Actin-based vesicle dynamics and exocytosis during wound wall formation in characean internodal cells

Re-Establishment Techniques and Transplantations of Charophytes to Support Threatened Species

Re-establishment of submerged macrophytes and especially charophyte vegetation is a common aim in lake management. If revegetation does not happen spontaneously, transplantations may be a suitable option. Only rarely have transplantations been used as a tool to support threatened submerged macrophytes and, to a much lesser extent, charophytes. Such actions have to consider species-specific life strategies. K-strategists mainly inhabit permanent habitats, are perennial, have low fertility and poor dispersal ability, but are strong competitors and often form dense vegetation. R-strategists are annual species, inhabit shallow water and/or temporary habitats, and are richly fertile. They disperse easily but are weak competitors. While K-strategists easily can be planted as green biomass taken from another site, rare R-strategists often must be reproduced in cultures before they can be planted on-site. In Sweden, several charophyte species are extremely rare and fail to (re)establish, though apparently suitable habitats are available. Limited dispersal and/or lack of diaspore reservoirs are probable explanations. Transplantations are planned to secure the occurrences of these species in the country. This contribution reviews the knowledge on life forms, dispersal, establishment, and transplantations of submerged macrophytes with focus on charophytes and gives recommendations for the Swedish project.

Brefeldin A inhibits clathrin-dependent endocytosis and ion transport in Chara internodal cells

BACKGROUND

The Characeae are multicellular green algae, which are closely related to higher plants. Their internodal cells are a convenient model to study membrane transport and organelle interactions.

RESULTS

In this study, we report on the effect of brefeldin A (BFA), an inhibitor of vesicle trafficking, on internodal cells of Chara australis. BFA induced the commonly observed agglomeration of Golgi bodies and trans Golgi network into 'brefeldin compartments' at concentrations between 6 and 500 μM and within 30-120 min treatment. In contrast to most other cells, however, BFA inhibited endocytosis and significantly decreased the number of clathrin-coated pits and clathrin-coated vesicles at the plasma membrane. BFA did not inhibit secretion of organelles at wounds induced by puncturing or local light damage but prevented the formation of cellulosic wound walls probably because of insufficient membrane recycling. We also found that BFA inhibited the formation of alkaline and acid regions along the cell surface ('pH banding pattern') which facilitates carbon uptake required for photosynthesis; we hypothesise that this is due to insufficient recycling of ion transporters. During long-term treatments over several days, BFA delayed the formation of complex 3D plasma membranes (charasomes). Interestingly, BFA had no detectable effect on clathrin-dependent charasome degradation. Protein sequence analysis suggests that the peculiar effects of BFA in Chara internodal cells are due to a mutation in the guanine-nucleotide exchange factor GNOM required for recruitment of membrane coats via activation of ADP-ribosylation factor proteins.

CONCLUSIONS AND SIGNIFICANCE

This work provides an overview on the effects of BFA on different processes in C. australis. It revealed similarities but also distinct differences in vesicle trafficking between higher plant and algal cells. It shows that characean internodal cells are a promising model to study interactions between seemingly distant metabolic pathways.

Identification and characterization of LIM gene family in Brassica rapa

BACKGROUND

LIM (Lin-11, Isl-1 and Mec-3 domains) genes have been reported to trigger the formation of actin bundles, a major higher-order cytoskeletal assembly, in higher plants; however, the stress resistance related functions of these genes are still not well known. In this study, we collected 22 LIM genes designated as Brassica rapa LIM (BrLIM) from the Brassica database, analyzed the sequences, compared them with LIM genes of other plants and analyzed their expression after applying biotic and abiotic stresses in Chinese cabbage.

RESULTS

Upon sequence analysis these genes were confirmed as LIM genes and found to have a high degree of homology with LIM genes of other species. These genes showed distinct clusters when compared to other recognized LIM proteins upon phylogenetic analysis. Additionally, organ specific expression of these genes was observed in Chinese cabbage plants, with BrPLIM2a, b, c, BrDAR1, BrPLIM2e, f and g only being expressed in flower buds. Furthermore, the expression of these genes (except for BrDAR1 and BrPLIM2e) was high in the early flowering stages. The remaining genes were expressed in almost all organs tested. All BrDAR genes showed higher expression in flower buds compared to other organs. These organ specific expressions were clearly correlated with the phylogenetic grouping. In addition, BrWLIM2c and BrDAR4 responded to Fusarium oxysporum f. sp. conglutinans infection, while commonly two BrDARs and eight BrLIMs responded to cold, ABA and pH (pH5, pH7 and pH9) stress treatments in Chinese cabbage plants.

CONCLUSION

Taken together, the results of this study indicate that BrLIM and BrDAR genes may be involved in resistance against biotic and abiotic stresses in Brassica.

Characean internodal cells as a model system for the study of cell organization

Giant internodal cells of characean green algae have been widely used for studying cellular physiology. This review emphasizes their significance for understanding cytoarchitecture and cytoplasmic reorganization. The cytoarchitecture of internodal cells undergoes pronounced, cytoskeleton-dependent changes during development and in response to environmental cues. Under bright light, internodes develop alternating bands of acid and alkaline pH at their surface that correlate with the differential size and abundance of cortical organelles and, in the genus Chara, with the size and distribution of convoluted plasma membrane domains known as charasomes. Wounding induces responses ranging from chloroplast detachment to deposition of wound walls. These properties and the possibility for mechanical manipulation make the internodal cell ideal for exploring plasma membrane domains, organelle interactions, vesicle trafficking, and local cell wall deposition. The significance of this model system will further increase with the application of molecular biological methods in combination with metabolomics and proteomics.

Cytoplasmic streaming in plant cells emerges naturally by microfilament self-organization

Many cells exhibit large-scale active circulation of their entire fluid contents, a process termed cytoplasmic streaming. This phenomenon is particularly prevalent in plant cells, often presenting strikingly regimented flow patterns. The driving mechanism in such cells is known: myosin-coated organelles entrain cytoplasm as they process along actin filament bundles fixed at the periphery. Still unknown, however, is the developmental process that constructs the well-ordered actin configurations required for coherent cell-scale flow. Previous experimental works on streaming regeneration in cells of Characean algae, whose longitudinal flow is perhaps the most regimented of all, hint at an autonomous process of microfilament self-organization driving the formation of streaming patterns during morphogenesis. Working from first principles, we propose a robust model of streaming emergence that combines motor dynamics with both microscopic and macroscopic hydrodynamics to explain how several independent processes, each ineffectual on its own, can reinforce to ultimately develop the patterns of streaming observed in the Characeae and other streaming species.

The characean internodal cell as a model system for studying wound healing

This work describes the characean internodal cell as a model system for the study of wound healing and compares wounds induced by certain chemicals and UV irradiation with wounds occurring in the natural environment. We review the existing literature and define three types of wound response: (1) cortical window formation characterised by disassembly of microtubules, transient inhibition of actin-dependent cytoplasmic streaming and chloroplast detachment, (2) fibrillar wound walls characterised by exocytosis of vesicles carrying wall polysaccharides and membrane-bound cellulose synthase complexes coupled with endocytosis of surplus membrane and (3) amorphous, callose- and membrane-containing wound walls characterised by exocytosis of vesicles and endoplasmic reticulum cisternae in the absence of membrane recycling. We hypothesize that these three wound responses reflect the extent of damage, probably Ca(2+) influx, and that the secretion of Ca(2+) -loaded endoplasmic reticulum cisternae is an emergency reaction in case of severe Ca(2+) load. Microtubules are not required for wound healing but their disassembly could have a signalling function. Transient reorganisation of the actin cytoskeleton into a meshwork of randomly oriented filaments is required for the migration of wound wall forming organelles, just as occurs in tip-growing plant cells. New data presented in this study show that during the deposition of an amorphous wound wall numerous actin rings are present, which may indicate specific ion fluxes and/or a storage form for actin. In addition, we present new evidence for the exocytosis of FM1-43-stained organelles, putative endosomes, required for plasma membrane repair during wound healing. Finally, we show that quickly growing fibrillar wound walls, even when deposited in the absence of microtubules, have a highly ordered helical structure of consistent handedness comprised of cellulose microfibrils.

Turnover of Phosphatidic Acid through Distinct Signaling Pathways Affects Multiple Aspects of Pollen Tube Growth in Tobacco

Phosphatidic acid (PA) is an important intermediate in membrane lipid metabolism that acts as a key component of signaling networks, regulating the spatio-temporal dynamics of the endomembrane system and the cytoskeleton. Using tobacco pollen tubes as a model, we addressed the signaling effects of PA by probing the functions of three most relevant enzymes that regulate the production and degradation of PA, namely, phospholipases D (PLD), diacylglycerol kinases (DGKs), and lipid phosphate phosphatases (LPPs). Phylogenetic analysis indicated a highly dynamic evolution of all three lipid-modifying enzymes in land plants, with many clade-specific duplications or losses and massive diversification of the C2-PLD family. In silico transcriptomic survey revealed increased levels of expression of all three PA-regulatory genes in pollen development (particularly the DGKs). Using specific inhibitors we were able to distinguish the contributions of PLDs, DGKs, and LPPs into PA-regulated processes. Thus, suppressing PA production by inhibiting either PLD or DGK activity compromised membrane trafficking except early endocytosis, disrupted tip-localized deposition of cell wall material, especially pectins, and inhibited pollen tube growth. Conversely, suppressing PA degradation by inhibiting LPP activity using any of three different inhibitors significantly stimulated pollen tube growth, and similar effect was achieved by suppressing the expression of tobacco pollen LPP4 using antisense knock-down. Interestingly, inhibiting specifically DGK changed vacuolar dynamics and the morphology of pollen tubes, whereas inhibiting specifically PLD disrupted the actin cytoskeleton. Overall, our results demonstrate the critical importance of all three types of enzymes involved in PA production and degradation, with strikingly different roles of PA produced by the PLD and DGK pathways, in pollen tube growth.

Actin-dependent deposition of putative endosomes and endoplasmic reticulum during early stages of wound healing in characean internodal cells

We investigated the behaviour of organelles stained with FM1-43 (putative endosomes) and/or LysoTracker Red (LTred; acidic compartments) and of the endoplasmic reticulum (ER) during healing of puncture and UV-induced wounds in internodal cells of Nitella flexilis and Chara corallina. Immediately after puncture, wounds were passively sealed with a plug of solid vacuolar inclusions, onto which a bipartite wound wall was actively deposited. The outer, callose-containing amorphous layer consisted of remnants of FM1-43- and LTred-labelled organelles, ER cisternae and polysaccharide-containing secretory vesicles, which became deposited in the absence of membrane retrieval (compound exocytosis). During formation of the inner cellulosic layer, exocytosis of secretory vesicles with the newly formed plasma membrane is coupled to endocytosis via coated vesicles. Migration of FM1-43- and LTred-stained organelles, ER and secretory vesicles towards the cell cortex and deposition of a bipartite wound wall could also be induced by spot-like irradiation with ultraviolet light. Cytochalasin D reversibly inhibited the accumulation and deposition of organelles. Our study indicates that active actin-dependent deposition of putative recycling endosomes is required for wound healing (plasma membrane repair) and supports the hypothesis that deposition of ER cisternae helps to restore wounding-disturbed Ca(2+) metabolism.

The actin multigene family in Populus: organization, expression and phylogenetic analysis

Despite the significance of actin in plant growth and development, little is known of the structure, expression and evolution of the actin gene family in woody plants. In this study, we systematically examined the diversification of the actin gene family in Populus by integrating genomic organization, expression, and phylogeny data. Genome-wide analysis of the Populus genome indicated that actin is a multigene family consisting of eight members, all predicted to encode 377-amino acid polypeptides that share high sequence homology ranging from 94.2 to 100% identity. Microarray and real-time PCR expression analysis showed that the PtrACT family members are differentially expressed in different tissues, exhibiting overlapping and unique expression patterns. Of particular interest, all PtrACT genes have been found to be preferentially expressed in the stem phloem and xylem, suggesting that poplar PtrACTs are involved in the wood formation. Gene structural and phylogenetic analyses revealed that the PtrACT family is composed of two main subgroups that share an ancient common ancestor. Extremely high intraspecies synonymous nucleotide diversity of pi(syn) = 0.01205 was detected, and the pi(non-syn)/pi(syn) ratio was significantly less than 1; therefore, the PtACT1 appears to be evolving in Populus, primarily under purifying selection. We demonstrated that the actin gene family in Populus is divided into two distinct subgroups, suggesting functional divergence. The results reported here will be useful in conducting future functional genomics studies to understand the detailed function of actin genes in tree growth and development.

Plasma membrane-associated SCAR complex subunits promote cortical F-actin accumulation and normal growth characteristics in Arabidopsis roots

The ARP2/3 complex, a highly conserved nucleator of F-actin polymerization, and its activator, the SCAR complex, have been shown to play important roles in leaf epidermal cell morphogenesis in Arabidopsis. However, the intracellular site(s) and function(s) of SCAR and ARP2/3 complex-dependent actin polymerization in plant cells remain unclear. We demonstrate that putative SCAR complex subunits BRK1 and SCAR1 are localized to the plasma membrane at sites of cell growth and wall deposition in expanding cells of leaves and roots. BRK1 localization is SCAR-dependent, providing further evidence of an association between these proteins in vivo. Consistent with plasma membrane localization of SCAR complex subunits, cortical F-actin accumulation in root tip cells is reduced in brk1 mutants. Moreover, mutations disrupting the SCAR or ARP2/3 complex reduce the growth rate of roots and their ability to penetrate semi-solid medium, suggesting reduced rigidity. Cell walls of mutant roots exhibit abnormal structure and composition at intercellular junctions where BRK1 and SCAR1 are enriched in the adjacent plasma membrane. Taken together, our results suggest that SCAR and ARP2/3 complex-dependent actin polymerization promotes processes at the plasma membrane that are important for normal growth and wall assembly.

FM dyes label sterol-rich plasma membrane domains and are internalized independently of the cytoskeleton in characean internodal cells

We applied the endocytic markers FM1-43, FM4-64 and filipin to internodal cells of the green alga Chara corallina. Both FM dyes stained stable, long-living plasma membrane patches with a diameter of up to 1 microm. After 5 min, FM dyes labeled cortical, trembling structures up to 500 nm in size. After 15 min, FM dyes localized to endoplasmic organelles up to 1 microm in diameter, which migrated actively along actin bundles or participated in cytoplasmic mass streaming. After 30-60 min, FM fluorescence appeared in the membrane of small, endoplasmic vacuoles but not in that of the central vacuole. Some of the FM-labeled organelles were also stained by neutral red and lysotracker yellow, indicative of acidic compartments. Filipin, a sterol-specific marker, likewise labeled plasma membrane domains which co-localized with the FM patches. However, internalization of filipin could not be observed. KCN, cytochalasin D, latrunculin B and oryzalin had no effect on size, shape and distribution of FM- and filipin-labeled plasma membrane domains. Internalization of FM dyes was inhibited by KCN but not by drugs which interfere with the actin or microtubule cytoskeleton. Our data indicate that the plasma membrane of characean internodal cells contains discrete domains which are enriched in sterols and probably correspond to clusters of lipid rafts. The inhibitor experiments suggest that FM uptake is active but independent of actin filaments, actin polymerization and microtubules. The possible function of the sterol-rich, FM labeled plasma membrane areas and the significance of actin-independent FM internalization (via endocytosis or energy-dependent flippases) are discussed.

Wide-ranging effects of eight cytochalasins and latrunculin A and B on intracellular motility and actin filament reorganization in characean internodal cells

Numerous forms of cytochalasins have been identified and, although they share common biological activity, they may differ considerably in potency. We investigated the effects of cytochalasins A, B, C, D, E, H and J and dihydrocytochalasin B in an ideal experimental system for cell motility, the giant internodal cells of the characean alga Nitella pseudoflabellata. Cytochalasins D (60 microM) and H (30 microM) were found to be most suited for fast and reversible inhibition of actin-based motility, while cytochalasins A and E arrested streaming at lower concentrations but irreversibly. We observed no clear correlation between the ability of cytochalasins to inhibit motility and the actual disruption of the subcortical actin bundle tracks on which myosin-dependent motility occurs. Indeed, the actin bundles remained intact at the time of streaming cessation and disassembled only after one to several days' treatment. Even when applied at concentrations lower than that required to inhibit cytoplasmic streaming, all of the cytochalasins induced reorganization of the more labile cortical actin filaments into actin patches, swirling clusters or short rods. Latrunculins A and B arrested streaming only after disrupting the subcortical actin bundles, a process requiring relatively high concentrations (200 microM) and very long treatment periods of >1 d. Latrunculins, however, worked synergistically with cytochalasins. A 1 h treatment with 15 nM latrunculin A and 4 microM cytochalasin D induced reversible fragmentation of subcortical actin bundles and arrested cytoplasmic streaming. Our findings provide insights into the mechanisms by which cytochalasins and latrunculins interfere with characean actin to inhibit motility.

Living markers for actin block myosin-dependent motility of plant organelles and auxin

Expression-based techniques using recombinant actin-binding proteins (ABPs) have been developed as advantageous means of visualising actin filaments. As actin function is linked to the movement of cellular cargoes, and overexpression of ABPs may compete with endogenous cytoskeletal proteins, such as myosins, secondary effects on cellular motility might be observed during actin visualisation. Cytoplasmic streaming and auxin transport were chosen as examples of cargo movement and investigated in two Arabidopsis thaliana lines stably transformed with fluorescently labelled talin (GFP-mTn) or fimbrin (GFP-FABD2). In both lines, the maximal streaming velocity of organelles was reduced to 80% in hypocotyl epidermal cells, where actin was broadly equally labelled by both ABPs. In contrast, observations of streaming and actin organisation during treatments with cytochalasin D (CD) suggested GFP-mTn-labelled actin to remain more stable. Furthermore, basipetal auxin transport was undisturbed in the GFP-FABD2 line but reduced by GFP-mTn. Remarkably, treatments with CD and 2,3-butanedione monoxime, which immobilizes myosin by impairing its ATPase, produced not only failures in organelle movement but also in basipetal auxin transport in the wild-type. These observations suggest that myosin is involved in processes of auxin translocation. In parallel, reduced motility in transgenic plants may be explained by a disturbed acto-myosin interplay, if overexpressed ABPs block the processive movement of myosin along actin filaments. This report shows that the use of live markers for actin visualisation may affect motility of cellular compounds and underlines the general need for critical investigation of actin-related processes in wild-type as well as transgenic plants prior to further interpretation.

Imaging of dynamic secretory vesicles in living pollen tubes of Picea meyeri using evanescent wave microscopy

Evanescent wave excitation was used to visualize individual, FM4-64-labeled secretory vesicles in an optical slice proximal to the plasma membrane of Picea meyeri pollen tubes. A standard upright microscope was modified to accommodate the optics used to direct a laser beam at a variable angle. Under evanescent wave microscopy or total internal reflection fluorescence microscopy, fluorophores localized near the surface were excited with evanescent waves, which decay exponentially with distance from the interface. Evanescent waves with penetration depths of 60 to 400 nm were generated by varying the angle of incidence of the laser beam. Kinetic analysis of vesicle trafficking was made through an approximately 300-nm optical section beneath the plasma membrane using time-lapse evanescent wave imaging of individual fluorescently labeled vesicles. Two-dimensional trajectories of individual vesicles were obtained from the resulting time-resolved image stacks and were used to characterize the vesicles in terms of their average fluorescence and mobility, expressed here as the two-dimensional diffusion coefficient D2. The velocity and direction of vesicle motions, frame-to-frame displacement, and vesicle trajectories were also calculated. Analysis of individual vesicles revealed for the first time, to our knowledge, that two types of motion are present, and that vesicles in living pollen tubes exhibit complicated behaviors and oscillations that differ from the simple Brownian motion reported in previous investigations. Furthermore, disruption of the actin cytoskeleton had a much more pronounced effect on vesicle mobility than did disruption of the microtubules, suggesting that actin cytoskeleton plays a primary role in vesicle mobility.

Endocytosis and vesicle trafficking during tip growth of root hairs

The directional elongation of root hairs, "tip growth", depends on the coordinated and highly regulated trafficking of vesicles which fill the tip cytoplasm and are active in secretion of cell wall material. So far, little is known about the dynamics of endocytosis in living root hairs. We analyzed the motile behaviour of vesicles in the apical region of living root hairs of Arabidopsis thaliana and of Triticum aestivum by live cell microscopy. For direct observation of endocytosis and of the fate of endocytic vesicles, we used the fluorescent endocytosis marker dyes FM 1-43 and FM 4-64. Rapid endocytosis was detected mainly in the tip, where it caused a bright fluorescence of the apical cytoplasm. The internalized membranes proceeded through highly dynamic putative early endosomes in the clear zone to larger endosomal compartments in the subapical region that are excluded from the clear zone. The internalized cargo ended up in the dynamic vacuole by fusion of large endosomal compartments with the tonoplast. Before export to these lytic compartments, putative early endosomes remained in the apical zone, where they most probably recycled to the plasma membrane and back into the cytoplasm for more than 30 min. Endoplasmic reticulum was not involved in trafficking pathways of endosomes. Actin cytoskeleton was needed for the endocytosis itself, as well as for further membrane trafficking. The actin-depolymerizing drug latrunculin B modified the dynamic properties of vesicles and endosomes; they became immobilized and aggregated in the tip. Treatment with brefeldin A inhibited membrane trafficking and caused the disappearance of FM-containing vesicles and putative early endosomes from the clear zone; labelled structures accumulated in motile brefeldin A-induced compartments. These large endocytic compartments redispersed upon removal of the drug. Our results hence prove that endocytosis occurs in growing root hairs. We show the localization of endocytosis in the tip and indicate specific endomembrane compartments and their recycling.

RAR1, ROR1, and the actin cytoskeleton contribute to basal resistance to Magnaporthe grisea in barley

The fungus Magnaporthe grisea, the causal agent of rice blast disease, is a major pathogen of rice and is capable of producing epidemics on other cultivated cereals, including barley (Hordeum vulgare). We explored the requirements for basal resistance of barley against a compatible M. grisea isolate using both genetic and chemical approaches. Mutants of the RAR1 gene required for the function of major resistance gene-mediated resistance and mutants of the ROR1 and ROR2 genes required for full expression of cell-wall-penetration resistance against powdery mildew pathogens were examined for macroscopic and microscopic alterations in M. grisea growth and symptoms. RAR1 contributed to resistance in epidermis and mesophyll at different stages of fungal infection dependent on the MLO/mlo-5 status. Whereas no ROR2 effect was detected, ROR1 was found to contribute to cell-wall-penetration resistance, at least in the epidermis. Application of the actin agonist cytochalasin E promoted cell wall penetration by M. grisea in a dose-dependent manner, demonstrating an involvement of the actin cytoskeleton in penetration resistance.

La3+ uptake and its effect on the cytoskeleton in root protoplasts of Zea mays L

La(3+) ions are known to antagonize Ca(2+) and are used as a Ca(2+) channel blocker but little is known on the direct effects of La(3+). Micromolar La(3+) concentrations promoted root growth while higher concentrations were inhibitory. The uptake of La(3+) in maize root protoplasts revealed a membrane binding component (0.14 and 0.44 pmol min(-1) protoplast(-1) for 100 and 1,000 microM La(3+)) followed by a slower concentration and time-dependent uptake. Uptake was reduced by Ca(2+), but had no substantial effect on other ions. La(3+) shifted microtubule organization from random to parallel but caused aggregation of microfilaments. Our data suggest that La(3+) is taken up into plant cells and affects growth via stabilization of the cytoskeleton.

The receptor-like MLO protein and the RAC/ROP family G-protein RACB modulate actin reorganization in barley attacked by the biotrophic powdery mildew fungus Blumeria graminis f.sp. hordei

Cytoskeleton remodelling is a crucial process in determining the polarity of dividing and growing plant cells, as well as during interactions with the environment. Nothing is currently known about the proteins, which regulate actin remodelling during interactions with invading pathogens. The biotrophic powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh) invades susceptible barley (Hordeum vulgare L.) by penetrating epidermal cells, which remain intact during fungal development. In contrast, resistant host plants prevent infection by inhibiting penetration through apoplastic mechanisms, which require focusing defence reactions on the site of attack. We stained actin filaments in a susceptible Mlo-genotype and a near-isogenic race-non-specifically resistant barley mlo5-mutant genotype using fluorescence-labelled phalloidin after chemical fixation. This revealed that the actin cytoskeleton is differentially reorganized in susceptible and resistant hosts challenged by Bgh. Actin filaments were polarized towards the sites of attempted penetration in the resistant host, whereas when susceptible hosts were penetrated, a more subtle reorganization took place around fungal haustoria. Strong actin filament focusing towards sites of fungal attack was closely associated with successful prevention of penetration. Actin focusing was less frequent and seemingly delayed in susceptible wild-type barley expressing the susceptibility factor MLO. Additionally, single cell overexpression of a constitutively activated RAC/ROP G-protein, CA RACB, another potential host susceptibility factor and hypothetical actin cytoskeleton regulator, partly inhibited actin reorganization when under attack from Bgh, whereas knockdown of RACB promoted actin focusing. We conclude that RACB and, potentially, MLO are host proteins involved in the modulation of actin reorganization and cell polarity in the interaction of barley with Bgh.

DISTORTED2 encodes an ARPC2 subunit of the putative Arabidopsis ARP2/3 complex

Arabidopsis trichomes are unicellular, branched structures that have highly constrained requirements for the cytoskeleton. The 'distorted group' genes function downstream from microtubule-based branch initiation, and are required during the actin-dependent phase of polarized stalk and branch expansion. Of the eight known 'distorted group' genes, a subset encode homologs of ARP2/3 complex subunits. In eukaryotic cells, the seven-protein ARP2/3 complex nucleates actin filament networks that push on the plasma membrane and organelles. In plants cells, the existence and function of an ARP2/3 complex is unclear. In this paper, we report that DISTORTED2 (DIS2) encodes a paralogue of the ARP2/3 complex subunit ARPC2. DIS2 has ARPC2 activity, based on its ability to rescue the growth defects of arpc2 (arc35Delta) null yeast cells. Like known ARPC2s, DIS2 physically interacts with ARPC4. Mutations in DIS2 cause a distorted trichome phenotype, defects in cell-cell adhesion, and a modest reduction in shoot FW. The actin cytoskeleton in dis2 trichomes is extensive, but developing branches fail to generate and maintain highly organized cytoplasmic actin bundles.

Myo4p and She3p are required for cortical ER inheritance in Saccharomyces cerevisiae

Myo4p is a nonessential type V myosin required for the bud tip localization of ASH1 and IST2 mRNA. These mRNAs associate with Myo4p via the She2p and She3p proteins. She3p is an adaptor protein that links Myo4p to its cargo. She2p binds to ASH1 and IST2 mRNA, while She3p binds to both She2p and Myo4p. Here we show that Myo4p and She3p, but not She2p, are required for the inheritance of cortical ER in the budding yeast Saccharomyces cerevisiae. Consistent with this observation, we find that cortical ER inheritance is independent of mRNA transport. Cortical ER is a dynamic network that forms cytoplasmic tubular connections to the nuclear envelope. ER tubules failed to grow when actin polymerization was blocked with the drug latrunculin A (Lat-A). Additionally, a reduction in the number of cytoplasmic ER tubules was observed in Lat-A–treated and myo4Δ cells. Our results suggest that Myo4p and She3p facilitate the growth and orientation of ER tubules.

Fusion between phagosomes, early and late endosomes: a role for actin in fusion between late, but not early endocytic organelles

Actin is implicated in membrane fusion, but the precise mechanisms remain unclear. We showed earlier that membrane organelles catalyze the de novo assembly of F-actin that then facilitates the fusion between latex bead phagosomes and a mixture of early and late endocytic organelles. Here, we correlated the polymerization and organization of F-actin with phagosome and endocytic organelle fusion processes in vitro by using biochemistry and light and electron microscopy. When membrane organelles and cytosol were incubated at 37 degrees C with ATP, cytosolic actin polymerized rapidly and became organized into bundles and networks adjacent to membrane organelles. By 30-min incubation, a gel-like state was formed with little further polymerization of actin thereafter. Also during this time, the bulk of in vitro fusion events occurred between phagosomes/endocytic organelles. The fusion between latex bead phagosomes and late endocytic organelles, or between late endocytic organelles themselves was facilitated by actin, but we failed to detect any effect of perturbing F-actin polymerization on early endosome fusion. Consistent with this, late endosomes, like phagosomes, could nucleate F-actin, whereas early endosomes could not. We propose that actin assembled by phagosomes or late endocytic organelles can provide tracks for fusion-partner organelles to move vectorially toward them, via membrane-bound myosins, to facilitate fusion.

Arabidopsis CROOKED encodes for the smallest subunit of the ARP2/3 complex and controls cell shape by region specific fine F-actin formation

The generation of a specific cell shape requires differential growth, whereby specific regions of the cell expand more relative to others. The Arabidopsis crooked mutant exhibits aberrant cell shapes that develop because of mis-directed expansion, especially during a rapid growth phase. GFP-aided visualization of the F-actin cytoskeleton and the behavior of subcellular organelles in different cell-types in crooked and wild-type Arabidopsis revealed that localized expansion is promoted in cellular regions with fine F-actin arrays but is restricted in areas that maintain dense F-actin. This suggested that a spatiotemporal distinction between fine versus dense F-actin in a growing cell could determine the final shape of the cell. CROOKED was molecularly identified as the plant homolog of ARPC5, the smallest sub-unit of the ARP2/3 complex that in other organisms is renowned for its role in creating dendritic arrays of fine F-actin. Rescue of crooked phenotype by the human ortholog provides the first molecular evidence for the presence and functional conservation of the complex in higher plants. Our cell-biological and molecular characterization of CROOKED suggests a general actin-based mechanism for regulating differential growth and generating cell shape diversity.

Loss of actin cytoskeletal function and EDS1 activity, in combination, severely compromises non-host resistance in Arabidopsis against wheat powdery mildew

Plant immunity against the majority of the microbial pathogens is conveyed by a phenomenon known as non-host resistance (NHR). This defence mechanism affords durable protection to plant species against given species of phytopathogens. We investigated the genetic basis of NHR in Arabidopsis against the wheat powdery mildew fungus Blumeria graminis f. sp. tritici (Bgt). Both primary and appressorial germ tubes were produced from individual Bgt conidia on the surface of the Arabidopsis leaves. Attempted infection occasionally resulted in successful penetration, which led to the development of an abnormal unilateral haustorium. Inoculation of a series of Arabidopsis defence-related mutants with Bgt resulted in the attenuation of reactive oxygen intermediate (ROI) production and salicylic acid (SA)-dependent defence gene expression in eds1, pad4 and nahG plants, which are known to be defective in some aspects of host resistance. Furthermore, Bgt often developed bilateral haustoria in the mutant Arabidopsis lines that closely resembled those formed in wheat. A similar decrease in NHR was observed following treatment of the wild-type Arabidopsis plants with cytochalasin E, an inhibitor of actin microfilament polymerisation. In eds1 mutants, inhibition of actin polymerisation severely compromised NHR in Arabidopsis against Bgt. This permitted completion of the Bgt infection cycle on these plants. Therefore, actin cytoskeletal function and EDS1 activity, in combination, are major contributors to NHR in Arabidopsis against wheat powdery mildew.

Microtubules and microfilaments in cell morphogenesis in higher plants

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

Involvement of the mitogen-activated protein kinase SIMK in regulation of root hair tip growth

Mitogen-activated protein kinases (MAPKs) are involved in stress signaling to the actin cytoskeleton in yeast and animals. We have analyzed the function of the stress-activated alfalfa MAP kinase SIMK in root hairs. In epidermal cells, SIMK is predominantly nuclear. During root hair formation, SIMK was activated and redistributed from the nucleus into growing tips of root hairs possessing dense F-actin meshworks. Actin depolymerization by latrunculin B resulted in SIMK relocation to the nucleus. Conversely, upon actin stabilization with jasplakinolide, SIMK co-localized with thick actin cables in the cytoplasm. Importantly, latrunculin B and jasplakinolide were both found to activate SIMK in a root-derived cell culture. Loss of tip-focused SIMK and actin was induced by the MAPK kinase inhibitor UO 126 and resulted in aberrant root hairs. UO 126 inhibited targeted vesicle trafficking and polarized growth of root hairs. In contrast, overexpression of gain-of-function SIMK induced rapid tip growth of root hairs and could bypass growth inhibition by UO 126. These data indicate that SIMK plays a crucial role in root hair tip growth.

Actin cytoskeleton in plants: from transport networks to signaling networks

The plant actin cytoskeleton is characterized by a high diversity in regard to gene families, isoforms, and degree of polymerization. In addition to the most abundant F-actin assemblies like filaments and their bundles, G-actin obviously assembles in the form of actin oligomers composed of a few actin molecules which can be extensively cross-linked into complex dynamic meshworks. The role of the actomyosin complex as a force generating system - based on principles operating as in muscle cells - is clearly established for long-range mass transport in large algal cells and specialized cell types of higher plants. Extended F-actin networks, mainly composed of F-actin bundles, are the structural basis for this cytoplasmic streaming of high velocities On the other hand, evidence is accumulating that delicate meshworks built of short F-actin oligomers are critical for events occurring at the plasma membrane, e.g., actin interventions into activities of ion channels and hormone carriers, signaling pathways based on phospholipids, and exo- and endocytotic processes. These unique F-actin arrays, constructed by polymerization-depolymerization processes propelled via synergistic actions of actin-binding proteins such as profilin and actin depolymerizing factor (ADF)/cofilin are supposed to be engaged in diverse aspects of plant morphogenesis. Finally, rapid rearrangements of F-actin meshworks interconnecting endocellular membranes turn out to be especially important for perception-signaling purposes of plant cells, e.g., in association with guard cell movements, mechano- and gravity-sensing, plant host-pathogen interactions, and wound-healing.