Root hair formation: F-actin-dependent tip growth is initiated by local assembly of profilin-supported F-actin meshworks accumulated within expansin-enriched bulges

Rapid identification of a natural knockout allele of ARMADILLO REPEAT-CONTAINING KINESIN1 that causes root hair branching by mapping-by-sequencing

In Arabidopsis (Arabidopsis thaliana), branched root hairs are an indicator of defects in root hair tip growth. Among 62 accessions, one accession (Heiligkreuztal2 [HKT2.4]) displayed branched root hairs, suggesting that this accession carries a mutation in a gene of importance for tip growth. We determined 200- to 300-kb mapping intervals using a mapping-by-sequencing approach of F2 pools from crossings of HKT2.4 with three different accessions. The intersection of these mapping intervals was 80 kb in size featuring not more than 36 HKT2.4-specific single nucleotide polymorphisms, only two of which changed the coding potential of genes. Among them, we identified the causative single nucleotide polymorphism changing a splicing site in ARMADILLO REPEAT-CONTAINING KINESIN1. The applied strategies have the potential to complement statistical methods in high-throughput phenotyping studies using different natural accessions to identify causative genes for distinct phenotypes represented by only one or a few accessions.

Gel-based comparative phosphoproteomic analysis on rice embryo during germination

Seed germination is a well regulated process, which incorporates many events including signal transduction, mobilization of reserves, reactive oxygen species scavenging and cell division. Although many transcriptomic and proteomic studies have been conducted on this process, regulation of protein modification has not been studied. To better understand the mechanism, a gel-based comparative phosphoproteomic study was performed on rice embryo during the germination process. In total, 168 protein spots exhibited significantly changed Pro-Q staining intensity during germination. Using matrix-assisted laser deionization-time of flight/time of flight mass spectrometry (MALDI-TOF/TOF MS) analysis, 193 proteins were identified. By combining Pro-Q and Coomassie brilliant blue stain intensity analyses, 109 proteins were verified to be phosphorylation regulation proteins. Functional analyses indicated that phosphorylation of proteins involved in stress response and storage was gradually enhanced. Phosphorylation of signal transduction proteins was mainly activated during the early stage of germination, while stress response and storage protein phosphorylation were enhanced at the late stage. Enzyme assays proved that the phosphorylation of fructokinase, pyruvate kinase, malate dehydrogenase, GDP-mannose 3,5-epimerase1, ascorbate peroxidase and glutathione S-transferase could consistently enhance their activity. This study showed the dynamic changes of protein phosphorylation status in rice embryo during germination and provided new insight into understanding the mechanism underlying this process.

Root hairs

Roots hairs are cylindrical extensions of root epidermal cells that are important for acquisition of nutrients, microbe interactions, and plant anchorage. The molecular mechanisms involved in the specification, differentiation, and physiology of root hairs in Arabidopsis are reviewed here. Root hair specification in Arabidopsis is determined by position-dependent signaling and molecular feedback loops causing differential accumulation of a WD-bHLH-Myb transcriptional complex. The initiation of root hairs is dependent on the RHD6 bHLH gene family and auxin to define the site of outgrowth. Root hair elongation relies on polarized cell expansion at the growing tip, which involves multiple integrated processes including cell secretion, endomembrane trafficking, cytoskeletal organization, and cell wall modifications. The study of root hair biology in Arabidopsis has provided a model cell type for insights into many aspects of plant development and cell biology.

Actin-Dependent and -Independent Functions of Cortical Microtubules in the Differentiation of Arabidopsis Leaf Trichomes

Arabidopsis thaliana tortifolía2 carries a point mutation in α-tubulin 4 and shows aberrant cortical microtubule dynamics. The microtubule defect of tortifolia2 leads to overbranching and right-handed helical growth in the single-celled leaf trichomes. Here, we use tortifolia2 to further our understanding of microtubules in plant cell differentiation. Trichomes at the branching stage show an apical ring of cortical microtubules, and our analyses support that this ring is involved in marking the prospective branch site. tortifolia2 showed ectopic microtubule bundles at this stage, consistent with a function for microtubules in selecting new branch sites. Overbranching of tortifolia2 required the C-terminal binding protein/brefeldin A-ADP ribosylated substrate protein ANGUSTIFOLIA1, and our results indicate that the angustifolia1 mutant is hypersensitive to alterations in microtubule dynamics. To analyze whether actin and microtubules cooperate in the trichome cell expansion process, we generated double mutants of tortifolia2 with distorted1, a mutant that is defective in the actin-related ARP2/3 complex. The double mutant trichomes showed a complete loss of growth anisotropy, suggesting a genetic interaction of actin and microtubules. Green fluorescent protein labeling of F-actin or microtubules in tortifolia2 distorted1 double mutants indicated that F-actin enhances microtubule dynamics and enables reorientation. Together, our results suggest actin-dependent and -independent functions of cortical microtubules in trichome differentiation.

The circular F-actin bundles provide a track for turnaround and bidirectional movement of mitochondria in Arabidopsis root hair

Background

The movement of organelles in root hairs primarily occurs along the actin cytoskeleton. Circulation and “reverse fountain” cytoplasmic streaming constitute the typical forms by which most organelles (such as mitochondria and the Golgi apparatus) in plant root hair cells engage in bidirectional movement. However, there remains a lack of in-depth research regarding the relationship between the distribution of the actin cytoskeleton and turnaround organelle movement in plant root hair cells.

Results

In this paper, Arabidopsis seedlings that had been stably transformed with a GFP-ABD2-GFP (green fluorescent protein-actin-binding domain 2-green fluorescent protein) construct were utilized to study the distribution of bundles of filamentous (F)-actin and the directed motion of mitochondria along these bundles in root hairs. Observations with a confocal laser scanning microscope revealed that there were widespread circular F-actin bundles in the epidermal cells and root hairs of Arabidopsis roots. In root hairs, these circular bundles primarily start at the sub-apical region, which is the location where the turnaround movement of organelles occurs. MitoTracker probes were used to label mitochondria, and the dynamic observation of root hair cells with a confocal laser scanning microscope indicated that turnaround mitochondrial movement occurred along circular F-actin bundles.

Conclusions

Relevant experimental results demonstrated that the circular F-actin bundles provide a track for the turnaround and bidirectional movement of mitochondria.

Visualization of highly dynamic F-actin plus ends in growing phaseolus vulgaris root hair cells and their responses to Rhizobium etli nod factors

Legume plants secrete signaling molecules called flavonoids into the rhizosphere. These molecules activate the transcription of rhizobial nod genes, which encode proteins involved in the synthesis of signaling compounds named Nod factors (NFs). NFs, in turn, trigger changes in plant gene expression, cortical cell dedifferentiation and mitosis, depolarization of the root hair cell membrane potential and rearrangement of the actin cytoskeleton. Actin polymerization plays an important role in apical growth in hyphae and pollen tubes. Using sublethal concentrations of fluorescently labeled cytochalasin D (Cyt-Fl), we visualized the distribution of filamentous actin (F-actin) plus ends in living Phaseolus vulgaris and Arabidopsis root hairs during apical growth. We demonstrated that Cyt-Fl specifically labeled the newly available plus ends of actin microfilaments, which probably represent sites of polymerization. The addition of unlabeled competing cytochalasin reduced the signal, suggesting that the labeled and unlabeled forms of the drug bind to the same site on F-actin. Exposure to Rhizobium etli NFs resulted in a rapid increase in the number of F-actin plus ends in P. vulgaris root hairs and in the re-localization of F-actin plus ends to infection thread initiation sites. These data suggest that NFs promote the formation of F-actin plus ends, which results in actin cytoskeleton rearrangements that facilitate infection thread formation.

Live microscopy analysis of endosomes and vesicles in tip-growing root hairs

Tip growth is one of the most preferable models in the study of plant cell polarity; cell wall deposition is restricted mainly to a certain area of the cell, and cell expansion at this specific area leads to the development of tubular outgrowth. Tip-growing root hairs are well-established systems for such studies, because their lateral position within the root makes them easily accessible for experimental approaches and microscopic observations. Fundamental structural and molecular processes driving tip growth are exocytosis, endocytosis, and all aspects of vesicular and endosomal dynamic trafficking, as related to targeted membrane flow. Study of vesicles and endosomes in living root hairs, however, is rather difficult, due to their small size and due to the resolution limits of conventional light microscopes. Here we present noninvasive approaches for visualizing vesicular and endosomal compartments in the tip of growing root hairs using electronic light microscopy, contrast-enhanced video light microscopy, and confocal laser scanning microscopy (CLSM). These methods allow utilizing the maximum resolution of the light microscope. Together with protocols for appropriate preparation of living plant samples, the described methods should help improve our understanding on how tiny vesicles and endosomes support the process of tip growth in root hairs.

Immunofluorescent localization of MAPKs in Steedman's wax sections

Signals of different nature are transduced in cells through signal transduction pathways, where mitogen-activated protein kinases (MAPKs) play an important role as signaling molecules. Views into intracellular localization of MAPKs are critical for the understanding of their spatial and temporal functions, like activation-based relocation, compartmentation, or interactions with local substrates. Localization of MAPKs in cells is thus very useful cell biological approach, extending complex mode of cell signaling characterization in plants. Here, we present a method for subcellular immunofluorescence localization of MAPKs using protein- or phospho-specific antibodies, performed on sectioned fixed plant samples. It is based on embedding of samples in the Steedman's wax, a low-melting point polyester wax embedding medium, which maintains high antigenicity of studied proteins. In addition, exposure of dewaxed sections to antibodies allows for their efficient penetration. Altogether, it makes this simple method a good tool in the efficient subcellular localization of diverse proteins, including plant MAPKs.

Actin polymerization drives polar growth in Arabidopsis root hair cells

In plants, the actin cytoskeleton is a prime regulator of cell polarity, growth, and cytoplasmic streaming. Tip growth, as observed in root hairs, caulonema, and pollen tubes, is governed by many factors, including calcium gradients, exocytosis and endocytosis, reactive oxygen species, and the cytoskeleton. Several studies indicate that the polymerization of G-actin into F-actin also contributes to tip growth. The structure and function of F-actin within the apical dome is variable, ranging from a dense meshwork to sparse single filaments. The presence of multiple F-actin structures in the elongating apices of tip-growing cells suggests that this cytoskeletal array is tightly regulated. We recently reported that sublethal concentrations of fluorescently labeled cytochalasin could be used to visualize the distribution of microfilament plus ends using fluorescence microscopy, and found that the tip region of the growing root hair cells of a legume plant exhibits a clear response to the nodulation factors secreted by Rhizobium. (1) In this current work, we expanded our analysis using confocal microscopy and demonstrated the existence of highly dynamic fluorescent foci along Arabidopsis root hair cells. Furthermore, we show that the strongest fluorescence signal accumulates in the tip dome of the growing root hair and seems to be in close proximity to the apical plasma membrane. Based on these findings, we propose that actin polymerization within the dome of growing root hair cells regulates polar growth.

Profilin as a regulator of the membrane-actin cytoskeleton interface in plant cells

Membrane structures and cytoskeleton dynamics are intimately inter-connected in the eukaryotic cell. Recently, the molecular mechanisms operating at this interface have been progressively addressed. Many experiments have revealed that the actin cytoskeleton can interact with membranes through various discrete membrane domains. The actin-binding protein, profilin has been proven to inhibit actin polymerization and to promote F-actin elongation. This is dependent on many factors, such as the profilin/G-actin ratio and the ionic environment of the cell. Additionally, profilin has specific domains that interact with phosphoinositides and poly-L-proline rich proteins; theoretically, this gives profilin the opportunity to interact with membranes, and a large number of experiments have confirmed this possibility. In this article, we summarize recent findings in plant cells, and discuss the evidence of the connections among actin cytoskeleton, profilin and biomembranes through direct or indirect relationships.

Plant cell shape: modulators and measurements

Plant cell shape, seen as an integrative output, is of considerable interest in various fields, such as cell wall research, cytoskeleton dynamics and biomechanics. In this review we summarize the current state of knowledge on cell shape formation in plants focusing on shape of simple cylindrical cells, as well as in complex multipolar cells such as leaf pavement cells and trichomes. We summarize established concepts as well as recent additions to the understanding of how cells construct cell walls of a given shape and the underlying processes. These processes include cell wall synthesis, activity of the actin and microtubule cytoskeletons, in particular their regulation by microtubule associated proteins, actin-related proteins, GTP'ases and their effectors, as well as the recently-elucidated roles of plant hormone signaling and vesicular membrane trafficking. We discuss some of the challenges in cell shape research with a particular emphasis on quantitative imaging and statistical analysis of shape in 2D and 3D, as well as novel developments in this area. Finally, we review recent examples of the use of novel imaging techniques and how they have contributed to our understanding of cell shape formation.

Root apex transition zone as oscillatory zone

Root apex of higher plants shows very high sensitivity to environmental stimuli. The root cap acts as the most prominent plant sensory organ; sensing diverse physical parameters such as gravity, light, humidity, oxygen, and critical inorganic nutrients. However, the motoric responses to these stimuli are accomplished in the elongation region. This spatial discrepancy was solved when we have discovered and characterized the transition zone which is interpolated between the apical meristem and the subapical elongation zone. Cells of this zone are very active in the cytoskeletal rearrangements, endocytosis and endocytic vesicle recycling, as well as in electric activities. Here we discuss the oscillatory nature of the transition zone which, together with several other features of this zone, suggest that it acts as some kind of command center. In accordance with the early proposal of Charles and Francis Darwin, cells of this root zone receive sensory information from the root cap and instruct the motoric responses of cells in the elongation zone.

Targeting and regulation of cell wall synthesis during tip growth in plants

Root hairs and pollen tubes are formed through tip growth, a process requiring synthesis of new cell wall material and the precise targeting and integration of these components to a selected apical plasma membrane domain in the growing tips of these cells. Presence of a tip-focused calcium gradient, control of actin cytoskeleton dynamics, and formation and targeting of secretory vesicles are essential to tip growth. Similar to cells undergoing diffuse growth, cellulose, hemicelluloses, and pectins are also deposited in the growing apices of tip-growing cells. However, differences in the manner in which these cell wall components are targeted and inserted in the expanding portion of tip-growing cells is reflected by the identification of elements of the plant cell wall synthesis machinery which have been shown to play unique roles in tip-growing cells. In this review, we summarize our current understanding of the tip growth process, with a particular focus on the subcellular targeting of newly synthesized cell wall components, and their roles in this form of plant cell expansion.

Cotton annexin proteins participate in the establishment of fiber cell elongation scaffold

Cotton plant is one of the most important economic crops in the world which supplies natural fiber for textile industry. The crucial traits of cotton fiber quality are fiber length and strength, which are mostly determined by the fiber elongation stage. Annexins are assumed to be involved in regulating fiber elongation, but direct evidences remain elusive. Recently, we have investigated the activities of fiber-specific expressed annexins AnGb5/6 and their interacted proteins in cotton. AnGb5 and 6 can interact reciprocally to generate a protein macro-raft in cell membrane. This macro-raft is probably a stabilized scaffold for Actin1 organization. The actin assembling direction and density are correlated with AnGb6 gene expression and fiber expanding rate among three fiber length genotypes. These results suggest that annexins may act as the adaptor that linked fiber cell membrane to actin assembling. Due to the strong Ca (2+) and lipid binding ability of annexins, these results also indicate that annexins complex may function as an intermediate to receive Ca (2+) or lipid signals during fiber elongation.

Translational Regulation of Cytoplasmic mRNAs

Translation of the coding potential of a messenger RNA into a protein molecule is a fundamental process in all living cells and consumes a large fraction of metabolites and energy resources in growing cells. Moreover, translation has emerged as an important control point in the regulation of gene expression. At the level of gene regulation, translational control is utilized to support the specific life histories of plants, in particular their responses to the abiotic environment and to metabolites. This review summarizes the diversity of translational control mechanisms in the plant cytoplasm, focusing on specific cases where mechanisms of translational control have evolved to complement or eclipse other levels of gene regulation. We begin by introducing essential features of the translation apparatus. We summarize early evidence for translational control from the pre-Arabidopsis era. Next, we review evidence for translation control in response to stress, to metabolites, and in development. The following section emphasizes RNA sequence elements and biochemical processes that regulate translation. We close with a chapter on the role of signaling pathways that impinge on translation.

Invasive cells in animals and plants: searching for LECA machineries in later eukaryotic life

Invasive cell growth and migration is usually considered a specifically metazoan phenomenon. However, common features and mechanisms of cytoskeletal rearrangements, membrane trafficking and signalling processes contribute to cellular invasiveness in organisms as diverse as metazoans and plants – two eukaryotic realms genealogically connected only through the last common eukaryotic ancestor (LECA). By comparing current understanding of cell invasiveness in model cell types of both metazoan and plant origin (invadopodia of transformed metazoan cells, neurites, pollen tubes and root hairs), we document that invasive cell behavior in both lineages depends on similar mechanisms. While some superficially analogous processes may have arisen independently by convergent evolution (e.g. secretion of substrate- or tissue-macerating enzymes by both animal and plant cells), at the heart of cell invasion is an evolutionarily conserved machinery of cellular polarization and oriented cell mobilization, involving the actin cytoskeleton and the secretory pathway. Its central components - small GTPases (in particular RHO, but also ARF and Rab), their specialized effectors, actin and associated proteins, the exocyst complex essential for polarized secretion, or components of the phospholipid- and redox- based signalling circuits (inositol-phospholipid kinases/PIP2, NADPH oxidases) are aparently homologous among plants and metazoans, indicating that they were present already in LECA.

Reviewer: This article was reviewed by Arcady Mushegian, Valerian Dolja and Purificacion Lopez-Garcia.

Brown algae as a model for plant organogenesis

Brown algae are an extremely interesting, but surprisingly poorly explored, group of organisms. They are one of only five eukaryotic lineages to have independently evolved complex multicellularity, which they express through a wide variety of morphologies ranging from uniseriate branched filaments to complex parenchymatous thalli with multiple cell types. Despite their very distinct evolutionary history, brown algae and land plants share a striking amount of developmental features. This has led to an interest in several aspects of brown algal development, including embryogenesis, polarity, cell cycle, asymmetric cell division and a putative role for plant hormone signalling. This review describes how investigations using brown algal models have helped to increase our understanding of the processes controlling early embryo development, in particular polarization, axis formation and asymmetric cell division. Additionally, the diversity of life cycles in the brown lineage and the emergence of Ectocarpus as a powerful model organism, are affording interesting insights on the molecular mechanisms underlying haploid-diploid life cycles. The use of these and other emerging brown algal models will undoubtedly add to our knowledge on the mechanisms that regulate development in multicellular photosynthetic organisms.

Growth mechanisms in tip-growing plant cells

Tip growth is employed throughout the plant kingdom. Our understanding of tip growth has benefited from modern tools in molecular genetics, which have enabled the functional characterization of proteins mediating tip growth. Here we first discuss the evolutionary role of tip growth in land plants and then describe the prominent model tip-growth systems, elaborating on some advantages and disadvantages of each. Next we review the organization of tip-growing cells, the role of the cytoskeleton, and recent developments concerning the physiological basis of tip growth. Finally, we review advances in the understanding of the extracellular signals that are known to guide tip-growing cells.

Eduard Strasburger (1844-1912): founder of modern plant cell biology

Eduard Strasburger, director of the Botany Institute and the Botanical Garden at the University of Bonn from 1881 to 1912, was one of the most admirable scientists in the field of plant biology, not just as the founder of modern plant cell biology but in addition as an excellent teacher who strongly believed in "education through science." He contributed to plant cell biology by discovering the discrete stages of karyokinesis and cytokinesis in algae and higher plants, describing cytoplasmic streaming in different systems, and reporting on the growth of the pollen tube into the embryo sac and guidance of the tube by synergides. Strasburger raised many problems which are hot spots in recent plant cell biology, e.g., structure and function of the plasmodesmata in relation to phloem loading (Strasburger cells) and signaling, mechanisms of cell plate formation, vesicle trafficking as a basis for most important developmental processes, and signaling related to fertilization.

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

Arabidopsis Myosin XI-K Localizes to the Motile Endomembrane Vesicles Associated with F-actin

Plant myosins XI were implicated in cell growth, F-actin organization, and organelle transport, with myosin XI-K being a critical contributor to each of these processes. However, subcellular localization of myosins and the identity of their principal cargoes remain poorly understood. Here, we generated a functionally competent, fluorescent protein-tagged, myosin XI-K, and investigated its spatial distribution within Arabidopsis cells. This myosin was found to associate primarily not with larger organelles (e.g., Golgi) as was broadly assumed, but with endomembrane vesicles trafficking along F-actin. Subcellular localization and fractionation experiments indicated that the nature of myosin-associated vesicles is organ- and cell type-specific. In leaves, a large proportion of these vesicles aligned and co-fractionated with a motile endoplasmic reticulum (ER) subdomain. In roots, non-ER vesicles were a dominant myosin cargo. Myosin XI-K showed a striking polar localization at the tips of growing, but not mature, root hairs. These results strongly suggest that a major mechanism whereby myosins contribute to plant cell physiology is vesicle transport, and that this activity can be regulated depending on the growth phase of a cell.

Nitric oxide is essential for vesicle formation and trafficking in Arabidopsis root hair growth

The functions of nitric oxide (NO) in processes associated with root hair growth in Arabidopsis were analysed. NO is located at high concentrations in the root hair cell files at any stage of development. NO is detected inside of the vacuole in immature actively growing root hairs and, later, NO is localized in the cytoplasm when they become mature. Experiments performed by depleting NO in Arabidopsis root hairs indicate that NO is required for endocytosis, vesicle formation, and trafficking and it is not involved in nucleus migration, vacuolar development, and transvacuolar strands. The Arabidopsis G'4,3 mutant (double mutant nia1/nia2) is severely impaired in NO production and generates smaller root hairs than the wild type (WT). Root hairs from the Arabidopsis G'4,3 mutant show altered vesicular trafficking and are reminiscent of NO-depleted root hairs from the Arabidopsis WT. Interestingly, normal vesicle formation and trafficking as well as root hair growth is restored by exogenous NO application in the Arabidopsis G'4,3 mutant. All together, these results firmly support the essential role played by NO in the Arabidopsis root-hair-growing process.

In vivo visualization of RNA in plants cells using the λN₂₂ system and a GATEWAY-compatible vector series for candidate RNAs

The past decade has seen a tremendous increase in RNA research, which has demonstrated that RNAs are involved in many more processes than were previously thought. The dynamics of RNA synthesis towards their regulated activity requires the interplay of RNAs with numerous RNA binding proteins (RBPs). The localization of RNA, a mechanism for controlling translation in a spatial and temporal fashion, requires processing and assembly of RNA into transport granules in the nucleus, transport towards cytoplasmic destinations and regulation of its activity. Compared with animal model systems little is known about RNA dynamics and motility in plants. Commonly used methods to study RNA transport and localization are time-consuming, and require expensive equipment and a high level of experimental skill. Here, we introduce the λN₂₂ RNA stem-loop binding system for the in vivo visualization of RNA in plant cells. The λN₂₂ system consists of two components: the λN₂₂ RNA binding peptide and the corresponding box-B stem loops. We generated fusions of λN₂₂ to different fluorophores and a GATEWAY vector series for the simple fusion of any target RNA 5' or 3' to box-B stem loops. We show that the λN₂₂ system can be used to detect RNAs in transient expression assays, and that it offers advantages compared with the previously described MS2 system. Furthermore, the λN₂₂ system can be used in combination with the MS2 system to visualize different RNAs simultaneously in the same cell. The toolbox of vectors generated for both systems is easy to use and promises significant progress in our understanding of RNA transport and localization in plant cells.

Control of the actin cytoskeleton in root hair development

The development of root hair includes four stages: bulge site selection, bulge formation, tip growth, and maturation. The actin cytoskeleton is involved in all of these stages and is organized into distinct arrangements in the different stages. In addition to the actin configuration, actin isoforms also play distinct roles in the different stages. The actin cytoskeleton is regulated by actin-binding proteins, such as formin, Arp2/3 complex, profilin, actin depolymerizing factor, and villin. Some upstream signals, i.e. calcium, phospholipids, and small GTPase regulate the activity of these actin-binding proteins to produce the proper actin configuration. We constructed a working model on how the actin cytoskeleton is controlled by actin-binding proteins and upstream signaling in root hair development based on the current literature: at the tip of hairs, actin polymerization appears to be facilitated by Arp2/3 complex that is activated by small GTPase, and profilin that is regulated by phosphatidylinositol 4,5-bisphosphate. Meanwhile, actin depolymerization and turnover are likely mediated by villin and actin depolymerizing factor, which are stimulated by calcium. At the shank, actin cables are produced by formin and villin. Under the complicated interaction, the actin cytoskeleton is controlled spatially and temporally during root hair development.

Parallel up-regulation of the profilin gene family following independent domestication of diploid and allopolyploid cotton (Gossypium)

Cotton is remarkable among our major crops in that four species were independently domesticated, two allopolyploids and two diploids. In each case thousands of years of human selection transformed sparsely flowering, perennial shrubs into highly productive crops with seeds bearing the vastly elongated and abundant single-celled hairs that comprise modern cotton fiber. The genetic underpinnings of these transformations are largely unknown, but comparative gene expression profiling experiments have demonstrated up-regulation of profilin accompanying domestication in all three species for which wild forms are known. Profilins are actin monomer binding proteins that are important in cytoskeletal dynamics and in cotton fiber elongation. We show that Gossypium diploids contain six profilin genes (GPRF1-GPRF6), located on four different chromosomes (eight chromosomes in the allopolyploid). All but one profilin (GPRF6) are expressed during cotton fiber development, and both homeologs of GPRF1-GPRF5 are expressed in fibers of the allopolyploids. Remarkably, quantitative RT-PCR and RNAseq data demonstrate that GPRF1-GPRF5 are all up-regulated, in parallel, in the three independently domesticated cottons in comparison with their wild counterparts. This result was additionally supported by iTRAQ proteomic data. In the allopolyploids, there This usage of novel should be fine, since it refers to a novel evolutionary process, not a novel discovery has been novel recruitment of the sixth profilin gene (GPRF6) as a result of domestication. This parallel up-regulation of an entire gene family in multiple species in response to strong directional selection is without precedent and suggests unwitting selection on one or more upstream transcription factors or other proteins that coordinately exercise control over profilin expression.

Emerging aspects of ER organization in root hair tip growth: lessons from RHD3 and Atlastin

Cell polarity is a fundamental aspect of eukaryotic cells. A central question for cell biologists is how the polarity of a cell is established and maintained. Root hairs are exceptionally polarized structures formed from specific root epidermal cells. The morphogenesis of root hairs is characterized by the localized cell growth in a small dome at the tip of the hair, a process called tip growth. Root hairs are thus an attractive model system to study the establishment and maintenance of cell polarity in eukaryotes. Research on Arabidopsis root hairs has identified a plethora of molecular and cellular components that are important for root hair tip growth. Recently, studies on RHD3 and Atlastin have revealed a surprising similarity with respect to the role of the tubular ER network in tip growth of root hairs in plants and the axonal outgrowth of corticospinal neurons in neurological disorders known as hereditary spastic paraplegia (HSP). In this mini-review, we highlight recent progress in understanding of the function and regulation of RHD3 in the generation of the tubular ER network and discussed ways in which RHD3 could be involved in the establishment and maintenance of root hair tip growth.

Disarrangement of actin filaments and Ca²⁺ gradient by CdCl₂ alters cell wall construction in Arabidopsis thaliana root hairs by inhibiting vesicular trafficking

Cadmium (Cd), one of the most toxic heavy metals, inhibits many cellular and physiological processes in plants. Here, the involvement of cytoplasmic Ca²⁺ gradient and actin filaments (AFs) in vesicular trafficking, cell wall deposition and tip growth was investigated during root (hair) development of Arabidopsis thaliana in response to CdCl₂ treatment. Seed germination and root elongation were prevented in a dose- and time-dependent manner by CdCl₂ treatment. Fluorescence labelling and non-invasive detection showed that CdCl₂ inhibited extracellular Ca²⁺ influx, promoted intracellular Ca²⁺ efflux, and disturbed the cytoplasmic tip-focused Ca²⁺ gradient. In vivo labelling revealed that CdCl₂ modified actin organization, which subsequently contributed to vesicle trafficking. Transmission electron microscopy revealed that CdCl₂ induced cytoplasmic vacuolization and was detrimental to organelles such as mitochondria and endoplasmic reticulum (ER). Finally, immunofluorescent labelling and Fourier transform infrared (FTIR) analysis indicated that configuration/distribution of cell wall components such as pectins and cellulose was significantly altered in response to CdCl₂. Our results indicate that CdCl₂ induces disruption of Ca²⁺ gradient and AFs affects the distribution of cell wall components in root hairs by disturbing vesicular trafficking in A. thaliana.

Arabidopsis VILLIN4 is involved in root hair growth through regulating actin organization in a Ca2+-dependent manner

• Villin is one of the major actin filament bundling proteins in plants. The function of Arabidopsis VILLINs (AtVLNs) is still poorly understood in living cells. In this report, the biochemical activity and cellular function of AtVLN4 were examined. • The biochemical property of AtVLN4 was characterized by co-sedimentation assays, fluorescence microscopy and spectroscopy of pyrene fluorescence. The in vivo function of AtVLN4 was analysed by ectopically expressing it in tobacco pollen and examining the phenotypes of its T-DNA insertional plants. • Recombinant AtVLN4 protein exhibited multiple activities on actin, including actin filament bundling, calcium (Ca(2+))-dependent filament severing and barbed end capping. Expression of AtVLN4 in tobacco pollen induced the formation of supernumerary actin cables and reduced pollen tube growth. Loss of function of AtVLN4 resulted in slowing of root hair growth, alteration in cytoplasmic streaming routes and rate, and reduction of both axial and apical actin bundles. • Our results demonstrated that AtVLN4 is involved in root hair growth through regulating actin organization in a Ca(2+)-dependent manner.

Root hair-specific EXPANSIN A7 is required for root hair elongation in Arabidopsis

Expansins are non-hydrolytic cell wall-loosening proteins that are involved in the cell wall modifications that underlie many plant developmental processes. Root hair growth requires the accumulation of cell wall materials and dynamic cell wall modification at the tip region. Although several lines of indirect evidence support the idea that expansin-mediated wall modification occurs during root hair growth, the involvement of these proteins remains to be demonstrated in vivo. In this study, we used RNA interference (RNAi) to examine the biological function of Arabidopsis thaliana EXPANSIN A7 (AtEXPA7), which is expressed specifically in the root hair cell. The root hairspecific AtEXPA7 promoter was used to drive RNAi expression, which targeted two independent regions in the AtEXPA7 transcript. Quantitative reverse transcriptase-PCR analyses were used to examine AtEXPA7 transcript levels. In four independent RNAi transformant lines, RNAi expression reduced AtEXPA7 transcript levels by 25-58% compared to controls. Accordingly, the root hairs of RNAi transformant lines were 25-48% shorter than control plants and exhibited a broader range of lengths than the controls. Our results provide in vivo evidence that expansins are required for root hair tip growth.

Actin filament-organized local cortical endoplasmic reticulum aggregations in developing stomatal complexes of grasses

Endoplasmic reticulum (ER) immunolabeling in developing stomatal complexes and in the intervening cells of the stomatal rows (ICSRs) of Zea mays revealed that the cortical-ER forms distinct aggregations lining locally expanding wall regions. The polarized subsidiary cell mother cells (SMCs), displayed a cortical-ER-patch lining the wall region shared with the inducing guard cell mother cell (GMC), which disorganized during mitosis. In dividing SMCs, ER persisted in the preprophase band region and was unequally distributed in the mitotic spindle poles. The subsidiary cells (SCs) formed initially an ER-patch lining the common wall with the GMC or the young guard cells and afterwards an ER-ring in the junction of the SC wall with the neighboring ones. Distinct ER aggregations lined the ICSR wall regions shared with the SCs. The cortical-ER aggregations in stomatal cells of Z. mays were co-localized with actin filament (AF) arrays but both were absent from the respective cells of Triticum turgidum, which follow a different morphogenetic pattern. Experimental evidence showed that the interphase ER aggregations are organized by the respective AF arrays, while the mitotic ER aggregations by microtubules. These results revealed that AF and ER demarcated "cortical cytoplasmic domains" are activated below the locally expanding stomatal cell wall regions, probably via a mechanosensing mechanism triggered by the locally stressed plasmalemma/cell wall continuum. The probable role(s) of the local ER aggregations are discussed.

New insights into the mechanism of development of Arabidopsis root hairs and trichomes

Epidermis cell differentiation in Arabidopsis thaliana is a model system for understanding the mechanisms leading to the developmental end state of plant cells. Both root hairs and trichomes differentiate from epidermal cells and molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of root hairs and trichomes is regulated by similar molecular mechanisms. Molecular-genetic approaches have led to the identification of many genes that are involved in epidermal cell differentiation, most of which encode transcription factors that induce the expression of genes active in both root hair and trichome development. Control of cell growth after fate determination has also been studied using Arabidopsis mutants.

Root hair-specific EXPANSIN B genes have been selected for Graminaceae root hairs

Cell differentiation ultimately relies on the regulation of cell type-specific genes. For a root hair cell to undergo morphogenesis, diverse cellular processes including cell-wall loosening must occur in a root hair cell-specific manner. Previously, we identified and characterized root hairspecific cis-elements (RHE) from the genes encoding the cell wall-loosening protein EXPANSIN A (EXPA) which functions preferentially on dicot cell walls. This study reports two root hair-specific grass EXPB genes that contain RHEs. These genes are thought to encode proteins that function more efficiently on grass cell walls. The proximal promoter regions of two orthologous EXPB genes from rice (Oryza sativa; OsEXPB5) and barley (Hordeum vulgare; HvEXPB1) included RHE motifs. These promoters could direct root hair-specific expression of green fluorescent protein (GFP) in the roots of rice and Arabidopsis (Arabidopsis thaliana). Promoter deletion analyses demonstrated that the RHE motifs are necessary for root hairspecific expression of these EXPB promoters. Phylogenetic analysis of EXP protein sequences indicated that grass EXPBs are the only orthologs to these root hair-specific EXPBs, separating dicot EXPBs to distal branches of the tree. These results suggest that RHE-containing root hair-specific EXPB genes have evolved for grass-specific cell wall modification during root hair morphogenesis.

Structural sterols are involved in both the initiation and tip growth of root hairs in Arabidopsis thaliana

Structural sterols are abundant in the plasma membrane of root apex cells in Arabidopsis thaliana. They specifically accumulate in trichoblasts during the prebulging and bulge stages and show a polar accumulation in the tip during root hair elongation but are distributed evenly in mature root hairs. Thus, structural sterols may serve as a marker for root hair initiation and growth. In addition, they may predict branching events in mutants with branching root hairs. Structural sterols were detected using the sterol complexing fluorochrome filipin. Application of filipin caused a rapid, concentration-dependent decrease in tip growth. Filipin-complexed sterols accumulated in globular structures that fused to larger FM4-64-positive aggregates in the tip, so-called filipin-induced apical compartments, which were closely associated with the plasma membrane. The plasma membrane appeared malformed and the cytoarchitecture of the tip zone was affected. Trans-Golgi network/early endosomal compartments containing molecular markers, such as small Rab GTPase RabA1d and SNARE Wave line 13 (VTI12), locally accumulated in these filipin-induced apical compartments, while late endosomes, endoplasmic reticulum, mitochondria, plastids, and cytosol were excluded from them. These data suggest that the local distribution and apical accumulation of structural sterols may regulate vesicular trafficking and plasma membrane properties during both initiation and tip growth of root hairs in Arabidopsis.

Arabidopsis synaptotagmin SYT1, a type I signal-anchor protein, requires tandem C2 domains for delivery to the plasma membrane

The correct localization of integral membrane proteins to subcellular compartments is important for their functions. Synaptotagmin contains a single transmembrane domain that functions as a type I signal-anchor sequence in its N terminus and two calcium-binding domains (C(2)A and C(2)B) in its C terminus. Here, we demonstrate that the localization of an Arabidopsis synaptotagmin homolog, SYT1, to the plasma membrane (PM) is modulated by tandem C2 domains. An analysis of the roots of a transformant-expressing green fluorescent protein-tagged SYT1 driven by native SYT1 promoter suggested that SYT1 is synthesized in the endoplasmic reticulum, and then delivered to the PM via the exocytotic pathway. We transiently expressed a series of truncated proteins in protoplasts, and determined that tandem C(2)A-C(2)B domains were necessary for the localization of SYT1 to the PM. The PM localization of SYT1 was greatly reduced following mutation of the calcium-binding motifs of the C(2)B domain, based on sequence comparisons with other homologs, such as endomembrane-localized SYT5. The localization of SYT1 to the PM may have been required for the functional divergence that occurred in the molecular evolution of plant synaptotagmins.

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.

Advances in imaging RNA in plants

Increasing evidence shows that many RNAs are targeted to specific locations within cells, and that RNA-processing pathways occur in association with specific subcellular structures. Compartmentation of mRNA translation and RNA processing helps to assemble large RNA-protein complexes, while RNA targeting allows local protein synthesis and the asymmetric distribution of transcripts during cell polarisation. In plants, intercellular RNA trafficking also plays an additional role in plant development and pathogen defence. Methods that allow the visualisation of RNA sequences within a cellular context, and preferably at subcellular resolution, can help to answer important questions in plant cell and developmental biology. Here, we summarise the approaches currently available for localising RNA in vivo and address the specific limitations inherent with plant systems.

Dynamic coordination of cytoskeletal and cell wall systems during plant cell morphogenesis

Underlying the architectural complexity of plants are diverse cell types that, under the microscope, easily reveal relationships between cell structure and specialized functions. Much less obvious are the mechanisms by which the cellular growth machinery and mechanical properties of the cell interact to dictate cell shape. The recent combined use of mutants, genomic analyses, sophisticated spectroscopies, and live cell imaging is providing new insight into how cytoskeletal systems and cell wall biosynthetic activities are integrated during morphogenesis. The purpose of this review is to discuss the unique geometric properties and physical processes that regulate plant cell expansion, then to overlay on this mechanical system some of the recent discoveries about the protein machines and cellular polymers that regulate cell shape. In the end, we hope to make clear that there are many interesting opportunities to develop testable mathematical models that improve our understanding of how subcellular structures, protein motors, and extracellular polymers can exert effects at spatial scales that span cells, tissues, and organs.

Three cotton genes preferentially expressed in flower tissues encode actin-depolymerizing factors which are involved in F-actin dynamics in cells

To investigate whether the high expression levels of actin-depolymerizing factor genes are related to pollen development, three GhADF genes (cDNAs) were isolated and characterized in cotton. Among them, GhADF6 and GhADF8 were preferentially expressed in petals, whereas GhADF7 displayed the highest level of expression in anthers, revealing its anther specificity. The GhADF7 transcripts in anthers reached its peak value at flowering, suggesting that its expression is developmentally-regulated in anthers. The GhADF7 gene including the promoter region was isolated from the cotton genome. To demonstrate the specificity of the GhADF7 promoter, the 5'-flanking region, including the promoter and 5'-untranslated region, was fused with the GUS gene. Histochemical assays demonstrated that the GhADF7:GUS gene was specifically expressed in pollen grains. When pollen grains germinated, very strong GUS staining was detected in the elongating pollen tube. Furthermore, overexpression of GhADF7 gene in Arabidopsis thaliana reduced the viable pollen grains and, consequently, transgenic plants were partially male-sterile. Overexpression of GhADF7 in fission yeast (Schizosaccharomyces pombe) altered the balance of actin depolymerization and polymerization, leading to the defective cytokinesis and multinucleate formation in the cells. Given all the above results together, it is proposed that the GhADF7 gene may play an important role in pollen development and germination.

The association of the Arabidopsis actin-related protein2/3 complex with cell membranes is linked to its assembly status but not its activation

In growing plant cells, the combined activities of the cytoskeleton, endomembrane, and cell wall biosynthetic systems organize the cytoplasm and define the architecture and growth properties of the cell. These biosynthetic machineries efficiently synthesize, deliver, and recycle the raw materials that support cell expansion. The precise roles of the actin cytoskeleton in these processes are unclear. Certainly, bundles of actin filaments position organelles and are a substrate for long-distance intracellular transport, but the functional linkages between dynamic actin filament arrays and the cell growth machinery are poorly understood. The Arabidopsis (Arabidopsis thaliana) "distorted group" mutants have defined protein complexes that appear to generate and convert small GTPase signals into an Actin-Related Protein2/3 (ARP2/3)-dependent actin filament nucleation response. However, direct biochemical knowledge about Arabidopsis ARP2/3 and its cellular distribution is lacking. In this paper, we provide biochemical evidence for a plant ARP2/3. The plant complex utilizes a conserved assembly mechanism. ARPC4 is the most critical core subunit that controls the assembly and steady-state levels of the complex. ARP2/3 in other systems is believed to be mostly a soluble complex that is locally recruited and activated. Unexpectedly, we find that Arabidopsis ARP2/3 interacts strongly with cell membranes. Membrane binding is linked to complex assembly status and not to the extent to which it is activated. Mutant analyses implicate ARP2 as an important subunit for membrane association.

How to shape a cylinder: pollen tube as a model system for the generation of complex cellular geometry

Expansive growth in plant cells is a formidable problem for biophysical studies, and the mechanical principles governing the generation of complex cellular geometries are still poorly understood. Pollen, the male gametophyte stage of the flowering plants, is an excellent model system for the investigation of the mechanics of complex growth processes. The initiation of pollen tube growth requires first of all, the spatially confined formation of a protuberance. This process must be controlled by the mechanical properties of the cell wall, since turgor is a non-vectorial force. In the elongating tube, cell wall expansion is confined to the apex of the cell, requiring the tubular region to be stabilized against turgor-induced tensile stress. Tip focused surface expansion must be coordinated with the supply of cell wall material to this region requiring the precise, logistical control of intracellular transport processes. The advantage of such a demanding mechanism is the high efficiency it confers on the pollen tube in leading an invasive way of life.

Polar growth in pollen tubes is associated with spatially confined dynamic changes in cell mechanical properties

Cellular morphogenesis involves changes to cellular size and shape which in the case of walled cells implies the mechanical deformation of the extracellular matrix. So far, technical challenges have made quantitative mechanical measurements of this process at subcellular scale impossible. We used micro-indentation to investigate the dynamic changes in the cellular mechanical properties during the onset of spatially confined growth activities in plant cells. Pollen tubes are cellular protuberances that have a strictly unidirectional growth pattern. Micro-indentation of these cells revealed that the initial formation of a cylindrical protuberance is preceded by a local reduction in cellular stiffness. Similar cellular softening was observed before the onset of a rapid growth phase in cells with oscillating growth pattern. These findings provide the first quantitative cytomechanical data that confirm the important role of the mechanical properties of the cell wall for local cellular growth processes. They are consistent with a conceptual model that explains pollen tube oscillatory growth based on the relationship between turgor pressure and tensile resistance in the apical cell wall. To further confirm the significance of cell mechanics, we artificially manipulated the mechanical cell wall properties as well as the turgor pressure. We observed that these changes affected the oscillation profile and were able to induce oscillatory behavior in steadily growing tubes.

Nitric oxide modulates dynamic actin cytoskeleton and vesicle trafficking in a cell type-specific manner in root apices

NO is an important regulatory molecule in eukaryotes. Much of its effect is ascribed to the action of NO as a signalling molecule. However, NO can also directly modify proteins thus affecting their activities. Although the signalling functions of NO are relatively well recognized in plants, very little is known about its potential influence on the structural integrity of plant cells. In this study, the reorganization of the actin cytoskeleton, and the recycling of wall polysaccharides in plants via the endocytic pathway in the presence of NO or NO-modulating substances were analysed. The actin cytoskeleton and endocytosis in maize (Zea mays) root apices were visualized with fluorescence immunocytochemistry. The organization of the actin cytoskeleton is modulated via NO levels and the extent of such modulation is cell-type specific. In endodermis cells, actin cables change their orientation from longitudinal to oblique and cellular cross-wall domains become actin-depleted/depolymerized. The reaction is reversible and depends on the type of NO donor. Actin-dependent vesicle trafficking is also affected. This was demonstrated through the analysis of recycled wall material transported to newly-formed cell plates and BFA compartments. Therefore, it is concluded that, in plant cells, NO affects the functioning of the actin cytoskeleton and actin-dependent processes. Mechanisms for the reorganization of the actin cytoskeleton are cell-type specific, and such rearrangements might selectively impinge on the functioning of various cellular domains. Thus, the dynamic actin cytoskeleton could be considered as a downstream effector of NO signalling in cells of root apices.