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Nuclear migration during growth and development is a conserved phenomenon among many eukaryotic species. In Arabidopsis, movement of the nucleus is important for root hair growth, but the detailed mechanism behind this movement is not well known. Previous studies in different cell types have reported that the myosin XI-I motor protein is responsible for this nuclear movement by attaching to the nuclear transmembrane protein complex WIT1/WIT2. Here, we analyzed nuclear movement in growing root hairs of wild-type, myosin xi-i , and wit1 wit2 Arabidopsis lines in the presence of actin and microtubule-disrupting inhibitors to determine the individual effects of actin filaments and microtubules on nuclear movement. We discovered that forward nuclear movement during root hair growth can occur in the absence of myosin XI-I, suggesting the presence of an alternative actin-based mechanism that mediates rapid nuclear displacements. By quantifying nuclear movements with high temporal resolution during the initial phase of inhibitor treatment, we determined that microtubules work to dampen erratic nuclear movements during root hair growth. We also observed microtubule-dependent backwards nuclear movement when actin filaments were impaired in the absence of myosin XI-I, indicating the presence of complex interactions between the cytoskeletal arrays during nuclear movements in growing root hairs. [ABSTRACT FROM AUTHOR]

Plant organs control their growth orientation in response to gravity. Within gravity-sensing cells, the input (gravity sensing) and signal conversion (gravity signalling) progress sequentially. The cells contain a number of high-density, starch-accumulating amyloplasts, which sense gravity when they reposition themselves by sedimentation to the bottom of the cell when the plant organ is re-orientated. This triggers the next step of gravity signalling, when the physical signal generated by the sedimentation of the amyloplasts is converted into a biochemical signal, which redirects auxin transport towards the lower flank of the plant organ. This review focuses on recent advances in our knowledge of the regulatory mechanisms that underlie amyloplast sedimentation and the system by which this is perceived, and on recent progress in characterising the factors that play significant roles in gravity signalling by which the sedimentation is linked to the regulation of directional auxin transport. Finally, we discuss the contribution of gravity signalling factors to the mechanisms that control the gravitropic set-point angle. [ABSTRACT FROM AUTHOR]

Organelle movement is an evident feature of pollen tubes and is essential for the process of tube growth because it enables the proper distribution of organelles and the accumulation of secretory vesicles in the tube apex. Organelles move along the actin filaments through dynamic interactions with myosin but other proteins are probably responsible for control of this activity. The role of microtubules and microtubule-based motors is less clear and somewhat enigmatic. Nevertheless, the pollen tube is an excellent cell model in which to study and analyse the molecular mechanisms that drive and control organelle motility in relation to plant cell expansion. Current knowledge and the main scientific discoveries in this field of research over the last 20 years are summarized here. Future prospects in the study of the molecular mechanisms that mediate organelle transport and vesicle accumulation during pollen tube elongation are also discussed. [ABSTRACT FROM PUBLISHER]

Mitogen‐activated protein kinases (MAPKs) are ubiquitous phosphorylation enzymes involved in signal transduction, gene expression and activation of diverse cytoskeletal proteins. MAPKs participate in the regulation of a broad range of crucial cellular processes including cell survival, division, polarization, stress responses, and metabolism. Phosphorylation of cytoskeletal proteins usually results in the rearrangement of cytoskeletal arrays leading to morphological changes and cell polarization. On the other hand, some cytoskeletal motor proteins, such as kinesins, could activate MAPK members and participate in signal delivery to the proper cellular destination (e.g. during cell division). Moreover, changes in the integrity of cytoskeletal elements have direct impacts on MAPK activity. Recent evidence suggests that there is bi‐directional signalling between MAPK cascades and cytoskeleton. The focus here is on this cross‐talk between MAPK signalling and the cytoskeleton in various eukaryotic systems including yeast, plants, and mammals and a role is proposed for MAPKs as sensors monitoring the cytoskeleton‐dependent balance of forces within the cell. [ABSTRACT FROM PUBLISHER]

Actin filament (AF) organization was studied during the plasmolytic cycle in leaf cells of Chlorophyton comosum Thunb. In most cells the hyperosmotic treatment induced convex or concave plasmolysis and intense reorganization of the AF cytoskeleton. Thin cortical AFs disappeared and numerous cortical, subcortical and endoplasmic AFs arranged in thick and well‐organized bundles were formed. Plasmolysed cells displayed a significant increase in the overall AF content compared with the control cells. Cortical AF bundles were preferentially localized in the shrunken protoplast areas, lining the detached plasmalemma regions. The endoplasmic AF bundles were mainly found in the perinuclear cytoplasm and on the tonoplast surface. AFs also traversed some of the Hechtian strands. AF disorganization after cytochalasin B (CB) treatment induced dramatic changes in the pattern of plasmolysis, which lasted for a longer time and led to a greater decrease of the protoplast volume compared to the untreated cells. In many of the above cells the protoplasts assumed an ‘amoeboid’ form and were often subdivided into sub‐protoplasts. Soon after the removal of the plasmolytic solution both CB‐treated and untreated cells were deplasmolysed, while the AF cytoskeleton gradually reassumed the organization observed in the control cells. The findings of this study revealed for the first time in angiosperm cells that plasmolysis triggers an extensive reorganization of the AF cytoskeleton, which is involved in the regulation of protoplast shape and volume. The probable mechanism(s) leading to AF reorganization as well as the function(s) of the atypical AF arrays in plasmolysed cells are discussed. [ABSTRACT FROM PUBLISHER]

Organelle movement and positioning are essential for proper cellular function. A nucleus moves dynamically during cell division and differentiation and in response to environmental changes in animal, fungal, and plant cells. Nuclear movement is well-studied and the mechanisms have been mostly elucidated in animal and fungal cells, but not in plant cells. In prothallial cells of the fern Adiantum capillus-veneris and leaf cells of the flowering plant Arabidopsis thaliana, light induces nuclear movement and nuclei change their position according to wavelength, intensity, and direction of light. This nuclear photorelocation movement shows some common features with the photorelocation movement of chloroplasts, which is one of the best-characterized plant organelle movements. This review summarizes nuclear movement and positioning in plant cells, especially plant-specific nuclear photorelocation movement and discusses the relationship between nuclear photorelocation movement and chloroplast photorelocation movement., (© The Author 2013. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)