Three-dimensional reconstruction of tubular structure of vacuolar membrane throughout mitosis in living tobacco cells

Are vacuolar dynamics crucial factors for plant cell division and differentiation?

Vacuoles are the largest membrane-bound organelles in plant cells, critical for development and environmental responses. Vacuolar dynamics indicate reversible changes of vacuoles in morphology, size, or numbers. In this review, we summarize current understandings of vacuolar dynamics in different types of plant cells, biological processes associated with vacuolar dynamics, and regulators controlling vacuolar dynamics. Specifically, we point out the possibility that vacuolar dynamics play key roles in cell division and differentiation, which are controlled by the nucleus. Finally, we propose three routes through which vacuolar dynamics actively participate in nucleus-controlled cellular activities.

Arabidopsis flippase ALA3 is required for adjustment of early subcellular trafficking in plant response to osmotic stress

To compensate for their sessile lifestyle, plants developed several responses to exogenous changes. One of the previously investigated and not yet fully understood adaptations occurs at the level of early subcellular trafficking, which needs to be rapidly adjusted to maintain cellular homeostasis and membrane integrity under osmotic stress conditions. To form a vesicle, the membrane needs to be deformed, which is ensured by multiple factors, including the activity of specific membrane proteins, such as flippases from the family of P4-ATPases. The membrane pumps actively translocate phospholipids from the exoplasmic/luminal to the cytoplasmic membrane leaflet to generate curvature, which might be coupled with recruitment of proteins involved in vesicle formation at specific sites of the donor membrane. We show that lack of the AMINOPHOSPHOLIPID ATPASE3 (ALA3) flippase activity caused defects at the plasma membrane and trans-Golgi network, resulting in altered endocytosis and secretion, processes relying on vesicle formation and movement. The mentioned cellular defects were translated into decreased intracellular trafficking flexibility failing to adjust the root growth on osmotic stress-eliciting media. In conclusion, we show that ALA3 cooperates with ARF-GEF BIG5/BEN1 and ARF1A1C/BEX1 in a similar regulatory pathway to vesicle formation, and together they are important for plant adaptation to osmotic stress.

At the Nexus between Cytoskeleton and Vacuole: How Plant Cytoskeletons Govern the Dynamics of Large Vacuoles

Large vacuoles are a predominant cell organelle throughout the plant body. They maximally account for over 90% of cell volume and generate turgor pressure that acts as a driving force of cell growth, which is essential for plant development. The plant vacuole also acts as a reservoir for sequestering waste products and apoptotic enzymes, thereby enabling plants to rapidly respond to fluctuating environments. Vacuoles undergo dynamic transformation through repeated enlargement, fusion, fragmentation, invagination, and constriction, eventually resulting in the typical 3-dimensional complex structure in each cell type. Previous studies have indicated that such dynamic transformations of plant vacuoles are governed by the plant cytoskeletons, which consist of F-actin and microtubules. However, the molecular mechanism of cytoskeleton-mediated vacuolar modifications remains largely unclear. Here we first review the behavior of cytoskeletons and vacuoles during plant development and in response to environmental stresses, and then introduce candidates that potentially play pivotal roles in the vacuole-cytoskeleton nexus. Finally, we discuss factors hampering the advances in this research field and their possible solutions using the currently available cutting-edge technologies.

Vacuoles in Bryophytes: Properties, Biogenesis, and Evolution

Vacuoles are the most conspicuous organelles in plants for their indispensable functions in cell expansion, solute storage, water balance, etc. Extensive studies on angiosperms have revealed that a set of conserved core molecular machineries orchestrate the formation of vacuoles from multiple pathways. Usually, vacuoles in seed plants are classified into protein storage vacuoles and lytic vacuoles for their distinctive morphology and physiology function. Bryophytes represent early diverged non-vascular land plants, and are of great value for a better understanding of plant science. However, knowledge about vacuole morphology and biogenesis is far less characterized in bryophytes. In this review, first we summarize known knowledge about the morphological and metabolic constitution properties of bryophytes' vacuoles. Then based on known genome information of representative bryophytes, we compared the conserved molecular machinery for vacuole biogenesis among different species including yeast, mammals, Arabidopsis and bryophytes and listed out significant changes in terms of the presence/absence of key machinery genes which participate in vacuole biogenesis. Finally, we propose the possible conserved and diverged mechanism for the biogenesis of vacuoles in bryophytes compared with seed plants.

Actin and Microtubules Differently Contribute to Vacuolar Targeting Specificity during the Export from the ER

Plants rely on both actin and microtubule cytoskeletons to fine-tune sorting and spatial targeting of membranes during cell growth and stress adaptation. Considerable advances have been made in recent years in the comprehension of the relationship between the trans-Golgi network/early endosome (TGN/EE) and cytoskeletons, but studies have mainly focused on the transport to and from the plasma membrane. We address here the relationship of the cytoskeleton with different endoplasmic reticulum (ER) export mechanisms toward vacuoles. These emergent features of the plant endomembrane traffic are explored with an in vivo approach, providing clues on the traffic regulation at different levels beyond known proteins’ functions and interactions. We show how traffic of vacuolar markers, characterized by different vacuolar sorting determinants, diverges at the export from the ER, clearly involving different components of the cytoskeleton.

Vacuole Biogenesis in Plants: How Many Vacuoles, How Many Models?

Vacuoles are the largest membrane-bounded organelles and have essential roles in plant growth and development, but several important questions on the biogenesis and dynamics of lytic vacuoles (LVs) remain. Here, we summarize and discuss recent research and models of vacuole formation, and propose, with testable hypotheses, that besides inherited vacuoles, plant cells can also synthesize LVs de novo from multiple organelles and routes in response to growth and development or external factors. Therefore, LVs may be further classified into different subgroups and/or populations with different pH, cargos, and functions, among which multivesicular body (MVB)-derived small vacuoles are the main source for central vacuole formation in arabidopsis root cortical cells.

Actin Filament Disruption Alters Phragmoplast Microtubule Dynamics during the Initial Phase of Plant Cytokinesis

Plant growth and development relies on the accurate positioning of the cell plate between dividing cells during cytokinesis. The cell plate is synthetized by a specialized structure called the phragmoplast, which contains bipolar microtubules that polymerize to form a framework with the plus ends at or near the division site. This allows the transport of Golgi-derived vesicles toward the plus ends to form and expand the cell plate. Actin filaments play important roles in cell plate expansion and guidance in plant cytokinesis at the late phase, but whether they are involved at the early phase is unknown. To investigate this further, we disrupted the actin filaments in cell cycle-synchronized tobacco BY-2 cells with latrunculin B (LatB), an actin polymerization inhibitor. We observed the cells under a transmission electron microscope or a spinning-disk confocal laser scanning microscope. We found that disruption of actin filaments by LatB caused the membrane vesicles at the equatorial plane of the cell plate to be dispersed rather than form clusters as they did in the untreated cells. The midzone constriction of phragmoplast microtubules also was perturbed in LatB-treated cells. The live cell imaging and kymograph analysis showed that disruption of actin filaments also changed the accumulation timing of NACK1 kinesin, which plays a crucial role in cell plate expansion. This suggests that there are two functionally different types of microtubules in the phragmoplast. Together, our results show that actin filaments regulate phragmoplast microtubules at the initial phase of plant cytokinesis.

To Lead or to Follow: Contribution of the Plant Vacuole to Cell Growth

Cell division and cell elongation are fundamental processes for growth. In contrast to animal cells, plant cells are surrounded by rigid walls and therefore loosening of the wall is required during elongation. On the other hand, vacuole size has been shown to correlate with cell size and inhibition of vacuolar expansion limits cell growth. However, the specific role of the vacuole during cell elongation is still not fully resolved. Especially the question whether the vacuole is the leading unit during cellular growth or just passively expands upon water uptake remains to be answered. Here, we review recent findings about the contribution of the vacuole to cell elongation. In addition, we also discuss the connection between cell wall status and vacuolar morphology. In particular, we focus on the question whether vacuolar size is dictated by cell size or vice versa and share our personnel view about the sequential steps during cell elongation.

Vacuolar membrane structures and their roles in plant-pathogen interactions

Short review focussing on the role and targeting of vacuolar substructure in plant immunity and pathogenesis. Plants lack specialized immune cells, therefore each plant cell must defend itself against invading pathogens. A typical plant defense strategy is the hypersensitive response that results in host cell death at the site of infection, a process largely regulated by the vacuole. In plant cells, the vacuole is a vital organelle that plays a central role in numerous fundamental processes, such as development, reproduction, and cellular responses to biotic and abiotic stimuli. It shows divergent membranous structures that are continuously transforming. Recent technical advances in visualization and live-cell imaging have significantly altered our view of the vacuolar structures and their dynamics. Understanding the active nature of the vacuolar structures and the mechanisms of vacuole-mediated defense responses is of great importance in understanding plant-pathogen interactions. In this review, we present an overview of the current knowledge about the vacuole and its internal structures, as well as their role in plant-microbe interactions. There is so far limited information on the modulation of the vacuolar structures by pathogens, but recent research has identified the vacuole as a possible target of microbial interference.

NET4 Modulates the Compactness of Vacuoles in Arabidopsis thaliana

The dimension of the plants largest organelle—the vacuole—plays a major role in defining cellular elongation rates. The morphology of the vacuole is controlled by the actin cytoskeleton, but molecular players remain largely unknown. Recently, the Networked (NET) family of membrane-associated, actin-binding proteins has been identified. Here, we show that NET4A localizes to highly constricted regions of the vacuolar membrane and contributes to vacuolar morphology. Using genetic interference, we found that deregulation of NET4 abundance increases vacuolar occupancy, and that overexpression of NET4 abundance decreases vacuolar occupancy. Our data reveal that NET4A induces more compact vacuoles, correlating with reduced cellular and organ growth in Arabidopsis thaliana.

Connecting membranes to the actin cytoskeleton

In plants, the actin cytoskeleton plays a major role in organelle movement, cargo transport, maintaining cell polarity and controlling the morphogenesis of endomembrane systems. All of these events require a direct connection between membrane structures and the cytoskeleton. Our knowledge in this field has been greatly advanced by a few recent discoveries including the identification of the plant specific NETWORKED family of proteins, which can mediate such linkages. Other proteins that are known to regulate actin nucleation and polymerization are also likely to be involved, but many key questions still remain unanswered. In this paper, we will focus on recent research on the interfaces between the actin cytoskeleton and membranes of the endoplasmic reticulum, the vacuole and autophagosomes.

Vacuolar convolution: possible mechanisms and role of phosphatidylinositol 3,5-bisphosphate

The central or lytic vacuole is the largest intracellular organelle in plant cells, but we know unacceptably little about the mechanisms regulating its function in vivo. The underlying reasons are related to difficulties in accessing this organelle without disrupting the cellular integrity and to the dynamic morphology of the vacuole, which lacks a defined structure. Among such morphological changes, vacuolar convolution is probably the most commonly observed event, reflected in the (reversible) transformation of a large central vacuole into a structure consisting of interconnected bubbles of a smaller size. Such behaviour is observed in plant cells subjected to hyperosmotic stress but also takes place in physiological conditions (e.g. during stomatal closure). Although vacuolar convolution is a relatively common phenomenon in plants, studies aimed at elucidating its execution mechanisms are rather scarce. In the present review, we analyse the available evidence on the participation of the cellular cytoskeleton and ion transporters in vacuolar morphology dynamics, putting special emphasis on the available evidence of the role played by phosphatidylinositol 3,5-bisphosphate in this process.

The relationship between vacuolation and initiation of PCD in rice (Oryza sativa) aleurone cells

Vacuole fusion is a necessary process for the establishment of a large central vacuole, which is the central location of various hydrolytic enzymes and other factors involved in death at the beginning of plant programmed cell death (PCD). In our report, the fusion of vacuoles has been presented in two ways: i) small vacuoles coalesce to form larger vacuoles through membrane fusion, and ii) larger vacuoles combine with small vacuoles when small vacuoles embed into larger vacuoles. Regardless of how fusion occurs, a large central vacuole is formed in rice (Oryza sativa) aleurone cells. Along with the development of vacuolation, the rupture of the large central vacuole leads to the loss of the intact plasma membrane and the degradation of the nucleus, resulting in cell death. Stabilizing or disrupting the structure of actin filaments (AFs) inhibits or promotes the fusion of vacuoles, which delays or induces PCD. In addition, the inhibitors of the vacuolar processing enzyme (VPE) and cathepsin B (CathB) block the occurrence of the large central vacuole and delay the progression of PCD in rice aleurone layers. Overall, our findings provide further evidence for the rupture of the large central vacuole triggering the PCD in aleruone layers.

The evolution of the actin binding NET superfamily

The Arabidopsis Networked (NET) superfamily are plant-specific actin binding proteins which specifically label different membrane compartments and identify specialized sites of interaction between actin and membranes unique to plants. There are 13 members of the superfamily in Arabidopsis, which group into four distinct clades or families. NET homologs are absent from the genomes of metazoa and fungi; furthermore, in plantae, NET sequences are also absent from the genome of mosses and more ancient extant plant clades. A single family of the NET proteins is found encoded in the club moss genome, an extant species of the earliest vascular plants. Gymnosperms have examples from families 4 and 3, with a hybrid form of NET1 and 2 which shows characteristics of both NET1 and NET2. In addition to NET3 and 4 families, the NET1 and pollen-expressed NET2 families are found only as independent sequences in Angiosperms. This is consistent with the divergence of reproductive actin. The four families are conserved across Monocots and Eudicots, with the numbers of members of each clade expanding at this point, due, in part, to regions of genome duplication. Since the emergence of the NET superfamily at the dawn of vascular plants, they have continued to develop and diversify in a manner which has mirrored the divergence and increasing complexity of land-plant species.

The reorganization of actin filaments is required for vacuolar fusion of guard cells during stomatal opening in Arabidopsis

The reorganization of actin filaments (AFs) and vacuoles in guard cells is involved in the regulation of stomatal movement. However, it remains unclear whether there is any interaction between the reorganization of AFs and vacuolar changes during stomatal movement. Here, we report the relationship between the reorganization of AFs and vacuolar fusion revealed in pharmacological experiments, and characterizing stomatal opening in actin-related protein 2 (arp2) and arp3 mutants. Our results show that cytochalasin-D-induced depolymerization or phalloidin-induced stabilization of AFs leads to an increase in small unfused vacuoles during stomatal opening in wild-type (WT) Arabidopsis plants. Light-induced stomatal opening is retarded and vacuolar fusion in guard cells is impaired in the mutants, in which the reorganization and the dynamic parameters of AFs are aberrant compared with those of the WT. In WT, AFs tightly surround the small separated vacuoles, forming a ring that encircles the boundary membranes of vacuoles partly fused during stomatal opening. In contrast, in the mutants, most AFs and actin patches accumulate abnormally around the nuclei of the guard cells, which probably further impair vacuolar fusion and retard stomatal opening. Our results suggest that the reorganization of AFs regulates vacuolar fusion in guard cells during stomatal opening.

Mechanism of higher plant gravity sensing

Higher plants have developed statocytes, specialized tissues or cells for gravity sensing, and subsequent signal formation. Root and shoot statocytes commonly harbor a number of amyloplasts, and amyloplast sedimentation in the direction of gravity is a critical process in gravity sensing. However, the molecular mechanism underlying amyloplast-dependent gravity sensing is largely unknown. In this review, we mainly describe the molecular basis for the gravity sensing mechanism, i.e., the molecules and their functions involved in amyloplast sedimentation. Several analyses of statocyte images in living plant organs have implied differences in the regulation of amyloplast movements between root and shoot statocytes. Amyloplasts in shoot statocytes display not only sedimentable but upward, saltatory movements, but the latter are rarely observed in root statocytes. A series of genetic studies on shoot gravitropism mutants of Arabidopsis thaliana has revealed that two intracellular components, the vacuolar membrane (VM) and actin microfilaments (AFs), within the shoot statocyte play important roles in amyloplast dynamics. Flexible VM structures surrounding the amyloplasts seem to allow them to freely sediment toward the bottom of cells. In contrast, long actin cables mediate the saltatory movements of amyloplasts. Thus, amyloplasts in shoot statocytes undergo a dynamic equilibrium of movement, and a proper intracellular environment for statocytes is essential for normal shoot gravitropism. Further analyses to identify the molecular regulators of amyloplast dynamics, including sedimentation, may contribute to an understanding of the gravity sensing mechanism in higher plants.

A mutant defective in sexual development produces aseptate ascogonia

The transition from the vegetative to the sexual cycle in filamentous ascomycetes is initiated with the formation of ascogonia. Here, we describe a novel type of sterile mutant from Sordaria macrospora with a defect in ascogonial septum formation. This mutant, named pro22, produces only small, defective protoperithecia and carries a point mutation in a gene encoding a protein that is highly conserved throughout eukaryotes. Sequence analyses revealed three putative transmembrane domains and a C-terminal domain of unknown function. Live-cell imaging showed that PRO22 is predominantly localized in the dynamic tubular and vesicular vacuolar network of the peripheral colony region close to growing hyphal tips and in ascogonia; it is absent from the large spherical vacuoles in the vegetative hyphae of the subperipheral region of the colony. This points to a specific role of PRO22 in the tubular and vesicular vacuolar network, and the loss of intercalary septation in ascogonia suggests that PRO22 functions during the initiation of sexual development.

The coiled-coil protein VIG1 is essential for tethering vacuoles to mitochondria during vacuole inheritance of Cyanidioschyzon merolae

Vacuoles/lysosomes function in endocytosis and in storage and digestion of metabolites. These organelles are inherited by the daughter cells in eukaryotes. However, the mechanisms of this inheritance are poorly understood because the cells contain multiple vacuoles that behave randomly. The primitive red alga Cyanidioschyzon merolae has a minimum set of organelles. Here, we show that C. merolae contains about four vacuoles that are distributed equally between the daughter cells by binding to dividing mitochondria. Binding is mediated by VIG1, a 30-kD coiled-coil protein identified by microarray analyses and immunological assays. VIG1 appears on the surface of free vacuoles in the cytosol and then tethers the vacuoles to the mitochondria. The vacuoles are released from the mitochondrion in the daughter cells following VIG1 digestion. Suppression of VIG1 by antisense RNA disrupted the migration of vacuoles. Thus, VIG1 is essential for tethering vacuoles to mitochondria during vacuole inheritance in C. merolae.

Dynamics of vacuoles and actin filaments in guard cells and their roles in stomatal movement

Vacuoles and actin filaments are important cytoarchitectures involved in guard cell function. The changes in the morphology and number of vacuoles and the regulation of ion channel activity in tonoplast of guard cells are essential for stomatal movement. A number of studies have investigated the regulation of ion channels in animal and plant cells; however, little is known about the regulating mechanism for vacuolar dynamics in stomatal movement. Actin filaments of guard cells are remodelling with the changes in the stomatal aperture; however, the dynamic functions of actin filaments in stomatal movement remain elusive. In this paper, we summarize the recent developments in the understanding of the dynamics of actin filaments and vacuoles of guard cells during stomatal movement. All relevant studies suggest that actin filaments might be involved in stomatal movement by regulating vacuolar dynamics and the ion channels in tonoplast. The future study could be focused on the linker protein mediating the interaction between actin filaments and tonoplast, which will provide insights into the interactive function of actin and vacuole in stomatal movement regulation.

Microtubules regulate dynamic organization of vacuoles in Physcomitrella patens

Eukaryotic cells have developed several essential membrane components. In flowering plants, appropriate structures and distributions of the major membrane components are predominantly regulated by actin microfilaments. In this study, we have focused on the regulatory mechanism of vacuolar structures in the moss, Physcomitrella patens. The high ability of P. patens to undergo homologous recombination enabled us stably to express green fluorescent protein (GFP) or red fluorescent protein (RFP) fusion proteins, and the simple body structure of P. patens enabled us to perform detailed visualization of the intracellular vacuolar and cytoskeletal structures. Three-dimensional analysis and high-speed time-lapse observations revealed surprisingly complex structures and dynamics of the vacuole, with inner sheets and tubular protrusions, and frequent rearrangements by separation and fusion of the membranes. Depolymerization of microtubules dramatically affected these structures and movements. Dual observation of microtubules and vacuolar membranes revealed that microtubules induced tubular protrusions and cytoplasmic strands of the vacuoles, indicative of interactions between microtubules and vacuolar membranes. These results demonstrate a novel function of microtubules in maintaining the distribution of the vacuole and suggest a functional divergence of cytoskeletal functions in land plant evolution.

Chapter 3. New insights into plant vacuolar structure and dynamics

The plant vacuole is a multifunctional organelle and is essential for plant development and growth. The most distinctive feature of the plant vacuole is its size, which usually occupies over 80-90% of the cell volume in well-developed somatic cells, and is therefore highly involved in cell growth and plant body size. Recent progress in the visualization of the vacuole, together with developments in image analysis, has revealed the highly organized and complex morphology of the vacuole, as well as its dynamics. The plant vacuolar membrane (VM) forms not only a typically large vacuole but also other structures, such as tubular structures, transvacuolar strands, bulbs, and sheets. In higher plant cells, actin microfilaments are mainly located near the VM and are involved in vacuolar shape changes with the actin-myosin systems. Most recently, microtubule-dependent regulation of vacuolar structures in moss plant cells was reported, suggesting a diversity of mechanisms regulating vacuolar morphogenesis.

Acceleration of vacuolar regeneration and cell growth by overexpression of an aquaporin NtTIP1;1 in tobacco BY-2 cells

Aquaporin is a water channel that increases water permeability through membranous structures. In plants, vacuoles are essential organelles that undergo dynamic volume changes during cell growth. To understand the contribution of aquaporins to plant cell growth, we developed a transgenic tobacco BY-2 cell line overexpressing the tonoplast intrinsic protein (TIP), gammaTIP. Vacuolar membranes of isolated vacuoles from gammaTIP-overexpressing cells showed higher water permeation activities than those from wild-type cells. We then examined the role of gammaTIP in vacuolar regeneration of evacuolated tobacco BY-2 protoplasts (miniprotoplasts). Vacuolar regeneration from thin to thick tube-network vacuoles and subsequent development of large vacuoles was accelerated in miniprotoplasts of this cell line. A parallel increase in the rate of cell expansion indicated a tight relationship between vacuolar development and cellular volume increases. Interestingly, overexpression of tobacco gammaTIP also enhanced cell division. Thus, increased vacuolar aquaporin activity may accelerate both cell expansion and cell division by increasing water permeability through the vacuolar membrane.

Array and distribution of actin filaments in guard cells contribute to the determination of stomatal aperture

Actin filaments in guard cells and their dynamics function in regulating stomatal movement. In this study, the array and distribution of actin filaments in guard cells during stomatal movement were studied with two vital labeling, microinjection of alexa-phalloidin in Vicia faba and expression of GFP-mTn in tobacco. We found that the random array of actin filaments in the most of the closed stomata changed to a ring-like array after stomatal open. And actin filaments, which were throughout the cytoplasm of guard cells of closed stomata (even distribution), were mainly found in the cortical cytoplasm in the case of open stomata (cortical distribution). These results revealed that the random array and even distribution of actin filaments in guard cells may be required for keeping the closed stomata; similarly, the ring-like array and cortical distribution of actin filaments function in sustaining open stomata. Furthermore, we found that actin depolymerization, the trait of moving stomata, facilitates the transformation of actin array and distribution with stomatal movement. So, the depolymerization of actin filaments was favorable for the changes of actin array and distribution in guard cells and thus facilitated stomatal movement.

Arabidopsis protein disulfide isomerase-5 inhibits cysteine proteases during trafficking to vacuoles before programmed cell death of the endothelium in developing seeds

Protein disulfide isomerase (PDI) oxidizes, reduces, and isomerizes disulfide bonds, modulates redox responses, and chaperones proteins. The Arabidopsis thaliana genome contains 12 PDI genes, but little is known about their subcellular locations and functions. We demonstrate that PDI5 is expressed in endothelial cells about to undergo programmed cell death (PCD) in developing seeds. PDI5 interacts with three different Cys proteases in yeast two-hybrid screens. One of these traffics together with PDI5 from the endoplasmic reticulum through the Golgi to vacuoles, and its recombinant form is functionally inhibited by recombinant PDI5 in vitro. Peak PDI5 expression in endothelial cells precedes PCD, whereas decreasing PDI5 levels coincide with the onset of PCD-related cellular changes, such as enlargement and subsequent collapse of protein storage vacuoles, lytic vacuole shrinkage and degradation, and nuclear condensation and fragmentation. Loss of PDI5 function leads to premature initiation of PCD during embryogenesis and to fewer, often nonviable, seeds. We propose that PDI5 is required for proper seed development and regulates the timing of PCD by chaperoning and inhibiting Cys proteases during their trafficking to vacuoles before PCD of the endothelial cells. During this transitional phase of endothelial cell development, the protein storage vacuoles become the de facto lytic vacuoles that mediate PCD.

Cortical actin filaments at the division site of mitotic plant cells: a reconsideration of the 'actin-depleted zone'

The preprophase bands of microtubules and F-actin are primary markers of the division site for most plant cells. After preprophase band breakdown, the division site has been considered to be 'negatively' memorized by the local absence of cortical actin filaments. However, there have been reports of cortical F-actin at the division site of mitotic plant cells, calling into question its distribution and possible role there. In this article, previous and recent data on this issue are reviewed. It is proposed that the division site of mitotic plant cells is not devoid of F-actin but is traversed by scarce cortical actin filaments. The description of the division site as an 'actin exclusion zone' might therefore be attributed to a failure to preserve and/or image the notoriously sensitive actin filaments. Accordingly, the 'actin-depleted zone' should be considered as a site with fewer actin filaments than the rest of the cortical cytoplasm. Taking into account recent molecular data on division site components, a possible role for the scarcity of cortical actin filaments in establishing a zone of minimum mobility is also proposed.

Elicitor-induced cytoskeletal rearrangement relates to vacuolar dynamics and execution of cell death: in vivo imaging of hypersensitive cell death in tobacco BY-2 cells

Disintegration of the vacuolar membrane (VM) has been proposed to be a crucial event in various types of programmed cell death (PCD) in plants. However, its regulatory mechanisms are mostly unknown. To obtain new insights on the regulation of VM disintegration during hypersensitive cell death, we investigated the structural dynamics and permeability of the VM, as well as cytoskeletal reorganization during PCD in tobacco BY-2 cells induced by a proteinaceous elicitor, cryptogein. From sequential observations, we have identified the following remarkable events during PCD. Stage 1: bulb-like VM structures appear within the vacuolar lumen and the cortical microtubules are disrupted, while the cortical actin microfilaments are bundled. Simultaneously, transvacuolar strands including endoplasmic microtubules and actin microfilaments are gradually disrupted and the nucleus moves from the center to the periphery of the cell. Stage 2: cortical actin microfilament bundles and complex bulb-like VM structures disappear. The structure of the large central vacuole becomes simpler, and small spherical vacuoles appear. Stage 3: the VM is disintegrated and a fluorescent dye, BCECF, leaks out of the vacuoles just prior to PCD. Application of an actin polymerization inhibitor facilitates both the disappearance of bulb-like vacuolar membrane structures and induction of cell death. These results suggest that the elicitor-induced reorganization of actin microfilaments is involved in the regulation of hypersensitive cell death via modification of the vacuolar structure to induce VM disintegration.

Identification and mitotic partitioning strategies of vacuoles in the unicellular red alga Cyanidioschyzon merolae

Cyanidioschyzon merolae is considered as a suitable model system for studies of organelle differentiation, proliferation and partitioning. Here, we have identified and characterized vacuoles in this organism and examined the partitioning of vacuoles using fluorescence and electron microscopy. Vacuoles were stained with the fluorescent aminopeptidase substrate 7-amino-4-chloromethylcoumarin L: -arginine amide, acidotrophic dyes quinacrine and LysoTracker, and 4',6-diamidino-2-phenyl indole, which, at a high concentration, stains polyphosphate. Vacuoles have been shown to be approximately 500 nm in diameter with a mean of around five per interphase cell. The vacuolar H(+)-ATPase inhibitor concanamycin A blocked the accumulation of quinacrine in the vacuoles, suggesting the presence of the enzyme on these membranes. Electron microscopy revealed that the vacuoles were single membrane-bound organelles with an electron-dense substance, often containing a thick layer surrounding the membrane. Immunoelectron microscopy using an anti-vacuolar-H(+)-pyrophosphatase antibody revealed the presence of the enzyme on these membranes. In interphase cells, vacuoles were distributed in the cytoplasm, while in mitotic cells they were localized adjacent to the mitochondria. Filamentous structures were observed between vacuoles and mitochondria. Vacuoles were distributed almost evenly to daughter cells and redistributed in the cytoplasm after cytokinesis. The change in localization of vacuoles also happened in microtubule-disrupted cells. Since no actin protein or filaments have been detected in C. merolae, this result suggests an intrinsic mechanism for the movement of vacuoles that differs from commonly known mechanisms mediated by microtubules and actin filaments.

Intra-vacuolar reserves of membranes during stomatal closure: the possible role of guard cell vacuoles estimated by 3-D reconstruction

Stomatal apertures are regulated by morphological changes in guard cells which have been associated with guard cell vacuolar structures. To investigate the contribution of guard cell vacuoles to stomatal movement, we examined the dynamics of vacuolar membrane structures in guard cells and evaluated the changes in vacuolar volumes and surface areas during stomatal movement. Using a transgenic Arabidopsis line expressing green fluorescent protein (GFP)-AtVAM3, we have found that the guard cell vacuolar structures became complicated during stomatal closure with the appearance of numerous intra-vacuolar membrane structures. A three-dimensional (3-D) reconstruction using our originally developed software, REANT (reconstructor and analyzer of 3-D structure), and photobleaching analysis revealed the continuity of the vacuolar structures, even when they appeared to be compartmented in confocal images of closed stomata. Furthermore, calculations of the surface area by REANT revealed an increase in vacuolar surface area during stomatal closure but a decrease in the surface area of the guard cells. Movement of a vital staining dye, FM4-64, to the vacuolar membrane was accelerated during ABA-induced stomatal closure in Vicia faba. These results suggest that the guard cell vacuoles store some portion of the excess membrane materials produced during stomatal closure as intra-vacuolar structures.

Actin dynamics in papilla cells of Brassica rapa during self- and cross-pollination

The self-incompatibility system of the plant species Brassica is controlled by the S-locus, which contains S-RECEPTOR KINASE (SRK) and S-LOCUS PROTEIN11 (SP11). SP11 binding to SRK induces SRK autophosphorylation and initiates a signaling cascade leading to the rejection of self pollen. However, the mechanism controlling hydration and germination arrest during self-pollination is unclear. In this study, we examined the role of actin, a key cytoskeletal component regulating the transport system for hydration and germination in the papilla cell during pollination. Using rhodamine-phalloidin staining, we showed that cross-pollination induced actin polymerization, whereas self-pollination induced actin reorganization and likely depolymerization. By monitoring transiently expressed green fluorescent protein fused to the actin-binding domain of mouse talin, we observed the concentration of actin bundles at the cross-pollen attachment site and actin reorganization and likely depolymerization at the self-pollen attachment site; the results correspond to those obtained by rhodamine-phalloidin staining. We further showed that the coat of self pollen is sufficient to mediate this response. The actin-depolymerizing drug cytochalasin D significantly inhibited pollen hydration and germination during cross-pollination, further emphasizing a role for actin in these processes. Additionally, three-dimensional electron microscopic tomography revealed the close association of the actin cytoskeleton with an apical vacuole network. Self-pollination disrupted the vacuole network, whereas cross-pollination led to vacuolar rearrangements toward the site of pollen attachment. Taken together, our data suggest that self- and cross-pollination differentially affect the dynamics of the actin cytoskeleton, leading to changes in vacuolar structure associated with hydration and germination.

Differential organelle movement on the actin cytoskeleton in lily pollen tubes

We have examined the arrangement and movement of three major compartments, the endoplasmic reticulum (ER), mitochondria, and the vacuole during oscillatory, polarized growth in lily pollen tubes. These movements are dependent on the actin cytoskeleton, because they are strongly perturbed by the anti-microfilament drug, latrunculin-B, and unaffected by the anti-microtubule agent, oryzalin. The ER, which has been labeled with mGFP5-HDEL or cytochalasin D tetramethylrhodamine, displays an oscillatory motion in the pollen tube apex. First it moves apically in the cortical region, presumably along the cortical actin fringe, and then periodically folds inward creating a platform that transects the apical domain in a plate-like structure. Finally, the ER reverses its direction and moves basipetally through the central core of the pollen tube. When subjected to cross-correlation analysis, the formation of the platform precedes maximal growth rates by an average of 3 s (35-40 degrees ). Mitochondria, labeled with Mitotracker Green, are enriched in the subapical region, and their movement closely resembles that of the ER. The vacuole, labeled with carboxy-dichlorofluorescein diacetate, consists of thin tubules arranged longitudinally in a reticulate network, which undergoes active motion. In contrast to the mitochondria and ER, the vacuole is located back from the apex, and never extends into the apical clear zone. We have not been able to decipher an oscillatory pattern in vacuole motion. Because this motion is dependent on actin and not tubulin, we think this is due to a different myosin from that which drives the ER and mitochondria.

Recent progress in living cell imaging of plant cytoskeleton and vacuole using fluorescent-protein transgenic lines and three-dimensional imaging

In higher-plant cells, microtubules, actin microfilaments, and vacuoles play important roles in a variety of cellular events, including cell division, morphogenesis, and cell differentiation. These intracellular structures undergo dynamic changes in their shapes and functions during cell division and differentiation, and to analyse these sequential structural changes, the vital labelling technique, using the green-fluorescent protein or other fluorescent proteins, has commonly been used to follow the localisation and translocation of specific proteins. To visualise microtubules, actin filaments, and vacuoles, several strategies are available for selecting the appropriate fluorescent-protein fusion partner: microtubule-binding proteins, tubulin, and plus-end-tracking proteins are most suitable for microtubule labelling; the actin binding domain of mouse talin and plant fimbrin for actin microfilament visualisation; and the tonoplast-intrinsic proteins and syntaxin-related proteins for vacuolar imaging. In addition, three-dimensional reconstruction methods are indispensable for localising the widely distributed organelles within the cell. The maximum intensity projection method is suitable for cytoskeletal structures, while contour-based surface modelling possesses many advantages for vacuolar membranes. In this article, we summarise the recent progress in living cell imaging of the plant cytoskeleton and vacuoles using various fusions with green-fluorescent proteins and three-dimensional imaging techniques.

The ATP binding cassette transporter AtMRP5 modulates anion and calcium channel activities in Arabidopsis guard cells

Stomatal guard cells control CO(2) uptake and water loss between plants and the atmosphere. Stomatal closure in response to the drought stress hormone, abscisic acid (ABA), results from anion and K(+) release from guard cells. Previous studies have shown that cytosolic Ca(2+) elevation and ABA activate S-type anion channels in the plasma membrane of guard cells, leading to stomatal closure. However, membrane-bound regulators of abscisic acid signaling and guard cell anion channels remain unknown. Here we show that the ATP binding cassette (ABC) protein AtMRP5 is localized to the plasma membrane. Mutation in the AtMRP5 ABC protein impairs abscisic acid and cytosolic Ca(2+) activation of slow (S-type) anion channels in the plasma membrane of guard cells. Interestingly, atmrp5 insertion mutant guard cells also show impairment in abscisic acid activation of Ca(2+)-permeable channel currents in the plasma membrane of guard cells. These data provide evidence that the AtMRP5 ABC transporter is a central regulator of guard cell ion channel during abscisic acid and Ca(2+) signal transduction in guard cells.

Actin microfilaments regulate vacuolar structures and dynamics: dual observation of actin microfilaments and vacuolar membrane in living tobacco BY-2 Cells

Actin microfilaments (MFs) participate in many fundamental processes in plant growth and development. Here, we report the co-localization of the actin MF and vacuolar membrane (VM), as visualized by vital VM staining with FM4-64 in living tobacco BY-2 cells stably expressing green fluorescent protein (GFP)-fimbrin (BY-GF11). The MFs were intensively localized on the VM surface and at the periphery of the cytoplasmic strands rather than at their center. The co-localization of MFs and VMs was confirmed by the observation made using transient expression of red fluorescent protein (RFP)-fimbrin in tobacco BY-2 cells stably expressing GFP-AtVam3p (BY-GV7) and BY-2 cells stably expressing gamma-tonoplast intrinsic protein (gamma-TIP)-GFP fusion protein (BY-GG). Time-lapse imaging revealed dynamic movement of MF structures which was parallel to that of cytoplasmic strands. Disruption of MF structures disorganized cytoplasmic strand structures and produced small spherical vacuoles in the VM-accumulating region. Three-dimensional reconstructions of the vacuolar structures revealed a disconnection of these small spherical vacuoles from the large vacuoles. Real-time observations and quantitative image analyses demonstrated rapid movements of MFs and VMs near the cell cortex, which were inhibited by the general myosin ATPase inhibitor, 2,3-butanedion monoxime (BDM). Moreover, both bistheonellide A (BA) and BDM treatment inhibited the reorganization of the cytoplasmic strands and the migration of daughter cell nuclei at early G1 phase, suggesting a requirement for the acto-myosin system for vacuolar morphogenesis during cell cycle progression. These results suggest that MFs support the vacuolar structures and that the acto-myosin system plays an essential role in vacuolar morphogenesis.

The vacuole system is a significant intracellular pathway for longitudinal solute transport in basidiomycete fungi

Mycelial fungi have a growth form which is unique among multicellular organisms. The data presented here suggest that they have developed a unique solution to internal solute translocation involving a complex, extended vacuole. In all filamentous fungi examined, this extended vacuole forms an interconnected network, dynamically linked by tubules, which has been hypothesized to act as an internal distribution system. We have tested this hypothesis directly by quantifying solute movement within the organelle by photobleaching a fluorescent vacuolar marker. Predictive simulation models were then used to determine the transport characteristics over extended length scales. This modeling showed that the vacuolar organelle forms a functionally important, bidirectional diffusive transport pathway over distances of millimeters to centimeters. Flux through the pathway is regulated by the dynamic tubular connections involving homotypic fusion and fission. There is also a strongly predicted interaction among vacuolar organization, predicted diffusion transport distances, and the architecture of the branching colony margin.

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.

Vacuolar membrane dynamics in the filamentous fungus Aspergillus oryzae

Vacuoles in filamentous fungi are highly pleomorphic and some of them, e.g., tubular vacuoles, are implicated in intra- and intercellular transport. In this report, we isolated Aovam3, the homologue of the Saccharomyces cerevisiae VAM3 gene that encodes the vacuolar syntaxin, from Aspergillus oryzae. In yeast complementation analyses, the expression of Aovam3 restored the phenotypes of both Deltavam3 and Deltapep12 mutants, suggesting that AoVam3p is likely the vacuolar and/or endosomal syntaxin in A. oryzae. FM4-64 [N-(3-triethylammoniumpropyl)-4-(p-diethylaminophenyl-hexatrienyl)pyridinium dibromide] and CMAC (7-amino-4-chloromethylcoumarin) staining confirmed that the fusion protein of enhanced green fluorescent protein (EGFP) with AoVam3p (EGFP-AoVam3p) localized on the membrane of the pleomorphic vacuolar networks, including large spherical vacuoles, tubular vacuoles, and putative late endosomes/prevacuolar compartments. EGFP-AoVam3p-expressing strains allowed us to observe the dynamics of vacuoles with high resolutions, and moreover, led to the discovery of several new aspects of fungal vacuoles, which have not been discovered so far with conventional staining methods, during different developmental stages. In old hyphae, EGFP fluorescence was present in the entire lumen of large vacuoles, which occupied most of the cell, indicating that degradation of cytosolic materials had occurred in such hyphae via an autophagic process. In hyphae that were not in contact with nutrients, such as aerial hyphae and hyphae that grew on a glass surface, vacuoles were composed of small punctate structures and tubular elements that often formed reticulum-like networks. These observations imply the presence of so-far-unrecognized roles of vacuoles in the development of filamentous fungi.

Cell cycle-dependent changes in Golgi stacks, vacuoles, clathrin-coated vesicles and multivesicular bodies in meristematic cells of Arabidopsis thaliana: a quantitative and spatial analysis

Cytokinesis in plants involves both the formation of a new wall and the partitioning of organelles between the daughter cells. To characterize the cellular changes that accompany the latter process, we have quantitatively analyzed the cell cycle-dependent changes in cell architecture of shoot apical meristem cells of Arabidopsis thaliana. For this analysis, the cells were preserved by high-pressure freezing and freeze-substitution techniques, and their Golgi stacks, multivesicular bodies, vacuoles and clathrin-coated vesicles (CCVs) characterized by means of serial thin section reconstructions, stereology and electron tomography techniques. Interphase cells possess approximately 35 Golgi stacks, and this number doubles during G2 immediately prior to mitosis. At the onset of cytokinesis, the stacks concentrate around the periphery of the growing cell plate, but do not orient towards the cell plate. Interphase cells contain approximately 18 multivesicular bodies, most of which are located close to a Golgi stack. During late cytokinesis, the appearance of a second group of cell plate-associated multivesicular bodies coincides with the onset of CCV formation at the cell plate. During this period a 4x increase in CCVs is paralleled by a doubling in number and a 4x increase in multivesicular bodies volume. The vacuole system also undergoes major changes in organization, size, and volume, with the most notable change seen during early telophase cytokinesis. In particular, the vacuoles form sausage-like tubular compartments with a 50% reduced surface area and an 80% reduced volume compared to prometaphase cells. We postulate that this transient reduction in vacuole volume during early telophase provides a means for increasing the volume of the cytosol to accommodate the forming phragmoplast microtubule array and associated cell plate-forming structures.

Cell cycle synchronization of tobacco BY-2 cells

Synchronization is a powerful technique for understanding cell cycle events. Here, we describe the procedure for synchronizing tobacco bright yellow 2 (BY-2) cell line, with which an exceptionally high level of synchrony can be achieved. It basically relies on an "arrest-and-release" strategy using aphidicolin, an inhibitor of DNA replication, and propyzamide, a plant-microtubule disruptant. In a single-step process using aphidicolin alone, a cell population with about 70% of the cells at mitosis can be achieved, whereas by a two-step method using the two inhibitors sequentially, the level of synchrony can reach over 90%. The method of choice depends not only on the peak mitotic cell proportion but also on the cell cycle stage that is targeted for analysis. Both procedures take about 1.5 days, and cell cycle progression can be observed from the S phase to the next G1 phase at about 12 h after a 24 h-period treatment with aphidicolin.

A unique pattern of F-actin organization supports cytokinesis in vacuolated cells of Macrocystis pyrifera (Phaeophyceae) gametophytes

The organization of actin filaments and their role in cytokinesis was studied in regenerating protoplasts and thallus cells of gametophytes of the brown alga Macrocystis pyrifera. Before the onset of cytokinesis, a ring of actin filaments appeared on the putative cytokinetic plane just under the plasmalemma. Light and electron microscopy of cytokinetic cells revealed that large vacuoles occupy the space between the daughter nuclei, which very often are eccentrically positioned at the cell cortex. By the progress of cytokinesis, actin filament bundles emanating from the cytokinetic ring tend to form an actin plate that enters cytoplasmic pockets in which the cytokinetic diaphragm develops. The mechanism of this cytokinetic pattern that has not been reported so far for brown algae is discussed.

Morphometrical study of plant vacuolar dynamics in single cells using three-dimensional reconstruction from optical sections

In higher plants, vacuoles increase their volumes in accordance with cell enlargement and occupy most of the cell volume. However, quantitative analyses of vacuolar contributions during changes in cell morphology have been hampered by the inadequacies and frequent artifacts associated with current three-dimensional (3-D) reconstruction methods of images derived from light microscopy. To overcome the limitations of quantifying 3-D structures, we have introduced 3-D morphometrics into light microscopy, adopting a contour-based approach for which we have developed an interpolation method. Using this software, named REANT, the morphological and morphometrical changes in protoplasts and vacuoles during plasmolysis could be investigated. We employed the tobacco (Nicotiana tabacum) BY-2 cell line No.7, expressing a GFP-AtVam3p fusion protein, BY-GV7, using GFP as a marker of vacuolar membranes (VMs). By vital staining of the plasma membrane (PM) of cells, we simultaneously obtained optical sections of both the PM and VM. We, therefore, reconstructed the 3-D structures of protoplasts and vacuoles before and after plasmolysis. We were able to identify the appearance of elliptical structures of VMs in the vacuolar lumen, and to determine that they were derived from cytoplasmic strands. From the 3-D structures, the volumes and surface areas were measured at the single cell level. The shrinkage of vacuoles accounted for most of the decrease in protoplast volume, while the surface area of the vacuoles remained mostly unchanged. These morphometrical analyses suggest that the elliptical structures are reservoirs for excess VMs that result from the response to rapid decreases in vacuolar and protoplast volumes.

The dynamic changes of tonoplasts in guard cells are important for stomatal movement in Vicia faba

Stomatal movement is important for plants to exchange gas with environment. The regulation of stomatal movement allows optimizing photosynthesis and transpiration. Changes in vacuolar volume in guard cells are known to participate in this regulation. However, little has been known about the mechanism underlying the regulation of rapid changes in guard cell vacuolar volume. Here, we report that dynamic changes in the complex vacuolar membrane system play a role in the rapid changes of vacuolar volume in Vicia faba guard cells. The guard cells contained a great number of small vacuoles and various vacuolar membrane structures when stomata closed. The small vacuoles and complex membrane systems fused with each other or with the bigger vacuoles to generate large vacuoles during stomatal opening. Conversely, the large vacuoles split into smaller vacuoles and generated many complex membrane structures in the closing stomata. Vacuole fusion inhibitor, (2s,3s)-trans-epoxy-succinyl-l-leucylamido-3-methylbutane ethyl ester, inhibited stomatal opening significantly. Furthermore, an Arabidopsis (Arabidopsis thaliana) mutation of the SGR3 gene, which has a defect in vacuolar fusion, also led to retardation of stomatal opening. All these results suggest that the dynamic changes of the tonoplast are essential for enhancing stomatal movement.

Appearance of actin microfilament 'twin peaks' in mitosis and their function in cell plate formation, as visualized in tobacco BY-2 cells expressing GFP-fimbrin

The actin cytoskeleton of higher plants plays an essential role in plant morphogenesis and in maintaining various cellular activities. In this study we established a tobacco BY-2 cell line, stably transformed with a GFP-fimbrin actin-binding domain (ABD) 2 construct, that allows visualization of actin microfilaments (MFs) in living cells. Using this cell line, designated BY-GF11, we performed time-sequential observations of MF dynamics during cell-cycle progression. Detailed analyses revealed the appearance of a broad MF band in the late G2 phase that separated to form a structure corresponding to the so-called actin-depleted zone (ADZ) in mitosis. In BY-GF11, the MF structure at the cell cortex in mitosis appeared to form two bands rather than the ADZ. Measurements of fluorescent intensities of the cell cortex indicated an MF distribution that resembled two peaks, and we therefore named the structure MF 'twin peaks' (MFTP). The cell plate formed exactly within the valley between the MFTP at cytokinesis, and this cell-plate guidance was distorted by disruption of the MFTP by an inhibitor of actin polymerization. These results suggest that the MFTP originates from the broad MF band in the G2 phase and functions as a marker of cytokinesis.

Endocytosis and vesicle trafficking during tip growth of root hairs

The directional elongation of root hairs, "tip growth", depends on the coordinated and highly regulated trafficking of vesicles which fill the tip cytoplasm and are active in secretion of cell wall material. So far, little is known about the dynamics of endocytosis in living root hairs. We analyzed the motile behaviour of vesicles in the apical region of living root hairs of Arabidopsis thaliana and of Triticum aestivum by live cell microscopy. For direct observation of endocytosis and of the fate of endocytic vesicles, we used the fluorescent endocytosis marker dyes FM 1-43 and FM 4-64. Rapid endocytosis was detected mainly in the tip, where it caused a bright fluorescence of the apical cytoplasm. The internalized membranes proceeded through highly dynamic putative early endosomes in the clear zone to larger endosomal compartments in the subapical region that are excluded from the clear zone. The internalized cargo ended up in the dynamic vacuole by fusion of large endosomal compartments with the tonoplast. Before export to these lytic compartments, putative early endosomes remained in the apical zone, where they most probably recycled to the plasma membrane and back into the cytoplasm for more than 30 min. Endoplasmic reticulum was not involved in trafficking pathways of endosomes. Actin cytoskeleton was needed for the endocytosis itself, as well as for further membrane trafficking. The actin-depolymerizing drug latrunculin B modified the dynamic properties of vesicles and endosomes; they became immobilized and aggregated in the tip. Treatment with brefeldin A inhibited membrane trafficking and caused the disappearance of FM-containing vesicles and putative early endosomes from the clear zone; labelled structures accumulated in motile brefeldin A-induced compartments. These large endocytic compartments redispersed upon removal of the drug. Our results hence prove that endocytosis occurs in growing root hairs. We show the localization of endocytosis in the tip and indicate specific endomembrane compartments and their recycling.