Motile plant cell body: a "bug" within a "cage"

Tools for studying the cytoskeleton during plant cell division

The plant cytoskeleton regulates fundamental biological processes, including cell division. How to experimentally perturb the cytoskeleton is a key question if one wants to understand the role of both actin filaments (AFs) and microtubules (MTs) in a given biological process. While a myriad of mutants are available, knock-out in cytoskeleton regulators, when nonlethal, often produce little or no phenotypic perturbation because such regulators are often part of a large family, leading to functional redundancy. In this review, alternative techniques to modify the plant cytoskeleton during plant cell division are outlined. The different pharmacological and genetic approaches already developed in cell culture, transient assays, or in whole organisms are presented. Perspectives on the use of optogenetics to perturb the plant cytoskeleton are also discussed.

The botanical multiverse of Peter Barlow

Dr Peter Barlow, who died in 2017, was one of the most respected botanists and biologists of the latter half of the 20th Century. His interests covered a wide range of plant biological topics, e.g. root growth and development, plant cytoskeleton, effects of gravity, plant intelligence, pattern formation, and evolution of eukaryotic cells. Here we consider Peter's numerous contributions to the: elucidation of plant patterns; understanding of root biology; role of the plant cytoskeleton in growth and development; influence of the Moon on terrestrial vegetation; Cell Body concept; and plant neurobiology. In so doing we attempt not only to provide an overview of Peter's important work in many areas of plant biology, but also to place that work in the context of recent advances in plant and biological sciences.

The intracellular and intercellular cross-talk during subsidiary cell formation in Zea mays: existing and novel components orchestrating cell polarization and asymmetric division

Background

Formation of stomatal complexes in Poaceae is the outcome of three asymmetric and one symmetric cell division occurring in particular leaf protodermal cells. In this definite sequence of cell division events, the generation of subsidiary cells is of particular importance and constitutes an attractive model for studying local intercellular stimulation. In brief, an induction stimulus emitted by the guard cell mother cells (GMCs) triggers a series of polarization events in their laterally adjacent protodermal cells. This signal determines the fate of the latter cells, forcing them to divide asymmetrically and become committed to subsidiary cell mother cells (SMCs).

Scope

This article summarizes old and recent structural and molecular data mostly derived from Zea mays, focusing on the interplay between GMCs and SMCs, and on the unique polarization sequence occurring in both cell types. Recent evidence suggests that auxin operates as an inducer of SMC polarization/asymmetric division. The intercellular auxin transport is facilitated by the distribution of a specific transmembrane auxin carrier and requires reactive oxygen species (ROS). Interestingly, the local differentiation of the common cell wall between SMCs and GMCs is one of the earliest features of SMC polarization. Leucine-rich repeat receptor-like kinases, Rho-like plant GTPases as well as the SCAR/WAVE regulatory complex also participate in the perception of the morphogenetic stimulus and have been implicated in certain polarization events in SMCs. Moreover, the transduction of the auxin signal and its function are assisted by phosphatidylinositol-3-kinase and the products of the catalytic activity of phospholipases C and D.

Conclusion

In the present review, the possible role(s) of each of the components in SMC polarization and asymmetric division are discussed, and an overall perspective on the mechanisms beyond these phenomena is provided.

Increased endocytosis of fluorescent phospholipid in tobacco pollen in microgravity and inhibition by verapamil

Gravity sensing in plants occurs in specialised tissues, like in the columella in root tips or the endodermis in shoots. Generally, dense organelles, acting as statoliths, are thought to interact with the cytosekeleton and ion channels in gravitropism. We examined the possibility that tobacco pollen tubes (Nicotiana sylvestris) having an elaborate cytoskeleton could perceive gravity through interaction of the cytoskeleton and the endomembrane system and organelles. Using lipid endocytosis as a quantitative parameter, we show that endocytosis is increased transiently in microgravity within 3 min. This increase is inhibited by the calcium blocker verapamil, suggesting that calcium is lowered in the tip, which is known to increase endocytosis in the pollen tube.

Strasburger's legacy to mitosis and cytokinesis and its relevance for the Cell Theory

Eduard Strasburger was one of the most prominent biologists contributing to the development of the Cell Theory during the nineteenth century. His major contribution related to the characterization of mitosis and cytokinesis and especially to the discovery of the discrete stages of mitosis, which he termed prophase, metaphase and anaphase. Besides his observations on uninucleate plant and animal cells, he also investigated division processes in multinucleate cells. Here, he emphasised the independent nature of mitosis and cytokinesis. We discuss these issues from the perspective of new discoveries in the field of cell division and conclude that Strasburger's legacy will in the future lead to a reformulation of the Cell Theory and that this will accommodate the independent and primary nature of the nucleus, together with its complement of perinuclear microtubules, for the organisation of the eukaryotic cell.

Cell-cell channels, viruses, and evolution: via infection, parasitism, and symbiosis toward higher levels of biological complexity

Between prokaryotic cells and eukaryotic cells there is dramatic difference in complexity which represents a problem for the current version of the cell theory, as well as for the current version of evolution theory. In the past few decades, the serial endosymbiotic theory of Lynn Margulis has been confirmed. This results in a radical departure from our understanding of living systems: the eukaryotic cell represents de facto"cells-within-cell." Higher order "cells-within-cell" situations are obvious at the eukaryotic cell level in the form of secondary and tertiary endosymbiosis, or in the male and female gametophytes of higher plants. The next challenge of the current version of the cell theory is represented by the fact that the multicellular fungi and plants are, in fact, supracellular assemblies as their cells are not physically separated from each other. Moreover, there are also examples of alliances and mergings between multicellular organisms. Infection, especially the viral one, but also bacterial and fungal infections, followed by symbiosis, is proposed to act as the major force that drives the biological evolution toward higher complexity.

Cytoplasmic compartmental response to local mechanical stimulation of internal tissue cells

A convenient experimental system was established to test how cells derived from higher-plant internal tissues respond to mechanical stimulation. Short-term culture of tobacco ovules in vitro led to the generation of bar-shaped cells from the parenchyma tissue of the ovule funicle. These cells are still connected to the mother tissue and are almost undifferentiated. The cells are translucent, and one end protrudes from the funicle, making them easy to manipulate and observe. Mechanical stimulation tests performed on these cells indicated that the cells are less sensitive to mechanical stimulation than epidermal hair cells but still possess the ability to respond to stimulation. Interestingly, the cells showed a cytoplasmic compartmental response to the stimulation. The nucleus, some plastids, and mitochondria were organized into a responsive unit that moved in unison to the stimulated sites, whereas most of the other organelles were not notably influenced by the stimulation. This suggests that the cytoplasm is highly organized and functionally divided in response to environmental stimulation.

Cell number, cell growth, antheridiogenesis, and callose amount is reduced and atrophy induced by deoxyglucose in Anemia phyllitidis gametophytes

Fluorescence staining and morphometrical measurements revealed that callose was a component of newly formed cell plates of symmetrically dividing cells and asymmetrically dividing antheridial mother cells during gibberellic acid-induced antheridiogenesis as well as in walls of young growing cells of Anemia phyllitidis gametophytes. Callose in cell walls forms granulations characteristic of pit fields with plasmodesmata. 2-deoxy-D-glucose (DDG), eliminated callose granulations and reduced its amount estimated by measurements of fluorescence intensity. This effect was accompanied by reduction of antheridia and cell numbers as well as size and atrophy of particular cells and whole gametophytes. It is suggested that inhibition of glucose metabolism and/or signalling, might decrease callose synthesis in A. phyllitidis gametophytes leading to its elimination from cell plates of dividing cells and from walls of differentiating ones as well as from plasmodesmata resulting in inhibition of cytokinesis, cell growth and disruption of the intercellular communication system, thus disturbing developmental programs and leading to cell death.

Cytoskeleton and morphogenesis in brown algae

BACKGROUND

Morphogenesis on a cellular level includes processes in which cytoskeleton and cell wall expansion are strongly involved. In brown algal zygotes, microtubules (MTs) and actin filaments (AFs) participate in polarity axis fixation, cell division and tip growth. Brown algal vegetative cells lack a cortical MT cytoskeleton, and are characterized by centriole-bearing centrosomes, which function as microtubule organizing centres.

SCOPE

Extensive electron microscope and immunofluorescence studies of MT organization in different types of brown algal cells have shown that MTs constitute a major cytoskeletal component, indispensable for cell morphogenesis. Apart from participating in mitosis and cytokinesis, they are also involved in the expression and maintenance of polarity of particular cell types. Disruption of MTs after Nocodazole treatment inhibits cell growth, causing bulging and/or bending of apical cells, thickening of the tip cell wall, and affecting the nuclear positioning. Staining of F-actin using Rhodamine-Phalloidin, revealed a rich network consisting of perinuclear, endoplasmic and cortical AFs. AFs participate in mitosis by the organization of an F-actin spindle and in cytokinesis by an F-actin disc. They are also involved in the maintenance of polarity of apical cells, as well as in lateral branch initiation. The cortical system of AFs was found related to the orientation of cellulose microfibrils (MFs), and therefore to cell wall morphogenesis. This is expressed by the coincidence in the orientation between cortical AFs and the depositing MFs. Treatment with cytochalasin B inhibits mitosis and cytokinesis, as well as tip growth of apical cells, and causes abnormal deposition of MFs.

CONCLUSIONS

Both the cytoskeletal elements studied so far, i.e. MTs and AFs are implicated in brown algal cell morphogenesis, expressed in their relationship with cell wall morphogenesis, polarization, spindle organization and cytokinetic mechanism. The novelty is the role of AFs and their possible co-operation with MTs.

The Cytomatrix as a Cooperative System of Macromolecular and Water Networks

Water was called by Szent-Gyorgi "life's mater and matrix, mother and medium." This chapter considers both aspects of his statement. Many astrobiologists argue that some, if not all, of Earth's water arrived during cometary bombardments. Amorphous water ices of comets possibly facilitated organization of complex organic molecules, kick-starting prebiotic evolution. In Gaian theory, Earth retains its water as a consequence of biological activity. The cell cytomatrix is a proteinaceous matrix/lattice incorporating the cytoskeleton, a pervasive, holistic superstructural network that integrates metabolic pathways. Enzymes of metabolic pathways are ordered in supramolecular clusters (metabolons) associated with cytoskeleton and/or membranes. Metabolic intermediates are microchanneled through metabolons without entering a bulk aqueous phase. Rather than being free in solution, even major signaling ions are probably clustered in association with the cytomatrix. Chloroplasts and mitochondria, like bacteria and archaea, also contain a cytoskeletal lattice, metabolons, and channel metabolites. Eukaryotic metabolism is mathematically a scale-free or small-world network. Enzyme clusters of bacterial origin are incorporated at a pathway level that is architecturally archaean. The eucaryotic cell may be a product of serial endosymbiosis, a chimera. Cell cytoplasm is approximately 80% water. Water is indisputably a conserved structural element of proteins, essential to their folding, specificity, ligand binding, and to enzyme catalysis. The vast literature of organized cell water has long argued that the cytomatrix and cell water are an entire system, a continuum, or gestalt. Alternatives are offered to mainstream explanations of cell electric potentials, ion channel, enzyme, and motor protein function, in terms of high-order cooperative systems of ions, water, and macromolecules. This chapter describes some prominent concepts of organized cell water, including vicinal water network theory, the association-induction hypothesis, wave-cluster theory, phase-gel transition theories, and theories of low- and high-density water polymorphs.

The Arp2/3 complex nucleates actin arrays during zygote polarity establishment and growth

Previous work has demonstrated that dynamic actin arrays are important for axis establishment and polar growth in the fucoid zygote, Silvetia compressa. Transitions between these arrays are mediated by depolymerization of an existing array and polymerization of a new array. To begin to understand how polymerization of new arrays might be regulated, we investigated the role of the highly conserved, actin-nucleating, Actin-related protein 2/3 (Arp2/3) complex. Arp2, a subunit of the complex, was cloned and peptide antibodies were raised to the C-terminal domain. In immunolocalization studies of polarizing zygotes, actin and Arp2 colocalized around the nucleus and in a patch at the rhizoid pole. In germinated zygotes, a cone of Arp2 and actin extended from the nucleus to the subapex. Within the rhizoid tip, three structural zones were observed in the majority of zygotes: the extreme apex was devoid of label, the subapex was enriched for Arp2, and further back both actin and Arp2 were present. This zonation suggests that actin nucleation occurs at the leading edge of the cone, in the Arp2-enriched region. In two sets of experiments, we showed that tip zonation is important for growth. First, pharmacological treatments that disrupted Arp2/actin zonation arrested tip growth. Second, changes in the direction of tip growth during negative phototropism were preceded by a reorientation of the zonation in accordance with the new growth direction. This work represents the first investigation of Arp2/3 complex localization in tip-growing algal cells.

Organelle inheritance in plant cell division: the actin cytoskeleton is required for unbiased inheritance of chloroplasts, mitochondria and endoplasmic reticulum in dividing protoplasts

Nuclear inheritance is highly ordered, ensuring stringent, unbiased partitioning of chromosomes before cell division. In plants, however, little is known about the analogous cellular processes that might ensure unbiased inheritance of non-nuclear organelles, either in meristematic cell divisions or those induced during the acquisition of totipotency. We have investigated organelle redistribution and inheritance mechanisms during cell division in cultured tobacco mesophyll protoplasts. Quantitative analysis of organelle repositioning observed by autofluorescence of chloroplasts or green fluorescent protein (GFP), targeted to mitochondria or endoplasmic reticulum (ER), demonstrated that these organelles redistribute in an ordered manner before division. Treating protoplasts with cytoskeleton-disrupting drugs showed that redistribution depended on actin filaments (AFs), but not on microtubules (MTs), and furthermore, that an intact actin cytoskeleton was required to achieve unbiased organelle inheritance. Labelling the actin cytoskeleton with a novel GFP-fusion protein revealed a highly dynamic actin network, with local reorganisation of this network itself, appearing to contribute substantially to repositioning of chloroplasts and mitochondria. Our observations show that each organelle exploits a different strategy of redistribution to ensure unbiased partitioning. We conclude that inheritance of chloroplasts, mitochondria and ER in totipotent plant cells is an ordered process, requiring complex interactions with the actin cytoskeleton.

Mastoparan alters subcellular distribution of profilin and remodels F-actin cytoskeleton in cells of maize root apices

Indirect immunofluorescence localization of profilin in cells of maize root apices revealed that this abundant protein was present both in the cytoplasm and within nuclei. Nucleo-cytoplasmic partitioning of profilin exhibits tissue-specific and developmental features. Mastoparan-mediated activation of heterotrimeric G-proteins, presumably through triggering a phosphoinositide-signaling pathway based on phosphatidylinositol-4,5-bisphosphate (PIP(2)), induced relocalization of profilin from nuclei into the cytoplasm of root apex cells. In contrast, PIP(2) accumulated within nuclei of mastoparan-treated root cells. Intriguingly, cytoplasmic accumulation of profilin was associated with remodeling of F-actin arrays in root apex cells. Specifically, dense F-actin networks were dismantled and distinct actin patches became associated with the periphery of small vacuoles. On the other hand, disruption of F-actin with the G-actin sequestering agent latrunculin B does not affect the subcellular distribution of profilin or PIP(2). These data suggest that nuclear profilin can mediate a stimulus-response action on the actin cytoskeleton which is somehow linked to a phosphoinositide-signaling cascade.