132 T. Sano et al.
sensus Cdk site in AtMAP65-1 to a non-phosphorylatable form affected the
recruitment of MAP65 proteins to the central spindle at anaphase (Mao et al.
2005). During phragmoplast development, phosphorylation of MAP65-1 by
a NRK1/NTF6 MAP kinase regulated MT stability and hence progression
of cytokinesis (Sasabe et al. 2006). Recently, three Aurora kinase orthologs,
AtAUR1, AtAUR2 and AtAUR3, were identified inArabidopsis(Kawabe et al.
2005). In animal systems, the Aurora serine/threonine protein kinases reg-
ulate cell cycle events, including chromosome bi-orientation and segrega-
tion, in which the mitotic spindle MTs function (Andrews et al. 2003). GFP-
tagged AtAUR proteins demonstrated AtAUR1 and 2 to be localized to the
mitotic spindle and AtAUR3 to the chromosomes during cell division. More-
over, overexpression of AtAUR3 induced the disassembly of spindle MTs
(Kawabe et al. 2005). These findings indicate that the characteristics and func-
tions of plant Aurora kinases in mitotic events differ from those of other
eukaryotes.
4
Dynamics of Actin Microfilaments during Cell Cycle Progression
The actin microfilaments (MFs) of higher plants are involved in various
aspects of plant morphogenesis, including pollen tube growth, trichome de-
velopment and the formation of stomatal complexes (McCurdy et al. 2001).
During cell cycle progression, a dynamic change in the actin cytoskeleton has
been demonstrated by fluorescent-phalloidin labeling after fixation or by mi-
croinjection into various plant cells (Traas et al. 1987; Hasezawa et al. 1989,
Schmit and Lambert 1990, Cleary et al. 1992; Liu and Palevitz 1992). These re-
ports described three major MF structures from the G2 phase to mitosis; the
MF band in G2 phase, the so-called actin-depleted zone (ADZ) in mitosis, and
a structure that overlaps the phragmoplast. The ADZ was identified as a band
or zone in the middle of the cell from where cortical MF structures disap-
peared while adjacent cortical MF structures remained intact. This structure
appeared at late G2 to mitosis at the original position of the PPB after the PPB
had disappeared and has been proposed to function as a memory of the PPB
position (Cleary 1995; Hoshino et al. 2003).
Recent GFP techniques have again successfully labeled plant MFs by using
GFP fusions with actin-binding domains (ABD) from mouse talin or fimbrin
(Kost et al. 1998; Mathur et al. 1999; Sheahan et al. 2004a; Sano et al. 2005;
Voigt et al. 2005). The lower MF binding activity of the fimbrin ABD appears
to successfully allow non-invading visualization of MFs (Kovar et al. 2001;
Sheahan et al. 2004b) and a tobacco BY-2 cell line, stably transformed with
a GFP-fimbrin ABD2 construct (BY-GF), has allowed MF dynamics to be fol-
lowed during cell cycle progression (Fig. 2, Sano et al. 2005). Time-sequential
observations revealed that the MF band in the late G2 phase separated to form