Actin Microfilaments Regulate Vacuolar Structures and Dynamics: Dual Observation of Actin Microfilaments and Vacuolar Membrane in Living Tobacco BY-2 Cells

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Abstract
Actin microfilaments (MFs) participate in many fundamental processes in plant growth and development. Here, we report the co-localization of the actin MF and vacuolar membrane (VM), as visualized by vital VM staining with FM4-64 in living tobacco BY-2 cells stably expressing green fluorescent protein (GFP)–fimbrin (BY-GF11). The MFs were intensively localized on the VM surface and at the periphery of the cytoplasmic strands rather than at their center. The co-localization of MFs and VMs was confirmed by the observation made using transient expression of red fluorescent protein (RFP)–fimbrin in tobacco BY-2 cells stably expressing GFP–AtVam3p (BY-GV7) and BY-2 cells stably expressing γ-tonoplast intrinsic protein (γ-TIP)–GFP fusion protein (BY-GG). Time-lapse imaging revealed dynamic movement of MF structures which was parallel to that of cytoplasmic strands. Disruption of MF structures disorganized cytoplasmic strand structures and produced small spherical vacuoles in the VM-accumulating region. Three-dimensional reconstructions of the vacuolar structures revealed a disconnection of these small spherical vacuoles from the large vacuoles. Real-time observations and quantitative image analyses demonstrated rapid movements of MFs and VMs near the cell cortex, which were inhibited by the general myosin ATPase inhibitor, 2,3-butanedion monoxime (BDM). Moreover, both bistheonellide A (BA) and BDM treatment inhibited the reorganization of the cytoplasmic strands and the migration of daughter cell nuclei at early G 1 phase, suggesting a requirement for the acto-myosin system for vacuolar morphogenesis during cell cycle progression. These results suggest that MFs support the vacuolar structures and that the acto-myosin system plays an essential role in vacuolar morphogenesis.