Microfluidics to Follow Spatiotemporal Dynamics at the Nucleo-Cytoplasmic Interface During Plant Root Growth

Nuclear dynamics refers to global/local changes in the molecular and spatial organization of genomic DNA that can occur during development or in response to environmental stress signals and eventually impact genomic functions. In plants, nuclear dynamics relies notably on the connection of the nucleus with the cytoskeleton during development. It orchestrates genomic functions in response to developmental and environmental cues. This is particularly true in the plant root system, which is constantly exposed to a wide range of internal and external stimuli. Currently, studying nuclear dynamics in a growing root is challenging due to limitations regarding real-time imaging for quantitative analyses under controlled conditions. Microfluidic systems for plant cell studies are valuable analytical tools that provide precise control of culture conditions together with live-imaging capabilities at high temporal and spatial resolutions. Herein, we describe a microfluidic platform to unravel dynamically and noninvasively nuclear organization in the seedling root system exposed to various treatments. As exemplified here, our microfluidic platform can be conveniently used for real-time microscopy imaging and quantitative analysis of fine nuclear morphological changes upon modifying cytoskeleton dynamics. Importantly, our system can be applied to a wide variety of microscopic means including high-resolution microscopy to investigate diverse subcellular compartments or nuclear domains in Arabidopsis thaliana roots.

Cooperative actin filament nucleation by the Arp2/3 complex and formins maintains the homeostatic cortical array in Arabidopsis epidermal cells

Precise control over how and where actin filaments are created leads to the construction of unique cytoskeletal arrays within a common cytoplasm. Actin filament nucleators are key players in this activity and include the conserved actin-related protein 2/3 (Arp2/3) complex as well as a large family of formins. In some eukaryotic cells, these nucleators compete for a common pool of actin monomers and loss of one favors the activity of the other. To test whether this mechanism is conserved, we combined the ability to image single filament dynamics in the homeostatic cortical actin array of living Arabidopsis (Arabidopsis thaliana) epidermal cells with genetic and/or small molecule inhibitor approaches to stably or acutely disrupt nucleator activity. We found that Arp2/3 mutants or acute CK-666 treatment markedly reduced the frequency of side-branched nucleation events as well as overall actin filament abundance. We also confirmed that plant formins contribute to side-branched filament nucleation in vivo. Surprisingly, simultaneous inhibition of both classes of nucleator increased overall actin filament abundance and enhanced the frequency of de novo nucleation events by an unknown mechanism. Collectively, our findings suggest that multiple actin nucleation mechanisms cooperate to generate and maintain the homeostatic cortical array of plant epidermal cells.

Automatic extraction of actin networks in plants

The actin cytoskeleton is essential in eukaryotes, not least in the plant kingdom where it plays key roles in cell expansion, cell division, environmental responses and pathogen defence. Yet, the precise structure-function relationships of properties of the actin network in plants are still to be unravelled, including details of how the network configuration depends upon cell type, tissue type and developmental stage. Part of the problem lies in the difficulty of extracting high-quality, quantitative measures of actin network features from microscopy data. To address this problem, we have developed DRAGoN, a novel image analysis algorithm that can automatically extract the actin network across a range of cell types, providing seventeen different quantitative measures that describe the network at a local level. Using this algorithm, we then studied a number of cases in Arabidopsis thaliana, including several different tissues, a variety of actin-affected mutants, and cells responding to powdery mildew. In many cases we found statistically-significant differences in actin network properties. In addition to these results, our algorithm is designed to be easily adaptable to other tissues, mutants and plants, and so will be a valuable asset for the study and future biological engineering of the actin cytoskeleton in globally-important crops.

Light Microscopy Technologies and the Plant Cytoskeleton

The cytoskeleton is a dynamic and diverse subcellular filament network, and as such microscopy is an essential technology to enable researchers to study and characterize these systems. Microscopy has a long history of observing the plant world not least as the subject where Robert Hooke coined the term "cell" in his publication Micrographia. From early observations of plant morphology to today's advanced super-resolution technologies, light microscopy is the indispensable tool for the plant cell biologist. In this mini review, we will discuss some of the major modalities used to examine the plant cytoskeleton and the theory behind them.

The TOR complex controls ATP levels to regulate actin cytoskeleton dynamics in Arabidopsis

Cells must overcome energy shortage, and the ability to do so determines their fate. The ability of cells to coordinate their cellular activities and energy status is therefore important for all living organisms. One of the major energy drains in eukaryotic cells is the constant turnover of the actin cytoskeleton, which consumes ATP during the cycle of polymerization and depolymerization. We report that the TOR complex, a master regulatory hub that integrates cellular energy information to coordinate cell growth and metabolism, controls cellular ATP levels in plant cells. We further elucidate that low ATP levels cause reduced actin dynamics in plant cells. These findings provide insight into how plant cells handle low energy situations.

Energy is essential for all cellular functions in a living organism. How cells coordinate their physiological processes with energy status and availability is thus an important question. The turnover of actin cytoskeleton between its monomeric and filamentous forms is a major energy drain in eukaryotic cells. However, how actin dynamics are regulated by ATP levels remain largely unknown in plant cells. Here, we observed that seedlings with impaired functions of target of rapamycin complex 1 (TORC1), either by mutation of the key component, RAPTOR1B, or inhibition of TOR activity by specific inhibitors, displayed reduced sensitivity to actin cytoskeleton disruptors compared to their controls. Consistently, actin filament dynamics, but not organization, were suppressed in TORC1-impaired cells. Subcellular localization analysis and quantification of ATP concentration demonstrated that RAPTOR1B localized at cytoplasm and mitochondria and that ATP levels were significantly reduced in TORC1-impaired plants. Further pharmacologic experiments showed that the inhibition of mitochondrial functions led to phenotypes mimicking those observed in raptor1b mutants at the level of both plant growth and actin dynamics. Exogenous feeding of adenine could partially restore ATP levels and actin dynamics in TORC1-deficient plants. Thus, these data support an important role for TORC1 in coordinating ATP homeostasis and actin dynamics in plant cells.

intER-ACTINg: the structure and dynamics of ER and actin are interlinked

The actin cytoskeleton is the driver of gross ER remodelling and the movement and positioning of other membrane-bound organelles such as Golgi bodies. Rapid ER membrane remodelling is a feature of most plant cells and is important for normal cellular processes, including targeted secretion, immunity and signalling. Modifications to the actin cytoskeleton, through pharmacological agents such as Latrunculin B and phalloidin, or disruption of normal myosin function also affect ER structure and/or dynamics. Here, we investigate the impact of changes in the actin cytoskeleton on structure and dynamics on the ER as well as in return the impact of modified ER structure on the architecture of the actin cytoskeleton. By expressing actin markers that affect actin dynamics, or expressing of ER-shaping proteins that influence ER architecture, we found that the structure of ER-actin networks is closely inter-related; affecting one component is likely to have a direct effect on the other. Therefore, our results indicate that a complicated regulatory machinery and cross-talk between these two structures must exist in plants to co-ordinate the function of ER-actin network during multiple subcellular processes. In addition, when considering organelle structure and dynamics, the choice of actin marker is essential in preventing off-target organelle structure and dynamics modifications. This article is protected by copyright. All rights reserved.

Global survey of alternative splicing and gene modules associated with fertility regulation in a thermosensitive genic male sterile wheat

Thermosensitive genic male sterile (TGMS) wheat lines are the core of two-line hybrid systems. Understanding the mechanism that regulates male sterility in TGMS wheat lines is helpful for promoting wheat breeding. Several studies have obtained information regarding the mechanisms associated with male sterility at the transcriptional level, but it is not clear how the post-transcriptional process of alternative splicing might contribute to controlling male sterility. In this study, we performed genome-wide analyses of alternative splicing during the meiosis stage in TGMS line BS366 using PacBio and RNA-Seq hybrid sequencing. Cytological observations indicated that cytoskeleton assembly in pollen cells, calcium deposition in pollen and tapetal cells, and vesicle transport in tapetal cells were deficient in BS366. According to our cytological findings, 49 differentially spliced genes were isolated. Moreover, 25 long non-coding RNA targets and three bHLH transcription factors were identified. Weighted gene co-expression network analysis detected four candidate differentially spliced genes that had strong co-relation with the seed setting percentage, which is the direct representation of male sterility in BS366. In this study, we obtained comprehensive data regarding the alternative splicing-mediated regulation of male sterility in TGMS wheat. The candidates identified may provide the molecular basis for an improved understanding of male sterility.

Lipid Signaling Requires ROS Production to Elicit Actin Cytoskeleton Remodeling during Plant Innate Immunity

In terrestrial plants a basal innate immune system, pattern-triggered immunity (PTI), has evolved to limit infection by diverse microbes. The remodeling of actin cytoskeletal arrays is now recognized as a key hallmark event during the rapid host cellular responses to pathogen attack. Several actin binding proteins have been demonstrated to fine tune the dynamics of actin filaments during this process. However, the upstream signals that stimulate actin remodeling during PTI signaling remain poorly characterized. Two second messengers, reactive oxygen species (ROS) and phosphatidic acid (PA), are elevated following pathogen perception or microbe-associated molecular pattern (MAMP) treatment, and the timing of signaling fluxes roughly correlates with actin cytoskeletal rearrangements. Here, we combined genetic analysis, chemical complementation experiments, and quantitative live-cell imaging experiments to test the role of these second messengers in actin remodeling and to order the signaling events during plant immunity. We demonstrated that PHOSPHOLIPASE Dβ (PLDβ) isoforms are necessary to elicit actin accumulation in response to flg22-associated PTI. Further, bacterial growth experiments and MAMP-induced apoplastic ROS production measurements revealed that PLDβ-generated PA acts upstream of ROS signaling to trigger actin remodeling through inhibition of CAPPING PROTEIN (CP) activity. Collectively, our results provide compelling evidence that PLDβ/PA functions upstream of RBOHD-mediated ROS production to elicit actin rearrangements during the innate immune response in Arabidopsis.

Dynamic apico-basal enrichment of the F-actin during cytokinesis in Arabidopsis cells embedded in their tissues

Cell division is a tightly regulated mechanism, notably in tissues where malfunctions can lead to tumour formation or developmental defects. This is particularly true in land plants, where cells cannot relocate and therefore cytokinesis determines tissue topology. In plants, cell division is executed in radically different manners than in animals, with the appearance of new structures and the disappearance of ancestral mechanisms. Whilst F-actin and microtubules closely co-exist, recent studies mainly focused on the involvement of microtubules in this key process. Here, we used a root tracking system to image the spatio-temporal dynamics of both F-actin reporters and cell division markers in dividing cells embedded in their tissues. In addition to the F-actin accumulation at the phragmoplast, we observed and quantified a dynamic apico-basal enrichment of F-actin from the prophase/metaphase transition until the end of the cytokinesis.

Proteomic dissection of rice cytoskeleton reveals the dominance of microtubule and microfilament proteins, and novel components in the cytoskeleton-bound polysome

The plant cytoskeleton persistently undergoes remodeling to achieve its roles in supporting cell division, differentiation, cell expansion and organelle transport. However, the links between cell metabolism and cytoskeletal networks, particularly how the proteinaceous components execute such processes remain poorly understood. We investigated the cytoskeletal proteome landscape of rice to gain better understanding of such events. Proteins were extracted from highly enriched cytoskeletal fraction of four-week-old rice seedlings, and the purity of the fraction was stringently monitored. A total of 2577 non-redundant proteins were identified using both gel-based and gel-free approaches, which constitutes the most comprehensive dataset, thus far, for plant cytoskeleton. The data set includes both microtubule and microfilament-associated proteins and their binding proteins comprising hypothetical as well as novel cytoskeletal proteins. Further, various in-silico analyses were performed, and the proteins were functionally classified on the basis of their gene ontology. The catalogued proteins were validated through their sequence analysis. Extensive comparative analysis of our dataset with the non-redundant set of cytoskeletal proteins across plant species affirms unique as well as overlapping candidates. Together, these findings unveil new insights of how cytoskeletons undergo dynamic remodeling in rice to drive seedling development processes in rapidly changing in planta environment.

NETWORKED2-subfamily proteins regulate the cortical actin cytoskeleton of growing pollen tubes and polarised pollen tube growth

We have recently characterised NET2A as a pollen-specific actin-binding protein that binds F-actin at the plasma membrane of growing pollen tubes. However, the role of NET2 proteins in pollen development and fertilisation have yet to be elucidated. To further characterise the role of Arabidopsis NET2 proteins in pollen development and fertilisation, we analysed the subcellular localisation of NET2A over the course of pollen grain development and investigated the role of the NET2 family using net2 loss-of-function mutants. We observed NET2A to localise to the F-actin cytoskeleton in developing pollen grains as it underwent striking structural reorganisations at specific stages of development and during germination and pollen tube growth. Furthermore, net2 loss-of-function mutants exhibited striking morphological defects in the early stages of pollen tube growth, arising from frequent changes to pollen tube growth trajectory. We observed defects in the cortical actin cytoskeleton and actin-driven subcellular processes in net2 mutant pollen tubes. We demonstrate that NET2 proteins are essential for normal actin-driven pollen development highlighting an important role for the NET2 family members in regulating pollen tube growth during fertilisation.

Analysis of formin functions during cytokinesis using specific inhibitor SMIFH2

The phragmoplast separates daughter cells during cytokinesis by constructing the cell plate, which depends on interaction between cytoskeleton and membrane compartments. Proteins responsible for these interactions remain unknown, but formins can link cytoskeleton with membranes and several members of formin protein family localize to the cell plate. Progress in functional characterization of formins in cytokinesis is hindered by functional redundancies within the large formin gene family. We addressed this limitation by employing Small Molecular Inhibitor of Formin Homology 2 (SMIFH2), a small-molecule inhibitor of formins. Treatment of tobacco (Nicotiana tabacum) tissue culture cells with SMIFH2 perturbed localization of actin at the cell plate; slowed down both microtubule polymerization and phragmoplast expansion; diminished association of dynamin-related proteins with the cell plate independently of actin and microtubules; and caused cell plate swelling. Another impact of SMIFH2 was shortening of the END BINDING1b (EB1b) and EB1c comets on the growing microtubule plus ends in N. tabacum tissue culture cells and Arabidopsis thaliana cotyledon epidermis cells. The shape of the EB1 comets in the SMIFH2-treated cells resembled that of the knockdown mutant of plant Xenopus Microtubule-Associated protein of 215 kDa (XMAP215) homolog MICROTUBULE ORGANIZATION 1/GEMINI 1 (MOR1/GEM1). This outcome suggests that formins promote elongation of tubulin flares on the growing plus ends. Formins AtFH1 (A. thaliana Formin Homology 1) and AtFH8 can also interact with EB1. Besides cytokinesis, formins function in the mitotic spindle assembly and metaphase to anaphase transition. Our data suggest that during cytokinesis formins function in: (1) promoting microtubule polymerization; (2) nucleating F-actin at the cell plate; (3) retaining dynamin-related proteins at the cell plate; and (4) remodeling of the cell plate membrane.

Kinases and protein motifs required for AZI1 plastid localization and trafficking during plant defense induction

The proper subcellular localization of defense factors is an important part of the plant immune system. A key component for systemic resistance, lipid transfer protein (LTP)-like AZI1, is needed for the systemic movement of the priming signal azelaic acid (AZA) and a pool of AZI1 exists at the site of AZA production, the plastid envelope. Moreover, after systemic defense-triggering infections, the proportion of AZI1 localized to plastids increases. However, AZI1 does not possess a classical plastid transit peptide that can explain its localization. Instead, AZI1 uses a bipartite N-terminal signature that allows for its plastid targeting. Furthermore, the kinases MPK3 and MPK6, associated with systemic immunity, promote the accumulation of AZI1 at plastids during priming induction. Our results indicate the existence of a mode of plastid targeting possibly related to defense responses.

Glycyrrhizin, the active compound of the TCM drug Gan Cao stimulates actin remodelling and defence in grapevine

Actin remodelling by a membrane-associated oxidative process can sense perturbations of membrane integrity and activate defence. In the current work, we show that glycyrrhizin, a muscle relaxant used in Traditional Chinese Medicine, can activate oxidative burst and actin remodelling in tobacco BY-2 cells, which could be suppressed by diphenylene iodonium, an inhibitor of NADPH oxidases. Glycyrrhizin caused a dose-dependent delay of proliferation, and induced cell death, which was suppressed by addition of indole-acetic acid, a natural auxin that can mitigate RboH dependent actin remodelling. To test, whether the actin remodelling induced by glycyrrhizin was followed by activation of defence, several events of basal immunity were probed. We found that glycyrrhizin induced a transient extracellular alkalinisation, indicative of calcium influx. Furthermore, transcripts of phytoalexins genes, were activated in cells of the grapevine Vitis rupestris, and this induction was followed by accumulation of the glycosylated stilbene α-piceid. We also observed that glycyrrhizin was able to induce actin bundling in leaves of a transgenic grape, especially in guard cells. We discuss these data in frame of a model, where glycyrrhizin, through stimulation of RboH, can cause actin remodelling, followed by defence responses, such as calcium influx, induction of phytoalexins transcripts, and accumulation of stilbene glycosides.

MTV proteins unveil ER- and microtubule-associated compartments in the plant vacuolar trafficking pathway

The factors and mechanisms involved in vacuolar transport in plants, and in particular those directing vesicles to their target endomembrane compartment, remain largely unknown. To identify components of the vacuolar trafficking machinery, we searched for Arabidopsis modified transport to the vacuole (mtv) mutants that abnormally secrete the synthetic vacuolar cargo VAC2. We report here on the identification of 17 mtv mutations, corresponding to mutant alleles of MTV2/VSR4, MTV3/PTEN2A MTV7/EREL1, MTV8/ARFC1, MTV9/PUF2, MTV10/VPS3, MTV11/VPS15, MTV12/GRV2, MTV14/GFS10, MTV15/BET11, MTV16/VPS51, MTV17/VPS54, and MTV18/VSR1 Eight of the MTV proteins localize at the interface between the trans-Golgi network (TGN) and the multivesicular bodies (MVBs), supporting that the trafficking step between these compartments is essential for segregating vacuolar proteins from those destined for secretion. Importantly, the GARP tethering complex subunits MTV16/VPS51 and MTV17/VPS54 were found at endoplasmic reticulum (ER)- and microtubule-associated compartments (EMACs). Moreover, MTV16/VPS51 interacts with the motor domain of kinesins, suggesting that, in addition to tethering vesicles, the GARP complex may regulate the motors that transport them. Our findings unveil a previously uncharacterized compartment of the plant vacuolar trafficking pathway and support a role for microtubules and kinesins in GARP-dependent transport of soluble vacuolar cargo in plants.

Auxin-induced actin cytoskeleton rearrangements require AUX1

The actin cytoskeleton is required for cell expansion and implicated in cellular responses to the phytohormone auxin. However, the mechanisms that coordinate auxin signaling, cytoskeletal remodeling and cell expansion are poorly understood. Previous studies examined long-term actin cytoskeleton responses to auxin, but plants respond to auxin within minutes. Before this work, an extracellular auxin receptor - rather than the auxin transporter AUXIN RESISTANT 1 (AUX1) - was considered to precede auxin-induced cytoskeleton reorganization. In order to correlate actin array organization and dynamics with degree of cell expansion, quantitative imaging tools established baseline actin organization and illuminated individual filament behaviors in root epidermal cells under control conditions and after indole-3-acetic acid (IAA) application. We evaluated aux1 mutant actin organization responses to IAA and the membrane-permeable auxin 1-naphthylacetic acid (NAA). Cell length predicted actin organization and dynamics in control roots; short-term IAA treatments stimulated denser and more parallel, longitudinal arrays by inducing filament unbundling within minutes. Although AUX1 is necessary for full actin rearrangements in response to auxin, cytoplasmic auxin (i.e. NAA) stimulated a lesser response. Actin filaments became more 'organized' after IAA stopped elongation, refuting the hypothesis that 'more organized' actin arrays universally correlate with rapid growth. Short-term actin cytoskeleton response to auxin requires AUX1 and/or cytoplasmic auxin.

A chemical screen identifies two novel small compounds that alter Arabidopsis thaliana pollen tube growth

BACKGROUND

During sexual reproduction, pollen grains land on the stigma, rehydrate and produce pollen tubes that grow through the female transmitting-tract tissue allowing the delivery of the two sperm cells to the ovule and the production of healthy seeds. Because pollen tubes are single cells that expand by tip-polarized growth, they represent a good model to study the growth dynamics, cell wall deposition and intracellular machineries. Aiming to understand this complex machinery, we used a low throughput chemical screen approach in order to isolate new tip-growth disruptors. The effect of a chemical inhibitor of monogalactosyldiacylglycerol synthases, galvestine-1, was also investigated. The present work further characterizes their effects on the tip-growth and intracellular dynamics of pollen tubes.

RESULTS

Two small compounds among 258 were isolated based on their abilities to perturb pollen tube growth. They were found to disrupt in vitro pollen tube growth of tobacco, tomato and Arabidopsis thaliana. We show that these 3 compounds induced abnormal phenotypes (bulging and/or enlarged pollen tubes) and reduced pollen tube length in a dose dependent manner. Pollen germination was significantly reduced after treatment with the two compounds isolated from the screen. They also affected cell wall material deposition in pollen tubes. The compounds decreased anion superoxide accumulation, disorganized actin filaments and RIC4 dynamics suggesting that they may affect vesicular trafficking at the pollen tube tip.

CONCLUSION

These molecules may alter directly or indirectly ROP1 activity, a key regulator of pollen tube growth and vesicular trafficking and therefore represent good tools to further study cellular dynamics during polarized-cell growth.

Anionic Lipids: A Pipeline Connecting Key Players of Plant Cell Division

How cells position their division plane is a critical component of cell division. Indeed, it defines whether the two daughter cells divide symmetrically (with equal volumes) or not, and as such is critical for cell differentiation and lineage specification across eukaryotes. However, oriented cell divisions are of special significance for organisms with cell walls, such as plants, because their cells are embedded and cannot relocate. Correctly positioning the division plane is therefore of prevailing importance in plants, as it controls not only the occurrence of asymmetric cell division, but also tissue morphogenesis and organ integrity. While cytokinesis is executed in radically different manners in animals and plants, they both rely on the dynamic interplay between the cytoskeleton and membrane trafficking to precisely deliver molecular components to the future site of cell division. Recent research has shown that strict regulation of the levels and distribution of anionic lipids, which are minor components of the cell membrane's lipids, is required for successful cytokinesis in non-plant organisms. This review focused on the recent evidence pointing to whether such signaling lipids have roles in plant cell division.

Actin Cytoskeleton as Actor in Upstream and Downstream of Calcium Signaling in Plant Cells

In plant cells, calcium (Ca²⁺) serves as a versatile intracellular messenger, participating in several fundamental and important biological processes. Recent studies have shown that the actin cytoskeleton is not only an upstream regulator of Ca²⁺ signaling, but also a downstream regulator. Ca²⁺ has been shown to regulates actin dynamics and rearrangements via different mechanisms in plants, and on this basis, the upstream signaling encoded within the Ca²⁺ transient can be decoded. Moreover, actin dynamics have also been proposed to act as an upstream of Ca²⁺, adjust Ca²⁺ oscillations, and establish cytosolic Ca²⁺ ([Ca²⁺]_cyt) gradients in plant cells. In the current review, we focus on the advances in uncovering the relationship between the actin cytoskeleton and calcium in plant cells and summarize our current understanding of this relationship.

Visualizing and Quantifying In Vivo Cortical Cytoskeleton Structure and Dynamics

The cortical microtubule and actin meshworks play a central role in the shaping of plant cells. Transgenic plants expressing fluorescent protein markers specifically tagging the two main cytoskeletal systems are available, allowing noninvasive in vivo studies. Advanced microscopy techniques, in particular confocal laser scanning microscopy (CLSM), spinning disk confocal microscopy (SDCM), and variable angle epifluorescence microscopy (VAEM), can be nowadays used for imaging the cortical cytoskeleton of living cells with unprecedented spatial and temporal resolution. With the aid of free computing tools based on the publicly available ImageJ software package, quantitative information can be extracted from microscopic images and video sequences, providing insight into both architecture and dynamics of the cortical cytoskeleton.

An Immune-Responsive Cytoskeletal-Plasma Membrane Feedback Loop in Plants

Cell wall appositions (CWAs) are produced reactively by the plant immune system to arrest microbial invasion through the local inversion of plant cell growth. This process requires the controlled invagination of the plasma membrane (PM) in coordination with the export of barrier material to the volume between the plant PM and cell wall. Plant actin dynamics are essential to this response, but it remains unclear how exocytosis and the cytoskeleton are linked in space and time to form functional CWAs. Here, we show that actin-dependent trafficking to immune response sites of Arabidopsis thaliana delivers membrane-integrated FORMIN4, which in turn contributes to local cytoskeletal dynamics. Total internal reflection fluorescence (TIRF) microscopy combined with controlled induction of FORMIN4-GFP expression reveals a dynamic population of vesicular bodies that accumulate to form clusters at the PM through an actin-dependent process. Deactivation of FORMIN4 and its close homologs partially compromises subsequent defense and alters filamentous actin (F-actin) distribution at mature CWAs. The localization of FORMIN4 is stable and segregated from the dynamic traffic of the endosomal network. Moreover, the tessellation of FORMIN4 at the PM with meso-domains of PEN3 reveals a fine spatial segregation of destinations for actin-dependent immunity cargo. Together, our data suggest a model where FORMIN4 is a spatial feedback element in a multi-layered, temporally defined sequence of cytoskeletal response. This positional feedback makes a significant contribution to the distribution of actin filaments at the dynamic CWA boundary and to the outcomes of pre-invasion defense.

Disruption of actin filaments in Zea mays by bisphenol A depends on their crosstalk with microtubules

Bisphenol A (BPA) is a widespread environmental pollutant, reportedly harmful to living organisms. In plant cells, BPA was shown to disrupt microtubule (MT) arrays and perturb mitosis, but its effects on filamentous actin (F-actin) have not been explored. Here we studied the effects of BPA on actin filaments (AFs) in meristematic root tip and leaf cells of Zea mays, by fluorescent labeling and confocal microscopy. Considering the typical dynamic interaction between MTs and AFs, the effects on these two essential components of the plant cytoskeleton were correlated. It was found that BPA disorganized rapidly AFs in a concentration- and time-dependent manner. The fine filaments were first to be affected, followed by the subcortical bundles, resulting in rod- and ring-like conformations. The observed differences in sensitivity between protodermal and cortex cells were attributed to the deeper location of the latter. Depolymerization or stabilization of MTs by relevant drugs (oryzalin, taxol) revealed that AF susceptibility to BPA depends on MT integrity. Developing leaves required harder and longer treatment to be affected by BPA. Ontogenesis of stomatal complexes was highly disturbed, arrangement of AFs and MT arrays was disordered and accuracy of cell division sequence was deranged or completely arrested. The effect of BPA confirmed that subsidiary cell mother cell polarization is not mediated by F-actin patch neither of preprophase band organization. On the overall, it is concluded that AFs in plant cells constitute a subcellular target of BPA and their disruption depends on their crosstalk with MTs.

A Ca2+-dependent remodelled actin network directs vesicle trafficking to build wall ingrowth papillae in transfer cells

The transport function of transfer cells is conferred by an enlarged plasma membrane area, enriched in nutrient transporters, that is supported on a scaffold of wall ingrowth (WI) papillae. Polarized plumes of elevated cytosolic Ca2+ define loci at which WI papillae form in developing adaxial epidermal transfer cells of Vicia faba cotyledons that are induced to trans-differentiate when the cotyledons are placed on culture medium. We evaluated the hypothesis that vesicle trafficking along a Ca2+-regulated remodelled actin network is the mechanism that underpins this outcome. Polarized to the outer periclinal cytoplasm, a Ca2+-dependent remodelling of long actin bundles into short, thin bundles was found to be essential for assembling WI papillae but not the underlying uniform wall layer. The remodelled actin network directed polarized vesicle trafficking to sites of WI papillae construction, and a pharmacological study indicated that both exo- and endocytosis contributed to assembly of the papillae. Potential candidates responsible for the Ca2+-dependent actin remodelling, along with those underpinning polarized exo- and endocyotosis, were identified in a transcriptome RNAseq database generated from the trans-differentiating epidermal cells. Of most significance, endocytosis was controlled by up-regulated expression of a dynamin-like isoform. How a cycle of localized exo- and endocytosis, regulated by Ca2+-dependent actin remodelling, assembles WI papillae is discussed.

The cell wall of Arabidopsis thaliana influences actin network dynamics

In plant cells, molecular connections link the cell wall-plasma membrane-actin cytoskeleton to form a continuum. It is hypothesized that the cell wall provides stable anchor points around which the actin cytoskeleton remodels. Here we use live cell imaging of fluorescently labelled marker proteins to quantify the organization and dynamics of the actin cytoskeleton and to determine the impact of disrupting connections within the continuum. Labelling of the actin cytoskeleton with green fluorescent protein (GFP)-fimbrin actin-binding domain 2 (FABD2) resulted in a network composed of fine filaments and thicker bundles that appeared as a highly dynamic remodelling meshwork. This differed substantially from the GFP-Lifeact-labelled network that appeared much more sparse with thick bundles that underwent 'simple movement', in which the bundles slightly change position, but in such a manner that the structure of the network was not substantially altered during the time of observation. Label-dependent differences in actin network morphology and remodelling necessitated development of two new image analysis techniques. The first of these, 'pairwise image subtraction', was applied to measurement of the more rapidly remodelling actin network labelled with GFP-FABD2, while the second, 'cumulative fluorescence intensity', was used to measure bulk remodelling of the actin cytoskeleton when labelled with GFP-Lifeact. In each case, these analysis techniques show that the actin cytoskeleton has a decreased rate of bulk remodelling when the cell wall-plasma membrane-actin continuum is disrupted either by plasmolysis or with isoxaben, a drug that specifically inhibits cellulose deposition. Changes in the rate of actin remodelling also affect its functionality, as observed by alteration in Golgi body motility.

Direct Comparison of the Performance of Commonly Employed In Vivo F-actin Markers (Lifeact-YFP, YFP-mTn and YFP-FABD2) in Tobacco Pollen Tubes

In vivo markers for F-actin organization and dynamics are extensively used to investigate cellular functions of the actin cytoskeleton, which are essential for plant development and pathogen defense. The most widely employed markers are GFP variants fused to F-actin binding domains of mouse talin (GFP-mTn), Arabidopsis fimbrin1 (GFP-FABD2) or yeast Abp140 (Lifeact-GFP). Although numerous reports describing applications of one, or occasionally more, of these markers, are available in the literature, a direct quantitative comparison of the performance of all three markers at different expression levels has been missing. Here, we analyze F-actin organization and growth rate displayed by tobacco pollen tubes expressing YFP-mTn, YFP-FABD2 or Lifeact-YFP at different levels. Results obtained establish that: (1) all markers strongly affect F-actin organization and cell expansion at high expression levels, (2) YFP-mTn and Lifeact-YFP non-invasively label the same F-actin structures (longitudinally oriented filaments in the shank, a subapical fringe) at low expression levels, (3) Lifeact-YFP displays a somewhat lower potential to affect F-actin organization and cell expansion than YFP-mTn, and (4) YFP-FABD2 generally fails to label F-actin structures at the pollen tube tip and affects F-actin organization as well as cell expansion already at lowest expression levels. As pointed out in the discussion, these observations (1) are also meaningful for F-actin labeling in other cell types, which generally respond less sensitively to F-actin perturbation than pollen tubes, (2) help selecting suitable markers for future F-actin labeling experiments, and (3) support the assessment of a substantial amount of published data resulting from such experiments.

Challenge Integrity: The Cell-Penetrating Peptide BP100 Interferes with the Auxin-Actin Oscillator

Actin filaments are essential for the integrity of the cell membrane. In addition to this structural role, actin can modulate signaling by altering polar auxin flow. On the other hand, the organization of actin filaments is modulated by auxin constituting a self-referring signaling hub. Although the function of this auxin–actin oscillator is not clear, there is evidence for a functional link with stress signaling activated by the NADPH oxidase Respiratory burst oxidase Homolog (RboH). In the current work, we used the cell-penetrating peptide BP100 to induce a mild and transient perturbation of membrane integrity. We followed the response of actin to the BP100 uptake in a green fluorescent protein (GFP)-tagged actin marker line of tobacco Bright Yellow 2 (BY-2) cells by spinning disc confocal microscopy. We observed that BP100 enters in a stepwise manner and reduces the extent of actin remodeling. This actin ‘freezing’ can be rescued by the natural auxin IAA, and mimicked by the auxin-efflux inhibitor 1-napthylphthalamic acid (NPA). We further tested the role of the membrane-localized NADPH oxidase RboH using the specific inhibitor diphenyl iodonium (DPI), and found that DPI acts antagonistically to BP100, although DPI alone can induce a similar actin ‘freezing’ as well. We propose a working model, where the mild violation of membrane integrity by BP100 stimulates RboH, and the resulting elevated levels of reactive oxygen species interfere with actin dynamicity. The mitigating effect of auxin is explained by competition of auxin- and RboH-triggered signaling for superoxide anions. This self-referring auxin–actin–RboH hub might be essential for integrity sensing.

CASEIN KINASE1-LIKE PROTEIN2 Regulates Actin Filament Stability and Stomatal Closure via Phosphorylation of Actin Depolymerizing Factor

The opening and closing of stomata are crucial for plant photosynthesis and transpiration. Actin filaments undergo dynamic reorganization during stomatal closure, but the underlying mechanism for this cytoskeletal reorganization remains largely unclear. In this study, we identified and characterized Arabidopsis thaliana casein kinase 1-like protein 2 (CKL2), which responds to abscisic acid (ABA) treatment and participates in ABA- and drought-induced stomatal closure. Although CKL2 does not bind to actin filaments directly and has no effect on actin assembly in vitro, it colocalizes with and stabilizes actin filaments in guard cells. Further investigation revealed that CKL2 physically interacts with and phosphorylates actin depolymerizing factor 4 (ADF4) and inhibits its activity in actin filament disassembly. During ABA-induced stomatal closure, deletion of CKL2 in Arabidopsis alters actin reorganization in stomata and renders stomatal closure less sensitive to ABA, whereas deletion of ADF4 impairs the disassembly of actin filaments and causes stomatal closure to be more sensitive to ABA Deletion of ADF4 in the ckl2 mutant partially recues its ABA-insensitive stomatal closure phenotype. Moreover, Arabidopsis ADFs from subclass I are targets of CKL2 in vitro. Thus, our results suggest that CKL2 regulates actin filament reorganization and stomatal closure mainly through phosphorylation of ADF.

Arabidopsis FH1 Formin Affects Cotyledon Pavement Cell Shape by Modulating Cytoskeleton Dynamics

Plant cell morphogenesis involves concerted rearrangements of microtubules and actin microfilaments. We previously reported that FH1, the main Arabidopsis thaliana housekeeping Class I membrane-anchored formin, contributes to actin dynamics and microtubule stability in rhizodermis cells. Here we examine the effects of mutations affecting FH1 (At3g25500) on cell morphogenesis and above-ground organ development in seedlings, as well as on cytoskeletal organization and dynamics, using a combination of confocal and variable angle epifluorescence microscopy with a pharmacological approach. Homozygous fh1 mutants exhibited cotyledon epinasty and had larger cotyledon pavement cells with more pronounced lobes than the wild type. The pavement cell shape alterations were enhanced by expression of the fluorescent microtubule marker GFP-microtubule-associated protein 4 (MAP4). Mutant cotyledon pavement cells exhibited reduced density and increased stability of microfilament bundles, as well as enhanced dynamics of microtubules. Analogous results were also obtained upon treatments with the formin inhibitor SMIFH2 (small molecule inhibitor of formin homology 2 domains). Pavement cell shape in wild-type (wt) and fh1 plants in some situations exhibited a differential response towards anti-cytoskeletal drugs, especially the microtubule disruptor oryzalin. Our observations indicate that FH1 participates in the control of microtubule dynamics, possibly via its effects on actin, subsequently influencing cell morphogenesis and macroscopic organ development.

Actin3 promoter reveals undulating F-actin bundles at shanks and dynamic F-actin meshworks at tips of tip-growing pollen tubes

The dynamic actin cytoskeleton of pollen tubes is both the driver of the tip growth and the organizer of cell polarity. In order to understand this fast re-arranging cytoskeletal system, we need reliable constructs expressed under relevant promoters. Here we are reporting that the Lifeact reporter, expressed under the pollen-specific Actin3 promoter, visualizes very dynamic F-actin elements both in germinating pollen grains and tip-growing pollen tubes. Importantly, we have documented very active actin polymerization at the cell periphery, especially in the bulging area during pollen germination and in the apical clear zone. Expression of the Lifeact reporter under control of the pollen-specific Actin3 promoter revealed 2 new aspects: (i) long F-actin bundles in pollen tube shanks are dynamic, showing undulating movements, (ii) subapical 'actin collars' or 'fringes' are absent.

Allyl Isothiocyanate Inhibits Actin-Dependent Intracellular Transport in Arabidopsis thaliana

Volatile allyl isothiocyanate (AITC) derives from the biodegradation of the glucosinolate sinigrin and has been associated with growth inhibition in several plants, including the model plant Arabidopsis thaliana. However, the underlying cellular mechanisms of this feature remain scarcely investigated in plants. In this study, we present evidence of an AITC-induced inhibition of actin-dependent intracellular transport in A. thaliana. A transgenic line of A. thaliana expressing yellow fluorescent protein (YFP)-tagged actin filaments was used to show attenuation of actin filament movement by AITC. This appeared gradually in a time- and dose-dependent manner and resulted in actin filaments appearing close to static. Further, we employed four transgenic lines with YFP-fusion proteins labeling the Golgi apparatus, endoplasmic reticulum (ER), vacuoles and peroxisomes to demonstrate an AITC-induced inhibition of actin-dependent intracellular transport of or, in these structures, consistent with the decline in actin filament movement. Furthermore, the morphologies of actin filaments, ER and vacuoles appeared aberrant following AITC-exposure. However, AITC-treated seedlings of all transgenic lines tested displayed morphologies and intracellular movements similar to that of the corresponding untreated and control-treated plants, following overnight incubation in an AITC-absent environment, indicating that AITC-induced decline in actin-related movements is a reversible process. These findings provide novel insights into the cellular events in plant cells following exposure to AITC, which may further expose clues to the physiological significance of the glucosinolate-myrosinase system.

Arp2/3 complex subunit ARPC2 binds to microtubules

Arp2/3 complex plays a fundamental role in the nucleation of actin filaments (AFs) in yeasts, plants, and animals. In plants, the aberrant shaping and elongation of several types of epidermal cells observed in Arp2/3 complex knockout plant mutants suggest the importance of Arp2/3-mediated actin nucleation for various morphogenetic processes. Here we show that ARPC2, a core Arp2/3 complex subunit, interacts with both actin filaments (AFs) and microtubules (MTs). Plant GFP-ARPC2 expressed in Nicotiana tabacum BY-2 cells, leaf epidermal cells of Nicotiana benthamiana and root epidermal cells of Arabidopsis thaliana decorated MTs. The interaction with MTs was demonstrated by pharmacological approach selectively interfering with either AFs or MTs dynamics as well as by the in vitro co-sedimentation assays. A putative MT-binding domain of tobacco NtARPC2 protein was identified using the co-sedimentation of several truncated NtARPC2 proteins with MTs. Newly identified MT-binding ability of ARPC2 subunit of Arp2/3 complex may represent a new molecular mechanism of AFs and MTs interaction.

Plant cytokinesis-No ring, no constriction but centrifugal construction of the partitioning membrane

Plants have evolved a unique way of partitioning the cytoplasm of dividing cells: Instead of forming a contractile ring that constricts the plasma membrane, plant cells target membrane vesicles to the plane of division where the vesicles fuse with one another to form the partitioning membrane. Plant cytokinesis starts in the centre and progresses towards the periphery, culminating in the fusion of the partitioning membrane with the parental plasma membrane. This membrane dynamics is orchestrated by a specific cytoskeletal array named phragmoplast that originates from interzone spindle remnants. Here we review the properties of the process as well as molecules that play specific roles in that process.

Molecular principles of membrane microdomain targeting in plants

Plasma membranes (PMs) are heterogeneous lipid bilayers comprising diverse subdomains. These sites can be labeled by various proteins in vivo and may serve as hotspots for signal transduction. They are found at apical, basal, and lateral membranes of polarized cells, at cell equatorial planes, or almost isotropically distributed throughout the PM. Recent advances in imaging technologies and understanding of mechanisms that allow proteins to target specific sites in PMs have provided insights into the dynamics and complexity of their specific segregation. Here we present a comprehensive overview of the different types of membrane microdomain and describe the molecular modes that determine site-directed targeting of membrane-resident proteins at the PM.

Signaling to actin stochastic dynamics

Advances in microscopy techniques applied to living cells have dramatically transformed our view of the actin cytoskeleton as a framework for cellular processes. Conventional fluorescence imaging and static analyses are useful for quantifying cellular architecture and the network of filaments that support vesicle trafficking, organelle movement, and response to biotic stress. However, new imaging techniques have revealed remarkably dynamic features of individual actin filaments and the mechanisms that underpin their construction and turnover. In this review, we briefly summarize knowledge about actin and actin-binding proteins in plant systems. We focus on the quantitative properties of the turnover of individual actin filaments, highlight actin-binding proteins that participate in actin dynamics, and summarize the current genetic evidence that has been used to dissect specific aspects of the stochastic dynamics model. Finally, we describe some signaling pathways in which recent data implicate changes in actin filament dynamics and the associated cytoplasmic responses.

F-actin forms mobile and unwinding ring-shaped structures in germinating Arabidopsis pollen expressing Lifeact

The flowering plant pollen tube is the fastest elongating plant cell and transports the sperm cells for double fertilization. The highly dynamic formation and reorganization of the actin cytoskeleton is essential for pollen germination and pollen tube growth. To drive pollen-specific expression of fluorescent marker proteins, commonly the strong Lat52 promoter is used. Here we show by quantitative fluorescent analysis that the gametophyte-specific ARO1 promoter from Arabidopsis drives an about 3.5 times weaker transgene expression than the Lat52 promoter. In one third of the pollen of F-actin-labeled ARO1p:tagRFP-T-Lifeact transgenic lines we observed mobile ring-shaped actin structures in pollen grains and pollen tubes. Pollen tube growth, transgene transmission and seed production were not affected by tagRFP-T-Lifeact expression. F-actin rings were able to integrate into emerging actin filaments and they may reflect a particular physiological state of the pollen or a readily available storage form provided for rapid actin network remodeling.

Actin marker lines in grapevine reveal a gatekeeper function of guard cells

Resistance to abiotic and biotic stress is a central topic for sustainable agriculture, especially in grapevine, one of the field crops with the highest economic output per acreage. As early cellular factors for plant defense, actin microfilaments (AF) are of high relevance. We therefore generated a transgenic actin marker line for grapevine by expressing a fusion protein between green fluorescent protein and the second actin-binding domain of Arabidopsis (Arabidopsis thaliana) fimbrin, AtFIM1. Based on this first cytoskeletal-marker line in grapevine, the response of AFs to phytopathogenic microorganisms could be followed in vivo. Upon inoculation with fluorescently labeled strains of phytopathogenic bacteria, actin responses were confined to the guard cells. In contrast, upon contact with zoospores of Plasmopara viticola, not only the guard cells, but also epidermal pavement cells, where no zoospores had attached responded with the formation of a perinuclear actin basket. Our data support the hypothesis that guard cells act as pacemakers of defense, dominating the responses of the remaining epidermal cells.

The plant cytoskeleton, NET3C, and VAP27 mediate the link between the plasma membrane and endoplasmic reticulum

The cortical endoplasmic reticulum (ER) network in plants is a highly dynamic structure, and it contacts the plasma membrane (PM) at ER-PM anchor/contact sites. These sites are known to be essential for communication between the ER and PM for lipid transport, calcium influx, and ER morphology in mammalian and fungal cells. The nature of these contact sites is unknown in plants, and here, we have identified a complex that forms this bridge. This complex includes (1) NET3C, which belongs to a plant-specific superfamily (NET) of actin-binding proteins, (2) VAP27, a plant homolog of the yeast Scs2 ER-PM contact site protein, and (3) the actin and microtubule networks. We demonstrate that NET3C and VAP27 localize to puncta at the PM and that NET3C and VAP27 form homodimers/oligomers and together form complexes with actin and microtubules. We show that F-actin modulates the turnover of NET3C at these puncta and microtubules regulate the exchange of VAP27 at the same sites. Based on these data, we propose a model for the structure of the plant ER-PM contact sites.

HopW1 from Pseudomonas syringae disrupts the actin cytoskeleton to promote virulence in Arabidopsis

A central mechanism of virulence of extracellular bacterial pathogens is the injection into host cells of effector proteins that modify host cellular functions. HopW1 is an effector injected by the type III secretion system that increases the growth of the plant pathogen Pseudomonas syringae on the Columbia accession of Arabidopsis. When delivered by P. syringae into plant cells, HopW1 causes a reduction in the filamentous actin (F-actin) network and the inhibition of endocytosis, a known actin-dependent process. When directly produced in plants, HopW1 forms complexes with actin, disrupts the actin cytoskeleton and inhibits endocytosis as well as the trafficking of certain proteins to vacuoles. The C-terminal region of HopW1 can reduce the length of actin filaments and therefore solubilize F-actin in vitro. Thus, HopW1 acts by disrupting the actin cytoskeleton and the cell biological processes that depend on actin, which in turn are needed for restricting P. syringae growth in Arabidopsis.

The availability of filament ends modulates actin stochastic dynamics in live plant cells

A network of individual filaments that undergoes incessant remodeling through a process known as stochastic dynamics comprises the cortical actin cytoskeleton in plant epidermal cells. From images at high spatial and temporal resolution, it has been inferred that the regulation of filament barbed ends plays a central role in choreographing actin organization and turnover. How this occurs at a molecular level, whether different populations of ends exist in the array, and how individual filament behavior correlates with the overall architecture of the array are unknown. Here we develop an experimental system to modulate the levels of heterodimeric capping protein (CP) and examine the consequences for actin dynamics, architecture, and cell expansion. Significantly, we find that all phenotypes are the opposite for CP-overexpression (OX) cells compared with a previously characterized cp-knockdown line. Specifically, CP OX lines have fewer filament-filament annealing events, as well as reduced filament lengths and lifetimes. Further, cp-knockdown and OX lines demonstrate the existence of a subpopulation of filament ends sensitive to CP concentration. Finally, CP levels correlate with the biological process of axial cell expansion; for example, epidermal cells from hypocotyls with reduced CP are longer than wild-type cells, whereas CP OX lines have shorter cells. On the basis of these and other genetic studies in this model system, we hypothesize that filament length and lifetime positively correlate with the extent of axial cell expansion in dark-grown hypocotyls.

Live cell imaging reveals actin-cytoskeleton-induced self-association of the actin-bundling protein WLIM1

Crosslinking of actin filaments into bundles is essential for the assembly and stabilization of specific cytoskeletal structures. However, relatively little is known about the molecular mechanisms underlying actin bundle formation. The two LIM-domain-containing proteins define a novel and evolutionarily conserved family of actin-bundling proteins whose actin-binding and -crosslinking activities primarily rely on their LIM domains. Using TIRF microscopy, we describe real-time formation of actin bundles induced by tobacco NtWLIM1 in vitro. We show that NtWLIM1 binds to single filaments and subsequently promotes their interaction and zippering into tight bundles of mixed polarity. NtWLIM1-induced bundles grew by both elongation of internal filaments and addition of preformed fragments at their extremities. Importantly, these data are highly consistent with the modes of bundle formation and growth observed in transgenic Arabidopsis plants expressing a GFP-fused Arabidopsis AtWLIM1 protein. Using two complementary live cell imaging approaches, a close relationship between NtWLIM1 subcellular localization and self-association was established. Indeed, both BiFC and FLIM-FRET data revealed that, although unstable NtWLIM1 complexes can sporadically form in the cytosol, stable complexes concentrate along the actin cytoskeleton. Remarkably, disruption of the actin cytoskeleton significantly impaired self-association of NtWLIM1. In addition, biochemical analyses support the idea that F-actin facilitates the switch of purified recombinant NtWLIM1 from a monomeric to a di- or oligomeric state. On the basis of our data, we propose a model in which actin binding promotes the formation and stabilization of NtWLIM1 complexes, which in turn might drive the crosslinking of actin filaments.

ACTIN DEPOLYMERIZING FACTOR4 regulates actin dynamics during innate immune signaling in Arabidopsis

Conserved microbe-associated molecular patterns (MAMPs) are sensed by pattern recognition receptors (PRRs) on cells of plants and animals. MAMP perception typically triggers rearrangements to actin cytoskeletal arrays during innate immune signaling. However, the signaling cascades linking PRR activation by MAMPs to cytoskeleton remodeling are not well characterized. Here, we developed a system to dissect, at high spatial and temporal resolution, the regulation of actin dynamics during innate immune signaling in plant cells. Within minutes of MAMP perception, we detected changes to single actin filament turnover in epidermal cells treated with bacterial and fungal MAMPs. These MAMP-induced alterations phenocopied an ACTIN DEPOLYMERIZING FACTOR4 (ADF4) knockout mutant. Moreover, actin arrays in the adf4 mutant were unresponsive to a bacterial MAMP, elf26, but responded normally to the fungal MAMP, chitin. Together, our data provide strong genetic and cytological evidence for the inhibition of ADF activity regulating actin remodeling during innate immune signaling. This work is the first to directly link an ADF/cofilin to the cytoskeletal rearrangements elicited directly after pathogen perception in plant or mammalian cells.

Visualizing and quantifying the in vivo structure and dynamics of the Arabidopsis cortical cytoskeleton using CLSM and VAEM

The cortical microtubules, and to some extent also the actin meshwork, play a central role in the shaping of plant cells. Transgenic plants expressing fluorescent protein markers specifically tagging the two main cytoskeletal systems are available, allowing noninvasive in vivo studies. Advanced microscopy techniques, in particular confocal laser scanning microscopy (CLSM) and variable angle epifluorescence microscopy (VAEM), can be nowadays used for imaging the cortical cytoskeleton of living cells with unprecedented spatial and temporal resolution. With the aid of suitable computing techniques, quantitative information can be extracted from microscopic images and video sequences, providing insight into both architecture and dynamics of the cortical cytoskeleton.

The actin cytoskeleton in root hairs: all is fine at the tip

Filamentous actin forms characteristic bundles in plant cells that facilitate cytoplasmic streaming. In contrast, networks of actin exhibiting fast turnover are found especially near sites of rapid cell expansion. These networks may serve various functions including delivering and retaining vesicles while preventing penetration of organelles into the area where cell growth occurs thereby allowing fast turnover of vesicles to and from the plasma membrane. Root hairs elongate by polarized growth at their tips and the local accumulation of fine F-actin near the tip has provided valuable insight into the organization of these networks. Here we will sequentially focus on the role of the actin cytoskeleton in root hair tip growth and on how activities of different actin binding proteins in the apical part of growing root hairs contribute to build the fine F-actin configuration that correlates with tip growth.

Actin dynamics in the cortical array of plant cells

The actin cytoskeleton changes in organization and dynamics as cellular functions are reprogrammed following responses to diverse stimuli, hormones, and developmental cues. How this is choreographed and what molecular players are involved in actin remodeling continues to be an area of intense scrutiny. Advances in imaging modalities and fluorescent fusion protein reporters have illuminated the strikingly dynamic behavior of single actin filaments at high spatial and temporal resolutions. This led to a model for the stochastic dynamic turnover of actin filaments and predicted the actions and responsibilities of several key actin-binding proteins. Recently, aspects of this model have been tested using powerful genetic strategies in both Arabidopsis and Physcomitrella. Collectively, the latest data emphasize the importance of filament severing activities and regulation of barbed-end availability as key facets of plant actin filament turnover.

Regulation of cytoskeletal dynamics by phospholipase D and phosphatidic acid

Plants respond to diverse biotic and abiotic stimuli as well as to endogenous developmental cues. Many of these stimuli result in altered activity of phospholipase D (PLD), an enzyme that hydrolyzes structural phospholipids producing phosphatidic acid (PA). PA is a key signaling intermediate in animals, but its targets in plants are relatively uncharacterized. Recent studies have demonstrated that the cytoskeleton is a major target of PLD-PA signaling and identified a positive feedback loop between actin turnover and PLD activity. Moreover, two cytoskeletal proteins, capping protein and MAP65-1, have been identified as PA-binding proteins regulating actin and microtubule organization and dynamics. In this review, we highlight the role of the PLD-PA module as an important hub for housekeeping and stress-induced regulation of membrane-associated cytoskeletal dynamics.

The plant actin cytoskeleton responds to signals from microbe-associated molecular patterns

Plants are constantly exposed to a large and diverse array of microbes; however, most plants are immune to the majority of potential invaders and susceptible to only a small subset of pathogens. The cytoskeleton comprises a dynamic intracellular framework that responds rapidly to biotic stresses and supports numerous fundamental cellular processes including vesicle trafficking, endocytosis and the spatial distribution of organelles and protein complexes. For years, the actin cytoskeleton has been assumed to play a role in plant innate immunity against fungi and oomycetes, based largely on static images and pharmacological studies. To date, however, there is little evidence that the host-cell actin cytoskeleton participates in responses to phytopathogenic bacteria. Here, we quantified the spatiotemporal changes in host-cell cytoskeletal architecture during the immune response to pathogenic and non-pathogenic strains of Pseudomonas syringae pv. tomato DC3000. Two distinct changes to host cytoskeletal arrays were observed that correspond to distinct phases of plant-bacterial interactions i.e. the perception of microbe-associated molecular patterns (MAMPs) during pattern-triggered immunity (PTI) and perturbations by effector proteins during effector-triggered susceptibility (ETS). We demonstrate that an immediate increase in actin filament abundance is a conserved and novel component of PTI. Notably, treatment of leaves with a MAMP peptide mimic was sufficient to elicit a rapid change in actin organization in epidermal cells, and this actin response required the host-cell MAMP receptor kinase complex, including FLS2, BAK1 and BIK1. Finally, we found that actin polymerization is necessary for the increase in actin filament density and that blocking this increase with the actin-disrupting drug latrunculin B leads to enhanced susceptibility of host plants to pathogenic and non-pathogenic bacteria.

Oryza sativa actin-interacting protein 1 is required for rice growth by promoting actin turnover

Rapid actin turnover is essential for numerous actin-based processes. However, how it is precisely regulated remains poorly understood. Actin-interacting protein 1 (AIP1) has been shown to be an important factor by acting coordinately with actin-depolymerizing factor (ADF)/cofilin in promoting actin depolymerization, the rate-limiting factor in actin turnover. However, the molecular mechanism by which AIP1 promotes actin turnover remains largely unknown in plants. Here, we provide a demonstration that AIP1 promotes actin turnover, which is required for optimal growth of rice plants. Specific down-regulation of OsAIP1 increased the level of filamentous actin and reduced actin turnover, whereas over-expression of OsAIP1 induced fragmentation and depolymerization of actin filaments and enhanced actin turnover. In vitro biochemical characterization showed that, although OsAIP1 alone does not affect actin dynamics, it enhances ADF-mediated actin depolymerization. It also caps the filament barbed end in the presence of ADF, but the capping activity is not required for their coordinated action. Real-time visualization of single filament dynamics showed that OsAIP1 enhanced ADF-mediated severing and dissociation of pointed end subunits. Consistent with this, the filament severing frequency and subunit off-rate were enhanced in OsAIP1 over-expressors but decreased in RNAi protoplasts. Importantly, OsAIP1 acts coordinately with ADF and profilin to induce massive net actin depolymerization, indicating that AIP1 plays a major role in the turnover of actin, which is required to optimize F-actin levels in plants.

Dynamic actin controls polarity induction de novo in protoplasts

Cell polarity and axes are central for plant morphogenesis. To study how polarity and axes are induced de novo, we investigated protoplasts of tobacco Nicotiana tabacum cv. BY-2 expressing fluorescently-tagged cytoskeletal markers. We standardized the system to such a degree that we were able to generate quantitative data on the temporal patterns of regeneration stages. The synthesis of a new cell wall marks the transition to the first stage of regeneration, and proceeds after a long preparatory phase within a few minutes. During this preparatory phase, the nucleus migrates actively, and cytoplasmic strands remodel vigorously. We probed this system for the effect of anti-cytoskeletal compounds, inducible bundling of actin, RGD-peptides, and temperature. Suppression of actin dynamics at an early stage leads to aberrant tripolar cells, whereas suppression of microtubule dynamics produces aberrant sausage-like cells with asymmetric cell walls. We integrated these data into a model, where the microtubular cytoskeleton conveys positional information between the nucleus and the membrane controlling the release or activation of components required for cell wall synthesis. Cell wall formation is followed by the induction of a new cell pole requiring dynamic actin filaments, and the new cell axis is manifested as elongation growth perpendicular to the orientation of the aligned cortical microtubules.