Cytoskeletal and Vacuolar Dynamics During Plant Cell Division 129
demonstrated to be involved in secondary wall development during tracheary
element differentiation inArabidopsiscell culture expressing GFP-tubulin
(Oda et al. 2005; Oda and Hasezawa 2006).
3.2
Involvement of MT-Associated and Regulatory Proteins in Cell Division
Microtubules (MTs) show dynamic changes during cell cycle progression as
mentioned above. To regulate such MT dynamics, numerous proteins, known
as MT-associated proteins and regulatory proteins, are thought to be involved
(Sonobe et al. 2004), and here we describe recent findings regarding some of
these, especially involved in cell cycle progression:γ-tubulin, which is a key
factor of MT nucleation from the MT organizing center (MTOC); microtubule
associated proteins (MAPs), which appear to have roles in MT organization
and function; the 26S proteasome, which is a key component of cyclin de-
struction; and protein kinases, which regulate the activity of MAPs. Kinesins
and other kinesin-like motor proteins that are thought to be involved in MT
translocation are also described in this volume (Liu 2007, in this volume).
γ-Tubulin.In eukaryotic cells, MTs are thought to be nucleated and to
be organized from distinct structures called MTOCs, the biochemical and
structural features of which have been intensively studied using animal cell
centrosomes as models (Andersen 1999). In higher plant cells, however, no
distinct structures comparable to the animal centriolar MTOCs are found,
and consequently the nature of the plant MTOCs remains unclear (Vaughn
and Harper 1998). To explain these plant MTOC characteristics, the local-
ization ofγ-tubulin, considered a candidate protein in plant MTOCs, was
examined. Gamma-tubulin is a member of the tubulin superfamily and forms
γ-tubulin ring complexes (γ-TuRCs) to initiate MT assembly (Schiebel 2000;
Job et al. 2003). Electron microscopy suggested that theγ-tubulin in this com-
plex interacts directly withα-tubulin at the microtubule minus end (Keating
and Borisy 2000; Wiese and Zheng 2000; Moritz et al. 2000). Althoughγ-
tubulin is localized to a limited area of animal centrosomes and yeast spindle
poles, immuno-fluorescence microscopy indicated that in higher plant cells
these proteins are widely associated with MT structures of the PPB, mitotic
spindle and phragmoplast, as well as with the cell cortex and nuclear surface
(Liu et al. 1993, 1994, 1995).
The use of transgenic BY-2 cells stably expressing GFP-γ-tubulin not only
confirmed the above localization ofγ-tubulin but also revealed, through
time-sequential observations, that during MT reorganization at the M/G1 in-
terfaceγ-tubulin first accumulates on the daughter nuclear surfaces and then
spreads onto the cell cortex along with MTs elongating from the daughter nu-
clei (Kumagai et al. 2003). From these observations, two possible processes
can be considered in plant cells; either the MTOCs are extensively distributed
as isγ-tubulin, or that theγ-tubulin proteins localize to sites other than the