Localization of a centrin-like protein to higher plant plasmodesmata

Reticulons 3 and 6 interact with viral movement proteins

Plant reticulon (RTN) proteins are capable of constricting membranes and are vital for creating and maintaining tubules in the endoplasmic reticulum (ER), making them prime candidates for the formation of the desmotubule in plasmodesmata (PD). RTN3 and RTN6 have previously been detected in an Arabidopsis PD proteome and have been shown to be present in primary PD at cytokinesis. It has been suggested that RTN proteins form protein complexes with proteins in the PD plasma membrane and desmotubule to stabilize the desmotubule constriction and regulate PD aperture. Viral movement proteins (vMPs) enable the transport of viruses through PD and can be ER-integral membrane proteins or interact with the ER. Some vMPs can themselves constrict ER membranes or localize to RTN-containing tubules; RTN proteins and vMPs could be functionally linked or potentially interact. Here we show that different vMPs are capable of interacting with RTN3 and RTN6 in a membrane yeast two-hybrid assay, coimmunoprecipitation, and Förster resonance energy transfer measured by donor excited-state fluorescence lifetime imaging microscopy. Furthermore, coexpression of the vMP CMV-3a and RTN3 results in either the vMP or the RTN changing subcellular localization and reduces the ability of CMV-3a to open PD, further indicating interactions between the two proteins.

Energide-cell body as smallest unit of eukaryotic life

Background

The evolutionary origin of the eukaryotic nucleus is obscure and controversial. Currently preferred are autogenic concepts; ideas of a symbiotic origin are mostly discarded and forgotten. Here we briefly discuss these issues and propose a new version of the symbiotic and archaeal origin of the eukaryotic nucleus.

Scope and Conclusions

The nucleus of eukaryotic cells forms via its perinuclear microtubules, the primary eukaryotic unit known also as the Energide-cell body. As for all other endosymbiotic organelles, new Energides are generated only from other Energides. While the Energide cannot be generated de novo, it can use its secretory apparatus to generate de novo the cell periphery apparatus. We suggest that Virchow's tenet Omnis cellula e cellula should be updated as Omnis Energide e Energide to reflect the status of the Energide as the primary unit of the eukaryotic cell, and life. In addition, the plasma membrane provides feedback to the Energide and renders it protection via the plasma membrane-derived endosomal network. New discoveries suggest archaeal origins of both the Energide and its host cell.

RNA on the move: The plasmodesmata perspective

It is now widely accepted that plant RNAs can have effects at sites far away from their sites of synthesis. Cellular mRNA transcripts, endogenous small RNAs and defense-related small RNAs all move from cell to cell via plasmodesmata (PD), and may even move long distances in the phloem. Despite their small size, PD have complicated substructures, and the area of the pore available for RNA trafficking can be remarkably small. The intent of this review is to bring into focus the role of PD in cell-to-cell and long distance communication in plants. We consider how cellular RNAs could move through the cell to the PD and thence through PD. The protein composition of PD and the possible roles of PD proteins in RNA trafficking are also discussed. Recent evidence for RNA metabolism in organelles acting as a factor in controlling PD flux is also presented, highlighting new aspects of plant intra- and intercellular communication. It is clear that while the phenomenon of RNA mobility is common and essential, many questions remain, and these have been highlighted throughout this review.

Evolutionary Cell Biology of Proteins from Protists to Humans and Plants

During evolution, the cell as a fine-tuned machine had to undergo permanent adjustments to match changes in its environment, while "closed for repair work" was not possible. Evolution from protists (protozoa and unicellular algae) to multicellular organisms may have occurred in basically two lineages, Unikonta and Bikonta, culminating in mammals and angiosperms (flowering plants), respectively. Unicellular models for unikont evolution are myxamoebae (Dictyostelium) and increasingly also choanoflagellates, whereas for bikonts, ciliates are preferred models. Information accumulating from combined molecular database search and experimental verification allows new insights into evolutionary diversification and maintenance of genes/proteins from protozoa on, eventually with orthologs in bacteria. However, proteins have rarely been followed up systematically for maintenance or change of function or intracellular localization, acquirement of new domains, partial deletion (e.g. of subunits), and refunctionalization, etc. These aspects are discussed in this review, envisaging "evolutionary cell biology." Protozoan heritage is found for most important cellular structures and functions up to humans and flowering plants. Examples discussed include refunctionalization of voltage-dependent Ca²⁺ channels in cilia and replacement by other types during evolution. Altogether components serving Ca²⁺ signaling are very flexible throughout evolution, calmodulin being a most conservative example, in contrast to calcineurin whose catalytic subunit is lost in plants, whereas both subunits are maintained up to mammals for complex functions (immune defense and learning). Domain structure of R-type SNAREs differs in mono- and bikonta, as do Ca²⁺ -dependent protein kinases. Unprecedented selective expansion of the subunit a which connects multimeric base piece and head parts (V0, V1) of H⁺ -ATPase/pump may well reflect the intriguing vesicle trafficking system in ciliates, specifically in Paramecium. One of the most flexible proteins is centrin when its intracellular localization and function throughout evolution is traced. There are many more examples documenting evolutionary flexibility of translation products depending on requirements and potential for implantation within the actual cellular context at different levels of evolution. From estimates of gene and protein numbers per organism, it appears that much of the basic inventory of protozoan precursors could be transmitted to highest eukaryotic levels, with some losses and also with important additional "inventions."

Architecture and permeability of post-cytokinesis plasmodesmata lacking cytoplasmic sleeves

Plasmodesmata are remarkable cellular machines responsible for the controlled exchange of proteins, small RNAs and signalling molecules between cells. They are lined by the plasma membrane (PM), contain a strand of tubular endoplasmic reticulum (ER), and the space between these two membranes is thought to control plasmodesmata permeability. Here, we have reconstructed plasmodesmata three-dimensional (3D) ultrastructure with an unprecedented level of 3D information using electron tomography. We show that within plasmodesmata, ER-PM contact sites undergo substantial remodelling events during cell differentiation. Instead of being open pores, post-cytokinesis plasmodesmata present such intimate ER-PM contact along the entire length of the pores that no intermembrane gap is visible. Later on, during cell expansion, the plasmodesmata pore widens and the two membranes separate, leaving a cytosolic sleeve spanned by tethers whose presence correlates with the appearance of the intermembrane gap. Surprisingly, the post-cytokinesis plasmodesmata allow diffusion of macromolecules despite the apparent lack of an open cytoplasmic sleeve, forcing the reassessment of the mechanisms that control plant cell-cell communication.

The Plant Cellular Systems for Plant Virus Movement

Plasmodesmata (PDs) are specialized intercellular channels that facilitate the exchange of various molecules, including sugars, ribonucleoprotein complexes, transcription factors, and mRNA. Their diameters, estimated to be 2.5 nm in the neck region, are too small to transfer viruses or viral genomes. Tobacco mosaic virus and Potexviruses are the most extensively studied viruses. In viruses, the movement protein (MP) is responsible for the PD gating that allows the intercellular movement of viral genomes. Various host factors interact with MP to regulate complicated mechanisms related to PD gating. Virus replication and assembly occur in viral replication complex (VRC) with membrane association, especially in the endoplasmic reticulum. VRC have a highly organized structure and are highly regulated by interactions among the various host factors, proteins encoded by the viral genome, and the viral genome. Virus trafficking requires host machineries, such as the cytoskeleton and the secretory systems. MP facilitates the virus replication and movement process. Despite the current level of understanding of virus movement, there are still many unknown and complex interactions between virus replication and virus movement. While numerous studies have been conducted to understand plant viruses with regards to cell-to-cell movement and replication, there are still many knowledge gaps. To study these interactions, adequate research tools must be used such as molecular, and biochemical techniques. Without such tools, virologists will not be able to gain an accurate or detailed understanding of the virus infection process.

Localization of fluorescently tagged protein to plasmodesmata by correlative light and electron microscopy

Plasmodesmata (PD) are intercellular communication channels that form long, membrane-lined cylinders across cellular junctions. A fluorescent-tagging approach is most commonly used for an initial assessment to address whether a protein of interest may localize or associate with PD domain. However, owing to the dimension of PD being at nanoscale, PD-associated fluorescent signals are detected only as small spots scattered at the cell periphery, hence requiring additional confirmatory evidence. Immunogold labeling provides such information, but suitable antibodies are not always available and morphological preservation is often compromised with this approach. Here we describe an alternative approach using a correlative light and electron microscopy (CLEM) technique, which combines fluorescent imaging and transmission electron microscopy. By employing this method, a clear correlation between fluorescent speckles and the presence of individual or clusters of PD is achieved.

Plasmodesmata in integrated cell signalling: insights from development and environmental signals and stresses

To survive as sedentary organisms built of immobile cells, plants require an effective intercellular communication system, both locally between neighbouring cells within each tissue and systemically across distantly located organs. Such a system enables cells to coordinate their intracellular activities and produce concerted responses to internal and external stimuli. Plasmodesmata, membrane-lined intercellular channels, are essential for direct cell-to-cell communication involving exchange of diffusible factors, including signalling and information molecules. Recent advances corroborate that plasmodesmata are not passive but rather highly dynamic channels, in that their density in the cell walls and gating activities are tightly linked to developmental and physiological processes. Moreover, it is becoming clear that specific hormonal signalling pathways play crucial roles in relaying primary cellular signals to plasmodesmata. In this review, we examine a number of studies in which plasmodesmal structure, occurrence, and/or permeability responses are found to be altered upon given cellular or environmental signals, and discuss common themes illustrating how plasmodesmal regulation is integrated into specific cellular signalling pathways.

Plasmodesmata paradigm shift: regulation from without versus within

Plant cells are surrounded by cellulosic cell walls, creating a potential challenge to resource sharing and information exchange between individual cells. To overcome this, plants have evolved channels called plasmodesmata that provide cytoplasmic continuity between each cell and its immediate neighbors. We first review plasmodesmata basics-their architecture, their origin, the types of cargo they transport, and their molecular components. The bulk of this review discusses the regulation of plasmodesmata formation and function. Historically, plasmodesmata research has focused intensely on uncovering regulatory or structural proteins that reside within or immediately adjacent to plasmodesmata. Recent findings, however, underscore that plasmodesmata are exquisitely sensitive to signals far removed from the plasmodesmal channel itself. Signals originating from molecules and pathways that regulate cellular homeostasis-such as reactive oxygen species, organelle-organelle signaling, and organelle-nucleus signaling-lead to astonishing alterations in gene expression that affect plasmodesmata formation and function.

The cytoskeleton in plasmodesmata: a role in intercellular transport?

Actin and myosin are components of the plant cell cytoskeleton that extend from cell to cell through plasmodesmata (PD), but it is unclear how they are organized within the cytoplasmic sleeve or how they might behave as regulatory elements. Early work used antibodies to locate actin and myosin to PD, at the electron microscope level, or to pitfields (aggregations of PD in the cell wall), using immunofluorescence techniques. More recently, a green fluorescent protein (GFP)-tagged plant myosin VIII was located specifically at PD-rich pitfields in cell walls. Application of actin or myosin disrupters may modify the conformation of PD and alter rates of cell-cell transport, providing evidence for a role in regulating PD permeability. Intriguingly, there is now evidence of differentiation between types of PD, some of which open in response to both actin and myosin disrupters, and others which are unaffected by actin disrupters or which close in response to myosin inhibitors. Viruses also interact with elements of the cytoskeleton for both intracellular and intercellular transport. The precise function of the cytoskeleton in PD may change during cell development, and may not be identical in all tissue types, or even in all PD within a single cell. Nevertheless, it is likely that actin- and myosin-associated proteins play a key role in regulating cell-cell transport, by interacting with cargo and loading it into PD, and may underlie the capacity for one-way transport across particular cell and tissue boundaries.

Arabidopsis plasmodesmal proteome

The multicellular nature of plants requires that cells should communicate in order to coordinate essential functions. This is achieved in part by molecular flux through pores in the cell wall, called plasmodesmata. We describe the proteomic analysis of plasmodesmata purified from the walls of Arabidopsis suspension cells. Isolated plasmodesmata were seen as membrane-rich structures largely devoid of immunoreactive markers for the plasma membrane, endoplasmic reticulum and cytoplasmic components. Using nano-liquid chromatography and an Orbitrap ion-trap tandem mass spectrometer, 1341 proteins were identified. We refer to this list as the plasmodesmata- or PD-proteome. Relative to other cell wall proteomes, the PD-proteome is depleted in wall proteins and enriched for membrane proteins, but still has a significant number (35%) of putative cytoplasmic contaminants, probably reflecting the sensitivity of the proteomic detection system. To validate the PD-proteome we searched for known plasmodesmal proteins and used molecular and cell biological techniques to identify novel putative plasmodesmal proteins from a small subset of candidates. The PD-proteome contained known plasmodesmal proteins and some inferred plasmodesmal proteins, based upon sequence or functional homology with examples identified in different plant systems. Many of these had a membrane association reflecting the membranous nature of isolated structures. Exploiting this connection we analysed a sample of the abundant receptor-like class of membrane proteins and a small random selection of other membrane proteins for their ability to target plasmodesmata as fluorescently-tagged fusion proteins. From 15 candidates we identified three receptor-like kinases, a tetraspanin and a protein of unknown function as novel potential plasmodesmal proteins. Together with published work, these data suggest that the membranous elements in plasmodesmata may be rich in receptor-like functions, and they validate the content of the PD-proteome as a valuable resource for the further uncovering of the structure and function of plasmodesmata as key components in cell-to-cell communication in plants.

Opportunities and successes in the search for plasmodesmal proteins

The proteinaceous composition of plasmodesmata (PDs) is a puzzle for which pieces have proven particularly difficult to find. This review describes the numerous approaches that have been undertaken in the search for PD-associated proteins and what each has contributed to our understanding of PD structure and function. These approaches include immunolocalisation of known proteins, proteomic characterisation of PD-enriched tissue fractions, high-throughput screens of random cDNAs and mutant screens. In addition to components of the cytoskeleton, novel proteins with predicted or unknown functions have been identified. Many of these have properties that relate to the symplastic and/or apoplastic faces of the plasma membrane. Mutant screens have identified proteins involved in previously unconnected cell pathways such as ROS signalling, implicating ROS in PD formation and regulation. Proteins associated with callose synthesis and degradation have also been identified and characterised, providing considerable weight to the hypothesis that callose deposition around the neck of the PD pore is one mechanism by which the PD aperture is regulated. The techniques described in this review have been developed such that it is to be expected that a considerable number of new PD proteins will be identified in coming years to fill in further detail of the structure and functional mechanisms of these dynamic pores.

Redox regulation of intercellular transport

Plant cells communicate with each other via plasmodesmata (PDs) in order to orchestrate specific responses to environmental and developmental cues. At the same time, environmental signals regulate this communication by promoting changes in PD structure that modify symplastic permeability and, in extreme cases, isolate damaged cells. Reactive oxygen species (ROS) are key messengers in plant responses to a range of biotic and abiotic stresses. They are also generated during normal metabolism, and mediate signaling pathways that modulate plant growth and developmental transitions. Recent research has suggested the participation of ROS in the regulation of PD transport. The study of several developmental and stress-induced processes revealed a co-regulation of ROS and callose (a cell wall polymer that regulates molecular flux through PDs). The identification of Arabidopsis mutants simultaneously affected in cell redox homeostasis and PD transport, and the histological detection of hydrogen peroxide and peroxidases in the PDs of the tomato vascular cambium provide new information in support of this novel regulatory mechanism. Here, we describe the evidence that supports a role for ROS in the regulation of callose deposition and/or in the formation of secondary PD, and discuss the potential importance of this mechanism during plant growth or defense against environmental stresses.

Plasmodesmata viewed as specialised membrane adhesion sites

A significant amount of work has been expended to identify the elusive components of plasmodesmata (PD) to help understand their structure, as well as how proteins are targeted to them. This review focuses on the role that lipid membranes may play in defining PD both structurally and as subcellular targeting addresses. Parallels are drawn to findings in other areas of research which focus on the lateral segregation of membrane domains and the generation of three-dimensional organellar shapes from flat lipid bilayers. We conclude that consideration of the protein-lipid interactions in cell biological studies of PD components and PD-targeted proteins may yield new insights into some of the many open questions regarding these unique structures.

Closure of plasmodesmata in maize (Zea mays) at low temperature: a new mechanism for inhibition of photosynthesis

BACKGROUND AND AIMS

Photosynthesis is one of the processes most susceptible to low-temperature inhibition in maize, a tropical C4 crop not yet fully adapted to a temperate climate. C4 photosynthesis relies on symplasmic exchange of large amounts of photosynthetic intermediates between Kranz mesophyll (KMS) and bundle sheath (BS) cells. The aim of this study was to test the hypothesis that the slowing of maize photosynthesis at low temperature is related to ultrastructural changes in the plasmodesmata between KM and BS as well as BS and vascular parenchyma (VP) cells.

METHODS

Chilling-tolerant (CT) KW 1074 and chilling-sensitive (CS) CM 109 maize (Zea mays) lines were studied. The effect of moderate chilling (14 °C) on the rate of photosynthesis, photosynthate transport kinetics, and the ultrastructure of plasmodesmata linking the KMS, BS and VP cells were analysed. Additionally, the accumulation of callose and calreticulin was studied by the immunogold method.

KEY RESULTS

Chilling inhibited photosynthesis, photosynthate transfer to the phloem and photosynthate export from leaves in both lines. This inhibition was reversible upon cessation of chilling in the CT line but irreversible in the CS line. Simultaneously to physiological changes, chilling induced swelling of the sphincters of plasmodesmata linking KMS and BS cells and a decreased lumen of the cytoplasmic sleeve of plasmodesmata at the BS/VP interface in the CS line but not in the CT line. Accumulation of calreticulin, which occurred near the neck region of the closed plasmodesmata was observed after just 4 h of chilling and over-accumulation of callose at the KMS/BS and BS/VP interfaces occurred after 28 h of chilling.

CONCLUSIONS

Stronger chilling sensitivity of the CM 109 maize line compared with the KW 1074 line, shown by decreased photosynthesis and assimilate export from a leaf, is related to changes in the ultrastructure of leaf plasmodesmata at low temperature. The chain of reactions to chilling is likely to include calreticulin action resulting in rapid and efficient closure of the plasmodesmata at both KMS/BS and BS/VP interfaces. Callose deposition in a leaf was a secondary effect of chilling.

Arabidopsis TONNEAU1 proteins are essential for preprophase band formation and interact with centrin

Plant cells have specific microtubule structures involved in cell division and elongation. The tonneau1 (ton1) mutant of Arabidopsis thaliana displays drastic defects in morphogenesis, positioning of division planes, and cellular organization. These are primarily caused by dysfunction of the cortical cytoskeleton and absence of the preprophase band of microtubules. Characterization of the ton1 insertional mutant reveals complex chromosomal rearrangements leading to simultaneous disruption of two highly similar genes in tandem, TON1a and TON1b. TON1 proteins are conserved in land plants and share sequence motifs with human centrosomal proteins. The TON1 protein associates with soluble and microsomal fractions of Arabidopsis cells, and a green fluorescent protein-TON1 fusion labels cortical cytoskeletal structures, including the preprophase band and the interphase cortical array. A yeast two-hybrid screen identified Arabidopsis centrin as a potential TON1 partner. This interaction was confirmed both in vitro and in plant cells. The similarity of TON1 with centrosomal proteins and its interaction with centrin, another key component of microtubule organizing centers, suggests that functions involved in the organization of microtubule arrays by the centrosome were conserved across the evolutionary divergence between plants and animals.

Gene expression in developing watermelon fruit

Background

Cultivated watermelon form large fruits that are highly variable in size, shape, color, and content, yet have extremely narrow genetic diversity. Whereas a plethora of genes involved in cell wall metabolism, ethylene biosynthesis, fruit softening, and secondary metabolism during fruit development and ripening have been identified in other plant species, little is known of the genes involved in these processes in watermelon. A microarray and quantitative Real-Time PCR-based study was conducted in watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus] in order to elucidate the flow of events associated with fruit development and ripening in this species. RNA from three different maturation stages of watermelon fruits, as well as leaf, were collected from field grown plants during three consecutive years, and analyzed for gene expression using high-density photolithography microarrays and quantitative PCR.

Results

High-density photolithography arrays, composed of probes of 832 EST-unigenes from a subtracted, fruit development, cDNA library of watermelon were utilized to examine gene expression at three distinct time-points in watermelon fruit development. Analysis was performed with field-grown fruits over three consecutive growing seasons. Microarray analysis identified three hundred and thirty-five unique ESTs that are differentially regulated by at least two-fold in watermelon fruits during the early, ripening, or mature stage when compared to leaf. Of the 335 ESTs identified, 211 share significant homology with known gene products and 96 had no significant matches with any database accession. Of the modulated watermelon ESTs related to annotated genes, a significant number were found to be associated with or involved in the vascular system, carotenoid biosynthesis, transcriptional regulation, pathogen and stress response, and ethylene biosynthesis. Ethylene bioassays, performed with a closely related watermelon genotype with a similar phenotype, i.e. seeded, bright red flesh, dark green rind, etc., determined that ethylene levels were highest during the green fruit stage followed by a decrease during the white and pink fruit stages. Additionally, quantitative Real-Time PCR was used to validate modulation of 127 ESTs that were differentially expressed in developing and ripening fruits based on array analysis.

Conclusion

This study identified numerous ESTs with putative involvement in the watermelon fruit developmental and ripening process, in particular the involvement of the vascular system and ethylene. The production of ethylene during fruit development in watermelon gives further support to the role of ethylene in fruit development in non-climacteric fruits.

Tunneling nanotubes: a new route for the exchange of components between animal cells

Recently, highly sensitive nanotubular structures mediating membrane continuity between mammalian cells have been discovered. With respect to their peculiar architecture, these membrane channels were termed tunneling nanotubes (TNTs). TNTs could form de novo between animal cells leading to the generation of complex cellular networks. They have been shown to facilitate the intercellular transfer of organelles as well as, on a limited scale, of membrane components and cytoplasmic molecules. It has been proposed that TNTs represent a novel and general biological principle of cell-to-cell communication and it becomes increasingly apparent that they fulfill important functions in the physiological processes of multicellular organisms.

Cytokinesis in plant and animal cells: endosomes 'shut the door'

For many years, cytokinesis in eukaryotic cells was considered to be a process that took a variety of forms. This is rather surprising in the face of an apparently conservative mitosis. Animal cytokinesis was described as a process based on an actomyosin-based contractile ring, assembling, and acting at the cell periphery. In contrast, cytokinesis of plant cells was viewed as the centrifugal generation of a new cell wall by fusion of Golgi apparatus-derived vesicles. However, recent advances in animal and plant cell biology have revealed that many features formerly considered as plant-specific are, in fact, valid also for cytokinetic animal cells. For example, vesicular trafficking has turned out to be important not only for plant but also for animal cytokinesis. Moreover, the terminal phase of animal cytokinesis based on midbody microtubule activity resembles plant cytokinesis in that interdigitating microtubules play a decisive role in the recruitment of cytokinetic vesicles and directing them towards the cytokinetic spaces which need to be plugged by fusing endosomes. Presently, we are approaching another turning point which brings cytokinesis in plant and animal cells even closer. As an unexpected twist, new studies reveal that both plant and animal cytokinesis is driven not so much by Golgi-derived vesicles but rather by homotypically and heterotypically fusing endosomes. These are generated from cytokinetic cortical sites defined by preprophase microtubules and contractile actomyosin ring, which induce local endocytosis of both the plasma membrane and cell wall material. Finally, plant and animal cytokinesis meet together at the physical separation of daughter cells despite obvious differences in their preparatory events.

Proteomic identification of putative plasmodesmatal proteins from Chara corallina

Plasmodesmata are channels that bridge the cell walls of plant cells, allowing regulated transport of molecules between neighbouring cells. We have used a proteomic strategy to identify putative plasmodesmata-associated proteins in the giant-celled green alga Chara corallina. Proteins were extracted from the plasmodesmata-rich nodal complexes and the middle of the long internodal cells, which do not contain plasmodesmata. Comparison of protein spot patterns generated by two-dimensional gel electrophoresis of both the soluble and cell wall fractions from the two cell types was done. Fifty-eight spots that were common to the nodal and internodal soluble fractions were analysed by matrix assisted laser desorption/ionisation-time of flight mass spectrometry, and peptide mass fingerprint data were used to search the database. Matches were made to four of these spots, in each case to housekeeping proteins. Further, a number of nodal specific spots were identified, 11 from the soluble fraction and nine from the wall fraction. These spots were excised from the gels and analysed by liquid chromatography tandem mass spectrometry to obtain peptide sequence. Database searches suggest that these spots include homologues to previously identified plasmodesmata-associated proteins cp-wap13 and heat shock cognate 70, as well as RNA-binding proteins, eukaryotic initiation factor 4A and a beta-1,3-glucanase. Several spots remained unidentified providing exciting new candidate plasmodesmata-associated proteins.

Blastocladiella emersoniiexpresses a centrin similar toChlamydomonas reinhardtiiisoform not found in late-diverging fungi

Centrins are members of the calcium-binding EF-hand protein superfamily which can be divided into two subfamilies, probably associated with different functions: one related to Chlamydomonas reinhardtii centrin, CrCenp, and the other, represented by Saccharomyces cerevisiae isoform, ScCdc31p. ESTs encoding the two isoforms (BeCen1 and BeCen3) from the chytridiomycete Blastocladiella emersonii were isolated, and expression of the CrCenp-type centrin, BeCen1, was analyzed throughout the fungus life cycle. Becen1 mRNA levels increase transiently during sporulation and protein levels present a similar pattern. Immunolocalization studies seem to localize BeCen1 at the basal body zone and in the cytoplasm surrounding the nuclear cap, a zoospore organelle.

Plasmodesmata form and function

Intercellular transport via plasmodesmata controls cell fate decisions in plants, and is of fundamental importance in viral movement, disease resistance, and the spread of RNAi signals. Although plasmodesmata appear to be unique to plant cells, they may have structural and functional similarities to the newly discovered tunneling nanotubes that connect animal cells. Recently, proteins that localize to plasmodesmata have been identified, and a microtubule-associated protein was found to negatively regulate the trafficking of viral movement proteins. Other advances have delivered new insights into the function and molecular nature of plasmodesmata and have shown that protein trafficking through plasmodesmata is developmentally regulated, opening up the possibility that the genetic control of plasmodesmal function will soon be understood.

Plasmodesmata as a supracellular control network in plants

The evolution of intercellular communication had an important role in the increasing complexity of both multicellular and supracellular organisms. Plasmodesmata, the intercellular organelles of the plant kingdom, establish an effective pathway for local and long-distance signalling. In higher plants, this pathway involves the trafficking of proteins and various forms of RNA that function non-cell-autonomously to affect developmental programmes.

Macromolecular Transport and Signaling Through Plasmodesmata

Plasmodesmata (Pd) are channels in the plant cell wall that in conjunction with associated phloem form an intercellular communication network that supports the cell-to-cell and long-distance trafficking of a wide spectrum of endogenous proteins and ribonucleoprotein complexes. The trafficking of such macromolecules is of importance in the orchestration of non-cell autonomous developmental and physiological processes. Plant viruses encode movement proteins (MPs) that subvert this communication network to facilitate the spread of infection. These viral proteins thus represent excellent experimental keys for exploring the mechanisms involved in intercellular trafficking and communication via Pd.

Getting the message across: how do plant cells exchange macromolecular complexes?

A major pathway for macromolecular exchange in plants involves plasmodesmata (PD), the small pores that connect adjoining cells. This article considers the nature of macromolecular complexes (MCs) that pass through PD and the pathways and mechanisms that guide them to the PD pore. Recent cell-biological studies have identified proteins involved in the directional trafficking of MCs to PD, and yeast two-hybrid studies have isolated novel host proteins that interact with viral movement proteins. Collectively, these studies are yielding important clues in the search for components that compose the plant intercellular MC trafficking pathway. Here, they are placed in the context of a functional model that links the cytoskeleton, chaperones and secretory pathway in the intercellular trafficking of MCs.

Plasmodesmata at the crossroads between development, dormancy, and defense

Plants are frequently exposed to environmental stress and organisms that seek to benefit from their autotrophic nature. To cope with these challenges plants have developed stress-resistance mechanisms, which involve sensing, activation of signal transduction cascades, changes in gene expression, and physiological adjustment. Exposure to one kind of stress often leads to cross-tolerance, that is, resistance to different kinds of stresses. The search for a common underlying mechanism concentrates mostly on changes in cellular physiology and gene expression. We focus on the cross-protective measures that are taken at the level above the single cell. We argue that the controlled alterations in symplasmic permeability that underlie development also play a role in survival and defense strategies. In development, most of the alterations are transient and dynamic, whereas the more persistent alterations function predominantly in dormancy and defense and are under the control of two key enzymes: 1,3-β-D-glucan synthase and 1,3-β-D-glucanase. 1,3-β-D-Glucan synthase functions in the narrowing or closing of plasmodesmata, whereas 1,3-β-D-glucanase counteracts this process. We propose that the closing of symplasmic paths constitutes an unspecific but effective early measure in adaptation and defense, which is accompanied by specific strategies tailored to the various challenges plants face.Key words: cross-adaptation, dormancy sphincter, 1,3-β-D-glucanase, 1,3-β-D-glucan synthase, meristem, overwintering, plasmodesmata, virus movement.

Gamma-tubulin distribution during cortical microtubule reorganization at the M/G1 interface in tobacco BY-2 cells

Cortical microtubules are considered to regulate the direction of cellulose microfibril deposition. Despite their significant role in determining cell morphology, cortical microtubules completely disappear from the cell cortex during M phase and become reorganized at G1 phase. The mechanism by which these microtubules become properly formed again is, however, still unclear. We have proposed that the origin of cortical microtubules is on the daughter nuclear surface, but further cortical microtubule reorganization occurs at the cell cortex. Hence it is probable that the locations of microtubule organizing centers (MTOCs) are actively changing. However, the actual MTOC sites of cortical microtubules were not clearly determined. In this paper, we have examined the distribution of gamma-tubulin, one of the key molecules of MTOCs in various organisms, during cortical microtubule reorganization using both immunofluorescence and a GFP reporter system. Using a monoclonal antibody (clone G9) that recognizes highly conserved residues in y-tubulin, y-tubulin was found to be constitutively expressed and to be clearly localized to microtubule structures, such as the preprophase bands, spindles, and phragmoplasts, specific to each cell cycle stage. This distribution pattern was confirmed by the GFP reporter system. During cortical microtubule reorganization at the M to G1 transition phase, gamma-tubulin first accumulated at the daughter nuclear surfaces, and then seemed to spread onto the cell cortex along with microtubules elongating from the daughter nuclei. Based on the results, it was confirmed that daughter nuclear surfaces acted as origins of cortical microtubules, and that further reorganization occurred on the cell cortex.

Plasmodesmata: dynamic regulation and role in macromolecular cell-to-cell signaling

Recent studies have demonstrated the functional significance of intercellular RNA and protein trafficking in plant development, confirming the role of plasmodesmata (PD) in the mediation and control of intercellular communication via macromolecules. Small fluorescent tracer loading techniques and experiments involving the expression of proteins tagged with green fluorescent protein (GFP) have been used to investigate the mechanisms of PD targeting and trafficking, as well as to elucidate the dynamic and structural properties of these channels.

Plasmodesmata and plant cytoskeleton

Plant cell-to-cell communication is achieved by membranous conduits called plasmodesmata, which bridge the cytoplasm of neighboring cells. A growing body of immunolocalization data shows an association of the cytoskeleton machinery with plasmodesmata. The role of the cytoskeleton in the plasmodesmata-mediated transport has been well documented for virus movement. Because viruses are known to exploit existing host pathways and because the cytoskeleton is involved in intracellular trafficking, the cytoskeleton is thought to drive and target macromolecules to plasmodesmata. It is this link between plasmodesmata and the cytoskeleton that will be described here.

CYTOKINESIS AND BUILDING OF THE CELL PLATE IN PLANTS

Cytokinesis in plant cells is more complex than in animals, as it involves building a cell plate as the final step in generating two cells. The cell plate is built in the center of phragmoplast by fusion of Golgi-derived vesicles. This step imposes an architectural problem where ballooning of the fused structures has to be avoided to create a plate instead. This is apparently achieved by squeezing the vesicles into dumbbell-shaped vesicle-tubule-vesicle (VTV) structures with the help of phragmoplastin, a homolog of dynamin. These structures are fused at their ends in a star-shaped body creating a tubulovesicular "honeycomb-like" structure sandwiched between the positive ends of the phragmoplast microtubules. This review summarizes our current understanding of various mechanisms involved in budding-off of Golgi vesicles, delivery and fusion of vesicles to initiate cell plate, and the synthesis of polysaccharides at the forming cell plate. Little is known about the molecular mechanisms involved in determining the site, direction, and the point of attachment of the growing cell plate with the parental cell wall. These gaps may be filled soon, as many genes that have been identified by mutations are analyzed and functions of their products are deciphered.

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.

Calcium: silver bullet in signaling

Accumulating evidence suggests that Ca(2+) serves as a messenger in many normal growth and developmental process and in plant responses to biotic and abiotic stresses. Numerous signals have been shown to induce transient elevation of [Ca(2+)](cyt) in plants. Genetic, biochemical, molecular and cell biological approaches in recent years have resulted in significant progress in identifying several Ca(2+)-sensing proteins in plants and in understanding the function of some of these Ca(2+)-regulated proteins at the cellular and whole plant level. As more and more Ca(2+)-sensing proteins are identified it is becoming apparent that plants have several unique Ca(2+)-sensing proteins and that the downstream components of Ca(2+) signaling in plants have novel features and regulatory mechanisms. Although the mechanisms by which Ca(2+) regulates diverse biochemical and molecular processes and eventually physiological processes in response to diverse signals are beginning to be understood, recent studies have raised many interesting questions. Despite the fact that Ca(2+) sensing proteins are being identified at a rapid pace, progress on the function(s) of many of them is limited. Studies on plant 'signalome' - the identification of all signaling components in all messengers mediated transduction pathways, analysis of their function and regulation, and cross talk among these components - should help in understanding the inner workings of plant cell responses to diverse signals. New functional genomics approaches such as reverse genetics, microarray analyses coupled with in vivo protein-protein interaction studies and proteomics should not only permit functional analysis of various components in Ca(2+) signaling but also enable identification of a complex network of interactions.

Fine structure analysis of the yeast centrin, Cdc31p, identifies residues specific for cell morphology and spindle pole body duplication

Centrin/Cdc31p is a Ca2+-binding protein related to calmodulin found in the MTOC of diverse organisms. In yeast, Cdc31p localizes to the SPB where it interacts with Kar1p and is required for SPB duplication. Recent findings suggest that centrin also functions elsewhere in the cell. To dissect the functions of Cdc31p, we generated cdc31 mutations chosen only for temperature sensitivity, but otherwise unbiased as to phenotype. Three phenotypes of the cdc31 mutants, temperature sensitivity, G2/M arrest, and cell lysis, were not well correlated, indicating that the mutations may differentially affect Cdc31p's interactions with other proteins. Alleles near the C-terminal region exhibited high G2/M arrest and genetic interactions with kar1-Delta17, suggesting that this region modulates an SPB-related function. Alleles causing high lysis and reduced Kic1p kinase activity mapped to the middle of the gene, suggesting disruption of a KIC1-like function and defects in activating Kic1p. A third region conferred temperature sensitivity without affecting cell lysis or G2/M arrest, suggesting that it defines a third function. Mutations in the C-terminal region were also defective for interaction with Kic1p. Mapping the alleles onto a predicted structure of Cdc31p, we have identified surfaces likely to be important for interacting with both Kar1p and Kic1p.

Primary and secondary plasmodesmata: structure, origin, and functioning

In the multicellular organisms of higher plants, plasmodesmata provide pathways for intimate symplasmic communication between neighboring cells. The arguments summarized in the present review demonstrate that plasmodesmata are diverse and highly dynamic structures. Differences in the plasmodesmal origin and modifications of the plasmodesmal structure and functioning at the various cell interfaces are the basic means which give rise to a complicated and flexibile symplasmic network. This complex communication system is discussed to serve a significant role in the coordinated development and in the concerted physiological functioning of the cells within the plant tissues, organs, and organisms.

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.

Opening up the communication channels: recent insights into plasmodesmal function

The past year has seen significant advances in our understanding of the function and regulation of plasmodesmata. Notably, we have learned that plasmodesmata undergo dynamic changes during development and may participate in long-range communication through the transmission of RNA signals. Biochemical studies have enriched our understanding of a putative plasmodesmal receptor and of plant factors involved in viral cell-to-cell movement.

Characterization of microsome-associated tobacco BY-2 centrins

Centrin - higher plants - MTOCs - microtubules nucleation In most eukaryotic cells, the Ca(2+)-binding protein centrin is associated with structured microtubule-organizing centers (MTOCs) such as centrosomes. In these cells, centrin either forms centrosome-associated contractile fibers, or is involved in centrosome biogenesis. Our aim was to investigate the functions of centrin in higher plant cells which do not contain centrosome-like MTOCs. We have cloned two tobacco BY-2 centrin cDNAs and we show that higher plant centrins define a phylogenetic group of proteins distinct from centrosome-associated centrins. In addition, tobacco centrins were found primarily associated with microsomes and did not colocalize with gamma-tubulin, a known MTOC marker. While the overall level of centrin did not vary during the cell cycle, centrin was prominently detected at the cell plate during telophase. Our results suggest that in tobacco, the major portion of centrin is not MTOC-associated and could be involved in the formation of the cell plate during cytokinesis.