasomes. Interestingly, inhibition of the 26S proteasome led to aggregation of MP17 in aggresome-like structures. Formation of these structures could be inhibited by colchicine, as was shown for aggresomes in mammalian cells.

Plasmodesmata and the control of symplastic transport

In 1879 Eduard Tangle discovered cytoplasmic connections between cells in the cotyledons of Strychnos nuxvomica , which he interpreted to be protoplasmic contacts. This led him to hypothesize that ‘the protoplasmic bodies . . . are united by thin strands passing through connecting ducts in the walls, which put the cells into connection with each other and so unite them to an entity of higher order’ (Carr 1976). This challenged the then current view that cells functioned as autonomous units. It was after much research in many other species and cell types that Strasburger, in 1901, named these structures plasmodesmata (Carr 1976). During the division and differentiation of meristematic cells, plasmodesmata are formed across each developing cell plate, allowing cytoplasmic and endomembrane continuity to occur between all daughter cells, and ultimately, between all cells in a developing tissue (Mezitt & Lucas 1996). Those plasmodesmata that form during cell division are termed primary plasmodesmata (Jones 1976). Those that form de novo across existing cell walls are called secondary plasmodesmata (Ehlers & Kollmann 2001). The formation of secondary plasmodesmata allows cells to increase their potential for molecular trafficking and allows connections to be created between cells that are not related cytokinetically. As cells expand and differentiate, their fate determines the extent to which their cytoplasmic connectivity to other cells is maintained (Mezitt & Lucas 1996). Some cell types, such as those of the leaf mesophyll, remain closely connected to their neighbours, and may even lay down additional plasmodesmata to increase the continuity (Ding et al . 1992a). In other areas of the plant, for instance in vascular tissue, certain cells greatly reduce the number of plasmodesmata in their adjoining walls (Gamalei 1989). In this way, the cytoplasmic continuity can be altered depending on the tissue type (Botha & Evert 1988; Brown et al . 1995). However, although reductions in the number of plasmodesmata are common, only guard cells surrounding stomata (Erwee, Goodwin & van Bel 1985; Palevitz & Hepler 1985) and differentiating xylem elements (Lachaud & Maurousset 1996) lose all symplastic connections at maturity. In all other cells, some degree of intercellular connection is maintained. This plasmodesmal continuum that potentially exists throughout the whole plant is termed the symplast (Munch 1930). However, the symplast is not the open continuum that Munch originally hypothesized, but is divided into functional domains, each tightly regulated by different forms of plasmodesmata (Erwee & Goodwin 1985; Ehlers & Kollmann 2001). Plasmodesmata are now thought of as fluid, dynamic structures that can be modified both structurally and functionally to cope with the requirements of specific cells and tissues.

OBSERVATIONS ON THE HABIT, MORPHOLOGY AND ULTRASTRUCTURE OF CEPHALEUROS PARASITICUS (CHLOROPHYTA) AND A COMPARISON WITH C. VIRESCENS1

ABSTRACT Cephaleuros parasiticus Karsten, an endophyte of Magnolia grandiflora L. has been examined with light, and scanning and transmission electron microscopy. The discoid thalli are composed of filaments which ramify throughout the leaf tissues beneath the epidermis. Algal filaments do not penetrate host cells, but do produce black leaf spots which have been mistaken for those caused by the fungus Glomerella cingulate (Ston.) Spauld. and Schrenk. Two distinct thallus types occur, often simultaneously on a single leaf. One bears clusters of zoosporangiate branches which seasonally emerge through the ventral (and rarely, dorsal) surface of the leaf. In contrast, the other thallus type bears gametangia which break through the dorsal leaf surface. Zoosporangia and gametangia have never been found on the same thallus. The zoosporangia are smaller than, but almost identical in shape to, those of C. virescens Kunze. Simple plasmodesmata are present in crosswalls and acetolysis indicates that little or no sporopollenin is present in the cell walls. The ultrastructure of biflagellate gametes and quadriflagellate zoospores is virtually indistinguishable from that reported for C. virescens and similar to that reported for Phycopeltis and Trentepohlia. In both gametes and zoospores there are keeled flagella, overlapping and parallel basal bodies, two 3‐layered multilayered structures with microtubular splines, and two medial compound microtubular roots. Pyrenoids, eyespots, flagellar and body scales, striated roots (or rhizoplasts), and distal bands are absent. Two presumptive mating structures are present in each biflagellate gamete, and flagellar collars occur in both types of motile cells. The extreme similarity in motile cell ultrastructure revealed in this interspecific comparison parallels that similarity revealed in intergeneric comparisons.

The neck constriction in plasmodesmata

Simple plasmodesmata between mesophyll and bundle sheath cells in actively expanding leaves of Salsola kali L. and roots of Epilobium hirsutum L. are shown to possess specialized structures, called sphincters, around their neck regions. The sphineters are made visible by the combined effects of tannic acid and heavy metal staining; they are localized just outside that area of the plasmalemma, which forms the collar around the entrance to each plasmodesmos. This localization corresponds to a very active area of the plasmodesmos/olasmalemma complex (i.e. enzyme activity and/or presence of strongly reducing substances).Evidence is presented that these ring structures are structural equivalents to hypothetical sphincters performing some valve function; i.e. participating in the control of rates and directions of symplastic transport of solutes through plasmodesmata. The middle layer of the plasmalemma in the neck region is composed of closely-packed, globular subunits appearing in negative contrast. Apparently, these subunits correspond to particle clusters observed at the plasmodesmatal entrance in freeze-fracture preparations. They appear similar to particle clusters in animal tight junctions, and their possible function in providing electrical coupling via low resistance junctions between plant cells is discussed.

Ultrastructure of plasmodesmata and cross walls inCephaleuros, PhycopeltisandTrentepohlia(Chroolepidaceae; Chlorophyta)

Cross walls in Cephaleuros, Phycopeltis and Trentepohlia contain simple plasmodesmata approximately 40–50 nm in diameter. In all three genera, the plasmodesmata generally occur in only the central portion of the cross wall, thus forming a “central pit”. In the cross walls between vegetative cells of Phycopeltis, a ring of lightly stained material within a thickened border encircles the central pit. The cross walls between vegetative cells in Cephaleuros and Trentepohlia specimens examined thus far clearly lack the distinct arrangement characteristic of Phycopeltis; however, the cross walls between the zoosporangium and pedicel (i.e. the zoosporangium-pedicel septa) in Cephaleuros, Phycopeltis and Trentepohlia contain an inner ring of wall material which encircles the central pit. Despite some similarity between such septa and the cross walls of Phycopeltis vegetative cells, the latter are morphologically distinct. Ultrastructural observations clearly indicate that the central pits of the Chroolepidaceae a...

Organization of quadrifid and bifid hairs in the trap ofUtricularia monanthos

The ultrastructure of quadrifid and bifid hairs which line the inside of the trap in the bladderwortUtricularia monanthos are described. Both types of hair consist of a pedestal cell resting on a special epidermal cell which bears two and four terminal cells in bifids and quadrifids, respectively. The pedestal cell is a transfer cell with an extensive labyrinth of wall ingrowths and cytoplasm containing numerous well differentiated mitochondria. The wall ingrowths contain little structural material and appear to undergo changes in width depending on the activity of the trap. The proximal region of the lateral wall is devoid of ingrowths and is completely impregnated with opaque material slightly different in appearance from that of the adjoining impregnated walls. Numerous compound plasmodesmata connect the protoplast of the pedestal cell to that of the basal epidermal cell. The terminal cells of quadrifids and bifids show a striking differentiation of protoplast and wall associated with their specialization into stalk and arms. The protoplast of the stalk is narrow and contains mostly tubular ER. Abundant simple plasmodesmata traverse the wall between the foot of the stalk and the pedestal cell. The protoplast of the arm displays a large central vacuole, with the nucleus and numerous mitochondria concentrated towards the base of the arm. The walls of the stalk are thick and heavily impregnated with cuticular material in their outer regions, but in the arms the cuticle is very thin and consists of small separate cutin cystoliths. The walls of the arms exhibit short unbranched ingrowths. The structure and function of the hairs is discussed. Materials are probably absorbed by the arms and transported to the pedestal cell. It is suggested that once substances have passed through the cuticle of the arm they are transported by the terminal cells, either symplasticallyvia the protoplast or apoplastically through the walls into the labyrinth of the pedestal cell. Because of the continuous impregnation in the proximal half of the lateral wall of the pedestal cell, all substances must pass through the protoplast of the pedestal cell if they are to be transported into the walls of the trap.