The plant cytoskeleton: vacuoles and cell walls make the difference

Active transport enables protein condensation in cells

Multiple factors drive biomolecular condensate formation. In plants, condensation of the transcription factors AUXIN RESPONSE FACTOR 7 (ARF7) and ARF19 attenuates response to the plant hormone auxin. Here, we report that actin-mediated movement of cytoplasmic ARF condensates enhances condensation. Coarse-grained molecular simulations of active polymers reveal that applied forces drive the associations of macromolecules to enhance phase separation while giving rise to dense phases that preferentially accumulate motile molecules. Our study highlights how molecular motility can drive phase separation, with implications for motile condensates while offering insights into cellular mechanisms that can regulate condensate dynamics.

Elucidating the Subcellular Localization of GLRaV-3 Proteins Encoded by the Unique Gene Block in N. benthamiana Suggests Implications on Plant Host Suppression

Grapevine leafroll-associated virus 3 (GLRaV-3) is a formidable threat to the stability of the global grape and wine industries. It is the primary etiological agent of grapevine leafroll disease (GLD) and significantly impairs vine health, fruit quality, and yield. GLRaV-3 is a member of the genus Ampelovirus, Closteroviridae family. Viral genes within the 3' proximal unique gene blocks (UGB) remain highly variable and poorly understood. The UGBs of Closteroviridae viruses include diverse open reading frames (ORFs) that have been shown to contribute to viral functions such as the suppression of the host RNA silencing defense response and systemic viral spread. This study investigates the role of GLRaV-3 ORF8, ORF9, and ORF10, which encode the proteins p21, p20A, and p20B, respectively. These genes represent largely unexplored facets of the GLRaV-3 genome. Here, we visualize the subcellular localization of wildtype and mutagenized GLRaV-3 ORFs 8, 9, and 10, transiently expressed in Nicotiana benthamiana. Our results indicate that p21 localizes to the cytosol, p20A associates with microtubules, and p20B is trafficked into the nucleus to carry out the suppression of host RNA silencing. The findings presented herein provide a foundation for future research aimed at the characterization of the functions of these ORFs. In the long run, it would also facilitate the development of innovative strategies to understand GLRaV-3, mitigate its spread, and impacts on grapevines and the global wine industry.

The cell surface is the place to be for brassinosteroid perception and responses

Adjusting the microenvironment around the cell surface is critical to responding to external cues or endogenous signals and to maintaining cell activities. In plant cells, the plasma membrane is covered by the cell wall and scaffolded with cytoskeletal networks, which altogether compose the cell surface. It has long been known that these structures mutually interact, but the mechanisms that integrate the whole system are still obscure. Here we spotlight the brassinosteroid (BR) plant hormone receptor BRASSINOSTEROID INSENSITIVE1 (BRI1) since it represents an outstanding model for understanding cell surface signalling and regulation. We summarize how BRI1 activity and dynamics are controlled by plasma membrane components and their associated factors to fine-tune signalling. The downstream signals, in turn, manipulate cell surface structures by transcriptional and post-translational mechanisms. Moreover, the changes in these architectures impact BR signalling, resulting in a feedback loop formation. This Review discusses how BRI1 and BR signalling function as central hubs to integrate cell surface regulation.

Heat stress during male meiosis impairs cytoskeletal organization, spindle assembly and tapetum degeneration in wheat

The significance of heat stress in agriculture is ever-increasing with the progress of global climate changes. Due to a negative effect on the yield of staple crops, including wheat, the impairment of plant reproductive development triggered by high ambient temperature became a restraint in food production. Although the heat sensitivity of male meiosis and the following gamete development in wheat has long been recognized, a detailed structural characterization combined with a comprehensive gene expression analysis has not been done about this phenomenon. We demonstrate here that heat stress severely alters the cytoskeletal configuration, triggers the failure of meiotic division in wheat. Moreover, it changes the expression of genes related to gamete development in male meiocytes and the tapetum layer in a genotype-dependent manner. 'Ellvis', a heat-tolerant winter wheat cultivar, showed high spikelet fertility rate and only scarce structural aberrations upon exposure to high temperature. In addition, heat shock genes and genes involved in scavenging reactive oxygen species were significantly upregulated in 'Ellvis', and the expression of meiosis-specific and major developmental genes showed high stability in this cultivar. In the heat-sensitive 'Mv 17-09', however, genes participating in cytoskeletal fiber nucleation, the spindle assembly checkpoint genes, and tapetum-specific developmental regulators were downregulated. These alterations may be related to the decreased cytoskeleton content, frequent micronuclei formation, and the erroneous persistence of the tapetum layer observed in the sensitive genotype. Our results suggest that understanding the heat-sensitive regulation of these gene functions would be an essential contribution to the development of new, heat-tolerant cultivars.

Microtubule-associated proteins WDL5 and WDL6 play a critical role in pollen tube growth in Arabidopsis thaliana

Morphological response of cells to environment involves concerted rearrangements of microtubules and actin microfilaments. A mutant of WAVE-DAMPENED2-LIKE5 (WDL5), which encodes an ethylene-regulated microtubule-associated protein belonging to the WVD2/WDL family in Arabidopsis thaliana, shows attenuation in the temporal root growth reduction in response to mechanical stress. We found that a T-DNA knockout of WDL6, the closest homolog of WDL5, oppositely shows an enhancement of the response. To know the functional relationship between WDL5 and WDL6, we attempted to generate the double mutant by crosses but failed in isolation. Close examination of gametophytes in plants that are homozygous for one and heterozygous for the other revealed that these plants produce pollen grains with a reduced rate of germination and tube growth. Reciprocal cross experiments of these plants with the wild type confirmed that the double mutation is not inherited paternally. These results suggest a critical and cooperative function of WDL5 and WDL6 in pollen tube growth.

Function follows form: How cell size is harnessed for developmental decisions

Cell size has profound effects on biological function, influencing a wide range of processes, including biosynthetic capacity, metabolism, and nutrient uptake. As a result, size is typically maintained within a narrow, population-specific range through size control mechanisms, which are an active area of study. While the physiological consequences of cell size are relatively well-characterized, less is known about its developmental consequences, and specifically its effects on developmental transitions. In this review, we compare systems where cell size is linked to developmental transitions, paying particular attention to examples from plants. We conclude by proposing that size can offer a simple readout of complex inputs, enabling flexible decisions during plant development.

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

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

Nitric oxide: A core signaling molecule under elevated GHGs (CO2, CH4, N2O, O3)-mediated abiotic stress in plants

Nitric oxide (NO), an ancient molecule with multiple roles in plants, has gained momentum and continues to govern plant biosciences-related research. NO, known to be involved in diverse physiological and biological processes, is a central molecule mediating cellular redox homeostasis under abiotic and biotic stresses. NO signaling interacts with various signaling networks to govern the adaptive response mechanism towards stress tolerance. Although diverging views question the role of plants in the current greenhouse gases (GHGs) budget, it is widely accepted that plants contribute, in one way or another, to the release of GHGs (carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3)) to the atmosphere, with CH4 and N2O being the most abundant, and occur simultaneously. Studies support that elevated concentrations of GHGs trigger similar signaling pathways to that observed in commonly studied abiotic stresses. In the process, NO plays a forefront role, in which the nitrogen metabolism is tightly related. Regardless of their beneficial roles in plants at a certain level of accumulation, high concentrations of CO2, CH4, and N2O-mediating stress in plants exacerbate the production of reactive oxygen (ROS) and nitrogen (RNS) species. This review assesses and discusses the current knowledge of NO signaling and its interaction with other signaling pathways, here focusing on the reported calcium (Ca2+) and hormonal signaling, under elevated GHGs along with the associated mechanisms underlying GHGs-induced stress in plants.

Sly-miR159 regulates fruit morphology by modulating GA biosynthesis in tomato

Fruit morphology is an important agronomical trait of many crops. Here, we identify Sly-miR159 as an important regulator of fruit morphology in tomato, a model species of fleshy-fruit development. We show that Sly-miR159 functions through its target SlGAMYB2 to control fruit growth. Suppression of Sly-miR159 and overexpression of SlGAMYB2 result in larger fruits with a reduced length/width ratio, while loss of function of SlGAMYB2 leads to the formation of smaller and more elongated fruits. Gibberellin (GA) is a major phytohormone that regulates fruit development in tomato. We show the Sly-miR159-SlGAMYB2 pathway controls fruit morphology by modulating GA biosynthesis. In particular, we demonstrate that Sly-miR159 promotes GA biosynthesis largely through the direct repression of the GA biosynthetic gene SlGA3ox2 by SlGAMYB2. Together, our findings reveal the action of Sly-miR159 on GA biosynthesis as a previously unidentified mechanism that controls fruit morphology in tomato. Modulating this pathway may have potential applications in tomato breeding for manipulating fruit growth and facilitating the process of fruit improvement.

Differential Polarization Imaging of Plant Cells. Mapping the Anisotropy of Cell Walls and Chloroplasts

Modern light microscopy imaging techniques have substantially advanced our knowledge about the ultrastructure of plant cells and their organelles. Laser-scanning microscopy and digital light microscopy imaging techniques, in general—in addition to their high sensitivity, fast data acquisition, and great versatility of 2D–4D image analyses—also opened the technical possibilities to combine microscopy imaging with spectroscopic measurements. In this review, we focus our attention on differential polarization (DP) imaging techniques and on their applications on plant cell walls and chloroplasts, and show how these techniques provided unique and quantitative information on the anisotropic molecular organization of plant cell constituents: (i) We briefly describe how laser-scanning microscopes (LSMs) and the enhanced-resolution Re-scan Confocal Microscope (RCM of Confocal.nl Ltd. Amsterdam, Netherlands) can be equipped with DP attachments—making them capable of measuring different polarization spectroscopy parameters, parallel with the ‘conventional’ intensity imaging. (ii) We show examples of different faces of the strong anisotropic molecular organization of chloroplast thylakoid membranes. (iii) We illustrate the use of DP imaging of cell walls from a variety of wood samples and demonstrate the use of quantitative analysis. (iv) Finally, we outline the perspectives of further technical developments of micro-spectropolarimetry imaging and its use in plant cell studies.

Modelling Nuclear Morphology and Shape Transformation: A Review

As one of the most important cellular compartments, the nucleus contains genetic materials and separates them from the cytoplasm with the nuclear envelope (NE), a thin membrane that is susceptible to deformations caused by intracellular forces. Interestingly, accumulating evidence has also indicated that the morphology change of NE is tightly related to nuclear mechanotransduction and the pathogenesis of diseases such as cancer and Hutchinson–Gilford Progeria Syndrome. Theoretically, with the help of well-designed experiments, significant progress has been made in understanding the physical mechanisms behind nuclear shape transformation in different cellular processes as well as its biological implications. Here, we review different continuum-level (i.e., energy minimization, boundary integral and finite element-based) approaches that have been developed to predict the morphology and shape change of the cell nucleus. Essential gradients, relative advantages and limitations of each model will be discussed in detail, with the hope of sparking a greater research interest in this important topic in the future.

Identification, characterization, and expression analysis of cowpea (Vigna unguiculata [L.] Walp.) miRNAs in response to cowpea severe mosaic virus (CPSMV) challenge

Cowpea miRNAs and Argonaute genes showed differential expression patterns in response to CPSMV challenge Several biotic stresses affect cowpea production and yield. CPSMV stands out for causing severe negative impacts on cowpea. Plants have two main induced immune systems. In the basal system (PTI, PAMP-triggered immunity), plants recognize and respond to conserved molecular patterns associated with pathogens (PAMPs). The second type (ETI, Effector-triggered immunity) is induced after plant recognition of specific factors from pathogens. RNA silencing is another important defense mechanism in plants. Our research group has been using biochemical and proteomic approaches to learn which proteins and pathways are involved and could explain why some cowpea genotypes are resistant whereas others are susceptible to CPSMV. This current study was conducted to determine the role of cowpea miRNA in the interaction between a resistant cowpea genotype (BRS-Marataoã) and CPSMV. Previously identified and deposited plant microRNA sequences were used to find out all possible microRNAs in the cowpea genome. This search detected 617 mature microRNAs, which were distributed in 89 microRNA families. Next, 4 out of these 617 miRNAs and their possible target genes that encode the proteins Kat-p80, DEAD-Box, GST, and SPB9, all involved in the defense response of cowpea to CPSMV, had their expression compared between cowpea leaves uninoculated and inoculated with CPSMV. Additionally, the differential expression of genes that encode the Argonaute (AGO) proteins 1, 2, 4, 6, and 10 is reported. In summary, the studied miRNAs and AGO 2 and AGO4 associated genes showed differential expression patterns in response to CPSMV challenge, which indicate their role in cowpea defense.

Oryza sativa drought-, heat-, and salt-induced RING finger protein 1 (OsDHSRP1) negatively regulates abiotic stress-responsive gene expression

Plants are sessile and unable to avoid environmental stresses, such as drought, high temperature, and high salinity, which often limit the overall plant growth. Plants have evolved many complex mechanisms to survive these abiotic stresses via post-translational modifications. Recent evidence suggests that ubiquitination plays a crucial role in regulating abiotic stress responses in plants by regulating their substrate proteins. Here, we reported the molecular function of a RING finger E3 ligase, Oryza sativa Drought, Heat and Salt-induced RING finger protein 1 (OsDHSRP1), involved in regulating plant abiotic stress tolerance via the Ub/26S proteasome system. The OsDHSRP1 gene transcripts were highly expressed under various abiotic stresses such as NaCl, drought, and heat and the phytohormone abscisic acid (ABA). In addition, in vitro ubiquitination assays demonstrated that the OsDHSRP1 protein possesses a RING-H2 type domain that confers ligase functionality. The results of yeast two-hybrid (Y2H), in vitro pull-down, and bimolecular fluorescence complementation assays support that OsDHSRP1 is able to regulate two substrates, O. sativa glyoxalase (OsGLYI-11.2) and O. sativa abiotic stress-induced cysteine proteinase 1 (OsACP1). We further confirmed that these two substrate proteins were ubiquitinated by OsDHSRP1 E3 ligase and caused protein degradation via the Ub/26S proteasome system. The Arabidopsis plants overexpressing OsDHSRP1 exhibited hypersensitivity to drought, heat, and NaCl stress and a decrease in their germination rates and root lengths compared to the control plants because the degradation of the OsGLYI-11.2 protein maintained lower glyoxalase levels, which increased the methylglyoxal amount in transgenic Arabidopsis plants. However, the OsDHSRP1-overexpressing plants showed no significant difference when treated with ABA. Our finding supports the hypothesis that the OsDHSRP1 E3 ligase acts as a negative regulator, and the degradation of its substrate proteins via ubiquitination plays important roles in regulating various abiotic stress responses via an ABA-independent pathway.

Super-resolution imaging of microtubules in Medicago sativa

Study of microtubules on cellular and subcellular levels is compromised by limited resolution of conventional fluorescence microscopy. However, it is possible to improve Abbe's diffraction-limited resolution by employment of super-resolution microscopy methods. Two of them, described herein, are structured-illumination microscopy (SIM) and Airyscan laser scanning microscopy (AM). Both methods allow high-resolution imaging of cortical microtubules in plant cells, thus contributing to the current knowledge on plant morphogenesis, growth and development. Both SIM and AM provide certain advantages and characteristic features, which are described here. We present immunofluorescence localization methods for microtubules in fixed plant cells achieving high signal efficiency, superb sample stability and sub-diffraction resolution. These protocols were developed for whole-mount immunolabeling of root samples of legume crop species Medicago sativa. They also contain tips for optimal sample preparation of plants germinated from seeds as well as plantlets regenerated from somatic embryos in vitro. We describe in detail all steps of optimized protocols for sample preparation, microtubule immunolabeling and super-resolution imaging.

OsVPE3 Mediates GA-induced Programmed Cell Death in Rice Aleurone Layers via Interacting with Actin Microfilaments

BACKGROUND

Vacuolar processing enzymes (VPEs) have been identified as the enzymes that regulate vacuole-mediated programmed cell death (PCD) in plants. The mechanism that VPE regulates the PCD in rice aleurone layers remains unknown.

RESULTS

The aleurone layers treated with distilled water exerted caspase-1 and VPE activity, both of which were inhibited by the caspase-1 specific inhibitor Ac-YVAD-CMK but not by the caspase-3 specific inhibitor Ac-DEVD-CHO. However, the caspase-1 and caspase-3 inhibitors weakened the activity of caspase-3. Combined with the effects of endogenous gibberellin (GA) on the induction of OsVPEs, we suggest that the OsVPE3 in the aleurone layers, which exhibits caspase-1-like activity, is a key molecule in GA-induced PCD via regulating the protease with caspase-3-like activity. Many studies have confirmed that vacuolar fusion is an important feature of vacuole-mediated PCD in plants. In this experiment, the process of vacuole fusion was accompanied by changes in the structure of actin filaments (AFs), specifically, their depolymerization and polymerization. The process of vacuolar fusion was accelerated or delayed by the promotion or inhibition of the depolymerization of AFs, respectively. Here, the inhibition of OsVPE3 blocked the depolymerization of AFs and delayed the fusion of vacuoles, indicating that OsVPE3 can regulate the fusion of vacuoles in rice aleurone layers via mediating AFs. Furthermore, the depolymerization of AFs contributed to the up-regulation of OsVPE3 gene expression and VPE activity, resulting in accelerated PCD in rice aleurone layers. However, the inhibitor of VPE reversed the effects of AF depolymerization on the activity of VPE, then postponing the process of PCD, implying that AF can involve in GA-induced PCD of rice aleurone layers by mediating OsVPE3.

CONCLUSIONS

Together, activation of OsVPE3 and depolymerization of AFs shortened the process of vacuolation and PCD in rice aleurone layers, and OsVPE3 interacted with AFs during regulation.

Comprehensive Sequence Analysis of IQD Gene Family and their Expression Profiling in Grapevine (Vitis vinifera)

The plant-specific IQ67-domain (IQD) protein family members are downstream targets of calcium sensors, known to regulate plant growth and lateral organ polarity, and basal defense response against environmental cues. No systematic study of IQD gene family has been performed on grapevine. The public availability of grapevine genome enables us to perform identification, phylogeny, chromosomal orientation, and gene structure analysis of the IQD genes in grapevine. We identified 49 VvIQD genes (VvIQD1–VvIQD49) and further classified them into eight subgroups based on phylogenetic relationships. The 49 VvIQD genes were assigned to 19 different chromosomal positions. The collinear relationship between grapevine and Arabidopsis IQDs (VvIQD and AtIQD), and within grapevine VvIQDs, was highly conserved. In addition, most of duplicated gene pairs showed Ka/Ks ratio less than 1.00, indicating purifying selection within these gene pairs, implying functional discrepancy after duplication. Transcription profiling of VvIQD genes shed light on their specific role in grapevine tissue and organ development. The qRT-PCR validation of the 49 VvIQD genes in grape berry tissue from cultivars with distinct berry shape during developmental phases suggested candidate genes involved in the shape of grape berries. The subcellular prediction of VvIQD22, VvIQD23, VvIQD38, and VvIQD49 genes validated their localization in the nucleus and plasma membrane. The VvIQD49 protein interaction with VvCaM2 was also verified by bimolecular fluorescence complementation (BiFC) analysis in the plasma membrane. Our findings will be valuable for the functional genomic studies for desirable shape development of grape berries.

Sensory role of actin in auxin-dependent responses of tobacco BY-2

Polar auxin transport depends on the polar localization of auxin-efflux carriers. The cycling of these carriers between cell interior and plasma membrane depends on actin. The dynamic of actin not only affects auxin transport, but also changes the auxin-responsiveness. To study the potential link between auxin responsiveness and actin dynamics, we investigated developmental responses of the non-transformed BY-2 (Nicotiana tabacum L. cv Bright Yellow 2) cell line and the transgenic BY-2 strain GF11 (stably transformed BY-2 cells with a GFP-fimbrin actin-binding domain 2 construct). The developmental process was divided into three distinct stages: cell cycling, cell elongation and file disintegration. Several phenotypes were measured to monitor the cellular responses to different concentrations of exogenous natural auxin (Indole-3-acetic acid, IAA). We found that auxin stimulated and prolonged the mitotic activity, and delayed the exit from the proliferation phase. However, both responses were suppressed in the GF11 line. At the stationary phase of the cultivation cycle, auxin strongly accelerated the cell file disintegration. Interestingly, it was not suppressed but progressed to a more complete disintegration in the GF11 line. During the cultivation cycle, we also followed the organization of actin in the GF11 line and did not detect any significant difference in actin organization from untreated control or exogenous IAA treatment. Therefore, our findings indicate that the specific differences observed in the GF11 line must be linked with a function of actin that is not structural. It means that there is a sensory role of actin for auxin signaling.

Soybean actin-depolymerizing factor 2 interacts with Soybean mosaic virus-encoded P3 protein

Soybean mosaic virus (SMV), a member of the Potyvirus genus, is one of the most prevalent and devastating viral pathogens in soybean-growing regions worldwide. It is generally accepted that symptom development of a viral plant disease results from molecular interactions between the virus and its host plant. P3 protein is the most variable polyprotein in potyviruses, which potentially plays an important role in the process of the evolution of virus type specialization. However, P3 not only plays a major role in virus replication and movement, but it is also responsible for symptom development in SMV-infected plants. This study provides evidence that actin-depolymerizing factor 2 (designated as ADF2) of soybean interacts with SMV P3 via a two-hybrid yeast system by screening a soybean cDNA library. Bimolecular fluorescence complementation assay further confirmed the interaction, which occurred in both the cytomembrane and cytoskeleton of Nicotiana benthamiana cells. The results support the hypothesis that SMV P3 might have a role in virus movement within cells.

RISAP is a TGN-associated RAC5 effector regulating membrane traffic during polar cell growth in tobacco

RAC/ROP GTPases coordinate actin dynamics and membrane traffic during polar plant cell expansion. In tobacco (Nicotiana tabacum), pollen tube tip growth is controlled by the RAC/ROP GTPase RAC5, which specifically accumulates at the apical plasma membrane. Here, we describe the functional characterization of RISAP, a RAC5 effector identified by yeast (Saccharomyces cerevisiae) two-hybrid screening. RISAP belongs to a family of putative myosin receptors containing a domain of unknown function 593 (DUF593) and binds via its DUF593 to the globular tail domain of a tobacco pollen tube myosin XI. It also interacts with F-actin and is associated with a subapical trans-Golgi network (TGN) compartment, whose cytoplasmic position at the pollen tube tip is maintained by the actin cytoskeleton. In this TGN compartment, apical secretion and endocytic membrane recycling pathways required for tip growth appear to converge. RISAP overexpression interferes with apical membrane traffic and blocks tip growth. RAC5 constitutively binds to the N terminus of RISAP and interacts in an activation-dependent manner with the C-terminal half of this protein. In pollen tubes, interaction between RAC5 and RISAP is detectable at the subapical TGN compartment. We present a model of RISAP regulation and function that integrates all these findings.

Fluorescent labelling of the actin cytoskeleton in plants using a cameloid antibody

BACKGROUND

Certain members of the Camelidae family produce a special type of antibody with only one heavy chain. The antigen binding domains are the smallest functional fragments of these heavy-chain only antibodies and as a consequence have been termed nanobodies. Discovery of these nanobodies has allowed the development of a number of therapeutic proteins and tools. In this study a class of nanobodies fused to fluorescent proteins (chromobodies), and therefore allowing antigen-binding and visualisation by fluorescence, have been used. Such chromobodies can be expressed in living cells and used as genetically encoded immunocytochemical markers.

RESULTS

Here a modified version of the commercially available Actin-Chromobody® as a novel tool for visualising actin dynamics in tobacco leaf cells was tested. The actin-chromobody binds to actin in a specific manner. Treatment with latrunculin B, a drug which disrupts the actin cytoskeleton through inhibition of polymerisation results in loss of fluorescence after less than 30 min but this can be rapidly restored by washing out latrunculin B and thereby allowing the actin filaments to repolymerise. To test the effect of the actin-chromobody on actin dynamics and compare it to one of the conventional labelling probes, Lifeact, the effect of both probes on Golgi movement was studied as the motility of Golgi bodies is largely dependent on the actin cytoskeleton. With the actin-chromobody expressed in cells, Golgi body movement was slowed down but the manner of movement rather than speed was affected less than with Lifeact.

CONCLUSIONS

The actin-chromobody technique presented in this study provides a novel option for in vivo labelling of the actin cytoskeleton in comparison to conventionally used probes that are based on actin binding proteins. The actin-chromobody is particularly beneficial to study actin dynamics in plant cells as it does label actin without impairing dynamic movement and polymerisation of the actin filaments.

Phosphorylation-dependent trafficking of plasma membrane proteins in animal and plant cells

In both unicellular and multicellular organisms, transmembrane (TM) proteins are sorted to and retained at specific membrane domains by endomembrane trafficking mechanisms that recognize sorting signals in the these proteins. The trafficking and distribution of plasma membrane (PM)-localized TM proteins (PM proteins), especially of those PM proteins that show an asymmetric distribution over the PM, has received much attention, as their proper PM localization is crucial for elementary signaling and transport processes, and defects in their localization often lead to severe disease symptoms or developmental defects. The subcellular localization of PM proteins is dynamically regulated by post-translational modifications, such as phosphorylation and ubiquitination. These modificaitons mostly occur on sorting signals that are located in the larger cytosolic domains of the cargo proteins. Here we review the effects of phosphorylation of PM proteins on their trafficking, and present the key examples from the animal field that have been subject to studies for already several decades, such as that of aquaporin 2 and the epidermal growth factor receptor. Our knowledge on cargo trafficking in plants is largely based on studies of the family of PIN FORMED (PIN) carriers that mediate the efflux of the plant hormone auxin. We will review what is known on the subcellular distribution and trafficking of PIN proteins, with a focus on how this is modulated by phosphorylation, and identify and discuss analogies and differences in trafficking with the well-studied animal examples.

A cotton LIM domain-containing protein (GhWLIM5) is involved in bundling actin filaments

LIM-domain proteins play important roles in cellular processes in eukaryotes. In this study, a LIM protein gene, GhWLIM5, was identified in cotton. Quantitative RT-PCR analysis showed that GhWLIM5 was expressed widely in different cotton tissues and had a peak in expression during fiber elongation. GFP fluorescence assay revealed that cotton cells expressing GhWLIM5:eGFP fusion gene displayed a network distribution of eGFP fluorescence, suggesting that GhWLIM5 protein is mainly localized to the cell cytoskeleton. When GhWLIM5:eGFP transformed cells were stained with rhodamine-phalloidin there was consistent overlap in eGFP and rhodamine-palloidin signals, demonstrating that GhWLIM5 protein is colocalized with the F-actin cytoskeleton. In addition, high-speed cosedimentation assay verified that GhWLIM5 directly bound actin filaments, while low cosedimentation assay and microscopic observation indicated that GhWLIM5 bundled F-actin in vitro. Increasing amounts of GhWLIM5 protein were able to protect F-actin from depolymerization in vitro in the presence of Lat B (an F-actin depolymerizer). Our results contribute to a better understanding of the biochemical role of GhWLIM5 in modulating the dynamic F-actin network in cotton.

Organization of actin cytoskeleton during meiosis I in a wheat thermo-sensitive genic male sterile line

BS366 is a thermo-sensitive male sterile line of wheat (Triticum aestivum L.) for two-line hybrid breeding, which exhibits aberrant meiotic cytokinesis under low temperature. Through transcriptome analysis, a possible regulatory role for plant actin cytoskeleton was suggested. However, the organization of actin cytoskeleton in meiosis has been poorly understood so far. Here, fixed microsporocytes during meiosis were labeled with tetramethylrhodamine isothiocyanate-phalloidin and 4',6-diamidino-2-phenylindole. Quantities of fluorescent micrographs were captured using a confocal microscope, including the transient state from metaphase to telophase. We observed that actin filaments were abundant in typical kariokinetic spindle, central spindle (parallel microtubules or actin fibers between two separated chromosomes in anaphase), and phragmoplast. Interestingly, we identified the Chinese lantern-shaped actin phragmoplast in wheat meiosis for the first time. Under low temperature, phragmoplast actin filaments were chaotic and normal cell plate failed to form. These data provide new insights into the organization of actin filaments during male meiosis of plant and support a role of actin cytoskeleton in bringing about thermo-sensitive male sterility in wheat.

Identification of differentially expressed genes in a spontaneous altered leaf shape mutant of the navel orange [Citrus sinensis (L.) Osbeck]

Most of the economically important citrus cultivars have originated from bud mutations. Leaf shape and structure are important factors that impact plant photosynthesis. We found a spontaneous bud mutant exhibiting a narrow leaf phenotype in navel orange [Citrus sinensis (L.) Osbeck]. To identify and characterize the genes involved in the formation of this trait, we performed suppression subtractive hybridization (SSH) and macroarray analysis. A total of 221 non-redundant differentially expressed transcripts were obtained. These transcripts included cell wall- and microtubule-related genes and two transcription factor-encoding genes, yabby and wox, which are crucial for leaf morphogenesis. Many highly redundant transcripts were associated with stress responses, while others, encoding caffeic acid 3-O-methyltransferase (EC 2.1.1.68) and a myb-like transcription factor, might be involved in the lignin pathway, which produces a component of secondary walls. Furthermore, real-time quantitative RT-PCR was performed for selected genes to validate the quality of the expressed sequence tags (ESTs) from the SSH libraries. This study represents an attempt to investigate the molecular mechanism associated with a leaf shape mutation, and its results provide new clues for understanding leaf shape mutations in citrus.

Control of the mitotic cleavage plane by local epithelial topology

For nearly 150 years, it has been recognized that cell shape strongly influences the orientation of the mitotic cleavage plane (e.g., Hofmeister, 1863). However, we still understand little about the complex interplay between cell shape and cleavage-plane orientation in epithelia, where polygonal cell geometries emerge from multiple factors, including cell packing, cell growth, and cell division itself. Here, using mechanical simulations, we show that the polygonal shapes of individual cells can systematically bias the long-axis orientations of their adjacent mitotic neighbors. Strikingly, analyses of both animal epithelia and plant epidermis confirm a robust and nearly identical correlation between local cell topology and cleavage-plane orientation in vivo. Using simple mathematics, we show that this effect derives from fundamental packing constraints. Our results suggest that local epithelial topology is a key determinant of cleavage-plane orientation, and that cleavage-plane bias may be a widespread property of polygonal cell sheets in plants and animals.

Isolation and identification of cytoskeleton-associated prolamine mRNA binding proteins from developing rice seeds

The messenger RNA of the rice seed storage protein prolamine is targeted to the endoplasmic reticulum (ER) membranes surrounding prolamine protein bodies via a mechanism, which is dependent upon both RNA sorting signals and the actin cytoskeleton. In this study we have used an RNA bait corresponding to the previously characterized 5'CDS prolamine cis-localization sequence for the capture of RNA binding proteins (RBPs) from cytoskeleton-enriched fractions of developing rice seed. In comparison to a control RNA, the cis-localization RNA bait sequence led to the capture of a much larger number of proteins, 18 of which have been identified by tandem mass spectrometry. Western blots demonstrate that several of the candidate proteins analyzed to date show good to excellent specificity for binding to cis-localization sequences over the control RNA bait. Temporal expression studies showed that steady state protein levels for one RNA binding protein, RBP-A, paralleled prolamine gene expression. Immunoprecipitation studies showed that RBP-A is bound to prolamine and glutelin RNAs in vivo, supporting a direct role in storage protein gene expression. Using confocal immunofluorescence microscopy, RBP-A was found to be distributed to multiple compartments in the cell. In addition to the nucleus, RBP-A co-localizes with microtubules and is associated with cortical ER membranes. Collectively, these results indicate that employing a combination of in vitro binding and in vivo binding and localization studies is a valid strategy for the identification of putative prolamine mRNA binding proteins, such as RBP-A, which play a role in controlling expression of storage protein mRNAs in the cytoplasm.

Atomic-scale dynamics inside living cells explored by neutron scattering

Single-particle neutron spectroscopy has contributed important experimental data on molecular dynamics in biological systems. The technique provides information on atomic and molecular motions in macromolecules on the picosecond to the nanosecond time scale, which are essential to biological function. Here, we report on recent neutron measurements performed directly in living cells by using isotope labelling to explore the dynamics of specific cellular components. The paper proposes an integrated view of results on atomic-scale cell water dynamics, internal and global macromolecular motions and solvent isotope effect on macromolecular dynamics. The work established the specific usefulness of the neutron scattering technique to get insight into biologically relevant dynamical features, in particular through comparative measurements. The method developed can now be applied to look for dynamical signatures related to cell characteristics in many different cell types and organelles.

Generation and analysis of expressed sequence tags from six developing xylem libraries in Pinus radiata D. Don

BACKGROUND

Wood is a major renewable natural resource for the timber, fibre and bioenergy industry. Pinus radiata D. Don is the most important commercial plantation tree species in Australia and several other countries; however, genomic resources for this species are very limited in public databases. Our primary objective was to sequence a large number of expressed sequence tags (ESTs) from genes involved in wood formation in radiata pine.

RESULTS

Six developing xylem cDNA libraries were constructed from earlywood and latewood tissues sampled at juvenile (7 yrs), transition (11 yrs) and mature (30 yrs) ages, respectively. These xylem tissues represent six typical development stages in a rotation period of radiata pine. A total of 6,389 high quality ESTs were collected from 5,952 cDNA clones. Assembly of 5,952 ESTs from 5' end sequences generated 3,304 unigenes including 952 contigs and 2,352 singletons. About 97.0% of the 5,952 ESTs and 96.1% of the unigenes have matches in the UniProt and TIGR databases. Of the 3,174 unigenes with matches, 42.9% were not assigned GO (Gene Ontology) terms and their functions are unknown or unclassified. More than half (52.1%) of the 5,952 ESTs have matches in the Pfam database and represent 772 known protein families. About 18.0% of the 5,952 ESTs matched cell wall related genes in the MAIZEWALL database, representing all 18 categories, 91 of all 174 families and possibly 557 genes. Fifteen cell wall-related genes are ranked in the 30 most abundant genes, including CesA, tubulin, AGP, SAMS, actin, laccase, CCoAMT, MetE, phytocyanin, pectate lyase, cellulase, SuSy, expansin, chitinase and UDP-glucose dehydrogenase. Based on the PlantTFDB database 41 of the 64 transcription factor families in the poplar genome were identified as being involved in radiata pine wood formation. Comparative analysis of GO term abundance revealed a distinct transcriptome in juvenile earlywood formation compared to other stages of wood development.

CONCLUSION

The first large scale genomic resource in radiata pine was generated from six developing xylem cDNA libraries. Cell wall-related genes and transcription factors were identified. Juvenile earlywood has a distinct transcriptome, which is likely to contribute to the undesirable properties of juvenile wood in radiata pine. The publicly available resource of radiata pine will also be valuable for gene function studies and comparative genomics in forest trees.

Structural changes in the vacuole and cytoskeleton are key to development of the two cytoplasmic domains supporting single-cell C(4) photosynthesis in Bienertia sinuspersici

Bienertia sinuspersici Akhani has an unusual mechanism of C4 photosynthesis which occurs within individual chlorenchyma cells. To perform C4, the mature cells have two cytoplasmic compartments consisting of a central (CCC) and a peripheral (PCC) domain containing dimorphic chloroplasts which are interconnected by cytoplasmic channels. Based on leaf development studies, young chlorenchyma cells have not developed the two cytoplasmic compartments and dimorphic chloroplasts. Fluorescent dyes which are targeted to membranes or to specific organelles were used to follow changes in cell structure and organelle distribution during formation of C4-type chlorenchyma. Chlorenchyma cell development was divided into four stages: 1-the nucleus and chloroplasts occupy much of the cytoplasmic space and only small vacuoles are formed; 2-development of larger vacuoles, formation of a pre-CCC with some scattered chloroplasts; 3-the vacuole expands, cells have directional growth; 4-mature stage, cells have become elongated, with a distinctive CCC and PCC joined by interconnecting cytoplasmic channels. By staining vacuoles with a fluorescent dye and constructing 3D images of chloroplasts, and by microinjecting a fluorescence dye into the vacuole of living cells, it was demonstrated that the mature cell has only one vacuole, which is traversed by cytoplasmic channels connecting the CCC with the PCC. Immunofluorescent studies on isolated chlorenchyma cells treated with cytoskeleton disrupting drugs suspended in different levels of osmoticum showed that both microtubules and actin filaments are important in maintaining the cytoplasmic domains. With prolonged exposure of plants to dim light, the cytoskeleton undergoes changes and there is a dramatic shift of the CCC from the center toward the distal end of the cell.

Overexpression of PwTUA1, a pollen-specific tubulin gene, increases pollen tube elongation by altering the distribution of alpha-tubulin and promoting vesicle transport

Tubulin genes are intimately associated with cell division and cell elongation, which are central to plant secondary cell wall development. However, their roles in pollen tube polar growth remain elusive. Here, a TUA1 gene from Picea wilsonii, which is specifically expressed in pollen, was isolated. Semi-quantitative RT-PCR analysis showed that the amount of PwTUA1 transcript varied at each stage of growth of the pollen tube and was induced by calcium ions and boron. Transient expression analysis in P. wilsonii pollen indicated that PwTUA1 improved pollen germination and pollen tube growth. The pollen of transgenic Arabidopsis overexpressing PwTUA1 also showed a higher percentage of germination and faster growth than wild-type plants not only in optimal germination medium, but also in medium supplemented with elevated levels of exogenous calcium ions or boron. Immunofluorescence and electron microscopy showed alpha-tubulin to be enriched and more vesicles accumulated in the apex region in germinating transgenic Arabidopsis pollen compared with wild-type plants. These results demonstrate that PwTUA1 up-regulated by calcium ions and boron contributes to pollen tube elongation by altering the distribution of alpha-tubulin and regulating the deposition of pollen cell wall components during the process of tube growth. The possible role of PwTUA1 in microtubule dynamics and organization was discussed.

Synthetic lipid (DOPG) vesicles accumulate in the cell plate region but do not fuse

The cell plate is the new cell wall, with bordering plasma membrane, that is formed between two daughter cells in plants, and it is formed by fusion of vesicles (approximately 60 nm). To start to determine physical properties of cell plate forming vesicles for their transport through the phragmoplast, and fusion with each other, we microinjected fluorescent synthetic lipid vesicles that were made of 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DOPG) into Tradescantia virginiana stamen hair cells. During interphase, the 60-nm wide DOPG vesicles moved inside the cytoplasm comparably to organelles. During cytokinesis, they were transported through the phragmoplast and accumulated in the cell plate region together with the endogenous vesicles, even inside the central cell plate region. Because at this stage microtubules are virtually absent from that region, while actin filaments are present, actin filaments may have a role in the transport of vesicles toward the cell plate. Unlike the endogenous vesicles, the synthetic DOPG vesicles did not fuse with the developing cell plate. Instead, they redistributed into the cytoplasm of the daughter cells upon completion of cytokinesis. Because the redistribution of the vesicles occurs when actin filaments disappear from the phragmoplast, actin filaments may be involved in keeping the vesicles inside the developing cell plate region.

Tobacco mosaic virus movement protein functions as a structural microtubule-associated protein

The cell-to-cell spread of Tobacco mosaic virus infection depends on virus-encoded movement protein (MP), which is believed to form a ribonucleoprotein complex with viral RNA (vRNA) and to participate in the intercellular spread of infectious particles through plasmodesmata. Previous studies in our laboratory have provided evidence that the vRNA movement process is correlated with the ability of the MP to interact with microtubules, although the exact role of this interaction during infection is not known. Here, we have used a variety of in vivo and in vitro assays to determine that the MP functions as a genuine microtubule-associated protein that binds microtubules directly and modulates microtubule stability. We demonstrate that, unlike MP in whole-cell extract, microtubule-associated MP is not ubiquitinated, which strongly argues against the hypothesis that microtubules target the MP for degradation. In addition, we found that MP interferes with kinesin motor activity in vitro, suggesting that microtubule-associated MP may interfere with kinesin-driven transport processes during infection.

Cell shape development in plants

The shape of a plant cell has long been the cornerstone of diverse areas of plant research but it is only recently that molecular-genetic and cell-biological tools have been effectively combined for dissecting plant cell morphogenesis. Increased understanding of the polar growth characteristics of model cell types, the availability of many morphological mutants and significant advances in fluorescent-protein-aided live-cell visualization have provided the major impetus for these analyses. The cytoskeleton and its regulators have emerged as essential components of the scaffold involved in fabricating plant cell shape. In this article, I collate information from recent discoveries to derive a simple cytoskeleton-based operational framework for plant cell morphogenesis.

Signalling to actin assembly via the WASP (Wiskott-Aldrich syndrome protein)-family proteins and the Arp2/3 complex

The assembly of a branched network of actin filaments provides the mechanical propulsion that drives a range of dynamic cellular processes, including cell motility. The Arp2/3 complex is a crucial component of such filament networks. Arp2/3 nucleates new actin filaments while bound to existing filaments, thus creating a branched network. In recent years, a number of proteins that activate the filament nucleation activity of Arp2/3 have been identified, most notably the WASP (Wiskott-Aldrich syndrome protein) family. WASP-family proteins activate the Arp2/3 complex, and consequently stimulate actin assembly, in response to extracellular signals. Structural studies have provided a significant refinement in our understanding of the molecular detail of how the Arp2/3 complex nucleates actin filaments. There has also been much progress towards an understanding of the complicated signalling processes that regulate WASP-family proteins. In addition, the use of gene disruption in a number of organisms has led to new insights into the specific functions of individual WASP-family members. The present review will discuss the Arp2/3 complex and its regulators, in particular the WASP-family proteins. Emphasis will be placed on recent developments in the field that have furthered our understanding of actin dynamics and cell motility.

Microtubules of guard cells are light sensitive

Guard cells of stomata are characterized by ordered bundles of microtubules radiating from the ventral side toward the dorsal side of the cylindrical cell. It was suggested that microtubules play a role in directing the radial arrangement of the cellulose micro-fibrils of guard cells. However, the role of microtubules in daily cycles of opening and closing of stomata is not clear. The organization of microtubules in guard cells of Commelina communis leaves was studied by analysis of three-dimensional immunofluorescent images. It was found that while guard cell microtubules in the epidermis of leaves incubated in the light were organized in parallel, straight and dense bundles, in the dark they were less straight and oriented randomly near the stomatal pore. The effect of blue and red light on the organization of guard cell microtubules resembled the effects of white light and dark respectively. When stomata were induced to open in the dark with fusicoccin, microtubules remained in the dark configuration. Furthermore, when incubated in the light, guard cell microtubules were more resistant to oryzalin. Similarly, microtubules of Arabidopsis guard cells, expressing green fluorescent protein-tubulin alpha 6, were disorganized in the dark, but were organized in parallel arrays in the presence of white light. The dynamics of microtubule rearrangement upon transfer of intact leaves from dark to light was followed in single stomata, showing that an arrangement of microtubules typical for light conditions was obtained after 1 h in the light. Our data suggest that microtubule organization in guard cells is responsive to light signals.

Subunits of the chaperonin CCT are associated with Tetrahymena microtubule structures and are involved in cilia biogenesis

The cytosolic chaperonin CCT is a heterooligomeric complex of about 900 kDa that mediates the folding of cytoskeletal proteins. We observed by indirect immunofluorescence that the Tetrahymena TpCCTalpha, TpCCTdelta, TpCCTepsilon, and TpCCTeta-subunits colocalize with tubulin in cilia, basal bodies, oral apparatus, and contractile vacuole pores. TpCCT-subunits localization was affected during reciliation. These findings combined with atomic force microscopy measurements in reciliating cells indicate that these proteins play a role during cilia biogenesis related to microtubule nucleation, tubulin transport, and/or axoneme assembly. The TpCCT-subunits were also found to be associated with cortex and cytoplasmic microtubules suggesting that they can act as microtubule-associated proteins. The TpCCTdelta being the only subunit found associated with the macronuclear envelope indicates that it has functions outside of the 900 kDa complex. Tetrahymena cytoplasm contains granular/globular-structures of TpCCT-subunits in close association with microtubule arrays. Studies of reciliation and with cycloheximide suggest that these structures may be sites of translation and folding. Combined biochemical techniques revealed that reciliation affects the oligomeric state of TpCCT-subunits being tubulin preferentially associated with smaller CCT oligomeric species in early stages of reciliation. Collectively, these findings indicate that the oligomeric state of CCT-subunits reflects the translation capacity of the cell and microtubules integrity.

The architecture of polarized cell growth: the unique status of elongating plant cells

Polarity is an inherent feature of almost all prokaryotic and eukaryotic cells. In most eukaryotic cells, growth polarity is due to the assembly of actin-based growing domains at particular locations on the cell periphery. A contrasting scenario is that growth polarity results from the establishment of non-growing domains, which are actively maintained at opposite end-poles of the cell. This latter mode of growth is common in rod-shaped bacteria and, surprisingly, also in the majority of plant cells, which elongate along the apical-basal axes of plant organs. The available data indicate that the non-growing end-pole domains of plant cells are sites of intense endocytosis and recycling. These actin-enriched end-poles serve also as signaling platforms, allowing bidirectional exchange of diverse signals along the supracellular domains of longitudinal cell files. It is proposed that these actively remodeled end-poles of elongating plant cells remotely resemble neuronal synapses.

Microtubule cytoskeleton: a track record

The plant microtubule cytoskeleton forms unique arrays during cell division and morphogenesis. Recent studies have addressed the biogenesis, turnover, spatio-temporal organisation and cellular function of microtubules. The results suggest that both conserved eukaryotic mechanisms and plant-specific modifications determine microtubule dynamics and function.

Force and compliance: rethinking morphogenesis in walled cells

In the turgid cells of plants, protists, fungi, and bacteria, walls resist swelling; they also confer shape on the cell. These two functions are not unrelated: cell physiologists have generally agreed that morphogenesis turns on the deformation of existing wall and the deposition of new wall, while turgor pressure produces the work of expansion. In 1990, I summed up consensus in a phrase: "localized compliance with the global force of turgor pressure." My purpose here is to survey the impact of recent discoveries on the traditional conceptual framework. Topics include the recognition of a cytoskeleton in bacteria; the tide of information and insight about budding in yeast; the role of the Spitzenkörper in hyphal extension; calcium ions and actin dynamics in shaping a tip; and the interplay of protons, expansins and cellulose fibrils in cells of higher plants.

Microtubules and microfilaments in cell morphogenesis in higher plants

Microtubules and microfilaments play important roles in cell morphogenesis. The picture emerging from drug studies and molecular-genetic analyses of mutant higher plants defective in cell morphogenesis shows that the roles played by them remain the same in both tip-growing and diffuse-growing cells. Microtubules are important for establishing and maintaining growth polarity whereas actin microfilaments deliver the materials required for growth to specified sites. The recent cloning of several cell morphogenesis genes has revealed that conserved mechanisms as well as novel signal transduction pathways spatially organize the plant cytoskeleton.