Symplastic intercellular connectivity regulates lateral root patterning

Immunofluorescence detection of callose deposition around plasmodesmata sites

Accumulation of callose (β-1,3 glucans) at the plasmodesmata (PD) neck region dynamically regulates symplastic intercellular transport. Here we describe a 2-3-day immuno-labelling protocol to determine callose levels in the cell wall region at PD. The method relies on exposure of internal cell walls by hand-sectioning of the sample and digestion of the cell wall with enzymes in order to improve antibody penetration to deep tissue layers. By using this protocol, combined with high-resolution confocal imaging, we successfully detected PD-associated callose in Arabidopsis root apical meristem, vascular tissue, and developing lateral root primordia.

Imaging callose at plasmodesmata using aniline blue: quantitative confocal microscopy

Callose (β-1,3-glucan) is both structural and functional component of plasmodesmata (Pd). The turnover of callose at Pd controls the cell-to-cell diffusion rate of molecules through Pd. An accurate assessment of changes in levels of Pd-associated callose has become a first-choice experimental approach in the research of intercellular communication in plants.Here we describe a detailed and easy-to-perform procedure for imaging and quantification of Pd-associated callose using fixed plant tissue stained with aniline blue. We also introduce an automated image analysis protocol for non-biased quantification of callose levels at Pd from fluorescence images using ImageJ. Two experimental examples of Pd-callose quantification using the automated method are provided as well.

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.

Roles and regulation of plant cell walls surrounding plasmodesmata

In plants, the intercellular transport of simple and complex molecules can occur symplastically through plasmodesmata. These are membranous channels embedded in cell walls that connect neighbouring cells. The properties of the cell walls surrounding plasmodesmata determine their transport capacity and permeability. These cell wall micro-domains are enriched in callose and have a characteristic pectin distribution. Cell wall modifications, leading to changes in plasmodesmata structure, have been reported to occur during development and in response to environmental signals. Cell wall remodelling enzymes target plasmodesmata to rapidly control intercellular communication in situ. Here we describe current knowledge on the composition of cell walls at plasmodesmata sites and on the proteins and signals that modify cell walls to regulate plasmodesmata aperture.

The plasmodesmal protein PDLP1 localises to haustoria-associated membranes during downy mildew infection and regulates callose deposition

The downy mildew pathogen Hyaloperonospora arabidopsidis (Hpa) is a filamentous oomycete that invades plant cells via sophisticated but poorly understood structures called haustoria. Haustoria are separated from the host cell cytoplasm and surrounded by an extrahaustorial membrane (EHM) of unknown origin. In some interactions, including Hpa-Arabidopsis, haustoria are progressively encased by host-derived, callose-rich materials but the molecular mechanisms by which callose accumulates around haustoria remain unclear. Here, we report that PLASMODESMATA-LOCATED PROTEIN 1 (PDLP1) is expressed at high levels in Hpa infected cells. Unlike other plasma membrane proteins, which are often excluded from the EHM, PDLP1 is located at the EHM in Hpa-infected cells prior to encasement. The transmembrane domain and cytoplasmic tail of PDLP1 are sufficient to convey this localization. PDLP1 also associates with the developing encasement but this association is lost when encasements are fully mature. We found that the pdlp1,2,3 triple mutant is more susceptible to Hpa while overexpression of PDLP1 enhances plant resistance, suggesting that PDLPs enhance basal immunity against Hpa. Haustorial encasements are depleted in callose in pdlp1,2,3 mutant plants whereas PDLP1 over-expression elevates callose deposition around haustoria and across the cell surface. These data indicate that PDLPs contribute to callose encasement of Hpa haustoria and suggests that the deposition of callose at haustoria may involve similar mechanisms to callose deposition at plasmodesmata.

Flowers under pressure: ins and outs of turgor regulation in development

BACKGROUND

Turgor pressure is an essential feature of plants; however, whereas its physiological importance is unequivocally recognized, its relevance to development is often reduced to a role in cell elongation.

SCOPE

This review surveys the roles of turgor in development, the molecular mechanisms of turgor regulation and the methods used to measure turgor and related quantities, while also covering the basic concepts associated with water potential and water flow in plants. Three key processes in flower development are then considered more specifically: flower opening, anther dehiscence and pollen tube growth.

CONCLUSIONS

Many molecular determinants of turgor and its regulation have been characterized, while a number of methods are now available to quantify water potential, turgor and hydraulic conductivity. Data on flower opening, anther dehiscence and lateral root emergence suggest that turgor needs to be finely tuned during development, both spatially and temporally. It is anticipated that a combination of biological experiments and physical measurements will reinforce the existing data and reveal unexpected roles of turgor in development.

A mechanistic framework for noncell autonomous stem cell induction in Arabidopsis

Cell-cell communication is essential for multicellular development and, consequently, evolution has brought about an array of distinct mechanisms serving this purpose. Consistently, induction and maintenance of stem cell fate by noncell autonomous signals is a feature shared by many organisms and may depend on secreted factors, direct cell-cell contact, matrix interactions, or a combination of these mechanisms. Although many basic cellular processes are well conserved between animals and plants, cell-to-cell signaling is one function where substantial diversity has arisen between the two kingdoms of life. One of the most striking differences is the presence of cytoplasmic bridges, called plasmodesmata, which facilitate the exchange of molecules between neighboring plant cells and provide a unique route for cell-cell communication in the plant lineage. Here, we provide evidence that the stem cell inducing transcription factor WUSCHEL (WUS), expressed in the niche, moves to the stem cells via plasmodesmata in a highly regulated fashion and that this movement is required for WUS function and, thus, stem cell activity in Arabidopsis thaliana. We show that cell context-independent mobility is encoded in the WUS protein sequence and mediated by multiple domains. Finally, we demonstrate that parts of the protein that restrict movement are required for WUS homodimerization, suggesting that formation of WUS dimers might contribute to the regulation of apical stem cell activity.

Cellular events during interfascicular cambium ontogenesis in inflorescence stems of Arabidopsis

Development of cambium and its activity is important for our knowledge of the mechanism of secondary growth. Arabidopsis thaliana emerges as a good model plant for such a kind of study. Thus, this paper reports on cellular events taking place in the interfascicular regions of inflorescence stems of A. thaliana, leading to the development of interfascicular cambium from differentiated interfascicular parenchyma cells (IPC). These events are as follows: appearance of auxin accumulation, PIN1 gene expression, polar PIN1 protein localization in the basal plasma membrane and periclinal divisions. Distribution of auxin was observed to be higher in differentiating into cambium parenchyma cells compared to cells within the pith and cortex. Expression of PIN1 in IPC was always preceded by auxin accumulation. Basal localization of PIN1 was already established in the cells prior to their periclinal division. These cellular events initiated within parenchyma cells adjacent to the vascular bundles and successively extended from that point towards the middle region of the interfascicular area, located between neighboring vascular bundles. The final consequence of which was the closure of the cambial ring within the stem. Changes in the chemical composition of IPC walls were also detected and included changes of pectic epitopes, xyloglucans (XG) and extensins rich in hydroxyproline (HRGPs). In summary, results presented in this paper describe interfascicular cambium ontogenesis in terms of successive cellular events in the interfascicular regions of inflorescence stems of Arabidopsis.

Bergmann D, De Smet I, Sozzani R, Weijers D, Brady SM. Omics and modelling approaches for understanding regulation of asymmetric cell divisions in arabidopsis and other angiosperm plants

BACKGROUND

Asymmetric cell divisions are formative divisions that generate daughter cells of distinct identity. These divisions are coordinated by either extrinsic ('niche-controlled') or intrinsic regulatory mechanisms and are fundamentally important in plant development.

SCOPE

This review describes how asymmetric cell divisions are regulated during development and in different cell types in both the root and the shoot of plants. It further highlights ways in which omics and modelling approaches have been used to elucidate these regulatory mechanisms. For example, the regulation of embryonic asymmetric divisions is described, including the first divisions of the zygote, formative vascular divisions and divisions that give rise to the root stem cell niche. Asymmetric divisions of the root cortex endodermis initial, pericycle cells that give rise to the lateral root primordium, procambium, cambium and stomatal cells are also discussed. Finally, a perspective is provided regarding the role of other hormones or regulatory molecules in asymmetric divisions, the presence of segregated determinants and the usefulness of modelling approaches in understanding network dynamics within these very special cells.

CONCLUSIONS

Asymmetric cell divisions define plant development. High-throughput genomic and modelling approaches can elucidate their regulation, which in turn could enable the engineering of plant traits such as stomatal density, lateral root development and wood formation.

A phylogenetic approach to study the origin and evolution of plasmodesmata-localized glycosyl hydrolases family 17

Colonization of the land by plants required major modifications in cellular structural composition and metabolism. Intercellular communication through plasmodesmata (PD) plays a critical role in the coordination of growth and cell activities. Changes in the form, regulation or function of these channels are likely linked to plant adaptation to the terrestrial environments. Constriction of PD aperture by deposition of callose is the best-studied mechanism in PD regulation. Glycosyl hydrolases family 17 (GHL17) are callose degrading enzymes. In Arabidopsis this is a large protein family, few of which have been PD-localized. The objective here is to identify correlations between evolution of this protein family and their role at PD and to use this information as a tool to predict the localization of candidates isolated in a proteomic screen. With this aim, we studied phylogenetic relationship between Arabidopsis GHL17 sequences and those isolated from fungi, green algae, mosses and monocot representatives. Three distinct phylogenetic clades were identified. Clade alpha contained only embryophytes sequences suggesting that this subgroup appeared during land colonization in organisms with functional PD. Accordingly, all PD-associated GHL17 proteins identified so far in Arabidopsis thaliana and Populus are grouped in this 'embryophytes only' phylogenetic clade. Next, we tested the use of this knowledge to discriminate between candidates isolated in the PD proteome. Transient and stable expression of GFP protein fusions confirmed PD localization for candidates contained in clade alpha but not for candidates contained in clade beta. Our results suggest that GHL17 membrane proteins contained in the alpha clade evolved and expanded during land colonization to play new roles, among others, in PD regulation.

Symplastic intercellular transport from a developmental perspective

Plant cells have channel-like structures named plasmodesmata that allow for the symplastic molecular transport between neighbouring cells. The importance of plasmodesmata in whole plant development is well acknowledged. They mediate the cell-to-cell and vascular loading and unloading of metabolites, proteins, and other signalling molecules. However, it is still not clear how, mechanistically, these channels are regulated in response to developmental and environmental cues. This review aims to bring together knowledge acquired in recent years on plasmodesmata composition, regulation, and function. Progress in the discovery of factors that regulate symplastic transport and plant development in particular are discussed. This will hopefully highlight the challenges faced by the scientific community to unveil the mechanisms controlling symplastic communication during the formation and maintenance of plant meristems.

Plant Vascular Biology 2013: vascular trafficking

About 200 researchers from around the world attended the Third International Conference on Plant Vascular Biology (PVB 2013) held in July 2013 at the Rantapuisto Conference Center, in Helsinki, Finland (http://www.pvb2013.org). The plant vascular system, which connects every organ in the mature plant, continues to attract the interest of researchers representing a wide range of disciplines, including development, physiology, systems biology, and computational biology. At the meeting, participants discussed the latest research advances in vascular development, long- and short-distance vascular transport and long-distance signalling in plant defence, in addition to providing a context for how these studies intersect with each other. The meeting provided an opportunity for researchers working across a broad range of fields to share ideas and to discuss future directions in the expanding field of vascular biology. In this report, the latest advances in understanding the mechanism of vascular trafficking presented at the meeting have been summarized.

Auxin-callose-mediated plasmodesmal gating is essential for tropic auxin gradient formation and signaling

In plants, auxin functions as a master controller of development, pattern formation, morphogenesis, and tropic responses. A sophisticated transport system has evolved to allow the establishment of precise spatiotemporal auxin gradients that regulate specific developmental programs. A critical unresolved question relates to how these gradients can be maintained in the presence of open plasmodesmata that allow for symplasmic exchange of essential nutrients and signaling macromolecules. Here we addressed this conundrum using genetic, physiological, and cell biological approaches and identified the operation of an auxin-GSL8 feedback circuit that regulates the level of plasmodesmal-localized callose in order to locally downregulate symplasmic permeability during hypocotyl tropic response. This system likely involves a plasmodesmal switch that would prevent the dissipation of a forming gradient by auxin diffusion through the symplasm. This regulatory system may represent a mechanism by which auxin could also regulate symplasmic delivery of a wide range of signaling agents.

Plasmodesmata-mediated intercellular signaling during plant growth and development

Plasmodesmata (PD) are cytoplasmic channels that connect neighboring cells for cell-to-cell communication. PD structure and function vary temporally and spatially to allow formation of symplastic domains during different stages of plant development. Reversible deposition of callose at PD plays an important role in controlling molecular trafficking through PD by regulating their size exclusion limit. Previously, we reported several semi-dominant mutants for CALLOSE SYNTHASE 3 (CALS3) gene, which overproduce callose at PD in Arabidopsis. By combining two of these mutations in a LexA-VP16-ER (XVE)-based estradiol inducible vector system, a tool known as the "icals3m system" was developed to temporally obstruct the symplastic connections in a specified spatial domain. The system has been successfully tested and used, in combination with other methods, to investigate the route for mobile signals such as the SHR protein, microRNA165/6, and cytokinins in Arabidopsis roots, and also to understand the role of symplastic domain formation during lateral root development. We envision that this tool may also be useful for identifying tissue-specific symplastic regulatory networks and to analyze symplastic movement of metabolites.

Traffic monitors at the cell periphery: the role of cell walls during early female reproductive cell differentiation in plants

The formation of female gametes in plants occurs within the ovule, a floral organ that is also the precursor of the seed. Unlike animals, plants lack a typical germline separated from the soma early in development and rely on positional signals, including phytohormones, mobile mRNAs and sRNAs, to direct diploid somatic precursor cells onto a reproductive program. In addition, signals moving between plant cells must overcome the architectural limitations of a cell wall which surrounds the plasma membrane. Recent studies have addressed the molecular and histological signatures of young ovule cells and indicate that dynamic cell wall changes occur over a short developmental window. These changes in cell wall properties impact signal flow and ovule cell identity, thereby aiding the establishment of boundaries between reproductive and somatic ovule domains.

A spatial accommodation by neighboring cells is required for organ initiation in Arabidopsis

Lateral root formation in plants can be studied as the process of interaction between chemical signals and physical forces during development. Lateral root primordia grow through overlying cell layers that must accommodate this incursion. Here, we analyze responses of the endodermis, the immediate neighbor to an initiating lateral root. Endodermal cells overlying lateral root primordia lose volume, change shape, and relinquish their tight junction-like diffusion barrier to make way for the emerging lateral root primordium. Endodermal feedback is absolutely required for initiation and growth of lateral roots, and we provide evidence that this is mediated by controlled volume loss in the endodermis. We propose that turgidity and rigid cell walls, typical of plants, impose constraints that are specifically modified for a given developmental process.

Plasma membrane partitioning: from macro-domains to new views on plasmodesmata

Compartmentalization of cellular functions relies on partitioning of domains of diverse sizes within the plasma membrane (PM). Macro-domains measure several micrometers and contain specific proteins concentrated to specific sides (apical, basal, and lateral) of the PM conferring a polarity to the cell. Cell polarity is one of the driving forces in tissue and growth patterning. To maintain macro-domains within the PM, eukaryotic cells exert diverse mechanisms to counteract the free lateral diffusion of proteins. Protein activation/inactivation, endocytosis, PM recycling of transmembrane proteins and the role of diffusion barriers in macro-domains partitioning at PM will be discussed. Moreover, as plasmodesmata (PDs) are domains inserted within the PM which also mediate tissue and growth patterning, it is essential to understand how segregation of specific set of proteins is maintained at PDs while PDs domains are smaller in size compared to macro-domains. Here, we will present mechanisms allowing restriction of proteins at PM macro-domains, but for which molecular components have been found in PDs proteome. We will explore the hypothesis that partitioning of macro-domains and PDs may be ruled by similar mechanisms.

Callose homeostasis at plasmodesmata: molecular regulators and developmental relevance

Plasmodesmata are membrane-lined channels that are located in the plant cell wall and that physically interconnect the cytoplasm and the endoplasmic reticulum (ER) of adjacent cells. Operating as controllable gates, plasmodesmata regulate the symplastic trafficking of micro- and macromolecules, such as endogenous proteins [transcription factors (TFs)] and RNA-based signals (mRNA, siRNA, etc.), hence mediating direct cell-to-cell communication and long distance signaling. Besides this physiological role, plasmodesmata also form gateways through which viral genomes can pass, largely facilitating the pernicious spread of viral infections. Plasmodesmatal trafficking is either passive (e.g., diffusion) or active and responses both to developmental and environmental stimuli. In general, plasmodesmatal conductivity is regulated by the controlled build-up of callose at the plasmodesmatal neck, largely mediated by the antagonistic action of callose synthases (CalSs) and β-1,3-glucanases. Here, in this theory and hypothesis paper, we outline the importance of callose metabolism in PD SEL control, and highlight the main molecular factors involved. In addition, we also review other proteins that regulate symplastic PD transport, both in a developmental and stress-responsive framework, and discuss on their putative role in the modulation of PD callose turn-over. Finally, we hypothesize on the role of structural sterols in the regulation of (PD) callose deposition and outline putative mechanisms by which this regulation may occur.

Plasmodesmata-associated proteins: can we see the whole elephant?

Encased in rigid cell walls, plant cells have evolved unique channel structures, plasmodesma (Pd), to create a pathway for molecular exchange between adjacent cells. Pd are basically cytoplasmic channels through the cell wall, which are lined by plasma membrane, and contain a modified strand of ER that spans them. These structures provide cytoplasmic and membrane continuity between connected cells, and that continuity is utilized for short and long distance molecular trafficking. Pd sphincters, made from constricting the Pd openings by outer layers of callose, together with the ER strand that occludes the Pd lumen set the upper limit for the size of molecules that can freely diffuse through the cytoplasmic component of the Pd channel. This limit, called the size exclusion limit (SEL), is a major factor that restricts macromolecular transport through Pd.

Traffic control in the root: keeping root branching in check

While the importance of intercellular communication in plants is well documented, the role of symplastic transport during the formation of new meristems in roots has remained unexplored. In this issue of Developmental Cell, Benitez-Alfonso et al. (2013) show that callose-dependent cell-to-cell connectivity determines root branching in Arabidopsis thaliana.

Root apex transition zone as oscillatory zone

Root apex of higher plants shows very high sensitivity to environmental stimuli. The root cap acts as the most prominent plant sensory organ; sensing diverse physical parameters such as gravity, light, humidity, oxygen, and critical inorganic nutrients. However, the motoric responses to these stimuli are accomplished in the elongation region. This spatial discrepancy was solved when we have discovered and characterized the transition zone which is interpolated between the apical meristem and the subapical elongation zone. Cells of this zone are very active in the cytoskeletal rearrangements, endocytosis and endocytic vesicle recycling, as well as in electric activities. Here we discuss the oscillatory nature of the transition zone which, together with several other features of this zone, suggest that it acts as some kind of command center. In accordance with the early proposal of Charles and Francis Darwin, cells of this root zone receive sensory information from the root cap and instruct the motoric responses of cells in the elongation zone.

Plasmodesmata dynamics are coordinated by intracellular signaling pathways

Membrane-lined channels called plasmodesmata (PD) connect the cytoplasts of adjacent plant cells across the cell wall, permitting intercellular movement of small molecules, proteins, and RNA. Recent genetic screens for mutants with altered PD transport identified genes suggesting that chloroplasts play crucial roles in coordinating PD transport. Complementing this discovery, studies manipulating expression of PD-localized proteins imply that changes in PD transport strongly impact chloroplast biology. Ongoing efforts to find genes that control root and stomatal development reveal the critical role of PD in enforcing tissue patterning, and newly discovered PD-localized proteins show that PD influence development, intracellular signaling, and defense against pathogens. Together, these studies demonstrate that PD function and formation are tightly integrated with plant physiology.

Callose deposition and symplastic connectivity are regulated prior to lateral root emergence

Root growth is critical for the effective exploitation of the rhizosphere and productive plant growth. Our recent work¹ showed that root architecture was dependent upon the degree of symplastic connectivity between neighboring cells during the specification of lateral root primordia and was affected by genes regulating callose deposition at plasmodesmata (PD). Here we provide additional evidence that both symplastic connectivity and callose are also important during the later phase of lateral root development: emergence. Callose immunolocalization assays indicated that transient symplastic isolation of the primordium occur immediately prior to emergence through the overlaying tissues to produce the mature lateral root.¹ Here we could corroborate these results by analyzing the mobility of a symplastic tracer and the expression of PD genes in lateral roots and in response to auxins. Moreover, we show that altering callose deposition affects the number of emerged lateral roots suggesting that PD regulation is important for emergence.

Receptor-mediated signaling at plasmodesmata

Plasmodesmata (PD) generate continuity between plant cells via the cytoplasm, endoplasmic reticulum (ER) and plasma membrane (PM), allowing movement of different classes of molecules between cells. Proteomic data indicates that the PD PM hosts many receptors and receptor kinases, as well as lipid raft and tetraspanin enriched microdomain associated proteins, suggesting the hypothesis that the PD PM is specialized with respect to both composition and function. PD-located receptor proteins and receptor kinases are responsible for perception of microbe associated molecular patterns at PD and initiate signaling that mediates changes to PD flux. In addition, developmentally relevant receptor kinases have different interactions dependent upon whether located at the PD PM or the cellular PM. The implications of these findings are that receptor-mediated signaling in PD membranes differs from that in the cellular PM and, in light the identification of PD-located proteins associated with membrane microdomains and the role of membrane microdomains in analogous signaling processes in animals, suggests that the PD PM contains specialized signaling platforms.