Microtubules and coated vesicles in guard-cell protoplasts ofAllium cepa L

Three-dimensional visualization of planta clathrin-coated vesicles at ultrastructural resolution

Biological systems are the sum of their dynamic three-dimensional (3D) parts. Therefore, it is critical to study biological structures in 3D and at high resolution to gain insights into their physiological functions. Electron microscopy of metal replicas of unroofed cells and isolated organelles has been a key technique to visualize intracellular structures at nanometer resolution. However, many of these methods require specialized equipment and personnel to complete them. Here, we present novel accessible methods to analyze biological structures in unroofed cells and biochemically isolated organelles in 3D and at nanometer resolution, focusing on Arabidopsis clathrin-coated vesicles (CCVs). While CCVs are essential trafficking organelles, their detailed structural information is lacking due to their poor preservation when observed via classical electron microscopy protocols experiments. First, we establish a method to visualize CCVs in unroofed cells using scanning transmission electron microscopy tomography, providing sufficient resolution to define the clathrin coat arrangements. Critically, the samples are prepared directly on electron microscopy grids, removing the requirement to use extremely corrosive acids, thereby enabling the use of this method in any electron microscopy lab. Secondly, we demonstrate that this standardized sample preparation allows the direct comparison of isolated CCV samples with those visualized in cells. Finally, to facilitate the high-throughput and robust screening of metal replicated samples, we provide a deep learning analysis method to screen the "pseudo 3D" morphologies of CCVs imaged with 2D modalities. Collectively, our work establishes accessible ways to examine the 3D structure of biological samples and provide novel insights into the structure of plant CCVs.

Fluorescent detection of fluid phase endocytosis allows for in vivo estimation of endocytic vesicle sizes in plant cells with sub-diffraction accuracy

Using the bright, photostable, charged and hydrophilic fluorescent dye Alexa 488 hydrazide to label the fluid phase around intact guard cells, we show that these cells incorporate the fluid phase during constitutive endocytosis against the high turgor. Mobile, cortical and diffraction-limited signals were not observed if a concentration <4 mm was used to stain the fluid phase, suggesting that endocytic vesicles had to be loaded with a minimal number of dye molecules to produce a signal above the background. To quantify the number of molecules taken up by the vesicles, we prepared liposomes, filled with various concentrations of Alexa 488 hydrazide, fractionated them according to their size and imaged them under identical conditions as the guard cells. From the size/intensity relations of these liposomes, we extrapolated the molecular brightness of Alexa 488 hydrazide. Using this calibration, the mean fluorescent intensity of single endocytic vesicles translates into a mean number of 573 Alexa 488 molecules. If a vesicle needs to take up 573 molecules from a 4 mm solution, it requires a diameter of at least 87 nm. This number provides the first in vivo estimate for the size of endocytic vesicles in intact, turgid plant cells.

Constitutive overexpression of a stress-inducible small GTP-binding protein PgRab7 from Pennisetum glaucum enhances abiotic stress tolerance in transgenic tobacco

The Rab GTPases are important components of endocytic network in plant cells. Endocytosis participates in the cell's reaction to extracellular stimuli by desensitizing, down-regulating or recycling receptors and membrane proteins. Rab7 is a small GTP-binding protein involved in intracellular vesicle trafficking from late endosome to the vacuole. We have isolated Rab7 cDNA from Pennisetum glaucum, a relatively drought-stress tolerant food grain crop grown commonly in India, during cDNA-subtractive hybridization of dehydration-stress treated plants. The PgRab7 ORF, encoding 207 aminoacids, was over-expressed in E. coli. The recombinant PgRab7 protein showed GTP-binding and GTPase activity. Transcript expression of PgRab7 gene was differentially up-regulated by different environmental stimuli such as cold, dehydration and NaCl and also by a plant hormone IAA. Overexpression of PgRab7 gene enhanced tolerance to NaCl and mannitol in transgenic tobacco. Transgenic plants also had increased alkaline phosphatase (ALP) activity. These results show that PgRab7 is a potential candidate gene for developing both salinity and dehydration tolerance in planta.

The role of the cytoskeleton in the morphogenesis and function of stomatal complexes

Microtubules (MTs) and actin filaments (AFs) form highly organized arrays in stomatal cells that play key roles in the morphogenesis of stomatal complexes. The cortical MTs controlling the orientation of the depositing cellulose microfibrils (CMs) and affecting the pattern of local wall thickenings define the mechanical properties of the walls of stomatal cells, thus regulating accurately their shape. Besides, they are involved in determination of the cell division plane. Substomatal cavity and stomatal pore formation are also MT-dependent processes. Among the cortical MT arrays, the radial ones lining the periclinal walls are of particular morphogenetic importance. Putative MT organizing centers (MTOCs) function at their focal regions, at least in guard cells (GCs), or alternatively, these regions either organize or nucleate cortical MTs. AFs are involved in cell polarization preceding asymmetrical divisions, in determination of the cell division plane and final cell plate alignment and probably in transduction of stimuli implicated in stomatal complex morphogenesis. Mature kidney-shaped GCs display radial AF arrays, undergoing definite organization cycles during stomatal movement. They are involved in stomatal movement, probably by controlling plasmalemma ion-channel activities. Radial MT arrays also persist in mature GCs, but a role in stomatal function cannot yet be attributed to them. Contents Summary 613 I. Introduction 614 II. Cytoskeleton and development of the stomatal complexes 614 III. Cytoskeleton and stomatal cell shaping 620 IV. Stomatal pore formation 624 V. Substomatal cavity formation 625 VI. Stomatal complex morphogenesis in mutants 626 VII. Cytoskeleton dynamics in functioning stomata 628 VIII. Mechanisms of microtubule organization in stomatal cells 631 IX. Conclusions-perspectives 634 References 635.

F-actin-dependent endocytosis of cell wall pectins in meristematic root cells. Insights from brefeldin A-induced compartments

Brefeldin A (BFA) inhibits exocytosis but allows endocytosis, making it a valuable agent to identify molecules that recycle at cell peripheries. In plants, formation of large intracellular compartments in response to BFA treatment is a unique feature of some, but not all, cells. Here, we have analyzed assembly and distribution of BFA compartments in development- and tissue-specific contexts of growing maize (Zea mays) root apices. Surprisingly, these unique compartments formed only in meristematic cells of the root body. On the other hand, BFA compartments were absent from secretory cells of root cap periphery, metaxylem cells, and most elongating cells, all of which are active in exocytosis. We report that cell wall pectin epitopes counting rhamnogalacturonan II dimers cross-linked by borate diol diester, partially esterified (up to 40%) homogalacturonan pectins, and (1-->4)-beta-D-galactan side chains of rhamnogalacturonan I were internalized into BFA compartments. In contrast, Golgi-derived secretory (esterified up to 80%) homogalacturonan pectins localized to the cytoplasm in control cells and did not accumulate within characteristic BFA compartments. Latrunculin B-mediated depolymerization of F-actin inhibited internalization and accumulation of cell wall pectins within intracellular BFA compartments. Importantly, cold treatment and protoplasting prevented internalization of wall pectins into root cells upon BFA treatment. These observations suggest that cell wall pectins of meristematic maize root cells undergo rapid endocytosis in an F-actin-dependent manner.

On the alignment of cellulose microfibrils by cortical microtubules: a review and a model

The hypothesis that microtubules align microfibrils, termed the alignment hypothesis, states that there is a causal link between the orientation of cortical microtubules and the orientation of nascent microfibrils. I have assessed the generality of this hypothesis by reviewing what is known about the relation between microtubules and microfibrils in a wide group of examples: in algae of the family Characeae, Closterium acerosum, Oocystis solitaria, and certain genera of green coenocytes and in land plant tip-growing cells, xylem, diffusely growing cells, and protoplasts. The salient features about microfibril alignment to emerge are as follows. Cellulose microfibrils can be aligned by cortical microtubules, thus supporting the alignment hypothesis. Alignment of microfibrils can occur independently of microtubules, showing that an alternative to the alignment hypothesis must exist. Microfibril organization is often random, suggesting that self-assembly is insufficient. Microfibril organization differs on different faces of the same cell, suggesting that microfibrils are aligned locally, not with respect to the entire cell. Nascent microfibrils appear to associate tightly with the plasma membrane. To account for these observations, I present a model that posits alignment to be mediated through binding the nascent microfibril. The model, termed templated incorporation, postulates that the nascent microfibril is incorporated into the cell wall by binding to a scaffold that is oriented; further, the scaffold is built and oriented around either already incorporated microfibrils or plasma membrane proteins, or both. The role of cortical microtubules is to bind and orient components of the scaffold at the plasma membrane. In this way, spatial information to align the microfibrils may come from either the cell wall or the cell interior, and microfibril alignment with and without microtubules are subsets of a single mechanism.

Plasma membrane-associated actin in bright yellow 2 tobacco cells. Evidence for interaction with microtubules

Plasma membrane ghosts form when plant protoplasts attached to a substrate are lysed to leave a small patch of plasma membrane. We have identified several factors, including the use of a mildly acidic actin stabilization buffer and the inclusion of glutaraldehyde in the fixative, that allow immunofluorescent visualization of extensive cortical actin arrays retained on membrane ghosts made from tobacco (Nicotiana tabacum L.) suspension-cultured cells (line Bright Yellow 2). Normal microtubule arrays were also retained using these conditions. Membrane-associated actin is random; it exhibits only limited coalignment with the microtubules, and microtubule depolymerization in whole cells before wall digestion and ghost formation has little effect on actin retention. Actin and microtubules also exhibit different sensitivities to the pH and K+ and Ca2+ concentrations of the lysis buffer. There is, however, strong evidence for interactions between actin and the microtubules at or near the plasma membrane, because both ghosts and protoplasts prepared from taxol-pretreated cells have microtubules arranged in parallel arrays and an increased amount of actin coaligned with the microtubules. These experiments suggest that the organization of the cortical actin arrays may be dependent on the localization and organization of the microtubules.

The generation and consolidation of a radial array of cortical microtubules in developing guard cells of Allium cepa L

The initiation and development of a radial array of microtubules (MTs) in guard cells of A. cepa was studied using immunofluorescence microscopy of tubulin in isolated epidermal layers. Soon after the completion of cytokinesis, MTs originate in the cortex adjacent to a central strip of the new, anticlinically oriented ventral wall separating the two guard cells. Cortical MTs extend from the mid-region of the central strip toward the cell edge where the ventral wall joins the inner periclinal wall. They then spread in a fan-like formation along the periclinal wall and gradually extend along the lateral and end walls as well. Many MTs criss-cross at various angles as they arc past the edge formed by the junction of the ventral and periclinal walls, but they do not terminate there, indicating that, contrary to previous report, the edge is not involved in MT initiation. Instead, the mid-region of the central strip appears to function as a planar MT-organizing zone. Initially, MTs radiate from this zone through the inner cytoplasm as well as the cortex. During cell expansion, however, the cortical MTs increasingly predominate and consolidate into relatively thick, long bundles, while the frequency of non-cortical MTs diminishes. The apparent density of MTs per unit surface area is maintained as the cells expand and gradually flex into an elliptical shape. The guard cells eventually separate completely at the pore site. The entire process is accomplished within about 12 h.

Microtubule-binding proteins from carrot : I. Initial characterization and microtubule bundling

Microtubules (MTs) participate in several processes of fundamental importance to growth and development in higher plants, yet little is known about the proteins with which they associate. Information about these molecules is important because they probably play a role in mediating functional and structural differences between various MT arrays. As a first step in gaining insight into this problem, we have isolated, from suspension-cultured cells of carrot (Daucus carota L.), non-tubulin proteins which bind to and affect microtubules (MTs) in vitro. These proteins were isolated using taxol-stabilized neuronal MTs as an affinity substrate. They cause MT bundling at substoichiometric concentrations, support the assembly of tubulin in vitro, and at low concentrations, decorate single MTs in a periodic fashion. The bundled MTs formed in vitro share similarities with those seen in situ in a variety of plant cells, including a center-center spacing of 34 nm, cold stability, resistance to anti-microtubule drugs, and sensitivity to calcium. The bundling activity is specific; other cationic proteins, as well as poly-L-lysine, do not behave in a similar manner. The bundling activity is insensitive to ATP. By assaying bundling activity with dark-field microscopy and employing standard biochemical procedures, a small number of polypeptides involved in the bundling process were identified. Affinity-isolated antibodies to one of these polypeptides (Mr=76000) were found to co-localize with MTs in the cortical array of protoplasts. Our findings are discussed with reference to the importance of these proteins in the cell and to their relationship to microtubule-associated proteins in other eukaryotes.

COATED VESICLES IN THE CYTOPLASM OF THE HOST CELLS IN OPHRYS LUTEA CAV. MYCORRHIZAS (ORCHIDACEAE)

The host cells from endomycorrhizas of Ophrys lutea Cav. contain many coated vesicles associated with dictyosomes, with a membrane system (probably partially coated reticulum) and with the host's sequestration plasmalemma. The contents of coated vesicles stain with neither the PATAg test for polysaccharides nor with ruthenium red. The involvement of coated vesicles in the host/endophyte interactions during the establishment of O. lutea endomycorrhizas is discussed.

The isolation of coated vesicles from protoplasts of soybean

Fractions enriched in coated vesicles were obtained from protoplasts derived from suspension cultured Glycine max (L.) Merr. cells. Initial enrichment was achieved by isopycnic centrifugation of a protoplast homogenate through a linear sucrose gradient in a vertical rotor. The coated-vesicle fractions from this gradient were pooled and centrifuged through a second linear sucrose gradient in a rate zonal fashion to remove the larger contaminating membrane vesicles. The most prominent polypeptide in the coated-vesicle fractions, plant "clathrin", had a relative molecular mass of approx. 190 kdalton as determined by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. Other enriched polypeptides included bands at 105, 100, 96, 64, 50, 38 and 32 kdalton. This method was compared with a procedure utilizing sucrose step gradients for preparing coated vesicles from soybean protoplasts. The effectiveness of the isopycnic-rate zonal centrifugation procedure was also tested for the preparation of bovine-brain coated vesicles.

Endocytosis of cationized ferritin by coated vesicles of soybean protoplasts

Soybean (Glycine max (L.) Merr.) protoplasts have been surface-labelled with cationized ferritin, and the fate of the label has been followed ultrastructurally. Endocytosis of the label occurs via the coated-membrane system. The pathway followed by the label, once it has been taken into the interior of the protoplast, appears to be similar to that found during receptor-mediated endocytosis in animal cells. Cationized ferritin is first seen in coated vesicles but rapidly appears in smooth vesicles. Labelled, partially coated vesicles are occasionally observed, indicating that the smooth vesicles may have arisen by the uncoating of coated vesicles. Structures which eventually become labelled with cationized ferritin include multivesicular bodies, dictyosomes, large smooth vesicles, and a system of partially coated reticula.

The endoplasmic reticulum in the cortex of developing guard cells: coordinate studies with chlorotetracycline and osmium ferricyanide

The distribution of endoplasmic reticulum (ER) was investigated in young guard cells of Vicia faba and Allium cepa in order to gain more information on the control of guard cell development. Young, living guard cells of V. faba fluoresce when exposed to 25-100 microM chlorotetracycline (CTC). Intense fluorescence is restricted to the cytoplasm between the nucleus and adjacent regions of the ventral and paradermal walls. Much of the fluorescence is fibrillar in appearance and seems to arise from endomembranes, but not from particulate organelles such as mitochondria and plastids. A similar fluorescence pattern is produced by the membrane probes oxytetracycline and N-phenyl-1-napthylamine. Procaine and dibucaine render the fluorescence highly prone to photobleaching. Fluorescence appears near the ventral wall during early stages of cell development but declines when the guard cells mature. Epidermal tissue of V. faba and A. cepa was examined in the electron microscope with the aid of osmium ferricyanide staining. ER appears to be concentrated in regions of the guard cell that exhibit intense CTC fluorescence, while no other organelles (e.g., mitochondria) are similarly distributed. Much of the ER consists of a tubular network in close proximity to the plasmalemma. Our results indicate that the ER becomes asymmetrically distributed in young guard cells adjacent to those regions of the cell wall that undergo extensive thickening during cell differentiation. Furthermore, these membranes appear to sequester divalent cations such as Ca2+.

Organelles associated with the plasma membrane of tobacco leaf protoplasts

Leaf protoplasts of tobacco (Nicotiana tabacum) heterozygous (Su/su) and normal (su/su) for the sulfur mutation exhibit a characteristic lag period before initiating cell wall formation. The early wall is composed of a network of Calcofluor positive fibrils. Large fragments of plasma membrane from freshly isolated protoplasts were examined by electron microscopy to determine the distribution of associated organelles. Coated vesicles and patches of coat material were present on the inner surface of the plasma membrane. The frequency of coated vesicles observed was considerably less than reported for protoplasts from suspension cultured tobacco cells. Very few microtubules were associated with the plasma membrane but fine filaments were frequently observed.