Inositol bisphosphate and inositol trisphosphate inhibit cell-to-cell passage of carboxyfluorescein in staminal hairs ofSetcreasea purpurea

Identification of symplasmic domains in the embryo and seed of Sedum acre L. (Crassulaceae)

MAIN CONCLUSION

Our study demonstrated that symplasmic communication between Sedum acre seed compartments and the embryo proper is not uniform. The presence of plasmodesmata (PD) constitutes the structural basis for information exchange between cells, and symplasmic communication is involved in the regulation of cell differentiation and plant development. Most recent studies concerning an analysis of symplasmic communication between seed compartments and the embryo have been predominantly performed on Arabidopsis thaliana. The results presented in this paper describe the analysis of symplasmic communication on the example of Sedum acre seeds, because the ultrastructure of the seed compartments and the embryo proper, including the PD, have already been described, and this species represents an embryonic type of development different to Arabidopsis. Moreover, in this species, an unusual electron-dense dome associated with plasmodesmata on the border between the basal cell/chalazal suspensor cells and the basal cell/the endosperm has been described. This prompted the question as to whether these plasmodesmata are functional. Thus, the aim of this study was to describe the movement of symplasmic transport fluorochromes between different Sedum seed compartments, with particular emphasis on the movement between the basal cell and the embryo proper and endosperm, to answer the following questions: (1) are seeds divided into symplasmic domains; (2) if so, are they stable or do they change with the development? The results have shown that symplasmic tracers movement: (a) from the external integument to internal integument is restricted; (b) from the basal cell to the other part of the embryo proper and from the basal cell to the endosperm is also restricted;

Staying Tight: Plasmodesmal Membrane Contact Sites and the Control of Cell-to-Cell Connectivity in Plants

Multicellularity differs in plants and animals in that the cytoplasm, plasma membrane, and endomembrane of plants are connected between cells through plasmodesmal pores. Plasmodesmata (PDs) are essential for plant life and serve as conduits for the transport of proteins, small RNAs, hormones, and metabolites during developmental and defense signaling. They are also the only pathways available for viruses to spread within plant hosts. The membrane organization of PDs is unique, characterized by the close apposition of the endoplasmic reticulum and the plasma membrane and spoke-like filamentous structures linking the two membranes, which define PDs as membrane contact sites (MCSs). This specialized membrane arrangement is likely critical for PD function. Here, we review how PDs govern developmental and defensive signaling in plants, compare them with other types of MCSs, and discuss in detail the potential functional significance of the MCS nature of PDs.

Functional Evaluation of Proteins in Watery and Gel Saliva of Aphids

Gel and watery saliva are regarded as key players in aphid-pIant interactions. The salivary composition seems to be influenced by the variable environment encountered by the stylet tip. Milieu sensing has been postulated to provide information needed for proper stylet navigation and for the required switches between gel and watery saliva secretion during stylet progress. Both the chemical and physical factors involved in sensing of the stylet's environment are discussed. To investigate the salivary proteome, proteins were collected from dissected gland extracts or artificial diets in a range of studies. We discuss the advantages and disadvantages of either collection method. Several proteins were identified by functional assays or by use of proteomic tools, while most of their functions still remain unknown. These studies disclosed the presence of at least two proteins carrying numerous sulfhydryl groups that may act as the structural backbone of the salivary sheath. Furthermore, cell-wall degrading proteins such a pectinases, pectin methylesterases, polygalacturonases, and cellulases as well as diverse Ca²⁺-binding proteins (e.g., regucalcin, ARMET proteins) were detected. Suppression of the plant defense may be a common goal of salivary proteins. Salivary proteases are likely involved in the breakdown of sieve-element proteins to invalidate plant defense or to increase the availability of organic N compounds. Salivary polyphenoloxidases, peroxidases and oxidoreductases were suggested to detoxify, e.g., plant phenols. During the last years, an increasing number of salivary proteins have been categorized under the term 'effector'. Effectors may act in the suppression (C002 or MIF cytokine) or the induction (e.g., Mp10 or Mp 42) of plant defense, respectively. A remarkable component of watery saliva seems the protein GroEL that originates from Buchnera aphidicola, the obligate symbiont of aphids and probably reflects an excretory product that induces plant defense responses. Furthermore, chitin fragments in the saliva may trigger defense reactions (e.g., callose deposition). The functions of identified proteins and protein classes are discussed with regard to physical and chemical characteristics of apoplasmic and symplasmic plant compartments.

A model system for analyzing intercellular communication through plasmodesmata using moss protonemata and leaves

Plant growth, development, and environmental responses require the proper regulation of intercellular movement of signals and nutrients. For this, plants have specialized cytoplasmic channels, the plasmodesmata (PD), which allow the symplasmic movement of micro- and macromolecules between neighboring cells. Internal and external signals spatio-temporally regulate the movement of molecules through the PD to control plant development and environmental responses. Although some aspects of targeted movement of molecules have been revealed, the mechanisms of non-targeted, diffusible flow of molecules through PD, and its regulation and function, remain poorly understood, particularly at the cellular level. Previously, we developed a system to quantitatively analyze non-targeted movement of a photoconvertible fluorescent protein, Dendra2, at the single-cell level in the filamentous protonemata tissue of the moss Physcomitrella patens. In protonemata, one-dimensional intercellular communication can be easily observed and quantitatively analyzed at the cellular level. In this review, we describe how protonemata and leaves of P. patens can be used to study symplasmic movement through PD, and discuss how this system can help improve our understanding of PD regulation and function in development and environmental responses in plants.

Quantitative imaging of directional transport through plasmodesmata in moss protonemata via single-cell photoconversion of Dendra2

Cell-to-cell transport of molecules in plants must be properly regulated for plant growth and development. One specialized mechanism that plants have evolved involves transport through plasmodesmata (PD), but when and how transport of molecules via PD is regulated among individual cells remains largely unknown, particularly at the single-cell level. Here, we developed a tool for quantitatively analyzing cell-to-cell transport via PD at a single-cell level using protonemata of Physcomitrella patens and a photoconvertible fluorescent protein, Dendra2. In the filamentous protonemal tissues, one-dimensional intercellular communication can be observed easily. Using this system, we found that Dendra2 was directionally transported toward the apex of the growing protonemata. However, this directional transport could be eliminated by incubation in the dark or treatment with a metabolic inhibitor. Thus, we propose that directional transport of macromolecules can occur via PD in moss protonemata, and may be affected by the photosynthetic and metabolic activity of cells.

Inhibitors of myosin, but not actin, alter transport through Tradescantia plasmodesmata

Actin and myosin are components of plasmodesmata, the cytoplasmic channels between plant cells, but their role in regulating these channels is unclear. Here, we investigated the role of myosin in regulating plasmodesmata in a well-studied, simple system comprising single filaments of cells which form stamen hairs in Tradescantia virginiana flowers. Effects of myosin inhibitors were assessed by analysing cell-to-cell movement of fluorescent tracers microinjected into treated cells. Incubation in the myosin inhibitor, 2,3-butanedione monoxime (BDM) or injection of anti-myosin antibodies increased cell-cell transport of fluorescent dextrans, while treatment with the myosin inhibitor N-ethylmaleimide (NEM) decreased cell-cell transport. Pretreatment with the callose synthesis inhibitor, deoxy-D: -glucose (DDG), enhanced transport induced by BDM treatment or injection of myosin antibodies but did not relieve NEM-induced reduction in transport. In contrast to the myosin inhibitors, cell-to-cell transport was unaffected by treatment with the actin polymerisation inhibitor, latrunculin B, after controlling for callose synthesis with DDG. Transport was increased following azide treatment, and reduced after injection of ATP, as in previous studies. We propose that myosin detachment from actin, induced by BDM, opens T. virginiana plasmodesmata whereas the firm attachment of myosin to actin, promoted by NEM, closes them.

Non-targeted and targeted protein movement through plasmodesmata in leaves in different developmental and physiological states

Plant cells rely on plasmodesmata for intercellular transport of small signaling molecules as well as larger informational macromolecules such as proteins. A green fluorescent protein (GFP) reporter and low-pressure microprojectile bombardment were used to quantify the degree of symplastic continuity between cells of the leaf at different developmental stages and under different growth conditions. Plasmodesmata were observed to be closed to the transport of GFP or dilated to allow the traffic of GFP. In sink leaves, between 34% and 67% of the cells transport GFP (27 kD), and between 30% and 46% of the cells transport double GFP (54 kD). In leaves in transition transport was reduced; between 21% and 46% and between 2% and 9% of cells transport single and double GFP, respectively. Thus, leaf age dramatically affects the ability of cells to exchange proteins nonselectively. Further, the number of cells allowing GFP or double GFP movement was sensitive to growth conditions because greenhouse-grown plants exhibited higher diffusion rates than culture-grown plants. These studies reveal that leaf cell plasmodesmata are dynamic and do not have a set size exclusion limit. We also examined targeted movement of the movement protein of tobacco mosaic virus fused to GFP, P30::GFP. This 58-kD fusion protein localizes to plasmodesmata, consistently transits from up to 78% of transfected cells, and was not sensitive to developmental age or growth conditions. The relative number of cells containing dilated plasmodesmata varies between different species of tobacco, with Nicotiana clevelandii exhibiting greater diffusion of proteins than Nicotiana tabacum.

Plasmodesmata: gatekeepers for cell-to-cell transport of developmental signals in plants

Cell walls separate individual plant cells. To enable essential intercellular communication, plants have evolved membrane-lined channels, termed plasmodesmata, that interconnect the cytoplasm between neighboring cells. Historically, plasmodesmata were viewed as facilitating traffic of low-molecular weight growth regulators and nutrients critical to growth. Evidence for macromolecular transport via plasmodesmata was solely based on the exploitation of plasmodesmata by plant viruses during infectious spread. Now plasmodesmata are revealed to transport endogenous proteins, including transcription factors important for development. Two general types of proteins, non-targeted and plasmodesmata-targeted, traffic plasmodesmata channels. Size and subcellular location influence non-targeted protein transportability. Superimposed on cargo-specific parameters, plasmodesmata themselves fluctuate in aperture between closed, open, and dilated. Furthermore, plasmodesmata alter their transport capacity temporally during development and spatially in different regions of the plant. Plasmodesmata are exposed as major gatekeepers of signaling molecules that facilitate or regulate developmental programs, maintain physiological status, and respond to pathogens.

Cytoplasmic acidification with butyric acid does not alter the ionic conductivity of plasmodesmata

The effect of lowering cytoplasmic pH on the ionic conductivity of higher-plant plasmodesmata was investigated with corn (Zea mays L. cv. Black Mexican Sweet) suspension culture cells. Exposure to butyric acid decreased the cytoplasmic pH by 0.8 units. Intercellular communication was monitored by electrophysiological techniques that allowed the measurement of membrane resistances of sister cells and the electrical resistance of the plasmodesmata connecting them. The decrease in cytoplasmic pH did not affect the resistance of plasmodesmata, despite the fact that the butyric acid treatment more than doubled the concentration of cytoplasmic calcium. This is discussed in light of previous findings that increases in cytoplasmic calcium increase the electrical resistance of plasmodesmata.

Aluminum-induced 1-->3-beta-D-glucan inhibits cell-to-cell trafficking of molecules through plasmodesmata. A new mechanism of aluminum toxicity in plants

Symplastic intercellular transport in plants is achieved by plasmodesmata (PD). These cytoplasmic channels are well known to interconnect plant cells to facilitate intercellular movement of water, nutrients, and signaling molecules including hormones. However, it is not known whether Al may affect this cell-to-cell transport process, which is a critical feature for roots as organs of nutrient/water uptake. We have microinjected the dye lucifer yellow carbohydrazide into peripheral root cells of an Al-sensitive wheat (Triticum aestivum cv Scout 66) either before or after Al treatment and followed the cell-to-cell dye-coupling through PD. Here we show that the Al-induced root growth inhibition is closely associated with the Al-induced blockage of cell-to-cell dye coupling. Immunofluorescence combined with immuno-electron microscopic techniques using monoclonal antibodies against 1-->3-beta-D-glucan (callose) revealed circumstantial evidence that Al-induced callose deposition at PD may responsible for this blockage of symplastic transport. Use of 2-deoxy-D-glucose, a callose synthesis inhibitor, allowed us to demonstrate that a reduction in callose particles correlated well with the improved dye-coupling and reduced root growth inhibition. While assessing the tissue specificity of this Al effect, comparable responses were obtained from the dye-coupling pattern in tobacco (Nicotiana tabacum) mesophyll cells. Analyses of the Al-induced expression of PD-associated proteins, such as calreticulin and unconventional myosin VIII, showed enhanced fluorescence and co-localizations with callose deposits. These results suggest that Al-signal mediated localized alterations to calcium homeostasis may drive callose formation and PD closure. Our data demonstrate that extracellular Al-induced callose deposition at PD could effectively block symplastic transport and communication in higher plants.

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.

Technical advance: near-infrared femtosecond laser pulses as a novel non-invasive means for dye-permeation and 3D imaging of localised dye-coupling in the Arabidopsis root meristem

We have used near-infrared femtosecond Titanium: Sapphire laser pulses as novel non-invasive means for dye loading into various cell types of the Arabidopsis root meristem, and by 3D imaging have assessed the extent of dye coupling between the meristematic cells. The post-embryonic primary root of Arabidopsis thaliana has an invariant ontogeny and fixed cellular organisation which makes it an attractive model system to study developmental events involving cell fate determination, cellular differentiation and pattern formation. Local intercellular communication and local transmission of positional signals are likely to play a pivotal role in cell proliferation and regulation of differentiation. We have therefore examined the extent to which the constituent cells in the root meristem are symplastically coupled. Following laser-assisted loading of membrane impermeate fluorescent dye propidium iodide (PI) in single cells, we show by time-lapse and 3D imaging that in the root tip all undifferentiated cells are dye-coupled. When PI is permeated into the central cells, it rapidly moved into the adjacent initials of the columella, cortex, pericycle and stele. Interestingly, when only either of the initials were loaded with the dye, it never moved into any of the central cells. Amongst the epidermal cells, the differentiated hair cells are symplastically isolated. Our data provide evidence (1) for differential dye-coupling behaviour between quiescent centre cells and the neighbouring initials; (2) that cells in the root are coupled during stages at which the cell-lineage pattern is formed and that it becomes progressively secluded as they differentiate and the pattern is fixed. Taken together, our NIR-laser mediated approach is highly efficient and has numerous potential applications for non-invasive permeation of dyes in different cell types.

Maize calreticulin localizes preferentially to plasmodesmata in root apex

Using a polyclonal antibody raised against calreticulin purified and sequenced from maize, we performed an immunocytological study to characterize putative domain-specific subcellular distributions of endoplasmic reticulum (ER)-resident calreticulin in meristematic cells of maize root tip. At the light microscopy level, calreticulin was immunolocalized preferentially at cellular peripheries, in addition to nuclear envelopes and cytoplasmic structures. Punctate labelling at the longitudinal walls and continuous labelling at the transverse walls was characteristic. Immunogold electron microscopy revealed plasmodesmata as the most prominently labelled cell periphery structure. In order to further probe the ER-domain-specific distribution of maize calreticulin at plasmodesmata, root apices were exposed to mannitol-induced osmotic stress. Plasmolysis was associated with prominent accumulations of calreticulin at callose-enriched plasmodesmata and pit fields while the contracting protoplasts were depleted of calreticulin. In contrast, other ER-resident proteins recognized by HDEL peptide and BiP antibodies localized exclusively to contracted protoplasts. This finding reveals that, in plasmolysed cells, calreticulin enriched ER domains at plasmodesmata and pit fields are depleted of other ER-resident proteins containing the HDEL retention peptide.

Localization of a myosin-like protein to plasmodesmata

Myosin has been localized to plasmodesmata in root tissues of Allium cepa, Zea mays and Hordeum vulgare using a polyclonal antibody to animal myosin in both fluorescence and electron microscopy. Labelling was also observed throughout the cytoplasm, mainly associated with the endoplasmic reticulum and plasma membrane. On Western blots, bands of 180 and 110 kDa were consistently labelled in all three species. These bands were also labelled when the blot was incubated in actin prior to staining with antibodies to actin, raising the possibility that either of these proteins (180 kDa or 110 kDa) may be present in plasmodesmata. Pre-treatment of the tissue with 2,3-butanedione monoxime (BDM), an inhibitor of actin-myosin motility, resulted in a strong constriction of the neck region of plasmodesmata. These results indicate that a myosin-like protein may be present in plasmodesmata and may also play a role in the regulation of transport at the neck region.

TRANSPORT OF PROTEINS AND NUCLEIC ACIDS THROUGH PLASMODESMATA

Despite a potentially key role in cell-to-cell communication, plant intercellular connections-the plasmodesmata-have long been a biological "black box." Little is known about their protein composition, regulatory mechanisms, or transport pathways. However, recent studies have shed some light on plasmodesmal function. These connections have been shown to actively traffic proteins and protein-nucleic acid complexes between plant cells. This review describes these transport processes-specifically, cell-to-cell movement of plant viruses as well as endogenous cellular proteins-and discusses their possible mechanism(s). For comparison and to provide a broader perspective on the plasmodesmal transport process, the current model for nuclear import, the only other known example of transport of large proteins and protein-nucleic acid complexes through a membrane pore, is summarized. Finally, the function of plasmodesmata as communication boundaries within plant tissue is discussed.

Plasmodesmata of maize root tips: structure and composition

A procedure is described for obtaining clean maize cell wall preparations that contain embedded plasmodesmata. Negative staining and rotary shadowing have been used with transmission electron microscopy to visualise the plasmodesmata in these isolated walls, and to assess the effects of simple biochemical treatments on plasmodesmal components. Light protease treatment removes material from the exposed ends of plasmodesmata but does not extract the plasmodesmal core, which lies within the cell wall. However, heavy proteolysis occasionally removes the complete plasmodesma, including its enclosing collar structure, from the wall. Extraction with urea has a similar effect. The collar itself appears not to be proteinaceous in composition, although protein may bind it into the wall. Callose is localised in the wall around plasmodesmata, but does not appear to be a constituent of the collar. The membrane components of the plasmodesma (plasma membrane and desmotubule) can be extracted with membrane-solubilising detergents. This treatment releases from the wall a small number of proteins that are regarded as being potentially of plasmodesmal origin. These results show that plasmodesmata from maize can be dissected biochemically and suggest a strategy for the characterisation of individual molecular components.

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.

Plasmodesmata and the supracellular nature of plants

In the classical formulation of Münch (1930), plasmodesmata are considered to form simple cytoplasmic bridges between neighbouring plant cells to create the symplasm. This concept has dominated, if not monopolized, the thinking of plant biologists and in particular plant physiologists over the last few decades. Recent advances in ultrastructural, physiological and molecular studies on plasmodesmata indicate that this simple view is in need of revision. Structurally, the higher plant plasmodesma has been revealed to be a supramolecular complex consisting of membranes and proteins. Functionally, evidence is at hand that this complex structure appears to have evolved not only to control the size exclusion limit for intercellular diffusion of metabolites and small molecules, but also to potentiate and regulate intercellular trafficking of macromolecules, including proteins and nucleic acids. In this regard, plasmodesmal transport may share parallel regulatory mechanisms with nucleocytoplasmic transport. Based on these findings, we advance the hypothesis that plants function as supracellular, rather than multicellular, organisms. As such, the dynamics of the plant body, including cell differentiation, tissue formation, organogenesis and specialized physiological function(s), is subject to plasmodesmal regulation. Plasmodesmata presumably accomplish such regulatory roles by trafficking informational molecules which orchestrate both metabolic activity and gene expression. Current and future studies on the evolutionary origin(s) of plasmodesmata are likely to provide valuable information in terms of the genetic and molecular basis for the supracellular nature of plants. Contents Summary 435 I. Introduction 436 II. Plasmodesmal formation, structure and biochemistry 436 III. Evolution of plasmodesmata 445 IV. Symplasmic dynamics 452 V. Plasniodesmal trafficking of macromolecules: parallels with nucleocytoplasmic transport 457 VI. Role of plasmodesmata in plant development 464 VII. Concluding remarks 469 Acknowledgements 470 References 470.

Studies on the longitudinal and lateral transport of IAA in the shoots of etiolated corn seedlings

The auxin, indole-3-acetic acid, and the symplastic probe, carboxyfluorescein diacetate, were applied to the cut mesocotyl base or coleoptile apex of etiolated Zea mays seedlings and their transport measured and tissue distribution determined. The longitudinal transport of indole-3-acetate was strongly basipolar, while that of carboxyfluorescein was essentially apolar. The longitudinal transport of IAA, like carboxyfluorescein, was mainly in the stele. IAA exhibited a much higher lateral mobility from stele to cortex than did carboxyfluorescein. Based on the calculation of moles probe/kg fw, IAA is 4 times more concentrated in the stele than in the cortex while CF is 24 times higher in concentration in the stele than in the cortex. The structure of the node and the mesocotyl regions just below the node, regions of maximum growth, were examined and plasmodesmatal structure and frequency in these regions determined. The plasmodesmatal frequency, about 3 per micrometer2, between all cell types of the mesocotyl was found to be about 5-8 fold higher than that found for the root. Hypotheses of lateral auxin transport are discussed.

How do plant virus nucleic acids move through intercellular connections?

In addition to their function in transport of water, ions, small metabolites, and growth factors in normal plant tissue, the plasmodesmata presumably serve as routes for cell-to-cell movement of plant viruses in infected tissue. Virus cell-to-cell spread through plasmodesmata is an active process mediated by specialized virus encoded movement proteins; however, the mechanism by which these proteins operate is not clear. We incorporate recent information on the biochemical properties of plant virus movement proteins and their interaction with plasmodesmata in a model for transport of nucleic acids through plasmodesmatal channels. We propose that only single stranded (ss) nucleic acids can be transported efficiently through plasmodesmata, and that movement proteins function as molecular chaperones for ss nucleic acids to form unfolded movement protein-ss nucleic acid complexes. These complexes are targeted to plasmodesmata. Plasmodesmatal permeability is then increased following interaction with movement protein and the entire movement complex or its nucleic acid component is translocated across the plasmodesmatal channel.

Tissue to tissue symplastic communication in the shoots of etiolated corn seedlings

Carboxyfluorescein, a symplastic probe, was applied to the cut mesocotyl base or coleoptile apex of etiolated Zea mays cv. Silver Queen seedlings and its transport measured and tissue distribution determined. Long-distance longitudinal symplastic transport of the carboxyfluorescein was mainly in the vascular stele. It moved laterally from the mesocotyl stele to the mesocotyl cortex but the presence of a weak barrier limited the movement. A partial symplastic barrier was also present near the coleoptile-mesocotyl node.