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

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

There are two classes of myosin, XI and VIII, in higher plants. Myosin XI moves actin filaments at high speed and its enzyme activity is also very high. In contrast, myosin VIII moves actin filaments very slowly with very low enzyme activity. Because most of these enzymatic and motile activities were measured using animal skeletal muscle α-actin, but not plant actin, they would not accurately reflect the actual activities in plant cells. We thus measured enzymatic and motile activities of the motor domains of two Arabidopsis myosin XI isoforms (MYA2, XI-B), and one Arabidopsis myosin VIII isoform (ATM1), by using three Arabidopsis actin isoforms (ACT1, ACT2, and ACT7). The measured activities were different from those measured by using muscle actin. Moreover, Arabidopsis myosins showed different enzymatic and motile activities when using different Arabidopsis actin isoforms. Our results suggest that plant actin should be used for measuring enzymatic and motile activities of plant myosins and that different actin isoforms in plant cells might function as different tracks along which affinities and velocities of each myosin isoform are modulated. [ABSTRACT FROM AUTHOR]

Long-distance communications in giant characean internodal cells involve cytoplasmic streaming as an effective means for transportation of regulatory substances. The local illumination of Chara corallina Klein ex C.L. Willdenow internodal cells with an intense 30 s pulse of white light caused a transient increase of modulated chlorophyll fluorescence in cell regions positioned downstream the cytoplasmic flow after a delay whose duration increased with the axial distance from the light source. No changes in fluorescence were observed in cell regions residing upstream of the light spot. The transient increase in actual fluorescence F0 in cell areas exposed to constant dim illumination at large distances from the brightly lit area indicates the transmission of photosynthetically active metabolite between chloroplasts separated by 1–5mm distances. The shapes of fluorescence transients were sensitive to retardation of cytoplasmic streaming by cytochalasin D and to variations in cyclosis velocity during gradual recovery of streaming after an instant arrest of cyclosis by elicitation of the action potential. Furthermore, the analysed fluorescence transients were skewed on the ascending or descending fronts depending on the position of light-modulated cytoplasmic package at the moment of streaming cessation with respect to the point of measurements. The observations are simulated in qualitative terms with a simplified streaming–diffusion model. [ABSTRACT FROM AUTHOR]

Purpose To provide a biophysical basis to estimate the effect of cytoplasmic flow in neurons, and assess their contribution to the drop in the Apparent Diffusion Coefficient (ADC) in a nerve tissue following extreme conditions, such as brain injury and epileptic seizures. Methods Three mechanisms are treated using the relevant physics of hydrodynamics and electrostatics: cargo induced streaming, electroosmosis, and membrane swelling. Results We begin by discussing the lack of experimental evidence on the necessary velocities required to influence the Magnetic Resonance (MR) experiments. This is followed by demonstrating that cargo induced streaming, a widely known phenomenon in plant cells, has a minor effect on the ADC in neurons. Subsequently, we suggest and analyze two additional mechanisms that may induce fluid displacement in neurons, and are related to the electrical activity: electroosmosis and membrane swelling. Conclusion Although these mechanisms may induce interesting fluid displacements, these cannot explain the significant drop in the ADC. We conclude by outlining the criteria that any future mechanism should meet to have an influence on standard diffusion-MR measurements. Magn Reson Med 78:746-753, 2017. © 2016 International Society for Magnetic Resonance in Medicine [ABSTRACT FROM AUTHOR]

Plasmolysis of hypocotyl cells of transgenic Arabidopsis thaliana and Nicotiana benthamiana diminishes the dynamics of the remodeling of the endoplasmic reticulum (ER) in the central protoplast, namely that withdrawn from the cell wall, and more persistent cisternae are formed, yet little change in the actin network in the protoplast occurs. Also, protein flow within the ER network in the protoplast, as detected with fluorescence recovery after photobleaching (FRAP), is not affected by plasmolysis. After plasmolysis, another network of strictly tubular ER remains attached to the plasma membrane-wall interface and is contained within the Hechtian strands and reticulum. FRAP studies indicate that protein flow within these ER tubules diminishes. Actin is largely absent from the Hechtian reticulum and the ER becomes primarily associated with altered, branched microtubules. The smaller volume of the central protoplast is accompanied by decreased movement rates of tubules, cisternae, and spheroid organelles, but this reduced movement is not readily reversed by the increase in volume that accompanies deplasmolysis. [ABSTRACT FROM AUTHOR]

Microscopy imaging allows detailed observations of intracellular movements and the acquisition of large datasets that can be fully analyzed only by automated algorithms. Here, we develop a computational method for the automatic identification and reconstruction of trajectories followed by subcellular particles captured in microscopy image data. The method operates on stacks of raw image data and computes the complete set of contained trajectories. The method utilizes topological data analysis and standard image processing techniques and makes no assumptions about the underlying dynamics besides continuity. We test the developed method successfully against artificial and experimental datasets. Application of the method on the experimental data reveals good agreement with manual tracking and benchmarking yields performance scores competitive to the existing state-of-the-art tracking methods. [ABSTRACT FROM AUTHOR]

Chloroplasts in vivo exposed to strong light export assimilates and excess reducing power to the cytoplasm for metabolic conversions and allocation to neighboring and distant organelles. The cytoplasmic streaming, being particularly fast in characean internodes, distributes the exported metabolites from brightly illuminated cell spots to light-limited regions, which is evident from the transient increase in chlorophyll fluorescence of shaded areas in response to illumination of distant cell regions situated upstream the liquid flow. It is not yet known whether long-distance communications between anchored chloroplasts are interfered by pH banding that commonly arises in characean internodes under the action of continuous or fluctuating light. In this study, microfluorometry, pH-microsensors, and local illumination were combined to examine long-distance transport and subsequent reentry of photosynthetic metabolites, including triose phosphates, into chloroplasts of cell regions producing external alkaline and acid bands. The lateral transmission of metabolic signals between distant chloroplasts was found to operate effectively in cell areas underlying acid zones but was almost fully blocked under alkaline zones. The rates of linear electron flow in chloroplasts of these regions were nearly equal under dim background light, but differed substantially at high light when availability of CO 2 , rather than irradiance, was the rate-limiting factor. Different productions of assimilates by chloroplasts underlying CO 2 -sufficient acid and CO 2 -deficient alkaline zones were a cause for contrasting manifestations of long-distance transport of photosynthetic metabolites. Nonuniform cytoplasmic pH in cells exhibiting pH bands might contribute to different activities of metabolic translocators under high and low pH zones. [ABSTRACT FROM AUTHOR]

We investigate the myosin XI-driven transport network in Arabidopsis using protein-protein interaction, subcellular localization, gene knockout, and bioinformatics analyses. The two major groups of nodes in this network are myosins XI and their membrane-anchored receptors (MyoB) that, together, drive endomembrane trafficking and cytoplasmic streaming in the plant cells. The network shows high node connectivity and is dominated by generalists, with a smaller fraction of more specialized myosins and receptors. We show that interaction with myosins and association with motile vesicles are common properties of the MyoB family receptors. We identify previously uncharacterized myosin-binding proteins, putative myosin adaptors that belong to two unrelated families, with four members each (MadA and MadB). Surprisingly, MadA1 localizes to the nucleus and is rapidly transported to the cytoplasm, suggesting the existence of myosin XI-driven nucleocytoplasmic trafficking. In contrast, MadA2 and MadA3, as well as MadB1, partition between the cytosolic pools of motile endomembrane vesicles that colocalize with myosin XI-K and diffuse material that does not. Gene knockout analysis shows that MadB1-4 contribute to polarized root hair growth, phenocopying myosins, whereas MadA1-4 are redundant for this process. Phylogenetic analysis reveals congruent evolutionary histories of the myosin XI, MyoB, MadA, and MadB families. All these gene families emerged in green algae and show concurrent expansions via serial duplication in flowering plants. Thus, the myosin XI transport network increased in complexity and robustness concomitantly with the land colonization by flowering plants and, by inference, could have been a major contributor to this process. [ABSTRACT FROM AUTHOR]