Season-associated modifications in symplasmic organization of the cambium in Populus nigra

BACKGROUND AND AIMS

Alterations of plasmodesma (PD) connectivity are likely to be very important for plant development. Here, the repetitive division pattern of cambial initials in Populus nigra 'italica' was studied to follow the development of the PD network during maturation. Furthermore, seasonal changes were investigated in order to trace indications for developmental and functional adaptations.

METHODS

Cambium samples of P. nigra twigs, collected in summer, autumn and spring, were chemically fixed for transmission electron microscopy. The parameters, PD density (number of PDs per square micrometre cell-wall area) and PD frequency (total number of PDs per average cell-wall area), were determined for radial and tangential cell interfaces deposited in chronological order.

KEY RESULTS

Data sets, presented in plasmodesmograms, show a strong variability in the PD network throughout the year. In summer, high PD numbers occur at the division wall which, after PD doubling by longitudinal fission, decline with further development both at the xylem and the phloem side. In autumn, the number of PDs at the division wall is low as they are in subsequent tangential interfaces. In spring, the first cell division coincides with a massive increase in PD numbers, in particular at the division wall. Only the radial walls between initials maintain their PD equipment throughout the year. This feature can be exploited for identification of the initial layer.

CONCLUSIONS

PD networks in the cambium go through a strict developmental programme depending on the season, which is associated with changing functional requirements. For instance, PD numbers correlate with proliferative activity and potential pathways for intercellular signalling. Increases in PD numbers are ascribed to longitudinal fission as a major mechanism, whereas the decline in older derivatives is ascribed to PD degradation.

The constitutive expression of Arabidopsis plasmodesmal-associated class 1 reversibly glycosylated polypeptide impairs plant development and virus spread

Arabidopsis class 1 reversibly glycosylated polypeptides (C1RGPs) were shown to be plasmodesmal-associated proteins. Transgenic tobacco (Nicotiana tabacum) plants constitutively expressing GFP tagged AtRGP2 under the control of the CaMV 35S promoter are stunted, have a rosette-like growth pattern, and in source leaves exhibit strong chlorosis, increased photoassimilate retention and starch accumulation that results in elevated leaf specific fresh and dry weights. Basal callose levels around plasmodesmata (Pd) of leaf epidermal cells in transgenic plants are higher than in WT. Such a phenotype is characteristic of virus-infected plants and some transgenic plants expressing Pd-associated viral movement proteins (MP). The local spread of Tobacco mosaic virus (TMV) is inhibited in AtRGP2:GFP transgenics compared to WT. Taken together these observations suggest that overexpression of the AtRGP2:GFP leads to a reduction in Pd permeability to photoassimilate, thus lowering the normal rate of translocation from source leaves to sink organs. Such a reduction may also inhibit the local cell-to-cell spread of viruses in transgenic plants. The observed reduction in Pd permeability could be due to a partial Pd occlusion caused either by the accumulation of AtRGP2:GFP fusion in Pd, and/or by constriction of Pd by the excessive callose accumulation.

Downregulating the sucrose transporter VpSUT1 in Verbascum phoeniceum does not inhibit phloem loading

Sucrose is loaded into the phloem in the minor veins of leaves before export. Two active, species-specific loading mechanisms have been proposed. One involves transporter-mediated sucrose transfer from the apoplast into the sieve element-companion cell complex, so-called apoplastic loading. In the putative second mechanism, sucrose follows an entirely symplastic pathway, and the solute concentration is elevated by the synthesis of raffinose and stachyose in the phloem, not by transporter activity. Several sucrose-transporting plants have been shown to be apoplastic loaders by downregulating sucrose transporter 1 (SUT1), leading to accumulation of sugars and leaf chlorosis. In this study we compared the effect of downregulating SUT1 in Nicotiana tabacum, a sucrose transporter, and Verbascum phoeniceum, a species that transports raffinose and stachyose. To test the effectiveness of RNAi downregulation, we measured SUT1 mRNA levels and sucrose-H(+) symport in leaf discs. Mild NtSUT1 downregulation in N. tabacum resulted in the pronounced phenotype associated with loading inhibition. In contrast, no such phenotype developed when VpSUT1 was downregulated in V. phoeniceum, leaving minimal sucrose transport activity. Only those plants with the most severe VpSUT1 downregulation accumulated more carbohydrate than usual and these plants were normal by other criteria: growth rate, photosynthesis, and ability to clear starch during the night. The results provide direct evidence that the mechanism of phloem loading in V. phoeniceum does not require active sucrose uptake from the apoplast and strongly supports the conclusion that the loading pathway is symplastic in this species.

Upregulation of the plant protein remorin correlates with dehiscence and cell maturation: a link with the maturation of plasmodesmata?

Remorins are plant-specific proteins found associated with plasma membrane microdomains, called lipid rafts. Recently, we have shown that this lipid raft marker also accumulated at plasmodesmata, likely within the plasma membrane lining these structures. Here, we have investigated the gene expression and protein accumulation patterns of remorin at the organ and cell type levels. We show that remorin level is significantly increased in dehiscent, mature and ageing tissues, as well as in source parts of the leaves, where mature branched plasmodesmata are in majority. These results suggest that remorin predominantly associates with mature branched plasmodesmata.

Viroid intercellular trafficking: RNA motifs, cellular factors and broad impacts

Viroids are noncoding RNAs that infect plants. In order to establish systemic infection, these RNAs must traffic from an initially infected host cell into neighboring cells and ultimately throughout a whole plant. Recent studies have identified structural motifs in a viroid that are required for trafficking, enabling further studies on the mechanisms of their function. Some cellular proteins interact with viroids in vivo and may play a role in viroid trafficking, which can now be directly tested by using a virus-induced gene silencing system that functions efficiently in plant species from which these factors were identified. This review discusses these recent advances, unanswered questions and the use of viroid infection as an highly productive model to elucidate mechanisms of RNA trafficking that is of broad biological significance.

Redox homeostasis regulates plasmodesmal communication in Arabidopsis meristems

Cell-to-cell communication is crucial for multicellular development, and in plants occurs through specialized channels called plasmodesmata (PD). In our recent manuscript we reported the characterization of a PD trafficking mutant, 'gfp arrested trafficking 1' (gat1), which carries a mutation in the thioredoxin-m3 (TRX-m3) gene. gat1 mutants showed restricted GFP transport from the phloem to the root meristem that appears to result from structural modifications in the PD channel. We found accumulation of reactive oxygen species (ROS) and callose, as well as a reduction in starch granules in the gat1 root meristem. Application of oxidants to wildtype plants and expression of our GFP reporter in the mutant root meristemless 1 (rml1) mimic the gat1 phenotype. Our results suggest that mutations in GAT1 cause ROS accumulation and induce the biosynthesis of callose, which in turn block PD transport. Therefore, we propose a model whereby GAT1/TRX-m3 is a component of a redox-regulated pathway that maintains PD permeability in Arabidopsis meristems.

Callose synthesis in higher plants

Callose is a polysaccharide in the form of beta-1,3-glucan with some beta-1,6-branches and it exists in the cell walls of a wide variety of higher plants. Callose plays important roles during a variety of processes in plant development and/or in response to multiple biotic and abiotic stresses. It is now generally believed that callose is produced by callose synthases and that it is degraded by beta-1,3-glucanases. Despite the importance of callose in plants, we have only recently begun to elucidate the molecular mechanism of its synthesis. Molecular and genetic studies in Arabidopsis have identified a set of genes that are involved in the biosynthesis and degradation of callose. In this mini-review, we highlight recent progress in understanding callose biosynthesis and degradation and discuss the future challenges of unraveling the mechanism(s) by which callose synthase operate.

Both the conserved GRAS domain and nuclear localization are required for SHORT-ROOT movement

Transcription factor movement is well established in plants. Since the initial report of KNOTTED movement, more than a dozen transcription factors have been shown to move in plants. However, the developmental significance of movement is not known. Using the SHORT-ROOT (SHR) transcription factor as a tool for studying cell-to-cell trafficking, we show that movement of SHR from its site of synthesis is necessary for normal development of the Arabidopsis root. We identify multiple regions of SHR that are required for intra- and intercellular movement of SHR, including a region that is necessary for movement but not activity. We made the surprising discovery that the capacity for intercellular movement may be conserved among other GRAS family proteins. Finally, we provide evidence that movement requires both cytoplasmic and nuclear localization, strongly suggesting a mechanistic link between nuclear transport and cell-to-cell movement.

Approaching the cellular mechanism that supports the intercellular spread of Tobacco mosaic virus

Plant viruses spread cell-to-cell in infected plants by exploiting plasmodesmata (PD), gatable channels in the cell wall that provide cytoplasmic passageways for the trafficking of informational macromolecules. Since it became known that the intercellular spread of Tobacco mosaic virus (TMV) depends on virus-encoded movement protein (MP), the mechanism by which this protein mediates in the targeting of this virus to PD is subject to intense studies. TMV movement occurs in a non-encapsidated form and thus promises to reveal important host functions involved in the intra-and intercellular trafficking of RNA molecules. We have recently presented new evidence that the cell-to-cell trafficking of TMV RNA (vRNA) involves the formation and intracellular trafficking of distinct MP particles. Upon assembly, these particles detach from cortical microtubule (MT) sites and then move with the flow of ER through the cell. During passage the particles continue to undergo transient interactions with MT which may guide the particles to their destination. The comprehensive analysis of particle composition may lead to important insights into the regulation of RNA transport in plants and may also reveal potential similarities to RNA transport mechanisms in animals and humans.

Symplastic domains in the Arabidopsis shoot apical meristem correlate with PDLP1 expression patterns

Symplastic domains in plants are defined by spatial limitations on cell-to-cell communication through plasmodesmata (Pds) and establish tissue boundaries necessary for metabolic and developmental programming. With the exception of the physical closure of Pds by callose, the cues and the processes for creating symplastic domains remain poorly understood. Recently, we identified a novel family of eight proteins, called Pd-located protein 1 (PDLP1). These proteins span the plasma membrane within Pds and likely form part of a signal transduction system that perceives external signals to regulate molecular trafficking between cells. For two members of this family that have high expression in the shoot apex we show that they have defined and partially overlapping tissue-specific expression patterns that correlate in part with previously defined symplastic domains. The importance of non-cell-autonomous proteins in shoot development and of the spatial rules that govern leaf and floral development highlight the need to have a clearer understanding of symplastic domains.

Plasmodesmata transport of GFP and GFP fusions requires little energy and transitions during leaf expansion

Plasmodesmata (Pd) are symplastic channels between neighboring plant cells and are key in plant cell-cell signaling. Viruses of proteins, nucleic acids, and a wide range of signaling macromolecules move across Pd. Protein transport Pd is regulated by development and biotic signals. Recent investigations utilizing the Arrhenius equation or Coefficient of conductivity showed that fundamental energetic measurements used to describe transport of proteins across membrane pores or the nuclear pore can also apply to protein movement across Pd. As leaves continue to expand, Pd transport of proteins declines which may result from changes in cell volume, Pd density or Pd structure.

Long-distance trafficking of macromolecules in the phloem

The fact that macromolecules such as proteins and mRNAs overcome the symplastic barriers between various tissue domains was first evidenced by the movement of plant viruses. We have recently demonstrated that viral infection disengages the symplastic restriction present between the sieve element-companion cell complex and neighboring cells in tobacco plants. As a result, green fluorescent protein, which was produced in mesophyll and bundle sheath cells, could traffic into the sieve tube and travel long distances within the vascular system. In this addendum we discuss the likely existence of a novel plant communication network in which macromolecules also act as long-distance trafficking signals. Plasmodesmata interconnecting sieve elements and companion cells as well as plasmodesmata connecting the sieve tube with neighboring cells may play a central role in establishing this communication network.

Distinct RNA motifs mediate systemic RNA trafficking

Systemic RNA trafficking is emerging as a new paradigm in gene regulation at the whole plant level. The mechanisms that ensure delivery of all sorts of RNAs to their respective cellular destinations remain poorly understood. Several lines of research suggest that specific sequence/structural motif(s) within an RNA directs its trafficking. The first motif was identified in Potato spindle tuber viroid (PSTVd), which mediates trafficking from bundle sheath to mesophyll in young tobacco leaves. Our recent work identified a tertiary structural motif in PSTVd that is required for trafficking from bundle sheath to phloem in Nicotiana benthamiana to initiate long-distance trafficking. This motif consists of a U/C cis Watson-Crick/Watson-Crick base pair with water insertion to widen the minor groove. Similar motifs that are structurally conserved in rRNAs serve as protein-binding sites. These data suggest that distinct RNA motifs interact with specific cellular factors for trafficking across various cellular boundaries to achieve systemic trafficking.

beta-1,3-Glucanases: Plasmodesmal Gate Keepers for Intercellular Communication

Plasmodesmata (Pd), coaxial membranous channels that connect adjacent plant cells, are not static, but show a dynamic nature and can be opened or closed. These controlled changes in Pd conductivity regulate plant symplasmic permeability and play a role both in development and defense processes. One of the mechanisms shown to produce these changes is the deposition and hydrolysis of callose by beta-1-3-synthase and glucanase, respectively. Recently we have identified the first beta-1,3-glucanase Arabidopsis enzyme that is associated to the macromolecular Pd complex, termed AtBG_pap. When fused to GFP, this previously identified GPI-anchored protein localizes to the ER and the plasma membrane where it appears in a punctuate pattern that colocalizes with callose present around Pd. In T-DNA insertion mutants that do not transcribe AtBG_pap, GFP cell-to-cell movement between epidermal cells is reduced and callose levels around Pd are elevated. In this addenda we review the plant developmental processes of symplasmic regulation that have been shown to include callose deposition and beta-1,3-glucanase activity, and suggest a role for AtBG_pap in these processes. Additionally, based on the ability of viral movement proteins (MPs) to interact with ankyrin repeat proteins, and together with our recent findings showing the involvement of viral particles in callose degradation, we also purpose a new model for the ability of viruses to overcome Pd-callose deposition, and mediate their cell-to-cell movement.

Plasmodesmal Targeting and Accumulation of TMV Movement Protein

The interaction between tobacco mosaic virus and its host plant cells has been intensively studied as a model for macromolecular trafficking. The observation that GFP-labelled TMV movement protein localises to microtubules led to the suggestion that microtubules are required for the cell to cell movement of the virus. In a recent paper we have demonstrated that the targeting of TMV movement protein to plasmodesmata requires the actin and ER networks, which supports previous evidence from our laboratory that showed that disruption of microtubules did not prevent cell to cell movement of TMV virus, and that a mutated movement protein, which did not localise to micro-tubules, showed enhanced viral movement. In this addendum we speculate where the TMV movement protein accumulates within plasmodesmata, and the relationship of this accumulation to the cell to cell movement of the virus.

The micromorphology of pit membranes in tracheary elements of ericales: new records of tori or pseudo-tori?

BACKGROUND AND AIMS

Intervascular pit membranes were examined within Ericales to determine the distribution and structure of torus-like thickenings.

METHODS

Forty-nine species representing 12 families of the order Ericales were investigated using light, scanning and transmission electron microscopy. They were compared with four species of Oleaceae to determine the true nature of the thickenings.

KEY RESULTS

Pit membranes with torus-like thickenings were observed in seven species of Ericaceae and were found to be amorphous, plasmodesmata-associated structures with an irregular distribution. These pseudo-tori show major differences compared with true tori with respect to their distribution and ultrastructure. Genuine tori, which are strongly correlated with round pit apertures in narrow tracheary elements, were found in two species of Osmanthus (Oleaceae).

CONCLUSIONS

The pseudo-tori found in some Ericaceae are considered to be similar to pit membrane thickenings previously recorded in Rosaceae. While true tori appear to be functionally significant in terms of efficiency and safety of water transport, the possible function of pseudo-tori could be associated with the role of plasmodesmata during differentiation of tracheids, fibre-tracheids or narrow vessels.

Effects of de- and rehydration on food-conducting cells in the moss Polytrichum formosum: a cytological study

BACKGROUND AND AIMS

Moss food-conducting cells (leptoids and specialized parenchyma cells) have a highly distinctive cytology characterized by a polarized cytoplasmic organization and longitudinal alignment of plastids, mitochondria, endoplasmic reticulum and vesicles along endoplasmic microtubules. Previous studies on the desiccation biology of mosses have focused almost exclusively on photosynthetic tissues; the effects of desiccation on food-conducting cells are unknown. Reported here is a cytological study of the effects of de- and rehydration on food-conducting cells in the desiccation-tolerant moss Polytrichum formosum aimed at exploring whether the remarkable subcellular organization of these cells is related to the ability of mosses to survive desiccation.

METHODS

Shoots of Polytrichum formosum were dehydrated under natural conditions and prepared for transmission and scanning electron microscopy using both standard and anhydrous chemical fixation protocols. Replicate samples were then fixed at intervals over a 24-h period following rehydration in either water or in a 10 microM solution of the microtubule-disrupting drug oryzalin.

KEY RESULTS

Desiccation causes dramatic changes; the endoplasmic microtubules disappear; the nucleus, mitochondria and plastids become rounded and the longitudinal alignment of the organelles is lost, though cytoplasmic polarity is in part retained. Prominent stacks of endoplasmic reticulum, typical of the hydrated condition, are replaced with membranous tubules arranged at right angles to the main cellular axis. The internal cytoplasm becomes filled with small vacuoles and the plasmalemma forms labyrinthine tubular extensions outlining newly deposited ingrowths of cell wall material. Whereas plasmodesmata in meristematic cells at the shoot apex and in stem parenchyma cells appear to be unaffected by dehydration, those in leptoids become plugged with electron-opaque material. Starch deposits in parenchyma cells adjoining leptoids are depleted in desiccated plants. Rehydration sees complete reestablishment over a 12- to 24-h period of the cytology seen in the control plants. Oryzalin effectively prevents leptoid recovery.

CONCLUSIONS

The results point to a key role of the microtubular cytoskeleton in the rapid re-establishment of the elaborate cytoplasmic architecture of leptoids during rehydration. The reassembly of the endoplasmic microtubule system appears to dictate the time frame for the recovery process. The failure of leptoids to recover normal cytology in the presence of oryzalin further underlines the key role of the microtubules in the control of leptoid cytological organization.

Identification of an interactor of cadmium ion-induced glycine-rich protein involved in regulation of callose levels in plant vasculature

Cadmium-induced glycine-rich protein (cdiGRP) is a cell wall-associated factor that increases callose levels in plant vasculature. To better understand the cdiGRP/callose regulation system, we identified a tobacco protein, GrIP (cdiGRP-interacting protein, GrIP), that associates with cdiGRP and localizes at the plant cell wall. Constitutive overexpression of GrIP enhanced the accumulation of the cdiGRP protein and callose in vasculature-associated cells with or without treatment with cadmium ions. That GrIP gene expression was not affected by cadmium ions indicated that GrIP does not directly modulate the callose levels induced by the treatment. Instead, GrIP most likely functions by further elevating the accumulated amount of cdiGRP, the expression of which is up-regulated by the cadmium ions. Interestingly, the levels of cdiGRP mRNA were not affected by constitutive expression of GrIP, demonstrating that the enhancement in cdiGRP protein accumulation by GrIP overexpression occurs posttranslationally. Collectively, these observations suggest that GrIP interacts with cdiGRP and increases its level of accumulation; in turn, the elevated amounts of cdiGRP induce callose deposits in the plant cell walls. Therefore, GrIP and cdiGRP represent sequentially acting factors in a biochemical pathway that regulates callose accumulation in the plant vasculature.

Eukaryotic cells and their cell bodies: Cell Theory revised

BACKGROUND

Cell Theory, also known as cell doctrine, states that all eukaryotic organisms are composed of cells, and that cells are the smallest independent units of life. This Cell Theory has been influential in shaping the biological sciences ever since, in 1838/1839, the botanist Matthias Schleiden and the zoologist Theodore Schwann stated the principle that cells represent the elements from which all plant and animal tissues are constructed. Some 20 years later, in a famous aphorism Omnis cellula e cellula, Rudolf Virchow annunciated that all cells arise only from pre-existing cells. General acceptance of Cell Theory was finally possible only when the cellular nature of brain tissues was confirmed at the end of the 20th century. Cell Theory then rapidly turned into a more dogmatic cell doctrine, and in this form survives up to the present day. In its current version, however, the generalized Cell Theory developed for both animals and plants is unable to accommodate the supracellular nature of higher plants, which is founded upon a super-symplasm of interconnected cells into which is woven apoplasm, symplasm and super-apoplasm. Furthermore, there are numerous examples of multinucleate coenocytes and syncytia found throughout the eukaryote superkingdom posing serious problems for the current version of Cell Theory.

SCOPE

To cope with these problems, we here review data which conform to the original proposal of Daniel Mazia that the eukaryotic cell is composed of an elemental Cell Body whose structure is smaller than the cell and which is endowed with all the basic attributes of a living entity. A complement to the Cell Body is the Cell Periphery Apparatus, which consists of the plasma membrane associated with other periphery structures. Importantly, boundary structures of the Cell Periphery Apparatus, although capable of some self-assembly, are largely produced and maintained by Cell Body activities and can be produced from it de novo. These boundary structures serve not only as mechanical support for the Cell Bodies but they also protect them from the hostile external environment and from inappropriate interactions with adjacent Cell Bodies within the organism.

CONCLUSIONS

From the evolutionary perspective, Cell Bodies of eukaryotes are proposed to represent vestiges of hypothetical, tubulin-based 'guest' proto-cells. After penetrating the equally hypothetical actin-based 'host' proto-cells, tubulin-based 'guests' became specialized for transcribing, storing and partitioning DNA molecules via the organization of microtubules. The Cell Periphery Apparatus, on the other hand, represents vestiges of the actin-based 'host' proto-cells which have become specialized for Cell Body protection, shape control, motility and for actin-mediated signalling across the plasma membrane.

Tobacco mosaic virus infection spreads cell to cell as intact replication complexes

Plant viruses encode movement proteins (MPs) that facilitate cell-cell transport of infection through plasmodesmata. Intracellular and intercellular spread of virus replication complexes (VRCs) of tobacco mosaic virus was followed in intact leaf tissue from 12 to 36 h post infection (hpi) by using confocal microscopy. From 12 hpi, VRCs in primary infected cells were associated with cortical endoplasmic reticulum, and at 14 hpi, exhibited high intracellular mobility ( approximately 160 nm/sec); mobility was slowed between 14 and 16 hpi ( approximately 40 nm/sec), and by 18 hpi, VRCs were stationary, adjacent to plasmodesmata. VRCs traversed the plasmodesmata between 18 and 20 hpi. The process of formation and movement of VRCs was repeated in adjacent cells in 3-4 h vs. 20 h from primary infected cells. The rapid intracellular movement of the VRCs and the spread to adjacent cells was blocked by inhibitors of filamentous actin and myosin, but not by inhibitors of microtubules. We propose a model whereby cell-cell spread of tobamovirus infection is accomplished by subviral replication complexes that initiate TMV replication immediately after entry to adjacent cells.

Maintaining apical dominance in the fern gametophyte

A kinetic model is developed for cell differentiation in the fern gametophyte to test hypotheses on the role of spatially patterned plasmodesmata networks in development. Of particular interest is the establishment and maintenance of apical cell type in a single cell, with concurrent suppression of this character in all other cells (apical dominance). Steps towards understanding apical cell localization in geometrically simple gametophytes may shed light on the establishment and maintenance of apical meristems in higher plants. The model, based on the plasmodesmata maps of Tilney and colleagues and involving kinetics for a requisite minimum of two morphogens. successfully produces the apical/non-apical cell differentiation patterns of normal development, and redifferentiation due to cell isolation, in six stages from 0-30 d of development. Our results indicate that increasing apical cell plasmodesmata number, as development progresses, is not required for effective transport across apical cell walls in maintaining apical dominance.

Development of the leafy shoot in Sphagnum (Bryophyta) involves the activity of both apical and subapical meristems

This light- and electron-microscope study of four species of Sphagnum reveals that stem elongation involves meristematic activities unique to the group and hitherto unrecognized. The internal tissue of the mature stem arises by the concerted activity of an apical (primary) and a subapical (secondary) meristem. The primary meristem comprises the immediate derivatives of the single apical cell. Following a small number of divisions, the primary derivatives differentiate into highly vacuolate parenchymatous cells with a storied arrangement. Subsequently, the large vacuoles are replaced by numerous small vacuoles and the cells then divide repeatedly, by transverse septa, producing files of about nine short cells. Finally, ninefold elongation of these secondary cells is responsible for extension growth of the main stem below the mature capitulum. An early step in primary differentiation is the confinement of pre-existing plasmodesmata to distinct pitted areas. Further enlargement of the cells during primary and secondary differentiation involves the thickening of non-pitted wall areas, followed by expansion and thinning out, while the pitted areas remain virtually unchanged. A cortical array of microtubules is regularly found in association with non-pitted wall areas, while the unexpanded pitted areas are associated with smooth endoplasmic reticulum showing continuity with desmotubules. Though sharing much the same cytology as the conducting cells in bryoid mosses, in terms of their development the central stem cells in Sphagnum are not homologous with those of other mosses. The unique mode of stem development may be an important factor in the ecological success of Sphagnum.

The leafy stems of Sphagnum (Bryophyta) contain highly differentiated polarized cells with axial arrays of endoplasmic microtubules

Contrary to the long-held belief that, internal to the cortical sterome, the central region of Sphagnum stems comprises unspecialized parenchyma, the present light- and electron-microscope study has revealed that these cells in fact have a highly specialized cytoplasmic organization. Their key features are: (a) the absence of large central vacuoles; (b) a spindle-shaped nucleus positioned internally; (c) a prominent axial system of endoplasmic microtubules associated with the nucleus, mitochondria, pleomorphic vacuoles, and membrane-bounded tubules and vesicles; (d) a distinct cytoplasmic polarization, with the cellular region near the capitulum being richer in organelles than the basal region; and (e) a high frequency of plasmodesmata in the cross walls with an enlarged median region containing no discernible desmotubule. Such a distinctive combination of cytological features has been hitherto only described for putative food-conducting cells in bryoid mosses. The results introduce a major new character common to Sphagnum and bryoid mosses and strongly suggest that this cytological organization underlines cellular specialization in symplasmic transport.

Development of water-conducting cells in the antipodal liverwort Symphyogyna brasiliensis (Metzgeriales)

The thallus of the metzgerialean liverwort Symphyogyna brasiliensis Nets contains a strand of dead thick-walled tells with helicoidally-arranged pits that arc presumably involved in water transport. During the first phase of differentiation these cells undergo a 13-16-fold elongation while remaining thin-walled and almost unchanged in diameter. During subsequent maturation the walls become strongly thickened by deposition of highly electron-opaque material on extraplasmodesmal areas and of transparent material forming collars around plasmodesmata. Whilst the growing wall shows an ordered microribrillar texture and is strongly reactive to PATAg staining for carbohydrates, the material associated with plasmodesmata is amorphous and PATAg-negative. A dense cortical array of microtobules (MTs) overlies the growing wall except in proximity to plasmodesmata, which are closely associated with tubular endoplasmic reticulum (ER). During cellular maturation plasmodesmata undergo extensive secondary elongation by incorporation of cortical ER supposedly continuous with desmotubules. Quantitative analysis of plasmodesmal frequencies in relation to cellular elongation and wall thickness indicates that there is no de novo formation of plasmodesmata. Cortical MTs, wall microfibrils and secondarily-modified plasmodesmata are consistently co-aligned, all forming helices of about 45°. During maturation the Golgi apparatus proliferates and a vast number of vesicles containing PATAg-positive material are produced from a membrane domain interpreted as trans Golgi network, whilst PATAg-negative vesicles are formed along the fenestrated margins of C& and media) dictyosomal cisternae. Exocytosis of PATAg-positive vesicles is confined to extraplasmodesmal areas. In ageing cells abundant fibrillar material, also positive to PATAg-test, accumulates within pleomorphic membrane-bounded tubules. Final cytoplasmic dissolution involves the lysis of all cellular membranes and the liberation of the membrane-bounded fibrillar material, that is subsequently deposited onto the walls. The eventual dissolution of the plugs of amorphous electron-transport material results in the formation of open pits. Similarities in the cytological mechanisms underlying pore development in water-conducting cells of Symphyogyna and in the sieve elements of angiosperms are discussed.

STRUCTURAL ASPECTS OF THE PRIMARY TISSUES OF THE CUCURBITA PEPO L. ROOT WITH SPECIAL REFERENCE TO THE PHLOEM

The primary phloem and associated tissues in the root of Cucurbita pepo L. were examined by light and transmission electron microscopy to provide information on the feasibility of symplastic transport from the sieve-tube members to the cortex in this organ. The structure, distribution and frequency of cytoplasmic connections between the various cell types of the root are reported as well as the cytological characteristics of the various cells. The protoplasts of contiguous cells of the root are joined by various numbers of cytoplasmic connections: plasmodesmata (simple and branched) between parenchymatous elements; pore-plasmodesmata between sieve-tube members and parenchymatous elements (companion, phloem parenchyma or pericycle cells in the Cucurbita root); and sieve-area pores between contiguous sieve elements. The plasmodesmata associated with secondary- and tertiary-state endodermal cells are not constricted by the suberin lamellae. Differences in the frequency of cytoplasmic connections between the various cell types of the primary phloem, pericycle and ground tissue do occur with the highest frequency between sieve-tube members and companion cells. The results indicate that the structure of the C. pepo root may be compatible with a symplastic pathway of phloem unloading and transport to the cells of the ground tissue.