Immunogold localization of the cell-wall-matrix polysaccharides rhamnogalacturonan I and xyloglucan during cell expansion and cytokinesis inTrifolium pratense L.; implication for secretory pathways

Four-dimensional quantitative analysis of cell plate development in Arabidopsis using lattice light sheet microscopy identifies robust transition points between growth phases

Cell plate formation during cytokinesis entails multiple stages occurring concurrently and requiring orchestrated vesicle delivery, membrane remodelling, and timely deposition of polysaccharides, such as callose. Understanding such a dynamic process requires dissection in time and space; this has been a major hurdle in studying cytokinesis. Using lattice light sheet microscopy (LLSM), we studied cell plate development in four dimensions, through the behavior of yellow fluorescent protein (YFP)-tagged cytokinesis-specific GTPase RABA2a vesicles. We monitored the entire duration of cell plate development, from its first emergence, with the aid of YFP-RABA2a, in both the presence and absence of cytokinetic callose. By developing a robust cytokinetic vesicle volume analysis pipeline, we identified distinct behavioral patterns, allowing the identification of three easily trackable cell plate developmental phases. Notably, the phase transition between phase I and phase II is striking, indicating a switch from membrane accumulation to the recycling of excess membrane material. We interrogated the role of callose using pharmacological inhibition with LLSM and electron microscopy. Loss of callose inhibited the phase transitions, establishing the critical role and timing of the polysaccharide deposition in cell plate expansion and maturation. This study exemplifies the power of combining LLSM with quantitative analysis to decode and untangle such a complex process.

Xyloglucan deficiency leads to a reduction in turgor pressure and changes in cell wall properties, affecting early seedling establishment

Xyloglucan is believed to play a significant role in cell wall mechanics of dicot plants. Surprisingly, Arabidopsis plants defective in xyloglucan biosynthesis exhibit nearly normal growth and development. We investigated a mutant line, cslc-Δ5, lacking activity in all five Arabidopsis cellulose synthase like-C (CSLC) genes responsible for xyloglucan backbone biosynthesis. We observed that this xyloglucan-deficient line exhibited reduced cellulose crystallinity and increased pectin levels, suggesting the existence of feedback mechanisms that regulate wall composition to compensate for the absence of xyloglucan. These alterations in cell wall composition in the xyloglucan-absent plants were further linked to a decrease in cell wall elastic modulus and rupture stress, as observed through atomic force microscopy (AFM) and extensometer-based techniques. This raised questions about how plants with such modified cell wall properties can maintain normal growth. Our investigation revealed two key factors contributing to this phenomenon. First, measurements of turgor pressure, a primary driver of plant growth, revealed that cslc-Δ5 plants have reduced turgor, preventing the compromised walls from bursting while still allowing growth to occur. Second, we discovered the conservation of elastic asymmetry (ratio of axial to transverse wall elasticity) in the mutant, suggesting an additional mechanism contributing to the maintenance of normal growth. This novel feedback mechanism between cell wall composition and mechanical properties, coupled with turgor pressure regulation, plays a central role in the control of plant growth and is critical for seedling establishment in a mechanically challenging environment by affecting shoot emergence and root penetration.

The plant cell wall-dynamic, strong, and adaptable-is a natural shapeshifter

Mythology is replete with good and evil shapeshifters, who, by definition, display great adaptability and assume many different forms-with several even turning themselves into trees. Cell walls certainly fit this definition as they can undergo subtle or dramatic changes in structure, assume many shapes, and perform many functions. In this review, we cover the evolution of knowledge of the structures, biosynthesis, and functions of the 5 major cell wall polymer types that range from deceptively simple to fiendishly complex. Along the way, we recognize some of the colorful historical figures who shaped cell wall research over the past 100 years. The shapeshifter analogy emerges more clearly as we examine the evolving proposals for how cell walls are constructed to allow growth while remaining strong, the complex signaling involved in maintaining cell wall integrity and defense against disease, and the ways cell walls adapt as they progress from birth, through growth to maturation, and in the end, often function long after cell death. We predict the next century of progress will include deciphering cell type-specific wall polymers; regulation at all levels of polymer production, crosslinks, and architecture; and how walls respond to developmental and environmental signals to drive plant success in diverse environments.

The phosphoinositide signature guides the final step of plant cytokinesis

Plant cytokinesis, which fundamentally differs from that in animals, requires the outward expansion of a plasma membrane precursor named the cell plate. How the transition from a cell plate to a plasma membrane occurs remains poorly understood. Here, we report that the acquisition of plasma membrane identity occurs through lateral patterning of the phosphatidylinositol 4,5-bisphosphate PI(4,5)P2 at the newly formed cell plate membrane. There, the phosphoinositide phosphatase SAC9 emerges as a key regulator, colocalizing with and regulating the function of the microtubule-associated protein MAP65-3 at the cell plate leading zone. In sac9-3 mutant, the polar distribution of PI(4,5)P2 at the cell plate is altered, leading to ectopic recruitment of the cytokinesis apparatus and formation of an additional cell plate insertion site. We propose that at the cell plate, SAC9 drives the depletion of PI(4,5)P2, which acts as a polar cue to spatially separate cell plate expansion from the acquisition of plasma membrane identity during final step of cytokinesis.

Building an extensible cell wall

This article recounts, from my perspective of four decades in this field, evolving paradigms of primary cell wall structure and the mechanism of surface enlargement of growing cell walls. Updates of the structures, physical interactions, and roles of cellulose, xyloglucan, and pectins are presented. This leads to an example of how a conceptual depiction of wall structure can be translated into an explicit quantitative model based on molecular dynamics methods. Comparison of the model's mechanical behavior with experimental results provides insights into the molecular basis of complex mechanical behaviors of primary cell wall and uncovers the dominant role of cellulose-cellulose interactions in forming a strong yet extensible network.

Plant cytokinesis and the construction of new cell wall

Cytokinesis in plants is fundamentally different from that in animals and fungi. In plant cells, a cell plate forms through the fusion of cytokinetic vesicles and then develops into the new cell wall, partitioning the cytoplasm of the dividing cell. The formation of the cell plate entails multiple stages that involve highly orchestrated vesicle accumulation, fusion, and membrane maturation, which occur concurrently with the timely deposition of polysaccharides such as callose, cellulose, and cross-linking glycans. This review summarizes the major stages in cytokinesis, endomembrane components involved in cell plate assembly and its transition to a new cell wall. An animation that can be widely used for educational purposes further summarizes the process.

A biophysical model for plant cell plate maturation based on the contribution of a spreading force

Plant cytokinesis, a fundamental process of plant life, involves de novo formation of a "cell plate" partitioning the cytoplasm of dividing cells. Cell plate formation is directed by orchestrated delivery, fusion of cytokinetic vesicles, and membrane maturation to form a nascent cell wall by timely deposition of polysaccharides. During cell plate maturation, the fragile membrane network transitions to a fenestrated sheet and finally a young cell wall. Here, we approximated cell plate sub-structures with testable shapes and adopted the Helfrich-free energy model for membranes, including a stabilizing and spreading force, to understand the transition from a vesicular network to a fenestrated sheet and mature cell plate. Regular cell plate development in the model was possible, with suitable bending modulus, for a two-dimensional late stage spreading force of 2-6 pN/nm, an osmotic pressure difference of 2-10 kPa, and spontaneous curvature between 0 and 0.04 nm-1. With these conditions, stable membrane conformation sizes and morphologies emerged in concordance with stages of cell plate development. To reach a mature cell plate, our model required the late-stage onset of a spreading/stabilizing force coupled with a concurrent loss of spontaneous curvature. Absence of a spreading/stabilizing force predicts failure of maturation. The proposed model provides a framework to interrogate different players in late cytokinesis and potentially other membrane networks that undergo such transitions. Callose, is a polysaccharide that accumulates transiently during cell plate maturation. Callose-related observations were consistent with the proposed model's concept, suggesting that it is one of the factors involved in establishing the spreading force.

Cell biology of primary cell wall synthesis in plants

Building a complex structure such as the cell wall, with many individual parts that need to be assembled correctly from distinct sources within the cell, is a well-orchestrated process. Additional complexity is required to mediate dynamic responses to environmental and developmental cues. Enzymes, sugars, and other cell wall components are constantly and actively transported to and from the plasma membrane during diffuse growth. Cell wall components are transported in vesicles on cytoskeletal tracks composed of microtubules and actin filaments. Many of these components, and additional proteins, vesicles, and lipids are trafficked to and from the cell plate during cytokinesis. In this review, we first discuss how the cytoskeleton is initially organized to add new cell wall material or to build a new cell wall, focusing on similarities during these processes. Next, we discuss how polysaccharides and enzymes that build the cell wall are trafficked to the correct location by motor proteins and through other interactions with the cytoskeleton. Finally, we discuss some of the special features of newly formed cell walls generated during cytokinesis.

Ultrastructural Observation of Cytokinesis and Plasmodesmata Formation in Brown Algae

Similar to land plant cells, brown algal cells possess plasmodesmata with minute cytoplasmic tunnels, which enable the direct connection between adjacent cells. Plasmodesmata are distributed depending on the association of their formation with cytokinesis. Primary plasmodesmata are formed during cytokinesis, while secondary plasmodesmata appear on the cell wall septum following cytokinesis. Typically, the plasmodesmata of brown algae are cylindrical without the penetration of desmotubules from the endoplasmic reticulum, and there are no morphological differences between primary and secondary plasmodesmata. This present chapter describes the observation of cytokinesis and primary plasmodesmata formation in brown algae using electron microscopy as well as the examination of polysaccharide distribution using antibodies and enzyme-gold probes.

Comparative transcriptome and DNA methylation analysis in temperature-sensitive genic male sterile wheat BS366

Background

Known as the prerequisite component for the heterosis breeding system, the male sterile line determines the hybrid yield and seed purity. Therefore, a deep understanding of the mechanism and gene network that leads to male sterility is crucial. BS366, a temperature-sensitive genic male sterile (TGMS) line, is male sterile under cold conditions (12 °C with 12 h of daylight) but fertile under normal temperature (20 °C with 12 h of daylight).

Results

During meiosis, BS366 was defective in forming tetrads and dyads due to the abnormal cell plate. During pollen development, unusual vacuolated pollen that could not accumulate starch grains at the binucleate stage was also observed. Transcriptome analysis revealed that genes involved in the meiotic process, such as sister chromatid segregation and microtubule-based movement, were repressed, while genes involved in DNA and histone methylation were induced in BS366 under cold conditions. MethylRAD was used for reduced DNA methylation sequencing of BS366 spikes under both cold and control conditions. The differentially methylated sites (DMSs) located in the gene region were mainly involved in carbohydrate and fatty acid metabolism, lipid metabolism, and transport. Differentially expressed and methylated genes were mainly involved in cell division.

Conclusions

These results indicated that the methylation of genes involved in carbon metabolism or fatty acid metabolism might contribute to male sterility in BS366 spikes, providing novel insight into the molecular mechanism of wheat male sterility.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08163-3.

Structure and Development of the Legume-Rhizobial Symbiotic Interface in Infection Threads

The intracellular infection thread initiated in a root hair cell is a unique structure associated with Rhizobium-legume symbiosis. It is characterized by inverted tip growth of the plant cell wall, resulting in a tunnel that allows invasion of host cells by bacteria during the formation of the nitrogen-fixing root nodule. Regulation of the plant-microbial interface is essential for infection thread growth. This involves targeted deposition of the cell wall and extracellular matrix and tight control of cell wall remodeling. This review describes the potential role of different actors such as transcription factors, receptors, and enzymes in the rearrangement of the plant-microbial interface and control of polar infection thread growth. It also focuses on the composition of the main polymers of the infection thread wall and matrix and the participation of reactive oxygen species (ROS) in the development of the infection thread. Mutant analysis has helped to gain insight into the development of host defense reactions. The available data raise many new questions about the structure, function, and development of infection threads.

Identification of two functional xyloglucan galactosyltransferase homologs BrMUR3 and BoMUR3 in brassicaceous vegetables

Xyloglucan (XyG) is the predominant hemicellulose in the primary cell walls of most dicotyledonous plants. Current models of these walls predict that XyG interacts with cellulose microfibrils to provide the wall with the rigidity and strength necessary to maintain cell integrity. Remodeling of this network is required to allow cell elongation and plant growth. In this study, homologs of Arabidopsis thaliana MURUS3 (MUR3), which encodes a XyG-specific galactosyltransferase, were obtained from Brassica rapa (BrMUR3) to Brassica oleracea (BoMUR3). Genetic complementation showed that BrMUR3 and BoMUR3 rescue the phenotypic defects of the mur3-3 mutant. Xyloglucan subunit composition analysis provided evidence that BrMUR3 and BoMUR3 encode a galactosyltransferase, which transfers a galactose residue onto XyG chains. The detection of XXFG and XLFG XyG subunits (restoration of fucosylated side chains) in mur3-3 mutants overexpressing BrMUR3 or BoMUR3 show that MUR3 from Brassica to Arabidopsis are comparable as they add Gal to the third xylosyl residue of the XXXG subunit. Our results provide additional information for functional dissection and evolutionary analysis of MUR3 genes derived from brassicaceous species.

PRPs localized to the middle lamellae are required for cortical tissue integrity in Medicago truncatula roots

A family of repetitive proline-rich proteins interact with acidic pectins and play distinct roles in legume root cell walls affecting cortical and vascular structure. A proline-rich protein (PRP) family, composed of tandemly repeated Pro-Hyp-Val-X-Lys pentapeptide motifs, is found primarily in the Leguminosae. Four distinct size classes within this family are encoded by seven tightly linked genes: MtPRP1, MtPRP2 and MtPRP3, and four nearly identical MtPRP4 genes. Promoter fusions to β-glucuronidase showed strong expression in the stele of hairy roots for all 4 PRP genes tested, with additional expression in the cortex for PRP1, PRP2 and PRP4. All except MtPRP4 are strongly expressed in non-tumorous roots, and secreted and ionically bound to root cell walls. These PRPs are absent from root epidermal cell walls, and PRP accumulation is highly localized within the walls of root cortical and vascular tissues. Within xylem tissue, PRPs are deposited in secondary thickenings where it is spatially exclusive to lignin. In newly differentiating xylem, PRPs are deposited in the regularly spaced paired-pits and pit membranes that hydraulically connect neighboring xylem elements. Hairpin-RNA knock-down constructs reducing PRP expression in Medicago truncatula hairy root tumors disrupted cortical and vascular patterning. Immunoblots showed that the knockdown tumors had potentially compensating increases in the non-targeted PRPs, all of which cross-react with the anti-PRP antibodies. However, PRP3 knockdown differed from knockdown of PRP1 and PRP2 in that it greatly reduced viability of hairy root tumors. We hypothesize that repetitive PRPs interact with acidic pectins to form block-copolymer gels that can play distinct roles in legume root cell walls.

Outside-in control - does plant cell wall integrity regulate cell cycle progression?

During recent years it has become accepted that plant cell walls are not inert objects surrounding all plant cells but are instead highly dynamic, plastic structures. They are involved in a large number of cell biological processes and contribute actively to plant growth, development and interaction with environment. Therefore, it is not surprising that cellular processes can control plant cell wall integrity (CWI) while, simultaneously, CWI can influence cellular processes. In yeast and animal cells such a bidirectional relationship also exists between the yeast/animal extracellular matrices and the cell cycle. In yeast, the CWI maintenance mechanism and a dedicated plasma membrane integrity checkpoint are mediating this relationship. Recent research has yielded insights into the mechanism controlling plant cell wall metabolism during cytokinesis. However, the knowledge regarding putative regulatory pathways controlling adaptive modifications in plant cell cycle activity in response to changes in the state of the plant cell wall are not yet identified. In this review, we summarize similarities and differences in regulatory mechanisms coordinating extracellular matrices and cell cycle activity in animal and yeast cells, discuss the available evidence supporting the existence of such a mechanism in plants and suggest that the plant CWI maintenance mechanism might also control cell cycle activity in plant cells.

The Ceratopteris (fern) developing motile gamete walls contain diverse polysaccharides, but not pectin

Unlike most plant cell walls, the five consecutive walls laid down during spermatogenesis in the model fern Ceratopteris contain sparse cellulose, lack pectin and are enriched with callose and hemicelluloses. Seed-free plants like bryophytes and pteridophytes produce swimming male gametes for sexual reproduction. During spermatogenesis, unique walls are formed that are essential to the appropriate development and maturation of the motile gametes. Other than the detection of callose and general wall polysaccharides in scattered groups, little is known about the sequence of wall formation and the composition of these walls during sperm cell differentiation in plants that produce swimming sperm. Using histochemistry and immunogold localizations, we examined the distribution of callose, cellulose, mannan and xylan-containing hemicelluloses, and homogalacturonan (HG) pectins in the special walls deposited during spermatogenesis in Ceratopteris. Five walls are produced in sequence and each has a unique fate. The first wall (W1) contains callose and sparse xylan-containing hemicelluloses. Wall two (W2) is thin and composed of cellulose crosslinked by xylan-containing hemicelluloses. The third wall (W3) is thick and composed entirely of callose, and the fourth wall (W4) is built of cellulose heavily crosslinked by galactoxyloglucan hemicelluloses. Wall five (W5) is an arabinogalactan protein (AGP)-rich matrix in which the gamete changes shape and multiple flagella elongate. We detected no esterified or unesterified HG pectins in any of the walls laid down during spermatogenesis. To consider evolutionary modifications in cell walls associated with motile gametes, comparisons are presented with male gametophyte and spermatogenous cell walls across plant groups.

Post-Golgi Trafficking and Transport of Cell Wall Components

The cell wall, a complex macromolecular composite structure surrounding and protecting plant cells, is essential for development, signal transduction, and disease resistance. This structure is also integral to cell expansion, as its tensile resistance is the primary balancing mechanism against internal turgor pressure. Throughout these processes, the biosynthesis, transport, deposition, and assembly of cell wall polymers are tightly regulated. The plant endomembrane system facilitates transport of polysaccharides, polysaccharide biosynthetic and modifying enzymes and glycoproteins through vesicle trafficking pathways. Although a number of enzymes involved in cell wall biosynthesis have been identified, comparatively little is known about the transport of cell wall polysaccharides and glycoproteins by the endomembrane system. This review summarizes our current understanding of trafficking of cell wall components during cell growth and cell division. Emerging technologies, such as vesicle glycomics, are also discussed as promising avenues to gain insights into the trafficking of structural polysaccharides to the apoplast.

Immunocytochemical detection of rhamnogalacturonan II on forming cell plates in cultured tobacco cells

Rhamnogalacturonan II (RG-II) is a region of pectin macromolecules that is present in plant primary cell walls. The RG-II region serves as the site of borate cross-linking within pectin, via which pectin macromolecules link together to form a gel. In this study, we examined whether RG-II is present in the cell plate, the precursor of primary cell walls that forms during cytokinesis. A structure inside dividing cells was labeled with a rabbit polyclonal anti-RG-II antibody and detected by immunofluorescence microscopy. An antibody against callose, a marker polysaccharide for the cell plate, also labeled the structure. In immunoelectron microscopy analyses using the anti-RG-II antibody, gold particles were distributed in electron-lucent vesicular structures that appeared to correspond to the forming cell plates in late anaphase cells. Together, these results suggest that RG-II is present in cell plates from the early phase of their assembly.

Remodeling of pectin and hemicelluloses in tomato pericarp during fruit growth

Tomato fruit texture depends on histology and cell wall architecture, both under genetic and developmental controls. If ripening related cell wall modifications have been well documented with regard to softening, little is known about cell wall construction during early fruit development. Identification of key events and their kinetics with regard to tissue architecture and cell wall development can provide new insights on early phases of texture elaboration. In this study, changes in pectin and hemicellulose chemical characteristics and location were investigated in the pericarp tissue of tomato (Solanum lycopersicon var Levovil) at four stages of development (7, 14 and 21day after anthesis (DPA) and mature green stages). Analysis of cell wall composition and polysaccharide structure revealed that both are continuously modified during fruit development. At early stages, the relative high rhamnose content in cell walls indicates a high synthesis of rhamnogalacturonan I next to homogalacturonan. Fine tuning of rhamnogalacturonan I side chains appears to occur from the cell expansion phase until prior to the mature green stage. Cell wall polysaccharide remodelling also concerns xyloglucans and (galacto)glucomannans, the major hemicelluloses in tomato cell walls. In situ localization of cell wall polysaccharides in pericarp tissue revealed non-ramified RG-I rich pectin and XyG at cellular junctions and in the middle lamella of young fruit. Blocks of non-methyl esterified homogalacturonan are detected as soon as 14 DPA in the mesocarp and remained restricted to cell corner and middle lamella whatever the stages. These results point to new questions about the role of pectin RGI and XyG in cell adhesion and its maintenance during cell expansion.

The middle lamella-more than a glue

In plant tissues, cells are glued to each other by a pectic polysaccharide rich material known as middle lamella (ML). Along with many biological functions, the ML plays a crucial role in maintaining the structural integrity of plant tissues and organs, as it prevents the cells from separating or sliding against each other. The macromolecular organization and the material properties of the ML are different from those of the adjacent primary cell walls that envelop all plant cells and provide them with a stiff casing. Due to its nanoscale dimensions and the extreme challenge to access the structure for material characterization, the ML is poorly characterized in terms of its distinct material properties. This review explores the ML beyond its functionality as a gluing agent. The putative molecular interactions of constituent macromolecules within the ML and at the interface between ML and primary cell wall are discussed. The correlation between the spatiotemporal distribution of pectic polysaccharides in the different portions of the ML and the subcellular distribution of mechanical stresses within the plant tissue are analyzed.

A Distinct Pathway for Polar Exocytosis in Plant Cell Wall Formation

Post-Golgi protein sorting and trafficking to the plasma membrane (PM) is generally believed to occur via the trans-Golgi network (TGN). In this study using Nicotiana tabacum pectin methylesterase (NtPPME1) as a marker, we have identified a TGN-independent polar exocytosis pathway that mediates cell wall formation during cell expansion and cytokinesis. Confocal immunofluorescence and immunogold electron microscopy studies demonstrated that Golgi-derived secretory vesicles (GDSVs) labeled by NtPPME1-GFP are distinct from those organelles belonging to the conventional post-Golgi exocytosis pathway. In addition, pharmaceutical treatments, superresolution imaging, and dynamic studies suggest that NtPPME1 follows a polar exocytic process from Golgi-GDSV-PM/cell plate (CP), which is distinct from the conventional Golgi-TGN-PM/CP secretion pathway. Further studies show that ROP1 regulates this specific polar exocytic pathway. Taken together, we have demonstrated an alternative TGN-independent Golgi-to-PM polar exocytic route, which mediates secretion of NtPPME1 for cell wall formation during cell expansion and cytokinesis and is ROP1-dependent.

Regulation of Meristem Morphogenesis by Cell Wall Synthases in Arabidopsis

The cell walls of the shoot apical meristem (SAM), containing the stem cell niche that gives rise to the above-ground tissues, are crucially involved in regulating differentiation. It is currently unknown how these walls are built and refined or their role, if any, in influencing meristem developmental dynamics. We have combined polysaccharide linkage analysis, immuno-labeling, and transcriptome profiling of the SAM to provide a spatiotemporal plan of the walls of this dynamic structure. We find that meristematic cells express only a core subset of 152 genes encoding cell wall glycosyltransferases (GTs). Systemic localization of all these GT mRNAs by in situ hybridization reveals members with either enrichment in or specificity to apical subdomains such as emerging flower primordia, and a large class with high expression in dividing cells. The highly localized and coordinated expression of GTs in the SAM suggests distinct wall properties of meristematic cells and specific differences between newly forming walls and their mature descendants. Functional analysis demonstrates that a subset of CSLD genes is essential for proper meristem maintenance, confirming the key role of walls in developmental pathways.

Nitric Oxide-Mediated Maize Root Apex Responses to Nitrate are Regulated by Auxin and Strigolactones

Nitrate (NO3 (-)) is a key element for crop production but its levels in agricultural soils are limited. Plants have developed mechanisms to cope with these NO3 (-) fluctuations based on sensing nitrate at the root apex. Particularly, the transition zone (TZ) of root apex has been suggested as a signaling-response zone. This study dissects cellular and molecular mechanisms underlying NO3 (-) resupply effects on primary root (PR) growth in maize, confirming nitric oxide (NO) as a putative modulator. Nitrate restoration induced PR elongation within the first 2 h, corresponding to a stimulation of cell elongation at the basal border of the TZ. Xyloglucans (XGs) immunolocalization together with Brefeldin A applications demonstrated that nitrate resupply induces XG accumulation. This effect was blocked by cPTIO (NO scavenger). Transcriptional analysis of ZmXET1 confirmed the stimulatory effect of nitrate on XGs accumulation in cells of the TZ. Immunolocalization analyses revealed a positive effect of nitrate resupply on auxin and PIN1 accumulation, but a transcriptional regulation of auxin biosynthesis/transport/signaling genes was excluded. Short-term nitrate treatment repressed the transcription of genes involved in strigolactones (SLs) biosynthesis and transport, mainly in the TZ. Enhancement of carotenoid cleavage dioxygenases (CCDs) transcription in presence of cPTIO indicated endogenous NO as a negative modulator of CCDs activity. Finally, treatment with the SLs-biosynthesis inhibitor (TIS108) restored the root growth in the nitrate-starved seedlings. Present report suggests that the NO-mediated root apex responses to nitrate are accomplished in cells of the TZ via integrative actions of auxin, NO and SLs.

How to let go: pectin and plant cell adhesion

Plant cells do not, in general, migrate. They maintain a fixed position relative to their neighbors, intimately linked through growth and differentiation. The mediator of this connection, the pectin-rich middle lamella, is deposited during cell division and maintained throughout the cell's life to protect tissue integrity. The maintenance of adhesion requires cell wall modification and is dependent on the actin cytoskeleton. There are developmental processes that require cell separation, such as organ abscission, dehiscence, and ripening. In these instances, the pectin-rich middle lamella must be actively altered to allow cell separation, a process which also requires cell wall modification. In this review, we will focus on the role of pectin and its modification in cell adhesion and separation. Recent insights gained in pectin gel mechanics will be discussed in relation to existing knowledge of pectin chemistry as it relates to cell adhesion. As a whole, we hope to begin defining the physical mechanisms behind a cells' ability to hang on, and how it lets go.

Polysaccharide deposition during cytokinesis: Challenges and future perspectives

De novo formation of a new cell wall partitions the cytoplasm of the dividing cell during plant cytokinesis. The development of the cell plate, a transient sheet-like structure, requires the accumulation of vesicles directed by the phragmoplast to the cell plate assembly matrix. Fusion and fission of the accumulated vesicles are accompanied by the deposition of polysaccharides and cell wall structural proteins; together, they are leading to the stabilization of the formed structure which after insertion into the parental wall lead to the maturation of the nascent cross wall. Callose is the most abundant polysaccharide during cell plate formation and during maturation is gradually replaced by cellulose. Matrix polysaccharides such as hemicellulose, and pectins presumably are present throughout all developmental stages, being delivered to the cell plate by secretory vesicles. The availability of novel chemical probes such as endosidin 7, which inhibits callose formation at the cell plate, has proved useful for dissecting the temporal accumulation of vesicles at the cell plate and establishing the critical role of callose during cytokinesis. The use of emerging approaches such as chemical genomics combined with live cell imaging; novel techniques of polysaccharide detection including tagged polysaccharide substrates, newly characterized polysaccharide antibodies and vesicle proteomics can be used to develop a comprehensive model of cell plate development.

Cell wall modification in tobacco by differential targeting of recombinant endoglucanase from Trichoderma reesei

BACKGROUND

The development of transgenic plants as a production platform for biomass-degrading enzymes is a promising tool for an economically feasible allocation of enzymes processing lignocellulose. Previous research has already identified a major limitation of in planta production such as interference with the structure and integrity of the plant cell wall resulting in a negative influence on plant growth and development.

RESULTS

Here, we describe the in planta expression of endoglucanase TrCel5A from the mesophilic fungus Trichoderma reesei with differential intracellular targeting and evaluate its impact on the tobacco cell wall composition. Targeting of the enzyme to the apoplast leads to distinct changes in cell polysaccharides such as glucose level in the matrix polysaccharides (MPS). These effects are combined with severe changes in plant development. Retention of TrCel5A in the endoplasmic reticulum (ER) could avoid visible effects on plant growth under the chosen conditions, but exhibits changes in the composition of the MPS.

CONCLUSIONS

These results give new insights into the complex interaction of heterologous cellulase expression with cell wall development and it outlines novel promising strategies to engineer plant cell walls for improved biomass processing.

XTH20 and XTH19 regulated by ANAC071 under auxin flow are involved in cell proliferation in incised Arabidopsis inflorescence stems

One week after partial incision of Arabidopsis inflorescence stems, the repair process in damaged tissue includes pith cell proliferation. Auxin is a key factor driving this process, and ANAC071, a transcription factor gene, is upregulated in the distal region of the incised stem. Here we show that XTH20 and the closely related XTH19, members of xyloglucan endotransglucosylase/hydrolases family catalyzing molecular grafting and/or hydrolysis of cell wall xyloglucans, were also upregulated in the distal part of the incised stem, similar to ANAC071. XTH19 was expressed in the proximal incision region after 3 days or after auxin application to the decapitated stem. Horizontal positioning of the plant with the incised side up resulted in decreased ProDR 5 :GUS, ANAC071, XTH20, and XTH19 expression and reduced pith cell proliferation. In incised stems of Pro35S :ANAC071-SRDX plants, expression of XTH20 and XTH19 was substantially and moderately decreased, respectively. XTH20 and XTH19 expression and pith cell proliferation were suppressed in anac071 plants and were increased in Pro35S :ANAC071 plants. Pith cell proliferation was also inhibited in the xth20xth19 double mutant. Furthermore, ANAC071 bound to the XTH20 and XTH19 promoters to induce their expression. This study revealed XTH20 and XTH19 induction by auxin via ANAC071 in the distal part of an incised stem and their involvement in cell proliferation in the tissue reunion process.

Plant cytokinesis is orchestrated by the sequential action of the TRAPPII and exocyst tethering complexes

Plant cytokinesis is initiated in a transient membrane compartment, the cell plate, and completed by a process of maturation during which the cell plate becomes a cross wall. How the transition from juvenile to adult stages occurs is poorly understood. In this study, we monitor the Arabidopsis transport protein particle II (TRAPPII) and exocyst tethering complexes throughout cytokinesis. We show that their appearance is predominantly sequential, with brief overlap at the onset and end of cytokinesis. The TRAPPII complex is required for cell plate biogenesis, and the exocyst is required for cell plate maturation. The TRAPPII complex sorts plasma membrane proteins, including exocyst subunits, at the cell plate throughout cytokinesis. We show that the two tethering complexes physically interact and propose that their coordinated action may orchestrate not only plant but also animal cytokinesis.

A survey of cellulose microfibril patterns in dividing, expanding, and differentiating cells of Arabidopsis thaliana

Cellulose microfibrils are critical for plant cell specialization and function. Recent advances in live cell imaging of fluorescently tagged cellulose synthases to track cellulose synthesis have greatly advanced our understanding of cellulose biosynthesis. Nevertheless, cellulose deposition patterns remain poorly described in many cell types, including those in the process of division or differentiation. In this study, we used field emission scanning electron microscopy analysis of cryo-planed tissues to determine the arrangement of cellulose microfibrils in various faces of cells undergoing cytokinesis or specialized development, including cell types in which cellulose cannot be imaged by conventional approaches. In dividing cells, we detected microfibrillar meshworks in the cell plates, consistent with the concentration at the cell plate of cellulose synthase complexes, as detected by fluorescently tagged CesA6. We also observed a loss of parallel cellulose microfibril orientation in walls of the mother cell during cytokinesis, which corresponded with the loss of fluorescently tagged cellulose synthase complexes from these surfaces. In recently formed guard cells, microfibrils were randomly organized and only formed a highly ordered circumferential pattern after pore formation. In pit fields, cellulose microfibrils were arranged in circular patterns around plasmodesmata. Microfibrils were random in most cotyledon cells except the epidermis and were parallel to the growth axis in trichomes. Deposition of cellulose microfibrils was spatially delineated in metaxylem and protoxylem cells of the inflorescence stem, supporting recent studies on microtubule exclusion mechanisms.

A study on antifungal activity of water-soluble chitosan against Macrophomina phaseolina

The objective of this study was to evaluate antifungal effect of water-soluble chitosan (s-chitosan) on Macrophomina phaseolina (M. phaseolina) causing jute seedling infection and monitor the change in activity of released enzymes during infection. The minimum inhibitory concentration (MIC) of s-chitosan for M. phaseolina was found at 12.5g/l and s-chitosan exhibited fungistatic mode of action against this pathogen. The application of s-chitosan (12.5g/l) during infection of jute seedlings by M. phaseolina inhibited fungal infection and length of the seedlings was found almost similar to seedlings without infection. M. phaseolina infected jute seedlings showed length of 22mm over 10 days of incubation and it increased to 58mm in presence of s-chitosan (12.5g/l) during incubation for 10 days. TEM study indicated presence of hyphae in the cortical and epidermal cells of fungus infected jute seedlings indicating colonization by the fungus and it disappeared after treatment with s-chitosan. The changes in enzyme profiles of jute seedling during prevention of fungal infection using s-chitosan helped in proper understanding of mode of action of s-chitosan as antifungal agent. The activity of defense related enzymes like chitosanase and peroxidase in infected seedlings was observed to be enhanced after treatment with s-chitosan.

Spatio-temporal analysis of cellulose synthesis during cell plate formation in Arabidopsis

During cytokinesis a new crosswall is rapidly laid down. This process involves the formation at the cell equator of a tubulo-vesicular membrane network (TVN). This TVN evolves into a tubular network (TN) and a planar fenestrated sheet, which extends at its periphery before fusing to the mother cell wall. The role of cell wall polymers in cell plate assembly is poorly understood. We used specific stains and GFP-labelled cellulose synthases (CESAs) to show that cellulose, as well as three distinct CESAs, accumulated in the cell plate already at the TVN stage. This early presence suggests that cellulose is extruded into the tubular membrane structures of the TVN. Co-localisation studies using GFP-CESAs suggest the delivery of cellulose synthase complexes (CSCs) to the cell plate via phragmoplast-associated vesicles. In the more mature TN part of the cell plate, we observed delivery of GFP-CESA from doughnut-shaped organelles, presumably Golgi bodies. During the conversion of the TN into a planar fenestrated sheet, the GFP-CESA density diminished, whereas GFP-CESA levels remained high in the TVN zone at the periphery of the expanding cell plate. We observed retrieval of GFP-CESA in clathrin-containing structures from the central zone of the cell plate and from the plasma membrane of the mother cell, which may contribute to the recycling of CESAs to the peripheral growth zone of the cell plate. These observations, together with mutant phenotypes of cellulose-deficient mutants and pharmacological experiments, suggest a key role for cellulose synthesis already at early stages of cell plate assembly.

Hemicellulose biosynthesis

One major component of plant cell walls is a diverse group of polysaccharides, the hemicelluloses. Hemicelluloses constitute roughly one-third of the wall biomass and encompass the heteromannans, xyloglucan, heteroxylans, and mixed-linkage glucan. The fine structure of these polysaccharides, particularly their substitution, varies depending on the plant species and tissue type. The hemicelluloses are used in numerous industrial applications such as food additives as well as in medicinal applications. Their abundance in lignocellulosic feedstocks should not be overlooked, if the utilization of this renewable resource for fuels and other commodity chemicals becomes a reality. Fortunately, our understanding of the biosynthesis of the various hemicelluloses in the plant has increased enormously in recent years mainly through genetic approaches. Taking advantage of this knowledge has led to plant mutants with altered hemicellulosic structures demonstrating the importance of the hemicelluloses in plant growth and development. However, while we are on a solid trajectory in identifying all necessary genes/proteins involved in hemicellulose biosynthesis, future research is required to combine these single components and assemble them to gain a holistic mechanistic understanding of the biosynthesis of this important class of plant cell wall polysaccharides.

Supercritical CO2 and ionic liquids for the pretreatment of lignocellulosic biomass in bioethanol production

Owing to high petroleum prices, there has been a major push in recent years to use lignocellulosic biomass as biorefinery feedstocks. Unfortunately, by nature's design, lignocellulosic biomass is notoriously recalcitrant. Cellulose is the most abundant renewable carbon source on the planet and comprises glucan polysaccharides which self-assemble into paracrystalline microfibrils. The extent of cellulose crystallinity largely contributes to biomass recalcitrance. Additionally, cellulose microfibrils are embedded into both hemicellulose and lignin polymeric networks, making cellulose accessibility an additional obstacle. Pretreatment is necessary before enzymatic hydrolysis in order to liberate high yields of glucose and other fermentable sugars from biomass polysaccharides. This work discusses two pretreatment methods, supercritical CO2 and ionic liquids (ILs). Both methods utilize green solvents that do not emit toxic vapours. Mechanisms for destroying or weakening biomass recalcitrance have been explored. Various pretreatment operating parameters such as temperature, pressure, time, dry biomass/solvent ratio, water content, etc. have been investigated for the pretreatment of various biomass types such as corn stover, switchgrass, sugarcane bagasse, soft and hard wood. The two pretreatment methods have their pros and cons. For example, supercritical CO2 explosion pretreatment uses inexpensive CO2, but requires a high pressure. By comparison, while IL pretreatment does not require an elevated pressure, ILs are still too expensive for large-scale uses. Further research and development are needed to make the two green pretreatment methods practical.

Arabidopsis mannan synthase CSLA9 and glucan synthase CSLC4 have opposite orientations in the Golgi membrane

Several proteins encoded by the cellulose synthase-like (CSL) gene family are known to be processive glycan synthases involved in the synthesis of cell-wall polysaccharides. These include CSLA proteins, which synthesize β-(1→4)-linked mannans found in the walls of many plant species, and CSLC proteins, which are thought to synthesize the β-(1→4)-linked glucan backbone of xyloglucan, an abundant polysaccharide in the primary walls of many plants. CSLA and CSLC proteins are predicted to have multiple membrane spans, and their products (mannan and xyloglucan) accumulate in the Golgi lumen. Knowing where these proteins are located in the cell and how they are orientated in the membrane is important for understanding many aspects of mannan and xyloglucan biosynthesis. In this study, we investigate the subcellular localization and membrane protein topology of CSLA9 and CSLC4, the members of these two families that are most highly expressed in Arabidopsis. CSLA9 and CSLC4 are found predominantly in Golgi membranes, based on co-localization with the known ER/Golgi marker ERD2-YFP. The topology of epitope-tagged proteins was examined using protease protection experiments. Experiments were designed to determine the positions of both the protein termini and the active loop of the CSL proteins investigated. The topology of CSLA9 is characterized by an odd number of transmembrane domains (probably five) and an active site that faces the Golgi lumen. In contrast, CSLC4 has an even number of transmembrane domains (probably six) and an active site that faces the cytosol. The implications of these topologies on various aspects of hemicellulose biosynthesis are discussed.

Subcompartment localization of the side chain xyloglucan-synthesizing enzymes within Golgi stacks of tobacco suspension-cultured cells

Xyloglucan is the dominant hemicellulosic polysaccharide of the primary cell wall of dicotyledonous plants that plays a key role in plant development. It is well established that xyloglucan is assembled within Golgi stacks and transported in Golgi-derived vesicles to the cell wall. It is also known that the biosynthesis of xyloglucan requires the action of glycosyltransferases including α-1,6-xylosyltransferase, β-1,2-galactosyltransferase and α-1,2-fucosyltransferase activities responsible for the addition of xylose, galactose and fucose residues to the side chains. There is, however, a lack of knowledge on how these enzymes are distributed within subcompartments of Golgi stacks. We have undertaken a study aiming at mapping these glycosyltransferases within Golgi stacks using immunogold-electron microscopy. To this end, we generated transgenic lines of tobacco (Nicotiana tabacum) BY-2 suspension-cultured cells expressing either the α-1,6-xylosyltransferase, AtXT1, the β-1,2-galactosyltransferase, AtMUR3, or the α-1,2-fucosyltransferase AtFUT1 of Arabidopsis thaliana fused to green-fluorescent protein (GFP). Localization of the fusion proteins within the endomembrane system was assessed using confocal microscopy. Additionally, tobacco cells were high pressure-frozen/freeze-substituted and subjected to quantitative immunogold labelling using anti-GFP antibodies to determine the localization patterns of the enzymes within subtypes of Golgi cisternae. The data demonstrate that: (i) all fusion proteins, AtXT1-GFP, AtMUR3-GFP and AtFUT1-GFP are specifically targeted to the Golgi apparatus; and (ii) AtXT1-GFP is mainly located in the cis and medial cisternae, AtMUR3-GFP is predominantly associated with medial cisternae and AtFUT1-GFP mostly detected over trans cisternae suggesting that initiation of xyloglucan side chains occurs in early Golgi compartments in tobacco cells.

Membrane fusion process and assembly of cell wall during cytokinesis in the brown alga, Silvetia babingtonii (Fucales, Phaeophyceae)

During cytokinesis in brown algal cells, Golgi-derived vesicles (GVs) and flat cisternae (FCs) are involved in building the new cell partition membrane. In this study, we followed the membrane fusion process in Silvetia babingtonii zygotes using electron microscopy together with rapid freezing and freeze substitution. After mitosis, many FCs were formed around endoplasmic reticulum clusters and these then spread toward the future cytokinetic plane. Actin depolymerization using latrunculin B prevented the appearance of the FCs. Fusion of GVs to FCs resulted in structures that were thicker and more elongated (EFCs; expanded flat cisternae). Some complicated membranous structures (MN; membranous network) were formed by interconnection of EFCs and following the arrival of additional GVs. The MN grew into membranous sacs (MSs) as gaps between the MNs disappeared. The MSs were observed in patches along the cytokinetic plane. Neighboring MSs were united to form the new cell partition membrane. An immunocytochemical analysis indicated that fucoidan was synthesized in Golgi bodies and transported by vesicles to the future cytokinetic plane, where the vesicles fused with the FCs. Alginate was not detected until the MS phase. Incubation of sections with cellulase-gold showed that the cellulose content of the new cross wall was not comparable to that of the parent cell wall.

Endocytic and secretory traffic in Arabidopsis merge in the trans-Golgi network/early endosome, an independent and highly dynamic organelle

Plants constantly adjust their repertoire of plasma membrane proteins that mediates transduction of environmental and developmental signals as well as transport of ions, nutrients, and hormones. The importance of regulated secretory and endocytic trafficking is becoming increasingly clear; however, our knowledge of the compartments and molecular machinery involved is still fragmentary. We used immunogold electron microscopy and confocal laser scanning microscopy to trace the route of cargo molecules, including the BRASSINOSTEROID INSENSITIVE1 receptor and the REQUIRES HIGH BORON1 boron exporter, throughout the plant endomembrane system. Our results provide evidence that both endocytic and secretory cargo pass through the trans-Golgi network/early endosome (TGN/EE) and demonstrate that cargo in late endosomes/multivesicular bodies is destined for vacuolar degradation. Moreover, using spinning disc microscopy, we show that TGN/EEs move independently and are only transiently associated with an individual Golgi stack.

A specialized outer layer of the primary cell wall joins elongating cotton fibers into tissue-like bundles

Cotton (Gossypium hirsutum) provides the world's dominant renewable textile fiber, and cotton fiber is valued as a research model because of its extensive elongation and secondary wall thickening. Previously, it was assumed that fibers elongated as individual cells. In contrast, observation by cryo-field emission-scanning electron microscopy of cotton fibers developing in situ within the boll demonstrated that fibers elongate within tissue-like bundles. These bundles were entrained by twisting fiber tips and consolidated by adhesion of a cotton fiber middle lamella (CFML). The fiber bundles consolidated via the CFML ultimately formed a packet of fiber around each seed, which helps explain how thousands of cotton fibers achieve their great length within a confined space. The cell wall nature of the CFML was characterized using transmission electron microscopy, including polymer epitope labeling. Toward the end of elongation, up-regulation occurred in gene expression and enzyme activities related to cell wall hydrolysis, and targeted breakdown of the CFML restored fiber individuality. At the same time, losses occurred in certain cell wall polymer epitopes (as revealed by comprehensive microarray polymer profiling) and sugars within noncellulosic matrix components (as revealed by gas chromatography-mass spectrometry analysis of derivatized neutral and acidic glycosyl residues). Broadly, these data show that adhesion modulated by an outer layer of the primary wall can coordinate the extensive growth of a large group of cells and illustrate dynamic changes in primary wall structure and composition occurring during the differentiation of one cell type that spends only part of its life as a tissue.

The timely deposition of callose is essential for cytokinesis in Arabidopsis

The primary plant cell wall is laid down over a brief period of time during cytokinesis. Initially, a membrane network forms at the equator of a dividing cell. The cross-wall is then assembled and remodeled within this membrane compartment. Callose is the predominant luminal component of the nascent cross-wall or cell plate, but is not a component of intact mature cell walls, which are composed primarily of cellulose, pectins and xyloglucans. Widely accepted models postulate that callose comprises a transient, rapid spreading force for the expansion of membrane networks during cytokinesis. In this study, we clone and characterize an Arabidopsis gene, MASSUE/AtGSL8, which encodes a putative callose synthase. massue mutants are seedling-lethal and have a striking cytokinesis-defective phenotype. Callose deposition was delayed in the cell plates of massue mutants. Mutant cells were occasionally bi- or multi-nucleate, with cell-wall stubs, and we frequently observed gaps at the junction between cross-walls and parental cell walls. The results suggest that the timely deposition of callose is essential for the completion of plant cytokinesis. Surprisingly, confocal analysis revealed that the cell-plate membrane compartment forms and expands, seemingly as far as the parental wall, prior to the appearance of callose. We discuss the possibility that callose may be required to establish a lasting connection between the nascent cross-wall and the parental cell wall.

Analysis of the Golgi apparatus in Arabidopsis seed coat cells during polarized secretion of pectin-rich mucilage

Differentiation of the Arabidopsis thaliana seed coat cells includes a secretory phase where large amounts of pectinaceous mucilage are deposited to a specific domain of the cell wall. During this phase, Golgi stacks had cisternae with swollen margins and trans-Golgi networks consisting of interconnected vesicular clusters. The proportion of Golgi stacks producing mucilage was determined by immunogold labeling and transmission electron microscopy using an antimucilage antibody, CCRC-M36. The large percentage of stacks found to contain mucilage supports a model where all Golgi stacks produce mucilage synchronously, rather than having a subset of specialist Golgi producing pectin product. Initiation of mucilage biosynthesis was also correlated with an increase in the number of Golgi stacks per cell. Interestingly, though the morphology of individual Golgi stacks was dependent on the volume of mucilage produced, the number was not, suggesting that proliferation of Golgi stacks is developmentally programmed. Mapping the position of mucilage-producing Golgi stacks within developing seed coat cells and live-cell imaging of cells labeled with a trans-Golgi marker showed that stacks were randomly distributed throughout the cytoplasm rather than clustered at the site of secretion. These data indicate that the destination of cargo has little effect on the location of the Golgi stack within the cell.

The reb1-1 mutation of Arabidopsis. Effect on the structure and localization of galactose-containing cell wall polysaccharides

The Arabidopsis (Arabidopsis thaliana) root epidermal bulger1-1 (reb1-1) mutant (allelic to root hair defective1 [rhd1]) is characterized by a reduced root elongation rate and by bulging of trichoblast cells. The REB1/RHD1 gene belongs to a family of UDP-D-Glucose 4-epimerases involved in the synthesis of D-Galactose (Gal). Our previous study showed that certain arabinogalactan protein epitopes were not expressed in bulging trichoblasts of the mutant. In this study, using a combination of microscopical and biochemical methods, we have investigated the occurrence and the structure of three major Gal-containing polysaccharides, namely, xyloglucan (XyG), rhamnogalacturonan (RG)-I, and RG-II in the mutant root cell walls. Our immunocytochemical data show that swollen trichoblasts were not stained with the monoclonal antibody CCRC-M1 specific for alpha-L-Fucp-(1-->2)-beta-D-Galp side chains of XyG, whereas they were stained with anti-XyG antibodies specific for XyG backbone. In addition, analysis of a hemicellulosic fraction from roots demonstrates the presence of two structurally different XyGs in reb1-1. One is structurally similar to wild-type XyG and the other is devoid of fuco-galactosylated side chains and has the characteristic of being insoluble. Similar to anti-XyG antibodies, anti-bupleuran 2IIC, a polyclonal antibody specific for galactosyl epitopes associated with pectins, stained all root epidermal cells of both wild type and reb1-1. Similarly, anti-RG-II antibodies also stained swollen trichoblasts in the mutant. In addition, structural analysis of pectic polymers revealed no change in the galactosylation of RG-I and RG-II isolated from reb1-1 root cells. These findings demonstrate that the reb1-1 mutation affects XyG structure, but not that of pectic polysaccharides, thus lending support to the hypothesis that biosynthesis of Gal as well as galactosylation of complex polysaccharides is regulated at the polymer level.

Endocytosis of Cell Surface Material Mediates Cell Plate Formation during Plant Cytokinesis

Dividing plant cells perform a remarkable task of building a new cell wall within the cytoplasm in a few minutes. A long-standing paradigm claims that this primordial cell wall, known as the cell plate, is generated by delivery of newly synthesized material from Golgi apparatus-originated secretory vesicles. Here, we show that, in diverse plant species, cell surface material, including plasma membrane proteins, cell wall components, and exogenously applied endocytic tracers, is rapidly delivered to the forming cell plate. Importantly, this occurs even when de novo protein synthesis is blocked. In addition, cytokinesis-specific syntaxin KNOLLE as well as plasma membrane (PM) resident proteins localize to endosomes that fuse to initiate the cell plate. The rate of endocytosis is strongly enhanced during cell plate formation, and its genetic or pharmacological inhibition leads to cytokinesis defects. Our results reveal that endocytic delivery of cell surface material significantly contributes to cell plate formation during plant cytokinesis.