title: Single microfilaments mediate the early steps of microtubule bundling during preprophase band formation in onion cotyledon epidermal cells.
abstract: The preprophase band (PPB) is a cytokinetic apparatus that determines the site of cell division in plants. It originates as a broad band of microtubules (MTs) in G2 and narrows to demarcate the future division site during late prophase. Studies with fluorescent probes have shown that PPBs contain F-actin during early stages of their development but become actin depleted in late prophase. Although this suggests that actins contribute to the early stages of PPB formation, how actins contribute to PPB-MT organization remains unsolved. To address this question, we used electron tomography to investigate the spatial relationship between microfilaments (MFs) and MTs at different stages of PPB assembly in onion cotyledon epidermal cells. We demonstrate that the PPB actins observed by fluorescence microscopy correspond to short, single MFs. A majority of the MFs are bound to MTs, with a subset forming MT-MF-MT bridging structures. During the later stages of PPB assembly, the MF-mediated links between MTs are displaced by MT-MT linkers as the PPB MT arrays mature into tightly packed MT bundles. On the basis of these observations, we propose that the primary function of actins during PPB formation is to mediate the initial bundling of the PPB MTs.
authors.affiliation: Graduate School of Life Science, University of Hyogo, Himeji 671-2201, Japan Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo 113-8657, Japan.
authors.forename: Miyuki
authors.initials: M
authors.lastname: Takeuchi
authors.affiliation: Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan.
authors.forename: Ichirou
authors.initials: I
authors.lastname: Karahara
authors.affiliation: Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki 567-0047, Japan.
authors.forename: Naoko
authors.initials: N
authors.lastname: Kajimura
authors.affiliation: Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki 567-0047, Japan.
authors.forename: Akio
authors.initials: A
authors.lastname: Takaoka
authors.affiliation: National Institute for Physiological Sciences, Okazaki 444-8585, Japan.
authors.forename: Kazuyoshi
authors.initials: K
authors.lastname: Murata
authors.affiliation: RIKEN Center for Life Science Technologies, Kobe 650-0047, Japan.
authors.forename: Kazuyo
authors.initials: K
authors.lastname: Misaki
authors.affiliation: RIKEN Center for Life Science Technologies, Kobe 650-0047, Japan.
authors.forename: Shigenobu
authors.initials: S
authors.lastname: Yonemura
authors.affiliation: Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, CO 80309-0347.
authors.forename: L Andrew
authors.initials: LA
authors.lastname: Staehelin
authors.affiliation: Graduate School of Life Science, University of Hyogo, Himeji 671-2201, Japan mineyuki@sci.u-hyogo.ac.jp.
authors.forename: Yoshinobu
authors.initials: Y
authors.lastname: Mineyuki
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism*; D000199:Actins / Q000378:metabolism; D002455:Cell Division / Q000502:physiology; D018548:Cotyledon / Q000166:cytology / Q000378:metabolism*; D048749:Cytokinesis; D003599:Cytoskeleton / Q000378:metabolism; D055032:Electron Microscope Tomography; D008870:Microtubules / Q000378:metabolism*; D008938:Mitosis; D019697:Onions / Q000166:cytology / Q000378:metabolism*; D011418:Prophase; D014404:Tubulin / Q000378:metabolism
chemical_list: D000199:Actins; D014404:Tubulin