Role of cortical microtubules in the orientation of cellulose microfibril deposition in higher-plant cells

Machine learning and feature analysis of the cortical microtubule organization of Arabidopsis cotyledon pavement cells

The measurement of cytoskeletal features can provide valuable insights into cell biology. In recent years, digital image analysis of cytoskeletal features has become an important research tool for quantitative evaluation of cytoskeleton organization. In this study, we examined the utility of a supervised machine learning approach with digital image analysis to distinguish different cellular organizational patterns. We focused on the jigsaw puzzle-shaped pavement cells of Arabidopsis thaliana. Measurements of three features of cortical microtubules in these cells (parallelness, density, and the coefficient of variation of the intensity distribution of fluorescently labeled cytoskeletons [as an indicator of microtubule bundling]) were obtained from microscopic images. A random forest machine learning model was then used with these images to differentiate mutant and wild type, and Taxol-treated and control cells. Using these three metrics, we were able to distinguish wild type from bpp125 triple mutant cells, with approximately 80% accuracy; classification accuracy was 88% for control and Taxol-treated cells. Different features contributed most to the classification, namely, coefficient of variation for the wild-type/mutant cells and parallelness for the Taxol-treated/control cells. The random forest method used enabled quantitative evaluation of the contribution of features to the classification, and partial dependence plots showed the relationships between metric values and classification accuracy. While further improvements to the method are needed, our small-scale analysis shows the potential for this approach in large-scale screening analyses.

Lipid Rafts and Plant Gravisensitivity

The necessity to include plants as a component of a Bioregenerative Life Support System leads to investigations to optimize plant growth facilities as well as a better understanding of the plant cell membrane and its numerous activities in the signaling, transport, and sensing of gravity, drought, and other stressors. The cell membrane participates in numerous processes, including endo- and exocytosis and cell division, and is involved in the response to external stimuli. Variable but stabilized microdomains form in membranes that include specific lipids and proteins that became known as (detergent-resistant) membrane microdomains, or lipid rafts with various subclassifications. The composition, especially the sterol-dependent recruitment of specific proteins affects endo- and exo-membrane domains as well as plasmodesmata. The enhanced saturated fatty acid content in lipid rafts after clinorotation suggests increased rigidity and reduced membrane permeability as a primary response to abiotic and mechanical stress. These results can also be obtained with lipid-sensitive stains. The linkage of the CM to the cytoskeleton via rafts is part of the complex interactions between lipid microdomains, mechanosensitive ion channels, and the organization of the cytoskeleton. These intricately linked structures and functions provide multiple future research directions to elucidate the role of lipid rafts in physiological processes.

Bordered Pit Formation in Cell Walls of Spruce Tracheids

The process of pit formation in plants still has various questions unaddressed and unknown, which opens up many interesting and new research opportunities. The aim of this work was elucidation of the mechanism for the formation of bordered pits of the spruce (Picea abies (L.) Karst.) tracheid with exosomes participation and mechanical deformation of the cell wall. Sample sections were prepared from spruce stem samples after cryomechanical destruction with liquid nitrogen. The study methods included scanning electron microscopy and enzymatic treatment. Enzymatic treatment of the elements of the bordered pit made it possible to clarify the localization of cellulose and pectin. SEM images of intermediate stages of bordered pit formation in the radial and tangential directions were obtained. An asynchronous mechanism of formation of bordered-pit pairs in tracheids is proposed. The formation of the pit pair begins from the side of the initiator cell and is associated with enzymatic hydrolysis of the secondary cell wall and subsequent mechanical deformation of the primary cell walls. Enzymatic hydrolysis of the S₁ layer of the secondary cell wall is carried out by exosome-delivered endoglucanases.

Cell Wall Biology: Dual Control of Cellulose Synthase Guidance

Cortical microtubules can direct the orientation of newly synthesized cellulose fibres in plant cell walls. However, cell wall-mediated steering mechanisms have also been anticipated. New research reveals that cellulose synthesis may be directed by pre-existing cellulose fibres in the walls.

Tubulin cytoskeleton during microsporogenesis in the male-sterile genotype of Allium sativum and fertile Allium ampeloprasum L

KEY MESSAGE

Microsporogenesis in garlic. The male-sterile Allium sativum (garlic) reproduces exclusively in the vegetative mode, and anthropogenic factors seem to be the cause of the loss of sexual reproduction capability. There are many different hypotheses concerning the causes of male sterility in A.sativum; however, the mechanisms underlying this phenomenon have not been comprehensively elucidated.Numerous attempts have been undertaken to understand the causes of male sterility, but the tubulin cytoskeleton in meiotically dividing cells during microsporogenesis has never been investigated in this species. Using sterile A.sativum genotype L13 and its fertile close relative A. ampeloprasum (leek), we have analysed the distribution of the tubulin cytoskeleton during microsporogenesis. We observed that during karyokinesis and cytokinesis, in both meiotic divisions I and II, the microtubular cytoskeleton in garlic L13 formed configurations that resembled tubulin arrangement typical of monocots. However, the tubulin cytoskeleton in garlic was distinctly poorer (composed of a few MT filaments) compared with that found in meiotically dividing cells in A. ampeloprasum. These differences did not affect the course of karyogenesis, chondriokinesis, and cytokinesis, which contributed to completion of microsporogenesis, but there was no further development of the male gametophyte. At the very beginning of the successive stage of development of fertile pollen grains, i.e. gametogenesis, there were disorders involving the absence of a normal cortical cytoskeleton and dramatically progressive degeneration of the cytoplasm in garlic. Therefore,we suggest that, due to disturbances in cortical cytoskeleton formation at the very beginning of gametogenesis, the intracellular transport governed by the cytoskeleton might be perturbed, leading to microspore decay in the male-sterile garlic genotype.

Quantitative analysis of microtubule orientation in interdigitated leaf pavement cells

Leaf pavement cells are shaped like a jigsaw puzzle in most dicotyledon species. Molecular genetic studies have identified several genes required for pavement cells morphogenesis and proposed that microtubules play crucial roles in the interdigitation of pavement cells. In this study, we performed quantitative analysis of cortical microtubule orientation in leaf pavement cells in Arabidopsis thaliana. We captured confocal images of cortical microtubules in cotyledon leaf epidermis expressing GFP-tubulinβ and quantitatively evaluated the microtubule orientations relative to the pavement cell growth axis using original image processing techniques. Our results showed that microtubules kept parallel orientations to the growth axis during pavement cell growth. In addition, we showed that immersion treatment of seed cotyledons in solutions containing tubulin polymerization and depolymerization inhibitors decreased pavement cell complexity. Treatment with oryzalin and colchicine inhibited the symmetric division of guard mother cells.

Cobtorin target analysis reveals that pectin functions in the deposition of cellulose microfibrils in parallel with cortical microtubules

Cellulose and pectin are major components of primary cell walls in plants, and it is believed that their mechanical properties are important for cell morphogenesis. It has been hypothesized that cortical microtubules guide the movement of cellulose microfibril synthase in a direction parallel with the microtubules, but the mechanism by which this alignment occurs remains unclear. We have previously identified cobtorin as an inhibitor that perturbs the parallel relationship between cortical microtubules and nascent cellulose microfibrils. In this study, we searched for the protein target of cobtorin, and we found that overexpression of pectin methylesterase and polygalacturonase suppressed the cobtorin-induced cell-swelling phenotype. Furthermore, treatment with polygalacturonase restored the deposition of cellulose microfibrils in the direction parallel with cortical microtubules, and cobtorin perturbed the distribution of methylated pectin. These results suggest that control over the properties of pectin is important for the deposition of cellulose microfibrils and/or the maintenance of their orientation parallel with the cortical microtubules.

Ultrastructure of the siphonaceous green alga Halimeda cuneata, with emphasis on the cytoskeleton

Cytoplasm streaming is a fundamental process for the transport of molecules and organelles in plant cells. In vegetative filaments of the coenocytic green alga, Halimeda cuneata Hering, the spatial organisation of microtubules in the cytoplasmic layer, was observed under transmission electron microscopy. A cross section of a cortical filament shows a tubular cell wall enclosing a peripheral layer of cytoplasm with numerous chloroplasts, amyloplasts, nuclei, mitochondria and microtubules surrounding a small central vacuole. Towards the thallus medulla the central vacuole enlarges considerably and the cytoplasm becomes gradually reduced to a thin parietal layer, the number of organelles is reduced and the quantity of microtubules increases. Therefore, most of the thallus volume is occupied by the huge central vacuole which extends throughout the coenocytic filaments. Microtubules run longitudinally, being about 0.05 microm from each other. Some microtubules were observed in close association to cell organelles.

Biosynthesis of callose and cellulose by detergent extracts of tobacco cell membranes and quantification of the polymers synthesized in vitro

The conditions that favor the in vitro synthesis of cellulose from tobacco BY-2 cell extracts were determined. The procedure leading to the highest yield of cellulose consisted of incubating digitonin extracts of membranes from 11-day-old tobacco BY-2 cells in the presence of 1 mM UDP-glucose, 8 mM Ca(2+) and 8 mM Mg(2+). Under these conditions, up to nearly 40% of the polysaccharides synthesized in vitro corresponded to cellulose, the other polymer synthesized being callose. Transmission electron microscopy analysis revealed the occurrence of two types of structures in the synthetic reactions. The first type consisted of small aggregates with a diameter between 3 and 5 nm that associated to form fibrillar strings of a maximum length of 400 nm. These structures were sensitive to the acetic/nitric acid treatment of Updegraff and corresponded to callose. The second type of structures was resistant to the Updegraff reagent and corresponded to straight cellulose microfibrils of 2-3 nm in diameter and 200 nm to up to 5 microm in length. In vitro reactions performed on electron microscopy grids indicated that the minimal rate of microfibril elongation in vitro is 120 nm/min. Measurements of retardance by liquid crystal polarization microscopy as a function of time showed that small groups of microfibrils increased in retardance by up to 0.047 nm/min per pixel, confirming the formation of organized structures.

Chemical genetic screening identifies a novel inhibitor of parallel alignment of cortical microtubules and cellulose microfibrils

It is a well-known hypothesis that cortical microtubules control the direction of cellulose microfibril deposition, and that the parallel cellulose microfibrils determine anisotropic cell expansion and plant cell morphogenesis. However, the molecular mechanism by which cortical microtubules regulate the orientation of cellulose microfibrils is still unclear. To investigate this mechanism, chemical genetic screening was performed. From this screening, 'SS compounds' were identified that induced a spherical swelling phenotype in tobacco BY-2 cells. The SS compounds could be categorized into three classes: those that disrupted the cortical microtubules; those that reduced cellulose microfibril content; and thirdly those that had neither of these effects. In the last class, a chemical designated 'cobtorin' was found to induce the spherical swelling phenotype at the lowest concentration, suggesting strong binding activity to the putative target. Examining cellulose microfibril regeneration using taxol-treated protoplasts revealed that the cobtorin compound perturbed the parallel alignment of pre-existing cortical microtubules and nascent cellulose microfibrils. Thus, cobtorin could be a novel inhibitor and an attractive tool for further investigation of the mechanism that enables cortical microtubules to guide the parallel deposition of cellulose microfibrils.

Establishing and maintaining axial growth: wall mechanical properties and the cytoskeleton

Organ morphology depends on cell placement and directional cell expansion. Microtubules are involved in both of these processes so genetic approaches to understand the role microtubules play in organ expansion are not straightforward. Our use of the temperature-sensitive mor1-1 mutants led to the surprising discovery that Arabidopsis thaliana (L.) Heynh. root cells can establish and maintain transverse cellulose texture without well organized microtubule arrays. This work also demonstrated that cells can lose the ability to expand anisotropically without losing transversely oriented cellulose microfibrils. We suggest that microtubule disruption affects the cell's ability to generate long cellulose microfibrils, which may be essential for achieving growth anisotropy. Thus organ shape may depend not only on the orientation but also on the relative length of cellulose microfibrils during axis establishment and growth. More recent work has shown an important correlation between microtubule organization and the deposition patterns of the glycosylphosphatidylinositol (GPI)-anchored wall protein COBRA. Loss of microtubule organization is associated with the dissipation of transverse banding patterns of COBRA, suggesting that COBRA's function in maintaining anisotropic expansion may be microtubule-dependent.

Phospholipase d activation correlates with microtubule reorganization in living plant cells

A phospholipase D (PLD) was shown recently to decorate microtubules in plant cells. Therefore, we used tobacco BY-2 cells expressing the microtubule reporter GFP-MAP4 to test whether PLD activation affects the organization of plant microtubules. Within 30 min of adding n-butanol, a potent activator of PLD, cortical microtubules were released from the plasma membrane and partially depolymerized, as visualized with four-dimensional confocal imaging. The isomers sec- and tert-butanol, which did not activate PLD, did not affect microtubule organization. The effect of treatment on PLD activation was monitored by the in vivo formation of phosphatidylbutanol, a specific reporter of PLD activity. Tobacco cells also were treated with mastoparan, xylanase, NaCl, and hypoosmotic stress as reported activators of PLD. We confirmed the reports and found that all treatments induced microtubule reorganization and PLD activation within the same time frame. PLD still was activated in microtubule-stabilized (taxol) and microtubule-depolymerized (oryzalin) situations, suggesting that PLD activation triggers microtubular reorganization and not vice versa. Exogenously applied water-soluble synthetic phosphatidic acid did not affect the microtubular cytoskeleton. Cell cycle studies revealed that n-butanol influenced not just interphase cortical microtubules but also those in the preprophase band and phragmoplast, but not those in the spindle structure. Cell growth and division were inhibited in the presence of n-butanol, whereas sec- and tert-butanol had no such effects. Using these novel insights, we propose a model for the mechanism by which PLD activation triggers microtubule reorganization in plant cells.

Dynamic changes and the role of the cytoskeleton during the cell cycle in higher plant cells

In higher plant cells microtubules (MTs) show dynamic structural changes during cell cycle progression and play significant roles in cell morphogenesis. The cortical MT (CMT), preprophase band (PPB), and phragmoplast, all of which are plant-specific MT structures, can be observed during interphase, from the late G2 phase to prophase, and from anaphase to telophase, respectively. The CMT controls cell shape, either irreversibly or reversibly, by orientating cellulose microfibril (CMF) deposition in the cell wall; the PPB is involved in determining the site of division; and the phragmoplast forms the cell plate at cytokinesis. The appearance and disappearance of these MT structures during the cell cycle have been extensively studied by immunofluorescence microscopy using highly synchronized tobacco BY-2 cells. Indeed, these studies, together with visualization of MT dynamics in living plant cells using the green fluorescent protein, have revealed much about the modes of MT structural organization, for example, of CMTs at the M/G1 interphase. The microfilaments which also show dynamic changes during the cell cycle, being similar to MTs at particular stages and different at other stages, appear to play roles in supporting MTs. In this article, we summarize our ongoing research and that of related studies of the structure and function of the plant cytoskeleton during cell cycle progression.

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.

New techniques enable comparative analysis of microtubule orientation, wall texture, and growth rate in intact roots of Arabidopsis

This article explores root epidermal cell elongation and its dependence on two structural elements of cells, cortical microtubules and cellulose microfibrils. The recent identification of Arabidopsis morphology mutants with putative cell wall or cytoskeletal defects demands a procedure for examining and comparing wall architecture and microtubule organization patterns in this species. We developed methods to examine cellulose microfibrils by field emission scanning electron microscopy and microtubules by immunofluorescence in essentially intact roots. We were able to compare cellulose microfibril and microtubule alignment patterns at equivalent stages of cell expansion. Field emission scanning electron microscopy revealed that Arabidopsis root epidermal cells have typical dicot primary cell wall structure with prominent transverse cellulose microfibrils embedded in pectic substances. Our analysis showed that microtubules and microfibrils have similar orientation only during the initial phase of elongation growth. Microtubule patterns deviate from a predominantly transverse orientation while cells are still expanding, whereas cellulose microfibrils remain transverse until well after expansion finishes. We also observed microtubule-microfibril alignment discord before cells enter their elongation phase. This study and the new technology it presents provide a starting point for further investigations on the physical properties of cell walls and their mechanisms of assembly.

The cytoskeleton and growth polarity

We are currently witnessing the discovery of many novel proteins that are associated with cytoskeletal activity. Integrated analyses of growth, cytoskeletal and cell-wall patterns are yielding surprising results, which demand reflection on the current model for wall construction. Meanwhile, research on actin filament and microtubule activity during gravitropic bending and trichome morphogenesis is stimulating new ideas about the establishment and maintenance of polarity.