Exploring the Role of the Plant Actin Cytoskeleton: From Signaling to Cellular Functions

The plant actin cytoskeleton is characterized by the basic properties of dynamic array, which plays a central role in numerous conserved processes that are required for diverse cellular functions. Here, we focus on how actins and actin-related proteins (ARPs), which represent two classical branches of a greatly diverse superfamily of ATPases, are involved in fundamental functions underlying signal regulation of plant growth and development. Moreover, we review the structure, assembly dynamics, and biological functions of filamentous actin (F-actin) from a molecular perspective. The various accessory proteins known as actin-binding proteins (ABPs) partner with F-actin to finely tune actin dynamics, often in response to various cell signaling pathways. Our understanding of the significance of the actin cytoskeleton in vital cellular activities has been furthered by comparison of conserved functions of actin filaments across different species combined with advanced microscopic techniques and experimental methods. We discuss the current model of the plant actin cytoskeleton, followed by examples of the signaling mechanisms under the supervision of F-actin related to cell morphogenesis, polar growth, and cytoplasmic streaming. Determination of the theoretical basis of how the cytoskeleton works is important in itself and is beneficial to future applications aimed at improving crop biomass and production efficiency.

Plant cell division from the perspective of polarity

The orientation of cell division is a major determinant of plant morphogenesis. In spite of considerable efforts over the past decades, the precise mechanism of division plane selection remains elusive. The majority of studies on the topic have addressed division orientation from either a predominantly developmental or a cell biological perspective. Thus, mechanistic insights into the links between developmental and cellular factors affecting division orientation are particularly lacking. Here, I review recent progress in the understanding of cell division orientation in the embryo and primary root meristem of Arabidopsis from both developmental and cell biological standpoints. I offer a view of multilevel polarity as a central aspect of cell division: on the one hand, the division plane is a readout of tissue- and organism-wide polarities; on the other hand, the cortical division zone can be seen as a transient polar subcellular plasma membrane domain. Finally, I argue that a polarity-focused conceptual framework and the integration of developmental and cell biological approaches hold great promise to unravel the mechanistic basis of plant cell division orientation in the near future.

The Orchid Velamen: A Model System for Studying Patterned Secondary Cell Wall Development?

Understanding the mechanisms through which plants generate secondary cell walls is of more than academic interest: the physical properties of plant-derived materials, including timber and textiles, all depend upon secondary wall cellulose organization. Processes controlling cellulose in the secondary cell wall and their reliance on microtubules have been documented in recent decades, but this understanding is complicated, as secondary walls normally form in the plant’s interior where live cell imaging is more difficult. We investigated secondary wall formation in the orchid velamen, a multicellular epidermal layer found around orchid roots that consists of dead cells with lignified secondary cell walls. The patterns of cell wall ridges that form within the velamen vary between different orchid species, but immunolabelling demonstrated that wall deposition is controlled by microtubules. As these patterning events occur at the outer surface of the root, and as orchids are adaptable for tissue culture and genetic manipulation, we conclude that the orchid root velamen may indeed be a suitable model system for studying the organization of the plant cell wall. Notably, roots of the commonly grown orchid Laelia anceps appear ideally suited for developing this research.

Simple Whole-Mount Staining Protocol of F-Actin for Studies of the Female Gametophyte in Agavoideae and Other Crassinucellate Ovules

During plant sexual reproduction, F-actin takes part in the elongation of the pollen tube and the movement of sperm cells along with it. Moreover, F-actin is involved in the transport of sperm cells throughout the embryo sac when double fertilization occurs. Different techniques for analysis of F-actin in plant cells have been developed: from classical actin-immunolocalization in fixed tissues to genetically tagged actin with fluorescent proteins for live imaging of cells. Despite the implementation of live cell imaging tools, fixed plant tissue methods for cytoskeletal studies remain an essential tool for genetically intractable systems. Also, most of the work on live imaging of the cytoskeleton has been conducted on cells located on the plant's surface, such as epidermal cells, trichomes, and root hairs. In cells situated in the plant's interior, especially those from plant species with thicker organ systems, it is necessary to utilize conventional sectioning and permeabilization methods to allow the label access to the cytoskeleton. Studies about the role of F-actin cytoskeleton during double fertilization in plants with crassinucellate ovules (e.g., Agave, Yucca, Polianthes, Prochnyantes, and Manfreda) remain scarce due to the difficulties to access the female gametophyte. Here, we have developed a straightforward method for analysis of F-actin in the female gametophyte of different Agavoideae sub-family species. The procedure includes the fixation of whole ovules with formaldehyde, followed by membrane permeabilization with cold acetone, a prolonged staining step with rhodamine-phalloidin, and Hoechst 33342 as a counterstain and two final steps of dehydration of samples in increasing-concentration series of cold isopropanol and clarification of tissues with methyl salicylate. This technique allows the analysis of a large number of samples in a short period, cell positioning relative to neighbor cells is maintained, and, with the help of a confocal microscope, reconstruction of a single 3D image of F-actin structures into the embryo sac can be obtained.

Cortical microtubule orientation in Arabidopsis thaliana root meristematic zone depends on cell division and requires severing by katanin

Background

Transverse cortical microtubule orientation, critical for anisotropic cell expansion, is established in the meristematic root zone. Intending to elucidate the possible prerequisites for this establishment and factors that are involved, microtubule organization was studied in roots of Arabidopsis thaliana, wild-type and the p60-katanin mutants fra2, ktn1-2 and lue1. Transverse cortical microtubule orientation in the meristematic root zone has proven to persist under several regimes inhibiting root elongation. This persistence was attributed to the constant moderate elongation of meristematic cells, prior to mitotic division. Therefore, A. thaliana wild-type seedlings were treated with aphidicolin, in order to prevent mitosis and inhibit premitotic cell elongation.

Results

In roots treated with aphidicolin for 12 h, cell divisions still occurred and microtubules were transverse. After 24 and 48 h of treatment, meristematic cell divisions and the prerequisite elongation ceased, while microtubule orientation became random. In meristematic cells of the p60-katanin mutants, apart from a general transverse microtubule pattern, cortical microtubules with random orientation were observed, also converging at several cortical sites, in contrast to the uniform transverse pattern of wild-type cells.

Conclusion

Taken together, these observations reveal that transverse cortical microtubule orientation in the meristematic zone of A. thaliana root is cell division-dependent and requires severing by katanin.

CLASP promotes stable tethering of endoplasmic microtubules to the cell cortex to maintain cytoplasmic stability in Arabidopsis meristematic cells

Following cytokinesis in plants, Endoplasmic MTs (EMTs) assemble on the nuclear surface, forming a radial network that extends out to the cell cortex, where they attach and incorporate into the cortical microtubule (CMT) array. We found that in these post-cytokinetic cells, the MT-associated protein CLASP is enriched at sites of EMT-cortex attachment, and is required for stable EMT tethering and growth into the cell cortex. Loss of EMT-cortex anchoring in clasp-1 mutants results in destabilized EMT arrays, and is accompanied by enhanced mobility of the cytoplasm, premature vacuolation, and precocious entry into cell elongation phase. Thus, EMTs appear to maintain cells in a meristematic state by providing a structural scaffold that stabilizes the cytoplasm to counteract actomyosin-based cytoplasmic streaming forces, thereby preventing premature establishment of a central vacuole and rapid cell elongation.

Kinesins and Myosins: Molecular Motors that Coordinate Cellular Functions in Plants

Kinesins and myosins are motor proteins that can move actively along microtubules and actin filaments, respectively. Plants have evolved a unique set of motors that function as regulators and organizers of the cytoskeleton and as drivers of long-distance transport of various cellular components. Recent progress has established the full complement of motors encoded in plant genomes and has revealed valuable insights into the cellular functions of many kinesin and myosin isoforms. Interestingly, several of the motors were found to functionally connect the two cytoskeletal systems and thereby to coordinate their activities. In this review, we discuss the available genetic, cell biological, and biochemical data for each of the plant kinesin and myosin families from the context of their subcellular mechanism of action as well as their physiological function in the whole plant. We particularly emphasize work that illustrates mechanisms by which kinesins and myosins coordinate the activities of the cytoskeletal system.

Contribution of the cytoskeleton to mechanosensitivity reported by dinoflagellate bioluminescence

The cytoskeleton is crucial to cell mechanics and sensing the extracellular physical environment. The objective of this study was to examine the role of the cortical cytoskeleton in mechanosensitivity in a unicellular protist, the marine dinoflagellate Lingulodinium polyedra, using its intrinsic bioluminescence as a rapid reporter of mechanotransduction. Pharmacological treatments resolved effects due to immediate cytoskeleton disruption from those due to cytoskeletal remodeling during the light to dark phase transition. The cytoskeleton was visualized by confocal laser scanning microscopy of immunohistochemically labeled microtubules and phalloidin labeled F-actin, and mechanosensitivity assessed based on the bioluminescence response to mechanical stimulation measured during the dark phase. Latrunculin B treatment after the transition from the light to dark phase resulted in some disruption of cortical F-actin, no observed effect on the cortical microtubules, and partial inhibition of the bioluminescence response. Treatment with oryzalin, which depolarizes microtubules, completely disrupted the microtubule network and cortical F-actin, and partially inhibited bioluminescence. These results demonstrate that cells retain some mechanosensitivity despite a disrupted cytoskeleton; link mechanosensitivity to intact F-actin; show a close connection between F-actin and microtubules comprising the cortical cytoskeleton; confirm a strong contribution of the actin cytoskeleton to the translocation of scintillons, vesicles containing the luminescent chemistry; and support the role of the actin cytoskeleton in the association of scintillons with the vacuole membrane.

Fluorescence Imaging of the Cytoskeleton in Plant Roots

During the past two decades the use of live cytoskeletal probes has increased dramatically due to the introduction of the green fluorescent protein. However, to make full use of these live cell reporters it is necessary to implement simple methods to maintain plant specimens in optimal growing conditions during imaging. To image the cytoskeleton in living Arabidopsis roots, we rely on a system involving coverslips coated with nutrient supplemented agar where the seeds are directly germinated. This coverslip system can be conveniently transferred to the stage of a confocal microscope with minimal disturbance to the growth of the seedling. For roots with a larger diameter such as Medicago truncatula, seeds are first germinated in moist paper, grown vertically in between plastic trays, and roots mounted on glass slides for confocal imaging. Parallel with our live cell imaging approaches, we routinely process fixed plant material via indirect immunofluorescence. For these methods we typically use non-embedded vibratome-sectioned and whole mount permeabilized root tissue. The clearly defined developmental regions of the root provide us with an elegant system to further understand the cytoskeletal basis of plant development.

Differential responsiveness of cortical microtubule orientation to suppression of cell expansion among the developmental zones of Arabidopsis thaliana root apex

Τhe bidirectional relationship between cortical microtubule orientation and cell wall structure has been extensively studied in elongating cells. Nevertheless, the possible interplay between microtubules and cell wall elements in meristematic cells still remains elusive. Herein, the impact of cellulose synthesis inhibition and suppressed cell elongation on cortical microtubule orientation was assessed throughout the developmental zones of Arabidopsis thaliana root apex by whole-mount tubulin immunolabeling and confocal microscopy. Apart from the wild-type, thanatos and pom2-4 mutants of Cellulose SynthaseA3 and Cellulose Synthase Interacting1, respectively, were studied. Pharmacological and mechanical approaches inhibiting cell expansion were also applied. Cortical microtubules of untreated wild-type roots were predominantly transverse in the meristematic, transition and elongation root zones. Cellulose-deficient mutants, chemical inhibition of cell expansion, or growth in soil resulted in microtubule reorientation in the elongation zone, wherein cell length was significantly decreased. Combinatorial genetic and chemical suppression of cell expansion extended microtubule reorientation to the transition zone. According to the results, transverse cortical microtubule orientation is established in the meristematic root zone, persisting upon inhibition of cell expansion. Microtubule reorientation in the elongation zone could be attributed to conditional suppression of cell elongation. The differential responsiveness of microtubule orientation to genetic and environmental cues is most likely associated with distinct biophysical traits of the cells among each developmental root zone.

The N-terminal TOG domain of Arabidopsis MOR1 modulates affinity for microtubule polymers

Microtubule-associated proteins of the highly conserved XMAP215/Dis1 family promote both microtubule growth and shrinkage, and move with the dynamic microtubule ends. The plant homologue, MOR1, is predicted to form a long linear molecule with five N-terminal TOG domains. Within the first (TOG1) domain, the mor1-1 leucine to phenylalanine (L174F) substitution causes temperature-dependent disorganization of microtubule arrays and reduces microtubule growth and shrinkage rates. By expressing the two N-terminal TOG domains (TOG12) of MOR1, both in planta for analysis in living cells and in bacteria for in vitro microtubule-binding and polymerization assays, we determined that the N-terminal domain of MOR1 is crucial for microtubule polymer binding. Tagging TOG12 at the N-terminus interfered with its ability to bind microtubules when stably expressed in Arabidopsis or when transiently overexpressed in leek epidermal cells, and impeded polymerase activity in vitro. In contrast, TOG12 tagged at the C-terminus interacted with microtubules in vivo, rescued the temperature-sensitive mor1-1 phenotype, and promoted microtubule polymerization in vitro. TOG12 constructs containing the L174F mor1-1 point mutation caused microtubule disruption when transiently overexpressed in leek epidermis and increased the affinity of TOG12 for microtubules in vitro. This suggests that the mor1-1 mutant protein makes microtubules less dynamic by binding the microtubule lattice too strongly to support rapid plus-end tracking. We conclude from our results that a balanced microtubule affinity in the N-terminal TOG domain is crucial for the polymerase activity of MOR1.

New insights into the role of plant formins: regulating the organization of the actin and microtubule cytoskeleton

Formins are well-known as important regulators participating in the organization of the actin cytoskeleton in organisms. For many years in the past, research on plant formins is more difficult than that in other eukaryotic formins and is limited to class I formins. Nevertheless, positive progress has been made in plant formin research recently, especially the investigations on class II formins. New functions of plant formins are identified gradually, such as regulating cell division and affecting diffuse cell expansion. More significantly, plant formins are also verified to interact with microtubules in vivo and in vitro. They may probably function as linking proteins between microtubules and microfilaments to participate in various cellular processes.

Nanoscale and geometric influences on the microtubule cytoskeleton in plants: thinking inside and outside the box

The dynamic microtubule (MT) cytoskeleton found in the cell cortex of plants drives cell expansion via cell wall modifications. In the last decade, live cell imaging studies employing green fluorescent protein have helped unravel the mechanisms behind how cells arrange cortical MTs into complex arrays and shape cell expansion. In this review, we explore the reverse scenario: how cell geometry and organelles influence and constrain the organization and behavior of cortical MTs. This newly emerging principle explains how cells perceive local nanoscale structural input from MT-organizing centers, such as the nucleus, endomembranes, and cell edges, and translate this into global cell-wide order via MT self-organization. Studies primarily using the model plant Arabidopsis thaliana and tobacco BY-2 suspension cultures have broadened our understanding of how cells form not only elegant parallel arrays but also more complex MT configurations, including the prominent MT bundles found in preprophase bands, leaf epidermal cells, and developing xylem.

The cytoskeleton in plasmodesmata: a role in intercellular transport?

Actin and myosin are components of the plant cell cytoskeleton that extend from cell to cell through plasmodesmata (PD), but it is unclear how they are organized within the cytoplasmic sleeve or how they might behave as regulatory elements. Early work used antibodies to locate actin and myosin to PD, at the electron microscope level, or to pitfields (aggregations of PD in the cell wall), using immunofluorescence techniques. More recently, a green fluorescent protein (GFP)-tagged plant myosin VIII was located specifically at PD-rich pitfields in cell walls. Application of actin or myosin disrupters may modify the conformation of PD and alter rates of cell-cell transport, providing evidence for a role in regulating PD permeability. Intriguingly, there is now evidence of differentiation between types of PD, some of which open in response to both actin and myosin disrupters, and others which are unaffected by actin disrupters or which close in response to myosin inhibitors. Viruses also interact with elements of the cytoskeleton for both intracellular and intercellular transport. The precise function of the cytoskeleton in PD may change during cell development, and may not be identical in all tissue types, or even in all PD within a single cell. Nevertheless, it is likely that actin- and myosin-associated proteins play a key role in regulating cell-cell transport, by interacting with cargo and loading it into PD, and may underlie the capacity for one-way transport across particular cell and tissue boundaries.

Live cell imaging reveals structural associations between the actin and microtubule cytoskeleton in Arabidopsis

In eukaryotic cells, the actin and microtubule (MT) cytoskeletal networks are dynamic structures that organize intracellular processes and facilitate their rapid reorganization. In plant cells, actin filaments (AFs) and MTs are essential for cell growth and morphogenesis. However, dynamic interactions between these two essential components in live cells have not been explored. Here, we use spinning-disc confocal microscopy to dissect interaction and cooperation between cortical AFs and MTs in Arabidopsis thaliana, utilizing fluorescent reporter constructs for both components. Quantitative analyses revealed altered AF dynamics associated with the positions and orientations of cortical MTs. Reorganization and reassembly of the AF array was dependent on the MTs following drug-induced depolymerization, whereby short AFs initially appeared colocalized with MTs, and displayed motility along MTs. We also observed that light-induced reorganization of MTs occurred in concert with changes in AF behavior. Our results indicate dynamic interaction between the cortical actin and MT cytoskeletons in interphase plant cells.

Mitogen-activated protein kinase 4 is involved in the regulation of mitotic and cytokinetic microtubule transitions in Arabidopsis thaliana

• A mitogen-activated protein kinase kinase kinase (MAPKKK) double mutant, Arabidopsis homologue of nucleus and phragmoplast associated kinase (anp) anp2anp3, and the mitogen-activated protein kinase (MAPK) 4 mutant mpk4 of Arabidopsis thaliana show prominent cytokinetic defects. This prompted the analysis of mitotic and cytokinetic progression as a function of MAPK signalling. Mutants were compared with wild types untreated or treated with the specific MAPKK inhibitor PD98059. • This study included phenotype analysis, expression analysis of the MPK4 promoter, immunofluorescent localization of MPK4, tubulin and MAP65-1, and time-lapse microscopic visualization of the mitotic microtubule (MT) transitions in control, mutant and inhibitor-treated cells. • Mutant and inhibitor-treated cells showed defects in mitosis and cytokinesis, including aberrant spindle and phragmoplast formation and drastically delayed or abortive mitosis and cytokinesis. As a result, bi- and multinucleate cells were formed, ultimately disturbing the vegetative tissue patterning. MPK4 was localized to all stages of the expanding phragmoplast, in a pattern similar to that of its putative substrate MAP65-1. • In this study, MPK4 is shown to be involved in the regulation of mitosis/cytokinesis through modulation of the cell division plane and cytokinetic progression.

Inhibition of phospholipase C disrupts cytoskeletal organization and gravitropic growth in Arabidopsis roots

The phospholipase protein superfamily plays an important role in hormonal signalling and cellular responses to environmental stimuli. There is also growing evidence for interactions between phospholipases and the cytoskeleton. In this report we used a pharmacological approach to investigate whether inhibiting a member of the phospholipase superfamily, phospholipase C (PLC), affects microtubules and actin microfilaments as well as root growth and morphology of Arabidopsis thaliana seedlings. Inhibiting PLC activity using the aminosteroid U73122 significantly inhibited root elongation and disrupted root morphology in a concentration-dependent manner, with the response being saturated at 5 μM, whereas the inactive analogue U73343 was ineffective. The primary root appeared to lose growth directionality accompanied by root waving and formation of curls. Immunolabelling of roots exposed to increasingly higher U73122 concentrations revealed that the normal transverse arrays of cortical microtubules in the elongation zone became progressively more disorganized or depolymerized, with the disorganization appearing within 1 h of incubation. Likewise, actin microfilament arrays also were disrupted. Inhibiting PLC using an alternative inhibitor, neomycin, caused similar disruptions to both cytoskeletal organization and root morphology. In seedlings gravistimulated by rotating the culture plates by 90°, both U73122 and neomycin disrupted the normal gravitropic growth of roots and etiolated hypocotyls. The effects of PLC inhibitors are therefore consistent with the notion that, as with phospholipases A and D, PLC likewise interacts with the cytoskeleton, alters growth morphology, and is involved in gravitropism.

Mechanisms of self-organization of cortical microtubules in plants revealed by computational simulations

Microtubules confined to the two-dimensional cortex of elongating plant cells must form a parallel yet dispersed array transverse to the elongation axis for proper cell wall expansion. Some of these microtubules exhibit free minus-ends, leading to migration at the cortex by hybrid treadmilling. Collisions between microtubules can result in plus-end entrainment ("zippering") or rapid depolymerization. Here, we present a computational model of cortical microtubule organization. We find that plus-end entrainment leads to self-organization of microtubules into parallel arrays, whereas catastrophe-inducing collisions do not. Catastrophe-inducing boundaries (e.g., upper and lower cross-walls) can tune the orientation of an ordered array to a direction transverse to elongation. We also find that changes in dynamic instability parameters, such as in mor1-1 mutants, can impede self-organization, in agreement with experimental data. Increased entrainment, as seen in clasp-1 mutants, conserves self-organization, but delays its onset and fails to demonstrate increased ordering. We find that branched nucleation at acute angles off existing microtubules results in distinctive sparse arrays and infer either that microtubule-independent or coparallel nucleation must dominate. Our simulations lead to several testable predictions, including the effects of reduced microtubule severing in katanin mutants.

Anti-tropomyosin antibodies co-localise with actin microfilaments and label plasmodesmata

The actin cytoskeleton and associated actin-binding proteins form a complex network involved in a number of fundamental cellular processes including intracellular trafficking. In plants, both actin and myosin have been localised to plasmodesmata, and thus it is likely that other actin-binding proteins are also associated with plasmodesmata structure or function. A 75-kDa protein, enriched in plasmodesmata-rich cell wall extracts from the green alga Chara corallina, was sequenced and found to contain three peptides with similarity to the animal actin-binding protein tropomyosin. Western blot analysis with anti-tropomyosin antibodies confirmed the identity of this 75-kDa protein as a tropomyosin-like protein and further identified an additional 55-kDa protein, while immunofluorescence microscopy localised the antibodies to plasmodesmata and to the subcortical actin bundles and associated structures. The anti-tropomyosin antibodies detected a single protein at 42.5 kDa in Arabidopsis thaliana extracts and two proteins at 58.5 and 54 kDa in leek extracts, and these localised to plasmodesmata and the cell plate in A. thaliana and to plasmodesmata in leek tissue. Tropomyosin is an actin-binding protein thought to be involved in a range of functions associated with the actin cytoskeleton, including the regulation of myosin binding to actin filaments, but to date no tropomyosin-like proteins have been conclusively identified in plant genomes. Our data suggests that a tropomyosin-like protein is associated with plasmodesmata.

Sample preparation for fluorescence imaging of the cytoskeleton in fixed and living plant roots

During the past decade the use of live cytoskeletal probes has increased dramatically due to the introduction of the green fluorescent protein. However, to make full use of these live cell reporters it is necessary to implement simple methods to maintain plant specimens in optimal growing conditions during imaging. To image the cytoskeleton in living Arabidopsis root cells, we rely on a system involving coverslips coated with nutrient supplemented agar where the seeds are directly germinated. This coverslip system can be conveniently transferred to the stage of a confocal microscope with minimal disturbance to the growth of the seedling. Parallel to our live cell imaging approaches, we routinely process fixed plant material via indirect immunofluorescence. For these methods we typically use nonembedded vibratome-sectioned and whole mount permeabilized root tissue. The clearly defined developmental regions of the root provide us with an elegant system to further understand the cytoskeletal basis of plant development.

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.

Cortical microtubules mark the mucilage secretion domain of the plasma membrane in Arabidopsis seed coat cells

During their differentiation Arabidopsis thaliana seed coat cells undergo a brief but intense period of secretory activity that leads to dramatic morphological changes. Pectic mucilage is secreted to one domain of the plasma membrane and accumulates under the primary cell wall in a ring-shaped moat around an anticlinal cytoplasmic column. Using cryofixation/transmission electron microscopy and immunofluorescence, the cytoskeletal architecture of seed coat cells was explored, with emphasis on its organization, function and the large amount of pectin secretion at 7 days post-anthesis. The specific domain of the plasma membrane where mucilage secretion is targeted was lined by abundant cortical microtubules while the rest of the cortical cytoplasm contained few microtubules. Actin microfilaments, in contrast, were evenly distributed around the cell. Disruption of the microtubules in the temperature-sensitive mor1-1 mutant affected the eventual release of mucilage from mature seeds but did not appear to alter the targeted secretion of vesicles to the mucilage pocket, the shape of seed coat cells or their secondary cell wall deposition. The concentration of cortical microtubules at the site of high vesicle secretion in the seed coat may utilize the same mechanisms required for the formation of preprophase bands or the bands of microtubules associated with spiral secondary cell wall thickening during protoxylem development.

Arabidopsis dynamin-like protein DRP1A: a null mutant with widespread defects in endocytosis, cellulose synthesis, cytokinesis, and cell expansion

Dynamin-related proteins are large GTPases that deform and cause fission of membranes. The DRP1 family of Arabidopsis thaliana has five members of which DRP1A, DRP1C, and DRP1E are widely expressed. Likely functions of DRP1A were identified by studying rsw9, a null mutant of the Columbia ecotype that grows continuously but with altered morphology. Mutant roots and hypocotyls are short and swollen, features plausibly originating in their cellulose-deficient walls. The reduction in cellulose is specific since non-cellulosic polysaccharides in rsw9 have more arabinose, xylose, and galactose than those in wild type. Cell plates in rsw9 roots lack DRP1A but still retain DRP1E. Abnormally placed and often incomplete cell walls are preceded by abnormally curved cell plates. Notwithstanding these division abnormalities, roots and stems add new cells at wild-type rates and organ elongation slows because rsw9 cells do not grow as long as wild-type cells. Absence of DRP1A reduces endocytotic uptake of FM4-64 into the cytoplasm of root cells and the hypersensitivity of elongation and radial swelling in rsw9 to the trafficking inhibitor monensin suggests that impaired endocytosis may contribute to the development of shorter fatter roots, probably by reducing cellulose synthesis.

Improved imaging of actin filaments in transgenic Arabidopsis plants expressing a green fluorescent protein fusion to the C- and N-termini of the fimbrin actin-binding domain 2

The role of the actin cytoskeleton in plant development is intimately linked to its dynamic behavior. Therefore it is essential to continue refining methods for studying actin organization in living plant cells. The discovery of green fluorescent protein (GFP) has popularized the use of translational fusions of GFP with actin filament (F-actin) side-binding proteins to visualize in vivo actin organization in plants. The most recent of these live cell F-actin reporters are GFP fusions to the actin-binding domain 2 (ABD2) of Arabidopsis fimbrin 1 (ABD2-GFP). To improve ABD2-GFP fluorescence for enhanced in vivo F-actin imaging, transgenic Arabidopsis plants were generated expressing a construct with GFP fused to both the C- and N-termini of ABD2 under the control of the CaMV 35S promoter (35S::GFP-ABD2-GFP). The 35S::GFP-ABD2-GFP lines had significantly increased fluorescence compared with the original 35S::ABD2-GFP lines. The enhanced fluorescence of the 35S::GFP-ABD2-GFP-expressing lines allowed the acquisition of highly resolved images of F-actin in different plant organs and stages of development because of the reduced confocal microscope excitation settings needed for data collection. This simple modification to the ABD2-GFP construct presents an important tool for studying actin function during plant development.

MICROTUBULE ORGANIZATION 1 regulates structure and function of microtubule arrays during mitosis and cytokinesis in the Arabidopsis root

MICROTUBULE ORGANIZATION 1 (MOR1) is a plant member of the highly conserved MAP215/Dis1 family of microtubule-associated proteins. Prior studies with the temperature-sensitive mor1 mutants of Arabidopsis (Arabidopsis thaliana), which harbor single amino acid substitutions in an N-terminal HEAT repeat, proved that MOR1 regulates cortical microtubule organization and function. Here we demonstrate by use of live cell imaging and immunolabeling that the mor1-1 mutation generates specific defects in the microtubule arrays of dividing vegetative cells. Unlike the universal cortical microtubule disorganization in elongating mor1-1 cells, disruption of mitotic and cytokinetic microtubule arrays was not detected in all dividing cells. Nevertheless, quantitative analysis identified distinct defects in preprophase bands (PPBs), spindles, and phragmoplasts. In nearly one-half of dividing cells at the restrictive temperature of 30 degrees C, PPBs were not detected prior to spindle formation, and those that did form were often disrupted. mor1-1 spindles and phragmoplasts were short and abnormally organized and persisted for longer times than in wild-type cells. The reduced length of these arrays predicts that the component microtubule lengths are also reduced, suggesting that microtubule length is a critical determinant of spindle and phragmoplast structure, orientation, and function. Microtubule organizational defects led to aberrant chromosomal arrangements, misaligned or incomplete cell plates, and multinucleate cells. Antiserum raised against an N-terminal MOR1 sequence labeled the full length of microtubules in interphase arrays, PPBs, spindles, and phragmoplasts. Continued immunolabeling of the disorganized and short microtubules of mor1-1 at the restrictive temperature demonstrated that the mutant mor1-1(L174F) protein loses function without dissociating from microtubules, providing important insight into the mechanism by which MOR1 may regulate microtubule length.

Organization and function of the actin cytoskeleton in developing root cells

The actin cytoskeleton is a highly dynamic structure, which mediates various cellular functions in large part through accessory proteins that tilt the balance between monomeric G-actin and filamentous actin (F-actin) or by facilitating interactions between actin and the plasma membrane, microtubules, and other organelles. Roots have become an attractive model to study actin in plant development because of their simple anatomy and accessibility of some root cell types such as root hairs for microscopic analyses. Roots also exhibit a remarkable developmental plasticity and possess a delicate sensory system that is easily manipulated, so that one can design experiments addressing a range of important biological questions. Many facets of root development can be regulated by the diverse actin network found in the various root developmental regions. Various molecules impinge on this actin scaffold to define how a particular root cell type grows or responds to a specific environmental signal. Although advances in genomics are leading the way toward elucidating actin function in roots, more significant strides will be realized when such tools are combined with improved methodologies for accurately depicting how actin is organized in plant cells.

Hypersensitivity to cytoskeletal antagonists demonstrates microtubule-microfilament cross-talk in the control of root elongation in Arabidopsis thaliana

Elongation of diffusely expanding plant cells is thought to be mainly under the control of cortical microtubules. Drug treatments that disrupt actin microfilaments, however, can reduce elongation and induce radial swelling. To understand how microfilaments assist growth anisotropy, we explored their functional interactions with microtubules by measuring how microtubule disruption affects the sensitivity of cells to microfilament-targeted drugs. We assessed the sensitivity to actin-targeted drugs by measuring the lengths and diameters of expanding roots and by analysing microtubule and microfilament patterns in the temperature-sensitive Arabidopsis thaliana mutant microtubule organization 1 (mor1-1), along with other mutants that constitutively alter microtubule arrays. At the restrictive temperature of mor1-1, root expansion was hypersensitive to the microfilament-disrupting drugs latrunculin B and cytochalasin D, while immunofluorescence microscopy showed that low doses of latrunculin B exacerbated microtubule disruption. Root expansion studies also showed that the botero and spiral1 mutants were hypersensitive to latrunculin B. Hypersensitivity to actin-targeted drugs is a direct consequence of altered microtubule polymer status, demonstrating that cross-talk between microfilaments and microtubules is critical for regulating anisotropic cell expansion.